Earth cooled for 60 million years and nobody knew where the CO₂ went — now they do

For 60 million years, Earth’s atmosphere shed carbon dioxide and the planet cooled. Scientists had known this. What they could not explain was where the CO₂ actually went. “We have had remarkably little understanding of where that carbon ended up,” said Ros Rickaby of the University of Oxford, lead author of the new study. The answer, the team found, was buried under the seafloor — placed there by a chain of ocean chemistry that ran on sea level and phosphorus recycling, and acted as a slow planetary thermostat throughout the Cenozoic Era.

The study, published in the Proceedings of the National Academy of Sciences, reconstructed 60 million years of geological evidence: ancient sea levels, ocean oxygen concentrations, phosphate availability, and rates of organic carbon burial in continental-shelf sediments. What emerged was a previously uncharted feedback loop connecting four systems that had each been studied separately but never assembled into a single mechanism.

How the chain works

The thermostat begins with sea level. When seas stood roughly 10 to 40 meters above their modern level — the range the team identifies as the system’s “sweet spot” — something distinctive happens on continental shelves. Oxygen concentrations in those shallow waters drop below a critical threshold as organic matter decomposes, creating zones of oxygen-depleted water right where carbon-rich shelf sediments are most exposed.

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Those low-oxygen conditions do something specific to phosphorus. Phosphate ordinarily binds to sediment particles in the presence of oxygen; remove the oxygen, and phosphate releases back into the water column. That recycled phosphorus feeds a surge in marine algae and other plankton, whose growth pulls carbon from surface waters. When those organisms die and sink, they carry carbon with them. On the seafloor, under the low-oxygen conditions that triggered the phosphorus release in the first place, the organic carbon becomes buried rather than decomposed — and it stays buried for millions of years.

The result is a self-reinforcing loop: sea level sets the stage, low oxygen releases phosphorus, phosphorus drives marine productivity, productivity buries carbon, buried carbon draws down atmospheric CO₂. The pulse lasts as long as the right sea-level conditions hold — and can run for millions of years before the system rebalances.

“Our co-author, Christian Bjerrum, studied the connection among sea level, ocean oxygen and phosphate with a computer model two decades ago,” said Zunli Lu of Syracuse University. “We finally pieced together the geologic records necessary to test this hypothesis.”

When the thermostat switched off

The team’s record shows the mechanism did not run continuously. During the Eocene epoch, from roughly 56 to 34 million years ago, sea levels rose high enough to push beyond the sweet spot. When that happened, the oxygen minimum zones no longer overlapped with organic-rich continental shelf sediments — the two conditions that make the feedback work. The carbon burial pulse weakened. CO₂ lingered in the atmosphere rather than being drawn down. The Eocene was, by geological standards, a warm period: warm enough that tropical forests grew in what is now Wyoming, and crocodilians lived above the Arctic Circle.

The system’s return — tied to sea-level changes associated with Antarctic glaciation — coincided with the sustained cooling that defines the last 34 million years of Earth’s history.

“Our results suggest that enhanced burial of organic carbon in marine sediments played a much more important role than was previously appreciated,” Rickaby said.

What this cannot do

The thermostat operates on million-year timescales. Each cycle — from sea-level trigger to phosphorus release to carbon burial to measurable CO₂ drawdown — takes hundreds of thousands of years to complete. The CO₂ that humanity has added to the atmosphere since industrialization has accumulated over roughly 200 years. The geological mechanism cannot respond at that pace.

There is a structural mismatch, too: today’s oxygen minimum zones sit deeper than they did during peak Cenozoic operation. Modern continental shelves are narrower and their oxygen-poor margins are less well-positioned to interact with organic-rich sediments. The conditions that made the thermostat efficient for 60 million years do not currently align.

Researchers are explicit about this limit. The system stabilised a climate that had thousands of years to adjust; it was not designed — and cannot operate — as a rapid carbon sink.

Common questions about Earth’s CO₂ thermostat

What is an oxygen minimum zone? It is a layer of ocean water, typically at depths of 100 to 1,000 meters, where the concentration of dissolved oxygen drops sharply. Marine life decomposing organic matter consumes most of the available oxygen before water circulation can replenish it. In these zones, normal chemical reactions change: phosphorus that would otherwise stay bound to sediments is released back into the water.

How is this different from the ocean absorbing CO₂ directly? Direct ocean absorption dissolves CO₂ at the surface and keeps it in solution — it does not remove the carbon permanently and is reversible as ocean temperatures change. The mechanism described in this study buries organic carbon physically in seafloor sediment, locking it out of the atmosphere for geological timescales rather than human ones.

Did this process ever fail? The Eocene example shows it can weaken. When sea levels are too high — above the sweet spot — the feedback loop breaks because the oxygen minimum zones no longer reach the richest sediment deposits. CO₂ stays elevated. The planet warms. The mechanism restarts only when sea levels return to the right range.

Could this process be artificially triggered? The study does not address geoengineering. The feedback depends on sea level, the position of oxygen minimum zones, and the distribution of continental-shelf sediments — variables that operate on scales no human technology currently controls.

What comes next

The research opens a direct line of investigation into one of paleoclimatology’s older puzzles: why Earth’s carbon drawdown over the Cenozoic was not smooth. The geological record shows pulses of cooling separated by warmer intervals — a pattern the thermostat model now explains in terms of shifting sea levels moving the system in and out of its effective range.

For climate science today, the finding also adds specificity to what makes past warm periods different from the present one. The Eocene was warm because the thermostat was off. Current warming is happening despite the thermostat being in its structural range — because the rate of CO₂ addition has simply outpaced any geological response.

Reference: Rickaby et al., “Shelf-invading low-oxygen waters control Cenozoic organic carbon burial rates,” Proceedings of the National Academy of Sciences, 2026. DOI: 10.1073/pnas.2526409123

Tags: climate, carbon burial, Cenozoic era, ocean chemistry, phosphorus, PNAS

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