| Summary: | The reconstruction of past sea ice distribution in the Southern Ocean is crucial for an improved understanding of ice-ocean-atmosphere feedbacks and the evaluation of Earth system and Antarctic ice sheet models. The Western Antarctic Peninsula (WAP) is experiencing rapid warming and the associated decrease in sea ice cover contrasts the trend of growing sea ice extent in eastern Antarctica. To reveal the long-term sea ice history at the WAP under changing climate conditions we examined a marine sediment core from the eastern basin of the Bransfield Strait covering the last Deglacial and the Holocene. For sea ice reconstructions, we focused on the specific sea ice biomarker lipid IPSO25, a highly branched isoprenoid (HBI), and sea ice diatoms, whereas a phytoplankton-derived HBI triene (C25:3) and open ocean diatom assemblages reflect predominantly ice-free conditions. We further reconstruct ocean temperatures using glycerol dialkyl glycerol tetraether (GDGTs) and diatom assemblages, and compare our sea ice and temperature records with published marine sediment and ice core data. Our results document a retreat of the WAP ice shelf at 13.9 ka BP (before present). Maximum sea ice cover is observed during the Antarctic Cold Reversal, while a still extended but variable sea ice coverage characterized the core site during the early Holocene. An overall decreasing sea ice trend throughout the Middle Holocene is accompanied by a successive ocean warming and increasing phytoplankton productivity. The Late Holocene is characterized by unstable (winter) sea ice conditions and a further sea ice decline until 0.5 ka BP.
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