Oxygen in the Arabian Sea: Lessons from the Past for the Future of the Oceans

Oxygen in Ancient Oceans

About 16 million years ago, the Arabian Sea contained more oxygen than it does today, even though Earth's climate was warmer at that time. Research shows that ocean oxygen levels are influenced not only by temperature, but also by factors such as monsoon intensity, changes in ocean currents, and connections between seas. This highlights that the health of the oceans depends on many variables, not just global warming. Over millions of years, ocean oxygen levels can rise again, although the consequences for marine life remain unclear.

Looking to the Future Through the Lens of the Past

Ancient oceans can offer clues about the potential recovery of oxygen in the future. A recent study suggests that regions of the world’s oceans with low oxygen could restore their levels in the coming centuries, even as global temperatures continue to rise. Scientists from the University of Southampton (UK) and Rutgers University (USA) analyzed plankton fossils preserved in Arabian Sea sediments. Their findings revealed that during a period of intense global warming about 16 million years ago, oxygen levels in this region were higher than they are today. Significant oxygen depletion began only about four million years later, when the climate started to cool.

Why the Arabian Sea Was Different from Other Regions

Researchers found that the Arabian Sea, located off the west coast of India, evolved differently from similar low-oxygen regions in the Pacific Ocean. This contrast underscores the importance of regional factors such as strong monsoon winds, unique ocean circulation patterns, and water exchange with neighboring seas. These local conditions appear to have slowed the loss of oxygen in the Arabian Sea.

Modern Trends: Oxygen Loss Continues

Dissolved oxygen in the oceans is essential for sustaining marine life, supporting greater biodiversity and ecosystem stability. However, over the past 50 years, about two percent of oceanic oxygen has been lost every decade worldwide due to rising global temperatures. Dr. Alexandra Oderse from the University of Southampton notes that the Miocene Climatic Optimum (MCO), a period about 17–14 million years ago, had temperatures and atmospheric conditions similar to those predicted after the year 2100. Studying the oxygen regime of the sea during that time helps us understand how future events might unfold.

Plankton Fossils: Unlocking the History of Oxygen

To reconstruct ancient conditions, the research team studied microscopic plankton fossils—specifically foraminifera—collected from cores provided by the Ocean Drilling Program (ODP). Chemical signals preserved in their shells allow scientists to estimate oxygen levels in seawater over millions of years. The analysis showed that the oxygen minimum zone (OMZ) existed in the Arabian Sea from the early Miocene, about 19 million years ago, until around 12 million years ago, with oxygen concentrations remaining below 100 micromoles per kilogram of water.

Delayed Oxygen Depletion

Despite low oxygen levels, conditions were not extreme enough to trigger the release of nitrogen from seawater into the atmosphere—a process observed in the Arabian Sea today. This shift occurred only after 12 million years ago, indicating that the most severe oxygen depletion was delayed.

Today, parts of the Arabian Sea are considered "suboxic"—they support only limited marine life due to minimal oxygen saturation. In contrast, during the MCO, the region had a hypoxic zone—moderate oxygen levels that supported a broader range of organisms.

The Impact of Regional Factors

Dr. Anja Hess from George Mason University adds that the MCO is the closest analogue to the expected climate warming after 2100 under high emissions scenarios. Previous studies show that the eastern tropical Pacific was well-oxygenated during this period, unlike current trends toward deoxygenation. The Arabian Sea was also better oxygenated during the MCO, though not as much as the Pacific, and the decline in oxygen levels occurred with a delay of about two million years.

Why Predicting the Future State of the Oceans Is Difficult

Dr. Oderse concludes that the ongoing loss of oxygen in the oceans is largely determined by local oceanography. Global models that focus only on climate warming risk overlooking regional factors that can either amplify or offset overall trends. The study shows that the ocean’s response to climate warming is complex, and we must be prepared to adapt to changing marine conditions.