LATE MIDDLE TO LATE MIOCENE PALEOCEANOGRAPHY OF THE WESTERN CORAL SEA: FORAMINIFERAL POPULATION AND ISOTOPIC ANALYSIS OF OPD LEG 194, HOLE 1195B

Population and isotopic analyses of mixed layer (Globigerinoides sacculifer), thermocline (Globorotalia menardii), and benthic (Cibicidoides spp.) foraminifera from Ocean Drilling Program Hole 1195B are used to investigate late middle to late Miocene (~13-5.5 Ma) paleoceanography of the western Coral Sea. Preliminary data from the Marion Plateau off the northeast Australian margin (Hole 1195B, ~420m depth) are consistent with other tropical western Pacific paleoceanographic data. Oxygen and carbon isotopic records show several distinct intervals of paleoceanographic change, including two warming intervals. The interval from ~11.5-9.5 Ma is characterized by increased water column d¹⁸O gradients as surface and thermocline waters warmed and/or became fresher. This interval of d¹⁸O instability coincides with a time of rapid Antarctic ice sheet expansion and a major eustatic sea level fall (Mi5 isotope event). Foraminiferal population analyses indicate higher abundances of mixed layer taxa and lower abundances of thermocline taxa. Low productivity is indicated by depressed surface-deep d¹³C gradients and low abundances of Globigerina bulloides. Depleted d¹⁸O upper water column values, combined with assemblage data, indicate warming and thickening of the mixed layer. Warmer sea surface temperatures and a depressed thermocline may signal increased influence of the South Equatorial Current (SEC) as Australia and New Guinea moved northward, narrowing the Indonesian Seaway (IS). The restriction of the IS, combined with the late middle to early late Miocene eustatic sea level fall, may have reduced Indonesian Throughflow, resulting in a deepening of the western tropical Pacific thermocline and southward diversion of the SEC. Relative water column stability and cooler or more saline surface waters are recorded from ~9.5 to 8.5 Ma, followed by a surface warming trend from ~8.5-6.5 Ma. Increased surface-benthic and thermocline-benthic d¹³C gradients and elevated abundances of G. bulloides and Neogloboquadrina spp. indicate heightened productivity. A second phase of this surface warming trend from ~6.5-5.5 Ma is associated with elevated mixed layer-thermocline d¹⁸O gradients and increased abundances of Globorotalia spp. and high surface-benthic d¹³C, suggesting continued high productivity.