Middle Miocene isotope stratigraphy and paleoceanographic evolution of the northwest and southwest Australian margins (Wombat Plateau and Great Australian Bight)

The benthic stable isotope record from ODP Site 761 (Wombat Plateau, NW Australia, 2179.3 m water depth) documents complete recovery of the middle Miocene delta13C excursion corresponding to the climatic optimum and subsequent expansion of the East Antarctic Ice Sheet. The six main delta13C maxima of the ?Monterey Excursion" between 16.4 and 13.6 Ma and the characteristic stepped increase in delta18O between 14.5 and 13.9 Ma are clearly identified. The sedimentary record of the shallower ODP Sites 1126 and 1134 [Great Australian Bight (GAB), SWAustralia, 783.8 and 701 m water depth, respectively] is truncated by several unconformities. However, a composite benthic stable isotope curve for these sites provides a first middle Miocene bathyal record for southwest Australia. The delta18O and delta13C curves for Sites 1126 and 1134 indicate a cooler, better-ventilated water mass at ~700 m water depth in the Great Australian Bight since approximately 16 Ma. This cooler and younger water mass probably originated from a close southern source. Cooling of the bottom water at ~16 Ma started much earlier than at other sites of equivalent paleodepths in the central and western parts of the Indian Ocean. At Site 761, the delta18O curve shows an excellent match with the global sea level curve between ~11.5 and 15.1 Ma, and thus closely reflects changes in global ice volume. Prior to 15.1 Ma, the mismatch between the delta18O curve and the sea level curve indicates that delta18O fluctuations are mainly due to changes in bottom water temperature.