Reconstructing Microbial & Animal Associated Formation and Dissolution of Methane Seep Carbonates using U/Th Dates and Electron Microscope Imagery

Active venting of methane from organic matter buried below the seafloor supports a unique diversity of life in the overlying sediment and on the seafloor. The consumption of this methane by microbial consortia sustains animal symbionts and reduces the amount of methane reaching the atmosphere, representing a key methane sink in the marine carbon cycle. Microbially mediated sulfate-dependent anaerobic methane oxidation also precipitates authigenic carbonate rocks. Under anoxic conditions, these carbonates can form large outcropping rock features on the seafloor that provide hard substrate for seep-associated endemic symbiotic macro and micro fauna, affecting deep ocean biodiversity. In the presence of oxygen, however, microbial and animal activity promotes the dissolution of seep carbonates, returning carbon to the water and short-term carbon cycle. To examine how seep chemical and biologic activity affects carbonate formation and dissolution, we determined the age, composition, and growth structure of seep carbonates from a range of water depths, ambient oxygen concentrations, and methane flux environments. Carbonates were collected from Southern California Borderland (800 and 1020 m) and Aleutian Trench (2020 m) seeps and subsampled to allow comparisons across both km- and µm- scales. U/Th dating revealed carbonate ages ranging from 201±100 to 10,138±63 years. Micro-scale rock fabric texture, microbe-mineral paragenesis, and elemental composition were determined from Scanning Electron Microscope backscatter images and energy-dispersive x-ray detector spectrum maps along with thin-section Electron Probe Micro-Analyzer images. Micro-scale results are used to examine the microbial-mineral interactions visible through fossil and crystal inclusions and discontinuities. We contextualize the macro-scale growth histories of the dated carbonates by relating them to variations in glacial-interglacial associated sea level and methane hydrate stability; overlying water column productivity and circulation related oxygen availability; and tectonic or tidal associated methane flux. These preliminary results improve our understanding of long-term biological and chemical processes associated with seep carbonate formation and dissolution, and their implications for global carbon cycling.