Queen's University, Canada

In contrast to tropical carbonates, which are composed primarily of aragonite and high-Mg calcite, one of the distinguishing characteristics of cool-water carbonates is their predominately calcitic mineralogy. This fundamental difference has suggested that cool-water carbonates are much less susceptible to both meteoric and shallow marine burial diagenetic processes. We report mineralogical and geochemical data from a three site drilling transect on the upper slope (200-480 m water depth) of the South Australian margin. One of the surprising discoveries of ODP Leg 182 drilling was the extent of early marine burial diagenesis in these cool-water carbonates. At all three sites, a thick (>500 m) wedge of Quaternary sediments was cored. Sediments are unlithified to partially lithified bioclastic packstones and grainstones. The allochems are fine carbonate sand and silt composed of skeletal fragments, mainly bryozoans, ostracodes, benthic and planktonic foraminifer tests, tunicate sclerites, and siliceous sponge spicules. Lithification increases downcore at all sites, becoming partially lithified between 120-235 m below the seafloor. Surprisingly, sediments are polymineralic, composed of striking variations of high-Mg calcite, low-Mg calcite, aragonite, and dolomite. Aragonite and high-Mg calcite have their greatest abundances in the upper part of the cores. At the shallower water sites, aragonite, and in particular, high-Mg calcite content decreases downcore as low-Mg calcite and dolomite content increases. Downcore dolomite content ranges between 10-20%, reaching a maximum of 80%.Pore fluids at the three sites are characterized by the presence of a high salinity brine (up to three times seawater). The very high sedimentation rates, the associated influx of organic matter, and the high pore-water sulfate concentrations result in extensive sulfate reduction and extraordinary hydrogen sulfide production. We believe the high hydrogen sulfide concentrations, resulting from organic matter diagenesis in these low-iron carbonate sediments, help maintain low pore-water pH, and hence, lower carbonate mineral saturation states that may accelerate early diagenetic reactions.