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Abstract Polar marine ecosystems are particularly vulnerable to the effects of climate change. Warming temperatures, freshening seawater, and disruption to sea‐ice formation potentially all have cascading effects on food webs. New approaches are needed to better understand spatiotemporal interactions among biogeochemical processes at the base of Southern Ocean food webs. In marine systems, isoscapes (models of the spatial variation in the stable isotopic composition) of carbon and nitrogen have proven useful in identifying spatial variation in a range of biogeochemical processes, such as nutrient utilization by phytoplankton. Isoscapes provide a baseline for interpreting stable isotope compositions of higher trophic level animals in movement, migration, and diet research. Here, we produce carbon and nitrogen isoscapes across the entire Southern Ocean (>40°S) using surface particulate organic matter isotope data, collected over the past 50 years. We use Integrated Nested Laplace Approximation‐based approaches to predict mean annual isoscapes and four seasonal isoscapes using a suite of environmental data as predictor variables. Clear spatial gradients in δ¹³C and δ¹⁵N values were predicted across the Southern Ocean, consistent with previous statistical and mechanistic views of isotopic variability in this region. We identify strong seasonal variability in both carbon and nitrogen isoscapes, with key implications for the use of static or annual average isoscape baselines in animal studies attempting to document seasonal migratory or foraging behaviors. 

Abstract. Over recent decades Antarctic sea-ice extent has increased, alongsidewidespread ice shelf thinning and freshening of waters along the Antarcticmargin. In contrast, Earth system models generally simulate a decrease insea ice. Circulation of water masses beneath large-cavity ice shelves is notincluded in current Earth System models and may be a driver of thisphenomena. We examine a Holocene sediment core off East Antarctica thatrecords the Neoglacial transition, the last major baseline shift ofAntarctic sea ice, and part of a late-Holocene global cooling trend. Weprovide a multi-proxy record of Holocene glacial meltwater input, sedimenttransport, and sea-ice variability. Our record, supported by high-resolutionocean modelling, shows that a rapid Antarctic sea-ice increase during themid-Holocene (∼ 4.5 ka) occurred against a backdrop ofincreasing glacial meltwater input and gradual climate warming. We suggestthat mid-Holocene ice shelf cavity expansion led to cooling of surfacewaters and sea-ice growth that slowed basal ice shelf melting.Incorporating this feedback mechanism into global climate models will beimportant for future projections of Antarctic changes.