Earth has a natural mechanism that has kept the planet habitable for more than 100 million years, but scientists have struggled to explain exactly how it works. New research now identifies a missing piece: the relationship between sea level and phosphate, a nutrient that controls how much carbon gets buried on the ocean floor.
The study, published in the Proceedings of the National Academy of Sciences, was co-authored by Zunli Lu, professor of Earth and environmental sciences at Syracuse University's College of Arts and Sciences. It traces how fluctuating sea levels and dissolved oxygen levels controlled phosphate availability and atmospheric carbon dioxide over the last 60 million years.
"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," said lead author Ros Rickaby, 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 is 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. That starved the open ocean of the nutrient, marine productivity declined, less organic carbon was buried on the seafloor, and carbon dioxide built up in the atmosphere.
When sea levels fell, that dynamic reversed. Shrinking shelves released more phosphate into the water column, fueling blooms of marine life. As that organic matter sank and decomposed, it consumed oxygen from the water, creating low-oxygen zones. When those zones extended into contact with carbon-rich shelf sediments, they triggered a feedback loop. Oxygen-poor conditions caused more phosphate to be released from sediments, driving further organic carbon burial and pulling carbon dioxide out of the atmosphere.
The researchers identified a sea-level sweet spot, roughly 10 to 40 meters above modern sea level, where this feedback was most powerful. That range represents the zone where the connection between sea level, phosphate and carbon burial operated most efficiently.
"Our co-author, Christian Bjerrum, studied the connection among sea level, ocean oxygen and phosphate with a computer model two decades ago," Lu said. "We finally pieced together the geologic records necessary to test this hypothesis."
The findings have broad implications for understanding how Earth has regulated its own climate over geological time. They also add detail to scientific models of the long-term carbon cycle, which researchers use to understand both past climate conditions and the planet's future behavior under changing atmospheric conditions.
