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), C-13-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 1 3 C-depleted archaeal lipid biomarkers such as crocetane (delta C-13 similar to -80 parts per thousand) and PMI (similar to -100 parts per thousand), 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 C-13-depleted, regular C-21 to C-24 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. C-13-depleted 17 alpha(H),21 beta(H) and 17 beta(H),21 alpha(H)-hopanes (C-30-C-34) with 22S- and 22R- isomer couplets (> C-31) are present in all samples in distributions indicative of a moderate thermal maturity. Low delta C-13 values (-78 parts per thousand to -60 parts per thousand) suggest that these are derived from anaerobic bacteria involved in AOM. Notably, 22S-isomers are consistently enriched in C-13 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. (c) 2006 Elsevier Ltd. All rights reserved.