Estimating the Impact of Seep Methane Oxidation on Ocean pH and Dissolved Inorganic Radiocarbon Along the US Mid-Atlantic Bight

Ongoing ocean warming can release methane (CH4) currently stored in ocean sediments as free gas and gas hydrates. Once dissolved in ocean waters, this CH4 can be oxidized to carbon dioxide (CO2). While it has been hypothesized that the CO2 produced from aerobic CH4 oxidation could enhance ocean acidification, a previous study conducted in Hudson Canyon shows that CH4 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 CH4 seepage on CO2 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 CH4-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 CH4 concentration and active gas seepage. This suggests that aerobic CH4 oxidation has a greater influence on the dissolved inorganic pool in areas where CH4 concentrations are locally elevated, instead of displaying a cumulative effect downcurrent from widespread groupings of CH4 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 CH4-derived carbon is not negligible on the global scale and could contribute to deepwater acidification over longer time scales. Plain Language Summary Ocean acidity may be enhanced not only due to the oceanic uptake of atmospheric carbon dioxide but also through temperature-driven processes that can mobilize ancient carbon stores and generate additional carbon dioxide. One of these processes is the oxidation of methane derived from seafloor seepage. Here, we investigate if the widespread release and subsequent oxidation of seep methane into carbon dioxide could contribute to the acidification of deep waters along the U.S. Mid-Atlantic Bight. Our results suggest that over short time scales, methane's role in enhancing ocean acidification is small. However, over longer time periods, methane-derived carbon could contribute to deepwater acidification.