Simulated Ocean Oxygen Under Miocene Boundary Conditions

Abstract

The Miocene Climatic Optimum (MCO; 16.9–14.7 Ma) was a warm interval with atmospheric comparable to end‐of‐21st‐century projections under intermediate emission scenarios. Proxy records indicate the Oxygen Minimum Zone (OMZ) in the Eastern Tropical Pacific (ETP) was small or non‐existent during the MCO, expanding only when declined after 15 Ma. Conversely, the modern ETP OMZ is expanding under anthropogenic warming. The mechanisms behind these opposing trends remain unclear. Here, we use the Geophysical Fluid Dynamics Laboratory coupled model (GFDL CM2.1) with the offline biogeochemistry PCO2 model to simulate Miocene ocean oxygen across multiple levels and two paleogeographic configurations. Increasing produces widespread oxygen loss at shallow and intermediate depths. One configuration develops pervasive hypoxia throughout the water column in the Atlantic and Arctic, driven by stagnation and stratification associated with a collapsed Atlantic Meridional Overturning Circulation (AMOC). In the same configuration, the ETP OMZ contracts at higher due to weaker trade winds, reduced upwelling, and water mass transport changes through the open Central American Seaway (CAS) associated with the shutdown of North Atlantic Deep Water and Tethys Intermediate Water formation. The same circulation reorganization responsible for Atlantic deoxygenation thus contributes to ETP oxygenation, highlighting the spatial complexity of the marine oxygen response to warming. These results indicate Miocene ETP oxygen dynamics are not a perfect analog to future changes: the boundary conditions permitting ETP OMZ contraction (including an open CAS and collapsed AMOC) are absent from the modern ocean, limiting direct extrapolation from proxy evidence.