ECCENTRICITY IS THE DOMINANT ORBITAL FORCING FACTOR AT ODP SITE 693 ON THE ANTARCTIC CONTINENTAL MARGIN IN THE MIDDLE AND LATE PLIOCENE

Site 693 off of Cape Norwegia, in the Weddell Sea, Antarctica, lies seaward of one of the most stable ice sheet areas of East Antarctica (Deconto & Pollard, 2003). Two cores from ODP Site 693 at -71 degrees S, 2359 m water depth differ considerably in composition. Core 8R (60.4-70.1 mbsf, 3.58-4.18 Ma), is clayey mud with a small biogenic fraction consisting of radiolaria and diatoms. Core 2R (2.5-12.2 mbsf; 0.27-1.07 Ma), is clayey mud with a biogenic fraction consisting of foraminifera, few radiolaria and no diatoms. Neither core contains calcareous nannofossils. Age resolution is limited to paleomagnetic reversals in a hole with poorly recovered sediments, averaging 62%. The two cores chosen for this work were selected because they had full recovery. Both contain an approximately equal number of dropstones.

The changes in biogenic sediment can be attributed to local changes in preservation and/or productivity. Productivity changes can be driven by a) seasonality, b) wind-driven changes or c) thermal stratification. Each driving factor would be dominated by a different orbital forcing. Enhanced seasonality would be dominated by precession. Wind-driven changes would be driven by obliquity forcing, which controls the pole to equator thermal gradients. An eccentricity driven forcing would likely be manifested by a local thermal response that enhanced stratification. Principal component and wavelet analysis was performed on four different sediment properties. Split core measurements were made at 2 cm spacing for magnetic susceptibility and XRF elemental counts. Sediment sample analyses were made on fine fraction grain-size distribution and spectral reflectance of the total sample. In both cores, all four properties show a strong eccentricity forcing while some show a weak precession forcing. Speculation about how these changes result in such dramatic changes in the sediment record will be discussed.