Holocene Sea Ice and Organic Matter Dynamics in the Southern Chukchi Sea Revealed by Lipid Biomarkers

Arctic sea ice plays a critical role in regulating global climate and marine primary production, yet long-term records documenting its natural variability remain sparse in the Pacific sector of the Arctic Ocean. This limitation hampers our ability to establish a regionally coherent understanding of how sea ice responds to climatic and oceanographic forcing on centennial to millennial timescales. Here, we present a new biomarker-based reconstruction of Holocene sea ice and environmental change from the southern Chukchi Sea, north of the Bering Strait.

A 519-cm sediment core (SKQ-VC29) was recovered using a vibracorer and spans the last ~8.6 kyr, based on 17 AMS radiocarbon dates from shells and terrestrial macrofossils. Downcore concentrations of highly branched isoprenoids (HBIs) and sterols were quantified to reconstruct sea-ice conditions, marine productivity, and terrestrial organic matter (OM) inputs. Seasonal sea ice presence is inferred from IP25, a mono-unsaturated HBI produced by sea-ice diatoms, while open-water conditions and phytoplankton productivity are tracked using HBI III, brassicasterol, and dinosterol. These proxies are combined using the PIP25 index to provide a semi-quantitative reconstruction of sea-ice cover. Terrestrial inputs are assessed using vascular-plant sterols (campesterol and β-sitosterol), alongside bulk δ¹³C and C:N ratios.

The record indicates predominantly open-water conditions during the early to mid-Holocene, followed by the reappearance of seasonal sea ice at ~2.5 kyr BP—substantially later than in more northerly Arctic records. This delayed signal suggests that Neoglacial sea-ice expansion in the Pacific Arctic was spatially heterogeneous. Bulk OM proxies and declining β-sitosterol concentrations indicate a progressive reduction in terrestrial OM delivery through the Holocene, while marine productivity remains relatively stable. A pronounced shift at ~4 ka BP marks reduced organic carbon accumulation and broader environmental reorganization.

Together, these results improve spatial coverage of Holocene sea-ice reconstructions in the Pacific Arctic and highlight the complex, regionally variable nature of sea-ice evolution in a climatically sensitive gateway region.