Deeply buried organic soil rich in ancient carbon from fire ash and vegetation poses a previously unrecognised greenhouse gas threat, say scientists.
Disturbance of the soils through erosion, agriculture, deforestation, mining and other human activities risks releasing the carbon into the atmosphere, they claim.
Formed thousands of years ago, the soil layers lie many metres below the present day earth’s surface and until now have largely been ignored.
US lead researcher Erika Marin-Spiotta, from the University of Wisconsin-Madison, said: “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 centimetres.
“Our study is showing that we are potentially grossly underestimating carbon in soils.”
The scientists studied Brady soil, created between 13,500 and 15,000 years ago in what is now Nebraska, Kansas and other parts of the US Great Plains.
It was buried to a depth of more than six metres by a vast accumulation of wind-born dust, known as loess, as glaciers once covering much of North America began to retreat around 10,000 years ago.
Numerous wildfires and the rapid burial of dead vegetation contributed to the soil’s high carbon content.
“Most of the carbon was fire derived or black carbon,” said Marin-Spiotta. “It looks like there was an incredible amount of fire.”
The team stressed that similar soils are likely to be found in many other parts of the world.
There is a lot of carbon at depths where nobody is measuring
As humans increasingly disturb the land there was a danger that after being locked away for thousands of years, the carbon could be released to add to global warming.
The Brady soil consists of a three-foot thick dark ribbon providing a snapshot of an environment undergoing major and rapid change, said the scientists.
The retreat of the glaciers signalled the start of a warmer era with loess dust blanketing large swaths of the ancient landscape.
In some areas, including parts of the mid-western US and China, the dust deposits could be more than 50 metres deep.
The research is reported in the latest edition of the journal Nature Geoscience.