At modern cold seeps,the anaerobic oxidation of methane(AOM)is the dominant pathway for methane consumption in marine sediments.AOM,which is mediated by a consortium of methane oxidizing archaea and sulfate reducing bacteria,is proposed to be responsible for authigenic carbonate formation.A methane-derived carbonate chimney was collected from the Shenhu area, northern South China Sea.The membrane lipids and their very low carbon isotopic compositions(?115‰to?104‰)in the Shenhu chimney suggest the presence of an AOM process.Three specific archaeal and bacterial biomarkers were detected,including Ar,DAGE 1f,and monocyclic MDGD.Their strongly depleted??13C values(?115‰to?104‰),which are lower than those of the normal marine lipids in sediments,reveal biogenic methane as their origin.The carbonate deposits exhibiting a chimney structure indicate that a vigorous methane-rich fluid expulsion may have occurred at the seafloor.We propose that the decomposition of gas hydrates at depth is the likely cause of seepage and cold seep carbonate formation in the Shenhu area.
GE LuJIANG ShaoYongYANG TaoYANG JingHongWU NengYouZHANG GuangXueLIU Jian
The Shenhu area is one of the promise target areas for marine gas hydrate exploration in the northern margin of the South China Sea.Pore water samples were collected from two piston cores (8.75 and 8.52 m) in site HS-A and site HS-B in the Shenhu area,and their major anion and cation contents and trace element contents have been analyzed in this study.Cl- concentrations in pore waters are similar to the seawater value and no systematic change along depth profiles has been found for the Cl- content in both sites.In site HS-A,the SO 2- 4 contents show a limited range in pore waters from 0 to 3 m depths,but a dramatic decrease is documented in depth below 3 m.Other elements such as Ca 2+,Mg 2+ and Sr 2+ show similar variation patterns as the SO 2- 4,i.e.,no variation in 0-3 m,but large decrease in depth below 3 m.In site HS-B,both the SO 4 and Ca 2+,Mg 2+ and Sr 2+ display a decrease pattern along the depth profile,but in 0-3 m the variation is less significant than those in depth below 3 m.In both sites HS-A and HS-B,I contents show a dramatic increase along depth profiles.Calculations show that the SMI depths are very shallow (10 and 11 m) in both sites,and the sulfate flux and I flux are very high.These geochemical characteristics and anomalies at Shenhu are quite similar to those found in other gas hydrate locations in the world,and a genetic link is suggested to be related to the gas hydrate occurrence at depths in the study area.In summary,we suggest that combined geochemical parameters in shallow sediment pore waters are useful indicators to indicate the gas hydrate occurrence at depths.
YANG TaoJIANG ShaoYongGE LuYANG JingHongWU NengYouZHANG GuangXueLIU Jian
The Qiongdongnan Basin is one of the target areas for marine gas hydrate exploration in the northern margin of the South China Sea. In this study, major anion (e.g., SO42 , Cl, Br, I), cation (e.g., Ca, Mg, K, Na), and trace element (e.g., Sr, Ba) con- centrations of pore water samples collected from site HQ-1PC in the Qiongdongnan Basin were analyzed. These geochemical data suggest that the process of AOM (Anaerobic Oxidation of Methane) is dominant in sulfate-reduction zone in this site due to high upwelling iodine flux and strong microbial activities. The iodine-rich fluids, which may carry methane for the gas hydrate formation, pass through the gas hydrate stability zone, mix with brine released during gas hydrate formation, and cause the geochemical anomalies observed at site HQ-1PC. The pore water geochemical characteristics and anomalies in the Qiongdongnan Basin are quite similar to those found in other gas hydrate locations in the Shenhu area in the northern South China Sea, and a genetic link is suggested with the possibility of gas hydrate occurrence in the study area.
YANG TaoJIANG ShaoYongGE LuYANG JingHongWU NengYouZHANG GuangXueLIU JianCHEN DaoHua
