Cyclicity in Eocene Southern Ocean Sediments; Patterns From the Tasmanian Gateway, ODP 189, Site 1172.

Here we present major element, diatoms and dinocysts data from sediments of Middle to early Late Eocene age, recovered during ODP Leg 189. Sites were selected to obtain an early Paleogene record of rifting between Antarctica and the Southern Tasman Rise. This was a time of minimal or no connection between the Indian and Pacific Oceans. Analysis of these cores is allowing a better understanding of paleoceanographic and paleoclimatic conditions prior to the opening of the Tasmanian Gateway and subsequent Antarctic Circumpolar Current development. In the early Paleogene marine siliciclastic sediments, largely silty claystones were deposited off Tasmania (at 65° S paleolatitude) under a sluggish ocean circulation within a restricted environment in relatively shallow water. Most of the Tasmanian Promontory had not yet subsided and the Antarctic hinterland provided most of the sediments to the Tasman Basin, resulting in relatively high accumulation rates. In Holes 1172A and 1172D the Eocene magnetic polarity zones are well defined; calcareous microfossils are sporadic, whereas siliceous microfossils (notably diatoms) and organic walled microfossils (dinocysts, spores, and pollen) are consistently abundant. High-resolution X-ray fluorescence (XRF) Core Scanner data of the Middle Eocene at Site 1172, East Tasman Plateau, reveals clear cycles of varying length that are present throughout most of the sequence. Ca and Fe were selected as proxies for climate-paleoceanographic cycles, since these elements mirror changes in calcium carbonate/clay ratios. Our data match well with other proxy data (e.g., magnetic susceptibility, color reflectance) but show a significantly higher signal-to-noise ratio. The major element data better define the origin and nature of cycles (Milankovitch, runoff and SST variations), the derived cyclostratigraphy helps to understand and explain regional similarities and differences in sedimentation and climate, and probable influences of sea-level changes. Quantitative palynological and diatom analysis of selected intervals straddling the distinct cyclic patterns provides paleoecological information on changing conditions not only at the sea surface but also on the sea floor (benthic diatoms). Generally the dominant taxa are endemic. Cycles are characterized by massive compositional changes in the assemblages, indicative of changes in eutrophic state, sea surface temperature, runoff/sea level and associated hydrodynamic energy levels.