THE EOCENE-OLIGOCENE TRANSITION IN MENTELLE BASIN, SOUTHEAST INDIAN OCEAN

The accumulation of a long-lived ice sheet on Antarctica across the Eocene-Oligocene (E-O) boundary marks the beginning of late Cenozoic icehouse climate. The opening of ocean passageways around Antarctica and decreasing CO₂ levels are commonly invoked as ultimate forcing mechanisms, but there is considerable regional variation in geochemical and sedimentological records among sites. In particular, long-term changes in ocean circulation around Antarctica inferred from Nd-isotope records do not consistently coincide with the E/O boundary, and the timing of circulation changes varies among sites. This complexity is a challenge to interpretation but also provides an opportunity, if age constraints are sufficiently refined, to examine in detail oceanographic evolution during the transition.

Cores from IODP Site U1514 in the Mentelle Basin recovered an apparently complete E-O record from a previously undocumented portion of the SE Indian Ocean. U1514 is strategically located to investigate the effect of the opening Austral-Antarctic Gulf on deep-ocean connectivity, and orbitally forced sedimentation patterns may allow the development of a detailed age model. Sediments at U1514 are rich in siliceous microfossils throughout the transition. Foraminifera are relatively rare, but the abundance of planktonic foraminifera increases above the transition. Over the same interval, sedimentation rates drop from ~10m/m.y. to ~3 m/m.y. Preliminary oxygen isotopic records of bulk carbonate show the expected two step increase from ~0‰ to ~1.2‰ across the transition whereas carbon isotopes display a more intricate pattern perhaps reflecting changes in surface water productivity and/or surface circulation patterns. Initial Nd isotopic results suggest a long term ~2 unit increase with an ~1 unit negative excursion occurring at or near the level of the second increase in oxygen isotopic values. Ongoing studies will increase the resolution of the neodymium data, add stable isotopic data from benthic foraminifera, and incorporate elemental data from high-resolution XRF scanning. The goal of constructing this integrated data set is to constrain better relationships among sedimentation, fossil assemblages, circulation, weathering inputs, and climate evolution across the E-O transition.