Scientists have uncovered evidence of a long term natural climate regulation mechanism that may have helped keep Earth temperatures stable for the past 60 million years by controlling how much carbon dioxide remained in the atmosphere.
The study published in the Proceedings of the National Academy of Sciences (PNAS), suggests that interactions between sea level, ocean oxygen, phosphate availability and carbon burial acted as a natural “thermostat” that moderated Earth’s climate over millions of years.
Researchers from the University of Oxford and Syracuse University found that changing sea levels influenced the amount of phosphate available in the oceans. Phosphate is an essential nutrient for marine organisms, and its availability affected marine productivity and the burial of organic carbon in seafloor sediments.
According to the researchers, higher sea levels caused continental shelves to trap phosphate in shallow sediments, reducing the nutrient supply to the open ocean. This limited marine growth, decreased carbon burial and allowed more carbon dioxide to remain in the atmosphere, contributing to a warmer climate.
When sea levels fell, more phosphate entered the ocean, boosting marine productivity. As marine organisms died and sank to the seafloor, their decomposition reduced oxygen levels in the water, creating oxygen poor zones that released even more phosphate from sediments. This feedback increased the burial of organic carbon, removing more carbon dioxide from the atmosphere and promoting cooler global temperatures.
The study found that this process worked most effectively when sea levels were between 10 and 40 metres above present day levels. Within this range, oxygen poor waters overlapped with organic rich continental shelf sediments, allowing carbon burial to continue efficiently over millions of years.
Researchers tested the hypothesis using geological records spanning the past 60 million years, including carbon isotope data, phosphorus accumulation in deep sea sediments and oxygen measurements derived from microscopic marine fossils.
The findings also help explain why Earth remained exceptionally warm during the Eocene Epoch, around 56 to 34 million years ago. During that period, very high sea levels trapped phosphate on continental shelves, weakening the carbon burial process and allowing atmospheric carbon dioxide to remain elevated.
Scientists say the research provides new insight into Earth’s long term carbon cycle and highlights the important role of marine sediments in regulating atmospheric carbon dioxide over geological timescales. They note that the study examines natural climate processes operating over millions of years and does not alter the scientific understanding that current global warming is primarily driven by human caused greenhouse gas emissions.