Ocean sediments contain a record of past vegetation fires, called biomass burning by scientists -- and this record shows much more past burning, at least in Africa during the Pleistocene era, than researchers expected.
National Science Foundation (NSF)-funded marine geologists David Verardo and William Ruddiman of the University of Virginia in Charlottesville have found in their research on ocean sediments the first detailed marine record of late Pleistocene era fires on land. Their paper, "Late Pleistocene charcoal in tropical Atlantic deep-sea sediments: Climatic and geochemical significance," appears in the September issue of the journal GEOLOGY.
"Biomass burning, a result of fires started by lightning strikes, is important for its relationship to climate (drier climates have more burning) and its immense effect on vegetation, and therefore land ecosystems," says Connie Sancetta, program director in NSF's marine geology and geophysics program, which funded Verardo's research along with NSF's climate dynamics program.
The burning of trees and grasses on land produces charred particles, or charcoal. Charcoal may then be transported long distances by winds and rivers to coastal and ocean environments, where it's then preserved in ocean sediments. Explains Verardo, "Charcoal may be swept aloft in plumes rising from active fires, and transported by prevailing winds from the source area to the ocean within days to weeks of its initial formation. It eventually sinks and becomes part of the sediments at the bottom of the sea."
Verardo studied such sediments by analyzing a core taken from the bottom of the eastern tropical Atlantic Ocean. "It was full, much to my surprise, of a large amount of charcoal," he says. "Given the great distance to land and the regional slope of the sea floor, the charcoal in this core must have been brought there by winds." The charcoal is a mixture of particles from both rainforest and savanna trees, as well as grasslands. Research on modern-day air mass routes for the eastern tropical Atlantic region indicates that the sediment in the core lies below several large-scale circulation pathways originating in Africa.
Says Sancetta, "Verardo's work is important because he has uncovered a record of burning that goes beyond historical records, so we can study natural burning before human influence." This in turn will help scientists determine whether human-induced burning has different effects than natural burning, and how burning is related to changes in ecology, and perhaps evolution, on longer time scales. "Another implication of Verardo's work," adds Sancetta, "is that if a lot of the carbon [charcoal] in ocean sediments is terrestrial in origin, then it isn't marine in origin. And scientists have been studying the marine record as a major place to find out about past carbon dioxide changes. If they've been making the wrong assumptions about how much marine carbon is involved, then their estimates about carbon dioxide in the oceans may be wrong."
Current models have not been able to determine exactly how the total amount of carbon on earth is distributed. According to Verardo, "This new data sheds much-needed light on where the 'missing' carbon may lie: in ocean-bottom sediments laden with charcoal from fires on land."