Insights into Circum-Arctic sea ice variability from molecular geochemistry

Geological records of past sea ice, such as those contained in Arctic marine sediments, offer an opportunity to strengthen our understanding of long-term sea ice variability, provided unambiguous paleo-sea ice proxies can be developed. One such recently proposed proxy is IP₂₅, a highly branched isoprenoid alkene biosynthesized exclusively by sea-ice dwelling diatoms (Haslea spp.), which is well preserved in marine sediments and could be used to reconstruct past changes in spring sea-ice extent. However, little is known about regional-scale controls on IP₂₅ production in sea ice, limiting its wider applicability as a paleo-sea-ice proxy. To address this issue we examined the distributions of IP₂₅ and the marine productivity biomarkers dinosterol and brassicasterol in a suite of surface sediment samples distributed across the Arctic. We find a statistically significant, logarithmic relationship between IP₂₅ and spring sea ice cover in sediment samples from arctic and subarctic sites in the Pacific (n = 96, R² = 0.67, P < 0.0001) and the Atlantic n = 25, R² = 0.50, P < 0.0001), though the absolute concentrations of IP₂₅ are generally higher in the Atlantic (1.6-166.4 μg/g OC) than in the Pacific (0-38.5 μg/g OC) for equivalent sea-ice cover, and there are regional and basin-specific differences in the slope of the IP₂₅ - sea ice relationship. Using the P_DIP₂₅ index, which also includes a biomarker for marine productivity (e.g., dinosterol) the relationship with sea ice cover is improved in all of the study areas (R² = 0.86 and R² = 0.75 for Atlantic and Pacific, respectively) and most basin specific differences in the proxy-sea ice relationships are significantly reduced. However, there remain differences in the absolute magnitudes of the P_DIP₂₅ index for equivalent amounts of sea ice cover between the Atlantic and the Pacific, and with previously published data from Fram Strait. Nevertheless, this work demonstrates that the P_DIP₂₅ index can provide a valuable tool for sea ice reconstructions at local and regional scales.