| Summary: | Application of the sympagic diatom-produced, C25 highly branched isoprenoid (HBI) termed IP25 to paleo-sea ice reconstruction has confirmed its utility as a qualitative seasonal sea ice proxy. Combination of IP25 and a pelagic biomarker into the Phytoplankton-IP25 index (PIP25) has facilitated more detailed, semi-quantitative descriptions of sea ice conditions. Further work is motivated by challenges inherent to univariate methods (such as PIP25), and the availability of multiple HBIs characteristic of ice algal and pelagic production within sedimentary archives. This study investigated the potential of incorporating multiple biomarkers to characterise contrasting sea ice and productivity conditions in the contemporary Barents Sea, applying the findings for paleo-reconstructions encompassing both abrupt and gradual climate change. Multivariate analysis of HBIs in Barents Sea surface sediments characterised by contrasting overlying sea ice conditions revealed the potential of classification trees (CTs) as a robust method of biomarker-based sea ice reconstruction. Thus, IP25 and a C25:2 analogue produced by sea ice diatoms were characteristic of extensive spring sea ice cover, while pelagic C25:3 isomers defined marginally ice-covered and ice-free areas in both surface and downcore sediments. Further, CT models did not require a correction factor and allowed systematic selection of a pelagic counterpart to IP25, thereby alleviating some inherent limitations of PIP25. In addition to the CT model, an association between a ratio of HBI C25:3 isomers and spring diatom blooms in the Barents Sea was tentatively identified, characterised by distinct relative abundances of these pelagic HBIs in regions of different productivity regimes. Further work is needed to determine biological and/or community-driven controls on this HBI triene ratio as a potential diatom bloom indicator. Finally, complementary application of CT and PIP25 methods to marine sediment cores spanning the last ca. 26 cal kyr BP at the northern and western Barents Sea continental margins resulted in reconstruction of both sea ice conditions and diatom productivity trends. At the western continental slope, extensive sea ice conditions and high sympagic production during the Last Glacial Maximum (LGM) definitively confirmed the presence of productive polynya throughout this glacial interval. After perennial sea ice and near-zero productivity resulted from the collapse of the Barents Sea Ice Sheet (BSIS) at the onset of Heinrich Stadial 1 (HS1), the ecosystem eventually recovered after rapid ice retreat as a consequence of increased Atlantic Water (AW) and reduced meltwater surges. At the northern margin, conditions during the subsequent Younger Dryas stadial were significantly ameliorated relative to the western Barents Sea at this time, possibly indicating the absence of a proximal Svalbard ice sheet, with warm AW influence. Such inferences of sea ice and productivity dynamics accompanying massive, abrupt climate change during glacial-interglacial cycles are key prerequisites for improved comprehension of current and future climate change. Centre for Arctic Gas Hydrate, Environment, and Climate (CAGE) University of Plymouth
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