ALBERT

Description: Similarity estimators for irregular and age uncertain time series Climate of the Past Discussions, 9, 5299-5346, 2013 Author(s): K. Rehfeld and J. Kurths Paleoclimate time series are often irregularly sampled and age uncertain, which is an important technical challenge to overcome for successful reconstruction of past climate variability and dynamics. Visual comparison and interpolation-based linear correlation approaches have been used to infer dependencies from such proxy time series. While the first is subjective, not measurable and not suitable for the comparison of many datasets at a time, the latter introduces interpolation bias, and both face difficulties if the underlying dependencies are nonlinear. In this paper we investigate similarity estimators that could be suitable for the quantitative investigation of dependencies in irregular and age uncertain time series. We compare the Gaussian-kernel based cross correlation (gXCF, Rehfeld et al., 2011) and mutual information (gMI, Rehfeld et al., 2013) against their interpolation-based counterparts and the new event synchronization function (ESF). We test the efficiency of the methods in estimating coupling strength and coupling lag numerically, using ensembles of synthetic stalagmites with short, autocorrelated, linear and nonlinearly coupled proxy time series, and in the application to real stalagmite time series. In the linear test case coupling strength increases are identified consistently for all estimators, while in the nonlinear test case the correlation-based approaches fail. The lag at which the time series are coupled is identified correctly as the maximum of the similarity functions in around 60–55% (in the linear case) to 53–42% (for the nonlinear processes) of the cases when the dating of the synthetic stalagmite is perfectly precise. If the age uncertainty increases beyond 5% of the time series length, however, the true coupling lag is not identified more often than the others for which the similarity function was estimated. Age uncertainty contributes up to half of the uncertainty in the similarity estimation process. Time series irregularity contributes less, particularly for the adapted Gaussian-kernel based estimators and the event synchronization function. The introduced link strength concept summarizes the hypothesis test results and balances the individual strengths of the estimators: while gXCF is particularly suitable for short and irregular time series, gMI and the ESF can identify nonlinear dependencies. ESF could, in particular, be suitable to study extreme event dynamics in paleoclimate records. Programs to analyze paleoclimatic time series for significant dependencies are included in a freely available software toolbox.

Description: Impact of precession on the climate, vegetation and fire activity in southern Africa during MIS4 Climate of the Past Discussions, 9, 5391-5438, 2013 Author(s): M.-N. Woillez, G. Levavasseur, A.-L. Daniau, M. Kageyama, D. H. Urrego, and M.-F. Sánchez-Goñi The relationships between climate, vegetation and fires are a major subject of investigation in the context of climate change. In southern Africa, fire is known to play a crucial role in the existence of grasslands and Mediterranean-like biomes. Microcharcoal-based reconstructions of past fire activity in that region have shown a tight correlation between grass-fueled fires and the precessional cycle, with maximum fire activity during maxima of the climatic precession index. These changes have been interpreted as the result of changes in fuel load in response to precipitation changes in eastern southern Africa. Here we use the general circulation model IPSL_CM5A and the dynamical vegetation model LPJ-LMfire to investigate the response of climate, vegetation and fire activity to precession changes in southern Africa during Marine Isotopic Stage 4. We perform two climatic simulations, for a maximum and minimum of the precession index, and use a statistical downscaling method to increase the spatial resolution of the IPSL_CM5A outputs over southern Africa and perform high-resolution simulations of the vegetation and fire activity. Our results show an anti-correlation between the North and South African monsoons in response to precession changes. A decrease of the precession climatic index leads to a precipitation decrease in the summer rainfall area of southern Africa. The drying of climate leads to a decrease of vegetation cover and fire activity. Our results are in qualitative agreement with data and confirm that fire activity in southern Africa is strongly dependent on the vegetation type.

Description: Ensemble meteorological reconstruction using circulation analogues of 1781–1785 Climate of the Past Discussions, 9, 5157-5182, 2013 Author(s): P. Yiou, M. Boichu, R. Vautard, M. Vrac, S. Jourdain, E. Garnier, F. Fluteau, and L. Menut This paper uses a method of atmospheric flow analogues to reconstruct an ensemble of atmospheric variables (namely sea-level pressure, surface temperature and wind speed) between 1781 and 1785. The properties of this ensemble are investigated and tested against observations of temperature. The goal of the paper is to assess whether the atmospheric circulation during the Laki volcanic eruption (in 1783) and the subsequent winter were similar to the conditions that prevailed in the winter 2009/2010 and during spring 2010. We find that the three months following the Laki eruption in June 1783 barely have analogues in 2010. The cold winter of 1783/1784 yields circulation analogues in 2009/2010. Our analysis suggests that it is unlikely that the Laki eruption was responsible for the cold winter of 1783/1784, of the relatively short memory of the atmospheric circulation.

Description: Seasonal changes in glacial polynya activity inferred from Weddell Sea varves Climate of the Past Discussions, 9, 5123-5156, 2013 Author(s): D. Sprenk, M. E. Weber, G. Kuhn, V. Wennrich, T. Hartmann, and K. Seelos The Weddell Sea and the associated Filchner-Rønne Ice Shelf constitute key regions for global bottom-water production today. However, little is known about bottom-water production under different climate and ice-sheet conditions. Therefore, we studied core PS1795, which consists primarily of fine-grained siliciclastic varves that were deposited on contourite ridges in the southeastern Weddell Sea during the Last Glacial Maximum (LGM). We conducted high-resolution X-ray fluorescence (XRF) analysis and grain-size measurements with the RADIUS tool (Seelos and Sirocko, 2005) using thin sections to characterize the two seasonal components of the varves at sub-mm resolution to distinguish the seasonal components of the varves. Bright layers contain coarser grains that can mainly be identified as quartz in the medium to coarse silt grain size. They also contain higher amounts of Si, Zr, Ca, and Sr, as well as more ice-rafted debris (IRD). Dark layers, on the other hand, contain finer particles such as mica and clay minerals from the chlorite and illite groups. In addition, chemical elements, Fe, Ti, Rb, and K are elevated as well. Based on these findings as well as on previous analyses on neighbouring cores, we propose a model of glacially enhanced thermohaline convection in front of a grounded ice sheet that is supported by seasonally variable coastal polynya activity. Accordingly, katabatic (i.e. offshore blowing) winds removed sea ice from the ice edge, leading to coastal polynya formation. We suggest that glacial processes were similar to today with stronger katabatic winds and enhanced coastal polynya activity during the winter season. If this is correct, silty layers are likely glacial winter deposits, when brine rejection was increased, leading to enhanced bottom water formation and increased sediment transport. Vice versa, finer-grained clayey layers were then deposited during summer, when coastal polynya activity was likely reduced.

Description: Is blue intensity ready to replace maximum latewood density as a strong temperature proxy? A tree-ring case study on Scots pine from northern Sweden Climate of the Past Discussions, 9, 5227-5261, 2013 Author(s): J. A. Björklund, B. E. Gunnarson, K. Seftigen, J. Esper, and H. W. Linderholm At high latitudes, where low temperatures mainly limit tree-growth, measurements of wood density (e.g. Maximum Latewood Density, MXD) using the X-Ray methodology provide a temperature proxy that is superior to that of TRW. Density measurements are however costly and time consuming and have lead to experimentation with optical flatbed scanners to produce Maximum Blue Intensity (BI max ). BI max is an excellent proxy for density on annual scale but very limited in skill on centennial scale. Discolouration between samples is limiting BI max where specific brightnesses can have different densities. To overcome this, the new un-exploited parameter Δ blue intensity (ΔBI) was constructed by using the brightness in the earlywood (BI EW ) as background, (BI max − BI EW = ΔBI). This parameter was tested on X-Ray material (MXD − earlywood density = ΔMXD) and showed great potential both as a quality control and as a booster of climate signals. Unfortunately since the relationship between grey scale and density is not linear, and between-sample brightness can differ tremendously for similar densities, ΔBI cannot fully match ΔMXD in skill as climate proxy on centennial scale. For ΔBI to stand alone, the range of brightness/density offset must be reduced. Further studies are needed to evaluate this possibility, and solutions might include heavier sample treatment (reflux with chemicals) or image-data treatment (digitally manipulating base-line levels of brightness).

Description: Migrating subtropical front and Agulhas Return Current affect the southwestern Indian Ocean during the late Quaternary Climate of the Past Discussions, 9, 5521-5551, 2013 Author(s): D. K. Naik, R. Saraswat, N. Khare, A. C. Pandey, and R. Nigam The position of sub-tropical front (STF), Agulhas Current (AC) and Agulhas Return Current (ARC) controls the hydrography of southwestern Indian Ocean. Although, equator-ward migration of STF and reduction in Agulhas leakage has been reported during the last glacial period, the fate of ARC during the last glacial–interglacial cycle is not clear. Therefore, in order to understand changes in the position and strength of ARC during the last glacial–interglacial cycle, here we reconstruct hydrographic changes in the southwestern Indian Ocean from temporal variation in planktic foraminiferal abundance, stable isotopic ratio (δ 18 O) and trace metal ratio (Mg / Ca) of planktic foraminifera Globigerina bulloides in a core collected from the Agulhas Retroflection Region (ARR) in the southwestern Indian Ocean. Increased abundance of G. bulloides suggests that the productivity in the southwestern Indian Ocean increased during glacial period which confirms previous reports of high glacial productivity in the Southern Ocean. The increased productivity was likely driven by a combination of equator-ward migration of subtropical front and westerlies. Increase in relative abundance of Neogloboquadrina pachyderma Dextral suggests warming of ARR leading to strong thermocline in the southwestern Indian Ocean during the last glacial period. We suggest that the warming of Agulhas Retroflection Region was driven by strengthened ARC which shifted to the east of its present location, thus bringing warmer and saltier water to the southwestern Indian Ocean. Therefore, it is inferred that over the last glacial–interglacial cycle, the hydrography of southwestern Indian Ocean was driven by an eastward shift of retroflection region as well as migrating subtropical front.

Description: The response of the Peruvian Upwelling Ecosystem to centennial-scale global change during the last two millennia Climate of the Past Discussions, 9, 5479-5519, 2013 Author(s): R. Salvatteci, D. Gutiérrez, D. Field, A. Sifeddine, L. Ortlieb, I. Bouloubassi, M. Boussafir, H. Boucher, and F. Cetin The Tropical Pacific ocean-atmosphere system influences global climate on interannual, decadal, as well as at longer timescales. Given the uncertainties in the response of the Tropical Pacific to the ongoing greenhouse effect, it is important to assess the natural range of the Tropical Pacific climate variability in response to global natural changes, and to understand the underlying mechanisms. The Peruvian Upwelling Ecosystem (PUE) represents an ideal area to reconstruct past changes in ocean-atmosphere systems because productivity and subsurface oxygenation are strongly linked to changes in the strength of the Walker circulation. Throughout the last 2000 yr, warmer (the Roman Warm Period [RWP], the Medieval Climate Anomaly [MCA] and the Current Warm Period [CWP]), and colder (the Dark Ages Cold Period [DACP] and Little Ice Age [LIA]) intervals occurred with considerable changes around the globe. In order to reconstruct the PUE response to these climatic periods and reveal the underlying mechanisms, we use a multi-proxy approach including organic and inorganic proxies in finely laminated sediments retrieved off Pisco (~ 14° S), Peru. Our results indicate that the PUE exhibited a La Niña-like mean state during the warm periods, characterized by an intense OMZ and high marine productivity. During cold periods the PUE exhibited an El Niño-like mean state, characterized by a weak OMZ and low marine productivity. Comparing our results with other relevant paleoclimatic reconstructions revealed that changes in the strength of the Walker circulation and the expansion/contraction of the South Pacific Sub-tropical High controlled productivity and subsurface oxygenation in the PUE during the last two millennia. This indicate that large scale circulation changes are the driving forces in maintaining productivity and subsurface oxygenation off Peru at centennial time scales during the past two millennia.

Description: Cumulated insolation: a simple explanation of Milankovitch's forcing on climate changes Climate of the Past Discussions, 9, 5553-5568, 2013 Author(s): F. Marra The occurrence of the sudden melting of the ice sheets during the glacial terminations is explained in this paper as the consequence of the combined role of the minima and the maxima of mean summer insolation on the Northern Hemisphere, providing a new contribution to understand the mechanisms ruling glacial forcing. Indeed, no satisfactory answer has been provided so far to the question why one specific maximum, after a series of consecutive maxima of insolation, has the potentiality to trigger a deglaciation. The explanation proposed in this paper accounts for a pre-conditioning factor, represented by "mild" (warmer) minimum, followed by a sufficiently warm maximum as the conditions that cause the end of a glacial cycle. These conditions are realized whenever the sum of the values of each consecutive minima and maxima ("cumulated insolation") on the curve of mean summer insolation at 65° N exceeds 742 Watt m −2 . The comparison of the succession of these cumulated insolation values with the astronomically tuned Oxygen isotopes record provides a satisfactory match with the occurrence of all the glacial terminations in the last 800 ka.

Description: Chronology of Lake El'gygytgyn sediments Climate of the Past Discussions, 9, 3061-3102, 2013 Author(s): N. R. Nowaczyk, E. M. Haltia, D. Ulbricht, V. Wennrich, M. A. Sauerbrey, P. Rosén, H. Vogel, A. Francke, C. Meyer-Jacob, A. A. Andreev, and A. V. Lozhkin A 318 m long sedimentary profile drilled by the International Continental Scientific Drilling Program (ICDP) at Site 5011-1 in Lake El'gygytgyn, Far East Russian Arctic, has been analysed for its sedimentologic response to global climate modes by chrono-stratigraphic methods. The 12 km wide lake is sited in an 18 km large crater that was created by the impact of a meteorite 3.58 Ma ago. Since then sediments have been continuously deposited. For establishing their chronology, major reversals of the Earth's magnetic field provided initial tie points for the age model, confirming that the impact occurred in the earliest Gauss chron. Various stratigraphic parameters, reflecting redox conditions at the lake floor and climatic conditions in the catchment were tuned synchronously to Northern Hemisphere insolation variations and the marine oxygen isotope stack, respectively. Thus, a robust age model comprising more than 600 tie points could be defined. It could be shown that deposition of sediments in Lake El'gygytgyn occurred in concert with global climatic cycles. The upper ~160 m of sediments represent the past 3.3 Ma, equivalent to sedimentation rates of 4 to 5 cm ka −1 , whereas the lower 160 m represent just the first 0.3 Ma after the impact, equivalent to sedimentation rates in the order of 45 cm ka −1 .

Description: Late Cenozoic continuous aridification in the western Qaidam Basin: evidence from sporopollen records Climate of the Past Discussions, 9, 1485-1508, 2013 Author(s): Y. F. Miao, X. M. Fang, F. L. Wu, M. T. Cai, C. H. Song, Q. Q. Meng, and L. Xu Cenozoic climate changes in inner Asia provide a basis for understanding linkages between global cooling, the Tibetan Plateau uplift, and possibly the development of the East Asian monsoon. Based on the compiled palynological results from the western Qaidam Basin, this study reconstructed an 18 Ma record of changing vegetation and paleoclimates since the middle Miocene. Thermophilic taxa percentages were highest between 18 and 14 Ma and decreased after 14 Ma, corresponding closely with the Middle Miocene Climatic Optimum (MMCO) between 18 and 14 Ma and the following global climatic cooling. After 3.6 Ma, the thermophilic taxa percentages further decreased, showing the inevitable relations with the ice-sheets enlargement in the North Hemisphere. During the same period of time, the increase in xerophytic taxa percentages and decrease in conifers percentages imply aridification in both the basin and surrounding mountains since 18 Ma. These results indicate that global cooling mainly controlled the climate change from a relative warm-wet stage to a cold-dry stage during the late Cenozoic at the western Qaidam Basin, and that the Tibetan Plateau uplift also contributed in contrast to the East Asian summer monsoon.

Description: Post-Pliocene establishment of the present monsoonal climate in SW China: evidence from the late Pliocene Longmen megaflora Climate of the Past Discussions, 9, 1675-1701, 2013 Author(s): T. Su, F. M. B. Jacques, R. A. Spicer, Y.-S. Liu, Y.-J. Huang, Y.-W. Xing, and Z.-K. Zhou The paleoclimate of the late Pliocene Longmen flora from Yongping County located at the southeastern boundary of the Qinghai-Tibet Plateau was reconstructed using two leaf physiognomy based methods, i.e. Leaf Margin Analysis (LMA) and Climate Leaf Analysis Multivariate Program (CLAMP), to understand the paleoclimate condition and geographical pattern of monsoonal climate in southwestern China during the late Pliocene. The mean annual temperatures (MATs) estimated by LMA and CLAMP are 17.4 ± 3.3 °C and 17.4 ± 1.3 °C, respectively, compared with 15.9 °C at present. Meanwhile, the growing season precipitation (GSP) estimated by CLAMP is 1735.5 ± 217.7 mm in the Longmen flora, compared with 986.9 mm nowadays. The calculated monsoon index (MSI) of the Longmen flora is significantly lower than that of today. These results appear consistent with previous studies based on the coexistence approach (CA), and further suggest that there was a slightly warmer and much wetter climate during the late Pliocene than the present climate in western Yunnan. We conclude that the significant change of the monsoonal climate might have been resulted from the continuous uplift of mountains in western Yunnan, as well as the intensification of eastern Asian winter monsoon, both occurring concurrently in the post-Pliocene period.

Description: Holocene vegetation and climate changes in central Mediterranean inferred from a high-resolution marine pollen record (Adriatic Sea) Climate of the Past Discussions, 9, 1969-2014, 2013 Author(s): N. Combourieu-Nebout, O. Peyron, V. Bout-Roumazeilles, S. Goring, I. Dormoy, S. Joannin, L. Sadori, G. Siani, and M. Magny To understand the effects of future climate change on the ecology of the central Mediterranean we can look to the impacts of long-term, millennial to centennial-scale climatic variability on vegetation in the basin. Pollen data from the Adriatic Marine core MD 90-917 allows us to reconstruct vegetation and regional climate changes over the south central Mediterranean during the Holocene. Clay mineral ratios from the same core reflect the relative contributions of riverine (illite and smectite) and eolian (kaolinite) contributions to the site, and thus act as an additional proxy with which to test precipitation changes in the Holocene. Vegetation reconstruction shows vegetation responses to the late-Glacial Preboreal oscillation, most likely driven by changes in seasonal precipitation. Pollen-inferred temperature declines during the early-mid Holocene, but increases during the mid-late Holocene, similar to southern-western Mediterranean climatic patterns during the Holocene. Several short climatic events appear in the record, indicating the sensitivity of vegetation in the region to millennial-scale variability. Reconstructed summer precipitation shows a regional maximum between 8000 and 7000 cal yr BP similar to the general pattern across southern Europe. Two important shifts in vegetation occur at 7700 and between 7500 and 7000 yr. These vegetation shifts are linked to changes in seasonal precipitation and are correlated to increased river inputs respectively from the north (7700 event) and from the central Adriatic borderlands (7500–7000 event). These results reinforce the strengths of multi-proxy analysis and provide a deeper understanding of the role of precipitation and particularly the seasonality of precipitation in mediating vegetation change in the central Mediterranean during the Holocene.

Description: Southern Hemisphere orbital forcing and its effects on CO 2 and tropical Pacific climate Climate of the Past Discussions, 9, 1869-1900, 2013 Author(s): K. Tachikawa, A. Timmermann, L. Vidal, C. Sonzogni, and O. E. Timm The western Pacific warm pool (WPWP) is an important heat source for the atmospheric circulation and influences climate conditions worldwide. Understanding its sensitivity to past radiative perturbations may help better contextualize the magnitudes and patterns of current and projected tropical climate change. Here we present a new Mg/Ca-based sea surface temperature (SST) reconstruction over the past 400 kyr from the Bismarck Sea, off Papua New Guinea, along with results from a transient earth system model simulation. Our results document the primary influence of CO 2 forcing on glacial/interglacial WPWP SSTs and secondary effects due to changes in wind-driven tropical boundary currents. In addition to the SST, deep ocean temperature reconstructions from this core are linked with Southern Ocean temperature and sea-ice variations on timescales of ~ 23 kyr. It is proposed that Southern Hemisphere insolation changes serve as pacemaker for sea-ice variations in the Southern Ocean, which in turn modulate windstress curl-driven upwelling of carbon-rich waters, hence controlling atmospheric CO 2 and tropical WPWP temperatures.

Description: A regional climate palaeosimulation for Europe in the period 1500–1990 – Part 1: Model validation Climate of the Past Discussions, 9, 1803-1839, 2013 Author(s): J. J. Gómez-Navarro, J. P. Montávez, S. Wagner, and E. Zorita We present and analyse a high-resolution regional climate palaeosimulation encompassing the European region for the period 1500–1990. We use the regional model MM5 coupled to the global model ECHO-G. Both models were driven by reconstructions of three external factors: greenhouse gas concentrations, Total Solar Irradiance and volcanic activity. The simulation has been assessed in a recent period by comparing the model results with the Climate Research Unit (CRU) database. The results show that although the regional model is tightly driven by the boundary conditions, it is able to improve the reliability of the simulations, narrowing the differences to the observations, especially in areas of complex topography. Additionally, the evolution of the spatial distributions of temperature and precipitation through the last five centuries has been analysed. The mean values of temperature reflects the influence of the external forcings but, contrary to the results obtained under climate change scenario conditions, we found that higher-order momenta of the probability distribution of seasonal temperature and precipitation are hardly affected by changes in the external forcings

Description: North–south palaeohydrological contrasts in the central Mediterranean during the Holocene: tentative synthesis and working hypotheses Climate of the Past Discussions, 9, 1901-1967, 2013 Author(s): M. Magny, N. Combourieu Nebout, J. L. de Beaulieu, V. Bout-Roumazeilles, D. Colombaroli, S. Desprat, A. Francke, S. Joannin, O. Peyron, M. Revel, L. Sadori, G. Siani, M. A. Sicre, S. Samartin, A. Simonneau, W. Tinner, B. Vannière, B. Wagner, G. Zanchetta, F. Anselmetti, E. Brugiapaglia, E. Chapron, M. Debret, M. Desmet, J. Didier, L. Essallami, D. Galop, A. Gilli, J. N. Haas, N. Kallel, L. Millet, A. Stock, J. L. Turon, and S. Wirth On the basis of a multi-proxy approach and a strategy combining lacustrine and marine records along a north–south transect, data collected in the Central Mediterranean within the framework of a collaborative project have led to reconstruction of high-resolution and well-dated palaeohydrological records and to assessment of their spatial and temporal coherency. Contrasting patterns of palaeohydrological changes have been evidenced in the Central Mediterranean: south (north) of around 40° N of latitude, the middle part of the Holocene was characterised by lake-level maxima (minima), during an interval dated to ca. 10 300–4500 cal BP to the south and 9000–4500 cal BP to the north. Available data suggest that these contrasting palaeohydrological patterns operated throughout the Holocene, both on millennial and centennial scales. Regarding precipitation seasonality, maximum humidity in the Central Mediterranean during the middle part of the Holocene was characterised by humid winters and dry summers north of ca. 40° N, and humid winters and summers south of ca. 40° N. This may explain an apparent conflict between palaeoclimatic records depending on the proxies used for reconstruction as well as the synchronous expansion of tree species taxa with contrasting climatic requirements. In addition, south of ca. 40° N, the first millennium of the Holocene was characterised by very dry climatic conditions not only in the Eastern, but also in the Central and the Western Mediterranean zones as reflected by low lake levels and delayed reforestation. These results suggest that, in addition to the influence of the Nile discharge reinforced by the African monsoon, the deposition of Sapropel 1 has been favoured (1) by an increase in winter precipitation in the northern Mediterranean borderlands, and (2) by an increase in winter and summer precipitation in the southern Mediterranean area. The climate reversal following the Holocene climate optimum appears to have been punctuated by two major climate changes around 7500 and 4500 cal BP. In the Central Mediterranean, the Holocene palaeohydrological changes developed in response to a combination of orbital, ice-sheet and solar forcing factors. The maximum humidity interval in the south-central Mediterranean started at ca. 10 300 cal BP, in correlation with the decline (1) of the possible blocking effects of the North Atlantic anticyclone linked to maximum insolation, and/or (2) of the influence of the remnant ice sheets and fresh water forcing in the North Atlantic Ocean. In the north-central Mediterranean, the lake-level minimum interval began only around 9000 cal BP when the Fennoscandian ice-sheet disappeared and a prevailing positive NAO-type circulation developed in the North Atlantic area. The major palaeohydrological oscillation around 4500–4000 cal BP may be a non-linear response to the gradual decrease, with additional key seasonal and interhemispherical changes, in insolation. On a centennial scale, the successive climatic events which punctuated the entire Holocene in the central Mediterranean coincided with cooling events associated with deglacial outbursts in the North Atlantic area and decreases in solar activity during the interval 11 700–7000 cal BP, and to a possible combination of NAO-type circulation and solar forcing since ca. 7000 cal BP onwards. Thus, regarding the centennial-scale climatic oscillations, the Mediterranean Basin appears to have been strongly linked to the North Atlantic area and affected by solar activity over the entire Holocene. In addition to model experiments, a better understanding of forcing factors and past atmospheric circulation patterns behind the Holocene palaeohydrological changes in the Mediterranean area will require further investigation to establish additional high-resolution and well-dated records in selected locations around the Mediterranean Basin and in adjacent regions. Special attention should be paid to greater precision in the reconstruction, on millennial and centennial time scales, of changes in the latitudinal location of the limit between the northern and southern palaeohydrological Mediterranean sectors, depending on (1) the intensity and/or characteristics of climatic periods/oscillations (e.g. Holocene thermal maximum versus Neoglacial, as well as, for instance, the 8.2 ka event versus the 4 ka event or the Little Ice Age), and (2) on varying geographical conditions from the western to the eastern Mediterranean areas (longitudinal gradients).

Description: The last 7 millennia of vegetation and climate changes at Lago di Pergusa (central Sicily, Italy) Climate of the Past Discussions, 9, 2059-2094, 2013 Author(s): L. Sadori, E. Ortu, O. Peyron, G. Zanchetta, B. Vannière, M. Desmet, and M. Magny The aim of this study is to investigate climate changes and human activities under the lens of palynology. Based on a new high-resolution pollen sequence (PG2) from Lago di Pergusa (667 m a.s.l., central Sicily, Italy) covering the last 6700 yr, we propose a reconstruction of climate and landscape changes over the recent past in central Sicily. Compared to former studies from Lago di Pergusa (Sadori and Narcisi, 2001), this work provides a reconstruction of the evolution of vegetation and climate over the last millennia in central Sicily, indeed completing previous results with new data which is particularly detailed on the last 3000 yr. Joint actions of increasing dryness, climate oscillations, and human impact shaped the landscape of this privileged site. Lago di Pergusa, in fact, besides being the main inland lake of Sicily, is very sensitive to climate change and its territory was inhabited and exploited continuously since the prehistory. The lake sediments turned out to be a good observatory for the natural phenomena occurred in the last thousands of years. Results of the pollen-based study are integrated with changes in magnetic susceptibility and a tephra layer characterization. The tephra layer was shown to be related to the Sicanians' event, radiocarbon dated at 3055 ± 75 yr BP (Sadori and Narcisi, 2001). We performed palaeoclimate reconstructions by MAT and WA-PLS. Palaeoclimate reconstructions based on the core show important climate fluctuations throughout the Holocene. Climate reconstruction points out four phases of cooling and enhanced wetness in the last three millennia (2600–2000, 1650–1100, 850–550, 400–200 cal BP). This appears to be the evidence of local responses to global climate oscillations during the recent past.

Description: Dust and associated trace element fluxes in a firn core from the coastal East Antarctica and its linkages with the Southern Hemisphere climate variability over the last ~ 50 yr Climate of the Past Discussions, 9, 1841-1867, 2013 Author(s): C. M. Laluraj, M. Thamban, and K. Satheesan High-resolution records of dust and trace element fluxes were studied in a firn core from the coastal Dronning Maud Land (cDML) in East Antarctica to identify the influence of climate variability on accumulation of these components over the past ~ 50 yr. A doubling of dust deposition was observed since 1985, coinciding with a shift in the Southern Annular Mode (SAM) index to positive values and associated increase in the wind speed. Back-trajectories showed that an increase in dust deposition is associated with the air parcels originating from north-west of the site, possibly indicating its origin from the Patagonian region. Our results suggest that while multiple processes could have influenced the increased dust formation, shift in SAM had a dominant influence on its transport. It is observed that since the 1985s the strength of easterlies increased significantly over the cDML region, which could sink air and dust material to the region that were brought by the westerlies through mass compensation. The correlation between the dust flux and δ 18 O records further suggest that enhanced dust flux in the firn core occurred during periods of colder atmospheric temperature, which reduced the moisture content and increased dust fall. Interestingly, the timing and amplitude of the insoluble dust peaks matched remarkably well with the fluxes of Ba, Cr, Cu, and Zn confirming that dust was the main carrier/source of atmospheric trace elements to East Antarctica during the recent past.

Description: An inter-laboratory investigation of the Arctic sea ice biomarker proxy IP 25 in marine sediments: key outcomes and recommendations Climate of the Past Discussions, 9, 5263-5298, 2013 Author(s): S. T. Belt, T. A. Brown, L. Ampel, P. Cabedo-Sanz, K. Fahl, J. J. Kocis, G. Massé, A. Navarro-Rodriguez, J. Ruan, and Y. Xu We describe the results of an inter-laboratory investigation into the identification and quantification of the Arctic sea ice biomarker proxy IP 25 in marine sediments. 7 laboratories took part in the study, which consisted of the analysis of IP 25 in a series of sediment samples from different regions of the Arctic, sub-Arctic and Antarctic, additional sediment extracts and purified standards. The results obtained allowed 4 key outcomes to be determined. First, IP 25 was identified by all laboratories in sediments from the Canadian Arctic with inter-laboratory variation in IP 25 concentration being substantially larger than within individual laboratories. This greater variation between laboratories was attributed to the difficulty in accurately determining instrumental response factors for IP 25 , despite provision of appropriate standards. Second, the identification of IP 25 by 3 laboratories in sediment from SW Iceland that was believed to represent a blank, was interpreted as representing a better limit of detection or quantification for such laboratories, contamination or mis-identification. These alternatives could not be distinguished conclusively with the data available, although it is noted that the precision of these data was significantly poorer compared with the other IP 25 concentration measurements. Third, 3 laboratories reported the occurrence of IP 25 in a sediment sample from the Antarctic Peninsula even though this biomarker is believed to be absent from the Southern Ocean. This anomaly is attributed to a combined chromatographic and mass spectrometric interference that results from the presence of a di-unsaturated highly branched isoprenoid (HBI) pseudo-homologue of IP 25 that occurs in Antarctic sediments. Finally, data are presented that suggest that extraction of IP 25 is consistent between Automated Solvent Extraction (ASE) and sonication methods and that IP 25 concentrations based on 7-hexylnonadecane as an internal standard are comparable using these methods. Recoveries of some more unsaturated HBIs and the internal standard 9-octylheptadecene, however, were lower with the ASE procedure, possibly due to partial degradation of these more reactive chemicals as a result of higher temperatures employed with this method. For future measurements, we recommend the use of reference sediment material with known concentration(s) of IP 25 for determining and routinely monitoring instrumental response factors. Given the significance placed on the presence (or otherwise) of IP 25 in marine sediments, some further recommendations pertaining to quality control are made that should also enable the two main anomalies identified here to be addressed.

Description: Expressions of climate perturbations in western Ugandan crater lake sediment records during the last 1000 yr Climate of the Past Discussions, 9, 5183-5226, 2013 Author(s): K. Mills, D. B. Ryves, N. J. Anderson, C. L. Bryant, and J. J. Tyler Equatorial East Africa has a complex, regional patchwork of climate regimes, with multiple interacting drivers. Recent studies have focussed on large lakes and reveal signals that are smoothed in both space and time, and, whilst useful at a continental scale, are of less relevance when understanding short-term, abrupt or immediate impacts of climate and environmental changes. Smaller-scale studies have highlighted spatial complexity and regional heterogeneity of tropical palaeoenvironments in terms of responses to climatic forcing (e.g. the Little Ice Age [LIA]) and questions remain over the spatial extent and synchroneity of climatic changes seen in East African records. Sediment cores from paired crater lakes in western Uganda were examined to assess ecosystem response to long-term climate and environmental change as well as testing responses to multiple drivers using redundancy analysis. These archives provide annual to sub-decadal records of environmental change. The records from the two lakes demonstrate an individualistic response to external (e.g. climatic) drivers, however, some of the broader patterns observed across East Africa suggest that the lakes are indeed sensitive to climatic perturbations such as a dry Mediaeval Climate Anomaly (MCA; 1000–1200 AD) and a relatively drier climate during the main phase of the LIA (1500–1800 AD); though lake levels in western Uganda do fluctuate. The relationship of Ugandan lakes to regional climate drivers breaks down c. 1800 AD, when major changes in the ecosystems appear to be a response to sediment and nutrient influxes as a result of increasing cultural impacts within the lake catchments. The data highlight the complexity of individual lake response to climate forcing, indicating shifting drivers through time. This research also highlights the importance of using multi-lake studies within a landscape to allow for rigorous testing of climate reconstructions, forcing and ecosystem response.

Description: Evaluation of seasonal climates of the Mediterranean and nothern Africa in the CMIP5 simulations Climate of the Past Discussions, 9, 5347-5389, 2013 Author(s): A. Perez-Sanz, G. Li, P. González-Sampériz, and S. P. Harrison We analyze the spatial expression of seasonal climates of the Mediterranean and northern Africa in pre-Industrial ( piControl ) and mid-Holocene ( midHolocene , 6 ka) simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Modern observations show four distinct precipitation regimes characterized by differences in the seasonal distribution and total amount of precipitation: an equatorial band characterized by a double peak in rainfall, the monsoon zone characterized by summer rainfall, the desert characterized by low seasonality and total precipitation, and the Mediterranean zone characterized by summer drought. Most models correctly simulate the position of the Mediterranean and the equatorial climates in the piControl simulations, but over-estimate the extent of monsoon influence and underestimate the extent of desert. However, most models fail to reproduce the amount of precipitation in each zone. Model biases in the simulated magnitude of precipitation are unrelated to whether the models reproduce the correct spatial patterns of each regime. In the midHolocene , the models simulate a reduction in winter rainfall in the equatorial zone, and a northward expansion of the monsoon with a significant increase in summer and autumn rainfall. Precipitation is slightly increased in the desert, mainly in summer and autumn, with northward expansion of the monsoon. Changes in the Mediterranean are small, although there is an increase in spring precipitation consistent with palaeo-observations of increased growing-season rainfall. Comparison with reconstructions shows that most models under-estimate the mid-Holocene changes in annual precipitation, except in the equatorial zone. Biases in the piControl have only a limited influence on midHolocene anomalies in ocean-atmosphere models; carbon-cycle models show no relationship between piControl bias and midHolocene anomalies. Biases in the prediction of the midHolocene monsoon expansion are unrelated to how well the models simulate changes in Mediterranean climate.

Description: Orbital and millennial-scale environmental changes between 64 and 25 ka BP recorded in Black Sea sediments Climate of the Past Discussions, 9, 5439-5477, 2013 Author(s): L. S. Shumilovskikh, D. Fleitmann, N. R. Nowaczyk, H. Behling, F. Marret, A. Wegwerth, and H. W. Arz High-resolution pollen and dinoflagellate cyst records from marine sediment core 25-GC1 were used to reconstruct vegetation dynamics in Northern Anatolia and surface conditions of the Black Sea between 64 and 25 ka BP. During this period, the dominance of Artemisia in the pollen record indicates a steppe landscape and arid climate conditions. However, the presence of temperate and warm-temperate arboreal pollen suggests the existence of glacial refugia in Northern Anatolia. A general cooling trend towards 25 ka BP is evidenced by the decrease of Quercus and increase of Pinus . There is evidence of orbital-driven vegetation dynamics in Northern Anatolia during 64–25 ka BP with spread of steppe during precession minima (insolation maxima) and development of forests during precession maxima (insolation minima). Dansgaard–Oeschger (D–O) events are characterized by a marked increase in temperate tree pollen, indicating a spread of forests due to warm and wet conditions in Northern Anatolia. The dominance of Pyxidinopsis psilata and Spiniferites cruciformis in the dinocyst record indicates a rather brackish Black Sea during the last glacial period. The decrease of marine indicators (marine dinocysts, acritachs) at ~ 54 ka BP and increase of freshwater algae ( Pediastrum, Botryococcus ) from 32 to 25 ka BP reveals freshening of the Black Sea surface water, related to orbital-driven arid/humid phases in the region, influencing hydrology and level changes of the Black Sea. D–O interstadials are characterized by high dinocyst concentrations and calcium carbonate content, as a result of an increase in primary productivity in the Black Sea. Heinrich events show a similar impact on the environment in Northern Anatolia/Black Sea region as D–O stadials.

Description: Vegetation responses to interglacial warming in the Arctic, examples from Lake El'gygytgyn, northeast Siberia Climate of the Past Discussions, 9, 245-267, 2013 Author(s): A. V. Lozhkin and P. M. Anderson Palynological data from Lake El'gygytgyn reveal responses of plant communities to a range of climatic conditions that can help assess the possible impact of global warming on arctoboreal ecosystems. Vegetation associated with climatic optima suggests two types of interglacial responses: one is dominated by deciduous taxa (the postglacial thermal maximum (PGTM) and marine isotope stage (MIS5)) and the second by evergreen conifers (MIS11, MIS31). The MIS11 forests show a similarity to Picea-Larix-Betula-Alnus forests of Siberia. While dark coniferous forest also characterizes MIS31, the pollen taxa show an affinity to the modern boreal forest of the lower Amur valley in the Russian Far East. Despite vegetation differences during the thermal maxima, all four glacial-interglacial transitions are alike, being dominated by deciduous woody taxa. Initially Betula shrub tundra established and was replaced by tundra with tree-sized shrubs (PGTM), Betula woodland (MIS5), or Betula-Larix (MIS11, MIS31) forest. The consistent occurrence of deciduous forest and/or high shrub tundra in all interglaciations as they approach or achieve maximum warmth underscores the significance of this biome for modeling efforts. The El'gygytgyn data also suggest the possible elimination or massive reduction of arctic plant communities under extreme warm-earth scenarios.

Description: The effect of precipitation seasonality on Eemian ice core isotope records from Greenland Climate of the Past Discussions, 9, 269-296, 2013 Author(s): W. J. van de Berg, M. R. van den Broeke, E. van Meijgaard, and F. Kaspar The previous interglacial (Eemian, 130–114 kyr BP) had a mean sea level highstand 4 to 7 m above the current level, and, according to climate proxies, a 2 to 6 K warmer Arctic summer climate. Greenland ice cores extending back into the Eemian show a reduced depletion in δ 18 O of about 3‰ for this period, which suggests a significant warming of several degrees over the Greenland ice sheet. Since the depletion in δ 18 O depends, among other factors, on the condensation temperature of the precipitation, we analyze climatological processes other than mean temperature changes that influence condensation temperature, using output of the regional climate model RACMO2. This model is driven by ERA-40 reanalysis and ECHO-G GCM boundaries for present-day, preindustrial and Eemian climate. The processes that affect the condensation temperature of the precipitation are analyzed using 6-hourly model output. Our results show that changes in precipitation seasonality can cause significant changes of up to 2 K in the condensation temperature that are unrelated to changes in mean temperature.

Description: Why could ice ages be unpredictable? Climate of the Past Discussions, 9, 1053-1098, 2013 Author(s): M. Crucifix It is commonly accepted that the variations of Earth's orbit and obliquity control the timing of Pleistocene glacial-interglacial cycles. Evidence comes from power spectrum analysis of palaeoclimate records and from inspection of the timing of glacial and deglacial transitions. However, we do not know how tight this control is. Is it, for example, conceivable that random climatic fluctuations could cause a delay in deglaciation, bad enough to skip a full precession or obliquity cycle and subsequently modify the sequence of ice ages? To address this question, seven previously published conceptual models of ice ages are analysed by reference to the notion of generalised synchronisation. Insight is being gained by comparing the effects of the astronomical forcing with idealised forcings composed of only one or two periodic components. In general, the richness of the astronomical forcing allows for synchronisation over a wider range of parameters, compared to periodic forcing. Hence, glacial cycles may conceivably have remained paced by the astronomical forcing throughout the Pleistocene. However, all the models examined here also show a range of parameters for which the structural stability of the ice age dynamics is weak. This means that small variations in parameters or random fluctuations may cause significant shifts in the succession of ice ages if the system were effectively in that parameter range. Whether or not the system has strong structural stability depends on the amplitude of the effects associated with the astronomical forcing, which significantly differ across the different models studied here. The possibility of synchronisation on eccentricity is also discussed and it is shown that a high Rayleigh number on eccentricity, as recently found in observations, is no guarantee of reliable synchronisation.

Description: East Asian monsoon climate simulated in the PlioMIP Climate of the Past Discussions, 9, 1135-1164, 2013 Author(s): R. Zhang, Q. Yan, Z. S. Zhang, D. Jiang, B. L. Otto-Bliesner, A. M. Haywood, D. J. Hill, A. M. Dolan, C. Stepanek, G. Lohmann, C. Contoux, F. Bragg, W.-L. Chan, M. A. Chandler, A. Jost, Y. Kamae, A. Abe-Ouchi, G. Ramstein, N. A. Rosenbloom, L. Sohl, and H. Ueda Based on the simulations with fifteen climate models in the Pliocene Model Intercomparison Project (PlioMIP), the regional climate of East Asia (focusing on China) during the mid-Pliocene is investigated in this study. Compared to the pre-industrial, the multi-model ensemble mean (MMM) of all models shows the East Asian summer wind (EASW) largely strengthens in monsoon China, and the East Asian winter wind (EAWW) strengthens in south monsoon China but slightly weakens in north monsoon China in mid-Pliocene. The MMM of all models also illustrates a warmer and wetter mid-Pliocene climate in China. The simulated weakened mid-Pliocene EAWW in north monsoon China and intensified EASW in monsoon China agree well with geological reconstructions. However, the model-model discrepancy in simulating mid-Pliocene East Asian monsoon climate, in particular EAWW, should be further addressed in the future work of PlioMIP.

Description: A reconstruction of radiocarbon production and total solar irradiance from the Holocene 14 C and CO 2 records: implications of data and model uncertainties Climate of the Past Discussions, 9, 1165-1235, 2013 Author(s): R. Roth and F. Joos Past atmospheric CO 2 concentrations reconstructed from polar ice cores combined with its Δ 14 C signature as conserved in tree-rings provide important information both on the cycling of carbon as well as the production of radiocarbon ( Q ) in the atmosphere. The latter is modulated by changes in the strength of the magnetic field enclosed in the solar wind and is a proxy for past changes in solar activity. We perform transient carbon-cycle simulations spanning the past 21 kyr using Bern3D-LPX, a fully featured Earth System Model of Intermediate Complexity (EMIC) with a 3-D ocean, sediment and a dynamic vegetation model. Using the latest atmospheric IntCal09/SHCal04 radiocarbon records, we reconstruct the Holocene radiocarbon fluxes and the total production rate. Our carbon-cycle based modern estimate of Q ≈ 1.7 atoms cm −2 s −1 is lower than previously reported by Masarik and Beer (2009) and more in line with Kovaltsov et al. (2012). Q is then translated into the solar modulation potential (Φ) using the latest geomagnetic field reconstruction and linked to a recent reanalysis of early instrumental data. In contrast to earlier reconstructions, our record suggests that periods of high solar activity (〉600 MeV) were quite common not only in recent millennia but throughout the Holocene. Solar activity in our decadally-smoothed record is during 28% of the time higher than the modern average of 650 MeV during the past 9 ka. But due to considerable uncertainties in the normalization of Φ to instrumental data, the absolute value of Φ remains weakly constrained. Further, our simulations with a spatially resolved model (taking the interhemispheric Δ 14 C gradient into account) show that reconstructions that rely on the Northern Hemisphere 14 C record only are biased towards low values during the Holocene. Notable deviations on decadal-to-centennial time scales are also found in comparison with earlier reconstructions. In a last step, past total solar irradiance (TSI) is quantified using a recently published Φ-TSI relationship yielding small changes in Holocene TSI of order 1 W m −2 with a Maunder Minimum irradiance reduction of 0.85 ± 0.17 W m −2 . Future extension of TSI using autoregressive modeling suggest a declining solar activity in the next decades towards average Holocene conditions. Past TSI changes are finally translated into changes in surfaces atmospheric temperature (SAT) by forcing the Bern3D-LPX model with our new TSI record, yielding SAT anomalies of less than 0.1 K.

Description: Northward advection of Atlantic water in the eastern Nordic Seas over the last 3000 yr: a coccolith investigation of volume transport and surface water changes Climate of the Past Discussions, 9, 1259-1295, 2013 Author(s): C. V. Dylmer, J. Giraudeau, F. Eynaud, K. Husum, and A. De Vernal Three marine sediment cores distributed along the Norwegian (MD95-2011), Barents Sea (JM09-KA11-GC), and Svalbard (HH11-134-BC) continental margins have been investigated in order to reconstruct changes in the poleward flow of Atlantic Waters (AW) and in the nature of upper surface water masses within the eastern Nordic Seas over the last 3000 yr. These reconstructions are based on a limited set of coccolith proxies: the abundance ratio between Emiliania huxleyi and Coccolithus pelagicus , an index of Atlantic vs. Polar-Arctic surface water masses; and Gephyrocapsa muellerae , a drifted coccolith species from the temperate North Atlantic, whose abundance changes are related to variations in the volume transport of the North Atlantic Current and its northernmost extension (the West Spitsbergen Current – WSC) off western Svalbard. The entire investigated area, from 66 to 77° N, was affected by an overall increase in volume flow of AW from 3000 cal yr BP to Present. The long-term modulation of westerlies strength and location which are essentially driven by the dominant mode of the North Atlantic Oscillation (NAO), is thought to explain the observed dynamics of poleward AW flow. The same mechanism also reconciles the recorded opposite zonal shifts in the location of the Arctic Front between the area off western Norway and the Barents Sea-eastern Fram Strait region. The Little Ice Age was governed by deteriorating conditions, with Arctic/Polar waters dominating in the surface off western Svalbard and western Barents Sea, possibly associated with both severe sea-ice conditions and a strongly reduced AW volume flow. A sudden short pulse of resumed high WSC flow interrupted this cold spell in eastern Fram Strait from 330 to 410 cal yr BP, with a a magnitude only surpassed by the one which characterizes the Modern Period. Our dataset not only confirms the high amplitude warming of surface waters at the turn of the 19th century off western Svalbard, it also shows that such a warming was primarily induced by an excess volume flow of AW which stands as unprecedented over the last 3000 yr.

Description: On the Milankovitch sensitivity of the Quaternary deep-sea record Climate of the Past Discussions, 9, 1237-1257, 2013 Author(s): W. H. Berger The response of the climate system to external forcing has become an item of prime interest in the context of global warming, especially with respect to the rate of melting land-based ice masses. The deep-sea record of ice-age climate change has been useful in assessing the sensitivity of the climate system to such forcing, notably to orbital forcing, which is well-known for the last several million years. When comparing response and forcing, one finds that sensitivity varies greatly through time, apparently in dependence on the state of the system. The changing stability of ice masses presumably is the underlying cause for the changing state of the system. A buildup of vulnerable ice masses within the latest Tertiary, when going into the ice ages, is conjectured to cause a stepwise increase of climate variability since the early Pliocene.

Description: Simulation of the Indian monsoon and its variability during the last millennium Climate of the Past Discussions, 9, 703-740, 2013 Author(s): S. Polanski, B. Fallah, S. Prasad, and U. Cubasch The general circulation model ECHAM5 has been used to simulate the Indian monsoon and its variability during the Medieval Warm Period (MWP; 900–1100 AD), the Little Ice Age (LIA; 1515–1715 AD) and for recent climate (REC; 1800–2000 AD). The focus is on the analysis of external drivers and internal feedbacks leading to extreme rainfall events over India from interannual to multidecadal time scale. An evaluation of spatiotemporal monsoon patterns with present-day observation data is in agreement with other state-of-the-art monsoon modeling studies. The simulated monsoon intensity on multidecadal time scale is weakened (enhanced) in summer (winter) due to colder (warmer) SSTs in the Indian Ocean. Variations in solar insolation are the main drivers for these SST anomalies, verified by very strong temporal anticorrelations between Total Solar Irradiance and All-India-Monsoon-Rainfall in summer monsoon months. The external solar forcing is coupled and overlain by internal climate modes of the ocean (ENSO and IOD) with asynchronous intensities and lengths of periods. In addition, the model simulations have been compared with a relative moisture index derived from paleoclimatic reconstructions based on various proxies and archives in India. In this context, the Lonar record in Central India has been highlighted and evaluated the first time. The simulated relative annual rainfall anomalies in comparison to present-day climate are in agreement (disagreement) with the reconstructed moisture index for MWP (LIA) climate. In order to investigate the interannual monsoon variability with respect to monsoon failures, dry summer monsoon composites for 30-yr-long periods of MWP, LIA and REC have been further analysed. Within dry years of LIA, the summer rainfall over India and surrounding oceans is less than in MWP indicating stronger drying conditions due to a stronger summer solar insolation forcing coupled with variations in ENSO. To quantify the ECHAM5 simulated long-term drought conditions within Monsoon Asia, the Palmer Drought Severity Index has been additionally estimated for recent climate showing strong pattern correlation between global SST anomalies and EOF variability signal of the drought index, whereas the temporal relationship is weak.

Description: Peak glacial 14 C ventilation ages suggest major draw-down of carbon into the abyssal ocean Climate of the Past Discussions, 9, 925-965, 2013 Author(s): M. Sarnthein, B. Schneider, and P. M. Grootes Ice core records demonstrate a glacial-interglacial atmospheric CO 2 increase of ~ 100 ppm. A transfer of ~ 530 Gt C is required to produce the deglacial rise of carbon in the atmosphere and terrestrial biosphere. This amount is usually ascribed to oceanic carbon release, although the actual mechanisms remained elusive, since an adequately old and carbon-enriched deep-ocean reservoir seemed unlikely. Here we present a new, though still fragmentary, ocean-wide 14 C dataset showing that during the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS-1) the 14 C age difference between ocean deep waters and the atmosphere exceeded the modern values by up to 1500 14 C yr, in the extreme reaching 5100 yr. Below 2000 m depth the 14 C ventilation age of modern ocean waters is directly linked to the concentration of dissolved inorganic carbon (DIC). We assume that the range of regression slopes of DIC vs. Δ 14 C remained constant for LGM times, which implies that an average LGM aging by ~ 600 14 C yr corresponded to a global rise by ~ 85–115 μmol DIC kg −1 in the deep ocean. Thus, the prolonged residence time of ocean deep waters indeed made it possible to absorb an additional ~ 730–980 Gt DIC, ~ 1/3 of which transferred from intermediate waters. We infer that LGM deep-water O 2 dropped to suboxic values of 〈 10 μmol kg −1 in the Atlantic sector of the Southern ocean, possibly also in the subpolar North Pacific. The transfer of aged deep-ocean carbon to the atmosphere and the ocean-atmosphere exchange are sufficient to account for the 190-‰ drop in atmospheric 14 C during the so-called HS-1 "Mystery Interval".

Description: Volcanic forcing for climate modeling: a new microphysics-based dataset covering years 1600–present Climate of the Past Discussions, 9, 967-1012, 2013 Author(s): F. Arfeuille, D. Weisenstein, H. Mack, E. Rozanov, T. Peter, and S. Brönnimann As the understanding and representation of the impacts of volcanic eruptions on climate have improved in the last decades, uncertainties in the stratospheric aerosol forcing from large eruptions are now not only linked to visible optical depth estimates on a global scale but also to details on the size, latitude and altitude distributions of the stratospheric aerosols. Based on our understanding of these uncertainties, we propose a new model-based approach to generating a volcanic forcing for General-Circulation-Model (GCM) and Chemistry-Climate-Model (CCM) simulations. This new volcanic forcing, covering the 1600–present period, uses an aerosol microphysical model to provide a realistic, physically consistent treatment of the stratospheric sulfate aerosols. Twenty-six eruptions were modeled individually using the latest available ice cores aerosol mass estimates and historical data on the latitude and date of eruptions. The evolution of aerosol spatial and size distribution after the sulfur dioxide discharge are hence characterized for each volcanic eruption. Large variations are seen in hemispheric partitioning and size distributions in relation to location/date of eruptions and injected SO 2 masses. Results for recent eruptions are in good agreement with observations. By providing accurate amplitude and spatial distributions of shortwave and longwave radiative perturbations by volcanic sulfate aerosols, we argue that this volcanic forcing may help refine the climate model responses to the large volcanic eruptions since 1600. The final dataset consists of 3-D values (with constant longitude) of spectrally resolved extinction coefficients, single scattering albedos and asymmetry factors calculated for different wavelength bands upon request. Surface area densities for heterogeneous chemistry are also provided.

Description: Inter-annual tropical Pacific climate variability in an isotope-enabled CGCM: implications for interpreting coral stable oxygen isotope records of ENSO Climate of the Past Discussions, 9, 741-773, 2013 Author(s): T. Russon, A. W. Tudhope, G. C. Hegerl, M. Collins, and J. Tindall Water isotope-enabled coupled atmosphere/ocean climate models allow for exploration of the relative contributions to coral stable oxygen isotope (δ 18 O coral ) variability arising from Sea Surface Temperature (SST) and the isotopic composition of seawater (δ 18 O sw ). The unforced behaviour of the isotope-enabled HadCM3 Coupled General Circulation Model affirms that the extent to which inter-annual δ 18 O sw variability contributes to that in model δ 18 O coral is strongly spatially dependent, ranging from being negligible in the eastern equatorial Pacific to accounting for 50% of δ 18 O coral variance in parts of the western Pacific. In these latter cases, a significant component of the inter-annual δ 18 O sw variability is correlated to that in SST, meaning that local calibrations of the effective local δ 18 O coral –SST relationships are likely to be essential. Furthermore, the relationship between δ 18 O sw and SST in the central and western equatorial Pacific is non-linear, such that the interpretation of model δ 18 O coral in the context of a linear dependence on SST alone may lead to overestimation (by up to 20%) of the SST anomalies associated with large El-Niño events. Intra-model evaluation of a salinity-based pseudo-coral approach shows that such an approach captures the first-order features of the model δ 18 O sw behaviour. However, the utility of the pseudo-corals is limited by the extent of spatial variability seen within the modelled slopes of the temporal salinity–δ 18 O sw relationship.

Description: Using paleo-climate comparisons to constrain future projections in CMIP5 Climate of the Past Discussions, 9, 775-835, 2013 Author(s): G. A. Schmidt, J. D. Annan, P. J. Bartlein, B. I. Cook, E. Guilyardi, J. C. Hargreaves, S. P. Harrison, M. Kageyama, A. N. LeGrande, B. Konecky, S. Lovejoy, M. E. Mann, V. Masson-Delmotte, C. Risi, D. Thompson, A. Timmermann, L.-B. Tremblay, and P. Yiou We present a description of the theoretical framework and "best practice" for using the paleo-climate model component of the Coupled Model Intercomparison Project (Phase 5) (CMIP5) to constrain future projections of climate using the same models. The constraints arise from measures of skill in hindcasting paleo-climate changes from the present over 3 periods: the Last Glacial Maximum (LGM) (21 thousand years before present, ka), the mid-Holocene (MH) (6 ka) and the Last Millennium (LM) (850–1850 CE). The skill measures may be used to validate robust patterns of climate change across scenarios or to distinguish between models that have differing outcomes in future scenarios. We find that the multi-model ensemble of paleo-simulations is adequate for addressing at least some of these issues. For example, selected benchmarks for the LGM and MH are correlated to the rank of future projections of precipitation/temperature or sea ice extent to indicate that models that produce the best agreement with paleoclimate information give demonstrably different future results than the rest of the models. We also find that some comparisons, for instance associated with model variability, are strongly dependent on uncertain forcing timeseries, or show time dependent behaviour, making direct inferences for the future problematic. Overall, we demonstrate that there is a strong potential for the paleo-climate simulations to help inform the future projections and urge all the modeling groups to complete this subset of the CMIP5 runs.

Description: Global and regional sea surface temperature trends during Marine Isotope Stage 11 Climate of the Past Discussions, 9, 837-890, 2013 Author(s): Y. Milker, R. Rachmayani, M. Weinkauf, M. Prange, M. Raitzsch, M. Schulz, and M. Kučera The Marine Isotope Stage (MIS) 11 (424–374 ka) was characterized by a protracted deglaciation and an unusually long climatic optimum. It remains unclear to what degree the climate development during this interglacial reflects the unusually weak orbital forcing or greenhouse gas trends. Previously, arguments about the duration and timing of the MIS11 climatic optimum and about the pace of the deglacial warming were based on a small number of key records, which appear to show regional differences. In order to obtain a global signal of climate evolution during MIS11, we compiled a database of 78 sea surface temperature (SST) records from 57 sites spanning MIS11, aligned these individually on the basis of benthic ( N = 28) or planktonic ( N = 31) stable oxygen isotope curves to a common time-frame and subjected 48 of them to an Empirical Orthogonal Function (EOF) analysis. The analysis revealed a high commonality among all records, with the principal SST trend explaining almost 49% of the variability. This trend indicates that on the global scale, the surface ocean underwent rapid deglacial warming during Termination V, in pace with carbon dioxide rise, followed by a broad SST optimum centered at ~ 410 kyr. The second EOF, which explained 19% of the variability, revealed the existence of a different SST trend, characterized by a delayed onset of the temperature optimum during MIS11 at ~ 398 kyr, followed by a prolonged warm period lasting beyond 380 kyr. This trend is most consistently manifested in the mid-latitude North Atlantic and Mediterranean Sea and is here attributed to the strength of the Atlantic meridional overturning circulation. A sensitivity analysis indicates that these results are robust to record selection and to age-model uncertainties of up to 3–6 kyr, but more sensitive to SST seasonal attribution and SST uncertainties 〉 1 °C. In order to assess the effect of orbital forcing on MIS11 SST trends, the annual and seasonal SST anomalies recorded in a total of 74 proxy records were compared with CCSM3 (Community Climate System Model, version 3) runs for three time slices representing orbital configuration extremes during the peak interglacial of MIS11. The modeled SST anomalies are characterized by a significantly lower variance compared to the reconstructions. Nevertheless, significant correlations between proxy and model data are found in comparisons on the seasonal basis, indicating that the model captures part of the long-term variability induced by astronomical forcing, which appears to have left a detectable signature in SST trends.

Description: Large spatial variations in coastal 14 C reservoir age – a case study from the Baltic Sea Climate of the Past Discussions, 9, 891-923, 2013 Author(s): B. C. Lougheed, H. L. Filipsson, and I. Snowball Coastal locations are highly influenced by input from freshwater river runoff, including sources of terrestrial carbon, which can be expected to modify the 14 C reservoir age, or R(t) , associated with marine water. In this Baltic Sea case study, pre-bomb museum collection mollusc shells of known calendar age, from 30 locations across a strategic salinity transect of the Baltic Sea, were analysed for 14 C, δ 13 C and δ 18 O. R(t) was calculated for all 30 locations. Seven locations, of which six are within close proximity of the coast, were found to have relatively higher R(t) values, indicative of hard-water effects. δ 13 C aragonite values were found to be indicative of hard-water influence only for certain locations, suggesting the possibility of different sources of old carbon in different locations. Whenever possible, the Macoma genus of mollusc was selected from the museum collections, in order to exclude species specific reservoir age effects as much as possible. When the Macoma samples are exclusively considered, and samples from hard-water locations excluded, a statistically significant correlation between Macoma R(t) and average salinity is found, indicating a two end-member linear mixing model between 14 C marine and 14 C runoff . A map of Baltic Sea Macoma aragonite R(t) for the late 19th and early 20th centuries is produced. Such a map can provide an estimate for contemporary Baltic Sea Macoma R(t) , although one must exercise caution when applying such estimates back in time or to 14 C dates obtained from different sample material. A statistically significant correlation is also found between δ 18 O aragonite and Macoma R(t) , suggesting that δ 18 O aragonite can be used to estimate Macoma palaeo- R(t) . The results of this Baltic Sea case study, which show that R(t) is affected by hydrographic conditions and local carbon inputs, have important consequences for other coastal and estuarine locations, where R(t) is also likely to significantly vary on spatial and temporal bases.

Description: The Irish famine of 1740–1741: causes and effects Climate of the Past Discussions, 9, 1013-1052, 2013 Author(s): S. Engler, J. Luterbacher, F. Mauelshagen, and J. Werner This paper advances the current debate on causes and effects of famines. Since Sen's food entitlement decline theory emerged in the 1980's, climate and environmental factors are widely excluded in famine analysis. Studying the causation and the processes of famines as well as the adaptations to it before the 20th century will enhance modern famine theories and lead to a rethinking of the role of climate/environmental aspects in current research. In our case study, the "Famine Vulnerability Analysis Model" (FVAM) serves as an explanatory model and will open up new perspectives on famines. Special emphasis will be put on the Europe-wide crises of 1740–1741, with a focus on the famine of the "great frost" in Ireland. The interaction of demographic, political, economic and environmental aspects is characteristic in this famine.

Description: A Last Glacial Maximum world-ocean simulation at eddy-permitting resolution – Part 1: Experimental design and basic evaluation Climate of the Past Discussions, 9, 297-328, 2013 Author(s): M. Ballarotta, L. Brodeau, J. Brandefelt, P. Lundberg, and K. Döös Most state-of-the-art climate models include a coarsely resolved oceanic component, which has difficulties in capturing detailed dynamics, and therefore eddy-permitting/eddy-resolving simulations have been developed to reproduce the observed World Ocean. In this study, an eddy-permitting numerical experiment is conducted to simulate the global ocean state for a period of the Last Glacial Maximum (LGM, ~ 26 500 to 19 000 yr ago) and to investigate the improvements due to taking into account these higher spatial scales. The ocean general circulation model is forced by a 49-yr sample of LGM atmospheric fields constructed from a quasi-equilibrated climate-model simulation. The initial state and the bottom boundary condition conform to the Paleoclimate Modelling Intercomparison Project (PMIP) recommendations. Before evaluating the model efficiency in representing the paleo-proxy reconstruction of the surface state, the LGM experiment is in this first part of the investigation, compared with a present-day eddy-permitting hindcast simulation as well as with the available PMIP results. It is shown that the LGM eddy-permitting simulation is consistent with the quasi-equilibrated climate-model simulation, but large discrepancies are found with the PMIP model analyses, probably due to the different equilibration states. The strongest meridional gradients of the sea-surface temperature are located near 40° N and S, this due to particularly large North-Atlantic and Southern-Ocean sea-ice covers. These also modify the locations of the convection sites (where deep-water forms) and most of the LGM Conveyor Belt circulation consequently takes place in a thinner layer than today. Despite some discrepancies with other LGM simulations, a glacial state is captured and the eddy-permitting simulation undertaken here yielded a useful set of data for comparisons with paleo-proxy reconstructions.

Description: Petrophysical characterization of the lacustrine sediment succession drilled in Lake El'gygytgyn, Far East Russian Arctic Climate of the Past Discussions, 9, 351-391, 2013 Author(s): A. C. Gebhardt, A. Francke, J. Kück, M. Sauerbrey, F. Niessen, V. Wennrich, and M. Melles Seismic profiles of Far East Russian Lake El'gygytgyn which was formed by a meteorite impact some 3.6 million years ago show a stratified sediment succession that can be separated into Subunits Ia and Ib at approximately 167 m below lake floor (= ∼ 3.17 Ma). The former is well-stratified, while the latter is acoustically more massive. The sediments are intercalated with frequent mass movement deposits mainly in the proximal parts, while the distal part is almost free of such deposits at least in the upper part. In spring 2009, a long core drilled in the lake center within the framework of the International Continental Scientific Drilling Program (ICDP) penetrated the entire lacustrine sediment succession down to ~ 320 m below lake floor and about 200 m further into the meteorite-impact related bedrock. Downhole logging data down to 390 m below lake floor show that the bedrock and the lacustrine part of the core differ largely in their petrophysical characteristics. The contact between the bedrock and the lacustrine sediments is not abrupt, but rather transitional with a mixture of impact-altered bedrock clasts in a lacustrine matrix with varying percentages. Physical and chemical proxies measured on the cores can be used to divide the lacustrine part into five different clusters. These can be plotted in a redox-condition vs. input type diagram with total organic carbon content and magnetic susceptibility values indicating anoxic or oxic conditions and with the Si/Ti ratio representing more clastic or more biogenic input. Plotting the clusters in this diagram allows identifying clusters that represent glacial phases (Cluster I), super interglacials (Cluster II), and interglacial phases (Clusters III and IV).

Description: Vegetation and climate development on the North American Atlantic Coastal Plain from 33 to 13 million years ago (IODP Expedition 313) Climate of the Past Discussions, 9, 6551-6603, 2013 Author(s): U. Kotthoff, D. R. Greenwood, F. M. G. McCarthy, K. Müller-Navarra, and S. P. Hesselbo We have investigated the palynology of sediment cores from Sites M0027 and M0029 of IODP Expedition 313 on the New Jersey shallow shelf, east coast of North America, spanning an age range of 33 to 13 million years before present. Additionally, a pollen assemblage from the Pleistocene was examined. The palynological results were statistically analyzed and complemented with pollen-based quantitative climate reconstructions. Transport-related bias of the pollen assemblages was identified via analysis of the ratio of terrestrial to marine palynomorphs, and considered when interpreting palaeovegetation and palaeoclimate from the pollen data. Results indicate that from the early Oligocene to the middle Miocene, the hinterland vegetation of the New Jersey shelf was characterized by oak-hickory forests in the lowlands and conifer-dominated vegetation in the highlands. The Oligocene witnessed several expansions of conifer forest, probably related to cooling events. The pollen-based climate data imply an increase in annual temperatures from ~12 °C to more than 15 °C during the Oligocene. The Mi-1 cooling event at the onset of the Miocene is reflected by an expansion of conifers and an annual temperature decrease by almost 3 °C, from 15 °C to 12.5 °C around 23 million years before present. Particularly low annual temperatures are also recorded for an interval around ~20 million years before present, which probably reflects the Mi-1aa cooling event. Generally, the Miocene ecosystem and climate conditions were very similar to those of the Oligocene in the hinterland of the New Jersey shelf. Miocene grasslands, as known from other areas in the USA during that time period, are not evident for the hinterland of the New Jersey shelf. Surprisingly, the palaeovegetation data for the hinterland of the New Jersey shelf do not show extraordinary changes during the Mid-Miocene climatic optimum at ~15 million years before present, except for a minor increase in deciduous-evergreen mixed forest taxa and a decrease in swamp forest taxa. Pollen-based annual temperature reconstructions show average annual temperatures of ~14 °C during the Mid-Miocene climatic optimum. We conclude that vegetation and regional climate in the hinterland of the New Jersey shelf did not react as sensitively to Oligocene and Miocene climate changes as other regions in North America or Europe. An additional explanation for the relatively low regional temperatures reconstructed for the Mid-Miocene climatic optimum could be an uplift of the Appalachian Mountains during the Miocene. The Pleistocene pollen assemblage probably derives from the Marine Isotope Chron 7 or 5e and shows climate conditions similar to present-day.

Description: 10 Be in late deglacial climate simulated by ECHAM5-HAM – Part 2: Isolating the solar signal from 10 Be deposition Climate of the Past Discussions, 9, 5627-5657, 2013 Author(s): U. Heikkilä, X. Shi, S. J. Phipps, and A. M. Smith This study investigates the effect of deglacial climate on the deposition of the solar proxy 10 Be globally, and at two specific locations, the GRIP site at Summit, Central Greenland, and the Law Dome site in coastal Antarctica. The deglacial climate is represented by three 30 yr time slice simulations of 10 000 BP (years before present = 1950 CE), 11 000 BP and 12 000 BP, compared with a preindustrial control simulation. The model used is the ECHAM5-HAM atmospheric aerosol–climate model, driven with sea surface temperatures and sea ice cover simulated using the CSIRO Mk3L coupled climate system model. The focus is on isolating the 10 Be production signal, driven by solar variability, from the weather or climate driven noise in the 10 Be deposition flux during different stages of climate. The production signal varies on lower frequencies, dominated by the 11yr solar cycle within the 30 yr time scale of these experiments. The climatic noise is of higher frequencies. We first apply empirical orthogonal functions (EOF) analysis to global 10 Be deposition on the annual scale and find that the first principal component, consisting of the spatial pattern of mean 10 Be deposition and the temporally varying solar signal, explains 64% of the variability. The following principal components are closely related to those of precipitation. Then, we apply ensemble empirical decomposition (EEMD) analysis on the time series of 10 Be deposition at GRIP and at Law Dome, which is an effective method for adaptively decomposing the time series into different frequency components. The low frequency components and the long term trend represent production and have reduced noise compared to the entire frequency spectrum of the deposition. The high frequency components represent climate driven noise related to the seasonal cycle of e.g. precipitation and are closely connected to high frequencies of precipitation. These results firstly show that the 10 Be atmospheric production signal is preserved in the deposition flux to surface even during climates very different from today's both in global data and at two specific locations. Secondly, noise can be effectively reduced from 10 Be deposition data by simply applying the EOF analysis in the case of a reasonably large number of available data sets, or by decomposing the individual data sets to filter out high-frequency fluctuations.

Description: Testing long-term summer temperature reconstruction based on maximum density chronologies obtained by reanalysis of tree-ring datasets from northernmost Sweden and Finland Climate of the Past Discussions, 9, 5659-5700, 2013 Author(s): V. V. Matskovsky and S. Helama Here we analysed the maximum latewood density (MXD) chronologies of two published tree-ring datasets: from Torneträsk region in northernmost Sweden (TORN, Melvin et al., 2013) and from northern Fennoscandia (FENN, Esper et al., 2012). We paid particular attention to the MXD low-frequency variations to reconstruct long-term summer (June–August, JJA) temperature history. We used published methods of tree-ring standardization: regional curve (RC) standardization, combined with signal-free (SF) implementation. Comparisons with a single-RC (RC1) and multiple-RC (RC2) were also carried out. We develop a novel method of standardization, the correction (C) implementation to SF (hence, RC1SFC or RC2SFC), tailored for detection of pure low-frequency signal in tree-ring chronologies. In this method, the error in RC1SF (or RC2SF) chronology, is analytically assessed and extracted to produce a RC1SFC or RC2SFC chronology. In TORN, the RC1SF chronology shows higher correlation with summer temperature (JJA) than RC1SFC, whereas in FENN the temperature signals of RC1SF chronology is improved by correction implementation (RC1SFC). The highest correlation between differently standardized chronologies for two datasets is obtained using FENN-RC2SFC and TORN-RC1 chronologies. Focusing on lowest frequencies, the importance of correction becomes obvious as the chronologies become progressively more correlative with RC1SFC and RC2SFC implementations. Subsampling the FENN data (which presents a higher number of samples than TORN dataset) to the chronology sample size of TORN data shows that the chronologies consistently bifurcate during the 7th, 9th, 17th and 20th centuries. We used the two MXD datasets to reconstruct summer temperature variations over the period −48–2010 calendar years. Our new reconstruction shows multi-decadal to multi-centennial variability with changes in the amplitude of the summer temperature of 2.6 °C in average during the Common Era.

Description: Persistent decadal-scale rainfall variability in the tropical South Pacific Convergence Zone through the past six centuries Climate of the Past Discussions, 9, 5593-5625, 2013 Author(s): C. R. Maupin, J. W. Partin, C.-C. Shen, T. M. Quinn, K. Lin, F. W. Taylor, J. L. Banner, K. Thirumalai, and D. J. Sinclair Observations and reconstructions of decadal-scale climate variability are necessary to place predictions of future global climate change into temporal context (Goddard et al., 2012). This is especially true for decadal-scale climate variability that originates in the Pacific Ocean (Deser et al., 2004; Dong and Lu, 2013). We focus here on the western tropical Pacific (Solomon Islands; ~ 9.5° S, ~ 160° E), a region directly influenced by: the South Pacific Convergence Zone (SPCZ), the West Pacific Warm Pool (WPWP), the Pacific Walker Circulation (PWC), and the Hadley Circulation. We calibrate δ 18 O variations in a fast growing stalagmite to local rainfall amount and produce a 600 yr record of rainfall variability from the zonally oriented, tropical portion of the SPCZ. We present evidence for large (~ 1.5 m), persistent and decade(s)-long shifts in total annual rainfall amount in the Solomon Islands since 1416 ± 5 CE. The timing of the decadal changes in rainfall inferred from the 20th century portion of the stalagmite δ 18 O record coincide with previously identified decadal shifts in Pacific ocean-atmosphere behavior (Clement et al., 2011; Deser et al., 2004). The 600 yr Solomons stalagmite δ 18 O record indicates that decadal oscillations in rainfall are a robust characteristic of SPCZ-related climate variability, which has important implications to water resource management in this region.

Description: The influence of atmospheric circulation on the mid-Holocene climate of Europe: a data-model comparison Climate of the Past Discussions, 9, 5569-5592, 2013 Author(s): A. Mauri, B. A. S. Davis, P. M. Collins, and J. O. Kaplan The atmospheric circulation is a key area of uncertainty in climate model simulations of future climate change, especially in mid-latitude regions such as Europe where atmospheric dynamics have a significant role in climate variability. It has been proposed that the mid-Holocene was characterized in Europe by a stronger westerly circulation in winter comparable with a more positive AO/NAO, and a weaker westerly circulation in summer caused by anti-cyclonic blocking near Scandinavia. Model simulations indicate at best only a weakly positive AO/NAO, whilst changes in summer atmospheric circulation have not been widely investigated. Here we use a new pollen-based reconstruction of European mid-Holocene climate to investigate the role of atmospheric circulation in explaining the spatial pattern of seasonal temperature and precipitation anomalies. We find that the footprint of the anomalies is entirely consistent with those from modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive AO/NAO) and a weak westerly circulation in summer (positive SCAND). We find little agreement between the reconstructed anomalies and those from a climate model simulation, which as with most model simulations shows a much greater sensitivity to local radiative forcing from top-of-the-atmosphere changes in solar insolation. Our findings are consistent with data-model comparisons on contemporary timescales that indicate that models underestimate the role of atmospheric circulation in climate change, whilst also highlighting the importance of atmospheric dynamics in explaining interglacial warming.

Description: Environmental and climatic changes in Central Chilean Patagonia since the Late Glacial (Mallín El Embudo, 44° S) Climate of the Past Discussions, 9, 5747-5784, 2013 Author(s): M. E. de Porras, A. Maldonado, F. A. Quintana, A. Martel-Cea, O. Reyes, and C. Méndez Multi-millennial environmental and climatic changes in Central Chilean Patagonia (44–49° S) during the Last Glacial–Interglacial cycle have been of particular interest as changes in the position and strength of the Southern Westerlies are the major forcing factor conditioning the environmental dynamics. Recent attempts to reconstruct regional environmental and climatic signals from Central Chilean Patagonia reveal some discrepancies and unclear issues among the records. This paper presents the 13 ka pollen and charcoal records from Mallín El Embudo (44°40' S; 71°42' W) located in the deciduous Nothofagus forest in the middle Río Cisnes valley. The paper aims to (1) establish the timing and magnitude of local vegetation changes and fire activity since the Late Glacial and (2) integrate these results at the regional scale in order to discuss the discrepancies and depict the Central Chilean Patagonia environmental and climatic dynamics since Late Glacial. Open landscapes dominated by grasses associated with scattered Nothofagus forest patches dominated middle Río Cisnes valley between 13–11.2 ka suggesting low effective moisture but also reflecting that landscape configuration after glacial retreat was still ongoing. At 11.2 ka, a sudden development of an open and quite dynamic Nothofagus forest probably associated to the synchronous high fire activity occurred suggesting a rise in effective moisture. Since 9.5 ka, the record reflects the presence of a closed Nothofagus forest related to higher/similar effective moisture conditions than before but under an unmarked precipitation seasonality. The forest experienced a slight canopy opening since 5.7 ka, probably due to slightly drier conditions than before followed by a sudden change around 4.2 ka associated with fire and volcanic disturbances. The recovery of an open Nothofagus forest related to slight wetter conditions (similar to present) occurred around 2 ka and persisted under highly variable climatic conditions up to 0.1 ka when massive forest burning and logging due to European settlements occurred. Central Chilean Patagonian climatic and environmental changes at millennial-centennial time scales since Late Glacial were driven by changes in the Southern Westerlies latitudinal shift and/or intensity but during the Late Holocene fire, volcanism and humans arise as major forcings contributing to environmental dynamics.

Description: Uncertainties in the modelled CO 2 threshold for Antarctic glaciation Climate of the Past Discussions, 9, 5701-5745, 2013 Author(s): E. Gasson, D. J. Lunt, R. DeConto, A. Goldner, M. Heinemann, M. Huber, A. N. LeGrande, D. Pollard, N. Sagoo, M. Siddall, and A. Winguth A frequently cited atmospheric CO 2 threshold for the onset of Antarctic glaciation of ~ 780 ppmv is based on a study using an ice sheet model and the GENESIS climate model. Proxy records suggest that atmospheric CO 2 concentrations passed through this threshold across the Eocene–Oligocene transition ~ 34 Ma. However, atmospheric CO 2 concentrations may have been close to this threshold earlier than this transition, which is used by some to suggest the possibility of Antarctic ice sheets during the Eocene. Here we investigate the climate model dependency of the threshold for Antarctic glaciation by performing offline ice sheet model simulations using the climate from a number of different climate models (HadCM3L, CCSM3, CESM1.0, GENESIS, FAMOUS, ECHAM5 and GISS_ER). These climate simulations are sourced from a number of independent studies, as such the boundary conditions, which are poorly constrained during the Eocene, are not identical between simulations. The results of this study suggest that the atmospheric CO 2 threshold for Antarctic glaciation is highly dependent on the climate model used and the climate model configuration. A large discrepancy between the climate model and ice sheet model grids for some simulations leads to a strong sensitivity to the lapse rate parameter. However, with the exception of HadCM3L and its reduced complexity version FAMOUS, the simulations suggest the growth of an intermediate sized ice sheet (〉 25 m sea level equivalent) for atmospheric CO 2 concentrations in the range of 560–920 ppmv, which is consistent with previous studies.

Description: A seasonality trigger for carbon injection at the Paleocene-Eocene thermal maximum Climate of the Past Discussions, 9, 5837-5854, 2013 Author(s): J. S. Eldrett, D. R. Greenwood, M. Polling, H. Brinkhuis, and A. Sluijs The Paleocene-Eocene thermal maximum (PETM) represents a ~170 kyr episode of anomalous global warmth ~56 Ma ago. The PETM is associated with rapid and massive injections of 13 C-depleted carbon into the ocean-atmosphere system reflected as a prominent negative carbon isotope excursion (CIE) in sedimentary components. Earth's surface and deep ocean waters warmed by ~5 °C, of which part may have occurred prior to the CIE. However, few records document continental climatic trends and changes in seasonality have not been documented. Here we present the first high-resolution vegetation reconstructions for the PETM, based on bioclimatic analysis of terrestrially-derived spore and pollen assemblages preserved in an expanded section from the Central North Sea. Our data indicate reductions in boreal conifers and an increase in mesothermal to megathermal taxa, reflecting a shift towards wetter and warmer climate. We also record an increase in summer temperatures, greater in magnitude than the rise in mean annual temperature changes. Within the CIE, vegetation varies significantly with initial increases in epiphytic and climbing ferns, and development of extensive wetlands, followed by abundance of Carya spp. indicative of broadleaf forest colonization. Critically, the change in vegetation we report occurs prior to the CIE, and is concomitant with anomalous marine ecological change, as represented by the occurrence of Apectodinium augustum . This suggests that amplifications of seasonal extremes triggered carbon injection.

Description: Regional climate model simulations for Europe at 6 k and 0.2 k yr BP: sensitivity to changes in anthropogenic deforestation Climate of the Past Discussions, 9, 5785-5836, 2013 Author(s): G. Strandberg, E. Kjellström, A. Poska, S. Wagner, M.-J. Gaillard, A.-K. Trondman, A. Mauri, H. J. B. Birks, A. E. Bjune, B. A. S. Davis, R. Fyfe, T. Giesecke, L. Kalnina, M. Kangur, J. O. Kaplan, W. O. van der Knaap, U. Kokfelt, P. Kuneš, M. Latałowa, L. Marquer, F. Mazier, A. B. Nielsen, B. Smith, H. Seppä, and S. Sugita This study aims to evaluate the direct effects of anthropogenic deforestation on simulated climate at two contrasting periods in the Holocene, ~6 k BP and ~0.2 k BP in Europe. We apply RCA3, a regional climate model with 50 km spatial resolution, for both time periods, considering three alternative descriptions of the past vegetation: (i) potential natural vegetation (V) simulated by the dynamic vegetation model LPJ-GUESS, (ii) potential vegetation with anthropogenic land cover (deforestation) as simulated by the HYDE model (V + H), and (iii) potential vegetation with anthropogenic land cover as simulated by the KK model (V + K). The KK model estimates are closer to a set of pollen-based reconstructions of vegetation cover than the HYDE model estimates. The climate-model results show that the simulated effects of deforestation depend on both local/regional climate and vegetation characteristics. At ~6 k BP the extent of simulated deforestation in Europe is generally small, but there are areas where deforestation is large enough to produce significant differences in summer temperatures of 0.5–1 °C. At ~0.2 k BP, simulated deforestation is much more extensive than previously assumed, in particular according to the KK model. This leads to significant temperature differences in large parts of Europe in both winter and summer. In winter, deforestation leads to lower temperatures because of the differences in albedo between forested and unforested areas, particularly in the snow-covered regions. In summer, deforestation leads to higher temperatures in central and eastern Europe since evapotranspiration from unforested areas is lower than from forests. Summer evaporation is already limited in the southernmost parts of Europe under potential vegetation conditions and, therefore, cannot become much lower. Accordingly, the albedo effect dominates also in summer, which implies that deforestation causes a decrease in temperatures. Differences in summer temperature due to deforestation range from −1 °C in south-western Europe to +1 °C in eastern Europe. The choice of anthropogenic land cover estimate has a significant influence on the simulated climate, but uncertainties in palaeoclimate proxy data for the two time periods do not allow for a thorough comparison with climate model results.

Description: Terrigenous input off northern South America driven by changes in Amazonian climate and the North Brazil Current retroflection during the last 250 ka Climate of the Past Discussions, 9, 5855-5898, 2013 Author(s): A. Govin, C. M. Chiessi, M. Zabel, A. O. Sawakuchi, D. Heslop, T. Hörner, Y. Zhang, and S. Mulitza We investigate changes in the delivery and oceanic transport of Amazon sediments related to terrestrial climate variations over the last 250 ka. We present high-resolution geochemical records from four marine sediment cores located between 5 and 12° N along the northern South American margin. The Amazon River is the sole source of terrigenous material for sites at 5 and 9° N, while the core at 12° N receives a mixture of Amazon and Orinoco detrital particles. Using an endmember unmixing model, we estimated the relative proportions of Amazon Andean material ("%-Andes", at 5 and 9° N) and of Amazon material ("%-Amazon", at 12° N) within the terrigenous fraction. The %-Andes and %-Amazon records exhibit significant precessional variations over the last 250 ka that are more pronounced during interglacials in comparison to glacial times. High %-Andes values observed during periods of high austral summer insolation reflect the increased delivery of suspended sediments by Andean tributaries and enhanced Amazonian precipitation, in agreement with western Amazonian speleothem records. However, low %-Amazon values obtained at 12° N during the same periods seem to contradict the increased delivery of Amazon sediments. We propose that reorganisations in surface ocean currents modulate the northwestward transport of Amazon material. In agreement with published records, the seasonal North Brazil Current retroflection is intensified (or prolonged in duration) during cold substages of the last 250 ka (which correspond to intervals of high DJF or low JJA insolation) and deflects eastward the Amazon sediment and freshwater plume.

Description: Model-data comparison and data assimilation of mid-Holocene Arctic sea-ice concentration Climate of the Past Discussions, 9, 6515-6549, 2013 Author(s): F. Klein, H. Goosse, A. Mairesse, and A. de Vernal The consistency between a new quantitative reconstruction of Arctic sea-ice concentration based on dinocyst assemblages and the results of climate models has been investigated for the mid-Holocene. The comparison shows that the simulated sea-ice changes are weaker and spatially more homogeneous than the recorded ones. Furthermore, although the model-data agreement is relatively good in some regions such as the Labrador Sea, the skill of the models at local scale is low. The response of the models follows mainly the increase in summer insolation at large scale. This is modulated by changes in atmospheric circulation leading to differences between regions in the models that are albeit smaller than in the reconstruction. Performing simulations with data assimilation using the model LOVECLIM amplifies those regional differences, mainly through a reduction of the southward winds in the Barents Sea and an increase in the westerly winds in the Canadian Basin of the Arctic. This leads to an increase in the ice concentration in the Barents and Chukchi Seas and a better agreement with the reconstructions. This underlines the potential role of atmospheric circulation to explain the reconstructed changes during the Holocene.

Description: Black shale deposition during Toarcian super-greenhouse driven by sea level Climate of the Past Discussions, 9, 4365-4384, 2013 Author(s): M. Hermoso, F. Minoletti, and P. Pellenard One of the most elusive aspects of the Toarcian Oceanic Anoxic Event (T-OAE) is the paradox between carbon isotopes that indicate intense global primary productivity and organic carbon burial at a global scale, and the delayed expression of anoxia in Europe. During the earliest Toarcian, no black shales were deposited in the European epicontinental seaways, and most organic carbon enrichment of the sediments postdated the T-OAE (defined by the overarching positive trend in the carbon isotopes). In the present studied, we have attempted to establish a sequence stratigraphy framework for Early Toarcian deposits recovered from a core drilled in the Paris Basin using a combination of mineralogical (quartz and clay relative abundance) and geochemical (Si, Zr, Ti and Al) measurements. Combined with the evolution in redox sensitive elements (Fe, V and Mo), the data suggest that expression of anoxia was hampered in European epicontinental seas during most of the T-OAE due to insufficient water depth that prevented stratification of the water column. Only the first stratigraphic occurrence of black shales in Europe corresponds to the "global" event. This interval is characterised by 〉 10% Total Organic Carbon (TOC) content that contains relatively low concentration of molybdenum compared to subsequent black shale horizons. Additionally, this first black shale occurrence is coeval with the record of the major negative Carbon Isotope Excursion (CIE), likely corresponding to a period of transient greenhouse intensification likely due to massive injection of carbon into the Atmosphere–Ocean system. As a response to enhanced weathering and riverine run-off, increased fresh water supply to the basin may have promoted the development of full anoxic conditions through haline stratification of the water column. In contrast, post T-OAE black shales were restricted to epicontinental seas (higher Mo to TOC ratios) during a period of relative high sea level, and carbon isotopes returning to pre-T-OAE values. Comparing palaeoredox proxies with the inferred sequence stratigraphy for Sancerre suggests that episodes of short-term organic carbon enrichment were primarily driven by third-order sea level changes. These black shales exhibit remarkably well-expressed higher-frequency cyclicities in the concentration of redox-sensitive elements such as iron or vanadium whose nature has still to be determined through cyclostratigraphic analysis.

Description: The East Asian winter monsoon variability in response to precession and inter-hemispheric heat balance Climate of the Past Discussions, 9, 4229-4261, 2013 Author(s): M. Yamamoto, H. Sai, M.-T. Chen, and M. Zhao The response of Asian monsoon variability to orbital forcing is still unclear, and all hypotheses are controversial. We present a record of the sea surface temperature difference (ΔSST) between the South China Sea and the other Western Pacific Warm Pool regions as a proxy for the intensity of the Asian winter monsoon, because the winter cooling of the South China Sea is caused by the cooling of surface water at the northern margin and the southward advection of cooled water due to winter monsoon winds. The ΔSST showed significant precession cycles during the last 150 kyr. In the precession cycle, the maximum winter monsoon intensity shown by the ΔSST corresponded to the May perihelion and was delayed behind the maximum ice volume. The East Asian winter monsoon was anti-phase with the Indian summer monsoon and the summer monsoon precipitation in central Japan. The timing of the maximum phase of the East Asian winter monsoon was different from previous results in terms of the March perihelion (ice volume maxima) and June perihelion (minimum of Northern Hemisphere winter insolation). We infer that the variation of the East Asian winter monsoon was caused by a physical mechanism of inter-hemispheric heat balance. The East Asian winter monsoon was intensified by the Northern Hemisphere cooling, which was caused by the combined effect of cooling by the ice volume forcing and the decrease in winter insolation, or by decreased heat transfer from the Southern Hemisphere to the Northern Hemisphere owing to the weak Indian summer monsoon at the May perihelion.

Description: Trace elements and cathodoluminescence of detrital quartz in Arctic marine sediments – a new ice-rafted debris provenance proxy Climate of the Past Discussions, 9, 4145-4189, 2013 Author(s): A. Müller and J. Knies The records of ice-rafted debris (IRD) provenance in the North Atlantic – Barents Sea allow the reconstruction of the spatial and temporal changes of ice-flow drainage patterns during glacial and deglacial periods. In this study a new approach to characterisation of the provenance of detrital quartz grains in the fraction 〉 500 μm of marine sediments offshore of Spitsbergen is introduced, utilizing scanning electron microscope backscattered electron and cathodoluminescence (CL) imaging, combined with laser ablation inductively-coupled plasma mass spectrometry. Based on their micro-inclusions, CL and trace element characteristics the investigated IRD grains can be classified into five distinct populations. Three of the populations are indicative of potential IRD provenance provinces in the Storfjord area including Barentsøya and Egdeøya. The results imply that under modern (interglacial) conditions IRD deposition along the western Spitsbergen margin is mainly governed by the East Svalbard Current controlling the ice-drift pattern. The presence of detrital quartz from local provinces, however, indicates that variations in IRD supply from western Spitsbergen may be quantified as well. In this pilot study it is demonstrated that this new approach applied on Arctic continental margin sediments, bears a considerable potential for the definition of the sources of IRD and thus of spatial/temporal changes in ice-flow drainage patterns during glacial/interglacial cycles.

Description: NGRIP temperature reconstruction from 10 to 120 kyr b2k Climate of the Past Discussions, 9, 4099-4143, 2013 Author(s): P. Kindler, M. Guillevic, M. Baumgartner, J. Schwander, A. Landais, and M. Leuenberger In order to reconstruct Greenland NGRIP temperature, measurements of δ 15 N from the beginning of the Holocene to Dansgaard–Oeschger (DO) event 8 have been performed. Together with previously measured and mostly published δ 15 N data, we are now able to present for the first time a NGRIP temperature reconstruction for the whole last glacial period (beginning of the Holocene back to 120 kyr) including every DO event based on δ 15 N isotope measurements using a firn densification and heat diffusion model. The detected temperature rises at DO events range from 5 °C (DO 25) up to 16.5 °C (DO 11), ± 3 °C. To bring measured and modelled data into agreement, we had to reduce the accumulation rate given by the ss09sea06bm time scale in some periods significantly, especially during the last glacial maximum (LGM). A comparison between reconstructed temperature and δ 18 O ice data confirms that the isotopic composition of the stadial was strongly influenced by seasonality. We continuously calculated α (δ 18 O ice to temperature sensitivity) on a 10 kyr running time window. α variations show an anticorrelation with obliquity, in agreement with a simple Rayleigh distillation model, and moreover seem to be influenced by Northern Hemisphere ice sheet volume.

Description: The impact of early Holocene Arctic Shelf flooding on climate in an atmosphere–ocean–sea–ice model Climate of the Past Discussions, 9, 4191-4227, 2013 Author(s): M. Blaschek and H. Renssen Glacial terminations are characterized by a strong rise in sea level related to melting ice sheets. This rise in sea level is not uniform all over the world, because regional effects (uplift and subsidence of coastal zones) are superimposed on global trends. During the early Holocene the Siberian Shelf became flooded before 7.5 ka BP and the coastline reached modern-day high stands at 5 ka BP. This area is currently known as a sea–ice production area and contributes significantly to the sea–ice exported from the Arctic through the Fram Strait. This leads to the following hypothesis: during times of rising sea levels, shelves become flooded, increasing sea–ice production on these shelves, increasing sea–ice volume and export through Fram Strait and causing the sea–ice extent to advance in the Nordic Seas, yielding cooler and fresher sea surface conditions. We have tested this hypothesis in an ocean–sea–ice–atmosphere coupled model of intermediate complexity (LOVECLIM). Our results of an early Holocene Siberian Shelf flooding show that in our model the Northern Hemisphere sea–ice production is increased (15%) and that the Northern Hemisphere sea–ice extent increases (14%) contrary to our hypothesis with lower sea–ice export through Fram Strait (−15%). The reason of this unexpected behaviour has its origin in a weakened polar vortex, induced by the land-ocean changes due to the shelf flooding, and a resulting decrease of zonality in the Nordic Seas pressure regime. Hence the winter Greenland high and the Icelandic low strengthen, yielding stronger winds on both sides of the Nordic Seas. Increased winds along the East Greenland Current support local sea–ice production and transport towards the south, resulting in a wider sea–ice cover and a southward shift of convection areas. The overall strength of the Atlantic Meridional Overturning Circulation is reduced by 4% and the heat transport in the Atlantic basin by 7%, resulting in an annual cooling pattern over the Nordic Seas by up to −4 °C. We find that the flooding of the Siberian shelf as a result of an orbital induced warming, causing Northern Hemisphere ice sheets to melt and global sea level to rise, causes a Nordic Seas cooling feedback opposed to this warming.

Description: A brief history of ice core science over the last 50 yr Climate of the Past Discussions, 9, 3711-3767, 2013 Author(s): J. Jouzel For about 50 yr, ice cores have provided a wealth of information about past climatic and environmental changes. Ice cores from Greenland, Antarctica and other glaciers, now emcompass a variety of timescales. However, the longer time scales (e.g. at least back to the Last Glacial period) are covered by deep ice cores the number of which is still very limited, seven from Greenland, with only one providing an undisturbed record of a part of the Last Interglacial Period, and a dozen from Antarctica with the longest record covering the last 800 000 yr. This article aims to summarize this successful adventure initiated by a few pioneers and their teams and to review key scientific results in focusing on climate (in particular water isotopes) and climate related (e.g. greenhouse gases) reconstructions. Future research is well taken into account by the four projects defined by IPICS. However it remains a challenge to get an intact record of the Last Interglacial in Greenland and to extend the Antarctic record through the mid-Pleistocene transition, if possible back to 1.5 Myr.

Description: Past freeze and thaw cycling in the margin of the El'gygytgyn Crater deduced from a 141 m long permafrost record Climate of the Past Discussions, 9, 3993-4034, 2013 Author(s): G. Schwamborn, H. Meyer, L. Schirrmeister, and G. Fedorov Past permafrost thaw and freeze has destabilised the basin slopes of Lake El'gygytgyn in the northeastern Eurasian Arctic. This has probably promoted the release of mass movements from the lake edge to the deeper basin as known from frequently occurring turbidite layers in the lake sediment column. The continuous sediment record from the Arctic spans the last 3.6 Ma and for much of this time permafrost dynamics and lake level changes likely have played a crucial role for sediment delivery to the lake. Changes in the ground ice hydrochemical composition (pH, δ 18 O, δD, electrical conductivity, Na + , Mg 2+ , Ca 2+ , K + , HCO 3 − , Cl − , SO 4 − ) of a 141 m long permafrost record from the western crater plain are examined to reconstruct repeated freeze and thaw cycles at the lake edge. Stable water isotope and major ion records of ground ice in the permafrost reflect both a synsedimentary palaeo-precipitation signal preserved in the near-surface permafrost (0.0 m to 9.1 m core depth) and a postdepositional record of talik thawing and refreezing in deeper layers of the core (9.1 to 141.0 m core depth). The lake marginal permafrost dynamics were controlled by lake level changes that episodically flooded the surfaces and induced thaw in the underlying frozen ground. At least three cycles of freeze and thaw during marine isotope stage (MIS) 7, possibly MIS 5, and the Allerød (AD) are identified and the hydrochemical data point to a vertical and horizontal talik refreezing through time.

Description: 10 Be in last deglacial climate simulated by ECHAM5-HAM – Part 1: Climatological influences on 10 Be deposition Climate of the Past Discussions, 9, 3681-3709, 2013 Author(s): U. Heikkilä, S. J. Phipps, and A. M. Smith Reconstruction of solar irradiance has only been possible for the Holocene so far. During the last deglaciation two solar proxies ( 10 Be and 14 C) deviate strongly, both of them being influenced by climatic changes in a different way. This work addresses the climate influence on 10 Be deposition by means of ECHAM5-HAM atmospheric aerosol-climate model simulations, forced by sea surface temperatures and sea ice extent created by the coupled climate system model CSIRO Mk3L. Three time slice simulations were performed during the last deglaciation: 10 000 BP ("10k"), 11 000 BP ("11k") and 12 000 BP ("12k"), each 30 yr long. The same 10 Be production rate was used in each simulation to isolate the impact of climate on 10 Be deposition. The changes are found to follow roughly the reduction in the greenhouse gas concentrations within the simulations. The 10k and 11k simulations produce a surface cooling which is symmetrically amplified in the 12k simulation. The precipitation rate is only slightly reduced at high latitudes, but there is a northward shift in the polar jet in the Northern Hemisphere and the stratospheric westerly winds are significantly weakened. These changes occur where the sea ice change is largest in the deglaciation simulations. This leads to a longer residence time of 10 Be in the stratosphere by 30 (10k and 11k) to 80 (12k) days, heavily increasing the atmospheric concentrations. Furthermore the shift of westerlies in the troposphere leads to an increase of tropospheric 10 Be concentrations, especially at high latitudes. The contribution of dry deposition generally increases, but decreases where sea ice changes are largest. In total, the 10 Be deposition rate changes by no more than 20% at mid- to high latitudes, but by up to 50% in the tropics. We conclude that on "long" time scales (a year to a few years), climatic influences on 10 Be deposition remain small even though atmospheric concentrations can vary significantly. Averaged over a longer period all 10 Be produced has to be deposited by mass conservation. This dominates over any climatic influences on 10 Be deposition. Snow concentrations, however, do not follow mass conservation and can potentially be impacted more by climate due to precipitation changes. Quantifying the impact of deglacial climate modulation on 10 Be in terms of preserving the solar signal locally is analysed in an accompanying paper (Heikkilä et al., 2013).

Description: Greenland accumulation and its connection to the large-scale atmospheric circulation in ERA-Interim and paleo-climate simulations Climate of the Past Discussions, 9, 3825-3870, 2013 Author(s): N. Merz, C. C. Raible, H. Fischer, V. Varma, M. Prange, and T. F. Stocker Accumulation and aerosol chemistry records from Greenland ice cores offer the potential to reconstruct variability in Northern Hemisphere atmospheric circulation over the last millennia. However, an important prerequisite for a reconstruction is the stable relationship between local accumulation at the ice core site with the respective circulation pattern throughout the reconstruction period. We address this stability issue by using a comprehensive climate model and performing time-slice simulations for the present, the pre-industrial, the early Holocene and the last glacial maximum (LGM). The relationships between accumulation, precipitation and atmospheric circulation are investigated on on various time-scales. The analysis shows that the relationship between local accumulation on the Greenland ice sheet and the large-scale circulation undergoes a significant seasonal cycle. As the weights of the individual seasons change, annual mean accumulation variability is not necessarily related to the same atmospheric circulation patterns during the different climate states. Within a season, local Greenland accumulation variability is indeed linked to a consistent circulation pattern, which is observed for all studied climate periods, even for the LGM, however these circulation patterns are specific for different regions on the Greenland ice sheet. The simulated impact of orbital forcing and changes in the ice-sheet topography on accumulation exhibits strong spatial variability emphasizing that accumulation records from different ice core sites cannot be expected to look alike since they include a distinct local signature. Accumulation changes between different climate periods are dominated by changes in the amount of snowfall and are driven by both thermodynamic and dynamic factors. The thermodynamic impact determines the strength of the hydrological cycle, and warmer temperatures are generally accompanied by an increase in Greenland precipitation. Dynamical drivers of accumulation changes are the large-scale circulation and the local orography having a distinct influence on the local flow characteristic and hence the amount of precipitation deposited in any Greenland region.

Description: Multi-decadal to-century NAO-like marine climate oscillations across the Denmark Strait (~66° N) over the last 2000 cal yr BP Climate of the Past Discussions, 9, 3871-3917, 2013 Author(s): J. T. Andrews and A. E. Jennings In the area of Denmark Strait (~66° N) the two modes of the North Atlantic Oscillation (NAO) are expressed in changes of the northward flux of Atlantic Water and the southward advection of Polar Water in the East Iceland Current. Proxies from marine cores along an environmental gradient from extensive to little or no drift ice, capture low frequency NAO-like variations over the last 2000 cal yr BP. Key proxies are the weight% of calcite, a measure of surface water stratification and nutrient supply, the weight% of quartz, a measure of drift ice transport, and grain-size. Records from Nansen and Kangerlussuaq fjords show variable ice-rafted debris (IRD) records but have distinct mineralogy associated with differences in the fjord catchment bedrock. High-resolution detrended records from Kangerlussuaq Trough show abrupt, significant multi-decadal changes (72 and 56 yr for calcite, and 94 and 65 yr for quartz), and parallel the 2000 yr Arctic summer temperature reconstructions. The calcite minimum occurred ca. 1550 AD whereas the quartz maxima occurred 200 yr earlier. Changes in calcite wt% from N and SW Iceland show similar abrupt changes to those in Kangerlussuaq Trough with an abrupt calcite peak ~1320 AD. Quartz values increased at two N Iceland sites in the last 500 yr whereas values declined in the East Greenland site.

Description: Persistent millennial-scale link between Greenland climate and northern Pacific Oxygen Minimum Zone under interglacial conditions Climate of the Past Discussions, 9, 3919-3952, 2013 Author(s): O. Cartapanis, K. Tachikawa, O. E. Romero, and E. Bard The intensity and/or extent of the northeastern Pacific Oxygen Minimum Zone (OMZ) varied in-phase with the Northern Hemisphere high latitude climate on millennial timescales during the last glacial period, indicating the presence of atmospheric and oceanic connections under glacial conditions. While millennial variability was observed for both the Greenland ice core and the northern Atlantic during the last interglacial period, the relationship with the northeastern Pacific OMZ has not yet been observed under warm interglacial conditions. Here we present a~new geochemical dataset, spanning the past 120 kyr, for major components (terrigenous fraction, marine organic matter, biogenic opal, and carbonates) by X-ray fluorescence scanning alongside with biological productivity and redox sensitive trace element content (Mo, Ni, Cd) of sediment core MD02-2508 at 23° N, retrieved from the northern limit of the modern OMZ. We evidenced high biological productivity from opal based on Si/Ti, and from Cd/Al, and Ni/Al ratios during last interglacial period. Highly resolved opal reconstruction presents millennial variability corresponding to all the Dansgaard–Oeschger interstadial events over the last interglacial, while the Mo/Al ratio indicates reduced oxygenation during these intervals. Extremely high opal content during warm interstadials suggests high diatom productivity. Despite the different climatic and oceanic background between glacial and interglacial periods, rapid variability in the northeastern Pacific OMZ seems to be tightly related to Northern Hemisphere high latitude climate mainly via atmospheric processes.

Description: Investigating the consistency between proxies and between proxies and models using data assimilation: a mid-Holocene case study Climate of the Past Discussions, 9, 3953-3991, 2013 Author(s): A. Mairesse, H. Goosse, P. Mathiot, H. Wanner, and S. Dubinkina The mid-Holocene (6 thousand years before present) is a key period to study the consistency between model results and proxy data as it corresponds to a standard test for models and a reasonable number of proxy records are available. Taking advantage of this relatively large amount of information, we have first compared a compilation of 50 air and sea surface temperature reconstructions with the results of three simulations performed with general circulation models and one carried out with LOVECLIM, a model of intermediate complexity. The conclusions derived from this analysis confirm that models and data agree on the large-scale spatial pattern but underestimate the magnitude of some observed changes and that large discrepancies are observed at the local scale. To further investigate the origin of those inconsistencies, we have constrained LOVECLIM to follow the signal recorded by the proxies selected in the compilation using a data assimilation method based on a particle filter. In one simulation, all the 50 proxies are used while in the other two, only the continental or oceanic proxies constrains the model results. This assimilation improves the consistency between model results and the reconstructions. In particular, this is achieved in a robust way in all the experiments through a strengthening of the westerlies at mid-latitude that warms up the Northern Europe. Furthermore, the comparison of the LOVECLIM simulations with and without data assimilation has also objectively identified 16 proxies whose reconstructed signal is either incompatible with the one recorded by some other proxies or with model physics.

Description: Biogeochemical properties and diagenetic changes during the past 3.6 Ma recorded by FTIR spectroscopy in the sediment record of Lake El'gygytgyn, Far East Russian Arctic Climate of the Past Discussions, 9, 2489-2515, 2013 Author(s): C. Meyer-Jacob, H. Vogel, M. Melles, and P. Rosén A number of studies have shown that Fourier transform infrared spectroscopy (FTIRS) can be applied to quantitatively assess lacustrine sediment constituents. In this study, we developed calibration models based on FTIRS for the quantitative determination of biogenic silica (BSi; n = 420; gradient: 0.9–56.5%), total organic carbon (TOC; n = 309; gradient: 0.02–2.89%), and total inorganic carbon (TIC; n = 153; gradient: 0.01–1.46%) in a 318 m long sediment record with a basal age of 3.6 Ma from Lake El'gygytgyn, Far East Russian Arctic. The developed partial least squares regression (PLSR) models yield high cross-validated (CV) R 2 CV = 0.85–0.91 and low root mean square error of cross-validation (RMSECV) (2.1–4.3% of the gradient for the different properties). The FTIRS-inferred concentrations of BSi, TOC, and TIC provide an initial insight into the climatic and environmental evolution at Lake El'gygytgyn throughout the late Pliocene and Quaternary showing a considerably high bioproductivity in the lake ecosystem between ~ 3.27–3.54 Ma during the early Pliocene warm period. Moreover, we found that the recorded FTIR spectra contain information on sample burial depth as a result of diagenetic changes (dehydration/dehydroxilation) of certain mineral phases. Despite the indicated post-depositional processes, the calibration models yield good statistical performances showing that general FTIRS models can be developed for several hundred meters long records extending several million years back in time. Our results highlight FTIRS to be a rapid, cost-effective alternative to conventional methods for quantification of biogeochemical properties.

Description: High-resolution glacial and deglacial record of atmospheric methane by continuous-flow and laser spectrometer analysis along the NEEM ice core Climate of the Past Discussions, 9, 2517-2556, 2013 Author(s): J. Chappellaz, C. Stowasser, T. Blunier, D. Baslev-Clausen, E.J. Brook, R. Dallmayr, X. Faïn, J.E. Lee, L.E. Mitchell, O. Pascual, D. Romanini, J. Rosen, and S. Schüpbach The Greenland NEEM (North Greenland Eemian Ice Drilling) operation in 2010 provided the first opportunity to combine trace-gas measurements by laser spectroscopic instruments and continuous-flow analysis along a freshly drilled ice core in a field based setting. We present the resulting atmospheric methane (CH 4 ) record covering the time period from 107.7 to 9.5 ka b2k (thousand years before 2000 AD). Companion discrete CH 4 measurements are required to transfer the laser spectroscopic data from a relative to an absolute scale. However, even on a relative scale, the high-resolution CH 4 dataset significantly improves our knowledge of past atmospheric methane concentration changes. New significant sub-millennial-scale features appear during interstadials and stadials, generally associated with similar changes in water isotopic ratios of the ice, a proxy for local temperature. In addition to the mid-point of Dansgaard/Oeschger (D/O) CH 4 transitions usually used for cross-dating, sharp definition of the start and end of these events brings precise depth markers (with ±20 cm uncertainty) for further cross-dating with other ice core or paleo records, e.g. speleothems. The method also provides an estimate of CH 4 rates of change. The onsets of D/O events in the methane signal show a more rapid rate of change than their endings. The rate of CH 4 increase associated with the onsets of D/O events progressively declines from 1.7 to 0.6 ppbv yr −1 in the course of Marine Isotope Stage 3. The largest observed rate of increase takes place at the onset of D/O event #21 and reaches 2.5 ppbv yr −1 .

Description: Evaluating climate field reconstruction techniques using improved emulations of real-world conditions Climate of the Past Discussions, 9, 3015-3060, 2013 Author(s): J. Wang, J. Emile-Geay, D. Guillot, J. E. Smerdon, and B. Rajaratnam Pseudoproxy experiments (PPEs) have become an essential framework for evaluating paleoclimate reconstruction methods. Most existing PPE studies assume constant proxy availability through time and uniform proxy quality across the pseudoproxy network. Real multi-proxy networks are, however, marked by pronounced disparities in proxy quality, and a steep decline in proxy availability back in time, either of which may have large effects on reconstruction skill. Additionally, an investigation of a real-world global multi-proxy network suggests that proxies are not exclusively indicators of local climate; rather, many are indicative of large-scale teleconnections. A suite of PPEs constructed from a millennium-length general circulation model simulation is thus designed to mimic these various real-world characteristics. The new pseudoproxy network is used to evaluate four climate field reconstruction (CFR) techniques: truncated total least square embedded within the regularized EM algorithm (RegEM-TTLS), the Mann et al. (2009) implementation of RegEM-TTLS (M09), canonical correlation analysis (CCA), and Gaussian graphical models embedded within RegEM (GraphEM). Each method's risk properties are also assessed via a 100-member noise ensemble. Contrary to expectation, it is found that reconstruction skill does not vary monotonically with proxy availability, but rather is a function of the type of climate variability (forced events vs. internal variability). The use of realistic spatiotemporal pseudoproxy characteristics also exposes large inter-method differences. Despite the comparable fidelity in reconstructing the global mean temperature, spatial skill varies considerably between CFR techniques. Both GraphEM and CCA efficiently exploit teleconnections, and produce consistent reconstructions across the ensemble. RegEM-TTLS and M09 appear advantageous for reconstructions on highly noisy data, but are subject to larger stochastic variations across different realizations of pseudoproxy noise. Results collectively highlight the importance of designing realistic pseudoproxy networks and implementing multiple noise realizations of PPEs. The results also underscore the difficulty in finding the proper bias-variance tradeoff for jointly optimizing the spatial skill of CFRs and the fidelity of the global mean reconstructions.

Description: On the low frequency component of the ENSO-Indian Monsoon relationship; a paired proxy perspective Climate of the Past Discussions, 9, 3103-3123, 2013 Author(s): M. Berkelhammer, A. Sinha, M. Mudelsee, H. Cheng, K. Yoshimura, and J. Biswas There are a number of clear examples in the instrumental period where positive El Niño events were coincident with a severely weakened summer monsoon over India (ISM). ENSO's influence on the Indian Monsoon has therefore remained the centerpiece of various predictive schemes of ISM rainfall for over a century. The teleconnection between the monsoon and ENSO has undergone a protracted weakening since the late 1980's suggesting the strength of ENSO's influence on the monsoon may vary considerably on multidecadal timescales. The recent weakening has specifically prompted questions as to whether this shift represents a natural mode of climate variability or a fundamental change in ENSO and/or ISM dynamics due to anthropogenic warming. The brevity of empirical observations and large systematic errors in the representation of these two systems in state-of-the-art general circulation models hamper efforts to reliably assess the low frequency nature of this dynamical coupling under varying climate forcings. Here we place the 20th century ENSO-Monsoon relationship in a millennial context by assessing the phase angle between the two systems across the time spectrum using a continuous tree-ring ENSO reconstruction from North America and a speleothem oxygen isotope (δ 18 O) based reconstruction of the ISM. The results suggest that in the high-frequency domain (≤ 15 yr), El Niño (La Niña) events persistently lead to a weakened (strengthened) monsoon consistent with the observed relationship between the two systems during the instrumental period. However, in the low frequency domain (≥ 60 yr), periods of strong monsoon are, in general, coincident with periods of enhanced ENSO variance. This relationship is opposite to which would be predicted dynamically and leads us to conclude that ENSO is not pacing the prominent multidecadal variability that has characterized the ISM over the last millennium.

Description: Paleo Agulhas rings enter the subtropical gyre during the penultimate deglaciation Climate of the Past Discussions, 9, 2095-2114, 2013 Author(s): P. Scussolini and E. van Sebille A maximum in the strength of Agulhas Leakage has been registered at the interface between Indian and South Atlantic oceans during glacial Termination II (T II), presumably transporting the salt and heat necessary to maintain the Atlantic Meridional Overturning Circulation (AMOC) at rates similar to the present day. However, it was never shown whether these were effectively incorporated in the South Atlantic gyre, or whether they retroflected into the Indian and/or Southern Oceans. To solve this question, we investigate the presence of paleo Agulhas rings from a sediment core on the central Walvis Ridge, almost 1800 km farther into the Atlantic basin than previously studied. Analysis of a 20 yr dataset from a global ocean circulation model allows us to relate density perturbations, at the depth of the thermocline, to the passage of individual rings over the core site. Using this relation from the numerical model as the basis for a proxy, we generate a time series of δ 18 O variability of Globorotalia truncatulinoides single specimens, revealing high levels of pycnocline depth variability at the site, suggesting enhanced numbers of Agulhas rings moving into the South Atlantic gyre around and before T II. Our record closely follows the published quantifications of Agulhas Leakage from the east of the Cape Basin, and thus shows that Indian Ocean waters entered the South Atlantic circulation. This provides crucial support to the view of a prominent role of the Agulhas Leakage in the shift from a glacial to an interglacial mode of AMOC.

Description: A high resolution record of atmospheric carbon dioxide and its stable carbon isotopic composition from the penultimate glacial maximum to the glacial inception Climate of the Past Discussions, 9, 2015-2057, 2013 Author(s): R. Schneider, J. Schmitt, P. Köhler, F. Joos, and H. Fischer The reconstruction of the stable carbon isotope evolution in atmospheric CO 2 (δ 13 C atm ), as archived in Antarctic ice cores, bears the potential to disentangle the contributions of the different carbon cycle fluxes causing past CO 2 variations. Here we present a highly resolved record of δ 13 C atm before, during and after the Marine Isotope Stage 5.5 (155 000 to 105 000 yr BP). The record was derived with a well established sublimation method using ice from the EPICA Dome C (EDC) and the Talos Dome ice cores in East Antarctica. We find an 0.4‰ offset between the mean δ 13 C atm level in the Penultimate (~140 000 yr BP) and Last Glacial Maximum (~22 000 yr BP), which can be explained by either (i) changes in the isotopic composition or (ii) intensity of the carbon input fluxes to the combined ocean/atmosphere carbon reservoir or (iii) by long-term peat buildup. Our isotopic data suggest that the carbon cycle evolution along Termination II and the subsequent interglacial was controlled by essentially the same processes as during the last 24 000 yr, but with different phasing and magnitudes. Furthermore, a 5000 yr lag in the CO 2 decline relative to EDC temperatures is confirmed during the glacial inception at the end of MIS 5.5 (120 000 yr BP). Based on our isotopic data this lag can be explained by terrestrial carbon release and carbonate compensation.

Description: A comparative study of large scale atmospheric circulation in the context of future scenario (RCP4.5) and past warmth (Mid Pliocene) Climate of the Past Discussions, 9, 1449-1483, 2013 Author(s): Y. Sun, G. Ramstein, C. Contoux, and T. Zhou The Pliocene climate (3.3 ~ 3.0 Ma) is often considered as the last sustained warm period with close enough geographic configurations compared to the present one and associated with atmospheric CO 2 concentration (405 ± 50 ppm) higher than the modern level. It is therefore suggested that the warm Pliocene climate may provide a plausible scenario for the future climate warming with the important advantage, that for mid-Pliocene, many marine and continental data are available. To investigate this issue, we selected RCP4.5 scenario, one of the current available future projections, to compare the pattern of tropical atmospheric response with past warm mid-Pliocene climate. We performed three OAGCM simulations (RCP4.5 scenario, mid-Pliocene and present day simulation) with the IPSL-CM5A model and investigated atmospheric tropical dynamics through Hadley and Walker cell responses to warmer conditions. Our results show that there is a damping of the Hadley cell intensity in the northern tropics and an increase in both subtropics. Moreover, northern and southern Hadley cells expand poleward. The response of Hadley cell is stronger for RCP4.5 scenario than for mid-Pliocene, but in very good agreement with the fact the atmospheric CO 2 concentration is higher in future scenario than mid-Pliocene (543 versus 405 ppm). Concerning the response of the Walker cell, we showed that, despite very large similarities, there are also some differences. i.e. the common features are for both scenarios: weakening of the ascending branch, leading to a suppression of the precipitation over the western tropical Pacific. The response of Walker cell is stronger in RCP4.5 scenario than mid-Pliocene but also depicts some major difference as an eastward shift of the rising branch of Walker cell in future scenario compared to the mid-Pliocene. In this paper, we explain the dynamics of the Hadley and Walker cell, and show that despite minor discrepancy, mid-Pliocene is certainly an interesting analogue for future climate changes in the tropical areas.

Description: Re-evaluation of the age model for North Atlantic Ocean Site 982 – arguments for a return to the original chronology Climate of the Past Discussions, 9, 2217-2233, 2013 Author(s): K. T. Lawrence, I. Bailey, and M. E. Raymo Recently, the veracity of the published chronology for the Pliocene section of North Atlantic Ocean Drilling Program Site 982 was called into question. Here, we examine the robustness of the original age model as well as the proposed age model revision. The proposed revision is predicated on an apparent mis-identification of the depth to the Gauss-Matuyama (G/M) polarity chronozone reversal boundary (2.581 Ma) based on preliminary shipboard paleomagnetic data and offers a new chronology which includes a hiatus between ~ 3.2 and 3 Ma. However, an even more accurate shore-based, u-channel-derived polarity chronozone stratigraphy for the past ~ 2.7 Ma supports the shipboard composite stratigraphy and demonstrates that the original estimate of the depth of the G/M reversal in the Site 982 record is correct. Thus, the main justification forwarded to support the revised chronology is not valid. We demonstrate that the proposed revision results in a pronounced anomaly in sedimentation rates proximal to the proposed hiatus, erroneous assignment of marine-isotope stages in the Site 982 Pliocene benthic stable oxygen isotope stratigraphy, and a markedly worse correlation of proxy records between this site and other regional paleoclimate data. We conclude that the original chronology for Site 982 is a far more accurate age-model than that which arises from the published revision. We strongly recommend the use of the original chronology for all future work at Site 982.

Description: Major dust events in Europe during marine isotope stage 5 (130–74 ka): a climatic interpretation of the "markers" Climate of the Past Discussions, 9, 2235-2276, 2013 Author(s): D.-D. Rousseau, M. Ghil, G. Kukla, A. Sima, P. Antoine, M. Fuchs, C. Hatté, F. Lagroix, M. Debret, and O. Moine At present, major dust storms are occurring at mid-latitudes in the Middle East and Asia, as well as at low latitudes in northern Africa and in Australia. Western Europe, though, does not experience such dramatic climate events, except for some African dust reaching it from the Sahara. This modern situation is of particular interest, in the context of future climate projections, since the present interglacial is usually interpreted, in this context, as an analog of the warm Eemian interval. European terrestrial records show, however, major dust events during the penultimate interglacial and early glacial. These events are easily observed in loess records by their whitish-color deposits, which lie above and below dark chernozem paleosols in Central European records of Marine Isotope Stage (MIS) 5 age. We describe here the base of the Dolni Vestonice (DV) loess sequence, Czech Republic, as the reference of such records. The dust is deposited during intervals that are characterized by poor vegetation – manifested by high δ 13 C values and low magnetic susceptibility – while the fine sand and clay in the deposits shows grain sizes that are clearly different from the overlying pleniglacial loess deposits. Some of these dust events have been previously described as "Markers" or Marker Silts (MS) by one of us (G. Kukla), and are dated at about 111–109 and 93–92 ka, with a third and last one slightly visible at about 75–73 ka. Other events correspond to the loess material of Kukla's cycles, and are described as eolian silts (ES); they are observed in the same DV sequence and are dated at about 106–105, 88–86, and 78.5–77 ka. The fine eolian deposits mentioned above, MS as well as ES, correspond to short events that lasted about 2 ka; they are synchronous with re-advances of the polar front over the North Atlantic, as observed in marine sediment cores. These deposits also correlate with important changes observed in European vegetation. Some ES and MS events appear to be coeval with significant dust peaks recorded in the Greenland ice cores, while others are not. This decoupling between the European eolian and Greenland dust depositions is of considerable interest, as it differs from the fully glacial situation, in which the Eurasian loess sedimentation mimics the Greenland dust record. Previous field observations supported an interpretation of MS events as caused by continental dust storms. We show here, by a comparison with speleothems of the same age found in the northern Alps, that different atmospheric-circulation modes seem to be responsible for the two categories of dust events, MS vs. ES.

Description: On the effect of orbital forcing on mid-Pliocene climate, vegetation and ice sheets Climate of the Past Discussions, 9, 1703-1734, 2013 Author(s): M. Willeit, A. Ganopolski, and G. Feulner We present results from modeling of the mid-Pliocene warm period (3.3–3 million years ago) using the Earth system model of intermediate complexity CLIMBER-2 analyzing the effect of changes in boundary conditions as well as of orbital forcing on climate. Firstly we performed equilibrium experiments following PlioMIP (Pliocene Model Intercomparison Project) protocol with a CO 2 concentration of 405 ppm, reconstructed mid-Pliocene orography and vegetation and a present day orbital configuration. Simulated global Pliocene warming is about 2.5 °C, fully consistent with results of atmosphere-ocean general circulation model simulations performed for the same modeling setup. A factor separation analysis attributes 1.5 °C warming to CO 2 , 0.3 °C to orography, 0.2 °C to ice sheets and 0.4 °C to vegetation. Transient simulations for the entire mid-Pliocene warm period with time-dependent orbital forcing as well as interactive ice sheets and vegetation give a global warming varying within the range 1.9–2.8 °C. Ice sheet and vegetation feedbacks in synergy act as amplifiers of the orbital forcing transforming seasonal insolation variations into an annual mean temperature signal. The effect of orbital forcing is more significant at high latitudes, especially during summer, when the warming over land varies in the wide range from 0–10 °C. The modeled ice sheet extent and vegetation distribution also show significant temporal variations. Modeled and reconstructed data for Northern Hemisphere sea surface temperatures and vegetation distribution show the best agreement if the reconstructions are assumed to be representative for the "warmest" periods during the orbital cycles. This suggests that low-resolution Pliocene paleoclimate reconstructions can reflect not only the impact of increased CO 2 concentrations and topography changes but also the effect of orbital forcing. Therefore, the climate (Earth system) sensitivity estimates from Pliocene reconstructions which do not account for the effect of orbital forcing can be biased toward high values.

Description: A 1500 yr warm-season temperature record from varved Lago Plomo, Northern Patagonia (47° S) and implications for the Pacific Decadal Oscillation (PDO) Climate of the Past Discussions, 9, 1771-1801, 2013 Author(s): J. Elbert, M. Jacques-Coper, M. Van Daele, R. Urrutia, and M. Grosjean High-resolution records of calibrated proxy data for the past 2000 yr are fundamental to place current changes into the context of pre-industrial natural forced and unforced variability. Although the need for regional spatially explicit comprehensive reconstructions is widely recognized, the proxy data sources are still scarce, particularly for the Southern Hemisphere and South America. We provide a 1500 yr long warm season temperature record from varved Lago Plomo, a proglacial lake of the Northern Patagonian Ice field in southern Chile (46°59' S, 72°52' W, 203 m). The thickness of the bright summer sediment layer relative to the dark winter layer (measured as total brightness; % reflectance 400–730 nm) is calibrated against warm season SONDJF temperature (1900–2009; r = 0.58, p (aut) = 0.056, RE = 0.52; CE = 0.15, RMSEP = 0.28 °C; five-year triangular filtered data). In Lago Plomo, warm summer temperatures lead to enhanced glacier melt and suspended sediment transport, which results in a thicker light summer layer and to brighter sediments (% total brightness). Although Patagonia shows pronounced regional differences in decadal temperature trends and variability, the 1500 yr temperature reconstruction from Lago Plomo compares favourably with other regional/continental temperature records but also emphasizes significant regional differences for which no data and information existed so far. The reconstruction shows pronounced sub-decadal–multi-decadal variability with cold phases in the 5th, 7th and 9th centuries, during parts of the Little Ice Age chronozone (16th and 18th centuries) and in the beginning of the 20th century. The most prominent warm phase is the 19th century which is as warm as the second half of the 20th century, emphasizing a delayed recent global warming in the Southern Hemisphere. The comparison between winter precipitation and summer temperature (inter-seasonal coupling) from Lago Plomo reveals alternating phases with parallel and contrasting decadal trends of winter precipitation and summer temperature and positive and negative running correlations (winterPP;summerTT) . In the 20th century the trend of this correlation changes at 1920, 1945 and 1975 AD, and the phases with positive (negative) correlations inferred from the lake sediments are also found as a regional robust pattern in reanalysis data, and coincide with the changes of the instrumental PDO index. Enhanced circumpolar flow around 60° S is proposed for positive phases of PDO which leads to the reversed coupling and contrasting decadal trends of winter precipitation and summer temperature during PDO positive phases. Our reconstruction of the inter-seasonal coupling back to 1530 AD reproduces many features of existing PDO reconstructions from the Pacific suggesting that Lago Plomo provides a record for the regional expression of the PDO in Patagonia.

Description: Coupled regional climate–ice sheet simulation shows limited Greenland ice loss during the Eemian Climate of the Past Discussions, 9, 1735-1770, 2013 Author(s): M. M. Helsen, W. J. van de Berg, R. S. W. van de Wal, M. R. van den Broeke, and J. Oerlemans During the last interglacial (Eemian, 130–115 kyr BP) eustatic global sea level likely peaked at 〉6 m above the present-day level, but estimates of the contribution of the Greenland ice sheet vary widely. Here we use an asynchronously two-way coupled regional climate–ice sheet model, which includes physically realistic feedbacks between the changing ice sheet topography and climate forcing. Our simulation results in a contribution from the Greenland ice sheet to the Eemian sea level highstand between 1.2 and 3.5 m, with a most likely value of 2.1 m. Simulated Eemian ice loss in Greenland is dominated by the rapid retreat of the southwestern margin: two-thirds of the ice loss occurred south of 70° N. The southern dome survived the Eemian and remained connected to the central dome. Large-scale ice sheet retreat is prevented in areas with high accumulation. Our results broadly agree with ice core inferred elevation changes and marine records, but it does not match with the ice-core derived temperature record from northern Greenland. During maximum Eemian summertime insolation, Greenland mass loss contributed ~0.5 m kyr −1 to sea level rise, 24% of the reconstructed total rate of sea level rise. Next to that, a difference of 〉3 m remains between our maximum estimate of the Greenland contribution and the reconstructed minimum value of the global eustatic Eemian highstand. Hence, the Antarctic ice sheet must also have contributed significantly to this sea level highstand.

Description: Impact of the Megalake Chad on climate and vegetation during the late Pliocene and the mid-Holocene Climate of the Past Discussions, 9, 1363-1396, 2013 Author(s): C. Contoux, A. Jost, G. Ramstein, P. Sepulchre, G. Krinner, and M. Schuster Given the growing evidence for megalakes in the geological record, assessing their impact on climate and vegetation is important for the validation of paleoclimate simulations and therefore the accuracy of model/data comparison in lacustrine environments. Megalake Chad (MLC) occurrences are documented for the mid-Holocene but also for the Mio-Pliocene (Schuster et al., 2009). The surface covered by water would have reached up to ~350 000 km 2 (Ghienne et al., 2002; Schuster et al., 2005; Leblanc et al., 2006) making it an important evaporation source, possibly modifying the climate and vegetation in the Chad basin. We investigated the impact of such a giant continental water area in two different climatic backgrounds within the Paleoclimate Model Intercomparison Project phase 3 (PMIP3): the late Pliocene (3.3 to 3 Ma, i.e. the mid-Piacenzian warm period) and the mid-Holocene (6 kyr BP). In all simulations including a MLC, precipitation is drastically reduced above the lake surface because deep convection is inhibited by colder air above the lake surface. Meanwhile, convective activity is enhanced around the MLC, because of the wind increase generated by the flat surface of the megalake, transporting colder and moister air towards the eastern shore of the lake. Effect of the MLC on precipitation and temperature is not sufficient to widely impact vegetation patterns. Nevertheless, tropical savanna is present in the Chad Basin in all climatic configurations, even without the MLC presence, showing that the climate itself is the driver of favourable environments for sustainable hominid habitats.

Description: Climate variability since MIS 5 in SW Balkans inferred from multiproxy analysis of Lake Prespa sediments Climate of the Past Discussions, 9, 1321-1362, 2013 Author(s): K. Panagiotopoulos, A. Böhm, M. J. Leng, B. Wagner, and F. Schäbitz The transboundary Lake Prespa (AL/FYROM/GR) has been recognized as a conservation priority wetland. The catchment area has a remarkably diverse flora that points to its refugial properties. A lake sediment core retrieved from a coring location in the northern part of the lake was investigated through geophysical, sedimentological, geochemical, and palynological analyses. Based on tephrochronology, radiocarbon and electron spin resonance (ESR) dating, and cross correlation with other Northern Hemisphere records, the age model suggests that the basal part of core Co1215 reaches back to 92 ka cal BP. Here we present the response of this mid-altitude site (849 m a.s.l.) to climate oscillations during this interval and assess its sensitivity to millennial-scale variability. Endogenic calcite precipitation occurred in Marine Isotope Stages (MIS) 5 and 1 and is synchronous with periods of increased primary production (terrestrial and/or lacustrine). Periods of pronounced phytoplankton blooms (inferred from green algae and dinoflagellate concentrations) are recorded in MIS 5 and MIS 1 and suggest that the trophic state and lake levels underwent substantial fluctuations. Three major phases of vegetation development are distinguished: the forested phases of MIS 5 and MIS 1 dominated by deciduous trees with higher temperatures and moisture availability, the open landscapes of MIS 3 with significant presence of temperate trees, and the pine dominated open landscapes of MIS 4 and MIS 2 with lower temperatures and moisture availability. Forest dynamics, cover and density are discussed in an altitudinal context and the existence of temperate tree refugia is examined.

Description: Siple Dome shallow ice cores: a study in coastal dome microclimatology Climate of the Past Discussions, 9, 2681-2715, 2013 Author(s): T. R. Jones, J. W. C. White, and T. Popp Ice cores at Siple Dome, West Antarctic receive the majority of their precipitation from Pacific Ocean moisture sources. Pacific climate patterns, particularly in response to the El Niño-Southern Oscillation, affect local temperature, atmospheric circulation, snow accumulation, and water isotope signals at Siple Dome. We examined borehole temperatures, accumulation, and water isotopes from a number of shallow ice cores recovered from a 60 km north–south transect of the Dome. The data (with coverage from 1920–1995) reveal a microclimate heavily influenced by ENSO and the location of the Amundsen Sea Low Pressure Area. The Dome Summit and Pacific Flank respond to La Niña conditions by warming, increased isotope ratios, higher deuterium excess, and increased snowfall. The Inland Flank responds to El Niño conditions and cold interior air masses by cooling, decreased isotope ratios, lower deuterium excess, and decreased snowfall. ENSO-type spectral signatures (2–7 yr) are present in all water isotope records, but are not similar in their power structures. A longer 300 yr wavelet analysis record from the Dome Summit shows a late 19th-century climate shift similar to that seen in South Pacific coral isotope records. Our analyses suggest that while an ENSO signal is evident at Siple Dome, the microclimate effect makes climate reconstruction problematic, a conclusion which should be considered at other West Antarctic coastal dome locations.

Description: Using ice-flow models to evaluate potential sites of million year-old ice in Antarctica Climate of the Past Discussions, 9, 2859-2887, 2013 Author(s): B. Van Liefferinge and F. Pattyn Finding suitable potential sites for an undisturbed record of million-year old ice in Antarctica requires a slow-moving ice sheet (preferably an ice divide) and basal conditions that are not disturbed by large topographic variations. Furthermore, ice should be thick and cold basal conditions should prevail, since basal melting would destroy the bottom layers. However, thick ice (needed to resolve the signal at sufficient high resolution) increases basal temperatures, which is a conflicting condition in view of finding a suitable drill site. In addition, slow moving areas in the center of ice sheets are also low-accumulation areas, and low accumulation reduces potential cooling of the ice through vertical advection. While boundary conditions such as ice thickness and accumulation rates are relatively well constraint, the major uncertainty in determining basal conditions resides in the geothermal heat flow (GHF) underneath the ice sheet. We explore uncertainties in existing GHF datasets and their effect on basal temperatures of the Antarctic ice sheet and propose an updated method based on Pattyn (2010) to improve existing GHF datasets in agreement with known basal temperatures and their gradients to reduce this uncertainty. Both complementary methods lead to a better comprehension of basal temperature sensitivity and a characterization of potential ice coring sites within these uncertainties.

Description: Mid and late Holocene dust deposition in western Europe: the Misten peat bog (Hautes Fagnes – Belgium) Climate of the Past Discussions, 9, 2889-2928, 2013 Author(s): M. Allan, G. Le Roux, N. Piotrowska, J. Beghin, E. Javaux, M. Court-Picon, N. Mattielli, S. Verheyden, and N. Fagel Dust deposition in southern Belgium is estimated from the geochemical signature of an ombrotrophic peatland. The Rare Earth Elements (REE) and lithogenic elements concentrations, as well as Nd isotopes, were determined by HR-ICP-MS and MC-ICP-MS respectively, in along a ~6 m peat section covering 5300 yr, from 30 BC to 5300 BC dated by the 14 C method. Changes in REE concentration in the peat correlate with those of Ti, Al, Sc and Zr that are lithogenic conservative elements, suggesting that REE are immobile in the studied peat bogs and can be used as tracers of dust deposition. Peat humification and testate amoebae were used to evaluate hydroclimatic conditions. The range of dust deposition varied from 0.03 to 4.0 g m −2 yr −1 . The highest dust fluxes were observed from 800 to 600 BC and from 3200 to 2800 BC and correspond to cold periods. The εNd values show a large variability of −5 to −13, identifying three major sources of dusts: local soils, distal volcanic and desert particles.

Description: Pre-LGM Northern Hemisphere paleo-ice sheet topography Climate of the Past Discussions, 9, 2557-2587, 2013 Author(s): J. Kleman, J. Fastook, K. Ebert, J. Nilsson, and R. Caballero We here reconstruct the paleotopgraphy of Northern Hemisphere ice sheets during the glacial maxima of marine isotope stages (MIS) 5b and 4. We employ two approaches, geologically based reconstruction and numerical modeling, in mutually supportive roles to arrive at probable ice sheet extents and topographies for each of these two time slices. For a physically based 3-D calculation based on geologically derived 2-D constraints, we use the University of Maine Ice Sheet Model (UMISM) to calculate ice-sheet thickness and topography. The approach and ice-sheet modeling strategy is designed to provide robust data sets of sufficient resolution for atmospheric circulation experiments for these previously elusive time periods. Two tunable parameters, a temperature scaling function applied to a spliced Vostok-GRIP record, and spatial adjustment of climatic pole position, were employed iteratively to achieve a good fit to geological constraints where such were available. The model credibly reproduces the first-order pattern of size and location of geologically indicated ice sheets during marine isotope stages (MIS) 5b (86.2 kyr model age) and 4 (64 kyr model age). From the interglacial state of two north-south obstacles to atmospheric circulation (Rocky Mountains and Greenland), by MIS 5b combined Quebec-Central Arctic and Scandinavian–Barents/Kara ice sheets had effectively increased the number of such highland obstacles to four. This number remained constant through MIS 4, but at the last glacial maximum (LGM) dropped to three, through the merging of the Cordilleran and the proto-Laurentide Ice Sheet to a single continent-wide North American ice sheet.

Description: Geochronological reconsiderations for the Eastern European key loess section at Stayky in Ukraine Climate of the Past Discussions, 9, 2629-2659, 2013 Author(s): A. Kadereit and G. A. Wagner Event-stratigraphical correlations between local/regional terrestrial sedimentary archives and marine or ice-core records providing the global climate history and time-scale are highly desirable for a deeper understanding of the effects of global climate change on a local/regional (palaeo-)environment. However, such correlations are not trivial, as the terrestrial records tend to be floating and fragmentary and usually show varying sedimentation rates. Therefore, a reliable chronometric framework is a necessary prerequisite for any event-stratigraphy involving terrestrial archives. In this respect, the age-model underlying the event-stratigraphical approach for the Eastern European key loess section at Stayky in Ukraine appears to need revision. Here we explain, why it is highly unlikely that the Middle Pleniglacial Vytachiv Soil developed during Greenland interstadial (GIS) 8, and why the embryonic soils in the upper part of the Upper Pleniglacial part of the loess section most likely post-date Heinrich 2 event. As a consequence, the revised age-model challenges the earlier suggested correlation of the suite of incipient soils above the Vytachiv Soil with Greenland Interstadials, which was supposed to start with GIS7 but for which matching from after GIS5 seems more likely. The revised chronology suggests that the transition from Middle to Upper Pleniglacial environmental conditions at the Eastern European key section occurred during the final phase of marine isotope stage (MIS) 3. Thus, the picture appears to be in accordance with that of the Western European key section at Nussloch in Germany pointing to a common driver of palaeo-environmental change in both regions, such as early Late Glacial Maximum (LGM) advances of the Arctic ice-shield or changes of the North Atlantic circulation and sea-ice distribution leading also to relevant changes of the palaeowind field.

Description: A high-resolution multi-proxy record of late Cenozoic environment change from central Taklimakan Desert, China Climate of the Past Discussions, 9, 2661-2680, 2013 Author(s): X. Wang, D. H. Sun, F. Wang, B. F. Li, S. Wu, F. Guo, Z. J. Li, Y. B. Zhang, and F. H. Chen The Taklimakan Desert in the Tarim Basin is the largest desert in Central Asia, and is regarded as one of the main dust sources to the Northern Hemisphere. Late Cenozoic sedimentary sequences with intercalated in-situ aeolian dune sands in this area preserve direct evidence for the Asian desertification. Herein, we report a high-resolution multi-proxy climatic record from the precise magnetostratigraphic dated Hongbaishan section in the central Taklimakan Desert. Our results show that a fundamental climate change, characterized by significant cooling, enhanced aridity, and intensified atmospheric circulation, occurred at 2.8 Ma. Good correlations between climate records in the dust sources and downwind areas suggest a broadly consistent climate evolution of northwestern China during the late Cenozoic, which is probably driven by the uplift of the Tibet Plateau and the Northern Hemisphere glaciation.

Description: High resolution measurements of carbon monoxide along a late Holocene Greenland ice core: evidence for in-situ production Climate of the Past Discussions, 9, 2817-2857, 2013 Author(s): X. Faïn, J. Chappellaz, R. H. Rhodes, C. Stowasser, T. Blunier, J. R. McConnell, E. J. Brook, S. Preunkert, M. Legrand, T. Desbois, and D. Romanini We present high-resolution measurements of carbon monoxide (CO) concentrations from continuous analysis of a shallow ice core from the North Greenland Eemian Ice Drilling project (NEEM-2011-S1). An Optical Feedback – Cavity Enhanced Absorption Spectrometer (OF-CEAS) was coupled to a continuous melter system during a 4-week laboratory-based measurement campaign. This analytical setup generates highly stable measurements of CO concentrations with an external precision of 7.8 ppbv (1 sigma) based on a comparison of replicate cores. The NEEM-2011-S1 CO record spans 1800 yr and exhibits highly variable concentrations at the scale of annual layers, ranging from 75 to 1327 ppbv. The most recent section of this record (i.e. since 1700 AD) agrees with existing discrete CO measurements from the Eurocore ice core and the deep NEEM firn. However, it is difficult to interpret in terms of atmospheric CO variation due to high frequency, high amplitude spikes in the data. 68% of the elevated CO spikes are observed in ice layers enriched with pyrogenic aerosols. Such aerosols, originating from boreal biomass burning emissions, contain organic compounds, which can be oxidized or photodissociated to produce CO in-situ. We suggest that elevated CO concentration features could present a new integrative proxy for past biomass burning history. Furthermore, the NEEM-2011-S1 record reveals an increase in baseline CO level prior to 1700 AD (129 m depth), with the concentration remaining high even for ice layers depleted in dissolved organic carbon (DOC). Overall, the processes driving in-situ production of CO within the NEEM ice are complex and may involve multiple chemical pathways.

Description: Biological proxies recorded in a Belukha ice core, Russian Altai Climate of the Past Discussions, 9, 2589-2627, 2013 Author(s): T. Papina, T. Blyacharchyuk, A. Eichler, N. Malygina, E. Mitrofanova, and M. Schwikowski Different biological proxies such as pollen, cysts, and diatoms were identified and quantified in the upper part of a Belukha ice core from the Russian Altai. The ice core from the Belukha glacier collected in 2001 (4062 m a.s.l., 49°48' N, 86° 34' E) was analyzed with annual resolution in the period 1964–2000. We used daily data of the frequency of synoptic patterns observed in the Northern Hemisphere along with daily data of precipitation to identify the main modern sources of biological proxies deposited at the Belukha glacier. Our analyses revealed that main sources of diatoms in the Belukha ice core are water bodies of the Aral, Caspian, and North Kazakhstan basins. Coniferous trees pollen originated from the taiga forest of the boreal zone of West Siberia and pollen of hardwoods and herbs from steppe and forest steppe vegetation in the Northern Altai and East Kazakhstan. Cysts of algae and spores of inferior plants were transported from local water bodies and forests. The identified source regions of the biological species are supported by back trajectory analyses and are in good agreement with emission source regions of the trace species in the ice core.

Description: Dynamic soil feedbacks on the climate of the mid-Holocene and the Last Glacial Maximum Climate of the Past Discussions, 9, 2717-2770, 2013 Author(s): M. Stärz, G. Lohmann, and G. Knorr State-of-the-art general circulation models (GCMs) are tested and challenged by the ability to reproduce paleoclimate key intervals. In order to account for climate changes associated with soil dynamics we have developed a soil scheme, which is asynchronously coupled to a state-of-the-art atmosphere ocean GCM with dynamic vegetation. We test the scheme for conditions representative of a warmer (mid-Holocene, 6 kyr before present, BP) and colder (Last Glacial Maximum, 21 kyr BP) than pre-industrial climate. The computed change of physical soil properties (i.e. albedo, water storage capacity, and soil texture) for these different climates leads to amplified global climate anomalies. Especially regions like the transition zone of desert/savannah and taiga/tundra, exhibit an increased response as a result of the modified soil treatment. In comparison to earlier studies, the inclusion of the soil feedback pushes our model simulations towards the warmer end in the range of mid-Holocene studies and beyond current estimates of global cooling during the Last Glacial Maximum based on PMIP2 (Paleoclimate Modelling Intercomparison Project 2) studies. The main impact of the interactive soil scheme on the climate response is governed by positive feedbacks, including dynamics of vegetation, snow, sea ice, local water recycling, which might amplify forcing factors ranging from orbital to tectonic timescales.

Description: Where to find 1.5 million yr old ice for the IPICS "Oldest Ice" ice core Climate of the Past Discussions, 9, 2771-2815, 2013 Author(s): H. Fischer, J. Severinghaus, E. Brook, E. Wolff, M. Albert, O. Alemany, R. Arthern, C. Bentley, D. Blankenship, J. Chappellaz, T. Creyts, D. Dahl-Jensen, M. Dinn, M. Frezzotti, S. Fujita, H. Gallee, R. Hindmarsh, D. Hudspeth, G. Jugie, K. Kawamura, V. Lipenkov, H. Miller, R. Mulvaney, F. Pattyn, C. Ritz, J. Schwander, D. Steinhage, T. van Ommen, and F. Wilhelms The recovery of a 1.5 Myr long ice core from Antarctica represents a keystone to our understanding of Quaternary climate, the progression of glaciation over this time period and the role of greenhouse gas cycles in this progression. Here we show that such old ice is most likely to exist in the plateau area of the East Antarctic Ice Sheet (EAIS) without stratigraphic disturbance and should be able to be recovered after careful pre-site selection studies. Based on a simple ice and heat flow model and glaciological observations, we conclude that positions in the vicinity of major domes and saddle positions on the East Antarctic Plateau will most likely have such old ice in store and represent the best study areas for dedicated reconnaissance studies in the near future. In contrast to previous ice core drill site selections, we strongly argue for significantly reduced ice thickness to avoid bottom melting, while at the same time maximizing the resolution and the distance of such old ice to the bedrock. For example for the geothermal heat flux and accumulation conditions at Dome C, an ice thickness lower than 2500 m would be required to find 1.5 Myr old ice. However, the final choice is strongly dependent on the local geothermal heat flux, which is largely unknown for the EAIS and has to be determined beforehand. In addition, the detailed bedrock topography and ice flow history for candidates of an Oldest Ice ice coring site has to be reconstructed. Finally, we argue strongly for rapid access drilling before any full deep ice coring activity commences to bring datable samples to the surface and to allow an age check of the oldest ice.

Description: Late Pliocene lakes and soils: a data – model comparison for the analysis of climate feedbacks in a warmer world Climate of the Past Discussions, 9, 3175-3207, 2013 Author(s): M. J. Pound, J. Tindall, S. J. Pickering, A. M. Haywood, H. J. Dowsett, and U. Salzmann Based on a synthesis of geological data we have reconstructed the global distribution of Late Pliocene soils and lakes which are then used as boundary conditions in a series of model experiments using the Hadley Centre General Circulation Model (HadCM3) and the BIOME4 mechanistic vegetation model. By combining our novel soil and lake reconstructions with a fully coupled climate model we are able to explore the feedbacks of soils and lakes on the climate of the Late Pliocene. Our experiments reveal regionally confined changes of local climate and vegetation in response to the new boundary conditions. The addition of Late Pliocene soils has the largest influence on surface air temperatures, with notable increases in Australia, southern North Africa and Asia. The inclusion of Late Pliocene lakes generates a significant increase in precipitation in central Africa, as well as seasonal increases in the Northern Hemisphere. When combined, the feedbacks on climate from Late Pliocene lakes and soils improve the data to model fit in western North America and southern North Africa.

Description: Assessing the impact of Laurentide Ice-Sheet topography on glacial climate Climate of the Past Discussions, 9, 3239-3306, 2013 Author(s): D. J. Ullman, A. N. LeGrande, A. E. Carlson, F. S. Anslow, and J. M. Licciardi Simulations of past climates require altered boundary conditions to account for known shifts in the Earth system. For the Last Glacial Maximum (LGM) and subsequent deglaciation, the existence of large Northern Hemisphere ice sheets provides a profound change in surface topography and albedo. While ice-sheet extent is fairly well known, numerous conflicting reconstructions of ice-sheet topography suggest that precision in this boundary condition is lacking. Here we use a high-resolution and oxygen-isotope-enabled fully-coupled global circulation model (GCM) (GISS ModelE2-R), along with two different reconstructions of the Laurentide Ice Sheet (LIS) that provide maximum and minimum estimates of LIS elevation, to assess the range of climate variability in response to uncertainty in this boundary condition. We present this comparison at two equilibrium time slices: the LGM, where differences in ice sheet topography are maximized, and 14 ka, where differences in maximum ice sheet height are smaller but still exist. Overall, we find significant differences in the climate response to LIS topography, with the larger LIS resulting in enhanced Atlantic meridional overturning circulation and warmer surface air temperatures, particularly over Northeast Asia and the North Pacific. These up and downstream effects are associated with differences in the development of planetary waves in the upper atmosphere, with the larger LIS resulting in a weaker trough over Northeast Asia that leads to the warmer temperatures and decreased albedo from snow and sea-ice cover. Differences between the 14 ka simulations are similar in spatial extent but smaller in magnitude, suggesting that climate is responding primarily to the larger difference in maximum LIS elevation in the LGM simulations. These results suggest that such uncertainty in ice-sheet boundary conditions alone may greatly impact the results of paleoclimate simulations and their ability to successfully simulate past climates, with implications for estimating climate sensitivity to greenhouse gas forcing utilizing past climate states.

Description: Salinity changes in the Agulhas leakage area recorded by stable hydrogen isotopes of C 37 alkenones during Termination I and II Climate of the Past Discussions, 9, 3209-3238, 2013 Author(s): S. Kasper, M. T. J. van der Meer, A. Mets, R. Zahn, J. S. Sinninghe Damsté, and S. Schouten At the southern tip of the African shelf, the Agulhas Current reflects back into the Indian Ocean causing so called "Agulhas rings" to spin off and release relatively warm and saline water into the South Atlantic Ocean. Previous reconstructions of the dynamics of the Agulhas current, based on paleo sea surface temperature and sea surface salinity proxies, inferred that Agulhas leakage from the Indian Ocean to the South Atlantic is reduced as a consequence of changes in wind fields related to a northwards migration of ice masses and the subtropical front during glacial stages. Subsequently, this might have led to a build-up of warm saline water in the southern Indian Ocean. To investigate this latter hypothesis, we reconstructed sea surface salinity changes using alkenone δ D, and paleo sea surface temperature using TEX H 86 and U K' 37 , from two sediment cores (MD02-2594, MD96-2080) located in the Agulhas leakage area during Termination I and II. Both U K' 37 and TEX H 86 temperature reconstructions infer an abrupt warming during the glacial terminations, which is different from the gradual warming trend previously reconstructed based on Mg/Ca ratios of Globigerina bulloides . These differences in temperature reconstructions might be related to differences in the growth season or depth habitat between organisms. A shift to more negative δ D alkenone values of approximately 14‰ during glacial Termination I and approximately 13‰ during Termination II is also observed. Approximately half of these shifts can be attributed to the change in global ice volume, while the residual isotopic shift is attributed to changes in salinity, suggesting relatively high salinities at the core sites during glacials, with subsequent freshening during glacial terminations. Approximate estimations suggest that δ D alkenone represents a salinity change of ca. 1.7–2 during Termination I and ca. 1.5–1.7 during Termination II. These estimations are in good agreement with the proposed changes in salinity derived from previously reported combined planktonic foraminifera δ 18 O values and Mg/Ca-based temperature reconstructions. Our results show that the δ D of alkenones is a potentially suitable tool to reconstruct salinity changes independent of planktonic foraminifera δ 18 O.

Description: Potential impact of the 74 ka Toba eruption on the Balkan region, SE Europe Climate of the Past Discussions, 9, 3307-3319, 2013 Author(s): B. Wagner, M. J. Leng, T. Wilke, A. Böhm, K. Panagiotopoulos, H. Vogel, J. Lacey, G. Zanchetta, and R. Sulpizio The 74 ka Toba eruption in Sumatra, Indonesia, is considered to be one of the largest volcanic events during the Quaternary. Tephra from the Toba eruption has been found in many terrestrial and marine sedimentary deposits, and acidity peaks related to the eruption have been used to synchronize ice core records from Greenland and Antarctica. Seismic profiles from Lake Prespa on the Balkan Peninsula, SE Europe, indicate a lake level lowstand, which is confirmed by the occurrence of an exceptional bivalve shell layer of Dreissena sp. in the sediment record, the isotope composition of the shells, as well as pollen data. ESR dating and extrapolation of the tephra and radiocarbon chronologies indicate that this lowstand occurred at 73.6 ± 7.7 ka. Our data, showing a short term shift to aridity superimposed on the general cooling trend at the end of MIS 5 and coupled with the chronological correspondence to the Toba eruption imply a distinct impact of the Toba eruption on environmental conditions in the northern Mediterranean region. The recovery from this dramatic environmental condition, in turn, may have triggered spatial expansion events in one of the lake's most abundant benthic species, the carino mussel Dreissena presbensis .

Description: A first chronology for the NEEM ice core Climate of the Past Discussions, 9, 2967-3013, 2013 Author(s): S. O. Rasmussen, P. Abbott, T. Blunier, A. Bourne, E. Brook, S. L. Buchardt, C. Buizert, J. Chappellaz, H. B. Clausen, E. Cook, D. Dahl-Jensen, S. Davies, M. Guillevic, S. Kipfstuhl, T. Laepple, I. K. Seierstad, J. P. Severinghaus, J. P. Steffensen, C. Stowasser, A. Svensson, P. Vallelonga, B. M. Vinther, F. Wilhelms, and M. Winstrup A stratigraphy-based chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core has been derived by transferring the annual layer counted Greenland Ice Core Chronology 2005 (GICC05) from the NGRIP core to the NEEM core using 787 match points of mainly volcanic origin identified in the Electrical Conductivity Measurement (ECM) and Dielectrical Profiling (DEP) records. Tephra horizons found in both the NEEM and NGRIP ice cores are used to test the matching based on ECM and DEP and provide additional horizons used for the time scale transfer. A thinning function reflecting the accumulated strain along the core has been determined using a Dansgaard–Johnsen flow model and an isotope-dependent accumulation rate parameterization. Flow parameters are determined from Monte Carlo analysis constrained by the observed depth-age horizons. In order to construct a chronology for the gas phase, the ice age–gas age difference (Δage) has been reconstructed using a coupled firn densification–heat diffusion model. Temperature and accumulation inputs to the Δage model, initially derived from the water isotope proxies, have been adjusted to optimize the fit to timing constraints from δ 15 N of nitrogen and high-resolution methane data during the abrupt onsets of interstadials. The ice and gas chronologies and the corresponding thinning function represent the first chronology for the NEEM core, and based on both the flow and firn modelling results, the accumulation history for the NEEM site has been reconstructed, providing the necessary basis for further analysis of the records from NEEM.

Description: Tree-ring inferred glacier mass balance variation in southeastern Tibetan Plateau and its linkage with climate variability Climate of the Past Discussions, 9, 3663-3680, 2013 Author(s): J. Duan, L. Wang, L. Li, and Y. Sun A large number of glaciers in the Tibetan Plateau (TP) have experienced wastage in recent decades. And the wastage is different from region to region, even from glacier to glacier. A better understanding of long-term glacier variations and their linkage with climate variability requires extending the presently observed records. Here we present the first tree-ring-based glacier mass balance (MB) reconstruction in the TP, performed at the Hailuogou Glacier in southeastern TP during 1865–2007. The reconstructed MB is characterized mainly by ablation over the past 143 yr, and typical melting periods occurs in 1910s–1920s, 1930s–1960s, 1970s–1980s, and the last 20 yr. After the 1900s, only a few short periods (i.e., 1920s–1930s, the 1960s and the late 1980s) is characterized by accumulation. These variations can be validated by the terminus retreat velocity of the Hailuogou Glacier and the ice-core accumulation rate in Guliya and respond well to regional and Northern Hemisphere temperature anomaly. In addition, the reconstructed MB is significantly and negatively correlated with August-September all-Indian monsoon precipitation (AIR) ( r 1871–2008 = −0.342, p 〈 0.0001). These results suggest that temperature variability is the dominant factor for the long-term MB variation at the Hailuogou Glacier. Indian summer monsoon precipitation doesn't affect the MB variation, yet the significant negative correlation between the MB and the AIR implies the positive effect of summer heating of the TP on Indian summer monsoon precipitation.

Description: The initiation of Neoproterozoic "snowball" climates in CCSM3: the influence of paleo-continental configuration Climate of the Past Discussions, 9, 3615-3662, 2013 Author(s): Y. Liu, W. R. Peltier, J. Yang, and G. Vettoretti We identify the "hard snowball" bifurcation point at which total sea ice cover of the oceans is expected by employing the comprehensive coupled climate model CCSM3 for two realistic Neoproterozoic continental configurations, namely a low-latitude configuration appropriate for the 720 Ma Sturtian glaciation and a higher southern latitude configuration more appropriate for the later 635 Ma Marinoan glaciation. We find that for the same total solar insolation (TSI) and atmospheric CO 2 concentration ( p CO 2 ), the most recent continental configuration is characterized by colder climate than the 720 Ma continental configuration and enters the hard snowball state more easily on account of the following four factors: the low heat capacity of land in the south polar region, the higher albedo of the snow covered land compared to that of sea ice, the more negative net cloud forcing near the ice front in the Northern Hemisphere (NH), and more importantly, the more efficient sea ice transport towards the equator in the NH due to the absence of blockage by continents. Beside the paleogeography, we also find the optical depth of aerosol to have a significant influence on this important bifurcation point. When the high value (recommended by CCSM3 but demonstrated to be a significant overestimate) is employed, the critical values of p CO 2 , beyond which a hard snowball will be realized, are between 80–90 ppmv and 140–150 ppmv for the Sturtian and Marinoan continental configurations, respectively. However, if a lower value is employed that enables the model to approximately reproduce the present-day climate, then the critical values of p CO 2 become 50–60 ppmv and 100–110 ppmv for the two continental configurations, respectively. All of these values are higher than previously obtained for the present-day geography (17–35 ppmv) using the same model, primarily due to the absence of vegetation, but are much lower than that obtained previously for the 635 Ma continental configuration using the ECHAM5/MPI-OM model in its standard configuration (∼500 ppmv). However, when the sea ice albedo in that model was reduced from 0.75 to a more appropriate value of 0.45, the critical p CO 2 becomes ∼204 ppmv, closer to but still higher than the values obtained here. Our results are similar to those obtained with the present-day geography (70–100 ppmv) when the most recent version of the NCAR model, CCSM4, is employed.

Description: Mending Milankovitch theory: obliquity amplification by surface feedbacks Climate of the Past Discussions, 9, 3769-3787, 2013 Author(s): C. R. Tabor, C. J. Poulsen, and D. Pollard Milankovitch theory states that orbitally induced changes in high-latitude summer insolation dictate the waxing and waning of ice-sheets. Accordingly, precession should dominate the ice-volume response because it most strongly modulates summer insolation intensity. However, Early Pleistocene (2.6–0.8 Ma) ice-volume proxy records vary almost exclusively at the frequency of the obliquity cycle. To explore this paradox, we use an Earth system model coupled with a dynamic ice-sheet to separate the climate responses to idealized transient orbits of obliquity and precession that maximize insolation changes. Our results show that positive surface albedo feedbacks between high-latitude annual-mean insolation, ocean heat flux and sea-ice coverage, and boreal forest/tundra exchange enhance the ice-volume response to obliquity forcing relative to precession forcing. These surface feedbacks, in combination with modulation of the precession cycle power by eccentricity, may explain the dominantly 41 kyr cycles in global ice volume of the Early Pleistocene.

Description: Consistency of the multi-model CMIP5/PMIP3-past1000 ensemble Climate of the Past Discussions, 9, 3789-3824, 2013 Author(s): O. Bothe, J. H. Jungclaus, and D. Zanchettin We present an assessment of the probabilistic and climatological consistency of the CMIP5/PMIP3 ensemble simulations for the last millennium relative to proxy-based reconstructions under the paradigm of a statistically indistinguishable ensemble. We evaluate whether simulations and reconstructions are compatible realizations of the unknown past climate evolution. A lack of consistency is diagnosed in surface air temperature data for the Pacific, European and North Atlantic regions. On the other hand, indications are found that temperature signals partially agree in the western tropical Pacific, the subtropical North Pacific and the South Atlantic. Deviations from consistency may change between sub-periods, and they may include pronounced opposite biases in different sub-periods. These distributional inconsistencies originate mainly from differences in multi-centennial to millennial trends. Since the data uncertainties are only weakly constrained, the frequent over-dispersive distributional relations prevent the formal rejection of consistency of the simulation ensemble.

Description: The impacts of Meltwater Pulse-1A in the South Atlantic Ocean deep circulation since the Last Glacial Maximum Climate of the Past Discussions, 9, 6375-6395, 2013 Author(s): J. M. Marson, I. Wainer, Z. Liu, and M. M. Mata Since 21 000 yr ago, the oceans have received large amounts of freshwater in pulses coming from the melting ice sheets. A specific event, known as meltwater pulse 1A (MWP-1A), has been identified in sea-level and temperature proxy records as responsible for the increase of ~20 m in sea level in less than 500 yr. Although its origin and timing are still under discussion, MWP-1A seems to have had a significant impact on several components of the climatic system. The present work aims to elucidate these impacts on the water mass distribution of the South Atlantic Ocean through the analysis of a transient simulation of the climate evolution from the Last Glacial Maximum to Present Day using a state-of-art CGCM, the National Center for Atmospheric Research Community Climate System Model version 3 (NCAR CCSM3). Results show that the freshwater discharge associated with the timing of MWP-1A was crucial to establish the present thermohaline structure associated with the North Atlantic Deep Water, marking the transition between a shallower and a deeper Atlantic Meridional Overturning Circulation.

Description: Holocene changes in African vegetation: tradeoff between climate and water availability Climate of the Past Discussions, 9, 6397-6427, 2013 Author(s): C. Hély, A.-M. Lézine, and APD contributors Although past climate change is well documented in West Africa through instrumental records, modeling activities, and paleo-data, little is known about regional-scale ecosystem vulnerability and long term impacts of climate on plant distribution and biodiversity. Here we use paleohydrological and paleobotanical data to discuss the relation between available surface water, monsoon rainfall and vegetation distribution in West Africa during the Holocene. The individual patterns of plant migration or community shifts in latitude are explained by differences among tolerance limits of species to rainfall amount and seasonality. Using the probability density function methodology, we show here that the widespread development of lakes, wetlands and rivers at the time of the "Green Sahara" played an additional role in forming a network of topographically defined water availability allowing for tropical plants to migrate north from 15 to 24° N (reached ca. 9 cal ka BP). The analysis of the spatio-temporal changes in biodiversity, through both pollen occurrence and diversity, shows that the core of the tropical rainbelt associated with the Intertropical Convergence Zone was centered at 15–20° N during the early Holocene wet period with comparatively drier/more seasonal climate conditions south of 15° N.

Description: Modeling of severe persistent droughts over eastern China during the last millennium Climate of the Past Discussions, 9, 6345-6373, 2013 Author(s): Y. Peng, C. Shen, H. Cheng, and Y. Xu We use proxy data and modeled data from 1000 yr model simulations with a variety of climate forcings to examine the occurrence of severe events of persistent drought over eastern China during the last millennium and to diagnose the mechanisms. Results show that the model was able to simulate many aspects of the low-frequency (periods greater than 10 yr) variations of precipitation over eastern China during the last millennium, including most of the severe persistent droughts such as those in the 1130s, 1200s, 1350s, 1430s, 1480s, and the late 1630s–mid-1640s. These six droughts are identified both in the proxy data and in the modeled data and are consistent with each other in terms of drought intensity, duration, and spatial coverage. Our analyses suggest that monsoon circulation can lock into a drought-prone mode that may last for years to decades and supports the suggestion that generally reduced monsoon in eastern Asia were associated with the land–sea thermal contrast. Study on the wavelet transform and spectral analysis reveals six well-captured events occurred all at the drought stages of statistically significant 15–35 yr timescale. A modeled data intercomparison suggests that solar activity is the primary driver in the occurrence of the 1130s, 1350s, 1480s, and late 1630s–mid-1640s droughts. Although the El-Niño–Southern Oscillation (ENSO) plays an important role in monsoon variability, a temporally consistent relationship between the droughts and SST pattern in the Pacific Ocean could not be found in the model. Our analyses also indicate that large volcanic eruptions play a role as an amplifier in the drought of 1635–1645 and caused the model to overestimate the decreasing trends in summer precipitation over eastern China during the mid-1830s and the mid-1960s.

Description: Pliocene to Pleistocene climate and environmental history of Lake El'gygytgyn, Far East Russian Arctic, based on high-resolution inorganic geochemistry data Climate of the Past Discussions, 9, 5899-5940, 2013 Author(s): V. Wennrich, P. S. Minyuk, V. Ya. Borkhodoev, A. Francke, B. Ritter, N. Nowaczyk, M. A. Sauerbrey, J. Brigham-Grette, and M. Melles The 3.6 Ma sediment record of Lake El'gygytgyn, Far East Russian Arctic, represents the longest continuous climate archive of the terrestrial Arctic. Its elemental composition monitored by X-ray fluorescence scanning exhibits significant changes since the Mid-Pliocene caused by climate driven variations in the primary production, postsedimentary diagenetic processes, and current activity in the lake as well as weathering processes in its catchment. During the Mid to Late Pliocene, warmer and wetter climatic conditions are reflected by elevated Si / Ti ratios, indicating enhanced diatom production in the lake. Prior to 3.3 Ma, this signal is highly masked by intensified detrital input from the catchment, visible in maxima of clastic-related proxies such as the K concentration. In addition, calcite formation in the early lake history points to enhanced nutrient flux into the lake caused by intensified weathering in its catchment. Its termination at ca. 3.3 Ma is supposed to be linked to the development of permafrost in the region triggered by a first cooling in the Mid-Pliocene. After ca. 3.0 Ma the elemental data suggest a gradual transition to Quaternary-style glacial / interglacial cyclicity. In the early Pleistocene, the cyclicity was first dominated by variations on the 41 ka obliquity band but experienced a change to a 100 ka eccentricity dominance after the Middle Pleistocene Transition at ca. 1.2 to 0.7 Ma. This clearly demonstrates the sensitivity of the Lake El'gygytgyn record to orbital forcing. A successive decrease of the baseline-levels of the redox-sensitive Mn / Fe ratio and magnetic susceptibility between 2.3 to 1.8 Ma reflects an overall change in the bottom water oxygenation due to an intensified occurrence of pervasive glacial episodes in the early Quaternary. The coincidence with major changes in the North Pacific and Bering Sea paleoceanography at ca. 1.8 Ma implies that the change in lake hydrology was caused by regional cooling and/or changes in the ocean-land moisture transport. Further rising TOC and TN values after ca. 1.6 Ma are attributed to a progressive intensification of the glacial intensity. In the course of the Quaternary glacial/interglacial sequence eight so-called "super-interglacials" occur. Their exceptional warm conditions are reflected by extreme Si / Ti peaks accompanied by lows in Ti, K, and Fe, thus indicating an extraordinary high lake productivity.

Description: Mass movement deposits in the 3.6 Ma sediment record of Lake El'gygytgyn, Far East Russian Arctic: classification, distribution and preliminary interpretation Climate of the Past Discussions, 9, 467-505, 2013 Author(s): M. A. Sauerbrey, O. Juschus, A. C. Gebhardt, V. Wennrich, N. R. Nowaczyk, and M. Melles This paper focuses on the characterization and genesis of mass movement deposits (MMD) in the Quaternary and Pliocene sediments of Lake El'gygytgyn, Far East Russian Arctic. The 320 m long sediment record was drilled by three partly overlapping holes at ICDP Site 5011-1 in the lake basin, representing the Quaternary almost completely, and the Pliocene down to 3.6 Ma with 52% recovery. Mass movement deposits were investigated in all three cores, based on macroscopical core descriptions, radiographic images, and high-resolution magnetic susceptibility and gamma-ray density. Five different types of MMDs were identified: turbidites, grain flow deposits, debrites, slumps and slides. These are formed by transitional mass movement processes, and thus, can be co-generic. An initial slope failure is thought to transform into a debris flow, deforms frontal sediments and partly disintegrates and dilutes into a turbidity flow. Turbidites are by far the most frequent MMD type in the lake center. They occur throughout the record in all pelagic sedimentary facies, but they are thinner in facies formed during cold climate conditions. All other MMDs, by contrast, incise exclusively the pelagic facies deposited during warm climates. In the 123 m thick Quaternary sediment record 238 mass movement events are identified, comprising 37% of the sediment length. Turbidites contribute 92% of the number of Quaternary MMDs, but only 32% of their thickness. In the Pliocene sediments between 123 and 320 m, additional 185 mass movement deposits are identified, which constitute 32% of the recovered sediments. The mean recurrence rate for MMDs is 11 ka and 5 ka in the Quaternary and Pliocene, respectively.

Description: Inorganic data from El'gygytgyn Lake sediments: stages 6–11 Climate of the Past Discussions, 9, 393-433, 2013 Author(s): P. S. Minyuk, V. Ya. Borkhodoev, and V. Wennrich Geochemical study was performed on sediment of deep drilling core from El'gygytgyn Lake, located in central Chukotka, northeastern Russia (67°30' N; 172°05' E). Major and rare elements were determined by X-ray fluorescence spectroscopy (XRF) on 600 samples covering the timeframe between ca. 450 and 125 ka corresponding to Marine Isotope Stages (MIS) 11 to 6. Inorganic geochemistry data indicates significant variations in the elemental compositions corresponding to the glacials and interglacials periods. Interglacial sediments are characterized by high contents of SiO 2 , Na 2 O, CaO, K 2 O, Sr and are depleted in Al 2 O 3 , Fe 2 O 3 , TiO 2 , MgO. Extreme enrichments in SiO 2 during MIS 11.3 and 9.3 are caused by an enhanced flux of biogenic silica (BSi). Geochemical structure of stage 11 shows very similar peculiarities to features of stage 11 from records of Lake Baikal/SE Siberia and Antarctic ice cores. High contents of TiO 2 , Fe 2 O 3 , MgO, Al 2 O 3 , LOI, Ni, Cr and Zr are typical for sediments of glacial stages, among those MIS 7.4 and 6.6 are the most marked. Peaks in the Fe 2 O 3 content and coinciding low Fe 2 O 3 /MnO ratios during glacials indicate reducing condition in the sediments. This is also supported by enrichments in P 2 O 5 and MnO, indicating an increased abundance of authigenic fine grained vivianite. Some elemental ratios indicate an enhanced alteration of glacial sediments accompanied by a loss of mobile elements, like Na, Ca, K and Sr. The higher alteration of sediments can presumably be traced back to changes in the sedimentation regime and diagenetic processes, and thus, reflects environmental changes.