What caused marine mass extinctions millions of years ago?

Once upon a time, a destructive “tag-team” of natural forces triggered catastrophic mass marine extinctions that reshaped the course of evolution on Earth, paving the way for new forms of life.

This powerful duo wasn’t made up of marauding warriors or mythical monsters but rather the elemental forces of nature we still encounter today: land and sea.

Acting in deadly harmony, these powerful elements transformed marine life, disrupted entire ecosystems, and altered the planet’s history forever.

Mass extinction events of marine species

In a recent study, researchers led by the University of Southampton present startling new explanations for these drastic events.

The scientists have shed new light on an age-old enigma: the sequence of traumatic environmental crises known as oceanic anoxic events that occurred between 185 and 85 million years ago.

During these events, the oceans were dangerously low on dissolved oxygen, throwing marine ecosystems into turmoil and leading to mass extinctions of marine species.

“Oceanic anoxic events (OAEs) are transient perturbations to the global carbon cycle, during which large regions of the oceans are depleted in dissolved oxygen,” noted the researchers.

“The resulting euxinic (anoxic and sulfidic) waters are highly toxic, leading to biological turnover events and, in extreme cases, mass extinctions of marine biota.”

Triggers for the extinction events

“Oceanic anoxic events were like hitting the reset button on the planet’s ecosystems. The challenge was understanding which geological forces hit the button,” said study lead author Professor Tom Gernon.

The study was an international effort, bringing together minds from universities in Australia, Canada, the Netherlands, and the United States.

The researchers set out to investigate how ocean chemistry was altered by the impact of tectonic shifts during the Mesozoic era, often known as the age of the dinosaurs.

The Mesozoic era marked the disintegration of Gondwana, a once vast landmass that dinosaurs claimed as their dominion.

“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 weathering volcanic rocks into the oceans,” explained Professor Gernon.

Land, sea, and mass marine extinctions

According to Professor Gernon, the team found evidence of multiple pulses of chemical weathering on both the seafloor and continents, which alternately disrupted the oceans. “It’s like a geological tag-team,” he added.

The timing of these weathering pulses eerily lined up with most oceanic anoxic events.

The researchers propose that the weathering-triggered influx of phosphorus to the ocean acted like a natural fertilizer, boosting the growth of marine organisms. But this seemingly beneficial fertilization had a disastrous flip side.

Widespread oxygen depletion

The surge in biological activity resulted in large amounts of organic matter sinking to the ocean floor, consuming significant quantities of oxygen in their wake.

This led to vast regions of the oceans becoming oxygen-depleted or anoxic, effectively becoming “dead zones” where most marine life could not survive.

“The anoxic events typically lasted around one to two million years and had profound impacts on marine ecosystems, the legacy of which even felt today,” said study co-author Professor Benjamin Mills.

These events also led to the accumulation of organic matter-rich rocks, which today form the largest source of commercial oil and gas reserves globally.

Echoes in today’s oceans

This mass extinction study not only offers answers to historical biological upheaval but also shines a spotlight on the dire effects of nutrient overloading in contemporary marine environments.

There is a haunting parallel with human activities today that have reduced oceanic oxygen levels by about two percent, causing a significant expansion in anoxic water masses.

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

A deadly symbiosis

Ultimately, the research illuminates a potent connection between 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,” noted the researchers. “Tearing continents apart can have profound repercussions for the course of evolution.”

The study is published in the journal Nature Geoscience.

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