STROMATOLITIC FABRIC OF AUTHIGENIC CARBONATE CRUSTS: RESULT OF ANAEROBIC METHANE OXIDATION AT COLD SEEPS IN 4,850 M WATER DEPTH

Abstract

Methane seepage leads to Mg-calcite and aragonite precipitation at a depth of 4,850 m on the Aleutian accretionary margin. Stromatolitic and oncoid growth structures imply encrustation of microorganisms (microbial mats) in the host sediment with a unique growth direction downward into the sediment, forming crust-shaped lithologies. Biomarker investigations of the residue after carbonate dissolution show strong enrichments in crocetane and archaeol, which contain extremely low delta(13)C values. This indicates the presence of methane-consuming archaea, and delta(13)C values of -42 to -51% PDB indicate that methane is the carbon source for the carbonate crusts. Thus, it appears that stromatolitic encrustations of methanotrophic anaerobic archaea probably occurs in a consortium with sulphate-reducing bacteria and that carbonate precipitation proceeds downward into the sediment, where ascending cold fluids provide a methane source. Strontium and oxygen isotope analyses as well as C-14 ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3,000 years ago.