Quantifying carbon sources in the formation of authigenic carbonates at gas hydrate sites in the Gulf of Mexico

Abstract

The northern slope of the Gulf of Mexico is known for extensive venting of methane and other hydrocarbons as related to active salt diapirism and associated fault conduits which link world-class subsurface reserves to seafloor seeps. These venting hydrocarbons fuel extensive micro- and macrofaunal cold seep communities. Of particular interest is the relationship between anaerobic methane-oxidizing archaea and sulfate-reducing bacteria. It has been suggested that sulfate-dependent anaerobic methane oxidation dominates carbon oxidation and attendant authigenic carbonate precipitation at these sites. To test this assumption, we have quantified the relative contributions of dissolved carbon dioxide (ΣCO 2) from a variety of sources—specifically seawater, organic matter, methane, and non-methane liquid and gaseous hydrocarbons—using the carbon isotope compositions of authigenic carbonates and a simple isotopic mass balance. Our model, and a small but representative suite of data from the Gulf, demonstrates that methane is a contributor but is not the dominant source of metabolic energy at the sites of active venting. Instead, oxidation of non-methane hydrocarbons appears to be the primary source of carbonate alkalinity. The secondary role played by methane oxidation has been independently recognized by other workers from organic biomarker relationships and from disparities observed between measured rates of sulfate reduction and methane oxidation. Despite the domination of the carbon reservoir by non-methane sources, oxygen isotope data for the authigenic carbonates bear the mark of appreciable gas hydrate dissociation. This study, rather than being an exhaustive survey of Gulf of Mexico seeps, is intended only to provide a template for the investigation of the abundant authigenic carbonate deposits distributed throughout the geologic record. As in the Gulf of Mexico, many modern and ancient cold seeps are characterized by a complex interplay of carbon sources readily preserved in the δ 13C values of carbonates.