Methane output from cold seep structures along the Central American continental margin
Methane seepage from different geological structures was investigated along the convergent margin off the Pacific coast of Costa Rica in order to identify the fate and the amount of methane discharging into the ocean. Thus, the water column above four mud extrusions and one of several embayments caused by the subduction of seamounts (scarps) was examined by means of methane concentration analyses, carbon isotopic characterization of the methane and current measurements. It was discovered, that most of the vent derived methane is dispersed and diluted by mixing with seawater having background methane-concentrations whereas only a part of the methane is oxidized. Besides, repeated measurements indicate long-term variations in methane seepage, which might be related to earthquake activity. The amount of methane released from mud extrusions and scarps offshore Costa Rica was estimated to be ~0.2‰ of the global methane output from the seafloor, which is reported to be in the order of 20 Tg yr-1. The estimated methane outputs present the first ones from an erosive convergent margin and are of great significance for assessments of the global methane output in general and of specific geological structures, e.g. mud extrusions, in particular.