Mysterious patches of black, fertile soil are scattered across the vast Amazon Rainforest. They stick out like a sore thumb given that the traditional soil that dominates the region is red.
Archaeologists and scientists have long thought that the soil is manmade, though had no way to prove it.
In recent months, however, extensive fieldwork and studies in the Amazon region have turned up compelling evidence to suggest that the suspicions are true.
It could, they say, change everything we know about not only the region and its history but the very organic matter that is vital to humans’ well-being.
Terra preta, also known as ‘Amazonian dark earth’, is found in many regions but in especially dense quantities around former human habitations.
Multiple researchers in a joint study from the Massachusetts Institute of Technology (MIT), the University of Florida, and the Federal University of Santa Catarina combined soil analyses, ethnographic observations, and interviews with modern Indigenous communities to conclude that terra preta was purposefully created by ancient Amazonians.
“This could change everything,” Lucas Silva, an environmental scientist at the University of Oregon who was not involved in the new study, told the journal, Science.
Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric and Planetary Sciences at MIT, added: “We argue here that people played a role in creating dark earth, and intentionally modified the ancient environment to make it a better place for human populations.”
The dark earth contains considerable amounts of stored carbon that has been collected over hundreds to thousands of years. With each generation enriching the soil with food scraps, charcoal, and waste, it became more concentrated in natural resources.
What has scientists excited is the unintentional carbon sequestration. It could, they say, be used as a way to mitigate the negative effects of climate change.
“Maybe we could adapt some of their indigenous strategies on a larger scale, to lock up carbon in soil, in ways that we now know would stay there for a long time,” wrote Samuel Goldberg, co-author of the study, in the journal, Science Advances.
Crops grow much better on terra preta because the dark soil is rich in phosphorus, nitrogen, and calcium content.
It is ordinarily found near archaeological sites and contains charcoal, organic matter from food remains like fish and animal bones, as well as artefacts such as pottery shards. It all hints that ancient civilisations were intentionally adding to and creating the rich soil, which, if true, is itself a breakthrough discovery.
Morgan Schmidt, an archaeologist and geographer at the Federal University of Santa Catarina, and his team looked at soil in the Kuikuro Indigenous Territory, on the upper Xingu River in the southeastern Amazon of Brazil.
There, they analysed soil from four archaeological sites and two historic villages occupied from 1973 to 1983. They also looked at one modern village, known as Kuikuro II. Radiocarbon dating of the lands found the oldest sample to be 5,000 years old, while other samples ranged from 300 to 1,000 years old.
Collecting soil from mounds bordering the ancient and historic villages, the scientists compared it with terra preta and found that soil from the residential areas contained more than double the organic carbon and was less acidic, making it more fertile. When they analysed soil from Kuikuro II, the team found a similar pattern.
Interviews with villages turned up more interesting information. Villagers referred to the soil as “eegepe” and described the process in which they created and cultivated the rich soil to improve its fertility.
One farmer said: “Charcoal and ash we sweep, gather it up, and then throw it where we will plant, to turn into beautiful eegepe, there we can plant sweet potatoes. When you plant where there is no eegepe, the soil is weak. That is why we throw the ash, [cassava] peelings, and… pulp.”
The results will prove vital in better understanding not only the rich cultural and historical practices of the people in the Amazon basin but also the future of humanity.
The study concludes that “modern sustainable agriculture and climate change mitigation efforts, inspired by the persistent fertility of ancient dark earth, can draw on traditional methods practised to this day by Indigenous Amazonians.”
