Sea level shifts tied to Earth's long-term carbon control

Scientists have traced a long-running climate feedback to changes in sea level, ocean nutrients and carbon burial, according to research described by Syracuse University. The study, published in Proceedings of the National Academy of Sciences, offers an explanation for how atmospheric carbon dioxide declined as Earth cooled over the past 60 million years.

The research focuses on phosphate, a form of phosphorus that marine organisms need to grow. Syracuse University said the team found that sea level changes affected how much phosphate reached the open ocean, which in turn shaped marine productivity, oxygen levels and the amount of organic carbon locked away in seafloor sediments.

Ros Rickaby of the University of Oxford led the study with Thomas J. Wood, Zunli Lu of Syracuse University and Christian J. Bjerrum of the University of Copenhagen, according to the journal reference. Rickaby said in a Syracuse University department report that scientists knew carbon dioxide fell as Earth cooled, but had little evidence for where that carbon went; the results point to a larger role for organic carbon burial in marine sediments than researchers had previously recognized.

How sea level changed the feedback

According to Syracuse University, high sea levels flooded broad continental shelves and allowed those shallow areas to trap phosphate in coastal sediments. With less phosphate available offshore, marine life became less productive, less organic matter sank to the seabed, and less carbon was buried.

The university said that under those conditions, ocean waters became more oxygen-rich and atmospheric carbon dioxide built up. That pattern is one reason the researchers connect very high sea levels with warmer long-term climate conditions.

When sea levels fell, the process shifted, according to the study summary. Smaller shelf areas allowed more phosphate to reach open waters, feeding more marine growth; after organisms died, their remains sank and decomposition used oxygen in the surrounding water.

Syracuse University said this helped create low-oxygen zones that could spread over organic-rich shelf sediments. Those low-oxygen waters caused sediments to release more phosphate, strengthening marine productivity and increasing the burial of organic carbon, which removed carbon from the ocean-atmosphere system.

Lu said in the Syracuse University report that Bjerrum had modeled links among sea level, oxygen and phosphate about two decades earlier, and that the new work assembled geological records needed to test the idea.

A narrow range for strong carbon burial

The researchers found the feedback was strongest when sea level stood about 10 to 40 meters above today’s level, according to Syracuse University. In that range, low-oxygen waters overlapped with carbon-rich continental shelf sediments, allowing high rates of organic carbon burial over long periods.

The team compared its proposed mechanism with 60 million years of geological records, including carbon isotopes, phosphorus accumulation in deep-sea sediments and iodine-to-calcium measurements used to reconstruct ancient ocean oxygen levels, according to Syracuse University.

Lu’s laboratory at Syracuse University carried out the iodine-to-calcium analyses, the university said. The method uses chemical signals preserved in foraminifera, microscopic marine organisms found in seafloor sediments, and the samples were measured with a National Science Foundation-funded mass spectrometer at Syracuse.

The Eocene epoch, about 56 million to 34 million years ago, fits the pattern described by the researchers, according to Syracuse University. During that interval, very high sea levels flooded shelves, phosphate stayed trapped in shallow sediments, the open ocean had fewer nutrients, less carbon was buried and Earth remained warm.

The researchers also propose that carbon-burial zones narrowed over geological time as low-oxygen waters moved deeper, according to Syracuse University. That change may have reduced swings in carbon burial and atmospheric carbon dioxide, making the climate system more stable over long timescales.

This story draws on original reporting from ScienceDaily.