Evaluation of the sea ice proxy IP25 against observational and diatom proxy data in the SW Labrador Sea

International audienceThe recent rapid decline in Arctic sea ice cover has increased the need to improve the accuracy of the sea ice component in climate models and to provide detailed long-term sea ice concentration records, which are only available via proxy data. Recently, the highly branched isoprenoid IP25, identified in marine sediments underlying seasonal sea ice, has emerged as a potential sea ice specific proxy for past sea ice cover. We tested the reliability of this biomarker as a sea ice proxy against observational sea ice data (sea ice concentrations from the global HadISST1 database) and against a more established sea ice proxy (sea ice diatom abundance in sediments) in the South-West (SW) Labrador Sea. Furthermore, our study location at the southern margin of Arctic sea ice drift provided a new environmental setting in which to further test the novel PIP25 index. Our two study sites are located North-East (NE) and South-East (SE) of Newfoundland where box cores covering the last ca 100-150 years were collected. IP25 concentrations are nearly an order of magnitude higher and sea ice diatoms more abundant in sediments from NE of Newfoundland, where sea ice prevails 2-4 months per year compared to the sediments SE of Newfoundland, where conditions are generally ice-free year round. The IP25 fluxes NE of Newfoundland agree well with multi-decadal North Atlantic Oscillation (NAO) trends in the study area, which in previous studies have been shown to affect the climatic and sea ice conditions in the region. When assessed against observational sea ice data, IP25 appears to be a more sensitive indicator of sea ice variability in this setting compared to sea ice diatoms and proved to be a robust and reliable proxy for reconstructing low-frequency variability in past sea ice concentrations. The PIP25 index results clearly differ from the observed sea ice data underlining that caution needs to be exercised when using the index in different environmental settings