A study reveals that the Arabian Sea had more dissolved oxygen 16 million years ago than today, despite global temperatures being warmer during the Miocene Climatic Optimum. This challenges simple assumptions about warming leading to immediate ocean deoxygenation. Regional factors like monsoons and currents delayed severe oxygen loss in the area.
Scientists from the University of Southampton in the UK and Rutgers University in the USA examined fossilized plankton, known as foraminifera, from sediment cores in the Arabian Sea. These samples, collected through the Ocean Drilling Program, preserve chemical signals that indicate past oxygen levels in seawater.
The research focuses on the Miocene Climatic Optimum, a period from about 17 to 14 million years ago when Earth's climate resembled projections for after 2100 under high-emissions scenarios. During this time, the Arabian Sea's oxygen concentrations exceeded today's levels, even as the planet warmed intensely. An oxygen minimum zone existed in the region from around 19 million years ago until about 12 million years ago, with levels staying below 100 micromoles per kilogram of water. However, conditions remained hypoxic—supporting a broader range of marine life—rather than the suboxic state seen now, which limits biodiversity.
Severe oxygen depletion, including the release of nitrogen into the atmosphere, only occurred after 12 million years ago, following a period of climate cooling. This timeline differed from the eastern tropical Pacific, where oxygenation was higher during the MCO but declined earlier.
"Oxygen dissolved in our oceans is essential for sustaining marine life, promoting greater biodiversity and stronger ecosystems. However, over the past 50 years, two percent of oxygen in the seas worldwide has been lost each decade as global temperatures rise," said co-lead author Dr. Alexandra Auderset, now at the University of Southampton and formerly at the Max Planck Institute of Chemistry in Mainz.
Regional influences, such as powerful monsoon winds, shifting ocean currents, and water exchanges with adjacent seas, played key roles in maintaining oxygen. Co-lead author Dr. Anya Hess, formerly of Rutgers University and Woods Hole Oceanographic Institution, noted: "The Arabian Sea was also better oxygenated during the MCO, but not as much as the Pacific, with moderate oxygenation and an eventual decline that lagged behind the Pacific by about 2 million years."
The findings, published in Communications Earth & Environment, highlight that ocean oxygen dynamics depend on more than just temperature. Dr. Auderset added: "Our results suggest that ocean oxygen loss, already underway today, is strongly shaped by local oceanography. Global models that focus solely on climate warming risk not capturing the regional factors that may either amplify or counteract those more general trends."
This complexity implies that future ocean conditions could vary regionally, potentially allowing some areas to recover oxygen over long timescales, though the impacts on marine ecosystems remain unclear.