The Geoglyph Sites of Acre, Brazil

Antiquity 2020 Vol. 94 (378): 1538–1556 https://doi.org/10.15184/aqy.2020.208 Research Article The geoglyph sites of Acre, Brazil: 10 000-year-old land-use practices and climate change in Amazonia Martti Pärssinen1,*, William Balée2, Alceu Ranzi3 & Antonia Barbosa4 1 Department of Cultures, University of Helsinki, Finland 2 Department of Anthropology, Tulane University, USA 3 Laboratório de Paleontologia, Universidade Federal do Acre, Brazil 4 Superintendência do Instituto do Patrimônio Histórico e Artístico Nacional no Acre, Rio Branco, Brazil * Author for correspondence: ✉ martti.parssinen@helsinki.fi Hypotheses concerning climatic change during the Amazonian Holocene often assume that the presence of ancient charcoal from forest fires indicates periods of drier climate in the past. These theories, however, neglect the possibility that such charcoal may result from early human activity. This article presents new evidence of anthropogenic ash and charcoal accumulation in the state of Acre, Brazil, dating back to c. 10 000 cal BP, which questions the value of charcoal as a proxy for phases of natural climate aridification. Carbon isotope (δ13C) values also suggest no signifi- cant changes in Holocene climate or vegetation. If these results are confirmed, previous studies on Ama- zonian Holocene climate will require re-evaluation. Keywords: Amazonia, Holocene, climate change, geoglyphs, land-use practices Introduction The discovery of hundreds of geometrical earthworks (geoglyphs) with an associated system of roads in the state of Acre, Brazil, has been labelled the “the most recent and dramatic discovery” in Amazonian archaeology (Mann 2008: 1148). Together with similar discoveries in the Lower Amazon, Central Amazonia, Xingu, and Mojos and Baures in Bolivia, the Acre geoglyphs have radically changed our understanding of the long-term human impact on the Amazonian rainforest: namely, that the Amazon floodplains and non-flooded terra firme hinterlands were not completely covered by pristine forest (see Balée 1989; Denevan 1992; Roosevelt et al. 1996; Heckenberger et al. 2003; Dias 2006; Erickson 2006; Neves Received: 5 November 2019; Revised: 26 February 2020; Accepted: 9 March 2020 © The Author(s), 2020. Published by Cambridge University Press on behalf of Antiquity Publications Ltd. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons. org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. 1538 The geoglyph sites of Acre, Brazil: land-use practices and climate change in Amazonia Figure 1. The area of known geoglyphs in South-western Amazonia discussed in the text (drawing by S. Saunaluoma, M. Pärssinen & W. Perttola). © The Author(s), 2020. Published by Cambridge University Press on behalf of Antiquity Publications Ltd 1539 Martti Pärssinen et al. 2007; Pärssinen et al. 2009; Schaan et al. 2012). To date, more than 450 prehistoric earthwork sites have been identified in the state of Acre alone (Figure 1), all of which seem to belong to the same ceramic tradition, with affiliations to ancient, Western Amazonian For- mative styles. Ceramics dating to c. 2000 BP and resembling other widespread Amazonian styles, including Incised-Rim, Polychrome and Corrugate styles, have also been recovered during excavations of sites, such as Severino Calazans and Tequinho (Pärssinen & Ranzi 2020). Calibrated radiocarbon dates indicate that the first Acre earthworks were initiated by c. 2500 BP, with construction continuing until 1000 BP. Some were still in use at the end of the thirteenth century AD and, according to recent radiocarbon dating, were re-used until as recently as the nineteenth century (Saunaluoma et al. 2018). Haffer (1969) initiated the debate on Amazonian savannah formation with the introduction of a biogeographic refuge theory. This theory assumed that the savannah expanded during the cooler, arid glacial periods of the Pleistocene, fragmenting the continuous lowland forest and forming smaller, isolated forest refuges. The resulting isolation stimulated biological differenti- ation and biodiversity. Our intention here is not to discuss the climatic aridity of the Pleistocene. Nevertheless, Meggers (1975, 1977) subsequently adopted this theory to explain patterns of cultural and linguistic diversity during the Holocene period, supposing that climatic oscillations would also have affected native lowland forest populations and the observed linguistic differen- tiation through isolation effects caused by expanding savannahs and isolated forest refuges. Although Meggers’s proposition has gained few followers, some correspondences with global climate fluctuation (especially Atlantic) are documented in the Amazonian evidence. The strength of the Atlantic monsoon winds and displacement of the Inter-Tropical Con- vergence Zone affect the South American summer monsoon, which usually brings a dry season (at its maximum in July) after a wet season (maximum in January). The circulation system of the South American summer monsoon brings moisture from the tropical Atlantic Ocean across the Amazon Basin (Marsh et al. 2018), and fluctuations normally cause opposite effects on different sides of the equator, between the southern and northern lowlands. The same may be true for Western and Eastern Amazonia (Marsh et al. 2018: fig. 2)—afact that may explain the various contradictory pieces of evidence for wet and dry periods when applied to all of Amazonia (e.g. Cruz et al. 2009: 210; Vonhof & Kaandorp 2010:204;this article). Although the extent to which Amazonia was covered with savannah during the Pleis- tocene remains under discussion (e.g. Colinvaux et al. 2000; Irion & Kalliola 2010), one may speculate as to whether less dramatic Holocene climatic oscillations, compared to the Pleisto- cene Ice Age, in global temperature and regional precipitation would have been sufficient to turn huge areas of savannah into a neo-tropical biome during the Holocene. While geochemical δ18O isotope analysis from Eastern Amazonia provides independent evidence of regional wetter and drier periods in the Holocene (Cruz et al. 2009), the extent to which this fluctuation changed the vegetation patterns over the entire area remains unknown. Much palaeobotanical evidence for radical climatic change is based on the assumption that the presence of open-land vegetation and charcoal from forest fires provides proof of much drier periods (e.g. Herma- nowski et al. 2012;Carsonet al. 2014). These circular theories, however, do not take into account the presence of humans in Amazonia throughout the Holocene, and that fires may have been used by Amerindians for clearing forests and maintaining open grasslands (see also Piperno 2011;Bushet al. 2015). Amazonia, or at least part of it, was already occupied © The Author(s), 2020. Published by Cambridge University Press on behalf of Antiquity Publications Ltd 1540 The geoglyph sites of Acre, Brazil: land-use practices and climate change in Amazonia by the Late Pleistocene period, c.13000–10 000 BP, and in the Early Holocene, c. 10 000– 8000 BP (Miller et al. 1992, cited in Neves 2007; Roosevelt et al. 1996; Roosevelt 2013; Lom- bardo et al. 2013;Watlinget al. 2018a). While the earthworks sites of Acre have so far yielded no evidence of Pleistocene ash and charcoal accumulations that could have been caused by prolonged natural or anthropogenic fires, thick Holocene ash and charcoal layers are present at many of these sites. In this article, we present evidence for such an accumulation from the Severino Calazans site in eastern Acre, dating to as early as c. 10 000 cal BP, and continuing, with some possible intervals, until the geoglyphs were abandoned. This represents some of the first evidence for an Early Holocene human presence from the Amazonian upland (terra firme)—far from the main riverine routes. Terra preta vs terra mulata Over the past 20 years, research on Amazonian Dark Earths—terra preta—has advanced our knowledge of ancient Amerindian landscape management practices, both in riverine bluffs and upland terra firme forests near Amazonia’s main riverine routes (see McMichael et al. 2014a: fig. 1). The dark earths are anthropogenic, highly fertile soils created by centuries of soil mulching over the otherwise poor Amazonian latosols. These anthropogenic black soils were created by Amerindians using fires to produce pyrogenic carbon, which lowers soil pH and gives stability to soil nutrients and micro-organisms (Woods & Denevan 2009). Although the presence or absence of dark earths has also been used in Amazonian studies as a measure of human occupation density (e.g. McMichael et al. 2012), it seems to be absent in Acre. Watling et al. (2017) have nevertheless proposed that at least 4000 years ago, the first inhabitants of Acre created small open patches within forests by burning, in order to attract useful trees, such as palms, long before the first earthwork-building projects commenced. Furthermore, the presence of only a few imported, small stone axes found during our archaeological investigations indicates that no local, large-scale tree-cutting activities were undertaken. Indeed, the cultural landscape may have been created through the use of controlled burning. An anthropogenic forest, for example, covers the geoglyph of Três Vertentes. This forest comprises numerous palm species, along with semi-domesticated and domesticated dicotyledonous species, including

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