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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

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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 δ 37Cl 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 δ 37Cl is–0.54‰ to +0.96‰, and positive δ 37Cl 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 δ 37Cl is also associated with the same mechanism.

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Acknowledgments

The authors thank Chris Eastoe who provided valuable suggestions on an earlier version of this paper. Two anonymous reviewers provided thoughtful reviews, which helped to significantly improve the manuscript.

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Correspondence to Shaoyong Jiang.

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Foundation item: The National Natural Science Foundation of China under contract Nos 41373002, 40903001 and 40903002.

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Li, Y., Jiang, S. & Yang, T. 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. Acta Oceanol. Sin. 36, 31–36 (2017). https://doi.org/10.1007/s13131-017-1013-3

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  • DOI: https://doi.org/10.1007/s13131-017-1013-3

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