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A study published in Nature Chemistry It explains how the breakup of the supercontinent Gondwana and subsequent volcanic activity led to anoxic conditions in the oceans between 185 and 85 million years ago. This event disrupted ocean chemistry, created oxygen-depleted “dead zones”, led to severe marine extinctions and changed Earth's evolutionary history.
Key Takeaways
Scientists have revealed how a “tag team” between oceans and continents devastated marine life millions of years ago and changed the course of evolution on Earth.
Their study reveals a new explanation for a series of severe environmental crises known as marine anoxia, which occurred between 185 million and 85 million years ago.
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These occurred when there was a severe reduction in dissolved oxygen in the ocean.
Experts from the University of Southampton, who led the study, said these events caused major biological upheaval, including mass extinctions of marine species.
The research results are: Nature Chemistry .
“Ocean anoxia was like hitting the reset button on Earth's ecosystems,” said lead author Tom Gernon, professor of geosciences at the University of Southampton.
“The challenge was to understand what geological forces pushed the button.”
The research was carried out by the University of Southampton in collaboration with researchers from the Universities of Leeds and Bristol in the UK, the University of Adelaide in Australia, Utrecht University in the Netherlands, the University of Waterloo in Canada and Yale University in the US.
The researchers investigated the influence of plate tectonic forces on ocean chemistry during the Jurassic and Cretaceous periods, collectively known as the Mesozoic Era.
This period in Earth's history is often referred to as the Age of the Dinosaurs and is well exposed along the cliffs of the Jurassic Coast on the south coast of England, Whitby in Yorkshire and Eastbourne in East Sussex, Prof Gernon said.
The team combined statistical analysis and sophisticated computer modelling to investigate how the ocean's chemical cycle may have responded to the breakup of the supercontinent Gondwana, the huge landmass once home to dinosaurs.
Prof Gernon added: “During the Mesozoic era, the continents broke apart and intense volcanic activity occurred around the world.”
“As the plates shifted and new ocean floor was formed, large amounts of phosphorus, a nutrient essential to life, were released into the ocean from weathered volcanic rocks.
“Importantly, we find evidence of multiple events of chemical weathering, both on the seafloor and on the continents, that alternately perturbed the oceans.
“It's like a geological tag team,” Prof Gernon said.
University experts found that the timing of these weathering waves coincides with most of the oceanic anoxic events recorded in the rock record.
They argue that weathering-related phosphorus entering the oceans acted as a natural fertilizer, promoting the growth of marine life.
But the researchers said the fertilization events took a toll on marine ecosystems.
Increased biological activity caused large amounts of organic matter to sink 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 adds: “This process ultimately left large swaths of the ocean anoxic, or oxygen-depleted, zones where most marine life died.”
“The anoxic conditions typically lasted for one to two million years and had profound effects on marine ecosystems, the effects of which are still being felt today.”
“The organic-rich rocks that accumulated during these events are the source of the world's largest commercial oil and gas reserves to date.”
The findings explain the causes of extreme biological chaos during the Mesozoic Era and highlight the devastating effects of nutrient overload on marine environments today.
The team explained that modern human activities have reduced the average oxygen level in the oceans by about 2%, significantly expanding the areas of oxygen-free water.
Professor Gernon added: “Studying geological events provides valuable insights that help us understand how Earth will respond to future climatic and environmental stresses.”
Overall, the team's findings reveal stronger-than-expected connections between Earth's solid interior and its surface environment and biosphere, especially during periods of tectonic and climatic upheaval.
“It is remarkable how a series of events occurring inside the Earth can have such profound, often devastating, effects on the surface,” Prof Gernon added.
“Continental breakup could have profound effects on evolutionary processes.”
reference: Gernon TM, Mills BJW, Hincks TK, et al. “Forcing of Mesozoic marine anoxic events by the solid Earth.” Nature Chemistry2024.doi: 10.1038/s41561-024-01496-0
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