Archaeal lipid concentration and isotopic data in Black Sea and underlying sediments

Archaeal lipids in marine sediments are important biomarkers widely used as indicators of paleoclimate and benthic archaeal ecology. However, the relative contributions of diverse sources of archaeal lipids based on planktonic deposition and benthic production, and the turnover of lipids are poorly constrained. To add constraints on sources and turnover of archaeal lipids in marine sediments, we determined the concentration and stable carbon isotopic compositions of intact polar lipids (IPLs) and core lipids (CLs) through the water column (N=9) and within an 8-m long sediment core (N=28) of the Black Sea. The δ13C values of lipids in the water column range from -19.9 to -43.1‰; lipids from the lower suboxic zone of the water column were primarily contributing signals to the surface sediment due to the similarity in δ13C values. δ13C values of crenarchaeol-based biphytane (BPcren) in both IPL- and CL-pools are indistinguishable in sediments during limnic deposition within the last glacial period, suggesting a long turnover time of plankton-derived IPL-crenarchaeol and the lack of production by sedimentary archaea. Intriguingly, caldarchaeol-derived BP0 from IPLs is on average 3‰ more negative relative to its CL analog. This offset is consistent with sedimentary archaea utilizing carbon pools that are isotopically distinct from those utilized by planktonic archaea. Using the δ13C value of lactate as a reference value for organic substrates utilized by heterotrophic archaea, we estimate that on average 34% of IPL-BP0 is produced by sedimentary archaea, with 28% occurring at the sediment surface. However, a negative deviation of the δ13C values of CL-derived BP0 between 100 and 600 cm below seafloor is consistent with the addition of hydrolytic products from the 13C-depleted IPL pool, which is supported by a decrease of IPL-caldarchaeol content relative to IPL-crenarchaeol. Isotope mass balance calculations suggest that on average 18% of CL-BP0 is derived from IPL degradation. Increasing 13C-depletions with depth of archaeol-derived phytane, specifically in the methanic zone, revealed a deviating source than the glycerol dialkyl glycerol tetraether-producing archaea. This trend corresponded to an increase in the IPL archaeol content, suggestive of archaeol being a sensitive marker of methane cycling archaea. Despite in situ sedimentary production of archaeal lipids, limited effects were found on CL-based proxies although IPL-based proxies were severely impacted. Our high-resolution analysis of both concentration and isotopes of CLs and IPLs gives insight into the sources and turnover of archaeal lipids and further constrains their application in the paleoenvironmental and biogeochemical studies in marine environments.