HR: 1340h
AN: B43E-1661    [Abstracts]
TI: Biogeochemical Controls on Authigenic Carbonate Formation at the Chapopote "Asphalt Volcano", Bay of Campeche
AU: * Naehr, T H
EM: thomas.naehr@tamucc.edu
AF: Texas A&M University-Corpus Christi, Department of Physical and Environmental Sciences, 6300 Ocean Drive, Corpus Christi, TX 78412-5869, United States
AU: Bohrmann, G
EM: gbohrmann@uni-bremen.de
AF: DFG Forschungszentrum Ozeanränder, Universität Bremen, Postfach 33 04 40, Bremen, 28334, Germany
AU: Birgel, D
EM: dbirgel@uni-bremen.de
AF: DFG Forschungszentrum Ozeanränder, Universität Bremen, Postfach 33 04 40, Bremen, 28334, Germany
AU: MacDonald, I R
EM: ian.macdonald@tamucc.edu
AF: Texas A&M University-Corpus Christi, Department of Physical and Environmental Sciences, 6300 Ocean Drive, Corpus Christi, TX 78412-5869, United States
AB: Unusual hydrocarbon seep features, so-called "asphalt volcanoes" were explored in the Bay of Campeche, southern Gulf of Mexico, in the spring of 2006. Guided by data from satellite imagery that showed evidence for persistent oil seeps in the region, we investigated lava-like flows of solidified asphalt along the rim of a dissected salt dome at a water depth of about 3000 m. Fresh asphalt contains copious thermogenic gas and gas hydrate. Slabs of authigenic carbonate form surface crusts with layers of oil pooled beneath. Sediments are anoxic with H₂S concentrations of 8 to 13 mM. Gas hydrate forms layers and mounds in the surface sediments. Alkalinity profiles show values from 29 to 35 mM, indicating oxidation of hydrocarbons by reduction of seawater sulfate. Molecular and isotopic compositions of gas hydrate and sediment headspace indicate moderately mature, thermogenic gas. Oily sediment extracts and asphalt pieces are composed of a degraded mixture of hydrocarbons with a peak at n-C₃₀ and a few resolved C₂₉ to C₃₂ hopanes.
Authigenic carbonate crusts from Chapopote are porous, aragonite-cemented mudstones. Peloidal textures are common, as are bivalve shells and at least two generations of aragonite-cemented intraclasts. The carbon isotopic composition of the authigenic aragonite cements varies between -28.6 ‰ and -17.9 ‰ (PDB), indicating a contribution of carbon from non-methane liquid hydrocarbons to the total pool of dissolved CO₂. δ¹⁸O values of the carbonates range from +3.2 ‰ to +4.5 ‰ (PDB), suggesting aragonite formation under near-equilibrium conditions in the shallow subsurface. Molecular fossils extracted from one carbonate sample contain abundant ¹³C-depleted archeal lipids, derived from anaerobic methanotrophs, suggesting that organisms mediating the anaerobic oxidation of methane are closely associated with carbonate authigenesis at the Chapopote asphalt seep site.
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL (0460)
DE: 4811 Chemosynthesis
DE: 4840 Microbiology and microbial ecology (0465)
DE: 8169 Sedimentary basin processes
SC: Biogeosciences [B]
MN: 2007 Fall Meeting