Biotic and abiotic degradation of the sea ice diatom biomarker IP25 and selected algal sterols in near-surface Arctic sediments

The organic geochemical IP25 (Ice Proxy with 25 carbon atoms) has been used as a proxy for Arctic sea ice in recent years. To date, however, the role of degradation of IP25 in Arctic marine sediments and the impact that this may have on palaeo sea ice reconstruction based on this biomarker have not been investigated in any detail. Here, we show that IP25 may be susceptible to autoxidation in near-surface oxic sediments. To arrive at these conclusions, we first subjected a purified sample of IP25 to autoxidation in the laboratory and characterised the oxidation products using high resolution gas chromatography-mass spectrometric methods. Most of these IP25 oxidation products were also detected in near-surface sediments collected from Barrow Strait in the Canadian Arctic, although their proposed secondary oxidation and the relatively lower abundances of IP25 in other sediments probably explain why we were not able to detect them in material from other parts of the region. A rapid decrease in IP25 concentration in some near-surface Arctic marine sediments, including examples presented here, may potentially be attributed to at least partial degradation, especially for sediment cores containing relatively thick oxic layers representing decades or centuries of deposition. An increase in the ratio of two common phytoplanktonic sterols - epi-brassicasterol and 24-methylenecholesterol - provides further evidence for such autoxidation reactions given the known enhanced reactivity of the latter to such processes reported previously. In addition, we provide some evidence that biodegradation processes also act on IP25 in Arctic sediments. The oxidation products identified in the present study will need to be quantified more precisely in downcore records in the future before the effects of degradation processes on IP25-based palaeo sea ice reconstruction can be fully understood. In the meantime, a brief overview of some previous investigations of IP25 in relatively shallow Arctic marine sediments suggests that overlying climate conditions were likely dominant over degradation processes, as evidenced from often increasing IP25 concentration downcore, together with positive relationships to known sea ice conditions. (C) 2018 Elsevier Ltd. All rights reserved.