For more than 100 million years, Earth has maintained a natural climate regulation system that has helped keep the planet habitable despite major environmental changes. A new study published in the Proceedings of the National Academy of Sciences identifies a previously overlooked relationship between sea level, ocean phosphate, and long-term carbon storage that helps explain how Earth’s carbon cycle has regulated atmospheric carbon dioxide over the past 60 million years. The research suggests that fluctuations in sea level influenced marine nutrient availability, biological productivity, and the burial of organic carbon, creating a powerful natural feedback that affected global climate.
The researchers found that changes in global temperature altered the size of polar ice sheets, causing sea levels to rise or fall. These sea-level changes determined how much phosphate, an essential nutrient for marine life, reached the open ocean. Because phosphate controls the growth of marine organisms, its availability influenced how much organic material was produced and eventually buried in seafloor sediments. This process became an important component of Earth’s carbon cycle, helping regulate the amount of carbon dioxide remaining in the atmosphere over geological timescales.
During periods of high sea level, extensive continental shelves trapped phosphate within shallow coastal sediments, reducing nutrient supplies in the open ocean. Lower phosphate availability limited marine productivity, resulting in fewer organisms transporting carbon to the seafloor after death. As a result, less organic carbon became buried in marine sediments, ocean waters remained relatively oxygen-rich, and more carbon dioxide accumulated in the atmosphere, contributing to warmer global conditions.
The opposite occurred when sea levels declined. As continental shelves became exposed, more phosphate entered the open ocean, stimulating marine productivity. The decomposition of greater amounts of sinking organic matter consumed oxygen, creating low-oxygen conditions in deeper waters. These oxygen-poor environments triggered the release of additional phosphate from marine sediments, strengthening a positive feedback that promoted even more biological growth and organic carbon burial. By removing larger amounts of carbon from the ocean-atmosphere system, this feedback lowered atmospheric carbon dioxide and strengthened Earth’s carbon cycle as a long-term climate regulator.
The study identified a sea-level “sweet spot” approximately 10 to 40 meters above present-day levels where carbon burial became most efficient. Under these conditions, low-oxygen waters overlapped with carbon-rich continental shelf sediments, maximizing phosphate recycling and allowing unusually large quantities of organic carbon to remain buried for millions of years. Researchers tested this hypothesis using 60 million years of geological evidence, including carbon isotope records, phosphorus accumulation in deep-sea sediments, and iodine-to-calcium measurements from fossilized foraminifera to reconstruct ancient ocean oxygen levels.
The Eocene Epoch illustrates the opposite situation. During this warm period, exceptionally high sea levels flooded continental shelves, trapping phosphate in shallow sediments and limiting nutrient availability in the open ocean. Marine productivity remained relatively low, carbon burial weakened, atmospheric carbon dioxide remained elevated, and Earth stayed considerably warmer because the feedback mechanism was largely inactive.
The researchers also suggest that the regions where carbon burial occurs have gradually become more restricted as low-oxygen waters shifted into deeper parts of the ocean over millions of years. This long-term evolution may have stabilized both atmospheric oxygen and carbon dioxide by reducing extreme fluctuations in carbon burial. The findings highlight phosphate as a previously underappreciated regulator of Earth’s carbon cycle and provide new insight into how interactions among sea level, ocean chemistry, marine ecosystems, and carbon storage have helped maintain a stable climate throughout much of Earth’s recent geological history.
https://www.sciencedaily.com/releases/2026/07/260715083527.htm