Recent thinning and loss of Antarctic ice shelves has been followed by near synchronous acceleration of ice flow that may eventually lead to sustained deflation and significant contraction in the extent of grounded and floating ice. Here, we present radiocarbon dates from foraminifera that constrain the time elapsed between a previously described paleo-ice-shelf collapse and the subsequent major grounding-line retreat in the Whales Deep Basin (WDB) of eastern Ross Sea. The dates indicate that West Antarctic Ice Sheet (WAIS) grounding-line retreat from the continental shelf edge was underway prior to 14.7 ± 0.4 cal kyr BP. A paleo-ice-shelf collapse occurred at 12.3 ± 0.2 cal kyr BP. The grounding position was maintained on the outer-continental shelf until at least 11.5 ± 0.3 cal kyr BP before experiencing a 200-km retreat. Given the age uncertainties, the major grounding-line retreat lagged ice-shelf collapse by at least two centuries and by as much as fourteen centuries. In the WDB, the centuries-long delay in the retreat of grounded ice was partly due to rapid aggradational stacking of an unusually large volume of grounding-zone-wedge sediment as ice-stream discharge accelerated following ice-shelf collapse. This new deglacial reconstruction shows that ongoing changes to ice shelves may trigger complex dynamics whose consequences are realized only after a significant lag.
We present new high-resolution absolute-dated stalagmite δ18O and δ13C records from the southeastern United States (SE US) spanning the last 12 thousand years (ka). A local relationship between annual rainfall amount and its amount-weighed δ18O composition exists on interannual timescales, driven mostly by an amount effect during summer and spring seasons, and by an isotopically depleted composition of fall and winter precipitation. Based on a novel interpretation of modern rainfall isotopic data, stalagmite δ18O variability is interpreted to reflect the relative contribution of summer and spring precipitation combined relative to combined fall and winter precipitation. Precipitation amount in the SE US increases during the Younger Dryas, the 8.2 ka and Little Ice Age abrupt cooling events. High precipitation during these events reflects enhancement of spring and summer precipitation while the contribution of fall and winter rainfall remained unchanged or decreased slightly. Results from this study support model simulation results that suggest increased precipitation in the SE US during Atlantic Meridional Overturning Circulation (AMOC) slowdown/shutdown (LeGrande et al., 2006; Renssen et al., 2002; Vellinga and Wood, 2002). In association with Northern Hemisphere mid-latitude cooling from the Early to mid-Holocene, annual precipitation in the SE US decreases, a pattern distinctive from that observed during abrupt cooling events related to AMOC shifts. Long-term hydroclimate change in the SE US is likely sensitive to summer insolation reduction as inferred for other tropical and subtropical regions. This study has implications for our understanding of the sensitivity of subtropical hydroclimate to factors both internal and external to the climate system in a warmer climate.
A newly recognized carbonate preservation event (CPE) dated between 11 and 12 Ma was identified in Hole U1361A during Integrated Ocean Drilling Program Expedition 318 in the Southern Ocean based on CaCO 3 content and the preservation of foraminiferal tests. High carbonate content occurs within the interval between 311.60 and 349.99 meters below seafloor, whereas CaCO 3 content was typically near 1 wt% for the rest of the core. In addition, benthic and planktonic foraminifers were relatively abundant. However, stable isotope measurements carried out within this interval contained unrealistic values, which we ascribe to postdepositional diagenetic alteration by the release of methane. This prohibited quantification of temperature, ice volume, and salinity. The CPE identified at Hole U1361A was also observed at Ocean Drilling Program Site 689, which is correlated to the Mi5 glaciation. We hypothesize that this Southern Ocean CPE and the diagenetic alteration of foraminifers were due to both the Mi5 glaciation and reorganization of ocean circulation.
We present new high-resolution absolute-dated stalagmite δ18O and δ13C records from the southeastern United States (SE US) spanning the last 12 thousand years (ka). A local relationship between annual rainfall amount and its amount-weighed δ18O composition exists on interannual timescales, driven mostly by an amount effect during summer and spring seasons, and by an isotopically depleted composition of fall and winter precipitation. Based on a novel interpretation of modern rainfall isotopic data, stalagmite δ18O variability is interpreted to reflect the relative contribution of summer and spring precipitation combined relative to combined fall and winter precipitation. Precipitation amount in the SE US increases during the Younger Dryas, the 8.2 ka and Little Ice Age abrupt cooling events. High precipitation during these events reflects enhancement of spring and summer precipitation while the contribution of fall and winter rainfall remained unchanged or decreased slightly. Results from this study support model simulation results that suggest increased precipitation in the SE US during Atlantic Meridional Overturning Circulation (AMOC) slowdown/shutdown (LeGrande et al., 2006; Renssen et al., 2002; Vellinga and Wood, 2002). In association with Northern Hemisphere mid-latitude cooling from the Early to mid-Holocene, annual precipitation in the SE US decreases, a pattern distinctive from that observed during abrupt cooling events related to AMOC shifts. Long-term hydroclimate change in the SE US is likely sensitive to summer insolation reduction as inferred for other tropical and subtropical regions. This study has implications for our understanding of the sensitivity of subtropical hydroclimate to factors both internal and external to the climate system in a warmer climate.
The evolution of human societies and ecosystems are intimately related to the degree of climate stability over time at a regional scale (IPBES, 2019;IPCC, 2014). The Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) estimates that within the next three decades 1 million species are threatened with extinction, and it highlights temperature and hydroclimate change as chief driving factors (IPBES, 2019). Large uncertainty remains, however, regarding our understanding of past hydroclimate variability and its underlying drivers, and therefore also, in our capacity to predict the future. Considerable disagreement among state-of-the-art climate model predictions of precipitation for the end of this century underlines the need to better understand the drivers of hydroclimate variability to help improve the climate forecast (Anandhi & Bentley, 2018).
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