Investigation of oceanic black carbon uncovers mystery in global carbon cycle

Researchers studied black carbon in several rivers, including the Northern Dvina in Russia.

In the global carbon cycle, “black carbon” — decay-resistant carbon molecules altered by exposure to fire or combustion — has long been presumed to originate on land and work its way to the ocean via rivers and streams.

An unexpected finding published today in Nature Communications by a collaborative group of scientists, including Florida State University Associate Professor Robert Spencer in the Department of Earth, Ocean and Atmospheric Science, challenges that long-held assumption and introduces a tantalizing new mystery: If oceanic black carbon is significantly different from the black carbon found in rivers, where did it come from?

“The signature of oceanic dissolved black carbon is very different from that of riverine dissolved black carbon, raising a host of fundamental questions,” said Sasha Wagner, a Rensselaer Polytechnic Institute assistant professor of earth and environmental sciences and lead author of the research. “Are there other sources of dissolved black carbon? Is it being degraded away in rivers, sequestered in sediments or altered beyond recognition before it reaches the open ocean? Is what we’ve measured actually fire-derived?”

By calling the origin of oceanic black carbon into question, the new research compounds a puzzle that the group has been exploring. Radiocarbon dating shows dissolved black carbon in the deep oceans to be as much as 20,000 years old, while calculations estimate that rivers could replace the entire amount of oceanic dissolved black carbon in about 500 years. If so much dissolved black carbon has been exported from rivers to the oceans, apparently for millennia, why don’t researchers find more of it?

In exploring these questions, the group used a new technique to analyze black carbon. Sources of black carbon have traditionally been tracked using a ratio between molecular proxies. That method is unreliable in aquatic environments though because the ratio is altered when exposed to sunlight.

The new approach incorporates carbon isotopes — variants of carbon that contain differing numbers of neutrons — to discern different sources of dissolved black carbon. By performing stable carbon isotope analysis on the individual proxy molecules, it becomes possible to track terrestrial sources of black carbon as it moves from soils to the ocean.

The technique examined a broad question, comparing samples taken from the Atlantic and Pacific Oceans with large rivers including the Amazon, Mississippi, Congo and two Arctic Rivers. The results show that oceanic dissolved black carbon contains a significantly higher proportion of carbon-13 (an isotope of carbon-12 that has one additional neutron) than dissolved black carbon found in global rivers.

“Riverine dissolved black carbon isn’t reaching the oceans, and that raises a lot of exciting alternatives we should explore,” Spencer said. “We already have some working possibilities as to this disconnect between riverine and oceanic black carbon values. As always, finding something unexpected in a scientific study is certainly going to stimulate a lot of future research.”

This research was supported by the National Science Foundation. In addition to Spencer and Wagner, the team included Jay Brandes and Kun Ma from the University of Georgia’s Skidaway Institute of Oceanography, Sarah Z. Rosengard from the University of British Columbia, Jose Mauro S. Moura from the Federal University of Western Para in Brazil and Aron Stubbins from Northeastern University.