SREL Reprint #2850

Free hydrocarbon gas, gas hydrate, and authigenic minerals in chemosynthetic communities of the northern Gulf of Mexico continental slope: relation to microbial processes

Roger Sassen1, Harry H. Roberts2, Robert Carney3, Alexei V. Milkoy4, Debra A. DeFreitas1, Brian Lanoil5, and Chaunlun Zhang6

1Geochemical and Environmental Research Group (GERG), Texas A&M University,
College Station, TX 77845, USA
2Department of Oceanography and Coastal Sciences, Louisiana State University,
Baton Rouge, LA 70803, USA
3Department of Coastal Ecology, Louisiana State Univer.rity, Baton Rouge, LA 70803. USA
4Department of Geology and Geophysics, Woods Hole Oceanographic Institution,
Woods Hole, MA 02543, USA
5Department of Environmental Sciences, University of California, Riverside, CA 92521, USA
6Marine Sciences Department, University of Georgia, Aiken, SC 29802 USA

Abstract: Research submersibles and piston cores were used to sample two chemosynthetic communities in the Gulf of Mexico continental slope at ~540 m water depth. Vent gas from the deep subsurface is the starting material from which other carbon pools are derived, including gas hydrate, free hydrocarbon gas in sediment, and authigenic carbonate rock. Gas crystallizes as exposed mounds of structure II gas hydrate and as massive vein-fillings in hemipelagic mud. Venting rates from gas hydrate mounds periodically increase after the temperature of the bottom water increases. Gas hydrate decomposition is largely restricted to exposed hydrate and at shallow depth in sediment. Overall, gas hydrate is accumulating, not decomposing at study sites. Most free gas in sediment appears to be destroyed in situ by anaerobic microbial oxidation in chemosynthetic communities, leading to sequestration of carbon as abundant authigenic carbonate rock depleted in 13C. Free methane is rapidly oxidized leaving residual methane enriched in 13C. Some in situ microbial CO2 reduction occurs, and methane depleted in 13C mixes with vent methane enriched in 13C. The C2-C5 hydrocarbons of vent gas initially most depleted in 13C (ethane, isobutane, isopentane) are least affected by microbial oxidation, whereas hydrocarbons initially enriched in 13C (propane, normal butane, nonnal pentane) are most affected. Anaerobic microbial oxidation of all C1-C5 hydrocarbon gases, not only methane, may be significant in chemosynthetic communities. Microbial processes contribute to the development and stability of chemosynthetic communities by providing required H2S. Anaerobic microbial processes lead to deposition of diagnostic authigenic minerals related to the carbon (carbonate minerals) and sulfur cycles (pyrite, elemental sulfur), altering the seafloor. Seafloor cementation favors fauna such as chemosynthetic tubeworms and seep mussels by forming carbonate hardgrounds on an otherwise unfavorable mud-dominated seafloor.

Keywords: Gulf of Mexico; Gas hydrates; Hydrocarbon gas; Methane oxidation

SREL Reprint #2850

Sassen, R., H. H. Roberts, R. Carney, A. V. Milkov, D. A. DeFreitas, B. Lanoil, and C. Zhang. 2004. Free hydrocarbon gas, gas hydrate, and authigenic minerals in chemosynthetic communities of the northern Gulf of Mexico continental slope: relation to microbial processes. Chemical Geology 205:195-217.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).