New Study Reveals How Earth Maintained Climate Control For 60 Million Years

New Study Reveals Earth’s Hidden Climate Control Feedback Over 60 Million Years

The findings were published in the Proceedings of the National Academy of Sciences. The research was co-authored by Zunli Lu, professor of Earth and environmental sciences in Syracuse University’s College of Arts and Sciences. Lead author Ros Rickaby, professor of Earth sciences at the University of Oxford, noted in a department news article that while atmospheric carbon dioxide decreased substantially as Earth cooled over the last 60 million years, researchers previously had remarkably little understanding of where that carbon ended up.

Phosphate and the Sea Level Sweet Spot for Carbon Burial

At the center of the study is phosphorus, particularly phosphate, which serves as a nutrient that marine organisms need to grow. When sea levels were high, shallow continental shelves covered a larger area, trapping phosphate in shallow sediments and leaving the open ocean relatively nutrient-poor.

The researchers discovered that strong carbon burial occurred within a relatively limited sea level window. When sea level stood about 10 to 40 meters above its present height, oxygen minimum zones aligned with organic-rich continental shelf sediments. In this sea level sweet spot, low-oxygen conditions released additional phosphate from the sediments, supporting more marine growth and increasing the burial of organic carbon. As more carbon was trapped beneath the seafloor, less carbon dioxide remained in the atmosphere, driving down atmospheric CO2.

Lu stated that co-author Christian Bjerrum originally studied the connection among sea level, ocean oxygen, and phosphate with a computer model two decades ago, and the team finally pieced together the geologic records necessary to test this hypothesis.

Testing Geological Records and Ancient Ocean Oxygen

The research team tested this pattern against 60 million years of geological evidence. Their records included carbon isotopes, phosphorus accumulation in deep-sea sediments, and a newly developed iodine-to-calcium proxy for estimating oxygen levels in ancient oceans.

Lu’s laboratory performed the iodine-to-calcium measurements. This method examines the preserved chemistry of foraminifera, which are microscopic marine organisms whose remains accumulate in seafloor sediments, to determine the oxygen content of ancient water. These samples were measured with a mass spectrometer at Syracuse University through funding from the National Science Foundation.

Why the Eocene Epoch Stayed Warm

The Eocene epoch, which spanned approximately 56 to 34 million years ago, provides a clear example of what happened when the carbon burial feedback stopped operating effectively. During this period, sea levels were extremely high and broad continental shelves were flooded.

Phosphate became trapped in shallow sediments while the open ocean remained highly oxygenated. Marine productivity remained lower and the mechanism that promoted organic carbon burial was largely inactive, keeping the Eocene warm.