Estimating the Impact of Seep Methane Oxidation on Ocean pH and Dissolved Inorganic Radiocarbon Along the U.S. Mid‐Atlantic Bight

Abstract

Abstract Ongoing ocean warming can release methane (CH 4 ) currently stored in ocean sediments as free gas and gas hydrates. Once dissolved in ocean waters, this CH 4 can be oxidized to carbon dioxide (CO 2 ). While it has been hypothesized that the CO 2 produced from aerobic CH 4 oxidation could enhance ocean acidification, a previous study conducted in Hudson Canyon shows that CH 4 oxidation has a small short‐term influence on ocean pH and dissolved inorganic radiocarbon. Here we expand upon that investigation to assess the impact of widespread CH 4 seepage on CO 2 chemistry and possible accumulation of this carbon injection along 234 km of the U.S. Mid‐Atlantic Bight. Consistent with the estimates from Hudson Canyon, we demonstrate that a small fraction of ancient CH 4 ‐derived carbon is being assimilated into the dissolved inorganic radiocarbon (mean fraction of 0.5 ± 0.4%). The areas with the highest fractions of ancient carbon coincide with elevated CH 4 concentration and active gas seepage. This suggests that aerobic CH 4 oxidation has a greater influence on the dissolved inorganic pool in areas where CH 4 concentrations are locally elevated, instead of displaying a cumulative effect downcurrent from widespread groupings of CH 4 seeps. A first‐order approximation of the input rate of ancient‐derived dissolved inorganic carbon (DIC) into the waters overlying the northern U.S. Mid‐Atlantic Bight further suggests that oxidation of ancient CH 4 ‐derived carbon is not negligible on the global scale and could contribute to deepwater acidification over longer time scales.