A series of intense environmental events 100 million years ago caused widespread extinction of marine species. Scientists have discovered how a “tag team” between oceans and continents millions of years ago wreaked havoc on marine life and dramatically altered the course of evolution on Earth.

Their research found a new explanation for a series of major environmental crises called ocean anoxic events that occurred between 185 and 85 million years ago, caused by a sudden depletion of dissolved oxygen in the seas.

Experts from the University of Southampton, who conducted the study, said these events triggered significant biological shocks, including mass extinctions of marine species. The findings were recently published in a journal Natural Geology.

Lead author Tom Gernon, Professor of Earth Sciences at Southampton, said: “Anoxic events in the ocean were like pressing the reset button on the planet’s ecosystems. The challenge was to understand which geological forces were pressing the button.”

The research was carried out by Southampton in collaboration with scientists from Leeds, Bristol in the UK, Adelaide in Australia, Utrecht in the Netherlands, Waterloo in Canada and Yale University in the US.

Geological forces behind marine destruction

The researchers examined the effects of plate tectonic forces on ocean chemistry during the Jurassic and Cretaceous periods, collectively known as the Mesozoic Era. Professor Gernon said this part of Earth’s history is often referred to as the age of dinosaurs and is known along the Jurassic Coast on the south coast of Britain, as well as Whitby Cliffs in Yorkshire and Eastbourne in East Sussex.

The team combined statistical analysis and advanced computer models to investigate how chemical cycles in the ocean might have actually responded to the breakup of the supercontinent Gondwana, a large landmass once inhabited by dinosaurs.

Professor Gernon added: “The Mesozoic era saw the break-up of this land mass, resulting in intense volcanic activity around the world. As tectonic plates shifted and new seafloor was formed, large amounts of phosphorus, an essential nutrient for life, were released into the oceans from the weathered volcanic rocks. Importantly, we found evidence of a great deal of chemical weathering, both on the seafloor and on the continents that were successively destroyed by the oceans.”

“It’s like a bunch of geological tags,” said Professor Gernon.

The university experts found that the timing of these weathering pulses overlaps with most oceanic anoxic events in rocks.

Impact on marine life and ecosystems

They suggest that phosphorus runoff into the ocean due to weathering acts as a natural fertiliser, speeding the growth of marine organisms. But the researchers said these fertilisation events were damaging to marine ecosystems.

Co-author Benjamin Mills, professor of Earth system evolution at the University of Leeds, said the increase in biological activity caused large amounts of organic matter to sink to the ocean floor, where it consumed large amounts of oxygen.

He added: “This process eventually led to parts of the oceans becoming anoxic, or depleted of oxygen, creating ‘dead zones’ where most marine life died. Anoxic events typically lasted one to two million years and had profound effects on marine ecosystems, the effects of which are still felt today. The organic-rich rocks deposited during these events are by far the largest source of commercial oil and gas reserves worldwide.”

Insights into modern environmental issues

In addition to explaining the cause of extreme biological upheavals in the Mesozoic, the study’s findings also highlight the devastating effects of nutrient overload on today’s marine environments.

The research team explained how modern human activities have reduced the average oxygen level in the ocean by approximately two percent, leading to the significant expansion of anoxic water bodies.

Professor Gernon added: “Studying geological events provides valuable information that can help us understand how the Earth might respond to future climate and environmental stresses.”

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

“It’s amazing how a chain of events on Earth can affect the surface, often with catastrophic consequences,” added Professor Gernon. “The breakup of continents could have serious consequences for evolution.”