Abstract:
The late middle Eocene through early Oligocene (42 to 29 Ma) represents an important transition period in Antarctic climates and Southern Ocean circulation. Recent Mg/Ca studies suggest that the positive excursion in benthic foraminiferal oxygen isotope values in the earliest Oligocene (the Oi-1 event) results largely from increased Antarctic ice volume. Southern Ocean surface waters, however, are also interpreted to have cooled at this time, based on increased "cool-water" nannofossil abundance and increased oxygen isotope values in planktic foraminifers. A significant early Oligocene increase in biosiliceous sedimentation also coincides with the Oi-1 event and is interpreted to indicate increased Southern Ocean upwelling and enhanced regional export productivity. Many studies have focused on Eocene-Oligocene boundary records in the Southern Ocean, with less attention directed towards long-term trends during the middle to late Eocene. The upper middle Eocene to lower Oligocene section of ODP Hole 748B, located on the southern Kerguelen Plateau, was examined in the present study. Two pulses in biosiliceous sedimentation are documented in Eocene cores from this hole (centered at approximately 42 and 35 Ma). Although minor in comparison, these events precede the major lower Oligocene increase in biosiliceous sedimentation associated with the Oi-1 isotope event. Additionally, these Eocene events are concomitant with increases in "cool-water" nannofossils, providing direct evidence that increased biosiliceous sedimentation is linked with local surface-water cooling at the site. The Eocene biosiliceous events at Site 748 include a component of radiolarians, sponge spicules, and siliceous plankton (diatoms, silicoflagellates, and ebridians). Increased biosilicification by this range of organisms indicates increased silica availability to all levels of the water column. Furthermore, these events are interpreted to represent the initial stages of deep-water production on the Antarctic shelves and associated upwelling driven by thermohaline circulation. This interpretation is supported by a long-term, middle to upper Eocene increase in benthic foraminiferal oxygen isotope values at Site 748.