CHEMOHERM BUILD-UPS ON HYDRATE RIDGE, CASCADIA MARGIN: A RESULT OF METHANE VENTING AND ANAEROBIC METHANE OXIDATION (AMO)

At Hydrate Ridge, two chemoherms, the so-called 'Pinnacle' and the 'South-East-Knoll', were discovered in 1999 during several research cruises (AT3-35B, SO143). These methane-derived carbonate mounds display very characteristic features. They rise 50 to 90 m above the seafloor with steep flanks. The morphology of the mounds is very irregular due to the porous structure of the primarily aragonitic precipitates. Cracks and fractures in the carbonate are often lined with chemosynthetic organisms (e.g. Calyptogena) and white or orange bacterial mats (Beggiatoa). In some places, small patches of bacterial mats cover larger areas of the chemoherm surface. The occurrence of these cold seep specific organisms proves an ascent of methane-rich fluids or free gas through fluid pathways in the chemoherm. At the SE-Knoll locality, bubbles of methane gas were observed visually as well as acoustically.

Carbon isotopic values give strong evidence for the formation of carbonate from methane by anaerobic methane oxidation (AMO) performed by a microbial consortium. The consortium in return depends on the availability of methane and sulfate. Archaea anaerobically oxidize the methane from the fluids while sulfate-reducing bacteria reduce the seawater sulfate. Beggiatoa mats covering the surface of the carbonates oxidize the sulfide which is produced during the process of sulfate reduction. AMO, which creates reducing micro-environments on the surface of the chemoherm, is essential for the precipitation of carbonate. Bicarbonate produced during AMO and increased alkalinity trigger the precipitation of new carbonate. Ideal habitats for the microbes are around the openings of fluid channels and along fractures where methane-rich fluids emanate. Therefore, the chemoherms grow upwards which is the main direction of the pathways for fluids and gas.