Scientists have revealed how a ‘tag team’ between the oceans and continents devastated marine life millions of years ago – and changed the course of evolution on Earth.
Their research has provided a new explanation for a series of serious environmental crises, the so-called oceanic anoxic events, that occurred between 185 and 85 million years ago.
These occurred when the seas became severely depleted of dissolved oxygen.
Experts from the University of Southampton, who led the research, said these events triggered significant biological upheavals, including mass extinctions of marine species.
The findings will be published today, Thursday, August 29 Natural Geosciences.
Lead author Tom Gernon, professor of earth sciences at Southampton, said: “Oceanic anoxic events were like pressing the reset button on the planet’s ecosystems.
“The challenge was to understand which geological forces were pushing the button.”
The research 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 tectonics on ocean chemistry during the Jurassic and Cretaceous periods, collectively known as the Mesozoic.
This chapter of Earth’s history is often referred to as the Age of Dinosaurs, said Prof Gernon, and is famously visible along the Jurassic Coast on Britain’s south coast, as well as along the cliffs of Whitby in Yorkshire and Eastbourne in East Sussex.
The team combined statistical analyzes and advanced computer modeling to explore how chemical cycles in the ocean could feasibly have responded to the breakup of the supercontinent Gondwana, the large landmass where dinosaurs once roamed.
Professor Gernon added: “The Mesozoic era witnessed the breakup of this landmass, which in turn brought intense volcanic activity worldwide.
‘As tectonic plates shifted and new seafloors formed, large amounts of phosphorus, a nutrient essential for life, were released from weathering of volcanic rocks in the oceans.
“Crucially, we found evidence of multiple pulses of chemical weathering on both the seafloor and the continents, alternately disrupting the oceans.
“It’s like a geological tag team,” said Prof. Gernon.
Experts from the universities found that the timing of these weathering pulses matched most oceanic anoxic events in the rock record.
They argue that the weathering-induced influx of phosphorus into the ocean has acted as a natural fertilizer, stimulating the growth of marine organisms.
However, the researchers said these fertilization episodes came at a great cost to marine ecosystems.
The increase in biological activity led to huge amounts of organic material sinking to the ocean floor, where it 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 parts of the oceans to become anoxic, or oxygen-depleted, creating ‘dead zones’ where most marine life perished.
‘The anoxic events typically lasted around one to two million years and had profound effects on marine ecosystems, the legacy of which is felt even today.
‘The organic matter-rich rocks that accumulated during these events are by far the largest source of commercial oil and gas reserves worldwide.
The study’s findings not only explain the cause of the extreme biological unrest in the Mesozoic Era, but also highlight the devastating effects that nutrient overload can have on today’s marine environment.
The team of researchers explained how current human activities have reduced the average oxygen level in the oceans by about two percent, leading to a significant increase in anoxic bodies of water.
Professor Gernon added: “Studying geological events provides valuable insights that can help us understand how the Earth may respond to future climate and ecological stresses.”
Overall, the team’s findings show a stronger-than-expected connection between Earth’s solid interior and the surface environment and biosphere, especially during periods of tectonic and climatic upheaval.
“It is remarkable how a series of events within the Earth can affect its surface, often with devastating consequences,” added Prof. Gernon.
“The tearing apart of continents could have profound consequences for the course of evolution.”
