Lessons From Ancient Ocean Oxygen

February 17, 2026 Staff

Ninety-four million years ago, large swaths of Earth’s oceans lacked a basic building block of life. From its sunlit regions down to its very depths, the ocean was devoid of oxygen throughout the Late Cretaceous period, a condition that persisted for nearly a million years.

The ocean’s oxygen levels have ebbed and flowed throughout geologic history, notably dropping during times of major changes in the Earth’s climate. Kate Davis, an associate professor in the Department of Marine, Earth and Atmospheric Sciences, is leading research into what those historical changes may indicate about our present.

Davis co-leads the PAGES PO2 working group, a team of international researchers who are studying how and why the ocean’s oxygen levels changed in the past. As part of the group’s efforts, Davis organized a three-day workshop in December to explore ancient hypoxic and anoxic events — times of very little or no oxygen in the ocean.

A group of people sit in a room listening to a guest lecturer at the front of the room

Kate Davis stands and speaks at a podium — Associate Professor Kate Davis co-leads the PAGES PO2 working group. As part of the group’s efforts, she organized a three-day workshop aimed at exploring the causes and consequences of past hypoxic events, as well as how oceans have recovered from such events.

The event, hosted at the North Carolina Museum of Natural Sciences, featured flash talks, poster presentations and guest lectures from ocean oxygen researchers around the world. Some traveled from as far as China and New Zealand to attend. Additionally, participants spent each afternoon in structured breakout discussions to identify major questions in the field moving forward.

“At its root, we’re asking what causes an extreme hypoxic event, and could we be causing one now or in the near future?” Davis said.

Experts like Davis, who was awarded the Sloan Research Fellowship for her work reconstructing past ocean oxygen, use fossils, core samples and Earth-system modeling to see that past instances of low oxygen in the ocean have occurred in tandem with climate change.

Hypoxic and anoxic events throughout history, however, haven’t always been consistent. Sometimes the ocean lost its oxygen when the climate or other environmental factors changed, but not always. Narrowing down the specific triggers for such events, therefore, is key.

“We have good geologic records of extreme events. We don’t have good geologic records of everything that happened in between,” Davis said. “We’re really lacking a lot of the boring background records, which is probably one of the difficulties in putting modern deoxygenation into context. We know when things got pushed to a real extreme, but we don’t know how often things have gotten knocked a little bit off-kilter like they are now.”

According to Davis, experts are currently observing a dramatic decrease in the ocean’s oxygen — about a 2% loss since the 1950s, which Davis described as “shocking.”

“We should all be alarmed by this,” she said.

A lack of oxygen in the ocean impacts everything from coral reefs to global nutrient cycles. Tuna, for example, are very active fish that require more oxygen. When oxygen levels drop farther down in the ocean, tuna lose some of their habitat.

“If you have an ocean with very little or no oxygen, it’s going to change who can live there and severely restrict the range of other things,” Davis said.

A person gestures to a powerpoint of their research projected onto a large screen.

A person sits in a crowd of people and asks a question while holding a microphone

A guest lecturer stands at a podium, laughing while discussing research — The workshop, titled, “Notable Hypoxic Events of the Geologic Past,” drew nearly 100 participants from around the world to share their research in the form of guest lectures and short “flash talk” presentations.

In addition to affecting basic biology, low oxygen is an indicator of another concerning trend: increased carbon dioxide (CO2) in the ocean. The same warm, sluggish waters that lead to low oxygen levels also lead to what is essentially a reservoir of CO2 in the ocean, which has its own negative implications for plants, animals and global climate change.

Davis said she doesn’t expect today’s climate change to trigger an anoxic event as dramatic as during the Cretaceous period, but those ancient occurrences may still help researchers manage their expectations.

“Ocean oxygen in the past is really the only template that we have for how it could change in the future, both naturally or due to human forcing,” Davis said. “Like so many parts of modern climate change, it’s probably a no-analog event; there probably isn’t a place we can point to in the geologic past and say we’re headed to again. But, knowing what the extreme is might help us understand where we fall on that continuum.”

A person presents their research poster to another person

An over-the-shoulder shot of a person looking at a research poster

A person smiles while talking with another person in a room with posters on display — The workshop also featured poster presentations and networking breaks.

At the December workshop, nearly 100 in-person and online participants planned out the next steps needed to strengthen understanding of hypoxic events. By the weekend’s close, participants left with new ideas for research proposals and potential collaborations — a great starting point, Davis said.

Ultimately, the researchers aim to release both a scientific review and perspectives article within the next two years that will reevaluate ocean oxygen data and just how much can be learned from it.

“Everyone is really excited to move ahead,” Davis said. “That feels like a success, bringing people together and identifying big questions. People then going out and doing the next concrete steps to answer those questions feels like a win.”

This post was originally published in Department of Marine, Earth, and Atmospheric Sciences.