BIOGEOCHEMICAL PATTERNS OF ANCIENT METHANE SEEPS

The oldest known seep deposits with macroinvertebrate taxa are Devonian in age. Although more and more examples continue to be discovered, the geological record of methane-related environments is still fragmentary and a matter of intense current research. In this contribution, characteristic features that testify to the biogeochemical processes in ancient methane-related settings are compiled. Carbonates typically precipitate through the microbial oxidation of methane, and specific carbonate fabrics and phases are produced, namely (i) inverted stromatactoid cavities, (ii) upside-down stromatolites, (iii) globular fabrics, (iv) botryoidal aragonite, (v) micritic nodules, (vi) fractures, (vii) clotted micrites, and (viii) constructive seams representing fossilised biofilms. The carbon of the early diagenetic carbonate phases was derived predominantly from the oxidation of methane which resulted in low b¹³C values. Pyrites enclosed in the seep limestones generally show low d³⁴S values and a significant variability of their isotopic composition on a small scale, both indicating bacterial sulphate reduction as the sulphide-supplying process. Furthermore, many authigenic seep carbonates show a remarkable preservation of source-specific organic substances. These biomarkers can reveal information on the taxa involved in the cycling of carbon at the ancient locations. Biomarker signatures in seep carbonates of different ages (Recent to Late Jurassic) and thermal maturities (low to moderate) are compiled, and their potential for tracing the modes of carbon cycling into the geological record is discussed. Typical compounds include isoprene-based lipids from archaea, and linear and monomethyl-branched carbon skeletons assigned to sulphate-reducing bacteria. Strong ¹³C-depletions of individual compounds, with d¹³C-values sometimes below -100 per mil vs. PDB, indicate an anabolic uptake of methane carbon by the source biota. The patterns are consistent with a syntrophic relationship of archaea and sulphate reducing bacteria in the anaerobic oxidation of methane, and the close association of these microbes observed at Recent methane seeps.