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In this study, we aimed to reconstruct spring (April–June) sea ice changes in thewesternArcticOcean over recent centuries (ca. the last 250 years) bymeasuring biomarker distributions in amulticore (ARA01B-03MUC) retrieved from theChukchi Shelf region and to evaluate outcomes against known ormodelled estimates of sea ice conditions. Specifically, we analyzed for the Arctic sea ice proxy IP25 and assessed the suitability of a further highly branched isoprenoid (HBI) lipid (HBI III), epibrassicasterol, and dinosterol as complementary biomarkers for usewith the so-called phytoplankton marker-IP25 index (PIP25; PIIIIP25, PBIP25, and PDIP25, respectively). The presence of IP25 throughout core ARA01B-03MUC confirms the occurrence of seasonal sea ice at the study site over recent centuries. Froma semi-quantitative perspective, all three PIP25 indices gave different trends, with some dependence on the balance factor c, a termused in the calculation of the PIP25 index. PIIIIP25-derived spring sea ice concentration (SpSIC) estimates using a c value of 0.63, determined previously from analysis of Barents Sea surface sediments, were likelymost reliable, since SpSIC values were high throughout the record (SpSIC>78%), consistent with themodern context for the Chukchi Sea and themean SpSIC record of the 41CMIP5 climatemodels over recent centuries. PBIP25-based SpSIC estimates were also high (SpSIC 108%−127%), albeit somewhat over-estimated, when using a c value of 0.023 obtained from a pan-Arctic distribution of surface sediments. In contrast, PDIP25 values using a pan-Arctic c value of 0.11, and PIP25 data based on themean biomarker concentrations from ARA01B-03MUC, largely underestimated sea ice conditions (SpSIC as low as 13%), and exhibited poor agreementwith instrumental records ormodel outputs. On the other hand, PBIP25 values using a c factor based onmean IP25 and epi-brassicasterol concentrations exhibited a decline towards the core top, which resembled recent decreasing changes in summer sea ice conditions for the Chukchi Sea; however, further work is needed to test the broader spatial generality of this observation.

Reconstructing spring sea ice concentration in the Chukchi Sea over recent centuries: insights into the application of the PIP25 index

Reconstructing spring sea ice concentration in the Chukchi Sea over recent centuries: insights into the application of the PIP₂₅ index