Little black carbon reaches ocean floor, study finds

Just a fraction of the carbon that finds its way into Earth’s oceans–the black soot and charcoal residue of fires–stays there for thousands of years. A first-of-its-kind analysis by UC Irvine, Rice University and the University of South Carolina also revealed how some black carbon breaks away and hitches a ride to the ocean floor on passing particles. “Our previous work showed that the black carbon in ocean sediments is ancient,” said lead investigator Ellen Druffel, the Fred Kavli Chair in Earth System Science at UC Irvine. “It’s anywhere from 2,000 to 5,500 years older than the organic carbon in the same sediments. That means that it either stays dissolved in the water for a long time before it’s deposited or stays trapped somewhere else–like the soil–for thousands of years before it enters the ocean. This new study offers the most complete picture yet of how black carbon finds its way into deep ocean sediments.” Scientific interest in black carbon is high, both because it’s a potential contributor to atmospheric climate change and because of its presence in natural “sinks” that store carbon out of harm’s way. “The environmental fate of black carbon is not well-understood,” said study co-author Caroline Masiello, a Rice biogeochemist who began studying the substance with Druffel in the 1990s. “Though we don’t fully understand how nature breaks it down and recycles it into the deep Earth, we know that it must, because otherwise we would be knee-deep in it.” The researchers used radiocarbon dating and other techniques to examine black carbon buried in seafloor sediments in the northeast Pacific Ocean about 20,000 years ago. The study’s lead author, graduate student Alysha Coppola of UC Irvine, also conducted extensive chemical analyses of the types and amounts of black carbon and its fate. To the researchers’ surprise, they discovered that most black carbon tends to stay dissolved in seawater and only occasionally gets snagged by a passing marine particle drifting to the seafloor. “We found that black carbon is taken up by particles that are produced in the water column,” Coppola said. “One example would be fecal pellets that are excreted by organisms. Another is a sticky ‘marine snow’ that falls slowly downward to the bottom and gets buried in sediments.” Lori Ziolkowski of the University of South Carolina also contributed to the study.