Environmental impact of geometric earthwork construction in pre-Columbian Amazonia

Abstract

There is considerable controversy over whether pre-Columbian (pre-A.D. 1492) Amazonia was largely "pristine" and sparsely populated by slash-and-burn agriculturists, or instead a densely populated, domesticated landscape, heavily altered by extensive deforestation and anthropogenic burning. The discovery of hundreds of large geometric earthworks beneath intact rainforest across southern Amazonia challenges its status as a pristine landscape, and has been assumed to indicate extensive pre-Columbian deforestation by large populations. We tested these assumptions using coupled local- and regional-scale paleoecological records to reconstruct land use on an earthwork site in northeast Bolivia within the context of regional, climate-driven biome changes. This approach revealed evidence for an alternative scenario of Amazonian land use, which did not necessitate labor-intensive rainforest clearance for earthwork construction. Instead, we show that the inhabitants exploited a naturally open savanna landscape that they maintained around their settlement despite the climatically driven rainforest expansion that began ∼2,000 y ago across the region. Earthwork construction and agriculture on terra firme landscapes currently occupied by the seasonal rainforests of southern Amazonia may therefore not have necessitated large-scale deforestation using stone tools. This finding implies far less labor--and potentially lower population density--than previously supposed. Our findings demonstrate that current debates over the magnitude and nature of pre-Columbian Amazonian land use, and its impact on global biogeochemical cycling, are potentially flawed because they do not consider this land use in the context of climate-driven forest-savanna biome shifts through the mid-to-late Holocene.

Keywords: Amazon rainforest; Amazonian archaeology; Anthropocene; human–environment interactions; paleoecology.

Fig. 1.

Map of site locations. (A) Present-day precipitation across Amazon basin during the three driest months of the year (26); solid black line delimits modern extent of Amazonian forest; approximate extent of SSAR/geometric earthwork region is within the <125-mm isohyet. Over 400 geometric earthworks have been discovered in eastern Acre, Brazil (location no. 3 in A), and many more across Iténez (location no. 1 in A, detail in B) and Riberalta (location no. 2 in A), northeast Bolivia, and the Upper Xingu (location no. 4 in A), Brazil. Previously published lake-sediment pollen records showing evidence for late Holocene biome shifts are represented by white circles: (a) Bella Vista, (b) Chaplin (27), (c) Yaguarú (28), (d) Carajas (29). Previously published lake-sediment pollen records in central and western Amazonia, which show stability of the forest biome and limited or no human impact over mid-to-late Holocene, are represented by gray circles: (e) Siberia (30), (f) Chalalán and Santa Rosa (22), (g) Gentry, Parker, Vargas, Werth (23). Black crosses represent soil-pit sampling locations/transects discussed in the main text, which show little evidence of pre-Columbian forest disturbance (20, 21). (B) Study area showing locations of LO and LG (detail in C), main rivers, and biome distribution. (C) Laguna Granja and Granja del Padre ring ditch.

Fig. 2.

LO pollen and charcoal records. Pollen data presented as percent abundance (of the total terrestrial pollen sum) and total concentrations per cubic centimeter. Charcoal presented as particles per cubic centimeter. “Total Arboreal” is the sum of all tree taxa. Dominant tree taxa (> 2% abundance) are shown. Plotted against calibrated radiocarbon years B.P. and individual radiocarbon dates shown on right (Fig. S1 and Table S1).

Fig. 3.

Aerial photo of ring ditch next to LG (taken 2008 by H.P.). Open ground around the ditch reflects modern, small-scale clearance for cattle pasture.

Fig. 4.

LG pollen and charcoal records. Pollen data presented as percent abundance (of the total terrestrial pollen sum) and total pollen concentrations. Charcoal presented as particles per cubic centimeter. Zea mays plotted as no. of grains per ∼0.4 cm³. “Total Arboreal” is the sum of all tree taxa. Dominant tree pollen types (>2% abundance) are shown. Dashed line indicates timing of regional forest expansion shown in LO. Plotted against calibrated years before present, with individual radiocarbon dates shown on the right (Fig. S2 and Table S1).