Middle Miocene benthic oxygen and carbon stable isotopes and Nd isotopes from Pacific ODP sites

We integrate micropaleontological and geochemical records (benthic stable isotopes, neodymium isotopes, benthic foraminiferal abundances and XRF-scanner derived elemental data) from well-dated Pacific Ocean successions(15-12.7Ma) to monitor circulation changes during the middle Miocene transition into a colder climate mode with permanent Antarctic ice cover. Together with previously published records, our results show improvement in deepwater ventilation and strengthening of the meridional overturning circulation following major ice expansion at 13.9 Ma. Neodymium isotope data reveal, however, that the provenance of intermediate and deepwater masses did not change markedly between 15 and 12.7 Ma. We attribute the increased d13C gradient between Pacific deep and intermediate water masses between 13.6 and 12.7 Ma to more vigorous entrainment of PacificCentral Water into the wind-driven ocean circulation due to enhanced production of intermediate and deep waters in the Southern Ocean. Prominent 100 kyr ventilation cycles after 13.9 Ma reveal that the deep Pacific remained poorly ventilated during warmer intervals at high eccentricity, whereas colder periods (low eccentricity) were characterized by a more vigorous meridional overturning circulation with enhanced carbonate preservation. The long-term d13C decline in Pacific intermediate and deepwater sites between 13.5 and 12.7 Ma reflects a global trend, probably related to a re-adjustment response of the global carbon cycle following the last 400 kyr carbon maximum (CM6) of the ''Monterey Excursion''.