The emission of low pH water from Gulf of Mexico seeps as revealed by δ13C–CO2 and methane oxidation data

Abstract

Seawater was collected around the MC118 hydrocarbon seep in the Gulf of Mexico and was used previously for a study of aerobic methane (CH₄) oxidation. During that experiment, changes in the dissolved concentrations and δ¹³C isotopes of CH₄ and CO₂ were recorded. Originally, the CO₂ concentrations and isotopes were recognized to qualitatively follow trends supporting the microbial conversion of CH₄ to CO₂via aerobic oxidation, however, no attempt was made to quantitatively explain this CO₂ data. The present study models the δ¹³C–CO₂ changes that occur as a result of CH₄ oxidation, accounting for the carbon already present as dissolved inorganic carbon (DIC), DIC added via CH₄ oxidation, and the pH of seawater. This study discovers that to accurately model the measured concentration and isotopic data for CO₂, the seawater emitted from this seep site must have a pH which is at most between 6.49 and 7.24, and possibly up to 0.43 ± 0.08 pH units lower. These results are corroborated by direct measurements of pH from seeps in the Mediterranean Sea. A first-order extrapolation indicates that while cold seeps in the Gulf of Mexico may be a source of low pH water influencing the carbon dynamics of the deep ocean environment, this influence is likely less than that of current surface ocean acidification caused by the infiltration of atmospheric CO₂.