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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 accumu-
lation 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 (δ
13
C) 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 dis-
covery”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
2007; Pärssinen et al. 2009; Schaan et al. 2012). To date, more than 450 prehistoric earth-
work sites have been identified in the state of Acre alone (Figure 1), all of which seem to
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
Antiquity 2020 Vol. 94 (378): 1538–1556
https://doi.org/10.15184/aqy.2020.208
1538
Figure 1. The area of known geoglyphs in South-western Amazonia discussed in the text (drawing by S. Saunaluoma, M. Pärssinen & W. Perttola).
The geoglyph sites of Acre, Brazil: land-use practices and climate change in Amazonia
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1539
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-
ationand biodiversity. Our intention here is not to discussthe 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 savannahinto a neo-tropical biome during the Holocene. While geochemical
δ
18
O 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 chan-
ged the vegetation patterns over the entire area remains unknown. Much palaeobotanical evi-
dence 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
by the Late Pleistocene period, c. 13 000–10 000 BP, and in the Early Holocene, c.
Martti Pärssinen et al.
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10 000–8000 BP (Miller et al. 1992, cited in Neves 2007; Roosevelt et al. 1996; Roosevelt
2013; Lombardo 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, dat-
ing to as early as c. 10 000 cal BP, and continuing, with some possible intervals, until the geo-
glyphs 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 dur-
ing 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 Brazil nuts (Bertholletia excelsa),
rubber trees (Hevea brasiliensis), caucho (Castilla ulei) and numerous moraceous (fig family)
vines and trees (Balée et al. 2014)—a‘legacy’of the geoglyph-building societies of the past
(Watling et al. 2017; Watling et al. 2018b).
According to Watling et al. (2017; see also Watling et al. 2018b), the earliest fires in the
geoglyph region have been recorded at the Fazenda Colorada site, dating to c. 4701–4535 cal
BC. In addition, Watling et al. (2017) recorded a piece of charcoal giving a radiocarbon age of
6984±33 BP (5981–5770 cal BC, according to the corrected IntCal13 calibration curve used
by Watling et al. 2017), although the authors question the reliability of this date due to the
sample’s uncertain stratigraphic position. While no human artefacts were found at Fazenda
Colorada, burning activities increased on such a large scale from 4500/4000 BP onward
(Watling et al. 2017) that they were undoubtedly associated with human presence and inten-
tional landscape modifications, including a decrease in bamboo and an increase in edible trees
such as palms. The notion of a 4500-year-old human presence in Acre had been previously
proposed by Schaan et al. (2012) based on our initial radiocarbon evidence from the Severino
The geoglyph sites of Acre, Brazil: land-use practices and climate change in Amazonia
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1541
Calazans site. Indeed, we suggest that the local inhabitants seem to have used these former
grassland patches as favoured sites upon which to build ditches, embankments and roads.
In this article, we present supporting evidence that the geoglyph-building populations of
Acre used fire in their horticultural practices and for clearing vegetation from ceremonial sites.
Although this activity did not produce Amazonian dark earths, the result resembles lighter-
coloured terra mulata soil (see e.g. Denevan 2001; Kämpf et al. 2003; Arroyo-Kalin 2010),
containing ash and tiny fragments of charcoal accumulated over a long period of time. While
this soil is less fertile than dark earth, it has a comparably low pH and stabilises soil nutrients.
Our primary research questions stem from this interpretation. If we accept that Acre had
savannah-like patches at least 4000 years ago (or even much earlier), then an obvious question
concerns whether the region had larger anthropogenic savannah formations, such as those
found widely today as a result of current burning activities conducted by modern ranchers
and farmers. Or were these pre-Colonial savannah areas created by climatic change, as pro-
posed by Pessenda et al. (1998) and Carson et al. (2014) in reference to the pre-Colonial
open grassland in nearby areas of Brazilian Rondônia and Bolivia? Finally, when did the
first Holocene savannah-like patches appear in the geoglyph zone of Acre? These questions
are critical, as they relate to the much broader and strongly debated issue of climatic fluctu-
ation and its effects on Amazonian vegetation, biodiversity and human history.
Excavations at the Severino Calazans site
The Severino Calazans site comprises a square, ditched enclosure, 230 × 230m that is
bisected and partially destroyed by the modern BR-317 highway (Figures 2–3). The
12m-wide ditch was dug on a plateau adjacent to a slope that descends towards the Iquiri
River. Most of the sediment excavated from the ditch was used to build an external bank.
Originally, the ditch would have been more than 5m lower than the top of the bank.
Attached to the north side of the main structure is a 100m-wide, earthen enclosure, with
an 8m-wide entrance at its northern mid-point. Approximately 700m to the south of the
main enclosure is an archaeological site known as Severino Batista (Figure 4), which com-
prises a sub-circular ditch approximately 80m in diameter, with rounded corners. The
ditch is flanked by earthen banks.
During our initial excavations at Severino Calazans in 2007, we noticed that the ash and
charcoal accumulation was surprisingly deep on the western side of the site. In this area, we
also discovered a circular, approximately 0.20m-wide pre-Ceramic hearth feature, 1.10m
below the ground surface (pit 4). In 2014, and later in 2017–2018, additional trenches
and test pits were excavated in different parts of the site to refine the chronology of Severino
Calazans. In total, four trenches measuring 12 × 1m (trenches 6A-F, 7A-F, 20A-F) and
12 × 2m (trench 13A-F), and eight pits measuring 2 × 1m (pits 1–5, 8, 9 and 10) were exca-
vated. We also excavated a further six 1 × 1m pits (pits 11–12 and 14–18), and two 2 × 2m
pits (19A–B) to the south, west and north of the enclosure. At nearby Severino Batista, we
excavated two 1 × 2m pits (pits 1–2). The trenches and all of the pits except pits 11 and 15–
18 contained cultural remains, predominantly comprising undecorated ceramics and burnt
clay, along with occasional fragments of grinding stones and burnt bone fragments. It is not-
able that whenever cultural layers with associated ceramics were identified, they always
Martti Pärssinen et al.
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Figure 2. Aerial photograph of the Severino Calazans site, looking south (photograph by D. Gurgel).
The geoglyph sites of Acre, Brazil: land-use practices and climate change in Amazonia
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1543
Figure 3. Plan of the earthworks at Severino Calazans (drawing by M. Pärssinen, S. Saunaluoma & W. Perttola).
Martti Pärssinen et al.
© The Author(s), 2020. Published by Cambridge University Press on behalf of Antiquity Publications Ltd
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Figure 4. Aerial photograph of the Severino Batista earthwork (photograph by D. Gurgel).
The geoglyph sites of Acre, Brazil: land-use practices and climate change in Amazonia
© The Author(s), 2020. Published by Cambridge University Press on behalf of Antiquity Publications Ltd
1545
continued well below the level containing ceramics and burnt clay, often without any observ-
able change in the soil colour until sterile soil was reached.
In general, the modern greyish surface layer of the Severino Calazans site was 0–0.10m
thick (Figure 5). Below this was a 0.10–0.60m-thick layer of yellowish red (5YR 5/8; 5YR
5/6 by the Munsell soil colour chart) or reddish yellow (5YR 6/8)/strong brown (7.5YR
5/6) soil, often containing ceramics, burnt clay, occasional fragments of grinding stones,
bones, charcoal, macrofossils and ash. This anthropogenic layer and materials resemble
terra mulata and are generally representative of ancient activity at the site. Beneath this
was a 0.10–1.20m-thick layer of red (2YR 4/6; 2.5YR 5/8) or yellowish red (5YR 5/8;
5YR 5/6) soil that also resembles terra mulata. This layer contained small fragments of char-
coal, ash, gravel and small conglomerate pebble stones, as well as occasional pieces of amber
and fossilised wood. The layer’s upper part (varying in depth between 0.10 and 0.60m)
contained few ceramic sherds. The underlying aceramic section varied in depth from
0.10–0.90m. The only exceptions to these measured depths were observed at the deepest
points of the three trenches, where the ceramic-containing layers reached approximately
4.70m below the current ground’s surface due to the presence of an ancient ditch. Finally,
the sterile soil layer was a red colour (2YR 4/8), with mixed clay and gravel, or with light
yellowish-brown silt spots (2.5Y 6/4).
Radiocarbon dates
Samples for radiocarbon dating were collected in 2007, 2014, 2017 and 2018 from the Sever-
ino Calazans and Severino Batista sites. These were analysed in the Ångström Laboratory of
the University of Uppsala, and calibrated using the OxCal v4.3 program, using the SHCal13
curve (Bronk Ramsey 2009; Hogg et al. 2013). The results are shown in Table 1.
The two samples (Ua-59605 and Ua-59604), taken from pits 19B and 19A, at 1.34m
below the surface, provide dates of 8238–7844 and 8203–7721 cal BC (at 95.4% confi-
dence), respectively. No lithics or other artefacts were discovered in this layer. Nevertheless,
in the western profile of pit 19A, a greyish soil lens stained by clay, ash and small pieces of
carbon—as is often found in human settlement sites in the Andes and Amazonia (see Pärs-
sinen & Siiriäinen 1997:fig. 3; Schaan 2004:figs 37, 45 & 47)—was recorded at the same
level, approximately 1.30m below the surface (Figure 5). Furthermore, a sample (Ua-50106)
taken from pit 12 (Figure 6), approximately 0.10m above the sterile soil layer, gives a date of
7543–7190 cal BC (at 95.4% confidence). Again, no human artefacts were present in this
layer. Unfortunately, it seems that not even similar durable lithic artefacts to those found,
for example, at Rondônia (Watling et al. 2018a) were used at Severino Calazans. This is per-
haps unsurprising, however, given the lack of local, natural lithic sources. Nevertheless, as the
date derives from the lower section of the very same soil layer that included ceramics in the
upper section, it is probable that the soil accumulated due to human activities. Furthermore,
the date fits into the chronological sequence provided by sample Ua-50102 (5999–5757 cal
BC) taken from trench 7, approximately 0.50m above the sterile layer.
Currently, we have no evidence for soil accumulation between c. 4000 and 3000 cal BC.
Evidence for fires and the presence of a (new?) intensive phase of soil and ash
accumulation began c. 2872–2581 cal BC and continued, with fluctuating intensity,
Martti Pärssinen et al.
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Figure 5. The western profile of pit 19A and the locations of four radiocarbon samples (denoted by the star symbols) listed
in Table 1 (drawing by M. Pärssinen).
The geoglyph sites of Acre, Brazil: land-use practices and climate change in Amazonia
© The Author(s), 2020. Published by Cambridge University Press on behalf of Antiquity Publications Ltd
1547
Table 1. Radiocarbon dates with δ
13
C values (Bronk Ramsey 2009; Hogg et al. 2013) from Severino
Batista and Severino Calazans (all collected by M. Pärssinen). All the pits, except 1, 17 and 18, are
denoted on Figure 3.
Lab. no. Pit Depth (m) Material δ
13
C‰VPDB BP date
Cal. date (94.6%
confidence)
Severino Batista
Ua-58146 1 0.95 Macrofossil −26.5 1295±30 AD 681–876
Ua-58147 1 1.10 Charcoal −27.2 1771±30 AD 240–383
Severino Calazans
Ua-58154 14 0.48 Charcoal −25.4 1512±30 AD 533–643
Ua-58158 15 0.60 Charcoal −25.5 1526±30 AD 543–645
Ua-58146 13D 0.95 Charcoal −26.2 1715±30 AD 250–429
Ua-58149 13D 1.55 Charcoal −27.7 1936±30 AD 42–206
Ua-50104 12 060 Charcoal −28.3 1970±35 48 BC–AD 202
Ua-37264 3 0.20–0.30 Charcoal −24.2 2050±35 101 BC–AD 66
Ua-58152 13D 4.05 Charcoal −27.4 2063±30 105 BC–AD 51
Ua-66031 19A 0.23 Charcoal −30.2 2074±30 136 BC–AD 34
Ua-58151 13D 3.83 Charcoal −26.6 2078±30 137–28 BC
Ua-58153 13C 1.50 Charcoal −28.3 2147±32 346–54 BC
Ua-37265 6B 0.50–0.60 Charcoal −27.8 2275±35 390–207 BC
Ua-59600 20A 1.33 Charcoal −29.3 2406±32 730–376 BC
Ua-59602 20A 1.56 Charcoal −28.8 2422±31 735–386 BC
Ua-59601 20A 1.45 Charcoal −28.1 2429±31 741–389 BC
Ua-59599 20A 1.20 Charcoal −29.2 2465±31 751–402 BC
Ua-37238 5 0.45 Charcoal −25.3 2915±35 1193–926 BC
Ua-50103 7D 4.00–4.10 Charcoal −28.0 2964±35 1258–1001 BC
Ua-59603 19A 0.85 Charcoal −26.9 3105±31 1416–1221 BC
Ua-58157 14 0.97 Charcoal −26.7 3410±31 1749–1534 BC
Ua-52903 12 1.05 Charcoal −27.6 3740±32 2201–1971 BC
Ua-58150 13D 2.32 Charcoal –3960±109 2856–2044 BC
Ua-66033 19B 0.90 Charcoal –26.4 3976±30 2565–2296 BC
Ua-37237 3 0.50 Charcoal −28.2 3990±40 2572–2298 BC
Ua-58156 14 0.80 Charcoal −26.1 4035±33 2622–2350 BC
Ua-50105 12 0.90 Charcoal −27.6 4086±36 2850–2469 BC
Ua-58155 14 0.67 Charcoal −26.4 4173±32 2872–2581 BC
Ua-66032 19A 1.20 Charcoal –25.6 5308±32 4231–3982 BC
Ua-66034 19B 1.11 Charcoal –25.0 6027±33 4991–4779 BC
Ua-50102 7F 0.85 Charcoal −26.5 7050±53 5999–5757 BC
Ua-50106 12 1.50 Charcoal −26.4 8391±59 7543–7190 BC
Ua-59604 19A 1.34 Charcoal −25.9 8849±40 8203–7721 BC
Ua-59605 19B 1.34 Charcoal −26.0 8932±37 8238–7844 BC
Severino Calazans: 200m north of pit 15
Ua-58159 17 0.48 Charcoal −27.6 3121±31 1429–1232 BC
Severino Calazans: 300m north of pit 15
Ua-58160 18 0.40 Charcoal −28.1 1620±30 AD 413–570
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until cal AD 681–876. In addition, four ceramic-associated radiocarbon samples from pit
20A provide evidence that the initial construction of the embankments situated inside of
the current ditches may have started c. 751–402 cal BC. Finally, various samples indicate
that the current ditched geoglyphs at Severino Calazans (samples from pits 3, 6B, 12,
13C, 13D, 14) and Severino Batista (sample from pit 1) were built and used between c.
350 BC and AD 850. Together with the radiocarbon dates and stratigraphy from Severino
Calazans described above, these samples further support pre-ceramic human activity in the
region, even though we have found no pre-ceramic lithic tools. With the exception of occa-
sional fragments of non-local grinding stones, lithics were also absent from the upper ceramic-
bearing layers. Most of the tools and utilitarian objects were therefore probably made from
local, organic materials.
In summary, we can propose no explanation for the accumulations of ash and charcoal at
the Severino Calazans and Severino Batista sites, other than that they must have resulted from
periodic, anthropogenic burning episodes. Although we have excavated test pits down to 3m
below the darker, ash- and charcoal-containing ‘cultural’soil, no charcoal has been recovered
from these sterile soils. Thus, the accumulation of the soil containing ash and charcoal is def-
initely a Holocene phenomenon. The two oldest samples—from pits 19B and 19A—give
dates of 8238–7844 and 8203–7721 cal BC, respectively. Hence, the first inhabitants
may indeed have arrived in the region c. 10 000 years ago. Compared to Watling et al.’s
(2017) evidence of heavy landscape transformation occurring during the transition period
between the Middle and Late Holocene (4500/4000 BP), this process seems to have started
centuries later at Fazenda Colorada and Jaco Sá than at Severino Calazans.
Whereas a period of intense burning began at Fazenda Colorada and Jaco Sá c. 4000 BP
(Watling et al. 2017), at Severino Calazans, such activity had already started by 4500 BP
(Table 1). Thus, the timing of these landscape-transforming activities appears to be localised
and varied. Nevertheless, it is notable that Pessenda et al. (1998) recorded similar but
larger-scale soil accumulation processes over the past 7000 years in Rondônia. Although
Pessenda et al.(1998) considered that these accumulations resulted from climatic change
and natural fires, the presence of humans in Rondônia 10 000–9500 years ago (Watling
et al. 2018a) suggests that at least part of the accumulation process at Rondônia may be attrib-
utable to human activity. There is no evidence at Severino Calazans for natural accumulation,
as there was no observable change in soil composition between the lower pre-ceramic
ash-and-charcoal layer and the ceramic-containing layer. It is also notable that the
soil-accumulation thickness varied greatly in different pits, indicating the localised and
controlled use of fire.
Carbon isotope δ
13
Cfluctuation and supposed climate change in
South-western Amazonia
Earlier we demonstrated that the accumulation of anthropogenic soil in Acre seems to have
started at the beginning of the Holocene. In this section, we analyse geochemical evidence for
Acrean forest structures across the entire Holocene according to data obtained from carbon
isotope (δ
13
C) records from our radiocarbon samples from Severino Calazans and other arch-
aeological sites in the region. If the first savannah-like patches had already appeared c. 10 000
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years ago in Acre, do we have evidence of much drier climatic periods in the Holocene that
would have accelerated environmental change towards the vast open savannah? As will be
seen, the answer seems to be ‘no’.
Figure 6. The northern and western profiles of pit 12 and the locations of four radiocarbon samples (denoted by the star
symbols) listed in Table 1 (drawing by M. Pärssinen).
Martti Pärssinen et al.
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1550
Researchers have traditionally studied climate oscillation by using variation in oxygen
isotope (δ
18
O) values of, for example, cave stalagmites and lake sediments. Seltzer et al.
(2000)notethatfluctuations in δ
13
C ratios correspond strongly with the variations in
δ
18
O values. In particular, it has been found that δ
13
C values predominantly reflect pre-
cipitation, with higher values indicating less precipitation and lower values, reflecting
more humid conditions (e.g. Kohn 2010). Most of the world’s vegetation comprises C
3
plants, including our samples of Amazonian vegetation and their seeds. These have their
own δ
13
C standards (O’Leary 1988: 334) that vary normally between −20 and −37‰.
C
3
plant values above −23‰are restricted to very dry environments, such as savannah
and desert. Furthermore, in savannah, subtropical grasslands and seasonally flooded trop-
ical environments, C
4
plant species may dominate. Their δ
13
C values are higher than C
3
plants ranging between −9and−16‰(Pessenda et al. 1998;Kohn2010;Hermenegildo
et al. 2017).
It is notable that the δ
13
C values of our samples from Severino Calazans (predominantly
small charred branches and sticks) indicate no significant changes in precipitation during the
10 000–1100 cal BP time period (Table 1). Hence, there is no evidence for a major shift in
vegetation towards open savannah. On the contrary, C
3
vegetation of the forest environment
continued to dominate and the small, savannah-like patches seemingly did not greatly affect
the general forest composition—not even during the geoglyph-building period. Similar
results have been obtained for the Late Holocene period at Fazenda Colorada and Jaco Sá
(Watling et al. 2017,2018b).
The same interpretation applies for the values of other samples collected by us and our
colleagues from different archaeological sites dating from 1631 cal BC–cal AD 1891 in east-
ern Acre and from nearby areas of tropical northern Bolivia (Siiriäinen 2003; Schaan et al.
2012; Saunaluoma 2013:30–31; Saunaluoma et al. 2018). Of these additional samples,
the only exception derives from the site of Balneário Quinauá in Acre, excavated by Sauna-
luoma (2012). There, one sample (Ua-37260), dated to cal AD 431–602, gives a value of
−13‰, which is typical of dry regions, such as Bolivian Chuquisaca. Nevertheless, this par-
ticular sample was obtained from the sooted surface of a ceramic sherd (Saunaluoma 2012),
and may originally have derived from a C
4
plant such as maize, which was known to be
exploited at the nearby Tequinho geoglyph site in Acre (Watling et al. 2015). Other than
this outlier, all the values from our total of 78 samples from South-western Amazonia fluc-
tuate around −27‰(consistently −23 to −30.6‰), without any clear long-term trends
observable. The variation is comparable with the results derived from forest and forest-
transition regions of the adjacent state of Rondônia, but differ radically from drier wooded
savannah (cerrado) and tropical, semi-deciduous forest (cerradão) sites in Rondônia. This
is probably due to the radically different type of vegetation (C
4
plants) (Pessenda et al. 1998).
Conclusions
Excavations in Severino Calazans produced new evidence of Early Holocene human presence
in the interfluvial terra firme tropical forest region of South-western Amazonia. It appears that
below the known geoglyph was acultural layer that pre-dates the geoglyph by more than 7000
years. Periodical forest burnings produced anthrosol that resembles terra mulata. Apparently
The geoglyph sites of Acre, Brazil: land-use practices and climate change in Amazonia
© The Author(s), 2020. Published by Cambridge University Press on behalf of Antiquity Publications Ltd
1551
these kinds of soil patches were used as favoured sites upon which geoglyph-type ditched
embankments were built.
During our excavation, we collected samples for radiocarbon dating and carbon isotope
(δ
13
C) measurements covering c. 9000 years. By combining these data with earlier measure-
ments (Siiriäinen 2003; Schaan et al. 2012; Saunaluoma 2013:30–31; Saunaluoma et al.
2018; this article) covering more than 3000 years, we can argue that the geochemical
δ
13
C data show no obvious long-term changes in precipitation or in the tropical forest com-
position in eastern Acre or northern Bolivia. On the contrary, the evidence presented here
strongly indicates periodic and repeated low-intensity burning events that opened small
savannah-like patches in the tropical forest of eastern Acre throughout the entire Holocene.
More precisely, our radiocarbon dates indicate that by the Early and Middle Holocene (c.
10 000–4200 BP), this part of Amazonia may already have been populated,and small patches
of the tropical forest (including bamboo-dominated C
3
forest; see McMichael et al.2014b)
may have been opened for human purposes. Watling et al. (2017,2018a) note an increase in
the number of phytoliths of edible palms after each Late Holocene major fire event at the
Fazenda Colorada and Jacó Sá sites. The same probably occurred at Severino Calazans,
which yielded evidence for edible oricuri (Attalea phalerata) and peach palm (Bactris gasipaes),
as well as Brazil nut (Bertholletia excelsa), during our archaeological excavations. Even today,
these three species, along with ivory nut palm (Phytelephas macrocarpa), are commonly
encountered close to ancient geoglyphs (Virtanen 2011).
Ancient Acrean land-use techniques may have been comparable with those of the modern
indigenous Kayapó (Posey 1985; Hecht & Posey 1989; Denevan 2001:68–69), which
include intensive swiddening to clean and fertilise agricultural patches. This technique was
based on the annual use of controlled burning of the upland (terra firme) forest in regions
of moderate or high seasonality, mixing dry and fresh vegetation. This system is still used
by some local Indigenous people, such as the Manchineri and the Apurinã in the states of
Acre and Amazonas (Pirjo K. Virtanen, pers. comm.).
We are aware that the evidence presented here is not yet conclusive and that we require
more archaeological data and additional radiocarbon and carbon isotope δ
13
C measurements
from Western Amazonia. In every instance, however, our results so far clearly suggest the
potential need for the re-evaluation of previous studies, which assume that charcoals are
evidence for radically drier periods in the South-western Amazonian Holocene and do not
take into account the possibility of anthropogenic activity. Further, such a re-evaluation
may also have consequences for our understanding of Amazonian climate change and even
for the modelling of past climate fluctuations on a global scale.
Acknowledgements
Denise Pahl Schaan co-directed the research project, but passed away in 2018. In Brazil,
the research was authorised by the Instituto do Patrimônio Histórico e Artístico Nacional
(IPHAN). The University of Helsinki, Instituto Iberoamericano de Finlandia, Universi-
dade Federal do Acre, Universidade Federal do Pará, Prefeitura de Rio Branco and the
Governo do Acre also contributed to the project. We wish to thank them all for their
support.
Martti Pärssinen et al.
© The Author(s), 2020. Published by Cambridge University Press on behalf of Antiquity Publications Ltd
1552
Funding statement
The research on pre-Columbian geometric earthworks in the Western Amazon was funded
by the Academy of Finland (decisions 2567481 and 297161).
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