Lipid biomarker patterns of methane-seep microbialites from the Mesozoic convergent margin of California

In order to reconstruct biogeochemical pathways at Mesozoic methane-seeps, a set of Late Jurassic (Tithonian) to Early Cretaceous (Aptian/Albian), 13C-depleted seep-limestones from forearc strata in western California were subjected to detailed molecular-isotopic biomarker analyses. Two of the microbial carbonate deposits are turbidite-hosted/fault-related, whereas one is hosted in serpentinite in a diapir-related setting. The limestones contain 13C-depleted archaeal lipid biomarkers such as crocetane (δ13C ∼ −80‰) and PMI (∼ −100‰), indicative of an involvement of anaerobic oxidation of methane (AOM) in carbonate precipitation. Isotopically depleted crocetane in the Tithonian sample represents the oldest reported occurrence of this compound at methane-seeps. In the set of samples, a series of strongly 13C-depleted, regular C21 to C24 isoprenoids possibly results from diagenetic alteration of archaeal sesterterpanylglycerol diethers as suggested by the presence of the putative intermediate 3,7,11,15,19-pentamethylicosanoic acid. 13C-depleted 17α(H),21β(H) and 17β(H),21α(H)-hopanes (C30-C34) with 22S- and 22R- isomer couplets (>C31) are present in all samples in distributions indicative of a moderate thermal maturity. Low δ13C values (−78‰ to −60‰) suggest that these are derived from anaerobic bacteria involved in AOM. Notably, 22S-isomers are consistently enriched in 13C relative to their 22R-counterparts. Our samples represent 70 myr of seepage activity and AOM along the Mesozoic margin of western California, filling the gap between the currently oldest methane-seep biomarker record from the Oxfordian (Late Jurassic) and the more widely recognised Cenozoic examples.