Earth's climate has remained habitable across deep time despite shifts in solar output and other pressures. A peer-reviewed study has now pinpointed one reason why: a marine sedimentary feedback loop that ties sea-level changes to the burial of organic carbon in ocean sediments.
The mechanism operated throughout the Cenozoic era, roughly the last 60 million years. It acted as a rectifier for perturbations in the carbon cycle, helping to stabilise atmospheric carbon dioxide and oxygen levels over multimillion-year timescales.
Researchers examined high-resolution records of carbon isotopes, phosphorus accumulation in deep-sea sediments, and iodine-to-calcium ratios used as a proxy for ancient ocean oxygen levels. These proxies trace how sea-level fluctuations influenced phosphate availability, marine productivity and the extent of low-oxygen zones.
How sea level shapes ocean chemistry
When sea levels stood higher, phosphate became trapped on flooded continental shelves. The open ocean received less of this key nutrient. Primary productivity fell, and the water column remained well oxygenated. Organic carbon burial rates stayed low.
At lower sea levels the opposite occurred. Shelf areas suitable for efficient burial of organic matter and phosphate shrank. Phosphate concentrations rose in the open ocean. Productivity increased. Low-oxygen zones developed in the water column, favouring the preservation and burial of organic carbon.
During the Eocene hothouse climate, organic carbon burial was suppressed in association with an oxygenated water column and low phosphate levels. In contrast, the Neogene saw multimillion-year pulses of enhanced organic carbon burial at intermediate sea-level highs of around 10 to 40 metres above present levels. These pulses were linked to redox recycling of phosphate within oxygen minimum zones.