Simulated Ocean Oxygen under Miocene Boundary Conditions

Investigating changes in ocean oxygenation during past warm climates advances our process understanding of biogeochemical and physical dynamics in the ocean and may inform our predictions of future changes. The Miocene Climatic Optimum (MCO) was a warm climate episode ~15, million years ago (Ma), with high atmospheric CO₂ concentrations that are comparable to end-of-century predictions for mid-range future emission scenarios. Proxy records suggest that the Oxygen Minimum Zone (OMZ) in the Eastern Tropical Pacific (ETP) was small or non-existent during the high-CO₂ MCO, and only expanded when CO₂ declined after 15 Ma. In contrast, the OMZ in the Eastern Pacific was already extensive in the recent preindustrial era, and is currently expanding further with increasing CO₂, due to ocean warming and stratification. Despite the importance of understanding the controls on Pacific OMZ extent under warm conditions, there are no existing model investigations of these opposing OMZ dynamics. Here, we use a climate model with an offline biogeochemical framework to investigate ocean oxygen concentrations during the Miocene for a range of CO₂ concentrations and two different topographic configurations. We compare results to available physical and biogeochemical proxies and assess which combination of boundary conditions best replicates recorded proxy trends. We find that for higher CO₂ concentrations, oxygen declines globally and OMZs expand, particularly in the Atlantic Ocean. However, for one of the topographic configurations, OMZs in the ETP contract under higher CO₂ concentrations. This contraction can be attributed to regionally reduced export production and remineralization rates, which are caused by weaker upwelling due to a southward shifted Hadley cell and correspondingly weaker southern hemisphere trade winds. This atmospheric response is driven by hemispheric asymmetries in warming due to changes in large scale ocean circulation. These results emphasize the complexity and spatial heterogeneity of the marine oxygen response to climate change.