OCEANOGRAPHIC CHANGES ACROSS THE OLIGOCENE/MIOCENE BOUNDARY (MI1 EVENT): ODP SITE 744, SOUTHERN KERGUELEN PLATEAU

The Mi1 event occurs roughly at the Oligocene/Miocene boundary. The ~1‰ oxygen isotope shift records a transient glaciation event with an unclear driver. A new multi-proxy records from ODP Site 744 (Southern Ocean) illustrate substantial changes in ocean currents during the lead-up to Mi1. Core 744A-12 (~28-22.9 Ma) has several short (kyr-scale) and long (myr-scale) hiatuses. These hiatuses have been identified using a combination of inflections in the mass of >63 µm sand size fractions relative to the rest of the sample, and breaks in the cyclicity in the bulk density, as seen in an evolutive harmonic analysis. These hiatuses have been accounted for in a new age model, constructed using a combination of paleomagnetic reversals, astrochronology, and stable isotope stratigraphy. Changes in grain size suggest multiple changes in winnowing strength along the Southern Kerguelen Plateau during the late Oligocene. Oxygen and carbon isotopes from benthic foraminifera and bulk sediment depict a series of changes in Southern Ocean bottom water. Sediment sand fraction counts show two intervals of possible increased productivity, with both large diatom valves (>63 µm) and benthic foraminifera increasing towards the Mi1 event. While these could suggest higher productivity, they could also indicate changes within the lysocline. Using a foram fragmentation index, we identify substantial changes within the lysocline, however the diatom pulses are not synchronous with these changes, suggesting they are independent of lysocline changes, and are in fact robust recorders of pulsed increases in productivity. There are then two likely explanations; one, an increase in upwelling, bringing up a more CO₂, nutrient-rich water mass, or two, an expansion of the Antarctic Polar Front northwards away from Antarctica, bringing its more silicate-rich sediments. The change in oceanographic situation occurred ~40-kyr prior to Mi1, possibly suggesting a role for the polar front in the onset of Mi1. During the event itself, an increase in carbonate preservation, noted both in a decrease in fragmentation, decline in diatoms, and increase in benthic foraminifera, suggests a southward shift in the polar front as ice growth occurred on Antarctica.