The new PNAS study shows that maximum organic carbon burial in marine sediments occurred when global sea level was about 33–131 feet (10–40 m) higher than today. High seas trapped phosphate on continental shelves, reducing offshore productivity and carbon burial, while lower seas boosted nutrient delivery and burial. Researchers used an iodine-to-calcium proxy in foraminifera, alongside carbon isotope and phosphorus records, to reconstruct these interactions over the past 60 million years.
Sea-Level 'Sweet Spot' Helped Lock Away Earth's 'Missing' Carbon, New Study Shows
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New research published in Proceedings of the National Academy of Sciences finds that a relatively narrow range of higher sea levels — roughly 33–131 feet (10–40 meters) above present — coincided with the greatest long-term burial of organic carbon in marine sediments. That deep burial, the authors argue, made the ocean floor a much larger carbon reservoir over tens of millions of years than scientists previously appreciated.
How Sea Level Controlled Carbon Burial
The study reconstructs how sea-level change, nutrient delivery (especially phosphate), ocean oxygenation, and organic carbon burial interacted over the past 60 million years. When global sea level was high, more phosphate was retained on shallow continental shelves instead of reaching offshore waters. Reduced nutrient supply lowered offshore biological productivity and the flux of organic carbon to the deep seafloor.
Conversely, lower sea levels allowed more phosphate to reach open-ocean regions, fueling greater marine production and increasing the amount of organic carbon exported to—and preserved in—deep-sea sediments. The greatest carbon burial therefore appeared only when several conditions aligned: an optimal sea-level band, sufficient nutrient delivery offshore, and the presence of low-oxygen waters that enhanced preservation of organic matter.
Evidence and Methods
The team—led by Rosalind E. M. Rickaby (University of Oxford) and Zunli Lu (Syracuse University)—used an iodine-to-calcium (I/Ca) proxy measured in foraminifera to estimate past ocean oxygen levels. Those measurements, performed in Lu's lab with U.S. National Science Foundation support, were compared with carbon isotope records and phosphorus accumulation in deep-sea sediments to reconstruct the coupled history of nutrient supply, oxygenation, and carbon burial.
"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," Rickaby said. "Our results suggest that enhanced burial of organic carbon in marine sediments played a much more important role than was previously appreciated."
Implications
The findings sharpen our understanding of the long-term carbon cycle and provide constraints that can improve climate models of how Earth responds to slow geologic forcings. The authors emphasize, however, that these processes operate over millions of years and cannot offset the rapid, human-caused rise in atmospheric CO2 today.
Bottom line: A sea-level "sweet spot" combined with nutrient and oxygen conditions amplified organic carbon burial on the seafloor, helping to remove CO2 from the atmosphere over geological time and contributing to long-term climate regulation.
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