PLEISTOCENE GRAIN-SIZE ANALYSIS AND MICROPALEONTOLOGY OF WEST TASMANIA SLOPE SEDIMENTS (ODP LEG 189, SITE 1168)

Ocean Drilling Program (ODP) Leg 189 examined the Cretaceous to Holocene geologic and paleoceanographic history of the Tasmanian Gateway. As part of the Leg, sediments were recovered from Site 1168 located on the West Tasmania Slope in the Sorell Basin of the Tasman Sea at water depths of 2463 m, approximately 50 km from Tasmania. This study focuses on Pleistocene sediments recovered from Site 1168, which are dominated by fine-grained nannofossil ooze and foraminifer-bearing nannofossil ooze, with minor amounts of clay. The purpose of this study is to conduct a low-resolution grain-size analysis of the Pleistocene sediments, correlating this textural information with foraminifer preservation at the Site and using these results as climate proxies.

Samples used in this study were obtained from the Gulf Coast IODP Repository. Approximately 20 cc of sediment was collected at a frequency of one per section throughout the Pleistocene section of Site 1168. Each sample was initially wet sieved, separating the sample at 62.5 µm. The >62.5 µm fraction was subsequently dry sieved into sand- and gravel-size classes. Sieve data was analyzed descriptively through graphical representation and various measures of central tendency, sorting, skewness, and kurtosis. Specific fractions were targeted for micropaleontologic study, including preliminary census information to assess whether the foraminifers were in situ or transported downslope and SEM analysis of tests to evaluate carbonate dissolution. The grain-size results were then compared to these micropaleontologic analyses.

Data collected thus far reveals trends in grain-size analysis. On average, 80% of the Pleistocene sediments were <62.5 µm, with ranges from 70-90%. Some samples exhibit an abundance of sand- and gravel-size sediment, when compared to the other samples. Specific intervals contain heterogeneous sediment, a combination of deeper-water foraminifers mixed with tests of shelfal foraminifers and fragments of bryozoans and mollusks, possibly indicating downslope transport. SEM analysis of foram tests was used to assess carbonate dissolution. Because the West Tasmania Slope is an important area for paleoceanographic study, this research provides information about the sedimentology, micropaleontology and paleoclimatology of the Southern Ocean.