First-year sea ice history in the Arctic Ocean inferred from ancient DNA of Polarella glacialis over the past 50,000 years

Sea ice is a crucial component of the climate system. It plays an important role in ocean circulation, the albedo of high latitude regions, and influences the biology and carbon cycling of marine ecosystems. Understanding how climatic changes have influenced sea ice in the past can inform us on past controls on polar environments and help us predict future changes. The use of multiple proxies at high resolution within sediment cores can identify fine scale climate changes in sea ice environments. Here we utilise the recently developed ancient DNA of the sea ice dwelling dinoflagellate Polarella glacialis to reconstruct first-year sea ice conditions. We trace P. glacialis quantitatively using droplet digital PCR (ddPCR), alongside biomarkers of sea-ice-associated and open-water phytoplankton. We utilise these proxies in combination to reconstruct past sea ice conditions in a sediment core from the Yermak Plateau (Arctic Ocean) dating back to MIS 3 (ca. 50,000 years ago). Our results indicate perennial and extensive sea ice coverage from MIS 3 until the end of the Last Glacial Maximum. We show that first-year sea ice, and seasonal sea ice were very variable during the Bølling-Allerød and Younger Dryas periods, including periods of open ocean during the Bølling-Allerød and permanent sea ice during the Younger Dryas. Within the Holocene we observe an increasing trend of P. glacialis DNA from the warm early Holocene to the cool late Holocene, suggesting an increase in first-year sea ice extent.