Variability of sea-ice cover and marine primary production in the Chukchi Sea over the Industrial Era: Insights from new lipid biomarker sedimentary records

Sea ice is an essential component of the Arctic climate system and a key player in Arctic amplification. The Chukchi Sea (western Arctic Ocean) is one of the largest marginal seas in the world and has become a key area to study ecosystem change with retreating sea ice over recent centuries. Here, we analyze a suite of geochemical tracers including total organic carbon (TOC), total nitrogen (TN), δ¹³C and δ¹⁵N and biomarker concentrations such as highly branched isoprenoids (HBIs) and sterols from a sediment core located on the Chukchi shelf to reconstruct the sea-ice variability and corresponding phytoplankton community changes over the past century, then further evaluate the role of potential key physical drivers (e.g., AO, PDO and AMO) on sea ice in this sensitive region. The sea-ice proxy HBIs record shows a transition from extensive sea ice in the late 19th century to Marginal Ice Zone (MIZ) in AD 1930–1990s and then moderate sea-ice cover since 1990s. Rising of all HBI abundances between AD 1865–1875 indicate a transient retreat of summer ice edge off the shelf followed by a return to near-perennial sea ice till 1920–1930 as revealed by the absence of HBIs and brassicasterol. Sea ice retreat occurred again in AD 1920–1930 and followed by colder decades in 1940s–1960s before a sustained decline since the 1990s. Temporal variation in the concentration of proportion of specific phytoplankton sterols indicate generally increased in marine primary productivity since the late 1970s, which was coinciding with the decrease of sea ice cover. Our results also suggest a minor role of PDO and AMO on seasonal sea ice as compared to AO since the 20th century in agreement with model experiments.