Hydrogen sulfide-rich hydrates and saline fluids in the continental marginof South Australia

During the drilling of the southern Australian continental margin (Leg 182 of the Ocean Drilling Program), fluids with unusually high salinities (to 1 06 parts per thousand) were encountered in Miocene to Pleistocene sediments . At three sites (1127, 1129, and 1131), high contents of H2S (to 15%), CH4 (50%), and CO2 (70%) were also encountered. These levels of H2S are the hi ghest yet reported during the history of either the Deep Sea Drilling Proje ct or the Ocean Drilling Program. The high concentrations of H2S and CH4 ar e associated with anomalous Na+/Cl- ratios in the pore waters. Although hyd rates were not recovered, and despite the shallow water depth of these site s (200-400 m) and relative warm bottom water temperatures (11-14 degreesC), we believe that these sites possess disseminated H2S-dominated hydrates. T his contention is supported by calculations using the measured gas: concent rations and temperatures of the cores, and depths of recovery. High concent rations of H2S necessary for the formation of hydrates under these conditio ns were provided by the abundant SO42- caused by the high salinities of the pore fluids, and the high concentrations of organic material. One hypothes is for the origin of these fluids is that they were formed on the adjacent continental shelf during previous lowstands of sea level and were forced in to the sediments under the influence of hydrostatic head.