Halide systematics in interstitial waters of ocean drilling sediment cores

The distribution of dissolved halides (Cl, Br, I) in interstitial waters of deep drill holes in the ocean is discussed for a number of environments: hemi-pelagic sediments; hydrate containing sediments; sediments affected by halide brines; and also in hydrothermal environments. The importance of the diagenesis of organic C in the production of I− and Br− is discussed, with reference to the observations of dissolved SO42- and NH4+ in the pore fluids. More importantly, in the zone of methano-genesis, below the SO42- reduction zone, and usually associated with high organic C accumulation rates, increases in I− and Br− are of importance and usually are much higher than the increases observed in the SO42- reduction zone. In pore fluids affected by dissolution of evaporites the Br/Cl is often a strong indicator of the degree of evaporation of seawater demonstrated from results obtained in the Mediterranean Sea. Dissolution of halites on the other hand leads to Br/Cl much lower than those of sea water. In hydrothermal sediments, there is no doubt that I− and Br− contents of the fluids are influenced by hydrothermal alteration of the sediments and their organic C, but signals are not clear. The present study provides an overview of previous DSDP and ODP studies of halide distributions in sedimentary pore waters, in addition to other contributions on halide distributions in sedimentary pore waters in this symposium volume.