Authigenic carbonates record some of the physical, chemical, and stable isotopic conditions existing in sediments at the time of carbonate precipitation. Carbonates persist in the rock record after nascent fluids are gone, and authigenic carbonate composition can be used to reconstruct diagenetic environments. Observations of environments in which carbonates are currently forming provide the necessary interpretive analogs. Sediment and pore fluids collected during coring operations of the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) have been a useful source of observations of various stages of diagenesis and associated mineral precipitates (Malone et al., 2002; Rodriguez et al., 2000; Hicks et al., 1996; Kelts and MacKenzie, 1982; Pisciotto and Mahoney, 1981).
During Leg 188 of the Ocean Drilling Program, we cored sediments offshore Antarctica, on the continental rise, slope, and shelf north of Prydz Bay during the months of January, February, and March 2000 (O'Brien, Cooper, Richter, et al., 2001). At Site 1165 on the continental rise (3537 m water depth), the expedition cored Miocene and younger pelagic and hemipelagic drift sediments. At Site 1166 on the continental shelf (475 m water depth), the cored sediments included organic carbon-rich rocks of Cretaceous (Turonian) age deposited during temperate climate conditions that supported onshore swamp vegetation. Thirteen carbonate nodules were collected and analyzed, ten from Site 1165 and three from Site 1166. The objective of this study is to relate the chemical and isotopic composition of carbonate nodules to diagenetic conditions at the time of carbonate precipitation and to show how sedimentologic conditions (primarily sedimentation rate) have influenced the diagenetic history of the cored sediments in this region.
Methane-containing cores and bottom-simulating seismic reflectors have been observed in drift sediments deposited off Antarctica (Hayes, Frakes, et al., 1975; Barker, Camerlenghi, Acton, et al., 1999). Possible occurrence of gas hydrate was anticipated for Leg 188. Although elevated methane contents were observed in Hole 1165B and pressure and temperature conditions were such that the methane hydrate would have been stable to a depth of ~470 mbsf, no positive evidence of methane hydrate presence was found. Dissolved methane concentrations in Hole 1165B apparently were insufficient to saturate the pore water and stabilize the hydrate phase. A principal result of this study is evidence that anaerobic methane oxidation processes may have operated to erode and deplete a once robust methanogenic zone, thus destroying any gas hydrates that may have been present.