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

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 fluids, we investigated the lipid biomarker inventory of this seep limestone. The dominant biomarkers are (super 13) C-depleted isoprenoids including tail-to-tail linked pentamethylicosane (delta (super 13) C value: -108 per mil), representing molecular fossils of methanotrophic archaea. This observation reveals that the seepage fluids contained methane. Because the seep carbonates have been found to be only moderately (super 13) C-depleted (delta (super 13) C values as low as -14 per mil), a significant contribution from a less (super 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 flow 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 flow. 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 first approximation of seepage intensity.