REVISED MIOCENE BIOSTRATIGRAPHY OF MARION PLATEAU, NE AUSTRALIA MARGIN, AND IMPLICATIONS FOR SEQUENCE STRATIGRAPHY AND PALEOCEANOGRAPHY

The early to middle Miocene (~25­­-13 Ma) was a major transition in the long-term cooling of the Cenozoic, as the extended warmth of the late early Miocene gave way to way to a permanent southern-hemisphere “icehouse” climate. The Marion Plateau (MP), a carbonate platform on the northeast margin of Australia drilled by ODP Leg 194, is an ideal location to study Miocene sea level and climate change and has been the focus of several previous studies. As we develop proxy data from events lasting <1myr, tight age control is essential. With a new high resolution nannofossil/planktic foraminiferal age model, a variety of new and revised proxies allow us to refine our picture of Miocene climate change at the Marion Plateau and propose three new early Miocene sequences. Detailed descriptions of Marion Plateau sequences can be found in John et al., 2011; our new biostratigraphy indicates three hiatuses and potential new sequences in the lower Miocene at 23.15 Ma, 21.98 Ma, and 20.80 Ma. The middle Miocene Mi3, Mi4, and Mi5 benthic foraminiferal oxygen isotope events track the stepwise growth of Antarctic icesheets. The largest of these events, characterized in deep sea paleoclimate records as strong positive oxygen isotope excursions, is Mi3 (also known as the middle Miocene climate transition – MMCT; 14.2-13.8 Ma) and is also linked to a negative carbon isotope excursion marking the end of the “Monterrey Excursion,” which began 16.9 Ma and ended at 13.4 Ma, one 400kyr eccentricity cycle following the onset of southern hemisphere glaciation. We were able to better date unconformities linked with excursions of the oxygen isotope record to show these events directly correspond to Mi1a, Mi1aa, and Mi2, and that the carbon isotope excursions in our records can confidently be identified as the Monterrey Excursion. Nannofossil assemblage data, planktic/benthic foraminifera ratios, and neritic fragment counts lead us to paleoceanographic and paleoclimatic interpretations as to where the Marion Plateau fits on the global picture of middle Miocene cooling.