Forward modeling the δ¹⁸O of foraminifera from North Pacific temperature and salinity fields to evaluate the potential fingerprints of Pliocene North Pacific Deep-Water Formation

Abstract

The global ocean absorbs significant amounts of human-induced CO₂ emissions and the associated heat that CO₂ traps in the atmosphere as a greenhouse gas. However, the long-term effects of warming oceans on climate and ocean circulation remains poorly understood. Climate model forecasts predict that ongoing global warming will lead to a more stratified ocean and weaker global ocean circulation in the near-term, with major ramifications for regional and global climates. However, they disagree on what will occur in the long-term as the deep ocean slowly adjusts to surface warming. To gain further insight into the long-term response of the ocean to global warming, our study focuses on the Pliocene Epoch (5.33 to 2.58 million years ago), the most recent geologic interval during which atmospheric carbon dioxide concentrations were similar to today, and temperatures were several degrees warmer. Recent studies suggest that the warm conditions of the Pliocene supported North Pacific Deep-Water formation and a Pacific Meridional Overturning Circulation, two phenomena that are absent in the modern ocean due to the freshness of surface waters in the subpolar North Pacific. The study builds a quantitative framework for how the presence of North Pacific Deep-Water formation during the Pliocene will imprint on the planktonic and benthic foraminiferal δ¹⁸O records that will be generated from 5 different sites in the North Pacific Ocean. These sites were chosen for their proximity to hypothesized Pliocene deep-water sites and deep-water circulation pathways that would allow them to detect a signal associated with a Pacific Meridional Overturning Circulation. We employ a forward modeling technique to simulate “synthetic” foraminiferal δ¹⁸O_carbonate records using temperature and salinity fields for 5 different sites in the Pacific Ocean. We create synthetic δ¹⁸O_carbonate time series’ using temperature and salinity fields from modern-day reanalysis data (ORAS5) and a Pre-industrial climate model (CESM) simulation, both of which don’t have North Pacific Deepwater formation, as well as a Pliocene simulation which has North Pacific Deepwater formation. These synthetic records quantify the fingerprints of Pliocene North Pacific Deep-Water Formation and will aid in the interpretation of the planktonic and benthic foraminiferal δ¹⁸O records that will be generated over the coming years for each site in the North Pacific Ocean. We acknowledge the feedback received in designing this research project from our collaborators on NSF award “Collaborative Research: Tracing Pacific Ocean circulation and ventilation during the warm Pliocene Epoch”, which the findings from our study will feed back into.