PLEISTOCENE COOL-WATER CARBONATES: FORAMS AND BRYOZOANS FROM 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 a result of this expedition, sediments were recovered from Site 1168 located on the West Tasmania Slope, in the Sorrell Basin of the Tasman Sea at water depths of 2483 m, approximately 50 km from Tasmania. This study focuses on the Pleistocene sediments (fine-grained nannofossil ooze and foraminifer-bearing nannofossil ooze, with minor amounts of clay). The goal of this study is to conduct a low-resolution study of the micropaleontology and sedimentology of those Pleistocene sediments.

We have presented low-resolution grain-size analysis previously (Zunk et al, 2006). In that study, we concluded that: 1)grain-size varied throughout the Pleistocene, 2) sediments are dominated by biogenic material of various types (foraminifers and bryozoans, primarily), and 3) with sea-level changes associated with glacial-interglacial periods in the Pleistocene, the amount of sediment brought from the shelf and slope to Site 1168 changes due to differing rates of submarine canyon incision/erosion and gravity flows.

Eleven, 20 cc sediment samples spaced at approximately 150,000 year intervals were obtained from the Gulf Coast Repository for the purpose of this study. Each sample was washed and sieved to separate it into three size fractions (0-63 micron, 63-125 micron, and >125 micron). In the Zunk et al, 2006 study, the samples were further dry sieved and separated by phi size (-1 up to 3). The samples separated by phi size have been picked to analyze the benthic foraminiferal population. There is abundant material in each sample for this. A preliminary analysis of the community (presence/absence data, diversity, etc.) of each sample is presented. This preliminary analysis, paying close attention to the possibility that foraminifera have been swept in from upslope, helps us determine whether the site warrants closer analysis of the community in the future. Site 1168 represents a cool-water carbonate setting, as evidence by the dominance of foraminifera, bryozoans, and mollusk fragments and the lack of hermatypic coral fragments. As previously mentioned, downslope transport is a dominant process at this site, so any micropaleontological analysis must be done with that in mind.