Molecular fossils reveal fluid composition and flow intensity at a Cretaceous seep

Abstract

An isolated Hauterivian marine limestone from the Crimean Peninsula containing masses of articulated specimens of the dimerelloid brachiopod Peregrinella has previously been interpreted to represent a hydrocarbon-seep deposit. In order to constrain the intensity of seepage and the composition of fl uids, we investigated the lipid biomarker inventory of this seep limestone. The dominant biomarkers are (13)C-depleted isoprenoids including tail-to-tail linked pentamethylicosane (delta(13)C value: -108 parts per thousand), representing molecular fossils of methanotrophic archaea. This observation reveals that the seepage fl uids contained methane. Because the seep carbonates have been found to be only moderately (13)C-depleted (delta(13)C values as low as -14 parts per thousand), a signifi cant contribution from a less (13)C-depleted carbon source than methane, probably marine carbonate, is apparent. Such a degree of admixture of marine carbonate is typical for seep limestones resulting from low fl ow rates. The observed biomarker pattern with the prominent occurrence of biphytanes, but lacking crocetane, reveals that the methanotrophic archaea at the Hauterivian seep site were similar to archaea of the ANME-1 cluster. Archaea of this cluster are known to be able to cope with lower methane concentrations than ANME-2 archaea; therefore ANME-1 archaea are better adapted to low seepage rates and diffusive fl ow. The Peregrinella limestone contains only a small amount of early diagenetic cement. Based on a comparison with biomarker patterns of other ancient seep deposits, it is apparent that diffusive seepage typically results in limestones with little cement, whereas advective, more intense seepage appears to favor cement precipitation. If applied with caution, this supposed relationship can be used as a fi rst approximation of seepage intensity.