Geologic intervals of sustained warmth such as the mid-Pliocene Warm Period can inform our understanding of future climate change, including the long-term consequences of oceanic uptake of anthropogenic carbon. Here we generate carbon isotope records and synthesize existing records to reconstruct the position of water masses and determine circulation patterns in the deep Pacific Ocean. We show that the mid-depth carbon isotope gradient in the North Pacific was reversed during the mid-Pliocene compared with today, which implies water flowed from north to south and deep water probably formed in the subarctic North Pacific Deep Water. An isotopically enabled climate model that simulates this North Pacific Deep Water reproduces a similar carbon isotope pattern. Modelled levels of dissolved inorganic carbon content in the North Pacific decrease slightly, although the amount of carbon stored in the ocean actually increases by 1.6% relative to modern due to an increase in dissolved inorganic carbon in the surface ocean. Although the modelled Pliocene ocean maintains a carbon budget similar to the present, the change in deep ocean circulation configuration causes pronounced downstream changes in biogeochemistry.
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The CESM 22.214.171.124 code used to run the climate simulations is available from https://svn-ccsm-models.cgd.ucar.edu/cesm1/release_tags/cesm1_2_2_1. The code modifications made to CESM 126.96.36.199 to include carbon isotopes following ref. 38 are available from https://github.com/nburls/FordEtAl2022_CISOmods. Figures with coastal outlines (for example, Figs. 1 and 2 and Extended Data Figs. 1 and 2) were created in Matlab with the M_maps package71.
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We thank J. Booth, J. Rolfe and J. Nicolson for their laboratory assistance and the International Ocean Discovery Program core repository for providing samples. This work benefited from conversations with L. Haynes and M. Lyle. We acknowledge high-performance computing support from Cheyenne (https://doi.org/10.5065/D6RX99HX) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the NSF. This work was supported by Natural Environment Research Council (NE/N015045/1; H.L.F.), the National Science Foundation (OCE-1844380 and OCE-2002448; N.J.B.) and a Sloan Ocean Fellowship (N.J.B.). H.L.F. personally thanks the breast cancer oncology, chemotherapy, radiotherapy and surgical team at St Bartholomew’s Hospital.
The authors declare no competing interests.
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Benthic isotope records of Prob-stack (A45), Site 1209 δ18O (B, blue circles) and δ13C (C, blue squares), and Site 1210 δ18O (D, orange circles) and δ13C (E, orange squares). The grey shading indicates the mid-Piacenzian warm period. Northern Hemisphere Glaciation (NHG) and Marine Isotope Stages KM3 and M2 are labelled.
Sea surface temperature (SST) anomaly maps for the mid-Pliocene Warm Period (mid-Piacenzian 3.264–3.025 Ma) and Early Pliocene (4–5 Ma).
Observed benthic δ13C and modelled zonal-mean (shaded contours in ‰) anomalies for the Early Pliocene (4–5 Ma). The benthic δ13C anomaly (‰) is calculated from the OC3 database20 with Late Holocene core top (diamonds) and modern δ13C ocean water (squares) values. Large symbols for open ocean sites are outlined in black, while symbols for marginal sites possibly influenced by boundary effects are small and outlined in grey. Horizontal basin-wide δ13C anomalies for 1000–1500 m (A), 2200–2800 m (B), and 3200–4000 m (C) water depth. The largest anomaly is found in the west due to western intensification of deep ocean circulation. The vertical cross section across the Pacific (D) shows the core and spatial extent of the PMOC.
Meridional overturning stream function in the Indo-Pacific ocean (in Sverdup, Sv) for the Pre-industrial (A), mid-Pliocene-like (mid-Piacenzian) (B), and Early Pliocene (C) model simulations. Contour interval is 2 Sv. Note the absence of active overturning cells in the preindustrial simulation other than the upper-ocean wind-driven cells.
Meridional ideal age in the Indo-Pacific ocean for the mid-Pliocene-like (mid-Piacenzian) (A), and Early Pliocene (B) model simulations minus Pre-industrial. Contour interval is 100 ideal years.
Pacific basin modelled DIC anomaly from the Pre-Industrial to the mid-Pliocene-like simulation.
Pacific basin modelled zonal mean DIC anomaly from the Pre-Industrial to the mid-Pliocene-like simulation.
Time series of modelled DIC anomaly from the Pre-Industrial to the mid-Pliocene-like simulation. Deep ocean is near equilibrium at ~2000 model run years.
Site information including site name, longitude, latitude and bottom water depth (m). OC3 data including pre-industrial δ13C natural ocean water column bottle data and Late Holocene δ13C from the benthic foraminifera genus Cibicides with the relevant search parameters for each site. Early Pliocene (4–5 Ma) δ13C values, standard deviation and number of data points within the interval. Calculated anomaly (Early Pliocene minus pre-industrial/Late Holocene). References used in the compilation
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Ford, H.L., Burls, N.J., Jacobs, P. et al. Sustained mid-Pliocene warmth led to deep water formation in the North Pacific. Nat. Geosci. 15, 658–663 (2022). https://doi.org/10.1038/s41561-022-00978-3