Forests on land kicked off life in the deep sea
- Forests that evolved on land around 400 million years ago played a crucial role in sparking life in the deep sea, according to new research.
- The study found that the deep ocean didn’t become permanently oxygenated until about 390 million years ago, much later than previously thought, and this change enabled larger and more active animals to colonize the deep sea.
- The transformation in deep-sea oxygenation was driven by the appearance of forests on land, which released carbon dioxide into the atmosphere, leading to an increase in atmospheric oxygen levels that eventually penetrated the ocean’s depths.
- The new study provides independent evidence for the timing and extent of oceanic oxygenation, using a novel proxy based on selenium isotopes recorded in sedimentary rocks from around the world.
- The research highlights the importance of oxygen in allowing for biodiversity, particularly among animals, and warns that human activities such as fertilizer runoff are depleting oxygen in many coastal waters today, leading to habitat and biodiversity loss.
New research digs into how forests sparked deep-sea life.
About 400 million years ago, the deeper part of the ocean was a difficult place to live. With low oxygen, it was nearly a biological dead zone—life was there, but only existing in the slow lane. However, over time it changed, becoming an environment rich with biodiversity. Today, whales, sharks, invertebrates and fish thrive there.
The new study in Proceedings of the National Academy of Sciences identifies when and how this shift from a deep-sea dead zone happened.
“This study reinforces the importance of oxygen to the radiation of life on this planet, particularly animal life,” says Linda Ivany, a coauthor and professor in the earth and environmental sciences department at Syracuse University.
“Over the past 15 years, we have seen more studies that connect the diversification of animals and their expansion into new habitats with the availability of oxygen in their settings.”
The researchers found that the deep ocean didn’t become permanently oxygenated until about 390 million years ago, much later than once thought. With more oxygen available for respiration, more and larger animals colonized the deep sea. Animals could be more energetic in places that were previously uninhabitable.
An expanded habitat for animals in the deep ocean meant evolutionary competition.
“Animals evolved different strategies to survive, which led to new species,” says lead author Kunmanee “Mac” Bubphamanee, a University of Washington doctoral student in Earth and space sciences.
“This window of diversification includes larger and more active animals, more predators, and more creative strategies to avoid becoming prey—it’s known as the mid-Paleozoic Marine Revolution,” says Ivany.
What drove this transformation in deep-sea oxygen and biodiversity? It was the appearance of the first forests on land.
Forests evolved around 400 million years ago. Woody plants soaked up carbon dioxide from the atmosphere, locking carbon in trunks, roots and sediments and leaving leftover oxygen to accumulate in the atmosphere. When trees died and decayed, they left behind nutrients in sediments that washed into waterways and the ocean. Over time, surplus oxygen and nutrients mixed in the sea and penetrated its deep waters. Interestingly, New York preserves some of the very oldest fossil forests ever described, and they come from just around the time that oxygen is rising in the oceans.
Scientists once thought that the oceans became permanently oxygenated more than 500 million years ago when the first animals appeared in the fossil record.
The new study, though, shows that ocean oxygenation happened in stages. Shallow waters near the shore were the first to be oxygenated, creating livable zones for early animals. Finally, during the Middle Devonian period—393 to 382 million years ago—enough oxygen had accumulated in the atmosphere and dissolved into seawater to enable the deep ocean to become permanently oxygen-rich.
Animals could get bigger because there was enough oxygen to consistently supply their metabolisms. And predators could now thrive in deeper waters for the same reason, making hunting and chasing possible. This set the stage for evolutionary bursts, including the rapid expansion of jawed vertebrates, or gnathostomes.
Over the past 15 or so years, several research teams have deployed geochemical tools such as iodine and molybdenum to reconstruct the ocean’s oxygen history. Each has suggested an increase in oxygen about this time, but none, alone, were without controversy. The new study adds yet another independent line of evidence, all pointing in the same direction, lending strong support to the contention that oceanic oxygen rose at this time.
The new study focused on a new proxy for measuring past oxygen—selenium, an element with several isotopes of distinct mass. Selenium isotopes are recorded in sediments in different proportions, depending on oxygen levels in the sea.
The team analyzed 97 sedimentary rock samples from five continents, dating from 252 to 541 million years ago. Ivany gathered some rock samples in New York State, which was often under the sea during this period. The researchers pulverized the rocks and measured selenium isotopes, drawing on sophisticated advances in isotopic geochemistry.
“The proportion of isotopes changed pretty dramatically about 390 million years ago,” indicating a rise in oxygenation to levels more like today, says Ivany, and this time it persisted. Earlier intervals of higher oxygen correlate to times when animals first appear and begin to diversify, but oxygenation was not high enough or did not last long enough to allow for animals to fully expand into new habitats and ecologies.
The timing of deep-ocean oxygenation explains why life diversified when it did. But this study isn’t just about solving a puzzle from Earth’s past.
Human activities—such as fertilizer runoff fueling massive plankton blooms—are depleting oxygen in many coastal waters today. Zones of very low oxygen, or “dead zones,” are expanding in ocean regions, driving habitat and biodiversity loss.
“This research drives home the importance of oxygen in allowing for biodiversity, of animals in particular,” says Ivany.
Source: Syracuse University
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