Tracing Ocean Oxygen Dynamics Through Time: A Miocene Perspective

Foraminifera-bound nitrogen isotopes (FB-δ¹⁵N) are a powerful tool for reconstructing past oxygen-deficient zones (ODZs). FB-δ¹⁵N record the strong isotopic fractionation associated with bacterial water column denitrification that occurs in oxygen-deficient environments, typically characterised by dissolved oxygen concentrations of less than ~5 µM. We applied this oxygen-sensitive proxy across multiple ocean basins during the Miocene, focusing on the Miocene Climatic Optimum (MCO) and Middle Miocene Climate Transition (MMCT), to study the expansions and contractions of tropical ODZs as a response to past global climate change.

Our multi-basin analysis indicates nuanced oxygen dynamics, including evidence of a persistent proto-ODZ in the Arabian Sea since ≥19.8 Ma. By integrating FB-δ¹⁵N with foraminiferal calcite trace element data (I/Ca, Mn/Ca), we generated the first temporal and spatial record of MMCT deoxygenation in the Arabian Sea. Combining these new data with regional palaeoceanographic proxies, we assess the roles of global climate, regional monsoonal activity, and tectonics in driving Arabian Sea hypoxia, recognising that the contributions of these factors varied in magnitude and timing.

In comparison, new preliminary data from the Atlantic and Pacific Oceans suggest synchronised yet regionally distinct ODZ responses during the MCO and subsequent cooling. Our high-resolution reconstructions of Pacific Ocean deoxygenation following the MMCT cooling indicate glacial/interglacial variations and provide critical new insights into potential marine oxygen deficient zone trajectories under future climate scenarios.