New technique unexpectedly finds that black carbon in rivers and oceans differs significantly.
In understanding 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 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 research published in Nature Communications actually compounds a puzzle that Wagner 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 moving downriver to the ocean, apparently for millennia, why don’t researchers find more of it?
In exploring such questions, Wagner developed a new technique for analyzing black carbon. Sources of black carbon have traditionally been tracked using a ratio between molecular proxies. But, given that the ratio is easily altered with exposure to sunlight, the method is unreliable when used in aquatic environments.
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