Anthropogenic dark earths of the Central Amazon region: remarks on their evolution and polygenetic composition

Abstract

Anthrosols are soils whose formation and characteristics have been enduringly influenced by the material effects of human action (Limbrey 1975; Eidt 1984; Woods 2003). Among others they include those whose surface horizon has been modified by topsoil disturbance and/or irrigation associated to different types of agriculture; those which have formed on human-transported, -manufactured or -mobilised sediments, including here landforms created or altered by humans; and soils whose surface horizons have become significantly transformed as a result of human-induced inputs (see also Dudal 2005). Anthrosols are ubiquitous on a planetary scale (FAO 1998): they vary in spatial extent from compost heaps that concentrate organic matter in the backyard of households to entire landscapes modified by agricultural or industrial activity. Below we focus on anthrosols variously known as terras pretas de índio or Amazonian Dark Earths (Sombroek 1966; Hilbert 1968; Woods and McCann 1999; Lehmann et al. 2003; Glaser and Woods 2004). We examine these soils from a geoarchaeological perspective because we believe that important aspects of the dynamics of past anthropogenic landscape transformations are recorded in them. Our approach focuses on relict signatures of past human agencies (e.g. French 2003; Davidson and Simpson 2005) in a soilscape that we understand as a ‘moving target’, i.e. one whose variability is dictated by its position in specific landscape evolutionary pathways. From this vantage point, with a sincere hope of contributing to the broader enterprise of understanding past anthropogenic landscape transformations in the Amazon basin, and in fond memory of Wim Sombroek, undoubtedly a pioneer in investigations of these soils, we offer below two interlinked sets of remarks about the physical and chemical characteristics of anthropogenic dark earths. We focus first on dimensions of the variability and evolution of the soil mantle of the central Amazon region that clearly impinge on the physical and chemical properties of these soils and, therefore, affect the nature of archaeological inferences that can be drawn from them.