Deep, ancient soils, dating to between 15,000 and 13,500 years old, contain significant amounts of carbon and could contribute to climate change as the carbon is released through agriculture, mining and other human activities, according to a group of scientists headed by Dr Erika Marin-Spiotta from the University of Wisconsin-Madison.
An eroding bluff on the U.S. Great Plains reveals a buried, carbon-rich layer of fossil soil. Image credit: Jospeh Mason.
The team analyzed a soil known as the Brady soil, which was formed more than 13,500 years ago in what is now Nebraska, Kansas and other parts of the Great Plains. It lies up to 6.5 m below the present-day surface and was buried by a vast accumulation of windborne dust known as loess beginning about 10,000 years ago.
“There is a lot of carbon at depths where nobody is measuring. It was assumed that there was little carbon in deeper soils. Most studies are done in only the top 30 cm. Our study is showing that we are potentially grossly underestimating carbon in soils,” Dr Marin-Spiotta explained.
The region where the Brady soil formed was not glaciated, but underwent radical change as the Northern Hemisphere’s retreating glaciers sparked an abrupt shift in climate, including changes in vegetation and a regime of wildfire that contributed to carbon sequestration as the soil was rapidly buried by accumulating loess.
“The world was getting warmer during the time the Brady soil formed. Warm-season prairie grasses were increasing and their expansion on the landscape was almost certainly related to rising temperatures,” said Dr Marin-Spiotta, who is the first author of a paper published in the journal Nature Geoscience.
“Most of the carbon in the soil was fire derived or black carbon. It looks like there was an incredible amount of fire.”
The scientists also found organic matter from ancient plants that, thanks to the thick blanket of loess, had not fully decomposed.
Rapid burial helped isolate the soil from biological processes that would ordinarily break down carbon in the soil. Such buried soils are not unique to the Great Plains and occur worldwide.
The study suggests that fossil organic carbon in buried soils is widespread and, as humans increasingly disturb landscapes through a variety of activities, a potential contributor to climate change as carbon that had been locked away for thousands of years in arid and semiarid environments is reintroduced to the environment.
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Erika Marin-Spiotta et al. Long-term stabilization of deep soil carbon by fire and burial during early Holocene climate change. Nature Geoscience, published online May 25, 2014; doi: 10.1038/ngeo2169
