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Past Methane Release Events and Environmental Conditions at the Upper Continental Slope of the South China Sea: Constraints by Seep Carbonates

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  • Authigenic carbonates and chemosymbiotic biota are archives of seepage 1 history and record paleo-environmental conditions at seep sites. Based on mineral and stable isotope compositions and U/Th-isotope systematics of seep carbonates and Calyptogena sp shell fragments from three seep sites located at 22°02' ~22°09'N, 118°43'~118°52'E (water depths: 473 m to 785 m) in the northeastern slope of the South China Sea, we obtained the timing of past methane release events and identified samples formed in contact with bottom seawater with negligible pore water influence, largely aragonitic chemoherm samples and shells, to reconstruct the palaeo-bottom water temperatures during their formation. Our results show that all methane release events occurred between 11.5±0.2 ka and 144.5±12.7 ka, when sea-levels were about 62 m to 104 m lower than today. The enhanced methane release during low sea-level stands is thought to be a modulating on reduced hydrostatic pressure, increased incision of canyons and the increase of sediment loading. The calculated past bottom water temperatures at one site (Site 3; water depth: 767 m to 771 m) during the periods of low sea-level stands between 11.5 ka and 65 ka were in the range of ~3.3 °C to ~4.0 °C, that is ~1.3 °C to ~2.2 °C colder than at present. The reliability of δ¹⁸O of seep carbonates and vent bivalve shells as a proxy for bottom water temperatures is critically assessed in light of seepage ¹⁸O-enriched fluids from gas hydrate and/or clay dehydration water. Our approach provides for the first time an independent estimate of past bottom water temperatures of the upper continental slope of the South China Sea.
  • This publication is contribution no. 264 of the Sonderforschungsbereich 574 “Volatiles and Fluids in Subduction Zones” at Kiel University. This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Springer and can be found at: http://www.springer.com/earth+sciences+and+geography/geology/journal/531
  • Keywords: Methane release events, Oxygen isotope, Bottom water temperature reconstruction, Seep carbonates, South China Sea, U/Th ages, O- and C-isotopes
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  • Han, X., E. Suess, V. Liebetrau, A. Eisenhauer, Y. Huang 2014. Past methane release events and environmental conditions at the upper continental slope of the South China Sea: constraints by seep carbonates. Int J Earth Sci (Geol Rundsch) (2014) 103:1873–1887; DOI 10.1007/s00531-014-1018-5
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  • 103
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  • 7
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  • This study was largely funded by the German Ministry of Science and Education (BMBF Az 03G0177A to E.S.) supplemented by considerable financial support for ship-time provided by the Geological Survey of China (GCS) through the Guangzhou Marine Geological Survey (GMGS). X. HAN was Visiting Scientist at GEOMAR in Kiel supported by SFB574; her subsequent work was supported through NSFC No. 40476050 and No. 40976040, 973 Program 2009CB21950607 and Zhejiang Provincial NSFC Grant No. R5110215. The work by E. Suess was partially supported through GMGS-grant D0990 and that of A. Eisenhauer and V. Liebetrau by the German GEOTECHNOLOGY initiative (COMET/TP Charon). X. Han and E. Suess wish to acknowledge support as Visiting Scientist and Courtesy Professor, respectively, at the College of Earth, Ocean and Atmospheric Sciences, Oregon State University.
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