Compound-specific carbon and hydrogen isotope analysis traces archaeal lipid signatures in cold seep marine systems

Abstract

Compound-specific stable isotope analyses of archaeal lipid biomarkers can provide useful information on biogeochemical cycling in marine systems. While most work has focused on carbon isotopes, the analysis of hydrogen isotope ratios (δ²H) remains underexplored for its potential to characterize sources of lipids and metabolic diversity in marine sedimentary systems. This study examines the δ²H values of archaeal biphytanes, derivatives of the parent isoprenoid glycerol dialkyl glycerol tetraether lipids (iGDGTs), as a tool to discriminate archaeal sources to sediments collected from Astoria Canyon, a cold-seep system along the Cascadia Margin. Using a mixing model applied to biphytane δ²H values obtained from individual iGDGTs, we estimate the δ²H signatures of the two primary endmember sources in this location: planktonic and benthic methane cycling communities. Additionally, we determine the structural biphytane combinations within each parent iGDGT. Our findings indicate that planktonic and benthic methane-cycling archaea both generate highly fractionated lipids, with modeled δ²H signatures of –308 ‰ and –222 ‰, respectively. When combined with reported compound-specific δ¹³C values for the corresponding iGDGTs, the dual-isotope approach allows us to determine the mean δ²H signatures of the two dominant archaeal communities, providing new endmember constraints for lipid source apportionment as well as the underlying biphytane structures comprising intact iGDGTs. These findings provide an interpretative framework for applying compound-specific stable isotopic approaches to disentangle sediment iGDGT sources and track methane cycling.