Br/Cl, I/Cl and chlorine isotopic compositions of pore water in shallow sediments: implications for the fluid sources in the Dongsha area, northern South China Sea

Abstract

The Dongsha area is one of the most promising target areas for gas hydrate exploration in the South China Sea (SCS). The study of pore water geochemistry has played a key role in Chinese gas hydrate exploration. Br/Cl, I/Cl and δ37Cl in pore water were applied here in tracing gas hydrate occurrence, chemical evolution of pore fluids and water/rock interactions in low temperature sediment environments. The samples were collected from Sites HD255PC and HD309PC in the Dongsha area in 2004. At Site HD255PC, we found the elevated Br/Cl, I/Cl and decreased SO4/Cl at the depth of 4–5 m, suggestive of a laterally migrated fluid probably generated from the gas hydrate occurrence. The range of δ37Cl is–0.54‰ to +0.96‰, and positive δ37Cl at 4–5 m interval should be related with different diffusion rates between 35Cl and 37Cl. At Site HD309PC, a laterally migrated fluid was also found at the depth of 3–4 m, with the Br/Cl two times to that of the seawater and decreased I/Cl, indicating the fluid has no relationship with the gas hydrate. In this site, the chlorine isotopic composition varies from–0.7‰ to +1.9‰. Extra high Br/Cl might relate with the deep generated fluid. At higher temperature and pressure, the Br/Cl of the fluid is elevated during the hydrous silicate formation, while positive δ37Cl is also associated with the same mechanism.