What Happens when Sea Ice Retreats, Peatlands Form, and a Landbridge Drowns? A Molecular View of the Last Deglacial from the Pacific-Arctic Gateway

Abstract:

Arctic peatlands and thermokarst lakes (TK) are thought to play a significant role in changing atmospheric methane concentration (AMC) during the last deglacial. However, there is debate concerning timing of their initiation and extent they drove variations in AMC. Models show sea ice cover (SIC) and sea surface temperatures (SSTs) can also play a significant role. Yet, changes in peatland/TK lake areal extent in response to those dynamics as continental shelves were submerged are often not considered. To examine such connections, we report on molecular proxies in marine records that reveal change in terrestrial organic matter (TOM) export, SIC, and SSTs as sea levels rose during the last 18 ka in the Pacific-Arctic Gateway. Here, TOM input to the ocean was tracked by measuring the flux of branched glycerol dialkyl glycerol tetraethers, n-alkyl lipids, and pentacyclic triterpenoids. SIC and SSTs were reconstructed using modern calibrations of highly branched isoprenoid alkene abundances in surface sediments from the Bering and Chukchi Seas. SSTs were also reconstructed based on the relative abundance of isoprenoid glycerol dialkyl glycerol tetraethers. Our sediment records reveal increased flux of TOM coincides with peatland/TK lake initiation, reduced SIC (~20%), and warmer SSTs (~4°C) as AMC increased during the Bølling-Allerød (BA). Terrestrial flux dramatically reduced as SIC increased (~50%) and SSTs cooled as AMC fell during the Younger Dryas. Most notably, TOM export rapidly rebounds as AMC abruptly rose throughout the Holocene Thermal Maximum (HTM), when SSTs warmed by ~3°C and SIC diminished and peatland areal extent increased. Using multi-proxy evidence in combination with a simple model that accounts for submergence of peatland/TK lake area, we estimate that the exposed Beringian shelf emitted an amount of CH₄ comparable to previously reported peatland emissions in Alaska during the BA and HTM. The GDGT-based methane index (MI) was <0.3 throughout our sediment records, suggesting destabilized marine gas hydrates were not the principal source of methane. Results of our study provide novel marine-based evidence for the timing of peatland and TK lake initiation and the role sea ice played in contributing to variations in AMC during deglacial sea level rise in the Arctic.