Changes in Pacific Ocean circulation following the Miocene onset of permanent Antarctic ice cover

Abstract

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 deep water ventilation and strengthening of the meridional overturning circulation following major ice expansion at ∼13.9Ma. Neodymium isotope data reveal, however, that the provenance of intermediate and deep water masses did not change markedly between 15 and 12.7Ma. We attribute the increased δ13C gradient between Pacific deep and intermediate water masses between ∼13.6 and 12.7Ma to more vigorous entrainment of Pacific Central Water into the wind-driven ocean circulation due to enhanced production of intermediate and deep waters in the Southern Ocean. Prominent 100kyr ventilation cycles after 13.9Ma 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 δ13C decline in Pacific intermediate and deep water sites between 13.5 and 12.7Ma reflects a global trend, probably related to a re-adjustment response of the global carbon cycle following the last 400kyr carbon maximum (CM6) of the “Monterey Excursion”.