Scientists have uncovered new evidence of a natural climate feedback system that may have helped regulate Earthโs temperature for tens of millions of years.
The study suggests that changes in sea level, ocean oxygen, phosphate availability, and carbon burial worked together like a hidden planetary thermostat. This system influenced how much carbon dioxide remained in the atmosphere and helped keep Earthโs climate within a range suitable for life.
The research was coauthored by Zunli Lu, professor of Earth and environmental sciences at Syracuse University, and published in the Proceedings of the National Academy of Sciences.
For the past 60 million years, atmospheric carbon dioxide has generally declined as Earth cooled. However, scientists have long debated where that carbon went. The new study suggests that much of it may have been locked away in marine sediments through increased burial of organic carbon.
At the center of the process is phosphate, a nutrient that marine organisms need to grow. When phosphate is available in the ocean, it can stimulate marine life. When those organisms die, some of their organic matter sinks to the seafloor and becomes buried, removing carbon from the atmosphere-ocean system.
The researchers found that sea level played a major role in controlling this process.
When sea levels were high, broad continental shelves trapped phosphate in shallow sediments before it could spread into the open ocean. With less phosphate available, marine productivity declined, less organic carbon was buried, and more carbon dioxide stayed in the atmosphere.
When sea levels fell, more phosphate entered the ocean. This boosted marine productivity and increased the amount of organic carbon sinking into sediments. As this material decomposed, it reduced oxygen levels in parts of the ocean, creating low-oxygen zones.
These low-oxygen waters then triggered a feedback loop. When they reached organic-rich shelf sediments, they released even more phosphate, fueling further marine growth and more carbon burial. As more carbon became trapped beneath the seafloor, atmospheric carbon dioxide declined.
The strongest carbon burial occurred within a narrow sea-level range, when oceans stood about 10 to 40 meters above todayโs level. In this โsweet spot,โ low-oxygen waters lined up with organic-rich continental shelf sediments, allowing the feedback system to operate for millions of years.
To test the idea, the team used geological records spanning 60 million years. These included carbon isotopes, phosphorus accumulation in deep-sea sediments, and a newer iodine-to-calcium method used to estimate oxygen levels in ancient oceans.
The findings also help explain why Earth remained warm during the Eocene epoch, about 56 to 34 million years ago. At that time, sea levels were very high, continental shelves were flooded, and phosphate was trapped in shallow sediments. This weakened the carbon burial system, allowing more carbon dioxide to remain in the atmosphere.
Over time, the researchers suggest, the range of ocean conditions that supported strong carbon burial became narrower. This may have helped stabilize atmospheric oxygen and carbon dioxide, making Earthโs climate system more resilient.
The study reveals how deeply connected Earthโs systems are. A shift in sea level can affect ocean nutrients, oxygen levels, carbon storage, atmospheric carbon dioxide, and global temperature.
In simple terms, Earthโs hidden thermostat may have worked by controlling how much carbon the oceans could bury beneath the seafloor.
Journal Reference:
Rickaby, R. E. M., Wood, T. J., Lu, Z., & Bjerrum, C. J. (2026). Shelf-invading low-oxygen waters control Cenozoic organic carbon burial rates. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.2526409123