Group 2i Isochrysidales produce characteristic alkenones reflecting sea ice distribution

<jats:title>Abstract</jats:title><jats:p>Alkenones are biomarkers produced solely by algae in the order Isochrysidales that have been used to reconstruct sea surface temperature (SST) since the 1980s. However, alkenone-based SST reconstructions in the northern high latitude oceans show significant bias towards warmer temperatures in core-tops, diverge from other SST proxies in down core records, and are often accompanied by anomalously high relative abundance of the C<jats:sub>37</jats:sub> tetra-unsaturated methyl alkenone (%C<jats:sub>37:4</jats:sub>). Elevated %C<jats:sub>37:4</jats:sub> is widely interpreted as an indicator of low sea surface salinity from polar water masses, but its biological source has thus far remained elusive. Here we identify a lineage of Isochrysidales that is responsible for elevated C<jats:sub>37:4</jats:sub> methyl alkenone in the northern high latitude oceans through next-generation sequencing and lab-culture experiments. This Isochrysidales lineage co-occurs widely with sea ice in marine environments and is distinct from other known marine alkenone-producers, namely <jats:italic>Emiliania huxleyi</jats:italic> and <jats:italic>Gephyrocapsa oceanica</jats:italic>. More importantly, the %C<jats:sub>37:4</jats:sub> in seawater filtered particulate organic matter and surface sediments is significantly correlated with annual mean sea ice concentrations. In sediment cores from the Svalbard region, the %C<jats:sub>37:4</jats:sub> concentration aligns with the Greenland temperature record and other qualitative regional sea ice records spanning the past 14 kyrs, reflecting sea ice concentrations quantitatively. Our findings imply that %C<jats:sub>37:4</jats:sub> is a powerful proxy for reconstructing sea ice conditions in the high latitude oceans on thousand- and, potentially, on million-year timescales.</jats:p>