Semi-quantitative estimates of paleo Arctic sea ice concentration based on source-specific highly branched isoprenoid alkenes: A further development of the PIP25 index

Previously reported concentration data for highly branched isoprenoid (HBI) alkenes and the sterol 24-methylcholesta-5,22E-dien-3β-ol (brassicasterol or epi-brassicasterol) obtained from 45 Barents Sea surface sediments reflecting contrasting sea ice conditions have been combined together in the form of the PIP25 index, with outcomes compared with recent mean decadal satellite-derived spring and summer sea ice concentrations. Reasonably good positive linear relationships were found between PIP25 indices and spring sea ice concentration, with the strongest correlation found when using a tri-unsaturated HBI (HBI III) as the open-water counterpart to the sea ice biomarker IP25, likely as a result of the greater sensitivity and selectivity of HBI III to the neighbouring sea ice conditions compared to epi-brassicasterol. The quality of the linear fits between PIIIIP25 (i.e., PIP25 based on HBI III) and spring sea ice concentration together with their respective spatial distributions were also far less dependent on the balance factor c, than for PBIP25 (i.e., PIP25 based on epi-brassicasterol), which may have important positive consequences for down-core semi-quantitative sea ice reconstruction, and when making comparisons between outcomes from different Arctic regions or climatic epochs. In contrast, consistently weak linear relationships were observed between individual biomarkers and spring or summer sea ice concentration, and between PIP25 values and summer sea ice concentration. However, we provide evidence that a lower limit threshold for PIIIIP25 might represent a useful qualitative proxy for the past occurrence of summer sea ice. Surface sediment biomarker data from other regions will be needed before the significance of our findings can be fully evaluated.