Evolution of the eastern equatorial Pacific oxygen minimum zone during the Miocene Climatic Optimum and Mid-Miocene Climatic Transition: Insights from I/Ca of planktonic foraminifera

Abstract:

The modern oxygen minimum zone (OMZ) in the eastern equatorial Pacific is among the largest and strongest in the world, reaching oxygen concentrations <1μmol/kg. As an area that vents large amounts of CO₂ and N₂ to the atmosphere, its persistence through geologic time has implications for global nutrient and carbon cycling. This study provides the first direct evidence for oxygenation changes in the OMZ beyond the Quaternary, during the climatically and tectonically complex Middle Miocene. This time records a climatic shift from the warm Miocene climatic optimum (17-14.8 Ma) to cooling during the Mid-Miocene climatic transition (MMCT; 14.8-12.8 Ma). Tectonically, the early stages of closure for two key equatorial gateways in the eastern and western equatorial Pacific, the Central American and Indonesian Seaways, respectively, had begun to affect ocean circulation. It remains unknown whether this would have allowed for a strong OMZ to develop in the eastern equatorial Pacific during the Middle Miocene. To determine the relative strength of the OMZ during the Middle Miocene, I/Ca values were measured in the calcitic tests of planktonic foraminifera from Miocene sediments from ODP Site 845, situated in the Guatemalan Basin (3704 m water depth) and underlying the OMZ. The I/Ca proxy relies on the fact that under hypoxic conditions iodate, the most abundant iodine species in oxic waters, is reduced to iodide. Because carbonate material incorporates iodate but not iodide, the tests of foraminifera living above the low-oxygen zone should record low I/Ca values. Results from Site 845 show high I/Ca values during the MCO (~16.1-14.9 Ma) and low I/Ca values during the MMCT (~14.9-11.6 Ma) suggesting that the OMZ was stronger during the MMCT than during the MCO. This is consistent with previously documented increased productivity and upwelling associated with this cooler, more glaciated time. High I/Ca suggest a relatively well ventilated MCO OMZ despite the warmer temperatures and high global organic carbon extraction associated with the Monterey event ~16.7-13.9 Ma. This implicates intermediate-water sources and local upwelling-induced productivity in the OMZ as the primary controls on the Miocene strength of the OMZ.