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

AbstractReliable 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.