Understanding lesser-known mass extinctions during the Age of Dinosaurs

Non-avian dinosaurs were wiped out by an asteroid which slammed into Earth 66 million years ago – which saw about 75% of plant and animal life wiped out – but other mass extinctions occurred during the “Age of Dinosaurs.”

Scientists are now uncovering how a “tag-team” of oceans and continents led to the devastation of marine life between 185 and 85 million years ago.

Research published in the journal Nature Geoscience explains a series of environmental catastrophes called oceanic anoxic events.

During these periods, oceans have much less dissolved oxygen, creating toxic waters.

These events caused mass marine extinctions at the time when dinosaurs ruled the land and altered the course of evolution on Earth.

“Oceanic anoxic events were like hitting the reset button on the planet’s ecosystems,” says lead author Tom Gernon from the UK’s University of Southampton. “The challenge was understanding which geological forces hit the button.”

Gernon’s team of international collaborators combined statistical analysis and computer models to measure how chemical cycles in Earth’s ancient oceans changed over millions of years.

They tracked how the ocean’s chemistry responded to the breakup of the supercontinent Gondwana which separated into the Africa, Australia, Antarctica, South America, Zealandia and the Indian subcontinent.

This breaking up of the massive landmass began during the Jurassic period (201–145 million years ago) and was completed in the Cretaceous (145–66 million years ago). The Jurassic and Cretaceous are the 2 latter of the 3 geological periods that make up the Mesozoic era, also known as the “Age of Dinosaurs”.

“The Mesozoic era witnessed the breakup of this landmass, in turn bringing intense volcanic activity worldwide,” Gernon explains. “As tectonic plates shifted and new seafloors formed, large amounts of phosphorus, a nutrient essential for life, were released from weathering volcanic rocks into the oceans.”

“Crucially, we found evidence of multiple pulses of chemical weathering on both the seafloor and continents, which alternately disrupted the oceans,” Gernon adds.

“It’s like a geological ‘tag-team’.”

The team found these pulses matched up with most of the oceanic anoxic events. They suggest that the influx of phosphorous into the ocean acted like a natural fertiliser, boosting the growth of marine organisms.

That sounds like a boon for marine life. But the authors of the new research say that it came at a cost for ocean ecosystems.

Increased biological activity led to huge amounts of organic matter sinking to the ocean floor, consuming large amounts of oxygen.

“This process eventually caused swathes of the oceans to become anoxic, or oxygen-depleted, creating ‘dead zones’ where most marine life perished,” says co-author Benjamin Mills from the University of Leeds also in the UK.

“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.”

Understanding these processes not also helps illuminate what happened millions of years ago but can inform the impact of human activity on vital marine ecosystems.

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