ALBERT

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

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

Description: The East Asian Monsoon exhibits a significant variability on timescales ranging from tectonic to centennial as inferred from Chinese loess, stalagmite and marine records. However, the relative contributions and plausible driving forces of the signals at different timescales remain poorly investigated. Here, we spectrally decompose time series data on loess grain size and speleothem δ18O records over the last two climatic cycles and correlate the decomposed components with possible driving parameters including the ice volume, insolation and North Atlantic cooling. Based on the spectral analysis of these two proxies, we tentatively identified six components of the signals corresponding to various forcing of ice volume (〉 50 kyr), obliquity (50–30 kyr), precession (30–9 kyr), North Atlantic cooling (9–3 kyr and 3–1 kyr), and a centennial residual. The relative contributions of each component differ significantly between loess grain size and speleothem δ18O records. Glacial and orbital components are dominant in the loess grain size, which implies that both ice volume and insolation have distinctive impacts on the winter monsoon variability in contrast to the predominant precession impact on the summer monsoon patterns. Moreover, the millennial components are evident with variances of 11 and 16% in the loess grain size and speleothem δ18O records, respectively. A comparison of the millennial-scale signals in these two proxies reveals that abrupt changes in the winter and summer monsoons over the last 260 kyr share common features and similar driving forces linked to high-latitude Northern Hemisphere climate.

Description: Holocene Eastern Mediterranean Sea sediments contain an organic-rich sapropel S1 layer that was formed in oxygen-depleted waters. The spatial distribution of this layer revealed that during S1 deposition deep waters were permanently anoxic below 1800 m in water depth. To provide further insight into past Eastern Mediterranean Sea circulation, a multi-proxy approach was applied to a core retrieved close to the 1800 m boundary (at 1780 m). We measured the bulk sediment elemental composition, the stable isotopic composition of the planktonic foraminifer Globigerinoides ruber, and the abundance of benthic foraminifera since the last deglaciation. The result indicates that authigenic U and Mo accumulation began around 13–12 cal ka BP, in concert with surface water freshening estimated from the G. ruber δ18O record. The onset of bottom/pore water oxygen depletion occurred prior to S1 deposition inferred from barium enrichment. In the middle of the S1 deposition period, between 9 and 8 cal ka BP, reduced authigenic V, Fe and As contents and Br / Cl ratio indicated short-term bottom water re-oxygenation. A sharp Mn peak and maximal abundance for benthic foraminifera marked a total recovery for circulation at approximately 7 cal ka BP. Based on our results and existing data, we suggest that S1 formation withinthe upper 1780 m of the Eastern Mediterranean Sea was preconditioned by reduced ventilation, resulting from excess fresh water inputs due to insolation changes under deglacial conditions, that initiated between 15 and 12 ka. Short-term re-oxygenation in the Levantine Basin is estimated to have affected bottom water below and above the anoxic boundary. We tentatively propose that complete ventilation recovery at the S1 termination was attained earlier within the upper 1780 m than at deeper water depths. Our results provided new constraints for eastern Mediterranean Sea thermohaline circulation.

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

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

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

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

Description: During Termination 1, millennial-scale weakening events of the Atlantic meridional overturning circulation (AMOC) supposedly produced major changes in sea surface temperatures (SST) of the western South Atlantic, and in mean air temperatures (MAT) over southeastern South America. It was suggested, for instance, that the Brazil Current (BC) would strengthen (weaken) and the North Brazil Current (NBC) would weaken (strengthen) during slowdown (speed-up) events of the AMOC. This anti-phase pattern was claimed to be a necessary response to the decreased North Atlantic heat piracy during periods of weak AMOC. However, the thermal evolution of the western South Atlantic and the adjacent continent is largely unknown and a compelling record of the BC-NBC anti-phase behavior remains elusive. Here we address this issue, presenting high temporal resolution SST and MAT records from the BC and southeastern South America, respectively. We identify a warming in the western South Atlantic during Heinrich Stadial 1 (HS1), which is followed first by a drop and then by increasing temperatures during the Bølling–Allerød, in-phase with an existing NBC record. Additionally, a similar SST evolution is shown by a southernmost eastern South Atlantic record, suggesting a South Atlantic-wide pattern in SST evolution during most of Termination 1. Over southeastern South America, our MAT record shows a two-step increase during Termination 1, synchronous with atmospheric CO2 rise (i.e., during the second half of HS1 and during the Younger Dryas), and lagging abrupt SST changes by several thousand years. This delay corroborates the notion that the long duration of HS1 was fundamental to drive the Earth out of the last glacial.

Description: The combined use of proxy records and climate modelling is invaluable for obtaining a better understanding of past climates. However, many methods of model-proxy comparison in the literature are fundamentally problematic because larger errors in the proxy tend to yield a "better" agreement with the model. Here we quantify model-proxy agreement as a function to proxy uncertainty using the overlapping coefficient OVL, which measures the similarity between two probability distributions. We found that the model-proxy agreement is poor (OVL 〈 50%) if the proxy uncertainty (σp) is greater than three times the model variability (σm), even if the model and proxy have similar mean estimates. Hence only proxies that fulfil the condition σp 〈 3σm should be used for detailed quantitative evaluation of the model performance.