Multi-proxy evidence for oceanographic changes in the Southern Ocean during the lead-up to the Oligocene/Miocene Boundary (Mi-1 Event)

Abstract:

The Mi-1 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 record from ODP Site 744 on the southern tip of the Kerguelan Plateau is the highest latitude site recording the Mi-1 event and is likely a sensitive recorder of Antarctic Circumpolar Current changes. Core 744A-12 (~28-22.9 Ma) has several short (kyr-scale) hiatuses, but using a combination of paleomagnetic reversals, astrochronology, and stable isotope stratigraphy a robust age model has been constructed. Oxygen and carbon isotopes from both benthic foraminifera and bulk sediment (~18-kyr resolution) depict a series of changes in bottom water. Sediment sand fraction counts show two intervals of possible increased productivity. Large diatom valves (> 63 µm) increase towards the Mi-1 event in pulses paced by obliquity. Benthic foraminifera, another proxy for productivity, increase prior to the Mi-1 event. While an increase in diatoms or benthic forams could suggest higher productivity, they can also be a sign of changes within the lysocline, as benthic forams are more hearty to dissolution and diatoms are immune to carbonate dissolution. 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-dominated sediments. The change in oceanographic situation occurred ~40-kyr prior to the onset of Mi-1 event, suggesting that a combination of current reconfiguring and orbital nodes may explain this transient event.