Earth has a natural thermostat that has kept the planet habitable for more than a hundred million years. Scientists have struggled to fully explain how it works, but new research identifies a missing link between phosphate availability and sea level. Temperature influenced the size of polar ice sheets and sea level. Sea level changes drove the availability of this nutrient and controlled how much carbon was buried in the ocean, which in turn regulates how much carbon dioxide stays in the atmosphere and how warm or cool the planet runs.
 co-authored by , professor of Earth and environmental sciences in the University’s , traces how fluctuating sea levels and dissolved oxygen content controlled the availability of phosphate in the ocean and atmospheric carbon dioxide accumulation stretching across the last 60 million years. The research was published in .
“We know that atmospheric carbon dioxide decreased substantially as Earth cooled over the last 60 million years, but we have had remarkably little understanding of where that carbon ended up,” says lead author , professor of Earth sciences at the University of Oxford, . “Our results suggest that enhanced burial of organic carbon in marine sediments played a much more important role than was previously appreciated.”
The key to the study is phosphorus, specifically phosphate, an essential nutrient for marine life that the researchers describe as a previously “invisible” piece of the puzzle. At high sea levels, broad continental shelves efficiently trapped phosphate in shallow sediments, starving the open ocean of the nutrient. With less phosphate available, marine productivity declined, less organic carbon was buried on the seafloor and the ocean became well-oxygenated—while carbon dioxide built up in the atmosphere.
As sea levels fell, that dynamic reversed. Shrinking shelves released more phosphate into the water column, fueling a bloom in marine life. As that organic matter sank and decomposed, it consumed oxygen from the water until low-oxygen zones began to emerge. When those low-oxygen zones extended into contact with carbon-rich shelf sediments, they triggered a feedback loop in which oxygen-poor conditions caused more phosphate to be released from sediments, driving further organic carbon burial and pulling CO2 out of the atmosphere.
“Our co-author, Christian Bjerrum, studied the connection among sea level, ocean oxygen and phosphate with a computer model two decades ago,” Lu says. “We finally pieced together the geologic records necessary to test this hypothesis.”
The researchers identified a sea-level “sweet spot,” roughly 10 to 40 meters above modern sea level, where this feedback was most powerful. At that range, oxygen minimum zones overlapped precisely with the organic-rich sediments of the continental shelf, maximizing carbon burial for millions of years at a time. The team matched these patterns against 60 million years of geological data, including carbon isotope records, phosphorus accumulation rates in deep-sea sediments and a novel iodine-to-calcium proxy developed to reconstruct past ocean oxygen levels.
Lu’s lab conducted the iodine-to-calcium measurements, a technique that uses the chemistry of ancient foraminifera, microscopic marine organisms preserved in seafloor sediments, to reconstruct oxygen conditions in the ancient water column. Samples were analyzed using a mass spectrometer at Syracuse University, funded by the National Science Foundation.
The Eocene epoch, which lasted from roughly 56 to 34 million years ago, stands out as a period when this carbon burial mechanism was effectively switched off. Sea levels were at their highest, shelves were flooded, phosphate was efficiently buried in shallow sediments and the ocean was highly oxygenated. Without the feedback loop, carbon accumulated in the atmosphere and the planet remained warm.
Over geological time, the study proposes, the zone for carbon burial has narrowed as oxygen minimum ranges have deepened—a process that has progressively stabilized both atmospheric oxygen and carbon dioxide. The oscillations between carbon burial and atmospheric accumulation have grown more muted, making Earth’s climate system increasingly resilient.
Key Takeaways From the Study:
- Phosphate, an essential nutrient for marine life, acted as a hidden regulator of Earth’s carbon cycle for the last 60 million years — but how it plays this role exactly has not been fully understood.
- Sea level controlled how much phosphate was available in the open ocean, which determined how much carbon was buried in seafloor sediments and how much carbon dioxide accumulated in the atmosphere.
- A sea-level “sweet spot” — roughly 10 to 40 meters above modern levels — maximized carbon burial for millions of years at a time, acting as a natural brake on warming and helping drive Earth’s transition to today’s cooler climate.
The research was conducted with collaborators at the University of Oxford (Rickaby and ) and the University of Copenhagen () and was supported by two National Science Foundation grants.
The new findings build on a body of research from Lu’s lab using the iodine-to-calcium proxy to reconstruct past ocean oxygen conditions. An earlier study, published in January in Nature Geoscience, used the same technique to reveal that —the exact reverse of today’s pattern—and that a planetary tipping point hundreds of millions of years ago flipped that distribution.
