Quantifying carbon sources in the formation of authigenic carbonates at gas hydrate sites in the Gulf of Mexico
Introduction
The Gulf of Mexico is an ideal natural laboratory for studies of the biogeochemical relationships among micro- and macrofaunal communities in oil and gas seep environments. The tectonic and depositional conditions in the Gulf favor formation of oil and gas and persistent upward migration into the overlying sediments and water column, where the hydrocarbons are utilized by heterotrophic, chemoautotrophic, and syntrophic biological communities Sassen et al., 1993, Sassen et al., 1999. Oil and gas discharge provides the energy source, and the buildup of oxidized carbon generated as a metabolic end-product drives the precipitation of authigenic carbonate. Organisms of particular interest are aerobic methanotrophs and sulfide-oxidizing bacteria (e.g., Beggiatoa spp.), anaerobic methane-oxidizing archaea and sulfate-reducing bacteria, and other unknown microorganisms that oxidize thermogenic hydrocarbon gases (C2 and longer) and crude oil Jannasch and Mottl, 1985, Sassen et al., 1993, Sassen et al., 1999.
The main goal of this study was to quantify the magnitude of reduced carbon oxidation and attendant carbonate precipitation linked specifically to sulfate-dependent anaerobic methane oxidation (Hoehler et al., 1994), which is well known in many seep settings. The overall effect of anaerobic methane-oxidizing archaea and sulfate-reducing bacteria is summarized in the following net chemical reaction (Valentine and Reeburgh, 2000, and references therein):
Pervasive crystallization of gas hydrates in the Gulf of Mexico is driven by the constant flux of hydrocarbon gas from the underlying reservoir through flow paths defined by widespread regional salt tectonism. Once hydrates crystallize, a variety of environmental changes result in destabilization and dissociation. In the presence of dissociating hydrates and venting gases, methane can be oxidized by archaea, and sulfate can be reduced by a variety of bacteria that use the reduced substrates formed during anaerobic oxidation of methane (H2, acetate or acetic acid; as summarized in Valentine and Reeburgh, 2000). The metabolic coupling between these organisms produces sulfide and dissolved inorganic carbon, including the potential for large increases in carbonate alkalinity that drive pervasive carbonate precipitation (e.g., calcite and aragonite):
The most common gas hydrate type within the Gulf of Mexico is structure II, in which methane and propane are the dominant hydrocarbon gases trapped in the lattice (Sassen et al., 1998). Bacterial oxidation of structure II gas hydrates can cause decomposition by removing hydrocarbons such as propane that are vital to maintaining hydrate stability Sassen et al., 1998, Sassen et al., 1999. Once hydrate destabilization occurs, the released methane is available for microbial oxidation. The resulting carbonate alkalinity (principally HCO3−) and sulfide (reaction 1) should bear the distinctive isotopic compositions of methane consumption and coupled sulfate reduction.
Pathways of carbon cycling are expressed in the isotopic compositions of authigenic mineral phases, such as δ13CCaCO3, δ18OCaCO3, and δ34Spyrite. Here, we have specifically developed a simple isotopic mass balance to assess the relative contributions of individual carbon pools in the formation of authigenic carbonates and in doing so demonstrate that hydrocarbons other than methane are the dominant carbon source at the seep sites investigated. This preliminary paper is not an exhaustive survey of hydrocarbon seeps in the Gulf of Mexico. Instead, we present a framework developed for our ongoing, integrated study of carbon and sulfur cycling at a wider range of gas hydrate localities in the Gulf of Mexico. This template should also have broader utility for the recognition and characterization of hydrocarbon cycling in ancient cold seep systems.
Section snippets
Regional geology
The northern shelf-slope region of the Gulf of Mexico is a passive continental margin characterized by over 10 km of Mesozoic–Cenozoic sediment well suited for the generation and accumulation of large oil and gas reserves. The tectonic/paleogeographic setting, in addition to favoring geologically recent formation of hydrocarbons within deep sediments, resulted in extensive salt deposits, which provide a unique environment for both hydrocarbon accumulation and migration. During the Late
Study location and sample description
Sediment samples were collected from a variety of locations in the Gulf of Mexico in 1997 and 1998 aboard the R/V Gyre. These sites are located in the Green Canyon (GC) area (27°2′–27°55′N and 91°33′–91°40′W; Fig. 1). Samples were taken using piston cores (6 m length, 7 cm diameter). In this study, we have concentrated on the samples collected at water depths ranging from 183 to 659 m (GC 49, GC 185, GC 233, and GC 234; Table 1). Core NBP (North of Brine Pool), the control site, was collected
Methods
A variety of geochemical techniques were used in this study, including isotopic measurements (δ13C, δ34S, and δ18O) and coulometric determinations of carbonate concentrations. A carbon isotope mass balance (details below) was developed to determine the relative contributions of carbon to authigenic carbonate formation from a variety of potential sources.
Results
The results of the geochemical analyses are summarized in Table 2. At each of the sites, the δ34S composition of TRIS, ranging from −5.2‰ to −24.9‰, confirms that bacterial SO42− reduction is occurring; however, whether this sulfate reduction is driven by methane oxidation or non-methane hydrocarbon oxidation is less clear and is addressed further through carbon isotope systematics. The measurements of TRIS, ranging from 0.04 to 1.62 wt.%, indicate the formation of iron sulfides in the
Carbon isotope mass balance
To evaluate the inputs of carbon from a variety of sources during the formation of authigenic carbonates, we have developed an isotope mass balance with relevance beyond this study area. Specifically, this model quantitatively tracks contributions to carbonate alkalinity using the signature, end-member isotopic compositions of the various carbon pools relative to the bulk isotopic composition of authigenic carbonates formed from this alkalinity. In this mathematical model, we simplify the
Discussion
Our results record a variety of processes driven by the oxidation of methane and/or non-methane liquid and gaseous hydrocarbons coupled to sulfate reduction in the hydrate/seep sediments Aharon and Fu, 2000, Sassen et al., 1993, Sassen et al., 1998, Sassen et al., 1999. In order to constrain the metabolic influences of the different carbon sources, we determined the relative contributions of the carbon pools as expressed in the δ13C of authigenic carbonates at a few representative sites in the
Summary
In this study, we investigated the relative contributions of dissolved inorganic carbon from a variety of potential sources as recorded in the bulk δ13C of authigenic CaCO3—that is, background inputs from seawater and organic matter coupled with additional contributions from the oxidation of methane and a variety of gaseous and liquid non-methane hydrocarbons. By using the final authigenic carbonate produced we were able to quantify the relative extents of anaerobic methane oxidation, which
Acknowledgements
Support for this project was provided by the National Science Foundation (OCE 0120610) (TWL, CLZ, and RS), the Petroleum Research Fund (CLZ) and the National Undersea Research Program (CLZ and RS). Special thanks go to Steve Sweet for providing sample logging, Yiliang Li for helping with carbonate sample preparation, Jon Fong and Steve Studley for assistance with sulfur isotope measurements, and Dylan Sullivan for carbon and oxygen isotope measurements. An additional thanks goes to David
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