How Earth's Seas Acted as a Thermostat for 60 Million Years

Imagine a planetary thermostat tucked away in the shallow margins of the oceans. Small shifts in sea level flipped it on and off, nudging marine life, chemistry, and ultimately the amount of carbon left to warm the atmosphere. Simple? No. Elegant? Yes.

Scientists from Syracuse University, the University of Oxford and collaborators have reconstructed a chain of cause and effect that links sea level, phosphate supply, ocean oxygen, and the burial of organic carbon — a sequence that appears to have helped keep Earth’s climate within a life-friendly band for roughly 60 million years. The new work, published in Proceedings of the National Academy of Sciences, stitches together old ideas and fresh geochemical evidence to reveal a feedback many researchers had suspected but could not prove.

So how did this hidden regulator work? Start with phosphate, the nutrient that fuels marine productivity. When sea level rose and flooded wide continental shelves, much of the phosphate became trapped in shallow sediments and never made it to the open ocean. Less nutrient meant lower biological productivity, fewer sinking particles, and less organic material getting sequestered on the seafloor. The ocean stayed relatively oxygen-rich and more carbon dioxide lingered in the atmosphere.

When sea level fell, shelves shrank and more phosphate reached deeper waters. Plankton bloomed. More organic matter sank. As microbes decomposed that material, they consumed oxygen and expanded oxygen-poor zones. Those low-oxygen conditions then mobilized additional phosphate from shelf sediments, feeding still more productivity and enhancing the burial of organic carbon. The loop reinforced itself. Over geologic time, that buried carbon was locked beneath sediments, removing CO2 from the atmosphere.

A narrow sea-level window—roughly 10 to 40 meters above present—maximized the burial of organic carbon for millions of years. When sea level sat in that sweet spot, oxygen minimum zones lined up with organic-rich continental shelves and the feedback operated at full strength. Step outside that window and the mechanism weakened or shut down.

The team tested this idea against a long, messy archive: carbon isotopes, records of phosphorus accumulation in deep-sea deposits, and a relatively new oxygen proxy based on iodine-to-calcium ratios in tiny foraminifera shells. Zunli Lu’s lab at Syracuse University measured the iodine-to-calcium signal using mass spectrometry funded by the National Science Foundation, providing direct clues about oxygen levels in ancient waters. Ros Rickaby, lead author and professor at Oxford, and coauthors then wove these records into a 60-million-year narrative.

This synthesis also resurrects an earlier hypothesis from Christian Bjerrum, who two decades ago used models to suggest the sea level–oxygen–phosphate link. What was once a theoretical loop now has empirical legs. The evidence implies that enhanced burial of organic carbon in marine sediments played a far larger role in long-term CO2 decline than many had appreciated.

There are vivid examples in Earth’s past. During the Eocene, roughly 56 to 34 million years ago, sea levels were unusually high. Continental shelves were flooded and phosphate lingered in shallow deposits rather than fueling open-ocean productivity. As a result, the ocean stayed well oxygenated, organic carbon burial waned, and atmospheric CO2 piled up — a recipe that kept the planet warm.

Over the last tens of millions of years, the story becomes one of narrowing opportunity. As oxygen minimum zones migrated deeper, the favorable alignment between low-oxygen water and organic-rich shelves tightened. That narrowing likely helped stabilize both atmospheric oxygen and carbon dioxide, smoothing swings between oceanic burial and atmospheric accumulation and increasing the climate system’s resilience.

The study also builds on other work from Lu’s group using the iodine-to-calcium proxy, which previously revealed surprising patterns of ancient ocean oxygenation, including episodes when tropical waters were more oxygen-rich than today. Together, these geochemical tools are sharpening our picture of how ocean chemistry and sea level combined to shape long-term climate.

What does this mean for now? The mechanism the researchers describe unfolded over millions of years, but it highlights how interconnected sea level, nutrient cycles and oxygen are in regulating carbon. Humans are altering those relationships rapidly — through greenhouse-driven sea-level rise, coastal development that changes sediment and nutrient fluxes, and massive redistribution of phosphorus via agriculture. The past may not map neatly onto the present, but it offers a sobering lesson: the ocean’s ability to bury carbon depends on a fragile alignment of conditions, and changing one part of the system can ripple through the whole.

For scientists, the next steps are clear: refine the proxies, expand records, and model how modern perturbations might tap—or short-circuit—this ancient thermostat. For everyone else, the takeaway is more reflective than prescriptive: Earth’s climate has been steered by subtle, long-running marine processes, and recognizing those invisible levers may be essential if we hope to understand what comes next.