Quantifying carbon sources in the formation of authigenic carbonates at gas hydrate sites in the Gulf of Mexico
Michael J. Fonnoloa,*, Timothy W. Lyonsa, Chuanlun
Zhangb, Cheryl Kelleya, Roger Sassenc,
Juske Horitad, David R. Cole d
aDepartment of Geological Sciences, University
of Missouri-Columbia, 101 Geological Sciences Building, Columbia, MO 65201,
USA
bSavannah River Ecology Laboratory and Marine Sciences Department,
University of Georgia, P.O. Box Drawer E, Aiken, SC29802, USA
cGeochemical and Environmental Research Group, Texas A&M
University, College Station, TX 77845, USA
dChemical Sciences Division, Oak Ridge National Laboratory,
Oak Ridge, TN 37831, USA.
Received 8 May 2003; received in revised form 16 September 2003; accepted
23 December 2003
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 (ECO2)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
reco~ized 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 modem and ancient cold seeps are characterized by a complex
interplay of carbon sources readily preserved in the S13C values
of carbonates.
Keywords: Gulf of Mexico; Authigenic carbonates; Methlme
SREL Reprint #2851
Formolo, M. J., T. W. Lyons, C. Zhang, C. Kelley, R. Sassen, J. Horita and D. R. Cole. 2004. Quantifying carbon sources in the formation of authigenic carbonates at gas hydrate sites in the Gulf of Mexico. Chemical Geology 205:253-264.
