Depleted δ13C Values in Salt Dome Cap Rock Organic Matter and Implications for Microbial Metabolism and Fixation

Abstract:

Salt domes occur throughout the Gulf Coast Region USA and are often associated with trapped hydrocarbons. These salt domes can be capped by sulfate and carbonate minerals that result from complex digenetic interactions in the subsurface. The specific natures of these interactions are poorly understood, in particular the role of microbes in facilitating mineralization and element cycling. Carbon isotope compositions of cap rock calcites (δ¹³C_carb) are highly variable and range from near neutral to less than –40‰ (VPDB) indicative of methane-sourced carbon. These low values and the common coexistence of elemental sulfur and metal sulfides have spurred hypotheses invoking microbial sulfate reduction as driving carbonate mineral authigenesis. Here, we present new organic carbon isotope (δ¹³C_org) data that, similar to δ¹³C_carb, exhibit depletions below –30 to –25‰. These δ¹³C_org values are lower than local liquid hydrocarbons and “normal” marine organic matter reflecting either microbial fixation of methane-sourced carbon or microbial fractionation from liquid hydrocarbon sources. The combined carbon isotope data (δ¹³C_carb and δ¹³C_org) indicate that methane likely plays an important role in microbial cycling in salt domes. The δ¹³C_org values are similar to those of anaerobic oxidation of methane (AOM) related communities from methane-sulfate controlled marine sediments. Ultimately, salt dome environments may share some important characteristics with AOM systems.