University of Texas at Dallas, United States

During the drilling of the southern Australian continental margin (Leg 182 of the Ocean Drilling Program), fluids with unusually high salinities (to 106%) were encountered in Miocene to Pleistocene sediments. At three sites (1127, 1129, and 1131), high contents of H (sub 2) S (to 15%), CH (sub 4) (50%), and CO (sub 2) (70%) were also encountered. These levels of H (sub 2) S are the highest yet reported during the history of either the Deep Sea Drilling Project or the Ocean Drilling Program. The high concentrations of H (sub 2) S and CH (sub 4) are associated with anomalous Na (super +) /Cl (super -) ratios in the pore waters. Although hydrates were not recovered, and despite the shallow water depth of these sites (200-400 m) and relative warm bottom water temperatures (11-14 degrees C), we believe that these sites possess disseminated H (sub 2) S-dominated hydrates. This contention is supported by calculations using the measured gas concentrations and temperatures of the cores, and depths of recovery. High concentrations of H (sub 2) S necessary for the formation of hydrates under these conditions were provided by the abundant SO (sub 4) (super 2-) caused by the high salinities of the pore fluids, and the high concentrations of organic material. One hypothesis 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 into the sediments under the influence of hydrostatic head.