At Site 1172, however, the presence of dissolved sulfate is likely inhibiting methanogenesis in the organic carbon-bearing Paleogene sediments. This observation is unusual because the TOC content of these low-gas sediments appears to be sufficient to have already driven sulfate concentrations to zero. In general, the fresher pore waters are located in the older part of the cored interval but are in different age sediments and are not restricted to specific lithologies. These fresher fluids are manifested in the Cl - profiles by multiple distinct maxima and minima rather than smoothly decreasing values. Low Cl - values in marine pore waters have been observed in environments ranging from accretionary prisms to passive continental margins.
Gas hydrate dissociation is a common cause of such profiles in continental margin settings.
Therefore, a more rigorous calculation of gas hydrate stability may extend the depth of the stability zone. At present, we cannot eliminate connate fluids as the possible source of fresher fluids. In fact, the jagged nature of the Cl - profiles suggests that the emplacement of fresher fluids has occurred relatively recently. Clay mineral reactions are a well-known source of fresh fluids, and clay content does increase in the older part of the sedimentary succession.