Carbonate mineralogical and isotopic analyses of TV-guided grab samples and TV-guided box corer samples from the Shumagin area

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 d13C values. This indicates the presence of methane-consuming archaea, and d13C values of -42 to -51 per mill 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 14C ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3,000 years ago.