ALBERT

Description: The sharp decline of East Asian summer monsoon at mid-Holocene indicated by the lake-wetland transition in the Sanjiang Plain, northeastern China Climate of the Past Discussions, 10, 4595-4622, 2014 Author(s): Z. Q. Zhang, G. P. Wang, X. G. Lv, H. J. Jia, and Q. H. Xu The timing of the waxing and wining of the East Asian summer monsoon during the Holocene is still under debate. In present study, we present the high-resolution grain-size and LOI records from a well-dated mud/peat profile to reveal the lake-wetland transition in the Sanjiang Plain and discuss its significance to Holocene monsoon evolutions. The results show that the shallow-water lakes have developed in low-lying areas of the plain before 4600 yr BP, corresponding to the Holocene monsoon maximum. Thereafter, the wetlands began to initiate with the extinction of the paleolakes, marking a lake-shrinking stage with the relative dry climate. Considering the prevalent monsoon climate in the Sanjiang Plain, we suggest the lake-wetland transition at 4600 yr BP indicate a sharp decline of the summer monsoon rather than the basin infilling process. Such a remarkable monsoon weakening event has been widely documented in northern China, and we associated it with the ocean–atmosphere interacting processes in low-latitude regions.

Description: A two thousand year annual record of snow accumulation rates for Law Dome, East Antarctica Climate of the Past Discussions, 10, 4469-4497, 2014 Author(s): J. Roberts, C. Plummer, T. Vance, T. van Ommen, A. Moy, S. Poynter, A. Treverrow, M. Curran, and S. George Accurate high resolution records of snow accumulation rates in Antarctica are crucial for estimating ice sheet mass balance and subsequent sea level change. Snowfall rates at Law Dome, East Antarctica, have been linked with regional atmospheric circulation to mid-latitudes as well as regional Antarctic snowfall. Here, we extend the Law Dome accumulation record from 750 to 2035 years, using recent annual layer dating that extends to AD −22. Accumulation rates were calculated as the ratio of measured to modelled layer thicknesses, multiplied by the long term mean accumulation rate. The modelled layer thicknesses were based on a power law vertical strain rate profile fitted to observed annual layer thickness. The periods AD 380–442, AD 727–783 and AD 1970–2009 have above average snow accumulation rates, while AD 663–704, AD 933–975 and AD 1429–1468 were below average. The calculated snow accumulation rates show good correlation with atmospheric reanalysis estimates, and significant spatial correlation over a wide expanse of East Antarctica, demonstrating that the Law Dome record captures larger scale variability across a large region of East Antarctica well beyond the immediate vicinity of the Law Dome summit. Spectral analysis reveals periodicities in the snow accumulation record which may be related to ENSO and Interdecadal Pacific Oscillation frequencies.

Description: Technical Note: Probabilistically constraining proxy age–depth models within a Bayesian hierarchical reconstruction model Climate of the Past Discussions, 10, 4499-4533, 2014 Author(s): J. P. Werner and M. P. Tingley Reconstructions of late-Holocene climate rely heavily upon proxies that are assumed to be accurately dated by layer counting, such as measurement on tree rings, ice cores, and varved lake sediments. Considerable advances may be achievable if time uncertain proxies could be included within these multiproxy reconstructions, and if time uncertainties were recognized and correctly modeled for proxies commonly treated as free of age model errors. Current approaches to accounting for time uncertainty are generally limited to repeating the reconstruction using each of an ensemble of age models, thereby inflating the final estimated uncertainty – in effect, each possible age model is given equal weighting. Uncertainties can be reduced by exploiting the inferred space–time covariance structure of the climate to re-weight the possible age models. Here we demonstrate how Bayesian Hierarchical climate reconstruction models can be augmented to account for time uncertain proxies. Critically, while a priori all age models are given equal probability of being correct, the probabilities associated with the age models are formally updated within the Bayesian framework, thereby reducing uncertainties. Numerical experiments show that updating the age-model probabilities decreases uncertainty in the climate reconstruction, as compared with the current de-facto standard of sampling over all age models, provided there is sufficient information from other data sources in the region of the time-uncertain proxy. This approach can readily be generalized to non-layer counted proxies, such as those derived from marine sediments.

Description: Factors controlling the last interglacial climate as simulated by LOVECLIM1.3 Climate of the Past Discussions, 10, 235-290, 2014 Author(s): M. F. Loutre, T. Fichefet, H. Goosse, P. Huybrechts, H. Goelzer, and E. Capron The last interglacial (LIG), also identified to the Eemian in Europe, began approximately at 130 kyr BP and ended at about 115 kyr BP (Before Present). More and more proxy-based reconstructions of the LIG climate become available even though they remain sparse. The major climate forcings during the LIG are rather well known and therefore models can be tested against paleoclimatic datasets and then used to better understand the climate of the LIG. However, models are displaying a large range of responses, being sometimes contradictory between them or with the reconstructed data. Here we would like to investigate causes of these differences. We focus on a single climate model, LOVECLIM, and we perform transient simulations over the LIG, starting at 135 kyr BP and run until 115 kyr BP. With these simulations, we test the role of the surface boundary conditions (the time-evolution of the Northern Hemisphere (NH) ice sheets) on the simulated LIG climate and the importance of the parameter sets (internal to the model, such as the albedos of the ocean and sea ice), which affect the sensitivity of the model. The magnitude of the simulated climate variations through the LIG remains too low for climate variables such as surface air temperature. Moreover, in the North Atlantic, the large increase summer sea surface temperature towards the peak of the interglacial occurs too early (at ~128 kyr BP) compared to the reconstructions. This feature as well as the climate simulated during the optimum of the LIG, between 130 and 121 kyr BP, are robust to changes in surface boundary conditions and parameter sets. The additional freshwater flux (FWF) from the melting NH ice sheets is responsible for a temporary abrupt weakening of the North Atlantic meridional overturning circulation, which causes a strong global cooling in annual mean. However, the changes in the configuration (extent and albedo) of the NH ice sheets during the LIG only slightly impact the simulated climate. Together, configuration of and FWF from the NH ice sheets greatly increase the magnitude of the temperature variations over continents as well as over the ocean at the beginning of the simulation and reduce the difference between the simulated climate and the reconstructions. At last, we show that the contribution from the parameter sets to the climate response is actually very modest.

Description: Southern high-latitude terrestrial climate change during the Paleocene–Eocene derived from a marine pollen record (ODP Site 1172, East Tasman Plateau) Climate of the Past Discussions, 10, 291-340, 2014 Author(s): L. Contreras, J. Pross, P. K. Bijl, R. B. O'Hara, J. I. Raine, A. Sluijs, and H. Brinkhuis Reconstructing the early Paleogene climate dynamics of terrestrial settings in the high southern latitudes is important to assess the role of high-latitude physical and biogeochemical processes in the global climate system. However, whereas a number of high-quality Paleogene climate records has become available for the marine realm of the high southern latitudes over the recent past, the long-term evolution of coeval terrestrial climates and ecosystems is yet poorly known. We here explore the climate and vegetation dynamics on Tasmania from the middle Paleocene to the early Eocene (60.7–54.2 Ma) based on a sporomorph record from Ocean Drilling Program (ODP) Site 1172 on the East Tasman Plateau. Our results show that three distinctly different vegetation types thrived on Tasmania under a high-precipitation regime during the middle Paleocene to early Eocene, with each type representing different temperature conditions: (i) warm-temperate forests dominated by gymnosperms that were dominant during the middle and late Paleocene; (ii) cool-temperate forests dominated by southern beech ( Nothofagus ) and araucarians across the middle/late Paleocene transition interval (~59.5 to ~59.0 Ma); and (iii) paratropical forests rich in ferns that were established during and in the wake of the Paleocene–Eocene Thermal Maximum (PETM). The transient establishment of cool-temperate forests lacking any frost-sensitive elements (i.e., palms and cycads) across the middle/late Paleocene transition interval indicates markedly cooler conditions, with the occurrence of frosts in winter, on Tasmania during that time. The integration of our sporomorph data with previously published TEX 86 -based sea-surface temperatures from ODP Site 1172 documents that the vegetation dynamics on Tasmania were closely linked with the temperature evolution in the Tasman sector of the Southwest Pacific region. Moreover, the comparison of our season-specific climate estimates for the sporomorph assemblages from ODP Site 1172 with the TEX 86 L - and TEX 86 H -based temperature data suggests a warm-season bias of both calibrations for the early Paleogene of the high southern latitudes.

Description: Constraining Holocene hydrological changes in the Carpathian-Balkan region using speleothem δ 18 O and pollen-based temperature reconstructions Climate of the Past Discussions, 10, 381-427, 2014 Author(s): V. Drăguşin, M. Staubwasser, D. L. Hoffmann, V. Ersek, B. P. Onac, and D. Veres Here we present a new speleothem isotope record (POM2) from Ascunsă Cave (Romania) that provides new data on past climate changes in the Carpathian-Balkan region from 8.2 ka until present. This paper describes an approach towards constraining the effect of temperature changes on calcite δ 18 O values in stalagmite POM2 over the course of the Middle Holocene (6–4 ka), and across the 8.2 and 3.2 ka rapid climate change events. Independent pollen temperature reconstructions are used to constrain the temperature-dependent component of total isotopic change in speleothem calcite. This includes the temperature-dependent composition of rain water attained during vapour condensation and during calcite precipitation at the given cave temperature. The only prior assumptions are that pollen-derived average annual temperature reflects average cave temperature, and that pollen-derived coldest and warmest month temperatures reflect the range of condensation temperatures of rain at the cave site. This approach constrains a range of values between which speleothem isotopic changes should be found if controlled only by surface temperature variations at the cave site. Deviations of measured δ 18 O c values from the calculated range are interpreted towards large-scale hydrologic change independent of local temperature. Following this approach, we show that an additional 0.6‰ enrichment of δ 18 O c in the POM2 stalagmite was caused by changing hydrological patterns in SW Romania during the Middle Holocene. Further, by extending the calculations to other speleothem records from around the entire Mediterranean Basin, it appears that all Eastern Mediterranean speleothems recorded a similar isotopic enrichment due to changing hydrology, whereas all changes recorded in speleothems from the Western Mediterranean are fully explained by temperature variation alone. This highlights a different hydrological evolution between the two sides of the Mediterranean. Our results also demonstrate that during the 8.2 ka event, POM2 stable isotope data fit the temperature-constrained isotopic variability, with only little hydrologic change at most. In the case of the 3.2 ka event, the hydrological factor is more evident. This implies a potentially different rainfall pattern in the Southern Carpathian region during this event at the end of the Bronze Age. This study brings new evidence for disturbances in Eastern Mediterranean hydrology during the Holocene, bearing importance for the understanding of climate pressure on agricultural activities in this area.

Description: Constraining the Last Glacial Maximum climate by data-model ( i LOVECLIM) comparison using oxygen stable isotopes Climate of the Past Discussions, 10, 105-148, 2014 Author(s): T. Caley, D. M. Roche, C. Waelbroeck, and E. Michel We use the fully coupled atmosphere-ocean three-dimensional model of intermediate complexity i LOVECLIM to simulate the climate and oxygen stable isotopic signal during the Last Glacial Maximum (LGM, 21 000 yr). By using a model that is able to explicitly simulate the sensor (δ 18 O), results can be directly compared with data from climatic archives in the different realms. Our results indicate that i LOVECLIM reproduces well the main feature of the LGM climate in the atmospheric and oceanic components. The annual mean δ 18 O in precipitation shows more depleted values in the northern and southern high latitudes during the LGM. The model reproduces very well the spatial gradient observed in ice core records over the Greenland ice-sheet. We observe a general pattern toward more enriched values for continental calcite δ 18 O in the model at the LGM, in agreement with speleothem data. This can be explained by both a general atmospheric cooling in the tropical and subtropical regions and a reduction in precipitation as confirmed by reconstruction derived from pollens and plant macrofossils. Data-model comparison for sea surface temperature indicates that i LOVECLIM is capable to satisfyingly simulate the change in oceanic surface conditions between the LGM and present. Our data-model comparison for calcite δ 18 O allows investigating the large discrepancies with respect to glacial temperatures recorded by different microfossil proxies in the North Atlantic region. The results argue for a trong mean annual cooling between the LGM and present (〉 6°C), supporting the foraminifera transfer function reconstruction but in disagreement with alkenones and dinocyst reconstructions. The data-model comparison also reveals that large positive calcite δ 18 O anomaly in the Southern Ocean may be explained by an important cooling, although the driver of this pattern is unclear. We deduce a large positive δ 18 Osw anomaly for the north Indian Ocean that contrasts with a large negative δ 18 Osw anomaly in the China Sea between the LGM and present. This pattern may be linked to changes in the hydrological cycle over these regions. Our simulation of the deep ocean suggests that changes in δ 18 Osw between the LGM and present are not spatially homogenous. This is supported by reconstructions derived from pore fluids in deep-sea sediments. The model underestimates the deep ocean cooling thus biasing the comparison with benthic calcite δ 18 O data. Nonetheless, our data-model comparison support a heterogeneous cooling of few degrees (2–4°C) in the LGM Ocean.

Description: Sensitivity simulations with direct radiative forcing by aeolian dust during glacial cycles Climate of the Past Discussions, 10, 149-193, 2014 Author(s): E. Bauer and A. Ganopolski Possible feedback effects between aeolian dust, climate and ice sheets are studied for the first time with an Earth system model of intermediate complexity over the late Pleistocene period. Correlations between climate variables and dust deposits suggest that aeolian dust potentially plays an important role for the evolution of glacial cycles. Here climatic effects from the dust direct radiative forcing (DRF) caused by absorption and scattering of solar radiation are investigated. Key factors controlling the dust DRF are the atmospheric dust distribution and the absorption-scattering efficiency of dust aerosols. Effective physical parameters in the description of these factors are varied within uncertainty ranges known from available data and detailed model studies. Although the parameters are reasonably constrained by use of these studies, the simulated dust DRF spans a wide uncertainty range related to nonlinear dependencies. In our simulations, the dust DRF is highly localized. Medium-range parameters result in negative DRF of several W m −2 in regions close to major dust sources and negligible values elsewhere. In case of high absorption efficiency, the local dust DRF can reach positive values and the global mean DRF can be insignificantly small. In case of low absorption efficiency, the dust DRF can produce a significant global cooling in glacial periods which leads to a doubling of the maximum glacial ice volume relative to the case with small dust DRF. DRF-induced temperature and precipitation changes can either be attenuated or amplified through a feedback loop involving the dust cycle. The sensitivity experiments suggest that depending on dust optical parameters the DRF has the potential to either damp or reinforce glacial–interglacial climate changes.

Description: Millennial Minimum Temperature Variations in the Qilian Mountains, China: evidence from Tree rings Climate of the Past Discussions, 10, 341-380, 2014 Author(s): Y. Zhang, X. Shao, Z.-Y. Yin, and Y. Wang A 1342 yr-long tree-ring chronology was developed from Qilian junipers in the central Qilian Mountains of the north-eastern Tibetan Plateau, China. The climatic implications of this chronology were investigated using simple correlation, partial correlation and response function analyses. The chronology was significantly positively correlated with temperature variables during the pre- and current growing seasons, especially with minimum temperature. The variability of the mean minimum temperature from January to August since 670 AD was then reconstructed based on the tree-ring chronology. The reconstruction explained 58.5% of the variance in the instrumental temperature records during the calibration period (1960–2011) and captured the variation patterns in minimum temperature at the annual to centennial time scales over the past millennium. The most recent 50 yr were the warmest period, while 1690–1880 was the coldest period since 670 AD. Comparisons with other temperature series from neighbouring regions and for the Northern Hemisphere as a whole supported the validity of our reconstruction and suggested that it provided a good regional representation of temperature change in the north-eastern Tibetan Plateau. The results of multi-taper spectral analysis showed the occurrence of significant quasi-periodic behaviour at a number of periods (2–3, 28.8–66.2, 113.6–169.5, and 500 yr), which were consistent with those associated with El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and solar activity. Some reconstructed cold events may have close relationship with the volcanic eruptions.

Description: Changing climatic response: a conceptual model for glacial cycles and the Mid-Pleistocene Transition Climate of the Past Discussions, 10, 1101-1127, 2014 Author(s): I. Daruka and P. D. Ditlevsen Milankovitch's astronomical theory of glacial cycles, attributing ice age climate oscillations to orbital changes in Northern Northern-Hemisphere insolation, is challenged by the paleoclimatic record. The climatic response to the variations in insolation is far from trivial. In general the glacial cycles are highly asymmetric in time, with slow cooling from the interglacials to the glacials (inceptions) and very rapid warming from the glacials to the interglacials (terminations). We shall refer to this fast-slow dynamics as the "saw-tooth" shape of the paleoclimatic record. This is non-linearly related to the time-symmetric variations in the orbital forcing. However, the most pronounced challenge to the Milankovitch theory is the Mid-Pleistocene Transition (MPT) occurring about one million years ago. During that event, the prevailing 41 kyr glacial cycles, corresponding to the almost harmonic obliquity cycle were replaced by longer saw-tooth shaped cycles with a time scale around 100 kyr. The MPT must have been driven by internal changes in climate response, since it does not correspond to any apparent changes in the orbital forcing. In order to identify possible mechanisms causing the observed changes in glacial dynamics, it is relevant to study simplified models with the capability of generating temporal behavior similar to the observed records. We present a simple oscillator type model approach, with two variables, a temperature anomaly and an ice volume analogous, climatic memory term. The generalization of the ice albedo feedback is included in terms of an effective multiplicative coupling between this latter climatic memory term (representing the internal degrees of freedom) and the external drive. The simple model reproduces the temporal asymmetry of the late Pleistocene glacial cycles and suggests that the MPT can be explained as a regime shift, aided by climatic noise, from a period 1 frequency locking to the obliquity cycle to a period 2–3 frequency locking to the same obliquity cycle. The change in dynamics has been suggested to be a result of a slow gradual decrease in atmospheric greenhouse gas concentration. The presence of chaos in the (non-autonomous) glacial dynamics and a critical dependence on initial conditions raises fundamental questions about climate predictability.

Description: Sediment sequence and site formation processes at the Arbreda Cave, NE Iberian Peninsula, and implications on human occupation and climate change during the Last Glacial Climate of the Past Discussions, 10, 1053-1100, 2014 Author(s): M. Kehl, E. Eckmeier, S. O. Franz, F. Lehmkuhl, J. Soler, N. Soler, K. Reicherter, and G.-C. Weniger The cave of Arbreda provides a detailed archaeological record of the Middle to Upper Palaeolithic and is a key site for studying human occupation and cultural transitions in NE Iberia. Recently, studies of lake archives and archaeological sites presented new evidence on climate changes in NE Iberia correlating with Heinrich events. It, therefore, needs to be determined whether climate signals can be identified in the cave sequence of Arbreda, and if so, whether these signals can be correlated with stratigraphic indicators suggesting the continuity or discontinuity of human occupation. We conducted a high-resolution sedimentological and geochemical study, including micromorphological investigations, to shed light on stratigraphy, processes of sediment accumulation and post-depositional alteration in the cave. Seven major sediment units were distinguished which partly correlate with archaeological levels. The lower part of the sequence including Mousterian levels J and K consists of fluvial deposits truncated by a sharp erosional disconformity between Mousterian levels J and I. Strong enrichment with phosphorus and strontium reflect zoogenic inputs. The transition from Mousterian to Archaic Aurignacian in levels I and H, respectively, is reflected by more gradual changes in colour, grain size, and geochemical composition. However, a peak in potentially wind-blown particles (40–125 μm in diameter) reflects higher aeolian input, and banded microstructure suggests reworking of sediments at the interface. Both properties correlate with low density of finds suggesting low intensity of human occupation related to a dry spell. More arid conditions than during the Holocene are indicated for the Gravettian to Solutrean levels. These findings are in agreement with previous palaeoclimatic interpretations as based on palaeontological proxies. The detailed multi-proxy analyses of the sequence adds to our understanding on sediment accumulation and alteration in the Cave of Arbreda. The transition from the Middle to Upper Palaeolithic probably includes a gap in human occupation. Assessing the significance and duration of this gap and correlating the climate signal requires three-dimensional reconstructions of find densities and more reliable geochronological control.

Description: SST phases in the open-ocean and margins of the tropical Pacific; implication on tropical climate dynamics Climate of the Past Discussions, 10, 1857-1899, 2014 Author(s): L.-J. Shiau, S. C. Clemens, M.-T. Chen, M. Yamamoto, and Y. Yokoyama The tropical Pacific exerts a major effect on the global climate system and might have driven large extra-tropical climate change. We present a 320 kyr high resolution U K' 37 -sea surface temperature (SST) record from core MD052928 (11°17.26' S, 148°51.60' E, water depth 2250 m) located off southeastern Papua New Guinea (PNG), in the western tropical Pacific. The age model of the core is based on AMS 14 C dating of planktic foraminifers and correlation of benthic to the LR04 stack. The U K' 37 -SST ranges from 26.5 to 29 °C, showing glacial–interglacial and millennial variations. We assess the phase of the MD052928 U K' 37 -SST as part of a synthesis of five other SST records from the tropical Pacific at the precession, obliquity, and eccentricity bands. The SST records can be separated into two groups when considering SST phase relative to changes in orbital forcing, ice volume and greenhouse gases (GHGs). SST maxima at open-ocean sites within primary equatorial current systems occur between obliquity maxima and methane (CH 4 ) maxima but early relative to ice volume minima and CO 2 maxima at the obliquity band. In contrast, SST maxima at continental margin sites change are in phase with ice minima and CO 2 maxima, likely influenced by the slow response of continental ice sheets and GHGs. At the precession band, the early group located on the Warm Pool area indicates a direct influenced by the local insolation, and with the similar phase progress as the obliquity band. These results indicate that the decreased high-low latitudes insolation gradient and increasing low latitude local insolation resulting in tropical Pacific SST rise. Higher SST would supply more moisture resulting in increased CH 4 in the tropical wetlands. This promotes increasing CO 2 and deglaciation leading to increase continental and continental margin surface temperatures.

Description: Late Holocene environmental reconstructions and the implications on flood events, typhoon patterns, and agriculture activities in NE Taiwan Climate of the Past Discussions, 10, 1977-2009, 2014 Author(s): L.-C. Wang, H. Behling, T.-Q. Lee, H.-C. Li, C.-A. Huh, L.-J. Shiau, and Y.-P. Chang In this study, we reconstructed the paleoenvironmental changes from a sediment archive of the floodplain lake in Ilan Plain of NE Taiwan on multi-decadal resolution for the last ca. 1900 years. On the basis of pollen and diatom records, we evaluated the record of past vegetation, floods, typhoons and agriculture activities of this area, which is sensitive to the hydrological conditions of the West Pacific. High sedimentation rates with low microfossil preservations reflected multiple flood events and humid climatic conditions during 100–1400 AD. A shortly interrupted dry phase can be found during 940–1010 AD. The driest phase corresponds to the Little Ice Age phase 1 (LIA1, 1400–1620 AD) with less disturbance by flood events, which enhanced the occurrence of wetlands (Cyperaceae) and diatom depositions. Humid phases with frequent typhoons are inferred by high percentages of Lagerstroemia and high ratios of planktonic/benthic diatoms, respectively, during 500–700 AD and Little Ice Age phase 2 (LIA2, 1630–1850 AD). The occurrences of cultivated Poaceae ( Oryza ) during 1250–1300 AD and the last ~400 years, reflect agriculture activities, which seems to implicate strongly with the environmental stability. Finally, we found flood events which dominated during the El Niño-like stage, but dry events as well as frequent typhoon events happened during the La Niña-like stage. After comparing our results with the reconstructed proxy for tropical hydrological conditions, we suggested that the local hydrology in coastal East Asia were strongly affected by the typhoon-triggered heavy rainfalls which were influenced by the variation of global temperature, expansion of the Pacific warm pool and intensification of ENSO events.

Description: Simulating ice core 10 Be on the glacial–interglacial timescale Climate of the Past Discussions, 10, 761-808, 2014 Author(s): C. Elsässer, D. Wagenbach, I. Levin, A. Stanzick, M. Christl, A. Wallner, S. Kipfstuhl, I. K. Seierstad, H. Wershofen, and J. Dibb 10 Be ice core measurements are an important tool for paleoclimate research, e.g. allowing for the reconstruction of past solar activity or variation in the natural 14 C production rate. However, especially on multi-millennial timescales, the share of production and climate induced variations of respective 10 Be ice core records is still up to debate. Here we present the first quantitative climatological model of the 10 Be ice concentration up to the glacial–interglacial timescale. The model approach is composed of (i) a coarse resolution global atmospheric transport model and (ii) a local 10 Be air–firn-transfer model. Extensive global-scale observational data of short-lived radionuclides as well as new polar 10 Be snow pit measurements are used for model calibration and validation. Being specifically configured for polar 10 Be, this tool thus allows for a straight-forward investigation of production and non-production related modulation of this nuclide. We find that the polar 10 Be ice concentration does not record a globally mixed cosmogenic production signal. In fact, the geomagnetic modulation of Greenland 10 Be is up to 50% lower than in case of the global atmospheric 10 Be inventory. Using geomagnetic modulation and revised Greenland snow accumulation rate changes as model input we simulate the observed Greenland Summit (GRIP and GISP2) 10 Be ice core records over the last 75 kyr (on the GICC05modelext timescale). We show that our basic model is capable to reproduce the largest portion of the observed 10 Be changes. However, model-measurements differences exhibit multi-millennial oscillations with amplitudes up to 87% of the mean observed Holocene 10 Be concentration. Focusing on the (12–37) kyr b2k (before the year 2000 AD) period, mean model-measurements differences of 30% cannot be imputed to production changes. However, unconsidered climate-induced changes could likely explain the model shortcomings. In fact, the 10 Be ice concentration is very sensitive to snow accumulation changes. Here the reconstructed Greenland Summit (GRIP) snow accumulation rate record would require revision of +28% to solely account for the (12–37) kyr b2k measurements-model differences.

Description: Climate variability and relationship with ocean fertility during the Aptian Stage Climate of the Past Discussions, 10, 689-738, 2014 Author(s): C. Bottini, E. Erba, D. Tiraboschi, H. C. Jenkyns, S. Schouten, and J. S. Sinninghe Damsté Several studies have been conducted to reconstruct temperature variations across the Aptian Stage, particularly during the Early Aptian Oceanic Anoxic Event (OAE)1a. There is a general consensus that a major warming characterized the OAE 1a, although some studies have provided evidence for transient "cold snaps" or cooler intervals during the event. The climatic conditions for the middle–late Aptian are less constrained, and a complete record through the Aptian is not available. Here we present a reconstruction of surface-water palaeotemperature and fertility based on calcareous nannofossil records from the Cismon and Piobbico cores (Tethys) and DSDP Site 463 (Pacific Ocean). The data, integrated with oxygen-isotope and TEX 86 records, provide a detailed picture of climatic and ocean fertility changes during the Aptian Stage, which are discussed in relation to the direct/indirect role of volcanism. Warm temperatures characterized the pre-OAE 1a interval followed by a maximum warming (of ~2–3 °C) during the early phase of anoxia under intense volcanic activity of the Ontong Java Plateau (OJP). A short-lived (~35 ky) cooling episode interrupted the major warming, following a rapid increase of weathering rates. Nannofossils indicate that eutrophic conditions were reached when temperatures were at their highest and OJP volcanism most intense, thus suggesting that continental runoff, together with increased input of hydrothermal metals, increased nutrient supply to the oceans. The latter part of OAE 1a was characterized by cooling events, probably promoted by CO 2 sequestration during burial of organic matter. In this phase, high productivity was probably maintained by N 2 -fixing cyanobacteria while nannofossil taxa indicating high fertility were rare. The end of anoxia coincided with the cessation of volcanism and a pronounced cooling. The mid-Aptian was characterized by high surface-water fertility and progressively decreasing temperatures, probably resulting from intense continental weathering drawing down p CO 2 . The lowest temperatures, combined with low fertility, were reached in the middle–late Aptian across the interval characterized by blooming of Nannoconus truittii . The data presented suggest that OJP activity played a direct role in inducing global warming during the early Aptian, whereas other mechanisms (weathering, deposition of organic matter) acted as feedback processes, favouring temporary cooler interludes.

Description: Water pH and temperature in Lake Biwa from MBT'/CBT indices during the last 282 000 years Climate of the Past Discussions, 10, 1153-1178, 2014 Author(s): T. Ajioka, M. Yamamoto, K. Takemura, and A. Hayashida We generated a 282 000-year record of water pH and temperature in Lake Biwa, central Japan, by analysing the methylation index (MBT') and cyclisation ratio (CBT) of branched tetraethers in sediments from piston and borehole cores to understand the responses of precipitation and air temperature in central Japan to the East Asian monsoon variability on the orbital timescale. Because water pH in Lake Biwa is determined by phosphorus input driven by precipitation, the record of water pH should indicate changes in summer precipitation in central Japan. The estimated pH showed significant periodicity at 19 and 23 ka (precession) and at 41 ka (obliquity). The variation in the estimated pH agrees with variation in the pollen temperature index. This indicates synchronous variation in summer air temperature and precipitation in central Japan, which contradicts the conclusions of previous studies. The variation in estimated pH was also synchronous with the variation of oxygen isotopes in stalagmites in China, suggesting that East Asian summer monsoon precipitation was governed by Northern Hemisphere summer insolation on orbital timescales. However, the estimated winter temperatures were higher during interglacials and lower during glacials, showing an eccentricity cycle. This suggests that the temperature variation reflected winter monsoon variability.

Description: Multi-proxy fingerprint of Heinrich event 4 in Greenland ice core records Climate of the Past Discussions, 10, 1179-1222, 2014 Author(s): M. Guillevic, L. Bazin, A. Landais, C. Stowasser, V. Masson-Delmotte, T. Blunier, F. Eynaud, S. Falourd, E. Michel, B. Minster, T. Popp, F. Prié, and B. M. Vinther Glacial climate was characterised by two types of abrupt events. Greenland ice cores record Dansgaard–Oeschger events, marked by abrupt warming in-between cold, stadial phases. Six of these stadials coincide with major Heinrich events (HE), identified from ice-rafted debris (IRD) and large excursions in carbon and oxygen stable isotopic ratios in North Atlantic deep sea sediments, documenting major ice sheet collapse events. This finding has led to the paradigm that glacial cold events are induced by the response of the Atlantic Meridional Overturning Circulation to such massive freshwater inputs, supported by sensitivity studies conducted with climate models of various complexities. This mechanism could however never be confirmed or infirmed because the exact timing of Heinrich events and associated low latitude hydrological cycle changes with respect to Greenland stadials has so far remained elusive. Here, we provide the first multi-proxy fingerprint of H4 within Stadial 9 in Greenland ice cores through ice and air proxies of low latitude climate and water cycle changes. Our new dataset demonstrates that Stadial 9 consists of three phases, characterised first by Greenland cooling during 550 ± 60 years (as shown by markers of Greenland temperature δ 18 O and δ 15 N), followed by the fingerprint of Heinrich Event 4 as identified from several proxy records (abrupt decrease in 17 O excess and Greenland methane sulfonic acid (MSA), increase in CO 2 and methane mixing ratio, heavier δ D-CH 4 and δ 18 O atm ), lasting 740 ± 60 years, itself ending approximately 390 ± 50 years prior to abrupt Greenland warming. Preliminary investigations on GS-13 encompassing H5, based on the ice core proxies δ 18 O, MSA, δ 18 O atm , CH 4 and CO 2 data also reveal a 3 phase structure, as well as the same sequence of events. The decoupling between stable cold Greenland temperature and low latitude HE imprints provides new targets for benchmarking climate model simulations and testing mechanisms associated with millennial variability.

Description: Variations in intermediate and deep ocean circulation in the subtropical northwestern Pacific from 26 ka to present based on a new calibration for Mg/Ca in benthic foraminifera Climate of the Past Discussions, 10, 1265-1303, 2014 Author(s): Y. Kubota, K. Kimoto, T. Itaki, Y. Yokoyama, Y. Miyairi, and H. Matsuzaki To understand variations in intermediate and deep ocean circulation in the North Pacific, bottom water temperatures (BWT), carbon isotopes (δ 13 C) of benthic foraminifera, and oxygen isotopes (δ 18 O) of seawater at a water depth of 1166 m were reconstructed from 26 ka to present. A new regional Mg/Ca calibration for the benthic foraminifera Cibicidoides wuellerstorfi was established to convert the benthic Mg/Ca value to BWT, based on twenty-six surface sediment samples and a core top sample retrieved around Okinawa Island. In addition, core GH08-2004, retrieved from 1166 m water depth east of Okinawa Island, was used to reconstruct water properties from 26 ka to present. During the Last Glacial Maximum (LGM), from 24 to 18 ka, BWT appeared to be relatively constant at approximately 2 °C, which is ~1.5–2 °C lower than today. One of the prominent features of our BWT records was a millennial-scale variation in BWT during the last deglaciation, with BWT higher during Heinrich event 1 (H1; ~17 ka) and the Younger Dryas (YD; ~12 ka) and lower during the Bølling/Allerød (B/A; ~14 ka). The record of seawater δ 18 O in core GH08-2004 exhibited a rapid increase in association with the rapid warming of BWT at 17 ka, likely due to the reduced precipitation in the North Pacific in response to less moisture transport from the equatorial Atlantic as a result of the collapse of the Atlantic Meridional Overturning Circulation. During the interval from 17 to 15 ka, the bottom water temperature tended to decrease in association with a decrease in the carbon isotope values of C. wuellerstorfi , likely as a result of increased upwelling of the older water mass that was stored in the abyssal Pacific during the glacial time. The timing of the increased upwelling coincided with the deglacial atmospheric CO 2 rise initiated at ~17 ka, and suggested that the increased upwelling in the subtropical northwestern Pacific from 17 to 15 ka contributed to the carbon release from the Pacific into the atmosphere.

Description: Two distinct decadal and centennial cyclicities forced marine upwelling intensity and precipitation during the late Early Miocene in Central Europe Climate of the Past Discussions, 10, 1223-1264, 2014 Author(s): G. Auer, W. E. Piller, and M. Harzhauser Within a 5.5 m-thick succession of Upper Burdigalian (Karpatian) sediments in the North Alpine Foreland Basin (NAFB; Austria), dated to CNP-zone NN4, a high-resolution section was logged continuously. 100 samples were taken with a resolution of ~10 mm per layer and analysed using an integrated multi-proxy approach. Earlier analyses of geochemistry and calcareous nannoplankton assemblages hint at small-scale, short-term variations in palaeoenvironmental conditions, such as water-column stratification, primary productivity, organic matter flux, bottom-water oxygenation, freshwater influx and changes in relative sea-level. The results indicate a highly dynamic shallow marine setting that was subject to high frequency environmental changes on a decadal to centennial scale. Time-series analyses on nine different proxy-datasets using REDFIT-analysis and Wavelet spectra were applied to resolve a possible cyclic nature of these variations. Analyses revealed that different proxies for precipitation, upwelling intensity, and over all productivity likely were controlled by different cyclicities. A best-fit adjustment of the likely sedimentation rates within the high-resolution section resulted in periodicities fitting well with the Lower (~65 yr) and Upper (~113 yr) Gleissberg cycle as well as the Suess/de Vries cycle (~211 yr). The section covers a timespan of ~1190 yr based on the correlation with solar cycles, which resulted in an estimated sedimentation rate of 575 mm kyr −1 . For the first time, short-term climate variability on a decadal to centennial scale is resolved in Lower Miocene shallow marine laminated sediments in a land-based section. The results hint at a close relationship between climate variability and solar forcing during the Late Burdigalian. Moreover, accepting that these cyclicities are indeed of solar origin, this would indicate that precipitation was driven by the two Gleissberg cycles, while upwelling was driven by the Suess cycle. Furthermore, proxies for primary productivity were influenced by both cycles, although the Suess cycle exerts dominant control, reflecting a stronger influence of upwelling on primary productivity.

Description: Variation in the Asian monsoon intensity and dry-wet condition since the Little Ice Age in central China revealed by an aragonite stalagmite Climate of the Past Discussions, 10, 1305-1335, 2014 Author(s): J.-J. Yin, D.-X. Yuan, H.-C. Li, H. Cheng, T.-Y. Li, R. L. Edwards, Y.-S. Lin, J.-M. Qin, W. Tang, Z.-Y. Zhao, and H.-S. Mii Highlight: this paper focuses on the climate variability in central China since 1300 AD, involving: 1. A well-dated, 1.5 year resolution stalagmite δ 18 O record from Lianhua Cave, central China; 2. Links of the δ 18 O record with regional dry-wet condition, monsoon intensity, and temperature over eastern China; 3. Correlations among drought events in the Lianhua record, solar irradiation, and ENSO index. We present a highly precisely 230 Th/U dated, 1.5 year resolution δ 18 O record of an aragonite stalagmite (LHD1) collected from Lianhua Cave in Wuling mountain area of central China. The comparison of the δ 18 O record with the local instrumental record and historical documents exhibits at least 15 drought events in the Wuling mountain and adjacent areas during the Little Ice Age, in which some of them were corresponding to megadrought events in the broad Asian monsoonal region of China. Thus, the stalagmite δ 18 O record reveals variations in the summer monsoon precipitation and dry-wet condition in Wuling mountain area. The eastern China temperature varied with the solar activity, showing higher temperature under stronger solar irradiation which produces stronger summer monsoon. During Maunder, Dalton and 1900 sunspot minima, more severe drought events occurred, indicating weakening of the summer monsoon when solar activity decreased on decadal time scales. On interannual time scale, dry conditions in the studying area were prevailing under El Niño condition, which is also supported by the spectrum analysis. Hence, our record illustrates the linkage of Asian summer monsoon precipitation to solar irradiation and ENSO: wetter condition under stronger summer monsoon during warm periods and vice versa; During cold periods, the Walker circulation will shift toward central Pacific under El Niño condition, resulting further weakening of Asian summer monsoon. However, the δ 18 O of LHD1 record is positively correlated with temperature after ~1940 AD which is opposite to the δ 18 O – temperature relationship in earlier time. This anomaly relationship might be caused by the greenhouse-gas forcing.

Description: Subsurface North Atlantic warming as a trigger of rapid cooling events: evidences from the Early Pleistocene (MIS 31–19) Climate of the Past Discussions, 10, 4033-4055, 2014 Author(s): I. Hernández-Almeida, F.-J. Sierro, I. Cacho, and J.-A. Flores Subsurface water column dynamics in the subpolar North Atlantic were reconstructed in order to improve the understanding of the cause of abrupt IRD events during cold periods of the Early Pleistocene. We used Mg / Ca-based temperatures of deep-dwelling ( Neogloboquadrina pachyderma sinistral) planktonic foraminifera and paired Mg / Ca-δ 18 O measurements to estimate the subsurface temperatures and δ 18 O of seawater at Site U1314. Carbon isotopes on benthic and planktonic foraminifera from the same site provide information about the ventilation and water column nutrient gradient. Mg / Ca-based temperatures and δ 18 O of seawater suggest increased temperatures and salinities during ice-rafting, likely due to enhanced northward subsurface transport of subtropical waters during periods of AMOC reduction. Planktonic carbon isotopes support this suggestion, showing coincident increased subsurface ventilation during deposition of ice-rafted detritus (IRD). Warm waters accumulated at subsurface would result in basal warming and break-up of ice-shelves, leading to massive iceberg discharges in the North Atlantic. Release of heat and salt stored at subsurface would help to restart the AMOC. This mechanism is in agreement with modelling and proxy studies that observe a subsurface warming in the North Atlantic in response to AMOC slowdown during the MIS3.

Description: ENSO flavors in a tree-ring δ 18 O record of Tectona grandis from Indonesia Climate of the Past Discussions, 10, 3965-3987, 2014 Author(s): K. Schollaen, C. Karamperidou, P. J. Krusic, E. R. Cook, and G. Helle Indonesia's climate is dominated by the equatorial monsoon system, and has been linked to El Niño–Southern Oscillation (ENSO) events that often result in extensive droughts and floods over the Indonesian archipelago. In this study we investigate ENSO-related signals in a tree-ring δ 18 O record (1900–2007) of Javanese teak. Our results reveal a clear influence of Warm Pool (central Pacific) El Niño events on Javanese tree-ring δ 18 O, and no clear signal of Cold Tongue (eastern Pacific) El Niño events. These results are consistent with the distinct impacts of the two ENSO flavors on Javanese precipitation, and illustrate the importance of considering ENSO flavors when interpreting palaeoclimate proxy records in the tropics.

Description: Atmospheric circulation controls on the inter-annual variability in precipitation isotope ratio in Japan Climate of the Past Discussions, 10, 3989-4032, 2014 Author(s): N. Kurita, Y. Fujiyoshi, T. Nakayama, Y. Matsumi, and H. Kitagawa This study explored the primary driver of variations of precipitation isotopes at multiple temporal scales (event, seasonal and inter-annual scales) to provide a greater depth of interpretation for isotope proxy records in Japan. A one-year record of the isotopic composition of event-based precipitation at Nagoya in central Japan showed less seasonal variation, but there is large isotopic variability on a storm-to-storm basis. In the summer, southerly flows transport isotopically enriched moisture from subtropical marine regions with the result that the rainfall produced by the subtropical air, or warm rainfall, was relatively enriched in heavy isotopes in comparison with the other rainfall events. In the winter, storm tracks are the dominant driver of storm-to-storm isotopic variation, and relatively lower isotopic values occurred when northerly winds in association with extratropical cyclones passing off the south coast of Japan (Nangan cyclone) brings cold precipitation. Using the historical 17 year record of monthly isotopes in precipitation at Tokyo station, we explored if the factors controlling event-scale isotopic variability can account for inter-annual isotopic variability. The relatively higher isotopes in summer precipitation were attributed to the higher contribution of the warm rainfall to the total summer precipitation. On the other hand, year-to-year variation of isotopic values in winter precipitation was negatively correlated with the relative ratio of the Nangan cyclone rainfall to the total winter precipitation. The 17 year precipitation history demonstrates that event-scale isotopic variability related to changes in meridional moisture transport is the primary driver of inter-annual isotopic variability in winter and summer precipitation. The meridional moisture transport to central Japan is likely linked to the activity of the western North Pacific subtropical high in summer and the intensity of the East Asian winter monsoon in winter. Therefore, isotope-based proxy records archived in central Japan may enable us to examine past atmospheric circulation changes in East Asia in response to climate variability.

Description: A 500 year seasonally resolved δ 18 O and δ 13 C, layer thickness and calcite fabric record from a speleothem deposited in equilibrium of the Han-sur-Lesse cave, Belgium Climate of the Past Discussions, 10, 4149-4190, 2014 Author(s): M. Van Rampelbergh, S. Verheyden, M. Allan, Y. Quinif, H. Cheng, L. Edwards, E. Keppens, and P. Claeys Speleothem δ 18 O and δ 13 C signals have already proven to enable climate reconstructions at high resolution. However, seasonally resolved speleothem records are still scarce and often difficult to interpret in terms of climate due to the multitude of factors that can affect the proxy signals. In this paper, a fast growing (up to 2 mm yr −1 ) seasonally laminated speleothem from the Han-sur-Lesse cave (Belgium) is analyzed for its δ 18 O and δ 13 C values, layer thickness and changes in calcite fabric. The studied part of the speleothem covers the most recent 500 years as indicated by layer counting and confirmed by 20 U/Th-ages. Epikarst recharge occurs mainly in winter and lesser during spring and fall. a good correlation can be established between lower winter temperatures and lower winter precipitation (DJF) based on the measured data by the Belgian meteorological institute since 1833 indicating that a dry winter is also a cold winter. Colder and dryer winters cause lower winter recharge and generally drier conditions in the cave. Lower winter recharge decreases the amount of isotopically light (δ 18 O) winter precipitation added to the epikarst in comparison to the heavier spring and fall waters, which leads to a net increase in δ 18 O value of the water in the epikarst. Increased δ 18 O values in the Proserpine are consequently interpreted to reflect colder and dryer winters. Higher δ 13 C signals are interpreted to reflect increased prior calcite precipitation (PCP) due to colder and dryer winters, when recharge is lower. Thinner layers and darker calcite relate to slower growth and occur when drip rates are low and when the drip water calcium ion concentration is low due to increased PCP, both caused by lower recharge during periods with colder and dryer winters. Exceptionally cold and dry winters cause the drip discharge to decrease under a certain threshold value inducing anomalies in the measured proxy records. Such anomalies occur from 1565 to 1610, from 1770 to 1800, from 1810 to 1860 and from 1880 to 1895 and correspond with exceptionally cold periods in proxy-based, historical and instrumental records and may relate to different factors such as negative winter NAO phases, lower solar irradiance and/or volcanic eruptions. When the discharge threshold is not reached, lower amplitude variations are observed such as between 1479 and 1565 and between 1730 and 1770 with two periods of relatively warmer and wetter winters. Between 1610 and 1730 a period of relatively cooler and dryer winters occurs and may relate to a decrease in solar irradiance during the Maunder Minimum (1640–1714). Seasonal δ 18 O variations indicate a 2.5 °C seasonality in cave air temperature during the two periods with warmer and wetter winters (1479–1565 and 1730–1770), and correspond to the cave air temperature seasonality observed today. a smaller 1.5 °C seasonality in cave air temperature occurs during the interval with colder and wetter winters between 1610 and 1730 and suggests colder summers. The δ 13 C seasonal changes suggest that the seasonality in discharge was lower than the one observed today with a short interval of increased seasonality between 1600 and 1660 reflecting stronger summer PCP-effects due to decreased winter recharge.

Description: Carbon isotope (δ 13 C) excursions suggest times of major methane release during the last 14 ka in Fram Strait, the deep-water gateway to the Arctic Climate of the Past Discussions, 10, 4191-4227, 2014 Author(s): C. Consolaro, T. L. Rasmussen, G. Panieri, J. Mienert, S. Bünz, and K. Sztybor We present results from a sediment core collected from a pockmark field on the Vestnesa Ridge (∼80° N) in the eastern Fram Strait. This is the only deep-water gateway to the Arctic, and one of the northernmost marine gas hydrate provinces in the world. Eight 14 C AMS dating reveals a detailed chronology for the last 14 ka BP. The δ 13 C record measured on the benthic foraminiferal species Cassidulina neoteretis shows two distinct intervals with negative values, as low as −4.37‰ in the Bølling–Allerød interstadials and as low as −3.41‰ in the early Holocene. After cleaning procedure designed to remove all authigenic carbonate coatings on benthic foraminiferal tests, the 13 C values are still negative (as low as −2.75‰). We have interpreted these negative carbon isotope excursions (CIEs) to record past methane release events, resulting from the incorporation of 13 C-depleted carbon from methane emissions into the benthic foraminiferal shells. The CIEs during the Bølling–Allerød interstadials and the early Holocene relate to periods of ocean warming, sea level rise and increased concentrations of methane (CH 4 ) in the atmosphere. CIEs with similar timing have been reported from other areas in the North Atlantic suggesting a regional event. The trigger mechanisms for such regional events remain to be determined. We speculate that sea-level rise and seabed loading due to high sediment supply in combination with increased seismic activity as a result of rapid deglaciation may have triggered the escape of significant amounts of methane to the seafloor and the water column above.

Description: Diminished greenhouse warming from Archean methane due to solar absorption lines Climate of the Past Discussions, 10, 4229-4256, 2014 Author(s): B. Byrne and C. Goldblatt Previous research has shown that methane may have been sustained at high concentrations in the Archean atmosphere, helping to offset lower insolation and solve the faint young sun problem. However, recent updates to the HITRAN line database have significantly increased the shortwave absorption by CH 4 in comparison to older versions of the database (e.g., HITRAN 2000). Here we investigate the climatological implications of strong shortwave CH 4 absorption in an Archean atmosphere rich in CH 4 . We show that the surface warming at CH 4 abundances 〉 10 -3 is greatly diminished relative to the HITRAN 2000 line data. Strong shortwave absorption also results in a warm stratosphere and lower tropopause. We discuss these results in the context of contemporary research of the Archean climate and how these results could affect the formation of stratospheric clouds and an organic haze.

Description: Optimisation of glaciological parameters for ice core chronology by implementing counted layers between identified depth levels Climate of the Past Discussions, 10, 3585-3616, 2014 Author(s): L. Bazin, B. Lemieux-Dudon, A. Landais, M. Guillevic, P. Kindler, F. Parrenin, and P. Martinerie A~recent coherent chronology has been built for 4 Antarctic ice cores and the NorthGRIP (NGRIP) Greenland ice core (Antarctic Ice Core Chronology 2012, AICC2012) using a bayesian approach for ice core dating (Datice). When building the AICC2012 chronology, and in order to prevent any confusion with official ice cores chronology, it has been imposed that the AICC2012 chronology for NGRIP should respect exactly the GICC05 chronology based on layer counting. However, such a strong tuning did not satisfy the hypothesis of independence of background parameters and observations for the NGRIP core as required by Datice. We present here the implementation in Datice of a new type of markers that is better suited to constraints deduced from layer counting: the markers of age-difference. Using this type of markers for NGRIP in a 5 cores dating exercise with Datice, we have performed several sensitivity tests and show that the new ice core chronologies obtained with these new markers do not differ by more than 400 years from AICC2012 for Antarctic ice cores and by more than 130 years from GICC05 for NGRIP over the last 60 000 years. With this new parameterization, the accumulation rate and lock-in depth associated with NGRIP are more coherent with independent estimates than those obtained in AICC2012. While these new chronologies should not be used yet as new ice core chronologies, the improved methodology presented here should be considered in the next coherent ice core dating exercise.

Description: Using results from the PlioMIP ensemble to investigate the Greenland Ice Sheet during the warm Pliocene Climate of the Past Discussions, 10, 3483-3535, 2014 Author(s): A. M. Dolan, S. J. Hunter, D. J. Hill, A. M. Haywood, S. J. Koenig, B. L. Otto-Bliesner, A. Abe-Ouchi, F. Bragg, W.-L. Chan, M. A. Chandler, C. Contoux, A. Jost, Y. Kamae, G. Lohmann, D. J. Lunt, G. Ramstein, N. A. Rosenbloom, L. Sohl, C. Stepanek, H. Ueda, Q. Yan, and Z. Zhang During the mid-Pliocene Warm Period (3.264 to 3.025 million years ago), global mean temperature was similar to that predicted for the end of this century, and atmospheric carbon dioxide concentrations were higher than pre-industrial levels. Sea level was also higher than today, implying a significant reduction in the extent of the ice sheets. Thus, the mid-Pliocene Warm Period provides a natural laboratory in which to investigate the long-term response of the Earth's ice sheets and sea level in a warmer-than-modern world. At present, our understanding of the Greenland ice sheet during the warmest intervals of the mid-Pliocene is generally based upon predictions using single climate and ice sheet models. Therefore, it is essential that the model dependency of these results is assessed. The Pliocene Model Intercomparison Project (PlioMIP) has brought together nine international modelling groups to simulate the warm climate of the Pliocene. Here we use the climatological fields derived from the results of the PlioMIP climate models to force an offline ice sheet model. We show Pliocene ice sheet reconstructions are highly dependent upon the forcing climatology used, with Greenland reconstructions ranging from an ice-free state to a near modern ice sheet. An analysis of surface albedo differences between the climate models over Greenland offers insights into the drivers of inter-model differences. As we demonstrate that the climate model dependency of our results is high, we highlight the necessity of data-based constraints in developing our understanding of the Pliocene Greenland ice sheet.

Description: Spatial climate dynamics in the Iberian Peninsula since 15 000 Yr BP Climate of the Past Discussions, 10, 3901-3930, 2014 Author(s): P. Tarroso, J. Carrión, M. Dorado-Valiño, P. Queiroz, L. Santos, A. Valdeolmillos-Rodríguez, P. Célio Alves, J. C. Brito, and R. Cheddadi The evolution of the climate in the Iberian Peninsula since the last glacial maximum is associated with distributional shifts of multiple species. We rely on this dynamic relationship between past climate and biodiversity patterns to quantify climate change using fossil pollen records widespread throughout the Iberian Peninsula and modern spatial distribution of plant taxa and climate. We have reconstructed spatial layers (1 ka interval) of January minimum temperature, July maximum temperature and minimum annual precipitation using a method based on probability density functions and covering the time period between 15 and 3 ka. A functional principal component analysis was used in order to summarise the spatial evolution of climate. Using a clustering method we have identified areas that share similar climate evolutions during the studied time period. The spatial reconstructions show a highly dynamic pattern in accordance with the main climatic trends. The four cluster areas we found exhibit different climate evolution over the studied period. The clustering scheme and patterns of change between millenia are coherent with the existence of multiple refugial areas in the Iberian Peninsula.

Description: Sensing Seasonality in the Arabian Sea: a coupled δ 18 O-Mg/Ca approach Climate of the Past Discussions, 10, 3847-3876, 2014 Author(s): W. Feldmeijer, L. J. de Nooijer, G.-J. Reichart, and G.M. Ganssen Millennial scale climate variability punctuates the record of Northern Hemisphere glacials in the form of Dansgaard–Oeschger cycles. The coldest episodes within these cycles have a marked impact on the location of the Inter-tropical Convergence Zone and thereby the intensity and extent of the Asian monsoon system. Arabian Sea oceanography is closely linked to the monsoon cycle and responds rapidly to changes therein. Seasonally varying upwelling intensity, adhesive mixing and the resulting variability on sea surface temperature and salinity, however, make it challenging to precisely reconstruct the behaviour of the Indian monsoon system through the Dansgaard–Oeschger cycles. To reconstruct impact of millennial-scale changes in the monsoon on the Arabian Sea, paired single-specimen Mg/Ca and stable oxygen isotope analyses were performed on three species of planktonic foraminifera from the northern as well as the western Arabian Sea. Mismatches between Mg/Ca- and δ 18 O-derived temperatures reflect changes in salinity caused by regional variability in the hydrological cycle. Comparison of Mg/Ca-derived temperatures from Globigerinoides ruber , Globigerina bulloides and Neogloboquadrina dutertrei over Heinrich Event 4 and Interstadial 8 allows for reconstructing the seasonal range in temperatures (minima and maxima) and vertical stratification of the water column during these intervals. Results show that Arabian Sea surface water temperatures were lower during the North Atlantic Heinrich Event 4, and that temperatures in the northern and western Arabian Sea differed relatively much. This implies that the north east monsoon (i.e. boreal winter) was enhanced during this interval. In contrast, the summer upwelling related species G. bulloides reveals lower temperatures during Interstadial 8 in the western Arabian Sea, showing an enhanced summer monsoon during the warmers stages of millennial-scale climate variability. Together, these results show that summer and winter monsoon intensities were thus also anti-phased on a millennial scale. An intensified NE monsoon might have contributed to enhanced mixing of surface waters in the northern Arabian Sea.

Description: Climatic information of Western Sahel (1535–1793 AD) in original documentary sources Climate of the Past Discussions, 10, 3877-3900, 2014 Author(s): V. Millán and F. S. Rodrigo The Sahel is the semi-arid transition zone between arid Sahara and humid tropical Africa, extending approximately 10–20° N from Mauritania in the West to Sudan in the East. The African continent, one of the most vulnerable regions to climate change, is subject to frequent droughts and famine. One climate challenge research is to isolate those aspects of climate variability that are natural from those that are related to human influences. Therefore, the study of climatic conditions before mid-19th century, when anthropogenic influence was of minor importance, is very interesting. In this work the frequency of extreme events, such as droughts and floods, in Western Sahel from the 16th to 18th centuries is investigated using documentary data. Original manuscripts with historical chronicles from Walata and Nema (Mauritania), Timbuktu and Arawan (Mali), and Agadez (Niger) have been analyzed. Information on droughts, intense rainfall, storms and floods, as well as socioeconomic aspects (famines, pests, scarcity, prosperity) has been codified in an ordinal scale ranging from −2 (drought and famines) to +2 (floods) to obtain a numerical index of the annual rainfall in the region. Results show wet conditions in the 17th century, as well as dry conditions in the 18th century (interrupted by a short wet period in the 1730s decade).

Description: Inferring paleo-accumulation records from ice-core data by an adjoint method: application to James Ross Island's ice core Climate of the Past Discussions, 10, 3821-3845, 2014 Author(s): C. Martín, R. Mulvaney, G. H. Gudmundsson, and H. Corr Ice cores contain a record of snow precipitation that includes information about past atmospheric circulation and mass imbalance in the polar regions. We present a novel adjoint method to reconstruct a climatic record by both optimally dating an ice-core and deriving from it a detailed accumulation history. The motivation of our work is the recent application of phase sensitive radar which measures the vertical velocity of an ice column. The velocity is dependent on the history of subsequent snow accumulation, compaction and compression; and in our inverse formulation of this problem, measured vertical velocity profiles can be utilized directly thereby reducing the uncertainty introduced by ice flow modelling. We first apply our method to synthetic data in order to study its capability and the effect of noise and gaps in the data on retrieved accumulation history. The method is then applied to the ice core retrieved from James Ross Island, Antarctica. We show that the method is robust and that the results depend on quality of the age-depth observations and the derived flow regime around the core site. The method facilitates the incorporation of increasing detail provided by ice-core analysis together with observed full-depth velocity in order to construct a complete climatic record of the polar regions.

Description: Reconciling reconstructed and simulated features of the winter Pacific–North-American pattern in the early 19th century Climate of the Past Discussions, 10, 4425-4468, 2014 Author(s): D. Zanchettin, O. Bothe, F. Lehner, P. Ortega, C. C. Raible, and D. Swingedouw Reconstructions of past climate behavior often describe prominent anomalous periods that are not necessarily captured in climate simulations. Here, we illustrate the contrast between an interdecadal strong positive phase of the winter Pacific/North American pattern (PNA) in the early 19th century that is described by a PNA reconstruction based on tree-rings from northwestern North America, and a slight tendency towards negative winter PNA anomalies during the same period in an ensemble of state-of-the-art coupled climate simulations. Additionally, a pseudo-proxy investigation with the same simulation ensemble allows assessing the robustness of PNA reconstructions using solely geophysical predictors from northwestern North America for the last millennium. The reconstructed early-19th-century positive PNA anomaly emerges as a potentially reliable feature, although it is subject to a number of sources of uncertainty and potential deficiencies. The pseudo-reconstructions demonstrate that the early-19th-century discrepancy between reconstructed and simulated PNA does not stem from the reconstruction process. Instead, reconstructed and simulated features of the early-19th-century PNA can be reconciled by interpreting the reconstructed evolution during this time as an expression of internal climate variability, hence unlikely to be reproduced in its exact temporal occurrence by a small ensemble of climate simulations. However, firm attribution of the reconstructed PNA anomaly is hampered by known limitations and deficiencies of coupled climate models and uncertainties in the early-19th-century external forcing and background climate conditions.

Description: Technical Note: How accurate can stalagmite formation temperatures be determined using vapour bubble radius measurements in fluid inclusions? Climate of the Past Discussions, 10, 3689-3713, 2014 Author(s): F. Spadin, D. Marti, R. Hidalgo-Staub, Y. Krüger, J. Rička, D. Fleitmann, and M. Frenz Stalagmites are natural archives containing detailed information on continental climate variability of the past. Microthermometric measurements of fluid inclusion homogenisation temperatures allow determination of stalagmite formation temperatures by measuring the radius of stable laser-induced vapour bubbles inside the inclusions. A reliable method for precisely measuring the radius of vapour bubbles is presented. The method is applied to stalagmite samples for which the formation temperature is known. An assessment of the bubble radius measurement accuracy and how this error influences the uncertainty in determining the formation temperature is provided. We demonstrate that the nominal homogenisation temperature of a single inclusion can be determined with an accuracy of ±0.25 °C, if the volume of the inclusion is larger than 10 5 μm 3 . Paleotemperatures can thus be determined within ±1.0 °C.

Description: Dust deposition in Antarctica in glacial and interglacial climate conditions: a modelling study Climate of the Past Discussions, 10, 3715-3753, 2014 Author(s): N. Sudarchikova, U. Mikolajewicz, C. Timmreck, D. O'Donnell, G. Schurgers, D. Sein, and K. Zhang The mineral dust cycle responds to climate variations and plays an important role in the climate system by affecting the radiative balance of the atmosphere and modifying biogeochemistry. Polar ice cores provide a unique information about deposition of aeolian dust particles transported over long distance. These cores are a paleoclimate proxy archive of climate variability thousands of years ago. The current study is a first attempt to simulate past interglacial dust cycles with a global aerosol-climate model ECHAM5-HAM. The results are used to explain the dust deposition changes in Antarctica in terms of quantitative contribution of different processes, such as emission, atmospheric transport and precipitation, which will help to interpret paleodata from Antarctic ice cores. The investigated periods include four interglacial time-slices such as the pre-industrial control (CTRL), mid-Holocene (6000 yr BP), last glacial inception (115 000 yr BP) and Eemian (126 000 yr BP). One glacial time interval, which is Last Glacial Maximum (LGM) (21 000 yr BP) was simulated as well as to be a reference test for the model. Results suggest an increase of mineral dust deposition globally, and in Antarctica, in the past interglacial periods relative to the pre-industrial CTRL simulation. Approximately two thirds of the increase in the mid-Holocene and Eemian is attributed to enhanced Southern Hemisphere dust emissions. Slightly strengthened transport efficiency causes the remaining one third of the increase in dust deposition. The moderate change of dust deposition in Antarctica in the last glacial inception period is caused by the slightly stronger poleward atmospheric transport efficiency compared to the pre-industrial. Maximum dust deposition in Antarctica was simulated for the glacial period. LGM dust deposition in Antarctica is substantially increased due to 2.6 times higher Southern Hemisphere dust emissions, two times stronger atmospheric transport towards Antarctica, and 30% weaker precipitation over the Southern Ocean. The model is able to reproduce the order of magnitude of dust deposition globally and in Antarctica for the pre-industrial and LGM climates.

Description: Freshwater discharge controlled deposition of Cenomanian-Turonian black shales on the NW European epicontinental shelf (Wunstorf, North Germany) Climate of the Past Discussions, 10, 3755-3786, 2014 Author(s): N. A. G. M. van Helmond, A. Sluijs, J. S. Sinninghe Damsté, G.-J. Reichart, S. Voigt, J. Erbacher, J. Pross, and H. Brinkhuis Global warming, changes in the hydrological cycle and enhanced marine primary productivity all have been invoked to have contributed to the occurrence of widespread ocean anoxia during the Cenomanian-Turonian Oceanic Anoxic Event (OAE2; ~ 94 Ma), but disentangling these factors on a regional scale has remained problematic. We generated palynological and organic geochemical records that allow the separation of these forcing factors in a core spanning the OAE2 from Wunstorf, Lower Saxony Basin (LSB; North Gemany), which exhibits cyclic black shale–marl alternations related to the orbital precession cycle. Despite the widely varying depositional conditions complicating the interpretation of the obtained records, TEX 86 H indicates that sea-surface temperature (SST) evolution in the LSB during OAE2 resembles that of previously studied sites throughout the proto-North Atlantic. Cooling during the so-called Plenus Cold Event interrupted black shale deposition during the early stages of OAE2. However, TEX 86 does not vary significantly across marl–black shale alternations, suggesting that temperature variations did not force the formation of the cyclic black shale horizons. Relative (i.e., with respect to marine palynomorphs) and absolute abundances of pollen and spores are elevated during phases of black shale deposition, indicative of enhanced precipitation and run-off. High abundances of cysts from inferred heterotrophic and euryhaline dinoflagellates supports high run-off, which likely introduced additional nutrients to the epicontinental shelf resulting in elevated marine primary productivity. We conclude that orbitally-forced enhanced precipitation and run-off, in tandem with elevated marine primary productivity, were critical in cyclic black shale formation on the northwest European epicontinental shelf and potentially for other OAE2 sections in the proto-Atlantic and Western Interior Seaway at similar latitudes as well.

Description: Technical Note: Are large error bars desirable? A note on quantitative model-proxy comparison Climate of the Past Discussions, 10, 4535-4552, 2014 Author(s): J. Liakka, J. T. Eronen, H. Tang, and F. T. Portmann The combined use of proxy records and climate modelling is invaluable for obtaining a better understanding of past climates. However, many methods of model-proxy comparison in the literature are fundamentally problematic because larger errors in the proxy tend to yield a "better" agreement with the model. Here we quantify model-proxy agreement as a function to proxy uncertainty using the overlapping coefficient OVL, which measures the similarity between two probability distributions. We found that the model-proxy agreement is poor (OVL 〈 50%) if the proxy uncertainty (σ p ) is greater than three times the model variability (σ m ), even if the model and proxy have similar mean estimates. Hence only proxies that fulfil the condition σ p 〈 3σ m should be used for detailed quantitative evaluation of the model performance.

Description: Thermal evolution of the western South Atlantic and the adjacent continent during Termination 1 Climate of the Past Discussions, 10, 4553-4593, 2014 Author(s): C. M. Chiessi, S. Mulitza, G. Mollenhauer, J. B. Silva, J. Groeneveld, and M. Prange During Termination 1, millennial-scale weakening events of the Atlantic meridional overturning circulation (AMOC) supposedly produced major changes in sea surface temperatures (SST) of the western South Atlantic, and in mean air temperatures (MAT) over southeastern South America. It was suggested, for instance, that the Brazil Current (BC) would strengthen (weaken) and the North Brazil Current (NBC) would weaken (strengthen) during slowdown (speed-up) events of the AMOC. This anti-phase pattern was claimed to be a necessary response to the decreased North Atlantic heat piracy during periods of weak AMOC. However, the thermal evolution of the western South Atlantic and the adjacent continent is largely unknown and a compelling record of the BC-NBC anti-phase behavior remains elusive. Here we address this issue, presenting high temporal resolution SST and MAT records from the BC and southeastern South America, respectively. We identify a warming in the western South Atlantic during Heinrich Stadial 1 (HS1), which is followed first by a drop and then by increasing temperatures during the Bølling–Allerød, in-phase with an existing NBC record. Additionally, a similar SST evolution is shown by a southernmost eastern South Atlantic record, suggesting a South Atlantic-wide pattern in SST evolution during most of Termination 1. Over southeastern South America, our MAT record shows a two-step increase during Termination 1, synchronous with atmospheric CO 2 rise (i.e., during the second half of HS1 and during the Younger Dryas), and lagging abrupt SST changes by several thousand years. This delay corroborates the notion that the long duration of HS1 was fundamental to drive the Earth out of the last glacial.

Description: A high-resolution δ 18 O record and Mediterranean climate variability Climate of the Past Discussions, 10, 4057-4084, 2014 Author(s): C. Taricco, G. Vivaldo, S. Alessio, S. Rubinetti, and S. Mancuso A~high-resolution, well-dated foraminiferal δ 18 O record from a shallow-water core drilled from the Gallipoli Terrace in the Gulf of Taranto (Ionian Sea), previously measured over the last two millennia, has been extended to cover 707 BC–1979 AD. Spectral analysis of this series, performed by Singular Spectrum Analysis (SSA) and other classical and advanced methods, strengthens the results obtained analysing the shorter δ 18 O profile, detecting the same highly significant oscillations of about 600 yr, 380 yr, 170 yr, 130 yr, and 11 yr, respectively explaining about 12%, 7%, 5%, 2% and 2% of the time series total variance, plus a millennial trend (18% of the variance). The comparison with the results of Multi-channel Singular Spectrum Analysis (MSSA) applied to a data set of 26 Northern Hemisphere (NH) temperature-proxy records shows that NH temperature anomalies share with our local record a long-term trend and a bicentennial cycle. These two variability modes, previously identified as temperature-driven, are the most powerful modes in the NH temperature data set. Both the long-term trends and the bicentennial oscillations, when reconstructed locally and hemispherically, show coherent phases. Also the corresponding local and hemispheric amplitudes are comparable, if changes in the precipitation-evaporation balance of the Ionian sea, presumably associated with temperature changes, are taken into account.

Description: The bivalve Glycymeris planicostalis as a high-resolution paleoclimate archive for Rupelian (Early Oligocene) of Central Europe Climate of the Past Discussions, 10, 4085-4127, 2014 Author(s): E. O. Walliser, B. R. Schöne, T. Tütken, J. Zirkel, K. I. Grimm, and J. Pross Current global warming is likely to result in a unipolar glaciated world with unpredictable repercussions on atmospheric and oceanic circulation patterns. These changes are expected to affect seasonality as well as the frequency and intensity of decadal climate oscillations. To better constrain the mode and tempo of the anticipated changes, climatologists require high-resolution proxy data of time intervals in the past, e.g. the Early Oligocene during which boundary conditions were similar to those predicted for the near future. As demonstrated by the present study, pristinely preserved shells of the long-lived bivalve mollusk Glycymeris planicostalis from the late Rupelian of the Mainz Basin, Germany, provide an excellent archive to reconstruct changes of sea surface temperature on seasonal to inter-annual time scales. Their shells grew uninterruptedly during winter and summer and therefore recorded the full seasonal temperature amplitude that prevailed in the Mainz Basin 30 Ma ago. Absolute sea surface temperature data were faithfully reconstructed from δ 18 O shell values assuming a δ 18 O water signature that was extrapolated from coeval sirenian tooth enamel. Extreme values ranged between 12.3 and 22.0°C and agree well with previous estimates based on planktonic foraminifera and shark teeth. However, summer and winter temperatures varied greatly on inter-annual time-scales. Winter and summer temperatures averaged over 40 annual increments of three specimens equaled 13.6 ± 0.8°C and 17.3 ± 1.2°C, respectively. Unless many samples are analyzed, this variability is hardly seen in foraminiferan tests. Our data also revealed decadal-scale oscillations of seasonal extremes which have – in the absence of appropriate climate archives – never been identified before for the Oligocene. This information can be highly relevant for numerical climate studies aiming to predict possible future climates in a unipolar glaciated or, ultimately, polar ice-free world.

Description: Sea level trends in South East Asian Seas (SEAS) Climate of the Past Discussions, 10, 4129-4148, 2014 Author(s): M. W. Strassburg, B. D. Hamlington, R. R. Leben, P. Manurung, J. Lumban Gaol, B. Nababan, S. Vignudelli, and K.-Y. Kim Southeast Asian Seas (SEAS) span the largest archipelago in the global ocean and provide a complex oceanic pathway connecting the Pacific and Indian Oceans. The SEAS regional sea level trends are some of the highest observed in the modern satellite altimeter record that now spans almost two decades. Initial comparisons of global sea level reconstructions find that 17 year sea level trends over the past 60 years exhibit good agreement in areas and at times of strong signal to noise associated decadal variability forced by low frequency variations in Pacific trade winds. The SEAS region exhibits sea level trends that vary dramatically over the studied time period. This historical variation suggests that the strong regional sea level trends observed during the modern satellite altimeter record will abate as trade winds fluctuate on decadal and longer time scales. Furthermore, after removing the contribution of the Pacific Decadal Oscillation (PDO) to sea level trends in the past twenty years, the rate of sea level rise is greatly reduced in the SEAS region. As a result of the influence of the PDO, the SEAS regional sea level trends during 2010s and 2020s are likely to be less than the global mean sea level (GMSL) trend if the observed oscillations in wind forcing and sea level persist. Nevertheless, long-term sea level trends in the SEAS will continue to be affected by GMSL rise occurring now and in the future.

Description: Northern Hemisphere control of deglacial vegetation changes in the Rufiji uplands (Tanzania) Climate of the Past Discussions, 10, 3931-3964, 2014 Author(s): I. Bouimetarhan, L. Dupont, H. Kuhlmann, J. Pätzold, M. Prange, E. Schefuß, and K. Zonneveld In tropical Eastern Africa, vegetation distribution is largely controlled by regional hydrology which has varied over the past 20 000 years. Therefore, accurate reconstructions of past vegetation and hydrological changes are crucial to better understand climate variability in the tropical Eastern African region. Through high-resolution pollen records from a marine sediment core recovered offshore the Rufiji River, our data show significant shifts in pollen assemblages during the last deglaciation identifying, through respective changes in both upland and lowland vegetation, specific responses of plant communities to atmospheric (precipitation) and coastal (coastal dynamics/sea level changes) alterations. Specifically, an interval of maximum pollen representation of dry and open vegetation occurred during the Northern Hemisphere cold Heinrich event 1 (H1) suggesting the expansion of drier upland vegetation under arid conditions. This dry spell is followed by an interval in which forest and humid woodland expanded, indicating a hydrologic shift towards more humid conditions. Droughts during H1 and the return to humid conditions around ~14.8 kyr BP in the uplands are primarily attributed to latitudinal shifts of the Intertropical Convergence Zone (ITCZ) driven by high-latitude Northern Hemisphere climatic fluctuations. Additionally, our results show that the lowland vegetation, consisting of a well developed salt marshes and mangroves in a successional pattern typical for vegetation occurring in intertidal habitats, has responded mainly to local coastal dynamics related to marine inundation frequencies and soil salinity in the Rufiji Delta as well as the local moisture availability. Lowland vegetation shows a substantial expansion of mangrove trees after ~14.8 kyr BP suggesting also an increased moisture availability and river runoff in the coastal area. The results of this study highlight the de-coupled climatic and environmental processes to which the vegetation in the uplands and the Rufiji Delta has responded during the last deglaciation.

Description: Climate history of the Southern Hemisphere Westerlies belt during the last glacial-interglacial transition revealed from lake water oxygen isotope reconstruction of Laguna Potrok Aike (52° S, Argentina) Climate of the Past Discussions, 10, 2417-2465, 2014 Author(s): J. Zhu, A. Lücke, H. Wissel, C. Mayr, D. Enters, K. J. Kim, C. Ohlendorf, F. Schäbitz, and B. Zolitschka The Southern Hemisphere westerly winds (SHW) play a crucial role in the large-scale ocean circulation and global carbon cycling. Accordingly, the reconstruction of its latitudinal position and intensity is essential for understanding global climatic fluctuations during the last glacial cycle. The southernmost part of the South American continent is of great importance for paleoclimate studies as the only continental mass intersecting a large part of the SHW belt. However, continuous proxy records back to the last Glacial are rare in southern Patagonia, owing to the Patagonian Ice Sheets expanding from the Andean area and the scarcity of continuous paleoclimate archives in extra-Andean Patagonia. Here, we present an oxygen isotope record from cellulose and purified bulk organic matter of aquatic moss shoots from the last glacial-interglacial transition preserved in the sediments of Laguna Potrok Aike (52° S, 70° W), a deep maar lake located in semi-arid, extra-Andean Patagonia. The highly significant correlation between oxygen isotope values of aquatic mosses and their host waters and the abundant well-preserved moss remains allow a high-resolution oxygen isotope reconstruction of lake water (δ 18 O lw ) for this lake. Long-term δ 18 O lw variations are mainly determined by δ 18 O changes of the source water of lake, surface air temperature and evaporative 18 O enrichment. Under permafrost conditions during the Glacial, the groundwater may not be recharged by regional precipitation. The isolated groundwater could have had much less negative δ 18 O values than glacial precipitation. The less 18 O depleted source water and prolonged lake water residence time caused by reduced interchange between in- and outflows could have resulted in the reconstructed glacial δ 18 O lw that was only ca. 3‰ lower than modern values. The significant two-step rise in reconstructed δ 18 O lw during the last deglaciation demonstrated the response of isotope composition of lake water to fundamental climatic shifts. Rapid deglacial warming is supposed to cause the 18 O enrichment of lake water by ca. 2‰ during the first rise between 17 600 and 15 600 cal BP by increasing temperature-induced evaporation and more 18 O enriched precipitation. After a millennial transition period of receding values by up to 0.7‰, the reconstructed δ 18 O lw resumed pronounced increase since 14 600 cal BP. This cumulative enrichment in 18 O of lake water could be interpreted as a response to the strengthened wind-driven evaporation, implying the intensification and establishment of the SHW at the latitude of Laguna Potrok Aike (52° S). During the early Holocene the SHW exerted its full influence on the lake water balance, reflected by reconstructed δ 18 O lw approaching modern values, indicating a strongly evaporative steppe climate in the Laguna Potrok Aike region.

Description: Enhanced 20th century heat transfer to the Arctic simulated in the context of climate variations over the last millennium Climate of the Past Discussions, 10, 2895-2924, 2014 Author(s): J. H. Jungclaus, K. Lohmann, and D. Zanchettin Oceanic heat transport variations, carried by the northward flowing Atlantic Water, strongly influence Arctic sea-ice distribution, ocean–atmosphere exchanges, and pan-Arctic temperatures. Paleoceanographic reconstructions from marine sediments near Fram Strait have documented a dramatic increase in Atlantic Water temperatures over the 20th century, unprecedented in the last millennium. Here we present results from Earth system model simulations over the last millennium that reproduce and explain reconstructed integrated quantities such as pan-Arctic temperature evolution during the pre-industrial millennium as well as the exceptional Atlantic Water warming in Fram Strait in the 20th century. The associated increase in ocean heat transfer to the Arctic can be traced back to changes in the ocean circulation in the sub-polar North Atlantic. An interplay between a weakening overturning circulation and a strengthening sub-polar gyre as a consequence of 20th century global warming is identified as driving mechanism for the pronounced warming along the Atlantic Water path toward the Arctic. Simulations covering the late Holocene provide a reference frame that allows us to conclude that the changes during the last century are unprecedented in the last 1150 years and that they cannot be explained by internal variability or natural forcing alone.

Description: An abrupt slowdown of Atlantic Meridional Overturning Circulation during 1915–1935 induced by solar forcing in a coupled GCM Climate of the Past Discussions, 10, 2519-2546, 2014 Author(s): P. Lin, Y. Song, Y. Yu, and H. Liu In this study, we explore an abrupt change of Atlantic Meridional Overturning Circulation (AMOC) apparent in the historical run simulated by the second version of the Flexible Global Ocean–Atmosphere–Land System model – Spectral Version 2 (FGOALS-s2). The abrupt change is noted during the period from 1915 to 1935, in which the maximal AMOC value is weakened beyond 6 Sv (1 Sv = 10 6 m 3 s −1 ). The abrupt signal first occurs at high latitudes (north of 46° N), then shifts gradually to middle latitudes (∼35° N) three to seven years later. The weakened AMOC can be explained in the following. The weak total solar irradiance (TIS) during early twentieth century decreases pole-to-equator temperature gradient in the upper stratosphere. The North polar vortex is weakened, which forces a negative North Atlantic Oscillation (NAO) phase during 1905–1914. The negative phase of NAO induces anomalous easterly winds in 50–70° N belts, which decrease the release of heat fluxes from ocean to atmosphere and induce surface warming over these regions. Through the surface ice–albedo feedback, the warming may lead to continuously melting sea ice in Baffin Bay and Davis Strait, which results in freshwater accumulation. This can lead to salinity and density reductions and then an abrupt slowdown of AMOC. Moreover, due to increased TIS after 1914, the enhanced Atlantic northward ocean heat transport from low to high latitudes induces an abrupt warming of sea surface temperature or upper ocean temperature in mid–high latitudes, which can also weaken the AMOC. The abrupt change of AMOC also appears in the PiControl run, which is associated with the lasting negative NAO phases due to natural variability.

Description: Deglacial ice–sheet meltdown: orbital pacemaking and CO 2 effects Climate of the Past Discussions, 10, 509-532, 2014 Author(s): M. Heinemann, A. Timmermann, O. E. Timm, F. Saito, and A. Abe-Ouchi Eighty thousand years of ice-sheet build-up came to a rapid end ~20–10 thousand years before present (ka BP), when ice sheets receded quickly, and global mean surface temperatures rose by about 4 °C. It still remains unresolved whether insolation changes due to variations of earth's tilt and orbit were sufficient to terminate glacial conditions. Using a coupled three-dimensional climate–ice-sheet model, we simulate the climate and Northern Hemisphere ice-sheet evolution from 78 to 0 ka BP in good agreement with sea level and ice topography reconstructions. Based on this simulation and a series of deglacial sensitivity experiments with individually varying orbital parameters and CO 2 , we find that enhanced calving led to a slow-down of ice-sheet growth already 5 to 8 ka prior to the Last Glacial Maximum (LGM), as evidenced by the change in curvature of the simulated and reconstructed ice volume time series. Increasing obliquity and precession then led to accelerated ice loss due to ablation and calving, thereby initiating the glacial termination. The deglacial sensitivity experiments further reveal that the ~100 ppmv rise of atmospheric CO 2 after ~18 ka BP was a key contributor to the deglaciation. Without it, the present-day ice volume would be comparable to that of the LGM and global mean temperatures would be about 3 °C lower than today. We further demonstrate that neither orbital forcing nor CO 2 forcing alone were sufficient to complete the deglaciation.

Description: Paleoclimate and weathering of the Tokaj (NE Hungary) loess-paleosol sequence: a comparison of geochemical weathering indices and paleoclimate parameters Climate of the Past Discussions, 10, 469-507, 2014 Author(s): A.-K. Schatz, T. Scholten, and P. Kühn The Tokaj loess-paleosol sequence in NE Hungary is one of the key sites for detailed paleoclimate reconstructions of the Quaternary in SE Europe. In this study, the geochemical composition of samples from the upper part of the sequence (45–21 ka) was analyzed and a variety of commonly used weathering indices and element ratios were applied to estimate weathering intensity. Further, similarities and differences between these weathering indices and their sensitivity to changes in paleoclimatic conditions were assessed. Results indicate that all of them accurately track changes in weathering intensity and are, with minor exceptions, very similar to each other. Based on different transfer functions for major and trace element concentrations (XRF), magnetic susceptibility (MS) and δ 13 C data, we calculated mean annual paleotemperature and mean annual paleoprecipitation for the time intervals of paleosol formation (45–27 ka) and dust deposition (27–21 ka). Results differ depending on the respective transfer function and method but largely agree with previously published paleoclimate data of the region. XRF- and δ 13 C-based results converge to a MAT of 8–10°C (paleosol) and 8–9°C (loess) and show a~MAP range of 685–879 mm a -1 (paleosol) and 572–700 mm a -1 (loess). MS-based results are most reliable with MATs of 8.4°C (paleosol) and 6.7°C (loess) and MAPs of 325–441 mm a -1 (paleosol) and 224 mm a -1 (loess).

Description: The role of the northward-directed (sub)surface limb of the Atlantic Meridional Overturning Circulation during the 8.2 ka Event Climate of the Past Discussions, 10, 665-687, 2014 Author(s): A. D. Tegzes, E. Jansen, and R. J. Telford The so-called "8.2 ka Event" has been widely regarded as a major climate perturbation over the Holocene. It is most readily identifiable in the oxygen-isotope records from Greenland ice cores as an approximately 160 yr-long cold interval between 8250–8090 yr BP. The prevailing view has been that the cooling over Greenland, and potentially over the northern North Atlantic at least, was triggered by the catastrophic final drainage of the Agassiz-Ojibway proglacial lake as part of the remnant Laurentide Ice Sheet collapsed over Hudson Bay at around 8420 ± 80 yr BP. The consequent freshening of surface waters in the northern North Atlantic Ocean and the Nordic Seas resulted in weaker overturning, hence reduced northward heat transport. Here we present proxy records from site JM97-MD95-2011 on the mid-Norwegian Margin indicating a (sharp) decline in the strength of the eastern branch of the Atlantic Inflow into the Nordic Seas immediately following a uniquely large drop in (sub)surface ocean temperatures coeval with the lake outbursts. We propose that the final drainage of Lake Agassiz-Ojibway was accompanied by a major iceberg discharge from Hudson Bay, which resulted in the cooling of the northward-directed northern Gulf Stream-North Atlantic Drift-Norwegian Atlantic Current system. Since our current-strength proxy records from the mid-Norwegian Margin do not evidence an exceptionally strong reduction in the main branch of the Atlantic Inflow into the Nordic Seas at the time, we argue that a chilled northward-directed (sub)surface-current system and an already colder background climate state could be the main factors responsible for the 8.2 ka climate perturbation.

Description: Natural periodicities and north–south hemispheres connection of fast temperature changes during the last glacial period: EPICA and NGRIP revisited Climate of the Past Discussions, 10, 1129-1152, 2014 Author(s): T. Alberti, F. Lepreti, A. Vecchio, E. Bevacqua, V. Capparelli, and V. Carbone We investigate both the European Project for Ice Coring in Antarctica (EPICA) and North Greenland Ice-Core Project (NGRIP) datasets to study the time evolution of the so-called Dansgaard–Oeschger events during the last glacial period. The Empirical Mode Decomposition (EMD) is used to extract the proper modes of both the datasets. It is shown that the time behaviour of Dansgaard–Oeschger events is captured by three EMD modes, while three more EMD modes can be used to describe the evolution at longer time scales. Using EMD signal reconstructions and a simple model based on the one-dimensional Langevin equations, it is argued that the occurrence of a Dansgaard–Oeschger event can be described as an excitation of the climate system within the same state, while the longer time scale behaviour appears to be due to transitions between different states. Finally, on the basis of a cross-correlation analysis performed on EMD reconstructions, evidence is presented that the Antarctic climate changes lead that of Greenland by a lag of ≈3.6 kyr.

Description: Observations of a stratospheric aerosol veil from a tropical volcanic eruption in December 1808: is this the "Unknown" ~1809 eruption? Climate of the Past Discussions, 10, 1901-1932, 2014 Author(s): A. Guevara-Murua, C. A. Williams, E. J. Hendy, A. C. Rust, and K. V. Cashman The "Unknown" eruption of 1808/1809 was the second most explosive SO 2 -rich volcanic eruption in the last two centuries, only eclipsed by the cataclysmic VEI 7 Tambora eruption in April 1815. However, no eyewitness accounts of the event, and therefore its location, or the atmospheric optical effects associated with its aerosols have been documented from historical records. Here we report on two meteorological observations dating from the end of 1808 that describe phenomena we attribute to volcanic-induced atmospheric effects caused by the Unknown eruption. The observations were made by two highly respected Latin American scientists. The first, Francisco José de Caldas, describes a stratospheric aerosol haze, a "transparent cloud that obstructs the sun's brilliance", that was visible over the city of Bogotá, Colombia, from 11 December 1808 to at least mid-February 1809. The second, made by physician José Hipólito Unanue in Lima, Peru, describes sunset after-glows (akin to well-documented examples known to be caused by stratospheric volcanic aerosols) from mid-December 1808 to February 1809. These two accounts provide direct evidence of a persistent stratospheric aerosol veil that spanned at least 2600 km into both Northern and Southern Hemispheres and establish that the source was a tropical volcano. Moreover, these observations confirm that the Unknown eruption, previously identified and tentatively assigned to February 1809 (±4 months) from analysis of ice core sulphate records, occurred in late November or early December 1808 (4 December 1808 ± 7 days). This date has important implications for the associated hemispheric climate impacts and temporal pattern of aerosol dispersal.

Description: Ocean Biogeochemistry in the warm climate of the Late Paleocene Climate of the Past Discussions, 10, 1933-1975, 2014 Author(s): M. Heinze and T. Ilyina The Late Paleocene is characterized by warm and stable climatic conditions which served as the background climate for the Paleocene-Eocene Thermal Maximum (PETM, ~55 million years ago). With respect to feedback processes in the carbon cycle, the ocean biogeochemical background state is of major importance for projecting the climatic response to a carbon perturbation related to the PETM. Therefore we use the Hamburg Ocean Carbon Cycle model HAMOCC, embedded into the ocean general circulation model of the Max Planck Institute for Meteorology, MPIOM, to constrain the ocean biogeochemistry of the Late Paleocene. We focus on the evaluation of modeled spatial and vertical distributions of the ocean carbon cycle parameters in a long-term warm steady-state ocean, based on a 560 ppm CO 2 atmosphere. Model results are discussed in the context of available proxy data and simulations of pre-industrial conditions. Our results illustrate that ocean biogeochemistry is shaped by the warm and sluggish ocean state of the Late Paleocene, which affects the strength and spatial variation of the different carbon pumps. Primary production is only slightly reduced in comparison to present-day; it is intensified along the equator, especially in the Atlantic. This enhances remineralization of organic matter, resulting in strong oxygen minimum zones and CaCO 3 dissolution in intermediate waters. We show that an equilibrium CO 2 exchange without increasing total alkalinity concentrations above today's values is achieved. Yet, the surface ocean pH and the saturation state with respect to CaCO 3 are lower than today. Our results indicate that under such conditions, the surface ocean carbonate chemistry is expected to be more sensitive to a carbon perturbation (i.e. the PETM) due to lower CO 3 2− concentration, whereas the deep ocean calcite sediments would be less vulnerable to dissolution due to the sluggish ocean.

Description: Interaction of ice sheets and climate during the past 800 000 years Climate of the Past Discussions, 10, 2547-2594, 2014 Author(s): L. B. Stap, R. S. W. van de Wal, B. de Boer, R. Bintanja, and L. J. Lourens During the Cenozoic, land ice and climate have interacted on many different time scales. On long time scales, the effect of land ice on global climate and sea level is mainly set by large ice sheets on North America, Eurasia, Greenland and Antarctica. The climatic forcing of these ice sheets is largely determined by the meridional temperature profile resulting from radiation and greenhouse gas (GHG) forcing. As response, the ice sheets cause an increase in albedo and surface elevation, which operates as a feedback in the climate system. To quantify the importance of these climate-land ice processes, a zonally-averaged energy balance climate model is coupled to five one-dimensional ice-sheet models, representing the major ice sheets. In this study, we focus on the transient simulation of the past 800 000 years, where a high-confidence CO 2 -record from ice cores samples is used as input in combination with Milankovitch radiation changes. We obtain simulations of atmospheric temperature, ice volume and sea level, that are in good agreement with recent proxy-data reconstructions. We examine long-term climate-ice sheet interactions by a comparison of simulations with uncoupled and coupled ice sheets. We show that these interactions amplify global temperature anomalies by up to a factor 2.6, and that they increase polar amplification by 94%. We demonstrate that, on these long time scales, the ice-albedo feedback has a larger and more global influence on the meridional atmospheric temperature profile than the surface-height temperature feedback. Furthermore, we assess the influence of CO 2 and insolation, by performing runs with one or both of these variables held constant. We find that atmospheric temperature is controlled by a complex interaction of CO 2 and insolation, and both variables serve as thresholds for Northern Hemispheric glaciation.

Description: Effect of the Ordovician paleogeography on the (in)stability of the climate Climate of the Past Discussions, 10, 2767-2804, 2014 Author(s): A. Pohl, Y. Donnadieu, G. Le Hir, J.-F. Buoncristiani, and E. Vennin The Ordovician is a particular Period during Earth History highlighted by abundant evidence for continental-size polar ice-sheets. Modelling studies published so far require a sharp CO 2 drawdown to initiate this glaciation. They mostly used non-dynamic slab mixed-layer ocean models. Here, we use a general circulation model with coupled components for ocean, atmosphere and sea ice to examine the response of Ordovician climate to changes in CO 2 and paleogeography. We conduct experiments for a wide range of CO 2 (from 16 to 2 times the preindustrial atmospheric CO 2 level (PAL)) and for two continental configurations (at 470 Ma and at 450 Ma) mimicking the Middle and the Late Ordovician conditions. We find that the temperature–CO 2 relationship is highly non-linear when ocean dynamics is taken into account. Two climatic modes are simulated as radiative forcing decreases. For high CO 2 concentrations (≥ 12 PAL at 470 Ma and ≥ 8 PAL at 450 Ma), a relative hot climate with no sea ice characterises the warm mode. When CO 2 is decreased to 8 PAL and 6 PAL at 470 and 450 Ma, a tipping-point is crossed and climate abruptly enters a runaway icehouse leading to a cold mode marked by the extension of the sea ice cover down to the mid-latitudes. At 450 Ma, the transition from the warm to the cold mode is reached for a decrease in atmospheric CO 2 from 8 to 6 PAL and induces a ~ 9 °C global cooling. We show that the tipping-point is due to the existence of a quasi-oceanic Northern Hemisphere, which in turn induces a minimum in oceanic heat transport located around 40° N. The peculiar shape of the oceanic heat transport in the Northern Hemisphere explains the potential existence of the warm and of the cold climatic modes. This major climatic instability potentially brings a new explanation to the sudden Late Ordovician Hirnantian glacial pulse that does not require any large CO 2 drawdown.

Description: Quantitative reconstruction of East Asian summer monsoon precipitation during the Holocene based on oxygen isotope mass-balance calculation in the East China Sea Climate of the Past Discussions, 10, 1447-1492, 2014 Author(s): Y. Kubota, R. Tada, and K. Kimoto The δ 18 O of seawater (δ 18 O w ), an indirect indicator of sea surface salinity, in the northern East China Sea (ECS) was reconstructed for the last 7 kyr using paired Mg/Ca ratio and δ 18 O of planktic foraminiferal tests. According to modern observation, interannual variations in sea surface salinity during summer in the northern part of the ECS are mainly controlled by the discharge from the Changjiang (Yangtze River), which reflects summer rainfall in the drainage area of the Changjiang. Thus, changes in the summer sea surface salinity in the northern ECS are interpreted as reflecting variations in the East Asian summer monsoon (EASM) precipitation in South China. This interpretation is confirmed by the strong relationship between salinity in the northern ECS and the discharge from the Changjiang during wet season (May–October) based on the observational salinity data from 1951 to 2000. On the other hand, it is difficult to estimate absolute salinity value in the past with high accuracy, because there are large uncertainties in salinity-δ 18 O w regression slope, end-member salinity, and δ 18 O w values. For this reason, in order to reconstruct the discharge in the past, we conducted δ 18 O w mass-balance calculation to estimate freshwater contribution to the surface water of the northern ECS during the Holocene. We assumed a simple mixing between two end-members: the seawater and the freshwater from the Changjiang. Temporal variations in the relative contribution of the freshwater from the Changjiang indicates that there was no long-term decreasing trend in the Changjiang freshwater discharge since the middle Holocene, but centennial to millennial scale variations were predominant. This suggests that changes in summer insolation in the Northern Hemisphere did not mainly control the changes in summer precipitation in the South China. Our results also indicate that variability of the Changjiang freshwater during the Holocene on sub-millennial timescale was much lower than interannual time scale, but similar to decadal time scale.

Description: Holocene environmental changes in the highlands of the southern Peruvian Andes (14° S) and their impact on pre-Columbian cultures Climate of the Past Discussions, 10, 1707-1746, 2014 Author(s): K. Schittek, B. Mächtle, F. Schäbitz, M. Forbriger, V. Wennrich, M. Reindel, and B. Eitel Within palaeoenvironmental studies, high-altitude peatlands of the Andes still remain relatively unexploited, although they offer an excellent opportunity for high-resolution chronologies, on account of their high accumulation rates and abundant carbon for dating. Especially in the central Andes, additional high-quality proxy records are still needed due to the lack of continuous and well-dated records, which show a significant variability on sub-centennial to decadal precision scales. To widen the current knowledge on climatic and environmental changes in the western Andes of southern Peru, we present a new, high-resolution 8600 year-long record from Cerro Llamoca peatland, a high-altitude Juncaceous cushion peatland in the headwaters of Río Viscas, a tributary to Río Grande de Nasca. A 10.5 m core of peat with intercalated sediment layers was examined for all kinds of microfossils, including fossil charred particles. We chose homogeneous peat sections for pollen analysis at a high temporal resolution. The inorganic geochemistry was analysed in 2 mm resolution using an ITRAX X-ray fluorescence (XRF) core scanner. We interpret the increase of Poaceae pollen in our record as an expansion of Andean grasslands during humid phases. Drier conditions are indicated by a significant decrease of Poaceae pollen and higher abundances of Asteraceae pollen. The results are substantiated by changes in arsenic contents and manganese/iron ratios, which turned out as applicable proxies for in situ palaeo-redox conditions. The mid-Holocene period of 8.6–5.6 ka is characterized by a series of episodic dry spells alternating with spells that are more humid. After a pronounced dry period at 4.6–4.2 ka, conditions generally shifted towards a more humid climate. We stress a humid/relatively stable interval between 1.8–1.2 ka, which coincides with the florescence of the Nasca culture in the Andean foreland. An abrupt turnover to a sustained dry period occurs at 1.2 ka, which coincides with the collapse of the Nasca/Wari society in the Palpa lowlands. Markedly drier conditions prevail until 0.75 ka, providing evidence for the presence of a Medieval Climate Anomaly. Moister but hydrologically highly variable conditions prevailed again after 0.75 ka, which allowed the re-expansion of tussock grasses in the highlands, increased discharge into the Andean foreland and the re-occupation of the settlements in the lowlands during this so-called Late Intermediate Period. On a supraregional scale, our findings can ideally be linked to and proofed by the archaeological chronology of the Nasca-Palpa region as well as other high-resolution marine and terrestrial palaeoenvironmental records. Our findings show that hydrological fluctuations, triggered by the changing intensity of the monsoonal tropical summer rains emerging from the Amazon Basin in the north-east, have controlled the climate in the study area.

Description: The impact of Sahara desertification on Arctic cooling during the Holocene Climate of the Past Discussions, 10, 1653-1673, 2014 Author(s): F. J. Davies, H. Renssen, M. Blaschek, and F. Muschitiello Since the start of the Holocene, temperatures in the Arctic have steadily declined. This has been accredited to the orbitally forced decrease in summer insolation reconstructed over the same period. However, we present climate modelling results here that indicate that up to 42% of the cooling in the Arctic, over the period 9–0 ka was a direct result of the desertification that occurred in the Sahara. Through a land–atmosphere teleconnection, increasing surface albedo in the Sahara leads to a regional increase in surface pressure, a weakening of the trade winds, the westerlies and the polar easterlies, which in turn reduces the meridional heat transported by the atmosphere to the Arctic. Additionally, through a series of targeted sensitivity experiments we explored the affects that using a modern cloud data set has upon mid and early Holocene climate simulations, and show that despite an apparent weakness in our model our original conclusions are robust. We conclude that interglacial climate is sensitive to changes in Sahara vegetation type, which has significance in the future debate of the response of the Sahara to climate change, considering the uncertainty surrounding future precipitation projections for this region.

Description: Identifying homogenous sub-periods in HadISD Climate of the Past Discussions, 10, 1567-1607, 2014 Author(s): R. J. H. Dunn, K. M. Willett, C. P. Morice, and D. E. Parker We report on preliminary steps in the homogenisation of HadISD, a sub-daily, station-based dataset covering 1973–2013. Using temperature, dewpoint temperature, sea-level pressure and wind speeds, change points are detected using the Pairwise Homogenisation Algorithm from Menne and Williams Jr. (2009). Monthly mean values and monthly mean diurnal ranges (temperature and dewpoint temperature) or monthly maximum values (wind speeds) are processed using the full network of 6103 stations in HadISD. Where multiple change points are detected within one year, they are combined and the average date used. Under the assumption that the underlying true population of inhomogeneity magnitudes is Gaussian, adjustments as small as around 0.5 °C, 0.5 hPa or 0.5 m s −1 have been successfully detected. No strong biases are present in the distributions of the adjustment values. The change point dates and adjustment values for each of the calculation methods will be provided alongside the dataset to allow users to select stations which have different levels of homogeneity. We give an example application of this change point information in calculating global temperature values from HadISD and comparing these to CRUTEM4. Removing the most inhomogeneous stations results in a better match between HadISD and CRUTEM4 when matched to the same coverage. However, further removals of stations with smaller and fewer inhomogeneities worsens the match.

Description: Sensitivity of the grassland-forest ecotone in East African open woodland savannah to historical rainfall variation Climate of the Past Discussions, 10, 1675-1706, 2014 Author(s): I. Ssemmanda, V. Gelorini, and D. Verschuren Fossil pollen records provide key insight into the sensitivity of terrestrial ecosystems to climate change at longer time scales. However, tracing vegetation response to relatively modest historical climate fluctuations is often complicated by the overriding signature of anthropogenic landscape disturbance. Here we use high-resolution pollen data from a ~200 year lake-sediment record in open woodland savannah of Queen Elisabeth National Park (southwestern Uganda) to assess the sensitivity of the tropical lowland grassland-forest ecotone to historical fluctuations in annual rainfall on the order of 10% lasting several decades. Specifically we trace vegetation response to three episodes of increased regional rainfall dated to the 1820s–1830s, ca. 1865–1890 and from 1962 to around 2000. During inferred wetter episodes we find increases in the relative pollen abundance from trees and shrubs of moist semi-deciduous forest ( Allophylus , Macaranga , Celtis , Alchornea ), riparian forest ( Phoenix reclinata ) and savannah woodland ( Myrica , Acalypha , Combretaceae/Melostomataceae) as well as local savannah taxa ( Acacia , Rhus type vulgaris , Ficus ), together creating strong temporary reductions in Poaceae pollen (to 45–55% of the terrestrial pollen sum). During intervening dry episodes, most notably the period ca. 1920–1962, Poaceae pollen attained values of 65–75%, and dryland herbs such as Commelina , Justicia type odora and Chenopodiaceae expanded at the expense of Asteraceae, Solanum -type, Swertia usumbarensis -type, and (modestly so) Urticaceae. Noting that the overall diversity of arboreal taxa remained high but their combined abundance low, we conclude that the landscape surrounding Lake Chibwera has been an open woodland savannah throughout the past 200 years, with historical rainfall variation exerting modest effects on local tree cover (mostly the abundance of Acacia and Ficus ) and the prevalence of damp soil areas promoting Phoenix reclinata . The strong apparent expansion of true forest trees during wet episodes can be explained partly by enhanced pollen influx via upland streams. Pollen from exotic trees and other cultural indicators appears from the 1970s onwards, but their combined influence fails to mask the region's natural vegetation dynamics.

Description: Global sensitivity analysis of Indian Monsoon during the Pleistocene Climate of the Past Discussions, 10, 1609-1651, 2014 Author(s): P. A. Araya-Melo, M. Crucifix, and N. Bounceur The sensitivity of Indian Monsoon to the full spectrum of climatic conditions experienced during the Pleistocene is estimated using the climate model HadCM3. The methodology follows a global sensitivity analysis based on the emulator approach of Oakley and O'Hagan (2004) implemented following a three-step strategy: (1) develop an experiment plan, designed to efficiently sample a 5-dimensional input space spanning Pleistocene astronomical configurations (3 parameters), CO 2 concentration and a Northern Hemisphere glaciation index, (2) develop, calibrate and validate an emulator of HadCM3, in order to estimate the response of the Indian Monsoon over the full input space spanned by the experiment design, and (3) estimate and interpret sensitivity diagnostics, including sensitivity measures, in order to synthesize the relative importance of input factors on monsoon dynamics, estimate the phase of the monsoon intensity response with respect to that of insolation, and detect potential non-linear phenomena. Specifically, we focus on four variables: summer (JJAS) temperature and precipitation over North India, and JJAS sea-surface temperature and mixed-layer depth over the north-western side of the Indian ocean. It is shown that precession controls the response of four variables: continental temperature in phase with June to July insolation, high glaciation favouring a late-phase response, sea-surface temperature in phase with May insolation, and continental precipitation in phase with July insolation, and mixed-layer depth in antiphase with the latter. CO 2 variations controls temperature variance with an amplitude similar to that of precession. The effect of glaciation is dominated by the albedo forcing, and its effect on precipitation competes with that of precession. Obliquity is a secondary effect, negligible on most variables except sea-surface temperature. It is also shown that orography forcing reduces the glacial cooling, and even has a positive effect on precipitation. As regards the general methodology, it is shown that the emulator provides a powerful approach, not only to express model sensitivity, but also to estimate internal variability (based on the nugget term introduced in the correlation function of the emulator) and detect anomalous simulations.

Description: Tree ring effects and ice core acidities clarify the volcanic record of the 1st millennium Climate of the Past Discussions, 10, 1799-1820, 2014 Author(s): M. G. L. Baillie and J. McAneney Various attempts have been made to link tree-ring and ice-core records, something vital for the understanding of the environmental response to major volcanic eruptions in the past. Here we demonstrate that, by taking note of the spacing between events, it is possible to clarify linkages between tree-response, as witnessed by frost rings in bristlecone pines from Western North America and volcanic acid deposition in ice cores. The results demonstrate that in the 6th and 7th centuries of the current era, and presumably for all earlier dates, the key European ice chronologies from the North Greenland Ice Core Project, namely Dye3, GRIP, NGRIP and NEEM appear to have been wrongly dated by 7 years, with the ice dates being too old. Similar offsets are observed for the Antarctic Law Dome and West Antarctic Ice Sheet Divide WDC06A ice-core chronologies that have been linked to the Greenland record. Importantly, the results clarify which frost rings in bristlecone pines are related to volcanic activity and which may be the result of other causes. In addition, it is possible to show that ice core researchers have used inappropriate linkages to tree effects to justify their chronology.

Description: Seasonal variations recorded in cave monitoring results and a 10 year monthly resolved speleothem δ 18 O and δ 13 C record from the Han-sur-Lesse cave, Belgium Climate of the Past Discussions, 10, 1821-1856, 2014 Author(s): M. Van Rampelbergh, S. Verheyden, M Allan, Y. Quinif, E. Keppens, and P. Claeys Speleothems provide paleoclimate information on multi-millennial to decadal scales in the Holocene. However seasonal or even monthly resolved records remain scarce. They require fast growing stalagmites and a good understanding of the proxy transfer function on very short time scales. The Proserpine stalagmite from the Han-sur-Less cave (Belgium) displays seasonal layers of 0.5 to 2 mm thickness that reconstruct paleoclimates at a monthly scale. Through a regular cave monitoring, we acquired a good understanding of how δ 18 O and δ 13 C signals in modern calcite reflect climate variations on sub-seasonal scale. Cave parameters vary seasonally in response to the activity of the vegetation cover and outside air temperature. From December to June, the cave remains in "winter-mode". Outside temperatures are cold inducing low cave air and water temperatures. Bio-productivity in the soil is limited leading to low p CO 2 , higher δ 13 C composition of the CO 2 in the cave air and high discharge due to the inactivity of the plant coverage. From June to December, the cave switches to "summer-mode" and the measured factors display an opposite behavior. The δ 18 O and δ 13 C signals of fresh calcite precipitated on glass slabs vary seasonally. Lowest δ 18 O values occur during the summer-mode when the δ 13 C values are high. The δ 18 O composition of the calcite is in equilibrium with the drip water δ 18 O and display seasonal variations due to changes in the cave air and water temperature. In contrast to the δ 18 O signal, δ 13 C values of the calcite precipitated on the glass slabs do not reflect equilibrium conditions. Highest δ 13 C values occur during summer, when discharge rates are low increasing the evaporation effect on the thin water film covering the stalagmite. This same antithetical behavior of the δ 18 O vs. the δ 13 C signals is seen in the monthly resolved speleothem record that covers the period between 1976 and 1985 AD. Dark layers are formed during summer, while light layers are formed during winter when calcite deposition occurs fast. The darker the color of a layer, the more compact its calcite structure, the more negative its δ 18 O signal and the more positive its δ 13 C signal.

Description: Reconstruction of recent climate change in Alaska from the Aurora Peak ice core, central Alaska Climate of the Past Discussions, 10, 1421-1446, 2014 Author(s): A. Tsushima, S. Matoba, T. Shiraiwa, S. Okamoto, H. Sasaki, D. J. Solie, and K. Yoshikawa A 180.17 m ice core was drilled at Aurora Peak in the central part of the Alaska Range, Alaska, in 2008 to allow reconstruction of centennial-scale climate change in the northern North Pacific. The 10 m-depth temperature in the borehole was −2.2 °C, which corresponded to annual mean air temperature at the drilling site. In this ice core, there were many melt-refrozen layers due to high temperature and/or strong insolation during summer seasons. We analyzed stable hydrogen isotopes (δD) and chemical species in the ice core. The ice core age was determined by annual counts of δD and seasonal cycles of Na + , and we used reference horizons of tritium peaks in 1963 and 1964, major volcanic eruptions of Mount Spurr in 1992 and Mount Katmai in 1912, and a large forest fire in 2004 as age controls. Here, we show that the chronology of the Aurora Peak ice core from 95.61 m w.eq. to the top corresponds to the period from 1900 to the summer season of 2008, with a dating error of ±3 years. We estimated that the mean accumulation rate from 1997 to 2007 (except for 2004) was 1.88 m w.eq per year. Our results suggest that temporal variation in δD and annual accumulation rates are strongly related to shifts in the Pacific Decadal Oscillation index (PDOI). The remarkable increase in annual precipitation since the 1970s has likely been the result of enhanced storm activity associated with shifts in the PDOI during winter in the Gulf of Alaska.

Description: Implication of methodological uncertainties for Mid-Holocene sea surface temperature reconstructions Climate of the Past Discussions, 10, 1747-1782, 2014 Author(s): I. Hessler, S. P. Harrison, M. Kucera, C. Waelbroeck, M.-T. Chen, C. Andersson, A. de Vernal, B. Fréchette, A. Cloke-Hayes, G. Leduc, and L. Londeix We present and examine a~multi-sensor global compilation of Mid-Holocene (MH) sea surface temperatures (SSTs), based on Mg/Ca and alkenone palaeothermometry and reconstructions obtained using planktonic foraminifera and organic-walled dinoflagellate cyst census counts. We assess the uncertainties originating from using different methodologies and evaluate the potential of MH SST reconstructions as a benchmark for climate-model simulations. The comparison between different analytical approaches (time frame, baseline climate) shows the choice of time window for the MH has a negligible effect on the reconstructed SST pattern, but the choice of baseline climate affects both the magnitude and spatial pattern of the reconstructed SSTs. Comparison of the SST reconstructions made using different sensors shows significant discrepancies at a regional scale, with uncertainties often exceeding the reconstructed SST anomaly. Apparent patterns in SST may largely be a reflection of the use of different sensors in different regions. Overall, the uncertainties associated with the SST reconstructions are generally larger than the MH anomalies. Thus, the SST data currently available cannot serve as a target for benchmarking model simulations.

Description: Evidence for the non-influence of salinity variability on the coral Sr/Ca paleothermometer Climate of the Past Discussions, 10, 1783-1798, 2014 Author(s): M. Moreau, T. Corrège, E. P. Dassié, and F. Le Cornec The influence of salinity in the incorporation of trace elements in the skeleton of calcareous organisms is still poorly known. Studies on foraminiferal Mg/Ca thermometry have suggested a bias due to Sea Surface Salinity (SSS) variations, leading to potential erroneous estimation of Mg/Ca-based Sea Surface Temperature (SST). Culture experiments seem to indicate that in three coral species (not including the widely used Porites genus), salinity does not influence the Sr/Ca thermometer. In this study, we test the salinity effect on coral Sr/Ca-based SST reconstructions at monthly and interannual timescales in open-ocean environmental conditions, using a large spatial compilation of published coral data (mainly based on the Porites genus) originating from the Western Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the China Sea and the Red Sea and adding a new Eastern Pacific coral Sr/Ca record from the Clipperton atoll. We use simple and multiple regressions between Sr/Ca on one hand and SST and SSS on the other hand at the various sites. We find no evidence for a salinity bias on the Sr/Ca SST proxy for the two studied timescales. This study reinforces the use of coral Sr/Ca as a reliable paleothermometer.

Description: Evolution of the large-scale atmospheric circulation in response to changing ice sheets over the last glacial cycle Climate of the Past Discussions, 10, 1381-1420, 2014 Author(s): M. Löfverström, R. Caballero, J. Nilsson, and J. Kleman We present modelling results of the atmospheric circulation at the cold periods of marine isotope stage 5b (MIS 5b), MIS 4 and the Last Glacial Maximum (LGM), as well as the interglacial. The paleo-simulations are forced by ice sheet reconstructions consistent with geological evidence and by appropriate insolation and greenhouse gas concentrations. The results suggest that the large-scale atmospheric winter circulation remained largely similar to the interglacial for a significant part of the glacial cycle. The proposed explanation is that the ice sheets were located in areas where their interaction with the mean flow is limited. However, the LGM Laurentide Ice Sheet induces a much larger planetary wave that leads to a zonalisation of the Atlantic jet. In summer, the ice sheet topography dynamically induces warm temperatures in Alaska and central Asia that inhibits the expansion of the ice sheets into these regions. The warm temperatures may also serve as an explanation for westward propagation of the Eurasian Ice Sheet from MIS 4 to the LGM.

Description: Multiproxy reconstruction for Kuroshio responses to Northern Hemispheric oceanic climate and Asian Monsoon since marine isotope stage 5.1 (∼88 ka) Climate of the Past Discussions, 10, 1337-1380, 2014 Author(s): X. Shi, Y. Wu, J. Zou, Y. Liu, S. Ge, M. Zhao, J. Liu, A. Zhu, X. Meng, Z. Yao, and Y. Han The Kuroshio, a western boundary current in the Western Pacific, plays a key role in regulating ocean and climate in the East Asia. The evolution of the Kuroshio and its branches has been the focus of paleoceanographic studies. In this study, we applied a multiproxy (grain size, planktonic foraminiferal species, δ 18 O, alkenone sea surface temperature (SST) and salinity) reconstruction from sediment core CSH1, which is located at the main axis of the Tsushima Warm Current, a branch of the Kuroshio, in the northern Okinawa Trough (OT). This study, for the first time, extended the paleoceanographic record of the Kuroshio to Marine Isotope Stage (MIS) 5.1 (∼88 ka) from the far northern site in the OT. The core CSH1 contains three volcanic layers, K-Ah, AT, and Aso-4, which are ideal chronostratigraphic markers for precise age controls of the core. Planktonic foraminiferal species identified from this core contain warm water species related to the Kuroshio and cold species related to subarctic water mass. The relative abundances of the warm water species are high during MIS 1 and MIS 5.1, while cold species are high during MIS 2. An organic biomarker proxy, alkenone SST measured from CSH1 ranges between 21 and 25 °C with higher values during interglacials (MIS 1, 3.3, 5.1) and interstadials, and lower values during glacials and Heinrich (H)/stadial events. Sea surface salinity (SSS) and the depth of thermocline (DOT) reconstructed based on foraminifera isotopes and faunas indicate dominant Kuroshio responses to Northern Hemispheric climate and Asian Monsoon (AM) since ∼88 ka. The CSH1 SSS appears to be mainly controlled by the local river runoff and open ocean water, while the DOT change seems to be closely related to the strength of Kuroshio and the latitudinal shift of subarctic frontal zone. Our records suggest that during MIS 1 and MIS 5.1 while global sea level was high, the Kuroshio was dominant; while during MIS 2, MIS 3, and MIS 4 with low sea level, stronger winter AM and more southward subarctic front played important roles in governing the hydrographic characteristics in the OT. Spectral analysis of our multiproxy hydrographic records shows a dominant period at ∼24 ka. Our multiproxy hydrographic records from site near the modern Tsushima Warm Current show homogenous regional responses mainly to the global sea level and Northern Hemispheric climate, and regional mechanisms such as the Kuroshio, AM and subarctic front, which are consistently invoked in the interpretations of other regional records from the OT.

Description: A paleoenvironmental reconstruction of the last 15 000 cal yr BP via Yellow Sea sediments using biomarkers and isotopic composition of organic matter Climate of the Past Discussions, 10, 1527-1565, 2014 Author(s): A. O. Badejo, B.-H. Choi, H.-G. Cho, H.-I. Yi, and K.-H. Shin This study is the first reconstruction of the paleoenvironment and paleovegetation during the Holocene (interglacial) and glacial periods of the Yellow Sea. We report the carbon isotopic and biomarker ( n -alkane and alkenone) compositions of organic matter from Yellow Sea sediments since the glacial period. Our findings show that the variability of the East Asian Monsoon (EAM) affected the sedimentary profile of total organic carbon (TOC), the stable isotopes of bulk organic carbon (δ 13 C org ), the atomic ratio of carbon and nitrogen (C/N ratio), and biomarker content. The sedimentary δ 13 C org profile along the core exhibited more negative δ 13 C org values under cold/dry climatic conditions (Younger and Oldest Dryas). The carbon preference index (CPI), the pristane to phytane ratio (Pr/Ph) and the pristane to n -C 17 ratio (Pr/ n -C 17 ) were used to determine the early stages of diagenesis along the sediment core. Two climatic conditions were distinguished (warm/humid and cold/dry) based on an n -alkane proxy, and the observed changes in δ 13 C of individual n -alkane (δ 13 C ALK ) between the Holocene and glacial periods were attributed to changes in plant distribution/type. Clear differences were not found in the calculated alkenone sea surface temperature (SST) between those of the Holocene and glacial periods. This anomaly during the glacial period might be attributed to the seasonal water mass distribution in the Yellow Sea or a seasonal shift in the timing of maximum alkenone production as well as the Bølling/Allerød interstadial.

Description: Holocene climate change, permafrost, and cryogenic carbonate formation: insights from a recently deglaciated, high-elevation cave in the Austrian Alps Climate of the Past Discussions, 10, 1493-1526, 2014 Author(s): C. Spötl and H. Cheng Cryogenically formed carbonate particles represent a rather new class of speleothems whose origin is directly linked to the presence of perennial ice in the subsurface. Recent studies concluded that dating these deposits provides important time constraints on the presence and the thickness of permafrost e.g. during the last glacial period. More precisely, these carbonates require the coexistence of water and ice and hence record episodes of permafrost thawing. To shed more light on the origin of the coarsely crystalline variety of these cryogenic cave carbonates – CCC coarse for short – we examined a high-elevation cave site in the western part of the Austrian Alps which is located in an area dominated by permafrost features and transformed from an ice cave into an essentially ice-free cave during the past decade. Two side chambers of the main gallery revealed cryogenic calcite deposits whose isotopic composition indicates that they formed in individual pools of water carved in ice which underwent very slow freezing under closed-system conditions, i.e. enclosed in ice. 230 Th dating shows that most of these carbonates formed ca. 2600 yr BP. Based on comparisons with other palaeoclimate archives in the Alps this thawing episode did not occur during a climate optimum, nor did CCC coarse form in this cave during e.g. the Roman or the Medieval Warm Periods. Our results suggest that the occurrence of CCC coarse , at least in mountain regions characterized by discontinuous permafrost, may be more stochastic than previously thought. Given the inherent heterogeneity of karst aquifers and the important role of localized water infiltration in modifying the thermal structure of the subsurface we caution against attributing CCC coarse occurrences solely to peak warming conditions, while confirming the unique significance of these deposits in providing robust age constraints on permafrost thawing episodes.

Description: Controls on fire activity over the Holocene Climate of the Past Discussions, 10, 4257-4275, 2014 Author(s): S. Kloster, T. Brücher, V. Brovkin, and S. Wilkenskjeld Changes in fire activity over the last 8000 years are simulated with a global fire model driven by changes in climate and vegetation cover. The changes were separated into those caused through variations in fuel availability, fuel moisture or wind speed which react differently to changes in climate. Disentangling these controlling factors helps to understand the overall climate control on fire activity over the Holocene. Globally the burned area is simulated to increase by 2.5% between 8000 and 200 cal yr BP with larger regional changes compensating on a global scale. Despite the absence of anthropogenic fire ignitions, the simulated trends in fire activity agree reasonably well with continental scale reconstructions from charcoal records, with the exception of Europe. For some regions the change in fire activity is predominantly controlled through changes in fuel availability (Australia-Monsoon, American Tropics/Subtropics). For other regions changes in fuel moisture are more important for the overall trend in fire activity (North America, Sub-Saharan Africa, Europe, Asia-Monsoon). In Sub-Saharan Africa, for example, changes in fuel moisture alone lead to an increase in fire activity between 8000 and 200 cal yr BP, while changes in fuel availability lead to a decrease. Overall, the fuel moisture control is dominating the simulated fire activity for Sub-Saharan Africa. The simulations clearly demonstrate that both changes in fuel availability and changes in fuel moisture are important drivers for the fire activity over the Holocene. Fuel availability and fuel moisture do, however, have different climate controls. As such observed changes in fire activity can not be related to single climate parameters such as precipitation or temperature alone. Fire models, as applied in this study, in combination with observational records can help to understand the climate control on fire activity, which is essential to project future fire activity.

Description: High resolution leaf wax carbon and hydrogen isotopic record of late Holocene paleoclimate in arid Central Asia Climate of the Past Discussions, 10, 4385-4424, 2014 Author(s): B. Aichner, S. J. Feakins, J. E. Lee, U. Herzschuh, and X. Liu Central Asia is located at the intersection of large scale atmospheric circulation systems. It is thus likely to be highly susceptible to changes in the dynamics of those systems, however little is still known about the regions paleoclimate history. Here we present carbon and hydrogen isotopic compositions of n-alkanoic acids from a late Holocene sediment core from Lake Karakuli (eastern Pamir, Xinjiang Province, China). Instrumental evidence and isotope-enabled climate model experiments with the Laboratoire de Météorologie Dynamique Zoom model version 4 (LMDZ4) demonstrate that δ D values of precipitation in the region are influenced by both temperature and precipitation amount. We find that those parameters are inversely correlated on an annual scale; i.e. climate varies between cool/wet and dry/warm over the last 50 years. Since the isotopic signals of these changes are in the same direction and therefore additive, isotopes in precipitation are sensitive recorders of climatic changes in the region. Additionally, we infer that plants are using year round precipitation (including snow-melt) and thus leaf wax δ D values must also respond to shifts in the proportion of moisture derived from westerly storms during late winter/early spring. Downcore results give evidence for a gradual shift to cooler and wetter climates between 3.5 and 2.5 cal kyr BP, interrupted by a warm/dry episode between 3.0–2.7 kyr BP. Further cool and wet episodes occur between 1.9–1.5 kyr BP and between 0.6–0.1 kyr BP, the latter coeval with the Little Ice Age. Warm and dry episodes between 2.5–1.9 kyr BP and 1.5–0.6 kyr BP coincide with the Roman Warm Period and Medieval Climate Anomaly, respectively. Finally, we find a drying tend in recent decades. Regional comparisons lead us to infer that the strength and position of the Westerlies, and wider Northern Hemispheric climate dynamics control climatic shifts in arid Central Asia.

Description: A tropical speleothem record of glacial inception, the South American summer monsoon from 125 to 115 ka Climate of the Past Discussions, 10, 4365-4384, 2014 Author(s): S. J. Burns, L. C. Kanner, H. Cheng, and R. L. Edwards Relatively few marine or terrestrial paleoclimate studies have focused on glacial inception, the transition from an interglacial to a glacial climate state. As a result, the timing and structure of glacial inception is not well known, nor is the spatial pattern of glacial inception in different parts of the world. Here we present results of a study of a speleothem from the Peruvian Andes that records changes in the intensity of South American Summer Monsoon (SASM) rainfall over the period from 125 to 115 ka. The results show that late in the last interglacial period, at 123 ka, SASM rainfall decreased, perhaps in response to a decrease in Northern Hemisphere ice cover. Then at 120.8 ka a rapid increase in SASM rainfall marks the end of the last interglacial. After a more gradual increase between 120 and 117 ka, a second abrupt increase occurs at 117 ka. This pattern of change is mirrored to a remarkable degree by changes in the East Asian Monsoon. It is interpreted to reflect both the a long term gradual response of the monsoons the orbitally-driven insolation changes and to rapid changes in Northern Hemisphere ice volume and temperature. Both monsoon systems are close to their full glacial conditions by 117 ka, before any significant decrease in atmospheric CO 2 .

Description: Twelve thousand years of dust: the Holocene global dust cycle constrained by natural archives Climate of the Past Discussions, 10, 4277-4363, 2014 Author(s): S. Albani, N. M. Mahowald, G. Winckler, R. F. Anderson, L. I. Bradtmiller, B. Delmonte, R. François, M. Goman, N. G. Heavens, P. P. Hesse, S. A. Hovan, K. E. Kohfeld, H. Lu, V. Maggi, J. A. Mason, P. A. Mayewski, D. McGee, X. Miao, B. L. Otto-Bliesner, A. T. Perry, A. Pourmand, H. M. Roberts, N. Rosenbloom, T. Stevens, and J. Sun Mineral dust plays an important role in the climate system by interacting with radiation, clouds, and biogeochemical cycles. In addition, natural archives show that the dust cycle experienced variability in the past in response to global and local climate change. The compilation of the DIRTMAP paleodust datasets in the last two decades provided a target for paleoclimate models that include the dust cycle, following a time slice approach. We propose an innovative framework to organize a paleodust dataset that moves on from the positive experience of DIRTMAP and takes into account new scientific challenges, by providing a concise and accessible dataset of temporally resolved records of dust mass accumulation rates and particle grain-size distributions. We consider data from ice cores, marine sediments, loess/paleosol sequences, lake sediments, and peat bogs for this compilation, with a temporal focus on the Holocene period. This global compilation allows investigation of the potential, uncertainties and confidence level of dust mass accumulation rates reconstructions, and highlights the importance of dust particle size information for accurate and quantitative reconstructions of the dust cycle. After applying criteria that help to establish that the data considered represent changes in dust deposition, 43 paleodust records have been identified, with the highest density of dust deposition data occurring in the North Atlantic region. Although the temporal evolution of dust in the North Atlantic appears consistent across several cores and suggest that minimum dust fluxes are likely observed during the Early to mid-Holocene period (6000–8000 years ago), the magnitude of dust fluxes in these observations is not fully consistent, suggesting that more work needs to be done to synthesize datasets for the Holocene. Based on the data compilation, we used the Community Earth System Model to estimate the mass balance and variability of the global dust cycle during the Holocene, with dust load ranging from 17.1 to 20.5 Tg between 2000 and 10 000 years ago, and a minimum in the Early to Mid-Holocene (6000–8000 years ago).

Description: Eastern Mediterranean Sea circulation inferred from the conditions of S1 sapropel deposition Climate of the Past Discussions, 10, 4647-4681, 2014 Author(s): K. Tachikawa, L. Vidal, M. Cornuault, M. Garcia, A. Pothin, C. Sonzogni, E. Bard, G. Menot, and M. Revel Holocene Eastern Mediterranean Sea sediments contain an organic-rich sapropel S1 layer that was formed in oxygen-depleted waters. The spatial distribution of this layer revealed that during S1 deposition deep waters were permanently anoxic below 1800 m in water depth. To provide further insight into past Eastern Mediterranean Sea circulation, a multi-proxy approach was applied to a core retrieved close to the 1800 m boundary (at 1780 m). We measured the bulk sediment elemental composition, the stable isotopic composition of the planktonic foraminifer Globigerinoides ruber , and the abundance of benthic foraminifera since the last deglaciation. The result indicates that authigenic U and Mo accumulation began around 13–12 cal ka BP, in concert with surface water freshening estimated from the G. ruber δ 18 O record. The onset of bottom/pore water oxygen depletion occurred prior to S1 deposition inferred from barium enrichment. In the middle of the S1 deposition period, between 9 and 8 cal ka BP, reduced authigenic V, Fe and As contents and Br / Cl ratio indicated short-term bottom water re-oxygenation. A sharp Mn peak and maximal abundance for benthic foraminifera marked a total recovery for circulation at approximately 7 cal ka BP. Based on our results and existing data, we suggest that S1 formation withinthe upper 1780 m of the Eastern Mediterranean Sea was preconditioned by reduced ventilation, resulting from excess fresh water inputs due to insolation changes under deglacial conditions, that initiated between 15 and 12 ka. Short-term re-oxygenation in the Levantine Basin is estimated to have affected bottom water below and above the anoxic boundary. We tentatively propose that complete ventilation recovery at the S1 termination was attained earlier within the upper 1780 m than at deeper water depths. Our results provided new constraints for eastern Mediterranean Sea thermohaline circulation.

Description: Multiscale monsoon variability during the last two climatic cycles inferred from Chinese loess and speleothem records Climate of the Past Discussions, 10, 4623-4646, 2014 Author(s): Y. Li, N. Su, L. Liang, L. Ma, Y. Yan, and Y. Sun The East Asian Monsoon exhibits a significant variability on timescales ranging from tectonic to centennial as inferred from Chinese loess, stalagmite and marine records. However, the relative contributions and plausible driving forces of the signals at different timescales remain poorly investigated. Here, we spectrally decompose time series data on loess grain size and speleothem δ 18 O records over the last two climatic cycles and correlate the decomposed components with possible driving parameters including the ice volume, insolation and North Atlantic cooling. Based on the spectral analysis of these two proxies, we tentatively identified six components of the signals corresponding to various forcing of ice volume (〉 50 kyr), obliquity (50–30 kyr), precession (30–9 kyr), North Atlantic cooling (9–3 kyr and 3–1 kyr), and a centennial residual. The relative contributions of each component differ significantly between loess grain size and speleothem δ 18 O records. Glacial and orbital components are dominant in the loess grain size, which implies that both ice volume and insolation have distinctive impacts on the winter monsoon variability in contrast to the predominant precession impact on the summer monsoon patterns. Moreover, the millennial components are evident with variances of 11 and 16% in the loess grain size and speleothem δ 18 O records, respectively. A comparison of the millennial-scale signals in these two proxies reveals that abrupt changes in the winter and summer monsoons over the last 260 kyr share common features and similar driving forces linked to high-latitude Northern Hemisphere climate.

Description: The WAIS-Divide deep ice core WD2014 chronology – Part 2: Methane synchronization (68–31 ka BP) and the gas age-ice age difference Climate of the Past Discussions, 10, 3537-3584, 2014 Author(s): C. Buizert, K. M. Cuffey, J. P. Severinghaus, D. Baggenstos, T. J. Fudge, E. J. Steig, B. R. Markle, M. Winstrup, R. H. Rhodes, E. J. Brook, T. A. Sowers, G. D. Clow, H. Cheng, R. L. Edwards, M. Sigl, J. R. McConnell, and K. C. Taylor The West Antarctic Ice Sheet (WAIS)-Divide ice core (WAIS-D) is a newly drilled, high-accumulation deep ice core that provides Antarctic climate records of the past ∼68 ka at unprecedented temporal resolution. The upper 2850 m (back to 31.2 ka BP) have been dated using annual-layer counting. Here we present a chronology for the deep part of the core (67.8–31.2 ka BP), which is based on stratigraphic matching to annual-layer-counted Greenland ice cores using globally well-mixed atmospheric methane. We calculate the WAIS-D gas age-ice age difference (Δage) using a combination of firn densification modeling, ice flow modeling, and a dataset of δ 15 N-N 2 , a proxy for past firn column thickness. The largest Δage at WAIS-D occurs during the last glacial maximum, and is 525 ± 100 years. Internally consistent solutions can only be found when assuming little-to-no influence of impurity content on densification rates, contrary to a recently proposed hypothesis. We synchronize the WAIS-D chronology to a linearly scaled version of the layer-counted Greenland Ice Core Chronology (GICC05), which brings the age of Dansgaard-Oeschger (DO) events into agreement with the U/Th absolutely dated Hulu speleothem record. The small Δage at WAIS-D provides valuable opportunities to investigate the timing of atmospheric greenhouse gas variations relative to Antarctic climate, as well as the interhemispheric phasing of the bipolar "seesaw".

Description: Geothermal evidence of the Late Pleistocene-Holocene orbital forcing (example from the Urals, Russia) Climate of the Past Discussions, 10, 3617-3629, 2014 Author(s): D. Y. Demezhko and A. A. Gornostaeva We use early obtained in the Middle Urals geothermal reconstruction of the ground surface temperature (GST) history to determine the surface heat flux (SHF) history over the past 35 kyr. A new algorithm of GST-SHF transformation was applied to solve this problem. The time scale of geothermal reconstructions has been corrected by comparing the estimated heat flux and annual insolation at the latitude of 60° N. The consistency of SHF and insolation changes on the interval 35–6 kyr BP (the linear correlation coefficient R = 0.99) points to orbital factors as the main cause of climatic changes during the Pleistocene–Holocene transition. The amplitude of SHF variations is about 1.3% of the insolation changes amplitude. The increase of carbon dioxide concentrations lagged by 2–3 kyr from the SHF increase and occurred synchronously with GST changes.

Description: Photic zone changes in the North West Pacific Ocean from MIS 4-5e Climate of the Past Discussions, 10, 3631-3660, 2014 Author(s): G. E. A. Swann and A. M. Snelling In comparison to other sectors of the marine system, the palaeoceanography of the subarctic North Pacific Ocean is poorly constrained. New diatom isotope records of δ 13 C, δ 18 O, δ 30 Si (δ 13 C diatom , δ 18 O diatom , δ 30 Si diatom ), are presented alongside existing geochemical and isotope records to document changes in photic zone conditions, including nutrient supply and the efficiency of the soft-tissue biological pump, between Marine Isotope Stage (MIS) 4 and MIS 5e. Peaks in opal productivity in MIS 5b/c and MIS 5e are both associated with the breakdown of the regional halocline stratification and increased nutrient supply to the photic zone. Whereas the MIS 5e peak is associated with low rates of nutrient utilisation, the MIS 5b/c peak is associated with significantly higher rates of nutrient utilisation. Both peaks, together with other smaller increases in productivity in MIS 4 and 5a culminate with a~significant increase in freshwater input which strengthens/re-establishes the halocline and limits further upwelling of sub-surface waters to the photic zone. Whilst δ 30 Si diatom and previously published records of diatom δ 15 N (δ 15 N diatom ) (Brunelle et al., 2007, 2010) show similar trends until the latter half of MIS 5a, the records become anti-correlated after this juncture and into MIS 4, suggesting a possible change in photic zone state such as may occur with a shift to iron or silicon limitation.

Description: Oxygen isotopic analyses of individual planktic foraminifera species: implications for seasonality in the western Arabian Sea Climate of the Past Discussions, 10, 3661-3688, 2014 Author(s): P. D. Naidu, N. Niitsuma, and S. Naik The variation of stable isotopes between individual shells of planktic foraminifera of a given species and size may provide short-term seasonal insight on Paleoceanography. In this context, oxygen isotope analyses of individual Globigerinoides sacculifer and Neogloboquadrina dutertrei were carried out from the Ocean Drilling Program Site 723A in the western Arabian Sea to unravel the seasonal changes for the last 22 kyr. δ 18 O values of single shells of G. sacculifer range from of 0.54 to 2.09‰ at various depths in the core which cover a time span of the last 22 kyr. Maximum inter-shell δ 18 O variability and high standard deviation is noticed from 20 to 10 kyr, whereas from 10 kyr onwards the inter shell δ 18 O variability decreased. The individual contribution of sea surface temperature (SST) and sea surface salinity (SSS) on the inter shell δ 18 O values of G. sacculifer were quantified. Maximum seasonal SST between 20 and 14 ka was caused due to weak summer monsoon upwelling and strong cold winter arid continental winds. Maximum SSS differences between 18 and 10 ka is attributed to the increase of net evaporation minus precipitation due to the shift of ITCZ further south. Overall, winter dominated SST signal in Greenland would be responsible to make a teleconnection between Indian monsoon and Greenland temperature. Thus the present study has wider implications in understanding wether the forcing mechanisms of tropical monsoon climate lies in high latitudes or in the tropics.

Description: Interannual climate variability seen in the Pliocene Model Intercomparison Project Climate of the Past Discussions, 10, 3787-3820, 2014 Author(s): C. M. Brierley Following proxy observations of weakened temperature gradients along the Equator in the early Pliocene, there has been much speculation about Pliocene climate variability. A major advance for our knowledge about the later Pliocene has been the coordination of modelling efforts through the Pliocene Model Intercomparison Project (PlioMIP). Here the changes in interannual modes of sea surface temperature variability will be presented across PlioMIP. Previously model ensembles have shown little consensus in the response of the El Niño–Southern Oscillation (ENSO) to imposed forcings – either for the past or future. The PlioMIP ensemble, however, shows surprising agreement with eight models simulating reduced variability and only one model indicating no change. The Pliocene's robustly weaker ENSO also saw a shift to lower frequencies. Model ensembles focussed at a wide variety of forcing scenarios have not yet shown this level of coherency. Nonetheless the PlioMIP ensemble does not show a robust response of either ENSO flavour or sea surface temperature variability in the Tropical Indian and North Pacific Oceans. Existing suggestions of ENSO properties linked to changes in zonal temperature gradient, seasonal cycle and the elevation of the Andes Mountains are investigated, yet prove insufficient to explain the coherent response. The reason for this surprisingly coherent signal warrants further investigation.

Description: Early Paleogene variations in the calcite compensation depth: new constraints using old boreholes across Ninetyeast Ridge in the Indian Ocean Climate of the Past Discussions, 10, 3163-3221, 2014 Author(s): B. S. Slotnick, V. Lauretano, J. Backman, G. R. Dickens, A. Sluijs, and L. Lourens Major variations in global carbon cycling occurred between 62 and 48 Ma. To better constrain the cause and magnitude of these changes, the community needs early Paleogene carbon isotope and carbonate accumulation records from widely separated deep-sea sediment sections, especially including the Indian Ocean. With the potential for renewed scientific drilling in the Indian Ocean, we examine lithologic, nannofossil assemblage, carbon isotope, and carbonate content records for late Paleocene – early Eocene sediment recovered at three existing sites spanning Ninetyeast Ridge: Deep Sea Drilling Project (DSDP) Sites 213 (deep, east), 214 (shallow, central), and 215 (deep, west). The sediment sections are not ideal, because they were recovered in single holes using rotary coring methods. Site 214 was very shallow during the late Paleocene, when it received significant amounts of neritic carbonate. The δ 13 C records at Sites 213 and 215 are similar to those generated at several locations in the Atlantic and Pacific. The prominent high in δ 13 C across the Paleocene carbon isotope maximum (PCIM) occurs at Site 215, and the prominent low in δ 13 C across the early Eocene Climatic Optimum (EECO) occurs at both Site 213 and Site 215. The Paleocene–Eocene thermal maximum (PETM) and the K/X event are found at Site 213 but not at Site 215, presumably because of coring gaps. Carbonate content at both Sites 213 and 215 drops to 〈 5% shortly after the first occurrence of Discoaster lodoensis and the early Eocene rise in δ 13 C (~ 52 Ma). This reflects a rapid shoaling of the calcite compensation depth (CCD), and likely a major decrease in the net flux of 13 C-depleted carbon to the ocean. Our work further constrains knowledge of the early Paleogene CCD, but more importantly suggests that excellent early Paleogene carbonate accumulation records might be recovered from the central Indian Ocean with future scientific drilling.

Description: Drilling disturbance and constraints on the onset of the Paleocene/Eocene boundary carbon isotope excursion in New Jersey Climate of the Past Discussions, 10, 3303-3325, 2014 Author(s): P. N. Pearson and E. Thomas The onset of the Paleocene/Eocene thermal maximum (PETM) and associated carbon isotope excursion (CIE; about 56 million years ago) was geologically abrupt but it is debated whether it took thousands of years or was effectively instantaneous. A significant new record of the onset of the CIE was published by Wright and Schaller (2013) who claimed that it could be resolved across 13 annual layers in a drill core through the Marlboro Clay at Millville, New Jersey (Ocean Drilling Program Leg 174X). Supporting evidence of similar layering was also reported from another New Jersey drill site, Wilson Lake B, and a photograph of the Marlboro Clay in outcrop. Such a short duration would imply an instantaneous perturbation of the atmosphere and surface ocean, and the impact of a comet or asteroid as the likely cause. However it was suggested by Pearson and Nicholas (2014) from the published photographs that the layers in the Marlboro Clay could be artifacts of drilling disturbance (so-called "biscuiting", wherein the formation is fractured into layers or "biscuits" and drilling mud is injected in between). Here we report new observations on the cores which support that interpretation, including concentric grooves on the surfaces of the biscuits caused by spinning in the bit, micro-fracturing at their edges, and injected drilling mud. We re-interpret the outcrop evidence as showing joints rather than sedimentary layers. We argue that foraminifer concentrations in the sediments are far too high for the layers to be annually deposited in turbid waters at depths of 40–70 m, indicating that the onset of the CIE in the Marlboro Clay likely took on the order of millennia, not years. Re-coring of Millville to minimize drilling disturbance and allow a higher resolution study of the carbon isotope excursion is highly desirable.

Description: Millennial meridional dynamics of Indo-Pacific Warm Pool during the last termination Climate of the Past Discussions, 10, 3397-3419, 2014 Author(s): L. Lo, C.-C. Shen, K.-Y. Wei, G. S. Burr, H.-S. Mii, M.-T. Chen, S.-Y. Lee, and M.-C. Tsai To develop an in-depth understanding of the natural dynamics of the Indo-Pacific Warm Pool (IPWP) during the last deglaciation, stacked North- (N-) and South-IPWP (S-IPWP) thermal and hydrological records over the past 23–10.5 thousand years (ka) were built using planktonic foraminiferal geochemical data from a new core, MD05-2925 (9.3° S, 151.5° E, water depth 1661 m) in the Solomon Sea and eleven previous sites. Ice-volume corrected seawater δ 18 O (δ 18 O SW-IVC ) stacks show that S-IPWP δ 18 O SW-IVC values are indistinguishable from their northern counterpart through glacial time. The N-IPWP SST stacked record features an increasing trend of 0.5 °C ka −1 since 18 ka. Its S-IPWP counterpart shows an earlier onset of temperature increase at 19 ka and a strong teleconnection to high-latitude climate in the Southern Hemisphere. Meridional SST gradients between N- and S-IPWP were 1.5 °C during the Bølling/Allerød period and 〈 0.5 °C during both Heinrich event 1 and the Younger Dryas due to a warmer S-IPWP. A warm S-IPWP during the cold events snapshots may possibly weaken the southern hemispheric convection branch of the Hadley Cell and reduce precipitation in the Asian Monsoon region.

Description: On-line and off-line data assimilation of palaeoclimate proxy data into a GCM using ensemble member selection Climate of the Past Discussions, 10, 3449-3482, 2014 Author(s): A. Matsikaris, M. Widmann, and J. Jungclaus Different ensemble-based data assimilation (DA) approaches for palaeoclimate reconstructions have been recently followed, but no systematic comparison among them has been attempted. We compare an off-line and an on-line ensemble-based method, with the testing period being the 17th century, which led into the Maunder Minimum. We use a low-resolution version of Max Planck Institute for Meteorology's model MPI-ESM, to assimilate the PAGES 2K continental temperature reconstructions. In the off-line approach the ensemble for the entire simulation period is generated first and then the ensemble is used in combination with the empirical information to produce the analysis. In contrast, in the on-line approach the ensembles are generated sequentially for sub-periods based on the analysis of previous sub-periods. Both schemes perform better than the simulations without DA. The on-line method would be expected to perform better if the assimilation led to states of the slow components of the climate system that are close to reality and the system had sufficient memory to propagate this information forward in time. In our comparison, which is based on analysing correlations and differences between the analysis and the proxy-based reconstructions, we find similar skill for both methods on continental and the hemispheric scales. This indicates either a lack of control of the slow components in our setup or a lack of information propagation on decadal timescales. Although the skill is similar and the on-line method is more difficult to implement, the temporal consistency of the analysis in on-line method makes it in general preferable.

Description: Using beryllium-10 to test the validity of past accumulation rate reconstruction from water isotope records in East Antarctic ice cores Climate of the Past Discussions, 10, 3421-3447, 2014 Author(s): A. Cauquoin, A. Landais, G. M. Raisbeck, J. Jouzel, L. Bazin, M. Kageyama, J.-Y. Peterschmitt, M. Werner, E. Bard, and ASTER Team Ice cores are exceptional archives which allow us to reconstruct a wealth of climatic parameters as well as past atmospheric composition over the last 800 ka in Antarctica. Inferring the variations of past accumulation rate in polar regions is essential both for documenting past climate and for ice core chronology. On the East Antarctic plateau, the accumulation rate is so small that annual layers cannot be identified and accumulation rate is mainly deduced from the water isotopic composition assuming constant temporal relationships between temperature, water isotopic composition and accumulation rate. Such assumption leads to large uncertainties on the reconstructed past accumulation rate. Here, we use high resolution beryllium-10 ( 10 Be) as an alternative tool for inferring past accumulation rate for the EPICA Dome C ice core, in East Antarctica. We present a high resolution 10 Be record covering a full climatic cycle over the period 269 to 355 kyr BP from MIS 9 to MIS 10 (Marine Isotope Stages). After correcting 10 Be for the estimated effect of the paleomagnetic field, we deduce that the classical estimation of accumulation rate variations from records of water isotopes agrees, with a possible underestimation of 16%, with the uncertainty on the temperature reconstruction from water isotopes in Antarctic ice cores. This is within their uncertainty of −10 to +30%. Finally, we show that the relationship between temperature and accumulation rate is comparable when using ice core data and results from several AGCM simulations run on glacial–interglacial conditions despite a larger spread in model outputs. These results indicate that the thermodynamic law linking moisture content in the air and temperature, as implemented in the different models, leads to realistic results even in polar regions, at the end of the water distillation trajectory.

Description: A GCM comparison of Plio–Pleistocene interglacial–glacial periods in relation to Lake El'gygytgyn, NE Arctic Russia Climate of the Past Discussions, 10, 3127-3161, 2014 Author(s): A. J. Coletti, R. M. DeConto, J. Brigham-Grette, and M. Melles Until now, the lack of time-continuous, terrestrial paleoenvironmental data from the Pleistocene Arctic has made model simulations of past interglacials difficult to assess. Here, we compare climate simulations of four warm interglacials at Marine Isotope Stage (MIS) 1 (9 ka), 5e (127 ka), 11c (409 ka), and 31 (1072 ka) with new proxy climate data recovered from Lake El'gygytgyn, NE Russia. Climate reconstructions of the Mean Temperature of the Warmest Month (MTWM) indicate conditions 2.1, 0.5 and 3.1 °C warmer than today during MIS 5e, 11c, and 31, respectively. While the climate model captures much of the observed warming during each interglacial, largely in response to boreal summer orbital forcing, the extraordinary warmth of MIS 11c relative to the other interglacials in the proxy records remain difficult to explain. To deconvolve the contribution of multiple influences on interglacial warming at Lake El'gygytgyn, we isolated the influence of vegetation, sea ice, and circum-Arctic land ice feedbacks on the climate of the Beringian interior. Simulations accounting for climate-vegetation-land surface feedbacks during all four interglacials show expanding boreal forest cover with increasing summer insolation intensity. A deglaciated Greenland is shown to have a minimal effect on Northeast Asian temperature during the warmth of stage 11c and 31 (Melles et al., 2012). A prescribed enhancement of oceanic heat transport into the Arctic ocean has some effect on Beringian climate, suggesting intrahemispheric coupling seen in comparisons between Lake El'gygytgyn and Antarctic sediment records might be related to linkages between Antarctic ice volume and ocean circulation. The exceptional warmth of MIS 11c remains enigmatic however, relative to the modest orbital and greenhouse gas forcing during that interglacial. Large Northern Hemisphere ice sheets during Plio-Pleistocene glaciation causes a substantial decrease in Mean Temperature of the Coldest Month (MTCM) and Mean Annual Precipitation (PANN) causing significant Arctic aridification. Aridification and cooling can be linked to a combination of mechanical forcing from the Laurentide and Fennoscandian ice sheets on mid-tropospheric westerly flow and expanded sea ice cover causing albedo-enhanced feedback.

Description: Using simulations of the last millennium to understand climate variability seen in paleo-observations: similar variation of Iceland-Scotland overflow strength and Atlantic Multidecadal Oscillation Climate of the Past Discussions, 10, 3255-3302, 2014 Author(s): K. Lohmann, J. Mignot, H. R. Langehaug, J. H. Jungclaus, D. Matei, O. H. Otterå, Y. Gao, T. L. Mjell, U. Ninnemann, and H. F. Kleiven A recent paleo-reconstruction of the strength of the Iceland-Scotland overflow during the last 600 years suggests that its low-frequency variability exhibits strong similarity with paleo-reconstructions of the Atlantic Multidecadal Oscillation (AMO). The underlying mechanism of the apparent covarying remains, however, unclear based on paleo-reconstructions alone. In this study we use simulations of the last millennium driven by external forcing reconstructions with three coupled climate models in order to investigate possible mechanisms underlying the apparent covarying. Two of the model simulations show a clear in-phase variation of Iceland-Scotland overflow strength and AMO index. Our analysis indicates that the basinwide AMO index in the externally forced simulations is dominated by the low-latitude SST variability and is not predominantly driven by variations in the strength of the Atlantic meridional overturning circulation (MOC). In the simulations, also a strong (weak) Iceland-Scotland overflow does generally not lead a strong (weak) MOC, suggesting that a large-scale link through the strength of the MOC is not sufficient to explain the (simulated) in-phase variation of Iceland-Scotland overflow strength and AMO index. Rather, a more local link through the influence of the Nordic Seas SST, which is positively correlated with the AMO index, on the Iceland-Scotland overflow strength is responsible for the (simulated) in-phase variation. The Nordic Seas surface state affects, via convective activity, the density structure and the sea surface height (SSH), and consequently the pressure north of the Iceland-Scotland-Ridge. In the model simulation showing a less clear in-phase variation of Iceland-Scotland overflow strength and AMO index, also the wind stress influences the Nordic Seas SSH anomalies associated with the anomalous overflow strength. The details of the mechanisms differ between the three models, underlining the importance of multi-model analysis. Our study demonstrates that paleo-climate simulations provide a useful tool to understand mechanisms and large-scale connections associated with the relatively sparse paleo-observations.

Description: Thenardite after mirabilite deposits as a cool climate indicator in the geological record: lower Miocene of Central Spain Climate of the Past Discussions, 10, 3223-3253, 2014 Author(s): M. J. Herrero, J. I. Escavy, and B. C. Schreiber Evaporites are commonly used as indicators of different paleoclimates and sedimentary environments, as well as being geological resources of great economic interest. Ordinarily the presence of evaporites is related to warm and arid environmental conditions, but there are evaporitic minerals, like mirabilite, that form by cooling and a concentration mechanism based on cooling and/or freezing. The diagenetic transformation of mirabilite into thenardite in the upper part of the lower Miocene unit of the Tajo basin (Spain) resulted in the largest reserves of this important industrial mineral in Europe. This unit was formed in a time period (~ 18.4 Ma) that, in other basins of the Iberian Peninsula, is characterized by the existence of particular mammal assemblages appropriate to a relatively cool and arid climate. Determining the origin of the thenardite deposits as related to the diagenetic alteration of a pre-existing mirabilite permits the establishment and characterization of the sedimentary environment where it was formed and also suggests use as a possible analogue with comparable deposits from extreme conditions such as Antarctica or Mars.

Description: Variability of summer humidity during the past 800 years on the eastern Tibetan Plateau inferred from δ 18 O of tree-ring cellulose Climate of the Past Discussions, 10, 3327-3356, 2014 Author(s): J. Wernicke, J. Grießinger, P. Hochreuther, and A. Bräuning We present an 800 years long δ 18 O chronology from the eastern part of the Tibetan Plateau (TP). The chronology dates back to 1193 AD and was sampled in 1996 AD from living Juniperus tibetica trees. The chronology is unique for eastern Tibet and provides a reliable archive for hydroclimatic reconstructions. Highly significant correlations were obtained with air moisture (relative humidity, vapour pressure and precipitation) during the summer season. We applied a linear transfer model to reconstruct the summer season relative humidity variation over the past 800 years. We identified more moist conditions at the termination of the Medieval Warm Period, an oscillating air humidity around the mean during the Little Ice Age and a sudden decrease of relative humidity since the 1870s. The late 19th century humidity decrease is in good accordance with several multiproxy hydroclimate reconstructions for south Tibet. On the other hand, since the end of the 19th century strong evidences for an increase in humidity on the northern TP is exhibited. Spatial correlation analysis with the North Atlantic Oscillation index (NAO) and the sea surface temperature (SST) of Niño region 3.4 reveal a weak and nonstationary relationship to the δ 18 O chronology. Instead, spatial correlations expose a dominating convective influence to the relative humidity reconstruction. Furthermore, wavelength analysis reveal good agreements between the significant cyclicities in our δ 18 O chronology and several moisture sensitive proxy archives.

Description: Nutrient utilisation and weathering inputs in the Peruvian upwelling region since the Little Ice Age Climate of the Past Discussions, 10, 3357-3396, 2014 Author(s): C. Ehlert, P. Grasse, D. Gutiérrez, R. Salvatteci, and M. Frank For this study two sediment cores from the Peruvian shelf covering the time period between the Little Ice Age (LIA) and present were examined for changes in productivity (biogenic opal concentrations (bSi)), nutrient utilisation (stable isotope compositions of silicon (δ 30 Si opal ) and nitrogen (δ 15 N sed )), as well as in ocean circulation and material transport (authigenic and detrital radiogenic neodymium (ε Nd ) and strontium ( 87 Sr/ 86 Sr) isotopes). For the LIA the proxies recorded weak primary productivity and nutrient utilisation reflected by low average bSi concentrations of ~10%, δ 15 N sed values of ~ +5‰ and intermediate δ 30 Si opal values of ~+0.97‰. At the same time the radiogenic isotope composition of the detrital sediment fraction indicates dominant local riverine input of lithogenic material due to higher rainfall in the Andean hinterland. These patterns were caused by permanent El Niño-like conditions characterized by a deeper nutricline, weak upwelling and low nutrient supply. At the end of the LIA, δ 30 Si opal dropped to low values of +0.6‰ and opal productivity reached its minimum of the past 650 years. During the following transitional period of time the intensity of upwelling, nutrient supply and productivity increased abruptly as marked by the highest bSi contents of up to 38%, by δ 15 N sed of up to ~ +7‰, and by the highest degree of silicate utilisation with δ 30 Si opal reaching values of +1.1‰. At the same time detrital ε Nd and 87 Sr/ 86 Sr signatures documented increased wind strength and supply of dust to the shelf due to drier conditions. Since about 1870, productivity has been high but nutrient utilisation has remained at levels similar to the LIA indicating significantly increased nutrient availability. Comparison between the δ 30 Si opal and δ 15 N sed signatures suggests that during the past 650 years the δ 15 N sed signature in the Peruvian Upwelling area has most likely primarily been controlled by surface water utilisation and not, as previously assumed, by subsurface nitrogen loss processes in the water column.

Description: Late Weichselian and Holocene paleoceanography of Storfjordrenna, southern Svalbard Climate of the Past Discussions, 10, 3053-3095, 2014 Author(s): M. Łącka, M. Zajączkowski, M. Forwick, and W. Szczuciński Multiproxy analyses (incl. benthic and planktonic foraminifera, δ 18 O and δ 13 C records, grain-size distribution, ice-rafted debris, XRF geochemistry and magnetic susceptibility) were performed on a 14 C dated marine sediment core from Storfjordrenna, off southern Svalbard. The sediments in the core cover the termination of Bølling–Allerød, the Younger Dryas and the Holocene, and they reflect general changes in the hydrology/climate of the European Arctic after the last glaciation. Grounded ice of the last Svalbard- Barents Sea Ice Sheet retreated from the coring site ca. 13 850 cal yr BP. During the transition from the sub-glacial to glacimarine setting, Arctic Waters dominated the hydrography in Storfjordrenna. However, the waters were not uniformly cold and experienced several warmer spells. A progressive warming and marked change in the nature of hydrology occurred during the early Holocene. Relatively warm and saline Atlantic Water started to dominate the hydrography from approx. 9500 cal yr BP. Even though the climate in eastern Svalbard was milder at that time than at present (smaller glaciers), there were two slight coolings observed in the periods of 9000–8000 cal yr BP and 6000–5500 cal yr BP. A change of the Storfjordrenna hydrology occurred at the beginning of late Holocene synchronously with glacier growth on land and enhanced bottom current velocities. Although cooling was observed in the surface water, Atlantic Water remained present in the deeper part of water column of Storfjordrenna.

Description: Laminated sediments in the Bering Sea reveal atmospheric teleconnections to Greenland climate on millennial to decadal timescales during the last deglaciation Climate of the Past Discussions, 10, 2467-2518, 2014 Author(s): H. Kuehn, L. Lembke-Jene, R. Gersonde, O. Esper, F. Lamy, H. Arz, and R. Tiedemann During the last glacial termination, the upper North Pacific Ocean underwent dramatic and rapid changes in oxygenation that lead to the transient intensification of Oxygen Minimum Zones (OMZs), recorded by the widespread occurrence of laminated sediments on circum-Pacific continental margins. We present a new laminated sediment record from the mid-depth (1100 m) northern Bering Sea margin that provides insight into these deglacial OMZ maxima with exceptional, decadal-scale detail. Combined ultrahigh-resolution micro-XRF data and sediment facies analysis of laminae reveals an alternation between predominantly terrigenous and diatom-dominated opal sedimentation. The diatomaceous laminae are interpreted to represent spring/summer productivity events that occur at the retreating sea ice margin. We identified five laminated sections in the deglacial part of our site. Laminae counts were carried out on these sections and correlated to the Bølling–Allerød and Preboreal phases in North Greenland Ice Core (NGRIP) oxygen isotope record, indicating an annual deposition of individual laminae couplets. The observed rapid intra-decadal intensifications of anoxia, in particular within the Bølling–Allerød, are tightly coupled to short-term warm events through increases in regional biogenic productivity. By correlating the counted laminated sections with Bering Sea Surface Temperature records (SST) and NGRIP δ 18 O data, we propose a deglacial minimum SST of 6–7 °C for the preservation of laminae, which we call the deglacial temperature threshold for anoxia occurrence, a process that strongly implies a close atmospheric teleconnection between the North Pacific and North Atlantic regions. We suggest that concomitant increases in Bering Sea biogenic productivity, in combination with oxygen-poor waters entering the Being Sea, drove down oxygen concentrations to values below 0.1 mL L -1 and caused laminae preservation. Calculated benthic-planktic ventilation ages show no significant variations throughout the last deglaciation, indicating that changes in formation rates or differing sources of North Pacific mid-depth waters are not prime candidates for strengthening the OMZ at our site. The age models established by our correlation procedure allow to determine calendar age control points for the Bølling–Allerød and the Preboreal that are independent of the initial radiocarbon-based chronology. Resulting calculated reservoir ages are 875 yr during the Bølling–Allerød, and 910–770 yr for the Younger Dryas and the Preboreal, respectively.

Description: Impact of the Last Glacial Cycle on Late-Holocene temperature and energy reconstructions from terrestrial borehole temperatures in North America Climate of the Past Discussions, 10, 2355-2396, 2014 Author(s): H. Beltrami, G. S. Matharoo, L. Tarasov, V. Rath, and J. E. Smerdon Reconstructions of past climatic changes from borehole temperature profiles are important independent estimates of temperature histories over the last millennium. There remain, however, multiple uncertainties in the interpretation of these data as climatic indicators and as estimates of the changes in heat content of the continental subsurface due to long-term climatic change. One of these uncertainties is associated with the often ignored impact of the last glacial cycle on the subsurface energy content, and on the estimate of the background quasi steady-state signal associated with the diffusion of accretionary energy from the Earth's interior. Here we provide the first estimate of the impact of the development of the Laurentide ice sheet on the estimates of energy and temperature reconstructions from measurements of terrestrial borehole temperatures in North America. We use basal temperature values from the data-calibrated Memorial University of Newfoundland Glacial Systems Model to quantify the extent of the perturbation to estimated steady-state temperature profiles and to derive spatial maps of the expected impacts on measured profiles over North America. Furthermore, we present quantitative estimates of the potential effects of temperature changes during the last glacial cycle on the borehole reconstructions over the last millennium for North America. The range of these possible impacts are estimated using synthetic basal temperatures for a period covering 120 ka to the present day that include the basal temperature history uncertainties from an ensemble of results from the calibrated numerical model. For all the locations, we find that within the depth ranges that are typical for available boreholes (≈600 m), the induced perturbations to the steady-state temperature profile are on the order of 10 mW m −2 , decreasing with greater depths. Results indicate that site-specific heat content estimates over North America can differ by as much as 50%, if the energy contribution of the last glacial cycle in those areas of North America that experienced glaciation is not taken into account when estimating recent subsurface energy change from borehole temperature data.

Description: Links between CO 2 , glaciation and water flow: reconciling the Cenozoic history of the Antarctic Circumpolar Current Climate of the Past Discussions, 10, 2397-2416, 2014 Author(s): J.-B. Ladant, Y. Donnadieu, and C. Dumas The timing of the onset of the Antarctic Circumpolar Current (ACC) is a crucial event of the Cenozoic because of its cooling and isolating effect over Antarctica. It is intimately related to the glaciations occurring throughout the Cenozoic from the Eocene–Oligocene (EO) transition (≈34 Ma) to the middle Miocene glaciations (≈13.9 Ma). However, the exact timing of the onset remains debated with evidence for a late Eocene set up contradicting others data pointing to an occurrence closer to the Oligocene–Miocene (OM) boundary. In this study, we show the potential impact of the Antarctic ice sheet on the initiation of a proto-ACC at the EO boundary. Our results reveal that the regional cooling effect of the ice sheet increases the sea ice formation, which disrupts the meridional density gradient in the Southern Ocean and leads to the onset of a circumpolar current and its progressive strengthening. We also suggest that subsequent variations in atmospheric CO 2 , ice sheet volumes and tectonic reorganizations may have affected the ACC intensity after the Eocene–Oligocene transition, which in turn may provide an explanation for the second initiation of the ACC at the Oligocene–Miocene boundary and may reconcile evidence supporting both early Oligocene and early Miocene onset of the ACC.

Description: The Global Monsoon across Time Scales: is there coherent variability of regional monsoons? Climate of the Past Discussions, 10, 2163-2291, 2014 Author(s): P. X. Wang, B. Wang, H. Cheng, J. Fasullo, Z. T. Guo, T. Kiefer, and Z. Y. Liu Monsoon has earned increasing attention from the climate community since the last century, yet only recently regional monsoons have been recognized as a global system. It remains a debated issue, however, as to what extent and at which time scales the global monsoon can be viewed as a major mode of climate variability. For this purpose a PAGES Working Group (WG) was set up to investigate the concept of the global monsoon and its future research directions. The WG's synthesis is presented here. On the basis of observation and proxy data, the WG found that the regional monsoons can vary coherently, although not perfectly, at various time scales, ranging from interannual, interdecadal, centennial and millennial, up to orbital and tectonics time scales, conforming the global monsoon concept across time scales. Within the global monsoon system each subsystem has its own features depending on its geographic and topographic conditions. Discrimination of global and regional components in the monsoon system is a key to reveal the driving factors of monsoon variations, hence the global monsoon concept helps to enhance our understanding and to improve future projection of the regional monsoons. This paper starts with a historical review of the global monsoon concept in both modern and paleo-climatology, and an assessment of monsoon proxies used in regional and global scales. The main body of the paper is devoted to a summary of observation data at various time scales, providing evidence for the coherent global monsoon system. The paper concludes with a projection of future monsoon shifts into a warming world. The synthesis will be followed by a companying paper to discuss driving mechanisms and outstanding issues in the global monsoon studies.

Description: Pliocene diatom and sponge spicule oxygen isotope ratios from the Bering Sea: isotopic offsets and future directions Climate of the Past Discussions, 10, 2087-2104, 2014 Author(s): A. M. Snelling, G. E. A. Swann, J. Pike, and M. J. Leng Oxygen isotope analyses of different size fractions of Pliocene diatoms (δ 18 O diatom ) from the Bering Sea show no evidence of an isotope offset and support the use of bulk diatom species samples for palaeoceanographic reconstructions. Additional samples containing concentrations of sponge spicules produce δ 18 O values several per mille lower than δ 18 O diatom with a calculated mean offset of 3.6‰ ± 0.7. This difference is significantly greater than modern day variations in water δ 18 O through the regional water column. Despite the potential for oxygen isotope disequilibrium within δ 18 O sponge , there appears to be some similarity between δ 18 O sponge and a global stacked benthic δ 18 O foram record. This highlights the potential for δ 18 O sponge in palaeoenvironmental research at sites where carbonates are not readily preserved.

Description: North African vegetation-precipitation feedback in early and mid-Holocene climate simulations with CCSM3-DGVM Climate of the Past Discussions, 10, 2055-2086, 2014 Author(s): R. Rachmayani, M. Prange, and M. Schulz The present study analyses the sign, strength and working mechanism of the vegetation-precipitation feedback over North Africa in middle (6 ka BP) and early Holocene (9 ka BP) simulations using the comprehensive coupled climate-vegetation model CCSM3-DGVM. The coupled model simulates enhanced summer rainfall and a northward migration of the West African monsoon trough along with an expansion of the vegetation cover for the early and middle Holocene compared to pre-industrial. It is shown that dynamic vegetation enhances the orbitally triggered summer precipitation anomaly by approximately 20% in the Sahara/Sahel region (10° N–25° N, 20° W–30° E) in both the early and mid-Holocene experiments compared to their fixed-vegetation counterparts. The primary vegetation-rainfall feedback identified here operates through surface latent heat flux anomalies by canopy evaporation and transpiration and their effect on the mid-tropospheric African Easterly Jet, whereas the effects of vegetation changes on surface albedo and local water recycling play a negligible role. Even though CCSM3-DGVM simulates a positive vegetation-precipitation feedback in the North African region, this feedback is not strong enough to produce multiple equilibrium climate-ecosystem states on a regional scale.

Description: Radiative forcings for 28 potential Archean greenhouse gases Climate of the Past Discussions, 10, 2011-2053, 2014 Author(s): B. Byrne and C. Goldblatt Despite reduced insolation in the late Archean, evidence suggests a warm climate which was likely sustained by a stronger greenhouse effect, the so-called Faint Young Sun Problem (FYSP). CO 2 and CH 4 are generally thought to be the mainstays of this enhanced greenhouse, though many other gases have been proposed. We present high accuracy radiative forcings for CO 2 , CH 4 and 26 other gases, performing the radiative transfer calculations at line-by-line resolution and using HITRAN 2012 line data for background pressures of 0.5, 1, and 2 bar. For CO 2 to resolve the FYSP alone, 0.21 bar is needed with 0.5 bar of atmospheric pressure, 0.13 bar with 1 bar of atmospheric pressures, or 0.07 bar with 2 bar of atmospheric pressure. For CH 4 , we find that near-infrared absorption is much stronger than previously thought, arising from updates to the HITRAN database. CH 4 radiative forcing peaks at 10.3, 9, or 8.3 W m −2 for background pressures of 0.5, 1 or 2 bar, likely limiting the utility of CH 4 for warming the Archean. For the other 26 HITRAN gases, radiative forcings of up to a few to 10 W m −2 are obtained from concentrations of 0.1–1 ppmv for many gases. We further calculate the reduction of radiative forcing due to gas overlap for the 20 strongest gases. We recommend the forcings provided here be used both as a first reference for which gases are likely good greenhouse gases, and as a standard set of calculations for validation of radiative forcing calculations for the Archean.

Description: The evolution of sub-monsoon systems in the Afro-Asian monsoon region during the Holocene – comparison of different transient climate model simulations Climate of the Past Discussions, 10, 2293-2353, 2014 Author(s): A. Dallmeyer, M. Claussen, N. Fischer, K. Haberkorn, S. Wagner, M. Pfeiffer, L. Jin, V. Khon, Y. Wang, and U. Herzschuh The recently proposed global monsoon hypothesis interprets monsoon systems as part of one global-scale atmospheric overturning circulation, implying a connection between the regional monsoon systems and an in-phase behaviour of all northern hemispheric monsoons on annual timescales (Trenberth et al., 2000). Whether this concept can be applied to past climates and variability on longer timescales is still under debate, because the monsoon systems exhibit different regional characteristics such as different seasonality (i.e. onset, peak, and withdrawal). To investigate the interconnection of different monsoon systems during the pre-industrial Holocene, five transient global climate model simulations have been analysed with respect to the rainfall trend and variability in different sub-domains of the Afro-Asian monsoon region. Our analysis suggests that on millennial timescales with varying orbital forcing, the monsoons do not behave as a tightly connected global system. According to the models, the Indian and North African monsoons are coupled, showing similar rainfall trend and moderate correlation in rainfall variability in all models. The East Asian monsoon changes independently during the Holocene. The dissimilarities in the seasonality of the monsoon sub-systems lead to a stronger response of the North African and Indian monsoon systems to the Holocene insolation forcing than of the East Asian monsoon and affect the seasonal distribution of Holocene rainfall variations. Within the Indian and North African monsoon domain, precipitation solely changes during the summer months, showing a decreasing Holocene precipitation trend. In the East Asian monsoon region, the precipitation signal is determined by an increasing precipitation trend during spring and a decreasing precipitation change during summer, partly balancing each other. A synthesis of reconstructions and the model results do not reveal an impact of the different seasonality on the timing of the Holocene rainfall optimum in the different sub-monsoon systems. They rather indicate locally inhomogeneous rainfall changes and show, that single palaeo-records should not be used to characterise the rainfall change and monsoon evolution for entire monsoon sub-systems.

Description: Investigating uncertainties in global gridded datasets of climate extremes Climate of the Past Discussions, 10, 2105-2161, 2014 Author(s): R. J. H. Dunn, M. G. Donat, and L. V. Alexander We assess the effects of different methodological choices made during the construction of gridded datasets of climate extremes, focusing primarily on HadEX2. Using global timeseries of the indices and their coverage, as well as uncertainty maps, we show that the choices which have the greatest effect are those relating to the station network used or which drastically change the values for individual grid boxes. The latter are most affected by the number of stations required in or around a grid box and the gridding method used. Most parametric changes have a small impact, on global and on grid box scales, whereas structural changes to the methods or input station networks may have large effects. On grid box scales, trends in temperature indices are very robust to most choices, especially in areas which have high station density (e.g. North America, Europe and Asia). Precipitation trends, being less spatially coherent, can be more susceptible to methodological changes, but are still clear in regions of high station density. Regional trends from all indices derived from areas with few stations should be treated with care. On a global scale, the linear trends over 1951–2010 from almost all choices fall within the statistical range of trends from HadEX2. This demonstrates the robust nature of HadEX2 and related datasets to choices in the creation method.

Description: Tree-ring reconstruction of seasonal mean minimum temperature at Mt. Yaoshan, China, since 1873 and its relevance to 20th-century warming Climate of the Past Discussions, 10, 859-894, 2014 Author(s): Y. Liu, Y. Zhang, H. Song, Y. Ma, Q. Cai, and Y. Wang It is very important to comprehend the climate variations in the vast regions of Central Plains of China. Current knowledge about climate changes of the past few hundred years in this region is primarily based on historical documents, and lack of evidences from the natural archives. However, these documents had somewhat artificially effects caused by the recorders, and not sufficient to fully understand natural climatic changes. In this paper, based on a significant correlation between the tree-ring width of Chinese Pine and observed instrumental data in the Mt. Yaoshan, China, we formulated a transfer function to reconstruct the mean minimum temperature (MMinT) from the previous December to the current June ( T min_DJ ) for the period 1873–2011. The reconstruction explained 39.8% of the instrumental variance during the calibration period of 1958–2011. High T min_DJ intervals with values greater than the 139 year average occurred in 1932–1965 and 1976–2006. The intervals 1878–1894 and 1906–1931 experienced a T min_DJ lower than the 139 year average. The ten highest T min_DJ years occurred after the 1950s, especially after 1996. A distinct upward trend in the T min_DJ series beginning in the 1910s was apparent, and the highest value occurred around 2000. The 20th-century warming signal was captured well by the Yaoshan T min_DJ temperature reconstruction, indicating that the temperature rise in the sensitive Central Plains of China region reflected the global temperature change. The T min_DJ reconstruction also matched several other temperature series in China with similar warm-cold patterns. The distinct spatial correlation between both observed and reconstructed series and CRU TS3.10 grid data indicates that our results may represent T min_DJ changes on a larger scale. The spatial correlation with sea surface temperature (SST) indicated that observed and reconstructed T min_DJ temperatures in the Mt. Yaoshan are closely linked to the West Pacific, Indian and North Atlantic Oceans as well as El Niño-Southern Oscillation (ENSO).