HR: 0800h
AN: B21D-0907    [Abstracts]
TI: Carbon Sources to Authigenic Carbonate Rock at Chemosynthetic Communities: Lower Slope of the Gulf of Mexico
AU: Sassen, R
EM: sassen@gerg.tamu.edu
AF: Texas A&M University / GERG, 833 Graham Road, College Station, TX 77845 United States
AU: Jung, W
EM: woodong@tamu.edu
AF: Texas A&M University / Geology & Geophysics Dept., Halbouty Building, College Station, TX 77843 United States
AU: * Zhang, C
EM: zhang@srel.edu
AF: University of Georgia / SREL, P.O. Box Drawer E, Aiken, SC 29802 United States
AU: DeFreitas, D A
EM: debz@gerg.tamu.edu
AF: Texas A&M University / GERG, 833 Graham Road, College Station, TX 77845 United States
AB: Flux of biogenic methane, crude oil and associated hydrocarbon gases occurs from the deep subsurface to the seafloor, water column, and atmosphere of the Gulf of Mexico slope. Chemosynthetic communities occur at sites of relatively high gas flux, frequently with gas hydrate, but always with authigenic carbonate rock \(ACR\). ACR contains carbonate carbon derived from microbial hydrocarbon oxidation that geologically sequesters much fossil carbon, perturbing the carbon cycle. ACR was collected using the ALVIN from sites with chemosynthetic communities in Alaminos Canyon, Atwater Valley, and the Florida Escarpment areas at water depths as much as 3.3 km. Bulk $\delta$13C was measured and carbonate petrology used to identify carbonate cements, normal marine carbonate, and non-carbonate components such as metal oxides and sulfides. ACR is depleted in 13C. However, the $\delta$13C of major hydrocarbon types is typically more depleted in 13C than the associated ACR. For example, the mean $\delta$13C of biogenic methane seeps in the Gulf slope is -74.0\permil PDB but the lightest bulk ACR measured in the study area is -46.6\permil PDB. Carbonate cements from hydrocarbon oxidation are shown to enclose skeletal remains of chemosynthetic fauna such as mussels, clams, as well as other fauna characterized by normal marine carbonate \(\sim 0\permil PDB\). The best explanation of why the $\delta$13C of ACR does not closely correspond to that of the hydrocarbon starting products is that normal marine carbon dilutes the $\delta$13C from hydrocarbon oxidation and thus affects the bulk isotopic properties of ACR.
DE: 4825 Geochemistry
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1055 Organic geochemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting