Abstract

Formed in the Southern Ocean, Antarctic Intermediate Water (AAIW) plays a fundamental role in the modern climate system as an important sink for atmospheric CO2, and as a mid-depth conduit supplying nutrient-rich waters to subtropical and tropical upwelling regions. During the transition from warm Pliocene 'greenhouse' conditions to Pleistocene 'icehouse' conditions around 3 Ma, model simulations suggest dramatic circulation changes in the Southern Ocean AAIW formation region due to cooling and expanded sea ice cover around Antarctica. However, the history and properties of AAIW over this time period are still poorly understood. Here, we trace AAIW source water provenance, temperature, and salinity changes over the last 4 Myr using Nd isotopes of Fe-Mn-oxyhydroxide encrusted foraminifera, benthic foraminiferal Mg/Ca, and stable isotopes from southwest Atlantic DSDP Site 516. Our results show that modern AAIW properties emerged gradually over the last 3 Myr, as evidenced by gradual cooling (6 degrees C) and freshening of intermediate waters. Over this same interval, epsilon Nd-values decrease and diverge away from those of Pacific-sourced waters and converge on values of present-day Circumpolar Deep Water (CDW). These observations are in accordance with model simulations indicating increased deep vertical mixing at the Polar Front associated with sea ice extension. These modified AAIW source waters penetrated increasingly northward into the Atlantic at intermediate depths, enhancing nutrient supply and contributing to surface cooling along subtropical coastal upwelling sites. These circulation changes may have facilitated ocean CO2 storage, providing a positive feedback amplifying global cooling and intensification of Northern Hemisphere Glaciation. (C) 2019 Elsevier Ltd. All rights reserved.