Reorganization and Strengthening of Ocean Circulation in the Oligocene: Stable Isotopic and Trace Element Records from Western North Atlantic Intermediate Waters

The Oligocene epoch (~33.7-23.8 Ma) is bracketed by major excursions (1.0-1.5‰) in benthic foraminiferal d18O values, called Oi-1 and Mi-1. Both excursions have been linked to a combination of ice-sheet expansion, sea-level fall, and cooling. Near the Eocene/Oligocene boundary, large (~1‰) interbasinal d18O gradients developed, reflecting a differentiation of deepwater source regions; together with the onset of widespread hiatuses and drift deposition, this indicates a substantial reorganization and strengthening of ocean circulation at this time. Substantial d18O fluctuations characterize the Oligocene, indicating that climate, sea level, and ice-sheet dynamics were unstable for many millions of years after establishment of the first continental-scale Antarctic ice sheet. Here, we present high-resolution benthic foraminiferal d18O and d13C records with Mg/Ca data from Atlantic Slope Project corehole 5 (ASP-5; 250 m present depth, southeast US continental slope) with comparisons to published records. Middle to upper bathyal benthic foraminiferal assemblages indicate that the paleobathymetry at ASP-5 was ~600m in the Oligocene. ASP-5 data record global isotopic events (e.g., Oi-1, Oi2a,b, Mi-1), and provide constraint on Oligocene intermediate waters in the western North Atlantic. ASP-5 isotopic values were similar to the North Atlantic and Pacific in the earliest Oligocene. Our data show that a d18O gradient developed between the deep and intermediate Atlantic in the early Oligocene, consistent with the development of interbasinal d18O gradients at the same time that reflect the reorganization of global ocean circulation patterns. A strong d13C depth gradient also developed between intermediate-depth ASP-5 and the deep North Atlantic in the early Oligocene. ASP-5 Mg/Ca data show that seafloor temperatures cooled from the Eocene (~16°C) to the Oligocene (~12-14°C; similar to modern temperatures in this region).