Earth appears to have a natural climate control system that has helped keep it habitable for over 100 million years. Scientists have long known that this system exists, but the mechanisms behind it have remained difficult to explain.
A new study points to a previously overlooked link between sea levels and ocean phosphate availability. Changes in global temperatures have affected the size of polar ice sheets and changed sea levels. These changes affected how much phosphate reaches the open ocean, how much carbon gets buried in ocean sediments, and how much carbon dioxide remains in the atmosphere.
Taken together, these processes helped determine whether the Earth became warmer or colder over long periods of time.
Sea level and the global carbon cycle
The new study was co-authored by Zunli Lu, professor of earth and environmental sciences in Syracuse University’s College of Arts and Sciences. Explore how changes in sea level and ocean oxygen conditions have affected phosphate availability and atmospheric carbon dioxide over the past 60 million years.
The survey results are Proceedings of the National Academy of Sciences.
“We know that as the Earth has cooled over the past 60 million years, there has been a significant reduction in carbon dioxide in the atmosphere, but there is surprisingly little understanding of where that carbon ends up,” lead author Ross Rickerby, professor of geosciences at the University of Oxford, said in a departmental news article. “Our findings suggest that the burial of organic carbon in marine sediments was enhanced and played a much more important role than previously recognized.”
Phosphate as a hidden climate regulator
The research focuses on phosphorus, especially phosphate, which is a nutrient necessary for the growth of marine life. Researchers explain that phosphates have been a hitherto “invisible” part of the climate puzzle.
When sea levels were higher, shallow continental shelves covered larger areas. These shelves trapped phosphates in coastal sediments, reducing the amount of nutrients left in the open ocean.
As phosphates in the water decreased, ocean productivity decreased. Less biological growth, less organic carbon sinking to the ocean floor, and less carbon being buried in sediments. Ocean waters also became rich in oxygen, and carbon dioxide accumulated in the atmosphere.
As a result, the Earth has become warmer.
Sea level collapse triggered carbon feedback
When sea levels fell, the process went in the opposite direction.
As the continental shelf shrank, more phosphate entered the water. The extra nutrients supported an explosion of marine life. When an organism dies, its carcass sinks and decomposes, consuming oxygen from the surrounding water.
Over time, a hypoxic zone formed in the ocean. When these zones reached carbon-rich sediments on the continental shelf, strong feedback processes were activated.
Less oxygen released more phosphate from the sediment. That additional phosphate promoted ocean growth, which led to more burial of organic carbon on the ocean floor. As more carbon is removed from the oceans and atmosphere, atmospheric CO2 has decreased.
“Our co-author Christian Bjerram used computer models to study the relationship between sea level, ocean oxygen, and phosphate 20 years ago,” Lu says. “We have finally pieced together the geological record needed to test this hypothesis.”
Zero meters above sea level is the sweet spot for carbon burial
The researchers found that this feedback reaches its maximum force when sea levels are approximately 10 to 40 meters above modern sea levels.
In this sea-surface “sweet spot,” low-oxygen waters overlapped organic-rich continental shelf sediments. This combination allowed unusually large amounts of carbon to remain buried for millions of years.
The researchers compared this pattern to 60 million years of geological evidence. The data included carbon isotope records, measurements of phosphorus accumulation in deep-sea sediments, and a new iodine-to-calcium method to reconstruct ancient ocean oxygen levels.
Reading ancient ocean oxygen
Lu’s lab performed the iodine-to-calcium measurements.
This method examines the chemistry of ancient foraminifera, microscopic marine organisms whose remains are preserved in sediments on the ocean floor. Their chemical composition allows scientists to estimate how much oxygen was present in the water when they were alive.
Samples were analyzed on a mass spectrometer at Syracuse University. The device was funded by the National Science Foundation.
Why was the Eocene so warm?
The Eocene epoch, which lasted from about 56 million years ago to 34 million years ago, provides a clear example of what happened when carbon burial feedbacks were largely inactive.
During that period, sea levels rose so high that large continental shelves were submerged. Phosphate became trapped in shallow sediments, leaving the open ocean relatively nutrient-poor.
Ocean productivity still declined, ocean oxygen levels increased, and the amount of organic carbon buried decreased. With the feedback mechanism effectively turned off, carbon dioxide accumulated in the atmosphere and the Earth remained warm.
a more stable climate system
The researchers propose that the zone where carbon burial occurs gradually narrowed over geological time as hypoxic waters moved deeper.
This long-term change may help stabilize both oxygen and carbon dioxide in the atmosphere. The fluctuations between carbon burial and atmospheric carbon accumulation have become less extreme, and Earth’s climate system has become more resistant to disruption.
Key takeaways from the study
Phosphate, an essential nutrient for marine life, has acted as a hidden regulator of the Earth’s carbon cycle for the past 60 million years, although its precise role was not fully understood.
Sea level affected the amount of phosphate reaching the open ocean. This controlled the productivity of the oceans, the amount of carbon buried in seafloor sediments, and the amount of carbon dioxide remaining in the atmosphere.
The ocean’s “sweet spot”, approximately 10 to 40 meters above modern sea level, produced the most intense carbon burial. This process has acted as a natural brake against warming for millions of years, helping move the Earth toward our current cooler climate.
The study included collaborators from the University of Oxford (Rickaby and Thomas Wood) and the University of Copenhagen (Christian J. Bjerrum). This research was supported by two National Science Foundation grants.
The discovery adds to Lu’s lab’s larger work using the iodine-to-calcium method to recreate ancient ocean oxygen conditions.
Early research published in January natural earth scienceused the same technique to show that tropical Proterozoic oceans were oxygen-rich. That pattern was the exact opposite of what exists today. Researchers also discovered that a planetary tipping point hundreds of millions of years ago caused a reversal of global oxygen distribution.
