HR: 14:55h
AN: B53C-06    [Abstracts]
TI: Sulfur and Trace Metal Chemistry of a Methane Charged Brine Pool and Adjacent Porewaters in the Northern Gulf of Mexico
AU: * Gilhooly, W P
EM: williamg@ucr.edu
AF: Department of Earth Sciences, University of California, Riverside 2207 Geology, Riverside, CA 92521-0423, United States
AU: Cable, J E
EM: jcable@lsu.edu
AF: Department of Oceanography and Coastal Sciences/CEI, Louisiana State University, Baton Rouge, LA 70803, United States
AU: Carney, R S
EM: rcarne1@lsu.edu
AF: Department of Oceanography and Coastal Sciences/CEI, Louisiana State University, Baton Rouge, LA 70803, United States
AU: Macko, S A
EM: sam8f@virginia.edu
AF: Department of Environmental Sciences, University of Virignia Clark Hall, Charlottesville, VA 22904, United States
AU: Lyons, T W
EM: timothyl@ucr.edu
AF: Department of Earth Sciences, University of California, Riverside 2207 Geology, Riverside, CA 92521-0423, United States
AB: A systematic study of a methane brine pool on the Louisiana continental slope reveals the extent to which steep chemical gradients associated with an anoxic hypersaline basin control the establishment and distribution of chemosynthetic organisms. The seep site, located in the Green Canyon lease block (GC233), provides habitat for a bivalve-dominated community of chemosynthetic mussels ( Bathymodiolus childressi). The pool is a brine-filled pockmark centered over a salt diapir buried within 500 m of the seafloor along which methane and vent fluids migrate to the surface. The depression slopes along its southern margin where brine overflows onto the seafloor. This study sought to establish the chemistry of the brine in an effort to better understand fluid transport to the seafloor and the extent to which brine influences chemosynthetic activity at a methane seep site. Ten sediment push cores were collected during submersible operations within the brine spillway and in upslope background sediments distal to the pool. Initial chemical analyses indicate the brine (128 ppt) is anoxic, chloride-rich (1994.8 mM) and sulfur-poor ([SO₄^2-] = 0.4 mM, [HS^-] = 8.2 uM). Steep porewater Cl^- and Sr²⁺ concentration gradients observed in sediments downslope of the brine pool clearly indicate mixing between brine and seawater end members. Porewater sulfur profiles from sediments within the brine outflow indicate complete sulfate consumption within 30 cm below seafloor and sulfide production as great as 5 mM. The paired isotopic composition of dissolved sulfate and sulfide (Δ³⁴S_SO4-HS = 40‰) is consistent with bacterial sulfate reduction, potentially driven by the anaerobic oxidation of methane or non-methane hydrocarbons. The brine was nearly devoid of dissolved Mo (22 nM) and enriched in Mn (6.3 uM), relative to measured seawater casts ([Mo] = 112 nM; [Mn] below detection). Dissolved Mo enrichments, up to 392.8 nM, in surficial sediments decrease with depth may indicate brine mixing coupled with accumulation of Mo in sulfidic sediments and subsequent removal during authigenic precipitation. The chemical cycling of sulfur and redox sensitive elements (Mo, Mn, and Fe) will better inform our understanding of modern seep formation and provide a baseline for comparison to ancient seep environments.
DE: 0404 Anoxic and hypoxic environments (4802, 4834)
DE: 0408 Benthic processes (4804)
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting