Abstract
In this study, we aimed to reconstruct spring (April–June) sea ice changes in the western Arctic Ocean over recent centuries (ca. the last 250 years) by measuring biomarker distributions in a multicore (ARA01B-03MUC) retrieved from the Chukchi Shelf region and to evaluate outcomes against known or modelled 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), epi-brassicasterol, and dinosterol as complementary biomarkers for use with 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. From a semi-quantitative perspective, all three PIP25 indices gave different trends, with some dependence on the balance factor c, a term used 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 likely most reliable, since SpSIC values were high throughout the record (SpSIC > 78%), consistent with the modern context for the Chukchi Sea and the mean SpSIC record of the 41 CMIP5 climate models 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 the mean biomarker concentrations from ARA01B-03MUC, largely underestimated sea ice conditions (SpSIC as low as 13%), and exhibited poor agreement with instrumental records or model outputs. On the other hand, PBIP25 values using a c factor based on mean 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.
Highlights
Over the last four decades, the Arctic Ocean has experienced a persistent loss of sea ice, which is one of its main characteristics (e.g. Stroeve et al 2007, Serreze and Stroeve 2015)
IP25 was detected throughout core ARA01B-3MUC with variable concentration (0.3− 2.6 μg g−1 total organic carbon (TOC)) and highest values towards the core top
The northern hemisphere sea ice biomarker IP25 was present throughout multicore ARA01B-03MUC retrieved from the Chukchi Shelf, reflecting the occurrence of seasonal sea ice at the core site over recent centuries
Summary
Over the last four decades, the Arctic Ocean has experienced a persistent loss of sea ice, which is one of its main characteristics (e.g. Stroeve et al 2007, Serreze and Stroeve 2015). Such trends have been based mainly on satellite passive-microwave records spanning the last 40 years (e.g. de Vernal et al 2013), some historical records extend back to ca. A number of biogenic or geochemical proxies have been developed and applied to reconstruct sea ice conditions in the past (de Vernal et al 2013 and references therein). IP25 has become an established northern hemisphere proxy for the qualitative occurrence of seasonal sea ice in various paleo records (e.g. Belt and Müller 2013, Belt 2018)
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