A groundbreaking study has revealed that the Arabian Sea was better oxygenated 16 million years ago than it is today, despite being under warmer climate conditions. This discovery, made by researchers from the University of Southampton and Rutgers University, suggests that the world’s oxygen-depleted seas might regain higher oxygen levels in the future, even as global temperatures continue to rise.
The study, published in the journal Communications Earth & Environment, examined fossilized plankton from the Arabian Sea. It found that during a period of significant global warming 16 million years ago, oxygen levels were higher than they are today. The sea only became severely oxygen-deficient four million years later, as the climate cooled. This finding challenges current assumptions about the relationship between climate change and ocean oxygenation.
Complex Interactions in Ocean Systems
The research highlights the complex interactions between monsoons, ocean circulation, and ocean gateways, which play a crucial role in ocean oxygenation. The Arabian Sea, located off the west coast of India, showed different behavior compared to a similar low-oxygen area in the Pacific. This suggests that local systems, such as strong winds, ocean currents, and outflows from marginal seas, may have delayed the onset of oxygen deficiency.
According to Dr. Alexandra Auderset, co-lead author of the study, “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.”
“The Miocene Climatic Optimum (MCO), a period approximately 17 to 14 million years ago, had similar temperatures and atmospheric conditions to those we predict will occur after 2100. We have taken a snapshot of sea oxygenation during the MCO to help understand how things might develop a hundred years or more from now,” Dr. Auderset explained.
Insights from Fossilized Plankton
The scientists examined tiny fossilized plankton called foraminifera (forams) extracted from core samples provided by the Ocean Drilling Program (ODP). These remains hold vital chemical information that can indicate oxygen concentrations in seawater over millions of years. Their research found that an Oxygen Minimum Zone (OMZ) existed in the Arabian Sea, with oxygen levels below about 100 micromol per kilogram of water, from the early Miocene (19 million years ago) to around 12 million years ago.
Dr. Anya Hess, co-lead author, noted that “The MCO is the closest comparison we have to climate warming beyond 2100 under a high-emissions scenario. One of our previous studies shows the eastern tropical Pacific was actually well oxygenated during this period, in contrast to the deoxygenation trend we see today.”
“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,” Dr. Hess added.
Implications for Future Ocean Conditions
These findings underscore the importance of considering regional oceanographic factors when predicting future ocean conditions. Dr. Auderset emphasized that “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.”
The study’s implications are profound. As the world grapples with the effects of climate change, understanding the complex interplay of factors affecting ocean oxygenation could inform strategies to mitigate its impact on marine ecosystems. The research suggests that while global warming poses a significant threat, there may be opportunities to adapt to changing ocean conditions.
As scientists continue to explore the ancient past for clues about our future, the hope is that these insights will guide efforts to preserve marine biodiversity and support resilient ecosystems in an era of unprecedented environmental change.
