Reaction buries carbon in the ocean faster than we thought

New research shows organic matter sulfurization, a process previously thought to occur over tens of thousands of years, can actually occur in a matter of hours or days. This change in timescales may have sizable implications for how scientists understand the past and future of the Earth’s climate.

About 94 million years ago, something happened that led to an unusually high amount of organic material being preserved in oceans around the world. The burial of this organic carbon—over about a half million years—pulled an enormous amount of CO2 out of the atmosphere and had a major impact on Earth’s climate.

The basic assumption has been that some combination of super-giant algae blooms and low levels of oxygen in the ocean allowed the organic carbon from these blooms to be preserved in sediments. But according to the new research, which appears in Nature Communications, there is another process that preserved this carbon: organic matter sulfurization reactions, which can occur on the timescale of just hours to days.

“We can even induce them in 24 hours in the lab,” says the study’s lead author Morgan Reed Raven, who conducted the research as a fellow at Washington University in St. Louis and is now an assistant professor in earth science at the University of California, Santa Barbara.

A ‘major mechanism’

The finding focused on a layer of sediment in the south of France from that time period, about 94 million years ago, known as the Ocean Anoxic Event 2 (OAE2). The site is more typical of other places and times on the planet than sites where many previous studies focused. For this reason, Raven says, “There are all sorts of places on Earth today where rapid sulfurization is on the table as a major mechanism for impacting how much carbon is preserved.”

The potential widespread nature of sulfurization as a manner of carbon preservation means that researchers may need to reevaluate our understanding of the history of oxygen in the ocean.

“People have known about sulfurization, but they thought it was slow and not that important environmentally.”

The amount of sedimentary carbon has acted as a kind of proxy for oxygen levels in the ocean. The more carbon in the sediment, the thinking went, the less oxygen was in the ocean. (If there’s no oxygen, there are no microbes or animals to eat organic material, so when that material dies, it accumulates in the ocean floor.)

“That is probably still correct,” says study coauthor David Fike, professor of earth and planetary sciences at Washington University. “But Morgan showed this other process. Even with oxygen in the system, if there is sulfur in the organic matter, nothing can easily eat it.” The matter, he says, will still be preserved in the sediment.

“People have known about sulfurization, but they thought it was slow and not that important environmentally,” Fike says. “What Morgan has been able to show is that it is a much more efficient and powerful way to lock up matter, to trap the organics.”

New info, better models

Going forward, he says, this work highlights an additional process that will be important to include in climate modeling.

“We hope that through this paper and others,” Fike says, “modelers will see this as an important process to incorporate into their systems.”

Raven has done research in a variety of environments, the results of which will be published in forthcoming papers.

“The hypotheses that came out of this paper do seem to be holding up,” she says. “And for understanding the formation of many extremely organic carbon-rich sediments, these rapid sulfur reactions are where the story is at.”

Source: Washington University in St. Louis