At a Glance
- New research confirms that the deep ocean did not become permanently oxygenated until the Middle Devonian, which is significantly later than some previous estimates had suggested.
- This dramatic rise in marine oxygen levels was likely driven by the global spread of woody vascular plants, which sequestered carbon and enriched the atmosphere with oxygen.
- The timing of this event directly coincides with the “mid-Paleozoic marine revolution,” a major evolutionary radiation of large, active jawed fish and other complex marine animals.
- Scientists also identified an earlier, temporary pulse of deep-ocean oxygen around 540 million years ago, which was followed by a long period of inhospitable anoxic conditions.
- These findings underscore the critical link between environmental oxygen and animal evolution, providing historical context for modern concerns about human-caused ocean deoxygenation and its impact on marine ecosystems.
New research indicates that around 390 million years ago, the rise of Earth’s first forests on land triggered a permanent increase in deep-ocean oxygen, enabling a major evolutionary leap for marine animals. This influx of oxygen, which has persisted to the present day, allowed fish and other complex organisms to colonize deep-water habitats for the first time, sparking a period of rapid diversification. The study, which helps settle a long-standing debate on the timing of ocean oxygenation, was published in the journal Proceedings of the National Academy of Sciences.
To determine when this crucial environmental shift occurred, scientists analyzed the geochemistry of 97 rock samples from five continents that were once part of ancient continental shelves. The team focused on selenium, an element whose chemical signature changes based on the presence of oxygen. In well-oxygenated water, the ratio of heavy to light forms of selenium, known as isotopes, varies significantly. By measuring these isotope ratios in pulverized rock, researchers could reconstruct the oxygen levels of the ancient deep ocean.
The data revealed two distinct oxygenation events. The first was a temporary spike around 540 million years ago, followed by a long return to low-oxygen conditions that were inhospitable to most animal life. The second, and more significant, event began in the Middle Devonian period between 393 and 382 million years ago, establishing the permanently oxygenated deep ocean we know today. This timing directly overlaps with the global spread of woody vascular plants, which researchers believe drove the change by sequestering carbon and releasing vast amounts of oxygen into the atmosphere.
This permanent oxygen increase provides a powerful explanation for the “mid-Paleozoic marine revolution,” a period marked by an explosion in the diversity and size of marine life, particularly among jawed fish—the ancestors of most modern vertebrates. The study highlights the profound connection between oxygen and animal evolution, issuing a stark warning for the present day. As human activities create expanding low-oxygen “dead zones” in modern oceans, this research highlights how quickly a balance struck nearly 400 million years ago can be disrupted.
References
- Bubphamanee, K., Kipp, M. A., Meixnerová, J., Stüeken, E. E., Ivany, L. C., Bartholomew, A. J., Algeo, T. J., Brocks, J. J., Dahl, T. W., Kinsley, J., Tissot, F. L. H., & Buick, R. (2025). Mid-Devonian ocean oxygenation enabled the expansion of animals into deeper-water habitats. Proceedings of the National Academy of Sciences, 122(35), e2501342122. https://doi.org/10.1073/pnas.2501342122
- Duke University. (2025, August 25). Rising deep-ocean oxygen levels likely opened up new marine habitats and spurred speciation. Phys.Org; Duke University. https://phys.org/news/2025-08-deep-ocean-oxygen-marine-habitats.html
