Land-sea “tag-team” devastated ocean life millions of years ago

A “tag-team” between the oceans and continents millions of years ago devasted marine life – and altered the course of evolution on Earth, say scientists.

The study by researchers at the University of Leeds and the University of Southampton has unearthed a new explanation for a string of severe environmental crises, called oceanic anoxic events, which happened between 185 and 85 million years ago.

These occurred when the seas became critically depleted of dissolved oxygen.

The events triggered significant biological upheavals, including mass extinctions of marine species.

The events typically had profound impacts on marine ecosystems, the legacy of which are even felt today.

Professor Benjamin Mills, School of Earth and Environment

The findings are published in Nature Geoscience.

Co-author Benjamin Mills, a Professor of Earth System Evolution in the School of Earth and Environment, said an increase in biological activity led to huge amounts of organic matter sinking to the ocean floor, where it consumed large quantities of oxygen.

“This process eventually caused swathes of the oceans to become anoxic, or oxygen-depleted, creating ‘dead zones’ where most marine life perished,” he explained.

“The anoxic events typically lasted around one to two million years and had profound impacts on marine ecosystems, the legacy of which are even felt today.

“The rocks rich in organic matter that accumulated during these events are by far the largest source of commercial oil and gas reserves globally.”

The researchers examined the impact of plate tectonic forces on ocean chemistry during the Jurassic and Cretaceous Periods, part of the Mesozoic era.

They combined statistical analyses and sophisticated computer models to explore how chemical cycles in the ocean could have feasibly responded to the breakup of the supercontinent Gondwana, the great landmass once roamed by the dinosaurs.

The Mesozoic era witnessed the breakup of this landmass, in turn bringing intense volcanic activity worldwide.

As tectonic plates shifted and new seafloors formed, large amounts of phosphorus, a nutrient essential for life, were released from volcanic rocks into the oceans.

Oceanic anoxic events were like hitting the reset button on the planet’s ecosystems.

Professor Tom Gernon, University of Southampton

Crucially, the team found evidence of multiple pulses of chemical release on both the seafloor and continents, which alternately disrupted the oceans – an effect which the researchers have described as being “like a geological tag team.”

Lead author Tom Gernon, a Professor of Earth Science at the University of Southampton, said: “Oceanic anoxic events were like hitting the reset button on the planet’s ecosystems.

“The challenge was understanding which geological forces hit the button.”

Experts from the universities found the timing of these phosphorus pulses matched up with most oceanic anoxic events in the rock record. They then used an ‘Earth System’ computer model, developed at the University of Leeds, to test how ocean chemistry would respond to these phosphorus inputs, and it was able to reproduce anoxic events from the geological past.

They propose that the influx of phosphorus to the ocean acted like a natural fertiliser, boosting the growth of marine organisms.

However, the researchers said these fertilisation episodes came at a major cost for marine ecosystems.

As well as explaining the cause of extreme biological turmoil in the Mesozoic, the study’s findings highlight the devastating effects that nutrient overloading can have on marine environments today.

The team of researchers explained how present-day human activities have reduced mean oceanic oxygen levels by about two per cent - leading to a significant expanse in anoxic water masses.

Prof Gernon added: “Studying geological events offers valuable insights that can help us grasp how the Earth may respond to future climatic and environmental stresses.”

Overall, the team’s findings reveal a stronger-than-expected connection between the Earth’s solid interior and its surface environment and biosphere, especially during periods of tectonic and climatic upheaval.

“It’s remarkable how a chain of events within the Earth can impact the surface, often with devastating effects”, added Professor Gernon.

“Tearing continents apart can have profound repercussions for the course of evolution”.