Predicting pre-Columbian anthropogenic soils in Amazonia

Abstract

The extent and intensity of pre-Columbian impacts on lowland Amazonia have remained uncertain and controversial. Various indicators can be used to gauge the impact of pre-Columbian societies, but the formation of nutrient-enriched terra preta soils has been widely accepted as an indication of long-term settlement and site fidelity. Using known and newly discovered terra preta sites and maximum entropy algorithms (Maxent), we determined the influence of regional environmental conditions on the likelihood that terra pretas would have been formed at any given location in lowland Amazonia. Terra pretas were most frequently found in central and eastern Amazonia along the lower courses of the major Amazonian rivers. Terrain, hydrologic and soil characteristics were more important predictors of terra preta distributions than climatic conditions. Our modelling efforts indicated that terra pretas are likely to be found throughout ca 154 063 km(2) or 3.2% of the forest. We also predict that terra preta formation was limited in most of western Amazonia. Model results suggested that the distribution of terra preta was highly predictable based on environmental parameters. We provided targets for future archaeological surveys under the vast forest canopy and also highlighted how few of the long-term forest inventory sites in Amazonia are able to capture the effects of historical disturbance.

Keywords: Amazonia; anthrosols; maxent algorithms; modified soils; pre-Columbian impacts; terra preta.

Figure 1.

Locations of previously published terra preta and terra preta-free sites, major archaeological sites (references = M [20], X [2], S [21], C [22], Llanos de Moxos [12], geoglyphs [3,18]), and river systems within Amazonia.

Figure 2.

Probabilities of terra preta occurrence based on predictive models. Legend of archaeological sites is the same as in figure 1. The black line indicates a potential cultural boundary where the probability of terra preta formation decreases and disappears and is replaced by alternative subsistence strategies in southwestern Amazonia (see text for details).

Figure 3.

Response curves for the geological and geographical variables (see Material and methods) included in the predictive model. Because many of these variables are correlated with each other (see the electronic supplementary material, figure S4), these response curves are based on a Maxent model using only the corresponding variable [54].

Figure 4.

Response curves for the soil variables [43] included in the predictive model. Because many of these variables are correlated with each other (see the electronic supplementary material, figure S4), these response curves are based on a Maxent model using only the corresponding variable [54]. S CEC, subsoil cation exchange content.

Figure 5.

Response curves for the climatic variables [42] included in the predictive model. Because many of these variables are correlated with each other (see Material and methods and the electronic supplementary material, figure S4), these response curves are based on a Maxent model using only the corresponding variable [54].

Figure 6.

(a) Terra preta probabilities in relation to locations of long-term forest inventories, large scale biosphere-atmosphere experiment in Amazonia (LBA) C-flux towers, geoglyphs, bamboo-dominated forests and major archaeological sites. The legend of archaeological sites is shown in figure 1. The location and spatial extent of panels (b) and (c) are shown as the red and black rectangles respectively. (b) Regional view of areas with forest inventory data and an increased probability of terra pretas. (c) Regional-scale view of the potential cultural boundary between ancient cultures that formed terra preta, and those that used alternative subsistence practices.