A new hypothesis for the origin of Amazonian Dark Earths

Lucas C R Silva,Scott D Young,Roberto Ventura Santos,Martin R Broadley,Lauren Hendricks,Barbara Bomfim,Vander De Freitas Melo,Jamie L Wright,Antônio Carlos Vargas Motta,Gilvan Coimbra Martins,Daniel G Gavin,Rodrigo Studart Corrêa,Julierme Zimmer Barbosa,Aleksander Westphal Muniz

doi:10.1038/s41467-020-20184-2

Abstract

Amazonian Dark Earths (ADEs) are unusually fertile soils characterised by elevated concentrations of microscopic charcoal particles, which confer their distinctive colouration. Frequent occurrences of pre-Columbian artefacts at ADE sites led to their ubiquitous classification as Anthrosols (soils of anthropic origin). However, it remains unclear how indigenous peoples created areas of high fertility in one of the most nutrient-impoverished environments on Earth. Here, we report new data from a well-studied ADE site in the Brazilian Amazon, which compel us to reconsider its anthropic origin. The amounts of phosphorus and calcium—two of the least abundant macronutrients in the region—are orders of magnitude higher in ADE profiles than in the surrounding soil. The elevated levels of phosphorus and calcium, which are often interpreted as evidence of human activity at other sites, correlate spatially with trace elements that indicate exogenous mineral sources rather than in situ deposition. Stable isotope ratios of neodymium, strontium, and radiocarbon activity of microcharcoal particles also indicate exogenous inputs from alluvial deposition of carbon and mineral elements to ADE profiles, beginning several thousands of years before the earliest evidence of soil management for plant cultivation in the region. Our data suggest that indigenous peoples harnessed natural processes of landscape formation, which led to the unique properties of ADEs, but were not responsible for their genesis. If corroborated elsewhere, this hypothesis would transform our understanding of human influence in Amazonia, opening new frontiers for the sustainable use of tropical landscapes going forward.

Highlights

Amazonian Dark Earths (ADEs) are unusually fertile soils characterised by elevated concentrations of microscopic charcoal particles, which confer their distinctive colouration
Recent findings suggest that domestication of native plant species dates back to >10,000 years ago in western Amazonia[12], with the emergence of complex societies that relied on soil management for agriculture occurring
Our results show that large sedentary populations would have had to manage soils thousands of years prior to the emergence of agriculture in the region or, more likely, that indigenous peoples used their knowledge to identify and preferentially settle areas of exceptionally high fertility before the onset of intensive land use in central Amazonia

Summary

Introduction

Amazonian Dark Earths (ADEs) are unusually fertile soils characterised by elevated concentrations of microscopic charcoal particles, which confer their distinctive colouration. We determined the source and timing of microcharcoal deposition using stable carbon isotope ratios and calibrated radiocarbon (14C) dates of ADE and Ultisol samples.

Results

Conclusion