Carbonate chimneys at the highly productive point Dume methane seep: Fine‐scale mineralogical, geochemical, and microbiological heterogeneity reflects dynamic and long‐lived methane‐metabolizing habitats.

Methane is a potent greenhouse gas that enters the marine system in large quantities at seafloor methane seeps. At a newly discovered seep site off the coast of Point Dume, CA, ~ meter‐scale carbonate chimneys host microbial communities that exhibit the highest methane‐oxidizing potential recorded to date. Here, we provide a detailed assessment of chimney geobiology through correlative mineralogical, geochemical, and microbiological studies of seven chimney samples in order to clarify the longevity and heterogeneity of these highly productive systems. U‐Th dating indicated that a methane‐driven carbonate precipitating system at Point Dume has existed for ~20 Kyr, while millimeter‐scale variations in carbon and calcium isotopic values, elemental abundances, and carbonate polymorphs revealed changes in carbon source, precipitation rates, and diagenetic processes throughout the chimneys' lifespan. Microbial community analyses revealed diverse modern communities with prominent anaerobic methanotrophs, sulfate‐reducing bacteria, and Anaerolineaceae; communities were more similar within a given chimney wall transect than in similar horizons of distinct structures. The chimneys represent long‐lived repositories of methane‐oxidizing communities and provide a window into how carbon can be transformed, sequestered, and altered over millennia at the Point Dume methane seep. [ABSTRACT FROM AUTHOR]