Methane anomalies in seawaters of the Ragay Gulf, Philippines: methane cycling and contributions to atmospheric greenhouse gases

Brief description

The vertical distribution of methane has been measured in the water column of a semi-enclosed basin, the Ragay Gulf, in the Philippines archipelago. The methane distribution is characterised by unusual mid-water and bottom-water plumes, between 80 and 100 m thick. The plumes are confined to water depths between about 100 and 220 m, where the temperature-depth (a proxy for seawater density) gradient is steepest. Plumes of high methane are trapped within the main thermocline; these are local features, persisting over kilometre-scale distances. Geochemical and geological evidence suggests that the elevated methane concentrations are thermogenic in origin (although an oxidised biogenic origin cannot be ruled out for some of the methane anomalies), and have migrated from the sea floor into the overlying water. The mid and bottom-water methane maxima support fluxes of methane from depth into surface waters and, subsequently, from the oceans to the atmosphere. The average supersaturation of methane in the top 5 m of the sea, at nine locations , was 206±16.5%; range 178- 237%. The average estimated sea- air flux was 101 nmole.cm-2.y-1; range 75- 129 nmole.cm-2.y-1, and probably represents a minimum flux, because of low wind speeds of < 10 knots. These fluxes, we suggest, are supported by seepage from the sea floor and represent naturally occurring fluxes of mostly fossil methane (in contrast to anthropogenic fossil methane), from the sea to the atmosphere. The estimated minimum fluxes of naturally occurring fossil methane are comparable to those biogenic fluxes measured elsewhere in the surface oceans , but are less than those naturally occurring methane inputs from sediments of the Barents Sea. Ragay Gulf fluxes are also less than anthropogenic fluxes measured in areas of petroleum exploration and development, such as the Texas and Louisiana, USA shelf areas.\n\nYou can also purchase hard copies of Geoscience Australia data and other products at http://www.ga.gov.au/products-services/how-to-order-products/sales-centre.html

Full description

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