Authigenic carbonates from methane seeps of the northern Congo fan: Microbial formation mechanism |
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| Authors: | Dong Feng Duofu Chen Jörn Peckmann Gerhard Bohrmann |
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| Institution: | 1. State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China;3. Key Laboratory of Carbonate Reservoir of CNPC, Department of Sedimentology and Hydrocarbon Accumulation, Southwest Petroleum University, Chengdu 610500, China;4. Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia;5. School of Earth and Environmental Sciences, The University of Queensland, Brisbane, QLD 4072, Australia;6. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;7. Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA;1. MARUM — Center for Marine and Environmental Sciences and Department of Geosciences, University of Bremen, 28334 Bremen, Germany;2. IFREMER, Unité de Recherche Géosciences Marines, 29280 Plouzané, France;3. Institute of Geology, University of Hamburg, 20146 Hamburg, Germany;4. State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, China;5. Department of Geodynamics and Sedimentology, Center for Earth Sciences, University of Vienna, 1090 Vienna, Austria |
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| Abstract: | Authigenic carbonates were collected from methane seeps at Hydrate Hole at 3113 m water depth and Diapir Field at 2417 m water depth on the northern Congo deep-sea fan during RV Meteor cruise M56. The carbonate samples analyzed here are nodules, mainly composed of aragonite and high-Mg calcite. Abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix. The δ13C values of the Hydrate Hole carbonates range from ?62.5‰ to ?46.3‰ PDB, while the δ13C values of the Diapir Field carbonate are somewhat higher, ranging from ?40.7‰ to ?30.7‰ PDB, indicating that methane is the predominant carbon source at both locations. Relative enrichment of 18O (δ18O values as high as 5.2‰ PDB) are probably related to localized destabilization of gas hydrate. The total content of rare earth elements (REE) of 5% HNO3-treated solutions derived from carbonate samples varies from 1.6 ppm to 42.5 ppm. The shale-normalized REE patterns all display positive Ce anomalies (Ce/Ce* > 1.3), revealing that the carbonates precipitated under anoxic conditions. A sample from Hydrate Hole shows a concentric lamination, corresponding to fluctuations in δ13C values as well as trace elements contents. These fluctuations are presumed to reflect changes of seepage flux. |
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