A large portion of human-induced carbon dioxide emissions, and the heat that they trap within Earth?s climate system, are being absorbed by the global ocean. The long-term impacts of this absorption on climate and ocean circulation are poorly understood. Forecasts of 21st-century climate change suggest that global warming will lead to a more stratified ocean and weaker global ocean circulation, which would have major ramifications for regional and global climate. However, recent studies suggest that ocean circulation was stronger, not more sluggish, during warm intervals earlier in Earth?s history. This potential discrepancy warrants a better characterization of ocean conditions during warmer-than-present climates of the past. To do so, the study will focus on intervals in the geologic past during which atmospheric carbon dioxide concentrations were similar to today, and temperatures were several degrees warmer. The most recent geologic interval that meets this criterion is the Pliocene Epoch (5.33 to 2.58 million years ago). The primary goal of this project is to generate paleoceanographic records to test models suggesting that there was a strong overturning circulation in the North Pacific Ocean during the Pliocene Epoch. This award will support the careers and training of five early-career scientists, and train and mentor high school and undergraduate students. The goals of this proposal are to: 1) test the recent hypothesis that there was North Pacific deep-water formation and an active Pacific Meridional Overturning Circulation present at times during the warm Pliocene; 2) trace the regional distribution of ocean ventilation; and 3) refine the use of redox, temperature, and productivity proxies in bulk sediments and foraminifera for reconstructing Pliocene North Pacific Ocean ventilation, nutrient availability, and water mixing. A multi-proxy approach will examine intermediate to deep ocean circulation patterns across two key geologic intervals: the mid-Pliocene warm period through the intensification of Northern Hemisphere glaciation (~2.5 to 3.3 Ma) and the early Pliocene (~4.9 to 5.1 Ma). This project will generate new records of i) redox and productivity proxies from bulk sediments, ii) redox, productivity, and water mass mixing proxies from benthic foraminifera, and iii) productivity and temperature proxies from planktic foraminifera, at four sites (IODP 882, 883, 887, and 1208) that cover a range of depths and locations across the North Pacific Ocean. In parallel, the study will analyze a series of coupled climate model simulations with active biogeochemical and d13C cycling. The model tracer, circulation, isotope, and ventilation age results will aid in the interpretation of the newly generated data. This multi-proxy approach, underpinned by climate model analysis, will help ensure the feasibility of the proxy-inferred dynamics. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.