Sustained mid-Pliocene warmth led to deep water formation in the North Pacific

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 in comparison to today, which implies water flowed from north to south and deep-water formation likely 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 (DIC) content in the North Pacific decreases slightly,  though the amount of carbon stored in the ocean actually increases by 1.6% relative to modern due to an increase in DIC 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.