Abstract. Over recent decades Antarctic sea-ice extent has increased, alongside widespread ice shelf thinning and freshening of waters along the Antarctic margin. In contrast, Earth system models generally simulate a decrease in sea ice. Circulation of water masses beneath large-cavity ice shelves is not included in current Earth System models and may be a driver of this phenomena. We examine a Holocene sediment core off East Antarctica that records the Neoglacial transition, the last major baseline shift of Antarctic sea ice, and part of a late-Holocene global cooling trend. We provide a multi-proxy record of Holocene glacial meltwater input, sediment transport, and sea-ice variability. Our record, supported by high-resolution ocean modelling, shows that a rapid Antarctic sea-ice increase during the mid-Holocene (∼ 4.5 ka) occurred against a backdrop of increasing glacial meltwater input and gradual climate warming. We suggest that mid-Holocene ice shelf cavity expansion led to cooling of surface waters and sea-ice growth that slowed basal ice shelf melting. Incorporating this feedback mechanism into global climate models will be important for future projections of Antarctic changes.

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A ntarctica's marine margin is a complex biological and oceanographic system in which sea-ice growth, Antarctic Bottom Water (AABW) formation and high primary productivity act as a significant CO 2 sink and ventilate the Southern Ocean 1,2 . High primary productivity occurs where nutrients are brought to the surface, including oceanographic fronts 3 , polynyas 4 , upwelling near the continental shelf break 5 and the marginal ice zone (MIZ) 6 , all of which are influenced by Antarctic wind fields. High productivity and export events around Antarctica occur with changing insolation and stratification associated with sea-ice break-up 7 . Large-scale subdecadal climate modes, specifically the El Niño-Southern Oscillation (ENSO), Southern Annular Mode (SAM) and Indian Ocean Dipole (IOD), are known to affect sea ice 8,9 and wind fields 10 around Antarctica. The teleconnection between ENSO (which varies at 2-7 year periods) and Antarctic sea-ice variability is largely driven by wind changes resulting from hemispheric-scale sea level pressure and 500 mbar height anomalies 8 . This teleconnection can be amplified or dampened by other subdecadal climate modes such as the IOD and SAM 9-12 . Collectively, these subdecadal climate modes alter meridional and zonal wind flows 9,10 that regulate sea-ice break-out 11 at 2-7 year periods, thus influencing primary productivity in Antarctica [13][14][15] . Clarifying how the annual cycle and subdecadal-scale climate modes have impacted past Antarctic coastal systems will inform models used to project future system response 16 .We investigate a 170 m sediment core recovered from the Adélie Basin (Integrated Ocean Drilling Program (IODP) Site U1357B) 17 along the Wilkes Land margin of East Antarctica (Fig. 1). The Adélie Basin is a region of high primary productivity near the MIZ. It also lies beneath and downstream of several large polynya systems and the westward-flowing Antarctic Coastal Current. The drill site targeted a high-sedimentation (~1.5-2 cm per year) drift deposit (Adélie Drift) dominated by pelagic biogenic deposition. It provides an ultra-high-resolution Holocene record of climate and oceanographic variability adjacent to the Mertz Polynya system, one of the largest exporters of sea ice and AABW along the East Antarctic margin 2 . Previously collected Antarctic cores have significantly lower sedimentation rates, and alternate between massive (bioturbated) and laminated diatom ooze 18,19 . They cannot resolve high-frequency change at subdecadal scales. However, U1357B is continuously laminated, and high sedimentation rates afford an unprecedented opportunity to assess subdecadal and annual changes at the Antarctic oceanic margin. An ultra-high-resolution record of marine biogenic bloomsThe east-west elongated Adélie Drift deposit formed parallel to the wind-driven Antarctic Coastal Current 2,20 . This current influences both surface and deep waters on the continental shelf 2,20 . Consequently, the mass accumulation rate (MAR) (Methods) in this drift is thought to reflec...

Abstract. The Antarctic coastal zone is an area of high primary productivity, particularly within coastal polynyas, where large phytoplankton blooms and drawdown of CO2 occur. Reconstruction of historical primary productivity changes and the associated driving factors could provide baseline insights on the role of these areas as sinks for atmospheric CO2, especially in the context of projected changes in coastal Antarctic sea ice. Here we investigate the potential for using carbon isotopes (δ13C) of fatty acids in marine sediments as a proxy for primary productivity. We use a highly resolved sediment core from off the coast of Adélie Land spanning the last ∼ 400 years and monitor changes in the concentrations and δ13C of fatty acids along with other proxy data from the same core. We discuss the different possible drivers of their variability and argue that C24 fatty acid δ13C predominantly reflects phytoplankton productivity in open-water environments, while C18 fatty acid δ13C reflects productivity in the marginal ice zone. These new proxies have implications for better understanding carbon cycle dynamics in the Antarctica coastal zone in future palaeoclimate studies.

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