In the age of dinosaurs, oceans lost their life-sustaining oxygen due to climate changes

Hundreds of meters beneath the ocean floor, the remnants of an ancient world exist as layers of rock and sediment that carry an alarming message: More than 100 million years ago, as dinosaurs fought and volcanoes bubbled, global temperatures were rising just like they are today. Then the ocean started running low on oxygen.

What triggered the change is that the Earth’s warming climate passed a tipping point, or a threshold, that turned ocean deoxygenation into a global phenomenon during the Early Cretaceous Period, a recent study published in Nature suggests. Many scientists believe this ocean deoxygenation had dire effects on sea creatures at the time.

Whether this could happen again in the modern era isn’t entirely clear. Temperatures and carbon dioxide levels were much higher in the Early Cretaceous period than today, but experts warn that there is still cause for concern, in part because climate change is accelerating much faster than it did millions of years ago.

Ancient drivers of climate change “are actually very slow compared to what we’re doing today,” said Ryan McKenzie, one of the study authors and an associate professor at the University of Hong Kong. “We’re much more efficient at putting carbon into the atmosphere.”

“What’s going on now is a harbinger of things that could be very nasty in the greenhouse future,” said Hugh Jenkyns, a geologist and emeritus professor at Oxford University who was part of a team that first identified the Early Cretaceous Period’s deoxygenated oceans in 1973. “The geologic record speaks to that with these oceanic anoxic events, which show you what happens with a planet ‘in extremis,’ when it’s suffered the slings and arrows of outrageous fortune.”

To reconstruct the events of the Early Cretaceous, McKenzie and other researchers tested rock samples from deep beneath the Pacific Ocean floor as well as an ancient sea basin now located high in the Italian Alps. The research team determined that volcanic activity during that period altered natural processes like the weathering of rocks and the removal of carbon dioxide from the atmosphere. These changes in turn caused global CO2 to increase past the climate threshold — about double the era’s baseline level of 1,000 parts per million — leading to ocean deoxygenation around the world.

Today, CO2 concentrations in the atmosphere are much lower, at about 420 parts per million, but they are up 50% since the Industrial Revolution and are now accelerating at an even faster rate. Meanwhile, low-oxygen “dead zones” are increasingly disrupting marine life in warm, shallow waters all over the world, including the mouth of the Mississippi River every summer.

The localized deoxygenation events of today offer some clues about how drastically Early Cretaceous sea creatures may have been affected as oxygen levels fell. “We’re getting those, for example, off the coast of British Columbia, and that’s obviously destroying the fishing grounds and things like that, because the fish won’t hang around,” Jenkyns said.

Because most modern ocean deoxygenation is still fairly localized, most fish that encounter low oxygen waters are able to swim away. But the fish that can escape — big billfish and tuna, for example — are then forced to crowd into higher oxygen waters. And for animals that aren’t able to flee, the threat is even more dire.

“For those that can’t move away and avoid low oxygen, they are likely to die,” said Lisa Levin, an emeritus professor of biological oceanography at the Scripps Institution of Oceanography. Both crowding and die-offs can lead to a decrease in local biodiversity, she added.

These oxygen-deprived areas form when temperatures rise, for the same reason carbonated soda goes flat when it’s warm, said Sunke Schmidtko, a physical oceanographer at the GEOMAR Helmholtz Centre for Ocean Research in Germany. “Warm waters do not take up as much gas as cold water. We all know that from our soda,” Schmidtko said. “When it’s warm, it loses the bubbly stuff, the carbon, immediately.”

The climate threshold described in the new study further clarifies the relationship between heat and widespread deoxygenation. While the findings are new, researchers have been studying the Early Cretaceous Period’s warm climate and deoxygenated oceans for decades. Pulling up rock samples from the Early Cretaceous in the early 1970s, Oxford’s Jenkyns and his collaborators were surprised to see black, carbon-rich sediments — a sign of a low oxygen environment.

It took a “leap of faith” to see this deoxygenation as a global rather than local phenomenon, Jenkyns said. Now, the period’s global ocean deoxygenation is both widely accepted and seen as a preview of what might happen as modern CO2 levels and temperatures continue to rise.

Schmidtko does not use ancient data in his own work but said it can fill gaps in our understanding of what happens next, as ocean temperatures and deoxygenation have increased quicker than expected in recent years.

“Over the past 50 years, global dissolved oxygen has decreased by around 2% and scientific data indicate that this trend is set to continue,” wrote Isabella Lövin, Minister for Environment and Climate and the Deputy Prime Minister of Sweden, in the preface to a 2019 publication on the mass effects of ocean deoxygenation. This percentage may sound small, but the open ocean alone has lost an estimated 77 billion metric tons of oxygen over the past 50 years, according to a 2018 study.

These fast rates of modern deoxygenation are unexpected, but McKenzie emphasized that studying the ancient oceans could help us to manage the uncertainty of what comes next. “Earth has gotten hot and it’s gotten cold,” he said, “and understanding how that happens … informs us on what we can expect today.”