Free hydrocarbon gas, gas hydrate, and authigenic minerals in chemosynthetic communities of the northern Gulf of Mexico continental slope: relation to microbial processes
Roger Sassena,*, Harry H. Robertsb, Robert Carneyc,
Alexei V. Milkoyd,1, Debra A. DeFreitasa, Brian
Lanoile, Chaunlun Zhangf
aGeochemical and Environmental Research Group
(GERG), Texas A&M University, College Station, TX 77845, USA
bDepartment of Oceanography and Coastal Sciences, Louisiana
State University, Baton Rouge, LA 70803. USA
cDepartment of Coastal Ecology, Louisiana State Univer.rity,
Baton Rouge, LA 70803. USA
dDepartment of Geology and Geophysics, Woods Hole Oceanographic
Institution, Woods Hole, MA 02543, USA
eDepartment of Environmental Sciences, University of California,
Riverside, CA 92521, USA
fMarine Sciences Department, University of Georgia, Atlanta,
GA 29802 USA
Received
10 October 2002; received in revised form 14 September 2003; accepted
23 December 2003
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 materia1 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 1east affected by microbial oxidation, whereas
hydrocarbons initially enriched in 13C (propane, norma1 butane,
nonnal pentane) are most affected. Anaerobic microbial oxidation of all
C1-C5 hydrocarbon gases, not only methane, may be
significant in chemosynthetic communities. Microbia1 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), a1tering 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.
