How Much Carbon is Stored in Ancient Amazonian Anthrosols?

Amazonian dark earth (ADE) has received global attention for its remarkable fertility and high carbon content in a region known for low-nutrient soils. ADE is an anthropic soil produced by human activities, including refuse disposal and crop cultivation, that concentrated charcoal, organic matter and nutrients. Evidence for modified soils reaches back to the terminal Pleistocene in the Amazon while the formation of ADE began by the mid-Holocene and appears to have increased substantially around 3000 years before present. ADE may be a major reservoir of soil carbon, but it is not known how widely distributed it is across the Amazon or how much total carbon it contains. We investigate the extent and composition of ADE in collaboration with the Kuikuro, an indigenous community in the Upper Xingu River Basin, Southeastern Amazon. Recent surveys combining satellite remote sensing with observations from soil pits suggest that modified soils in the Upper Xingu are much more extensive than previously thought. Areas of altered forest visible in satellite images in and around known archaeological sites appear to correlate well with known distributions of darkened and enriched anthropic soils. We test this correlation by quantifying the total carbon and soil fertility across the landscape in the study area. Combining ground truth data from systematic soil sampling with satellite images and machine learning techniques, we create predictive models to estimate the distribution and total carbon content of modified soils in the study area. In the unique context of the Upper Xingu, we explore the formation processes of ADE in a place where it is still being created, while building upon a solid foundation of archaeological research and partnership with the local indigenous community. In ongoing research, we plan to take what we learn from the Upper Xingu and apply it to other regions of the Amazon, using the methods to benefit local communities interested in mapping their cultural landscapes while gaining a better understanding of the magnitude and extent of anthropic soils. Determining the extent to which past human activities modified soils and sequestered carbon in Amazonia offers valuable insights for our efforts to understand human interactions with the environment and how to manage it.