Discrimination of Hydrate and Diagenetic Methane Inputs to the Ocean with Measurements of Natural 14CH4

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ABSTRACTOCE-0327776Methane clathrate hydrates are a major unknown in the present global methane budget. Independent estimates of the global methane hydrate reservoir size indicate that it contains 1019 g of methane, equivalent to 2000-times the Earth's atmospheric methane reservoir, or about 30% of the ocean DIC reservoir. Methane concentration, oxidation rate, and stable isotope distributions in the Black Sea water column indicate that methane turnover in the Black Sea is rapid (~20 y) and that hydrates, which are known to be present in Black Sea sediments, are an important water column methane source. Restricted circulation in the Black Sea allows using it as a biogeochemical bucket to integrate methane inputs and to obtain an estimate of the basal hydrate dissociation rate.In this project, researchers at the University of California - Irvine will attempt measurements of natural 14CH4 to discriminate between hydrate (radiocarbon-dead) methane and diagenetically-produced (modern or finite age) methane, increasing our understanding of the dissociation of hydrates and the importance of basal hydrate decomposition to the global methane budget. They will use the Cariaco Basin as a hydrate-free comparison environment to provide a means of understanding the multiple methane sources in the Black Sea and will undertake parallel measurements of d13C-CH4, d2H-CH4, and 14CH4 there. Vent inputs will be characterized with stable isotope and 14CH4 measurements on individual Black Sea vents as well as vents from the western coast of North America and the Gulf of Mexico. Finally, they will measure 14CH4 in waters in the Eastern Tropical North Pacific to test the suggestion that the deeper zone of the methane maximum present in this region may result from shelf seep and vent inputs. The low-blank, low-background method already developed by the investigators will permit measurement of 14CH4 in as little as 100 liters of seawater with a methane concentration as low as 2nM. Broader Impacts: Methane hydrates are under serious consideration as a possible future energy source. It is important to have a specific, well-understood tracer like 14CH4 to assess changes in natural hydrate fluxes as well as to assess potential losses associated with commercial development. This project will involve one graduate student and an undergraduate.