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Pronounced changes in the deep Atlantic circulation occurred during glacial stages, which affected the global carbon distribution and the biogeochemical cycles of other elements. Previous studies demonstrated that oceanic mercury is sensitive to the same kind of processes that affect the carbon cycle in glacial-interglacial time scales. We used Hg isotopes to elucidate Hg cycling during the last two glacial-interglacial transitions in the subtropical western South Atlantic. Mass-dependent fractionation (MDF, δ202Hg) and mass-independent fractionation (MIF, Δ199Hg) show significant variations and shift toward higher and lower values during the penultimate and Last Glacial Maximum periods, respectively. δ202Hg variability is in-phase with periods of lower atmospheric CO2 and benthic δ13C, suggesting that MDF is affected by the same mechanisms that led to atmospheric CO2 drawdown during glacial stages. In contrast, terrestrial Hg input to our site, boosted by lower sea level and exposure of the continental shelf, dominates the Δ199Hg variability. [ABSTRACT FROM AUTHOR]

Variation in d18O and d13C values in a speleothem from the Cango Caves in southernmost South Africa enable the construction of coherent regional composite records spanning the past 113,500 yr. Novel for the region in terms of both their length and detail, these records indicate environmental and climatic changes that both are consistent with records from the wider region and show a clear evolution from low- to high-latitude forcing dominance across the last glacial period. Prior to ca. 70 ka, the influence of direct low-latitude insolation forcing is expressed through increases in summer rainfall during austral summer insolation maxima. With the onset of Marine Isotope Stage 4, cooler global conditions and the development of high-latitude ice sheets appear to have supplanted direct insolation forcing as the dominant driver pacing patterns of environmental change, with records from the Southern and Northern Hemisphere tropics exhibiting a positive relationship until after the Last Glacial Maximum. These results highlight the complexity of South African climate change dynamics as a response to changing global boundary conditions and provide a critical reference for regional and global comparisons. [ABSTRACT FROM AUTHOR]

An accurate reconstruction of the Antarctic Ice Sheet is essential in order to develop an understanding of ice-sheet responses to global climate changes. However, the erosive nature of ice-sheet expansion and the difficulty of accessing much of Antarctica make it challenging to obtain field-based evidence of ice-sheet and sea-level changes before the Last Glacial Maximum. Limited sedimentary records from Lützow-Holm and Prydz Bays in East Antarctica demonstrate that the sea level during Marine Isotope Stage 3 was close to the present level despite the global sea-level drop lower than -40 m. We demonstrate glacial isostatic adjustment modeling with refined Antarctic Ice Sheet loading histories. Our experiments reveal that the Indian Ocean sector of the Antarctic Ice Sheet would have been required to experience excess ice loads before the Last Glacial Maximum in order to explain the observed sea-level highstands during Marine Isotope Stage 3. As such, we suggest that the Antarctic Ice Sheet partly reached its maximum thickness before the global Last Glacial Maximum. [ABSTRACT FROM AUTHOR]

The Qaidam Basin marks a crucial boundary between the Westerlies and the Asian summer monsoons. Previous studies in the Qaidam Basin have advanced our knowledge of the paleoclimate over glacial to interglacial cycles. However, our understanding of the paleoclimatic sensitivity of the Qaidam Basin to the relative strength of these two climatic driving forces remains limited due to the lack of regional paleoclimatic reconstructions. The Qaidam Basin is proposed as a regional and global eolian dust source during the glacial periods, during which a cold, dry climate is associated with the equatorward shift of the jet stream. On the contrary, paleoshoreline records suggest that a highstand lake stage prevailed in late Marine Isotope Stage 3 (MIS 3) and lasted until 15 ka. To address this conundrum, we have applied an integrated approach to reconstructing the regional paleoclimatic history by combining compound-specific isotope analysis, lake temperature reconstruction, and numerical modeling. Our results show varying paleoclimate associated with the dynamic climate boundary since 45 ka: (1) a wet climate during late MIS 3, when the Asian summer monsoons are strengthened under high summer insolation and penetrate further into Central Asia; (2) a general cold, dry but wetter than at present climate in the Last Glacial Maximum (LGM), when the Asian summer monsoons retreat and the Westerlies become dominant; and (3) three short periods of extreme aridity corresponding to the Younger Dryas and Heinrich 2 and 4 events, when the normal moisture transport via the Westerlies and Asian summer monsoons is interrupted. The numerical modeling supports an increase in the effective precipitation during the LGM due to reduced evaporation under low summer insolation. These results suggest that the Westerlies and Asian summer monsoons alternately controlled the climate in the Qaidam Basin in response to precessional forcing during the late Pleistocene. [ABSTRACT FROM AUTHOR]

Subaqueous calcite deposited on the walls of Devils Hole 2 cave (Nevada, USA) represents a unique archive for geochemical variations within the regional aquifer. Here, we present a 475,000-year record of initial 234U/238U activity ratios in delta notation (δ234U0). Results show a range in values from 1851-1616‰. Variations in δ234U0 coincide with interglacial-glacial cycles over the past 475,000 years. Maximum δ234U0 values correspond to the last five glacial intervals, during which southwest Nevada experienced cool, pluvial conditions. Minimum δ234U0 values correspond to interglacial intervals, during which this region experienced warm, arid conditions. We propose that an elevated water table during glacial periods inundated previously dry bedrock and basin sediments, thereby leaching excess 234U accumulated in these materials. We interpret Devils Hole 2 cave δ234U0 as a proxy for water-rock interactions in this regional aquifer, which is ultimately tied to the surface moisture conditions at recharge zones. The mechanism proposed here serves as a testable hypothesis and possible analogue for future subaqueous speleothem studies in similar hydrogeologic settings. Due to its unprecedented duration, the Devils Hole 2 cave δ234U0 record provides the first paleo-moisture record in southwest Nevada for marine isotope stages 10-12. In addition, high-precision δ234U measurements of modern groundwaters sampled from Devils Hole 2 cave are presented. [ABSTRACT FROM AUTHOR]

Although the general patterns of deglacial climate change are relatively well constrained, how, and to what magnitude, large parts of the Southern Hemisphere responded to deglacial forcings remains unknown, particularly for the early part of the last deglaciation. We investigate the timing and magnitude of early deglacial climate change using cosmogenic 10Be surface exposure ages of moraines deposited by glaciers in the Rio Guanaco Valley, adjacent to the Southern Patagonian Ice Field at 50°S. We demonstrate that the beginning of ice retreat from the local last glacial maximum occurred at 19.7 ± 1.1 ka, with significant retreat commencing at 18.9 ± 0.4 ka, concurrent with glacier retreat elsewhere in southern Patagonia and New Zealand and with warming of Southern Hemisphere middle to high latitudes. A third moraine shows that half of the deglacial retreat upvalley had occurred by 17.0 ± 0.3 ka. Equilibrium line altitudes and climate simulations show ~1.5 °C of warming in southern Patagonia between 18.9 ± 0.4 ka and 17.0 ± 0.3 ka, one-third of the total estimated deglacial warming relative to present. The climate model links this warming to retreat of Northern Hemisphere ice sheets ca. 19 ka through changes in ocean circulation that caused a bipolar seesaw response resulting in Southern Hemisphere warming and driving initial deglaciation across southern Patagonia. [ABSTRACT FROM AUTHOR]