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 PDIP25 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 PDIP25 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 PDIP25 index can provide a valuable tool for sea ice reconstructions at local and regional scales.

► Arctic-wide IP25 concentrations are assessed as a paleo sea ice proxy.
► Significant exponential relationship between IP25 and spring–summer sea ice.
► The IP25–sea ice relationship is improved by ratioing to productivity (PIP25 index).
► Regional differences remain in the PIP25 index – sea ice relationship.