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Holocene sea-ice conditions and circulation at the Chukchi-Alaskan margin, Arctic Ocean, inferred from biomarker proxies

Two sediment cores from the Chukchi Sea margin were investigated for the Arctic sea-ice biomarker IP25, along with marine and terrestrial sterols and glycerol dialkyl glycerol tetraethers (GDGTs). This is the first paleoclimatic application of IP25 in the Chukchi-Alaskan region of the Arctic, which is key for understanding Arctic–Pacific interactions and is experiencing rapid sea-ice retreat under present warming. Sea-ice and related circulation conditions were characterized in this study with a multicentury resolution for the long-term Holocene record to multidecadal for the last several centuries. Sea ice was found to be present during the entire record but with considerable spatial and temporal variability. After very low deglacial IP25 values, possibly related to permanent sea ice and/or an iceberg-dominated environment, cores from the upper slope and shelf show IP25 maxima, interpreted as representing a relative proximity to the sea-ice margin, in the early (ca. 8–9 ka) and middle (ca. 5–6 ka) Holocene, respectively. Along with isoprenoid GDGT distribution, this asynchronicity in sea-ice history probably reflects oceanographic evolution of the Chukchi margin affected by the Beaufort Gyre circulation and Pacific water inflow via Bering Strait. Data for the last several centuries, with elevated values of brassicasterol and terrestrial sterols covarying with dinosterol and IP25, are interpreted in terms of long-distance import by currents combined with diagenetic transformations. We infer that high-amplitude variability in the late ‘Little Ice Age’, starting in the late 18th century, is related to the intensity of the Alaskan Coastal Current. This interval is preceded by three centuries of presumably diminished Alaskan Coastal Current but overall increased Bering Strait Inflow resulting in reduced sea-ice cover according to dinocyst-based data.