News summary produced by Claude AI
Research published in Proceedings of the National Academy of Sciences reveals how Earth maintains a natural thermostat through interconnected processes involving ocean phosphate levels and carbon cycling. The study, led by researchers at the University of Oxford with collaboration from Syracuse University and the University of Copenhagen, demonstrates that sea level changes directly influenced the availability of phosphate, an essential nutrient for marine organisms, thereby affecting atmospheric carbon dioxide concentrations over the past 60 million years.
Phosphate emerges as a previously underappreciated regulator of Earth’s climate system. When sea levels were elevated, continental shelves expanded and trapped phosphate in coastal sediments, reducing nutrient availability in open ocean waters. This nutrient scarcity limited marine productivity, resulting in less organic carbon sinking to the seafloor and decreased carbon burial. Consequently, carbon dioxide accumulated in the atmosphere, warming the planet. Conversely, when sea levels fell, continental shelves contracted and released more phosphate into ocean waters, triggering a feedback mechanism that enhanced marine growth and increased organic carbon burial in seafloor sediments.
The researchers identified an optimal sea level range of 10 to 40 meters above current levels as a “sweet spot” where carbon burial processes operated most effectively. At this configuration, low-oxygen ocean zones overlapped with organic-rich sediments on continental shelves, enabling substantial quantities of carbon to become sequestered for extended periods. The team validated their hypothesis by analyzing 60 million years of geological evidence, including carbon isotope records and measurements using an iodine-to-calcium method to reconstruct ancient ocean oxygen conditions.
The Eocene epoch, spanning approximately 56 to 34 million years ago, exemplifies periods when this natural climate control weakened. During that time, extremely high sea levels flooded broad continental shelves, trapping phosphate and reducing marine productivity while keeping the atmosphere rich in carbon dioxide and the climate warm. The research suggests that over geological timescales, the zones where carbon burial occurs have gradually shifted deeper in the ocean, potentially stabilizing atmospheric conditions and making Earth’s climate system more resistant to extreme fluctuations.