PALEOCEANOGRAPHY AT ODP SITE 744: A HOLISTIC "PRECONDITIONING HYPOTHESIS" EXPLANATION FOR THE OLIGOCENE MIOCENE TRANSITION (Invited Presentation)

The Oligocene/Miocene Transition (OMT) is characterized by a ~1.0‰ benthic foraminiferal oxygen isotope excursion, referred to as the ‘Mi-1’ event (23.0 Ma), representing bottom water cooling, and an abrupt and transient glaciation on Antarctica. This study focuses on Ocean Drilling Program Site 744 on the Kerguelen Plateau in the Southern Ocean. Site 744 is the highest latitude record of ‘Mi-1’ drilled by scientific ocean drilling, though it is somewhat incomplete. Paloeceanography is investigated through sediment particle counts, benthic foraminiferal stable isotopes, and size fraction analyses. The data reveal the influence of both secular and orbital controls on the records. Changes in the depth of the lysocline at Site 744 are linked to eccentricity forced changes in carbonate productivity. Site 744 also displays a strong connection to the ~1.2 Ma obliquity amplitude cycle. Increased wind driven upwelling ~300-kyr prior to the ‘Mi-1’ peak oxygen isotope values drew warmer intermediate waters up to the surface, increasing local productivity. Through a detailed review of published chronostratigraphy and our stable isotope results, about 150-kyr of the ‘Mi-1’ excursion is missing in a hiatus at Site 744, which agrees with other data showing increased bottom water flow. Here we suggest that the OMT was caused by ‘preconditioning’ of the Southern Ocean. Multiple ocean gateways around Antarctica opened by the late Oligocene, allowing the full circulation of the Antarctic Circumpolar Current (ACC). A cooling step during the penultimate Oligocene obliquity low-amplitude node (~24.2 Ma) increased the pole-to-equator temperature gradient, increasing geostrophic winds. Increased windstress caused the fully open divergence zone around Antarctica to upwell more strongly. Progressively higher rates of productivity during this last 1.2 myr of the Oligocene, timed at ~405-kyr eccentricity band, sequestered CO₂ and preconditioned Antarctica for glaciation across the Oligocene/Miocene boundary. The glaciation was itself triggered by favorable orbital conditions (low eccentricity and low obliquity). Thus, ‘Mi-1’/OMT is driven by a mix of long- and short-term orbital changes and gateway configurations.