Southern Ocean carbon sink enhanced by sea-ice feedbacks at the Antarctic Cold Reversal

The Southern Ocean occupies some 14% of the planet’s surface and plays a fundamental role in the global carbon cycle and climate. It provides a direct connection to the deep ocean carbon reservoir through biogeochemical processes that include surface primary productivity, remineralisation at depth, and the upwelling of carbon-rich water masses. However, the role of these different processes in modulating past and future air-sea carbon flux remains poorly understood. A key period in this regard is the Antarctic Cold Reversal (ACR, 14.6-12.7 kyr BP), a period of mid- to high-latitude cooling that coincided with a sustained plateau in deglacial atmospheric rise in CO2 globally. Here we reconstruct high-latitude Southern Ocean surface productivity from marine derived aerosols captured in a highly-resolved horizontal ice core. Our multiproxy reconstruction reveals a coherent signal of enhanced productivity across the ACR. Transient climate modelling indicates this period coincided with maximum seasonal variability in sea-ice extent, implying that sea-ice biological feedbacks enhanced CO2 sequestration, creating a significant regional marine carbon sink that contributed to the sustained plateau in CO2 at the ACR. Our results highlights the role Antarctic sea ice plays in controlling global CO2, and demonstrates the need to incorporate such feedbacks in climate-carbon models

Seasonal changes in glacial polynya activity inferred from Weddell Sea varves

The Weddell Sea and the associated Filchner-Ronne 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, Fe, Ti, Rb, and K are elevated. Based on these findings as well as on previous analyses on neighbouring cores, we propose a model of enhanced thermohaline convection in front of a grounded ice sheet that is supported by seasonally variable coastal polynya activity during the LGM. 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. Under these conditions, lighter coarser-grained layers are likely glacial winter deposits, when brine rejection was increased, leading to enhanced bottom-water formation and increased sediment transport. Vice versa, darker finer-grained layers were then deposited during less windier season, mainly during summer, when coastal polynya activity was likely reduced

Southern Ocean bioproductivity during the last glacial cycle--new detection method and decadal-scale insight from the Scotia Sea

We present biogenic opal flux records from two deep-sea sites in the Scotia Sea (MD07- 3133 and MD07-3134) at decadal-scale resolution, covering the last glacial cycle. In addition to conventional and time-consuming biogenic opal measuring methods, we introduce new biogenic opal estimation methods derived from sediment colour b*, wet bulk density, Si/Ti-count ratio and Fourier transform infrared spectroscopy (FTIRS). All methods capture the biogenic opal amplitude; however, FTIRS–a novel method for marine sediment – yields the most reliable results. 230Th normalization data show strong differences in sediment focusing with intensified sediment focusing during glacial times. At MD07-3134 230Th normalized biogenic opal fluxes vary between 0.2 and 2.5 g cm22 kyr21. Our biogenic opal flux records indicate bioproductivity changes in the Southern Ocean, strongly influenced by sea ice distribution and also summer sea surface temperature changes. South of the Antarctic Polar Front, lowest bioproductivity occurred during the Last Glacial Maximum when upwelling of mid-depth water was reduced and sea ice cover intensified. Around 17 ka, bioproductivity increased abruptly, corresponding to rising atmospheric CO2 and decreasing seasonal sea ice coverag

Centennial-scale Holocene climate variations amplified by Antarctic Ice Sheet discharge

Proxy-based indicators of past climate change show that current global climate models systematically underestimate Holocene-epoch climate variability on centennial to multi-millennial timescales, with the mismatch increasing for longer periods1,2,3,4,5. Proposed explanations for the discrepancy include ocean–atmosphere coupling that is too weak in models6, insufficient energy cascades from smaller to larger spatial and temporal scales7, or that global climate models do not consider slow climate feedbacks related to the carbon cycle or interactions between ice sheets and climate4. Such interactions, however, are known to have strongly affected centennial- to orbital-scale climate variability during past glaciations8,9,10,11, and are likely to be important in future climate change12,13,14. Here we show that fluctuations in Antarctic Ice Sheet discharge caused by relatively small changes in subsurface ocean temperature can amplify multi-centennial climate variability regionally and globally, suggesting that a dynamic Antarctic Ice Sheet may have driven climate fluctuations during the Holocene. We analysed high-temporal-resolution records of iceberg-rafted debris derived from the Antarctic Ice Sheet, and performed both high-spatial-resolution ice-sheet modelling of the Antarctic Ice Sheet and multi-millennial global climate model simulations. Ice-sheet responses to decadal-scale ocean forcing appear to be less important, possibly indicating that the future response of the Antarctic Ice Sheet will be governed more by long-term anthropogenic warming combined with multi-centennial natural variability than by annual or decadal climate oscillations

Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation

Our understanding of the deglacial evolution of the Antarctic Ice Sheet (AIS) following the Last Glacial Maximum(26,000–19,000 years ago)1 is based largely on a few well-dated but temporally and geographically restricted terrestrial and shallow-marine sequences. .This sparseness limits our understanding of the dominant feedbacks between the AIS, Southern Hemisphere climate and global sea level. Marine records of iceberg-rafted debris (IBRD) provide a nearly continuous signal of ice-sheet dynamics and variability. IBRD records from the North Atlantic Ocean have been widely used to reconstruct variability in Northern Hemisphere ice sheets5, but comparable records from the Southern Ocean of the AIS are lacking because of the low resolution and large dating uncertainties in existing sediment cores. Here we present two well-dated, high-resolution IBRD records that capture a spatially integrated signal of AIS variability during the last deglaciation. We document eight events of increased iceberg flux from various parts of the AIS between 20,000 and 9,000 years ago, in marked contrast to previous scenarios which identified the main AIS retreat as occurring after meltwater pulse 1A and continuing into the late Holocene epoch. The highest IBRD flux occurred 14,600 years ago, providing the first direct evidence for an Antarctic contribution to meltwater pulse 1A. Climate model simulations with AIS freshwater forcing identify a positive feedback between poleward transport of Circumpolar Deep Water, subsurface warming and AIS melt, suggesting that small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise