Insights into circum-Arctic sea ice variability from molecular geochemistry : the IP₂₅ index
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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 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 [mu]g/g OC) than in the Pacific (0 - 38.5 [mu]g/g OC) for equivalent sea-ice cover, and there are regional and basin-specific differences in the slope of the IP₂₅ - sea ice relationship. After normalization of IP₂₅ concentrations to that of a productivity biomarker (e.g., dinosterol; the PDIP₂₅ index) the proxy-sea ice relationship in greatly improved for all regions (r² = 0.86 and r² = 0.75 for Atlantic and Pacific, respectively) and most of the basin specific differences in the rate of change of IP₂₅ with sea ice are removed. This suggests that productivity plays an important secondary role in controlling IP₂₅ concentrations. However, the use of the PDIP₂₅ index does not change the absolute differences in concentrations seen in the Atlantic and the Pacific, and previously published data from Fram Strait remain anomalous when compared to the rest of our data. This suggests that there are additional, yet unidentified controls on the IP₂₅ proxy - sea ice relationship, which may hinder the development of an Arctic-wide calibration but that the PDIP₂₅ index is a viable tool for local and regional sea ice reconstructions.