Hundreds of millions of years ago, Earth first forests played a pivotal role in a global environmental transformation, permanently oxygenating the deep oceans and sparking a revolution in marine life. A new study by researchers at Duke University and the University of Washington reveals that this “oxygen flood” enabled the expansion, diversification, and growth of large, jawed fish and other marine animals, fundamentally reshaping ocean ecosystems.
This research, published in the journal Proceedings of the National Academy of Sciences, pinpoints a lasting oxygen boost in the deep seas during the Middle Devonian period, approximately 393-382 million years ago. This event coincided with a remarkable burst of diversity in jawed fish, the ancestors of nearly all modern vertebrates.
Unlocking the Past with Selenium Isotopes
To track ancient oxygen levels, the researchers analyzed sedimentary rocks from around the world that formed on the deep seafloor. They focused on selenium isotopes, which serve as a reliable proxy for oxygen. In oxygen-rich marine environments, the ratio of heavy to light selenium isotopes varies widely. In oxygen-poor conditions, this ratio is much more consistent.
The team’s analysis of 97 rock samples, dating back over 250 million years, revealed a stepwise pattern of ocean oxygenation. While a brief, transient oxygenation event occurred about 540 million years ago, it was followed by a period where oxygen levels dropped. The permanent and lasting oxygen boost began during the Middle Devonian, an event the researchers directly link to the widespread emergence of woody plants on land. As these early forests flourished, they released vast amounts of oxygen into the atmosphere, which then dissolved into the deep ocean.
A Warning for Today
The study’s findings have important implications for modern-day oceans. Today’s oceans are facing a new threat from deoxygenation, driven largely by nutrient runoff from fertilizers and industrial activity. This process fuels massive algal blooms that consume oxygen when they decay, creating “dead zones.”
As lead author Michael Kipp noted, “This work shows very clearly the link between oxygen and animal life in the ocean. This was a balance struck about 400 million years ago, and it would be a shame to disrupt it today in a matter of decades.” The research serves as a stark warning that human activity could unravel an ecological balance that took millions of years to establish.