US: Light reflected by the Amazon rainforest’s vegetation could help an orbiting satellite find the elusive fertile patches of soil known as terra preta, or ‘black earth’, that mark archaeological sites where pre-Columbian populations settled.
Finding these rich patches of earth has been a challenge. They’re sprinkled throughout the enormous Amazon basin, hidden beneath an impenetrable forest, and embedded in a land with few roads.
So a team of scientists from the University of New Hampshire is testing whether satellite measurements of the light reflected by tree canopies could help researchers panning for black soil gold.
The black earth patches, otherwise invisible from above, mark the locations of pre-Columbian archaeological sites, remnants of a civilisation that lived in the Amazon for thousands of years. Wherever settlements sprang up, decades of discarded fish and animal bones, charcoal and other waste transformed the typically yellow and nutrient-poor Amazonian soil into nuggets of black gold.
“They’re super enriched with artifacts, charcoal – it’s like a giant compost,” paleoecologist Crystal McMichael of the University of New Hampshire said at the American Geophysical Union conference, where she presented the work. The sites vary in age from about 500 years to more than 2,000 years old. “They’ve retained nutrients for that long, which is incredible,” she says.
McMichael and her colleagues studied whether remote sensing could be used to help find archaeological sites. First, they assembled a database of known black earth and typical soil sites – about 2,900 of them — illustrated by the black and white circles in the map.
Then, they looked at data returned by the Hyperion spectrometer, which rides aboard the earth-orbiting satellite EO-1, run by NASA and the USGS.
Hyperion scans tree canopies at a range of wavelengths. The team sorted through about 1,600 scanned areas, removing those blocked by clouds or with improper location tags. Then, they found the scans that contained patches of known black and normal soils. In those data, they saw differences in the vegetation reflectance – five wavelengths, in particular, were diagnostic of the different soil types.
“You have a different set of species that grow on those super-enriched soils as compared to the ones that grow on those crappy soils,” McMichael said. “You can use it to map species distributions or target archaeological sites.”
Next, McMichael and colleagues hope to confirm the result using data from other orbiters, and test their ability to find previously unknown patches of black earth. If confirmed, remote spectral imaging could help archaeologists find these remnants of an ancient civilisation.