Biogeochemical controls on authigenic carbonate formation at the Chapopote “asphalt volcano”, Bay of Campeche |
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| Authors: | Thomas H Naehr Daniel Birgel Gerhard Bohrmann Ian R MacDonald Sabine Kasten |
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| Institution: | aTexas A&M University-Corpus Christi, 6300 Ocean Drive, Corpus Christi, TX 78412-5869, USA;bMARUM-Center for Marine Environmental Sciences and Department of Geosciences, University Bremen, PO Box 33 04 40, D-28334 Bremen, Germany;cAlfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany |
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| Abstract: | The Campeche Knolls in the Bay of Campeche, southern Gulf of Mexico, were investigated through detailed seafloor mapping, ROV surveys, and sediment and pore water sampling. The knolls are elongated, submarine hills created by salt tectonics with a positive relief of up to 800 m above the surrounding seafloor. Several of the knolls are associated with sea-surface oil slicks identified from satellite data, indicating the presence of hydrocarbon seeps on the seafloor. One of the knolls, named “Chapopote”, was studied in detail during two international research expeditions (SO174/2 and M67/2) and is characterized by extensive hydrocarbon seepage including large asphalt flows, oil and gas seeps, and seafloor gas hydrate deposits. Chemosymbiotic biological communities and authigenic carbonate deposits are associated with the seeps and are the result of both biogeochemical turnover and the interaction between downward-diffusing seawater and hydrocarbon-rich pore fluids at shallow sediment depth. Authigenic carbonates are characterized by aragonite, exhibit a porous texture, and are cemented by a matrix of microsparitic to sparitic aragonite. Macropores of the carbonates were completely filled with liquid oil. Carbonate microfabrics include peloidal or clotted fabrics that may indicate the existence of microenvironments resulting from microbial metabolism. Banded/botryoidal aragonite cements line the intra- and bioclasts and incompletely fill the pore spaces. The stable carbon isotopic composition of authigenic aragonite varies between − 28.6‰ and − 17.9‰ (PDB), identifying oil oxidation as the primary source of carbon to the DIC pool, while lipid biomarker data demonstrate the concurrent existence of microbial communities responsible for anaerobic oxidation of methane (AOM). These observations indicate the presence of additional, AOM-independent reactions responsible for carbon sequestration at hydrocarbon seeps and demonstrate the complexity of biogeochemical processes at seep sites in the Gulf of Mexico basin. Oxygen isotope data of authigenic aragonite vary from + 2.5‰ to + 3.8‰ (PDB), indicating carbonate precipitation in slight disequilibrium with the surrounding pore fluids. |
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| Keywords: | Asphalt Hydrocarbon seeps Authigenic carbonate Carbon isotopes Biomarkers Archaea |
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