Scientists have revealed how oceans and continents acted like a ‘relay team’ millions of years ago to devastate marine life and alter the course of evolution.
Their study has unearthed a new explanation for a series of severe environmental crises, called oceanic anoxic events, that occurred between 185 and 85 million years ago. They occurred when the seas became critically short of dissolved oxygen.
Experts from the University of Southampton, which led the study, said these events triggered major biological disruptions, including mass extinctions of marine species. The findings are published in Nature Geoscience.
Lead author Tom Gernon, professor of Earth Sciences at Southampton, said: “Ocean anoxic events were like hitting the reset button on the planet’s ecosystems. The challenge was to understand what geological forces pushed it.”
The study was carried out by Southampton in collaboration with academics from the universities of Leeds, Bristol in the UK, Adelaide in Australia, Utrecht in the Netherlands, Waterloo in Canada and Yale in the US. The researchers examined the impact of plate tectonic forces on ocean chemistry during the Jurassic and Cretaceous periods, collectively known as the Mesozoic era.
The team combined statistical analysis and sophisticated computer models to explore how chemical cycles in the ocean could feasibly have responded to the breakup of the supercontinent Gondwana, the vast landmass once roamed by dinosaurs.
Professor Gernon said: “The Mesozoic era saw the fragmentation of this landmass, which in turn brought with it intense volcanic activity around the world. As tectonic plates shifted and new seabed was formed, large amounts of phosphorus, an essential nutrient for life, were released from the eroded volcanic rocks into the oceans.
AS A GEOLOGICAL SURVEY TEAM
“Crucially, we found evidence of multiple pulses of chemical erosion on both the seafloor and the continents, alternately altering the oceans. “It’s like a geological relay team.”
The university experts found that the timing of these erosion pulses coincided with most oceanic anoxic events recorded in rocks. They propose that the erosion-related influx of phosphorus into the ocean acted as a natural fertilizer, boosting the growth of marine organisms.
However, the researchers said these bouts of fertilisation came at a significant cost to marine ecosystems. Increased biological activity caused huge amounts of organic matter to sink to the ocean floor, where they consumed large amounts of oxygen, said co-author Benjamin Mills, professor of Earth System Evolution at the University of Leeds.
He added: “This process eventually caused swathes of the oceans to become anoxic, or lacking oxygen, creating ‘dead zones’ where much marine life perished.
“Anoxic events typically lasted about one to two million years and had profound impacts on marine ecosystems, the legacy of which is felt even today. The organic-rich rocks that accumulated during these events are by far the largest source of commercial oil and gas reserves globally.”
In addition to explaining the cause of extreme biological upheaval in the Mesozoic, the study’s findings highlight the devastating effects that nutrient overload can have on modern marine environments.
The team of researchers explained how current human activities have reduced average ocean oxygen levels by about two percent, leading to a significant expansion of anoxic water masses.
Professor Gernon said: “Studying geological events provides valuable information that can help us understand how the Earth may respond to future climatic and environmental stresses.”
Overall, the team’s findings reveal a stronger-than-expected connection between Earth’s solid interior and its surface environment and biosphere, especially during periods of tectonic and climatic upheaval.
“It is amazing how a chain of events inside the Earth can affect the surface, often with devastating effects,” added Professor Gernon. “The separation of continents can have profound repercussions on the course of evolution.”
