North American Coasts Are Absorbing Large Amounts of Carbon

New Jersey shore. (Peter Roome, via flickr)

Coastal waters play an important role in the carbon cycle by absorbing carbon into sediments or transferring it to the open ocean, a new study confirms. The study used data from the eastern coast of North America to show that of the carbon entering coastal waters from rivers and the atmosphere in that region, about 20 percent is absorbed into wetland soils or nearshore sediments. About 80 percent flows on to the open ocean, which takes in much of the remainder. The results were reported in the journal Global Biogeochemical Cycles.

A team of scientists led by Raymond Najjar, professor of oceanography at Penn State University examined data tracking flows of organic and inorganic carbon into and out of coastal waters from the southern tip of Nova Scotia to the southern tip of Florida. Cycling of carbon in the open ocean and on land has been the focus of much research, but coastal waters, which bridge the two areas, have “fallen through the cracks,” Najjar said. The new research helps to establish how coastal waters influence atmospheric carbon dioxide levels and, in turn, climate. “It’s basically telling us that the coasts play a significant role in the global carbon cycle,” said coauthor Wade McGillis, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory.

The research involved 30 scientists from 20 institutions who specialize in various aspects of coastal oceanography. The team gathered data from dozens of published studies to quantify how much carbon enters, exits and is transformed within the region’s coastal waters. The study subdivided the waters into tidal wetlands, estuaries and continental shelves. Tidal wetlands and estuaries, despite being the smallest ecosystems in the study at 2.4 percent and 9 percent of the area, respectively, absorbed the majority of the carbon. Tidal wetlands took in 42 percent  and estuaries 38 percent, for a total of 80 percent of carbon burial in coastal waters.

Carbon burial usually starts with photosynthesis, through which carbon dioxide is converted to organic material in the form of plankton, marsh grass, mangroves, or sea grass. Eventually that material dies and settles to the bottom. But continental shelf waters are deeper than tidal wetlands and estuaries, so there is more time for bacteria and other animals to consume this dead matter before it can get buried.

Carbon burial is an important metric when it comes to predicting future atmospheric carbon dioxide levels because, once carbon is in the sediments, it has the potential to remain there and not contribute to the greenhouse effect. However, the fragility of the coastal zone, Najjar said, could be cause for concern. “As sea level continues to rise and disturb the coasts, some of the buried carbon could be respired and released to the atmosphere in the form of carbon dioxide,” he said. McGillis said the study shows that the amount of carbon absorbed by coasts already can vary widely from year to year, depending conditions on the land.

The study is the first synthesis to develop a cohesive view of the carbon cycle in a large coastal region. Other such studies are underway for the Pacific and Gulf of Mexico coasts.

NASA and the U.S. National Science Foundation funded the research.

(This story was adapted from a press release by Penn State University.)

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