Geochemical characteristics of pore water in shallow sediments from Shenhu area of South China Sea and their significance for gas hydrate occurrence

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 (4) (2-) 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 Ca2+, Mg2+ and Sr2+ show similar variation patterns as the SO (4) (2-) , i.e., no variation in 0-3 m, but large decrease in depth below 3 m. In site HS-B, both the SO4 and Ca2+, Mg2+ and Sr2+ 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.