Lipid biomarker patterns reflect different formation environments of mussel- and tubeworm-dominated seep carbonates from the Gulf of Mexico (Atwater Valley and Green Canyon)

Abstract

Mussels and tubeworms thriving at many methane seeps typically live in symbiosis with chemosynthetic, chiefly methanotrophic or thiotrophic bacteria. It has been shown that the activities of chemosymbiotic animals can result in large differences in the sedimentary environments of their habitats. Here, we put forward the concept that such environmental variability can be archived in the lipid biomarker inventories of authigenic carbonates forming in different, locally confined environments at seeps, mussel beds and tubeworm bushes in this case. To test this hypothesis, lipid biomarker patterns of carbonates from mussel and tubeworm environments from two seep sites (Atwater Valley 340 and Green Canyon 852) of the Gulf of Mexico were analyzed. Previous work revealed stronger carbon isotope fractionation between the methane source and biomarkers of anaerobic methane oxidizing archaea-2 (ANME-2)/sulfate-reducing Desulfosarcina/Desulfococcus (DSS) consortia than for ANME-1/DSS consortia, both performing anaerobic oxidation of methane (AOM). Similar δ13Cmethane values were found at the mussel and tubeworm sites from the same seeps and the local microbial consortia also appear to be largely similar based on the observed AOM biomarker inventories. Yet, a large average offset of 32‰ between the δ13C values of molecular fossils of sulfate-reducing bacteria (SRB) involved in AOM was observed, with lower values typifying tubeworm carbonates than mussel carbonates. This pattern is interpreted to reflect local effects on isotope fractionation caused by the chemosymbiotic metazoans at mussel- and tubeworm-dominated sites. At tubeworm-dominated sites, the excess sulfate produced by thiotrophic symbionts of tubeworms and pumped down into the sediment results in persistent production of AOM-derived bicarbonate and the enrichment of 12C in sub-surface sediments. Interestingly, tubeworm carbonates also contain high amounts of non-isoprenoidal dialkyl glycerol diethers (DAGEs) with extreme 13C depletions, representing compounds that derived from non-DSS cluster SRB. Most likely, 13C-depleted AOM-derived organic intermediates were used as carbon sources by the DAGE-producing non-DSS cluster SRB, possibly performing organoclastic sulfate reduction. Our study identifies significant variation in biomarker patterns between mussel and tubeworm carbonates at two seep sites in the Gulf of Mexico. Such variation allows to characterize different habitats at seeps, which are shaped by the interaction of chemosymbiotic seep metazoans and their symbionts with the local environment. Metazoan community composition apparently controls geobiological interaction in seep ecosystems to a large degree, which may allow tracing of the effects of chemosymbiosis into the rock record.