Jun 13 2019
Many people might recall the Earth’s carbon cycle from their grade school science class. The carbon cycle occurs something like this: plants release oxygen into the atmosphere after taking up carbon dioxide and changing it into organic carbon.
A new study finds clay minerals, such as those at the bottom of rivers and oceans, can bind with bits of decomposing plants and phytoplankton in a natural carbon-storing process. (Image credit: MIT)
This oxygen is inhaled and carbon dioxide is exhaled by complex life forms, including humans. When pathogens consume the decaying plants, they also ingest the carbon present within them, eventually converting and releasing this element back into the atmosphere in the form of carbon dioxide gas. The cycle thus continues.
The huge majority of the Earth’s carbon loops continuously via this cycle, fueled by respiration and photosynthesis. However, there is a “leak” in the cycle through which a small amount of organic carbon is continually escaping, the cause of which has mostly remained a mystery. Researchers are aware that through this leak, a small amount of carbon is continuously locked away and kept in the form of rock for a countless number of years.
Currently, scientists from MIT and elsewhere have come across evidence that might account for the slow and steady escape route of carbon.
In a paper recently reported in the journal Nature, the researchers have reported that a mechanism which they have dubbed “mineral protection” is mainly responsible for causing the organic carbon to leak out of the carbon cycle. In this kind of process, carbon, in the form of decayed bits of phytoplankton and plant material, gloms onto clay and other mineral particles, for example, at the bottom of an ocean or river, and is conserved in the form of sediments and, eventually, rock.
The presence of oxygen on Earth could be explained by the mineral protection mechanism. When something makes carbon to leak out of the carbon cycle, more amounts of oxygen will be left to build up in the atmosphere.
Fundamentally, this tiny leak is one reason why we exist It’s what allows oxygen to accumulate over geologic time, and it’s why aerobic organisms evolved, and it has everything to do with the history of life on the planet.
Daniel Rothman, Professor of Geophysics, Department of Earth, Atmospheric and Planetary Sciences, MIT
Co-authors of the paper include Jordon Hemingway, who headed the work as a graduate student at MIT and the Woods Hole Oceanographic Institution and is currently a postdoc at Harvard University, together with Sarah Rosengard, Katherine Grant, Valier Galy, Louis Derry, and Timothy Eglinton.
Burning dirt
Researchers have considered two major prospects for the way carbon is leaking out of the planet’s carbon cycle. The first possibility involves “selectivity,” the notion being that certain types of organic matter, owing to their molecular makeup, may be more difficult to break down when compared to others. On the basis of this idea, the carbon that is not ingested, and thus leaks out of the carbon cycle, has been “selected” to do so, depending on the molecular structure of the initial organic matter.
The second possibility has to do with “accessibility,” the idea being that some organic matter has a tendency to leak out of the Earth’s carbon cycle because some secondary process has made it inaccessible for consumption.
According to a few researchers, mineral protection could constitute that secondary process—interactions between clay-based minerals and organic carbon, with the former binding the two together in an unconsumable and inaccessible form.
In order to test which kind of mechanisms better explains the carbon leak of Earth, Hemingway examined sediment samples that were obtained from across the globe, each containing minerals and organic matter from many different coastal environments and rivers. In case mineral preservation accounts for locking away and conserving carbon over geologic timescales, then organic carbon attached to clay minerals should be able to last longer in the environment when compared to the unbound carbon, proposed Hemingway. Such carbon will be resistant to degradation by scavenging pathogens, or even other forces like extreme heat.
The investigators tested this concept by first burning each sediment sample and then determining the type and amount of organic carbon that continued to be there as they heated up the sample at increasingly higher temperatures. The team achieved this by utilizing a device that was developed by Hemingway as part of his PhD dissertation.
It’s been hypothesized that organic matter that sticks to mineral surfaces will stick around longer in the environment. But there was never a tool to directly quantify that.
Jordon Hemingway, Postdoctoral Fellow, Harvard University
“Beating up a natural process”
They eventually discovered that the organic matter that endured the highest temperatures and lasted the longest was attached to clay minerals. Most significantly, in a discovery that went against the notion of selectivity, the molecular structure of that organic matter no longer mattered—it was preserved as long as it was attached to clay.
The outcomes indicate accessibility, and mineral preservation to be more specific, as the major mechanism for the carbon’s escape route through the Earth’s cycle. To put this in simple terms, clay minerals all over the world are steadily and gradually drawing down small quantities of carbon, and preserving it for thousands of years.
“It’s this clay-bound protection that seems to be the mechanism, and it seems to be a globally coherent phenomenon,” Hemingways stated. “It’s a slow leak happening all the time, everywhere. And when you integrate that over geologic timescales, it becomes a really important sink of carbon.”
According to the researchers, mineral protection has made it viable to bury and store huge reservoirs of carbon in the Earth, and some of this mineral has been pressed and heated into petroleum over an infinite number of years.
At the planet’s geologic pace, this carbon conserved in rocks ultimately resurfaces via mountain uplift and slowly erodes, discharging the carbon dioxide gas back into the atmosphere very slowly.
“What we do today with fossil fuel burning is speeding up this natural process,” Rothman stated. “We’re getting it out of the ground and burning it right away, and we’re changing the rate at which the carbon that was leaked out is being returned to the system, by a couple orders of magnitude.”
Is it possible to harvest mineral preservation to sequester even more amounts of carbon, in an attempt to mitigate climate change induced by fossil fuels?
If we magically had the ability to take a fraction of organic matter in rivers or oceans and attach it to a mineral to hold onto it for 1,000 years, it could have some advantages. That’s not the focus of this study. But the longer soils can lock up organic matter, the slower their return to the atmosphere. You can imagine if you could slow that return process down just a little bit, it could make a big difference over 10 to 100 years.
Daniel Rothman, Professor of Geophysics, Department of Earth, Atmospheric and Planetary Sciences, MIT
The study was partly supported by NASA and the National Science Foundation.