Fossil soils long buried underground are rich in carbon, and may play a role in global warming, notes a new study from the University of Wisconsin Madison.
These soils once formed on the surface of the world, and became buried after millennia of accumulation of new material. This Brady soil is believed to be between 13,500 and 15,000 years old. The material formed on the Great Plains, mostly in Nebraska and Kansas. This fossil soil is now located over 20 feet underground, far deeper than most soil studies.
When frozen sheets from the last Ice Age began to recede from North America 10,000 years ago, silt-size sediment called loess blew through the region, carried by powerful winds. Soil was buried by this action, locking up vast quantities of carbon, in a layer roughly three feet thick.
"It was assumed that there was little carbon in deeper soils. Most studies are done in only the top [twelve inches]. Our study is showing that we are potentially grossly underestimating carbon in soils," Erika Marin-Spiotta, assistant professor of geography and the lead author of the new study, said.
Carbon likely layered into the soil after being released in vast forest fires. Marin-Spiotta believes the time following the Ice Age in the Midwest was marked by "an incredible amount of fire." Prairie grasses became abundant around the area with rising temperatures at the end of the widespread glaciation. These dry plants could have provided the fuel that fed the massive fires.
Plants trapped by the loess were locked underground before they could decompose. This also served to trap carbon underground. This process has also been observed elsewhere in the world. Loess accumulations in some areas of the Midwest and China have been measured at more than 150 feet deep.
Investigators believe that human activities are disrupting surface material, exposing such fossil soils to the environment. Once re-exposed to oxygen, weathering and microbes, the ancient material releases the carbon that was sequestered underground for 100 centuries.
Carbon is stored and released in the environment by a wide variety of mechanisms.
"Scientists have long known about the carbon storage capacity of soils, the potential for carbon sequestration, and that carbon in soil can be released to the atmosphere through microbial decomposition," Marin-Spiotta and her team wrote in press release announcing the study.
The National Science Foundation and the Wisconsin Alumni Research Foundation helped fund the study.
Investigation of fossil soils and the role they could play in the global carbon cycle was detailed in the journal Nature Geoscience.
