For tens of millions of years, Earth's climate may have been steadied by a narrow set of ocean conditions.
A new study links sea-level changes to the amount of carbon buried in marine sediments, providing another clue to how the planet's natural temperature regulation worked.
What happened?
According to SciTechDaily, the researchers found that ocean carbon burial was greatest when sea level stayed within a fairly limited band — about 33 to 131 feet (10 to 40 meters) above today's level. By influencing how much carbon dioxide remained in the atmosphere, that range may have helped keep the climate stable for long periods.
Published in Proceedings of the National Academy of Sciences, the study followed interactions among sea level, phosphate, ocean oxygen, and carbon burial across the past 60 million years.
Because phosphate is an essential nutrient for marine organisms, it plays a central role in the findings.
The authors included lead author Rosalind E. M. Rickaby, a professor of Earth sciences at the University of Oxford, and Syracuse University Earth and environmental sciences professor Zunli Lu.
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Higher seas meant more phosphate was held on continental shelves instead of reaching offshore waters.
With less nutrient supply, marine growth declined, and less carbon sank to the seafloor to be buried.
Lower sea levels shifted that balance. Under those conditions, more phosphate reached the ocean, supporting more marine life and leading to greater storage of organic carbon in sediments.
Why does it matter?
Scientists have known that atmospheric carbon dioxide fell as Earth cooled over geologic time, but the destination of that carbon has been much less clear. These results suggest the ocean floor was a far more important carbon reservoir than previously thought.
Rickaby said, "We know that atmospheric carbon dioxide decreased substantially as Earth cooled over the last 60 million years, but we have had remarkably little understanding of where that carbon ended up."
Rather than pointing to one simple control, the study indicates that climate stability relied on multiple feedbacks, with sea level and ocean chemistry needing to line up in the right way.
Peak carbon burial showed up only within a small "sweet spot," a zone where oxygen-poor waters reached organic-rich shelf sediments and intensified the feedback.
It gives scientists a better understanding of how natural carbon cycling works over long timescales.
While these processes move far too slowly to offset modern human-caused pollution, they can help researchers build better models for how climate systems respond to change.
What's being done?
Scientists are refining this picture with stronger tools for interpreting ancient oceans. In this study, one major technique was an iodine-to-calcium proxy that uses chemistry preserved in foraminifera, microscopic organisms found in seafloor sediments, to estimate oxygen levels in the distant past.
Lu's lab at Syracuse University performed those measurements with National Science Foundation support, and the researchers compared the resulting pattern with carbon isotopes and phosphorus accumulation in deep-sea sediments throughout the geologic record.
The study also extends earlier work from Lu's lab, including a January paper in Nature Geoscience that used the same method to reconstruct oxygen levels in much older tropical oceans.
"Our results suggest that enhanced burial of organic carbon in marine sediments played a much more important role than was previously appreciated," Rickaby said.
And as Lu put it, "We finally pieced together the geologic records necessary to test this hypothesis."
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