Oxygen depletion in the upper ocean is commonly associated with poor ventilation and storage of respired carbon, potentially linked to atmospheric CO2 levels. Iodine to calcium ratios (I/Ca) in recent planktonic foraminifera suggest that values less than ∼2.5 μmol mol−1 indicate the presence of O2-depleted water. Here we apply this proxy to estimate past dissolved oxygen concentrations in the near surface waters of the currently well-oxygenated Southern Ocean, which played a critical role in carbon sequestration during glacial times. A down-core planktonic I/Ca record from south of the Antarctic Polar Front (APF) suggests that minimum O2 concentrations in the upper ocean fell below 70 μmol kg−1 during the last two glacial periods, indicating persistent glacial O2 depletion at the heart of the carbon engine of the Earth's climate system. These new estimates of past ocean oxygenation variability may assist in resolving mechanisms responsible for the much-debated ice-age atmospheric CO2 decline.

Over recent decades outlet glaciers of the Amundsen Sea Embayment (ASE), West Antarctica, have accelerated, thinned, and retreated, and are now contributing approximately 10% to global sea level rise. All the ASE glaciers flow into ice shelves, and it is the thinning of these since the 1970s, and their ungrounding from “pinning points” that is widely held to be responsible for triggering the glaciers' decline. These changes have been linked to the inflow of warm Circumpolar Deep Water (CPDW) onto the ASE's continental shelf. CPDW delivery is highly variable and is closely related to the regional atmospheric circulation. The ASE is south of the Amundsen Sea Low (ASL), which has a large variability and which has deepened in recent decades. The ASL is influenced by the phase of the Southern Annular Mode, along with tropical climate variability. It is not currently possible to simulate such complex atmosphere‐ocean‐ice interactions in models, hampering prediction of future change. The current retreat could mark the beginning of an unstable phase of the ASE glaciers that, if continued, will result in collapse of the West Antarctic Ice Sheet, but numerical ice sheet models currently lack the predictive power to answer this question. It is equally possible that the recent retreat will be short‐lived and that the ASE will find a new stable state. Progress is hindered by incomplete knowledge of bed topography in the vicinity of the grounding line. Furthermore, a number of key processes are still missing or poorly represented in models of ice‐flow.

We compare multi-ice core data with 18 O model output for the early last interglacial Antarctic sea ice minimum. The spatial pattern of 18 O across Antarctica is sensitive to the spatial pattern of sea ice retreat. Local sea ice retreat increases the proportion of winter precipitation, depleting 18 O at ice core sites. However, retreat also enriches 18 O because of the reduced source-to-site distance for atmospheric vapor. The joint overall effect is for 18 O to increase as sea ice is reduced. Our data-model comparison indicates a winter sea ice retreat of 67, 59, and 43% relative to preindustrial in the Atlantic, Indian, and Pacific sectors of the Southern Ocean. A compilation of Southern Ocean sea ice proxy data provides weak support for this reconstruction. However, most published marine core sites are located too far north of the 128,000 years B.P. sea ice edge, preventing independent corroboration for this sea ice reconstruction. Plain Language Summary The Antarctic isotope and temperature maximum, which occurred approximately 128,000 years before present (B.P.) during the warmer than present last interglacial period, is associated with a major retreat of Antarctic sea ice. Understanding the details of this major sea ice retreat is crucial in order to understand the sensitivity of the Southern Hemisphere sea ice system and to evaluate the performance of climate model simulations in response to future warming. This work uses a multi-ice and ocean core data-model evaluation to assess the magnitude and spatial pattern of this sea ice retreat. Our results suggest that sea ice retreat was greatest in the Atlantic and Indian sectors of the Southern Ocean and less in the Pacific sector. These results may have had serious implications for the stability of marine terminating glaciers around the Antarctic Ice Sheet and their contribution to the last interglacial sea level rise. These results also support a hypothesized slowdown in northward ocean heat transport during the early last interglacial. Ice core stable water isotope (18 O and D) records from East Antarctica show an isotopic maximum, associated with peak Antarctic warmth, around 128,000 years ago (128 ka), that is, early in the LIG. The 18 O RESEARCH LETTER

Upper Quaternary marine sediments recovered from the West Antarctic continental margin are characterized by a distinct lithological succession allowing the reconstruction of past environmental changes. Massive, homogenous diamictons were deposited elsewhere on the margin during the last glacial period, when grounded ice masses advanced across the shelf. Sedimentological investigations using a multi-proxy approach and examination of the margin topography suggest that during this time a deformation till and subsequently a glaciomarine till were deposited on the shelf, while glaciogenic debris flows were deposited on the slope and rise. The comparison of the clay mineral assemblages in the matrix of the diamictons with clay mineral assemblages of potential source areas reveals distinct pathways of grounded ice flow on the shelf. Lithogenic sandy muds overlying the diamictons were deposited by meltwater flows and/or marine currents on the shelf, and by turbidity currents and marine currents on the slope and rise, respectively. The sedimentation of the sandy muds denotes a deglaciation stage, when grounded ice started to retreat from the shelf, but semi-permanent sea-ice coverage still hampered biological productivity. Bioturbated, foraminifer-bearing sediments were deposited in a glaciomarine setting during the present interglacial period, when sea-ice cover was only seasonally present and a marine current related to the southern boundary of the Antarctic Circumpolar Current winnowed the seafloor on the outer shelf, slope and rise. D

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