Sedimentary ancient DNA and HBI biomarkers as sea-ice indicators: A complementary approach in Antarctic fjord environments

Reliable high-resolution, pre-observational-period sea-ice datasets are rare but critical for contextualizing recent sea-ice declines and future scenarios. We combine sedimentary ancient DNA of the sea-ice dinoflagellate Polarella glacialis (Pgla-sedaDNA) with selected highly branched isoprenoid (HBI) biomarkers alongside other indicators to reconstruct sub-decadal sea-ice changes in a marine archive from the Antarctic Peninsula that extends to ~ 1900 CE. Pre-1940 CE, the continuously present sea-ice biomarker IPSO₂₅ yet absent Pgla-sedaDNA, along with low open-water biomarkers and total organic carbon (TOC), imply more prominent seasonal sea ice and lower productivity under cooler climate. Post-1940 CE, rising Pgla-sedaDNA and open-water HBIs under climate warming reflect young ice with a retreating sea-ice edge. Over the last two decades, lower Pgla-sedaDNA, higher open-water HBIs and TOC infer known warming, sea-ice reduction, and increased productivity. Our multiproxy-based palaeo-histories agree well with observational data, highlighting the potential of this combination of proxies for nuanced and long-term sea-ice reconstructions.