Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo

CATENA-02285; No of Pages 13 Catena xxx (2014) xxx–xxx

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Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo Leonor Rodrigues a,⁎, Umberto Lombardo a, Seraina Fehr a, Frank Preusser b, Heinz Veit a a b

Institute of Geography, University of Berne, Hallerstrasse 12, CH-3012 Bern, Switzerland Department of Physical Geography and Quaternary Geology, Stockholm University, 10691 Stockholm, Sweden

a r t i c l e

i n f o

Article history: Received 28 February 2014 Received in revised form 17 August 2014 Accepted 26 August 2014 Available online xxxx Keywords: Amazonia Llanos de Moxos Raised fields Pre-Columbian archaeology

a b s t r a c t Recent archaeological research suggests that some parts of the Amazon Basin were significantly modified by preColumbian populations. One of the most impressive examples of such transformations is the raised fields of south-western Amazonia, in the Llanos de Moxos in the Bolivian Lowlands. Despite a growing interest in raised field agriculture, due to the important role it seems to have played in the development of pre-Columbian complex societies, very few field-based investigations have been performed in the Amazon Basin. As a result, there is limited knowledge of how these fields were constructed, managed and within which time-frame they were in use. This study provides a new interpretation of how pre-Columbian raised fields were managed and a chronological sequence of their utilisation and eventual abandonment. Fieldwork was carried out in the indigenous community of Bermeo, in the vicinity of San Ignacio de Moxos, where some of the best preserved fields in the Llanos Moxos are found. Magnetic susceptibility and the geochemistry of the sediments, combined with radiocarbon and optically stimulated luminescence dating, show that the raised fields were in intermittent use since as early as AD 570–770. The original surface on which the fields were built and distinct periods of construction and use have been identified. The data suggests that raised fields were built during a few separate construction events, probably linked to periods of more frequent and severe floods. The study challenges the most widely accepted theory that suggests that pre-Columbians were able to cultivate these fields on a continuous basis by transferring nutrient-rich sediments from the canals to the fields. We conclude that pre-Columbians built raised fields to overcome periods of increased flooding, with the main objective of improving drainage. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The landscape of the Amazon Basin was significantly modified by humans during pre-Columbian times (Clement et al., 2010; Denevan, 2001; Hanagarth, 1993; Heckenberger, 2005; Lombardo et al., 2011b; Mann, 2000; Prümers, 2004). The Llanos de Moxos (LM) in the Bolivian Lowlands, with its great variety of earthworks, is one of the most important examples of pre-Columbian anthropogenic landscapes in the Amazon Basin (Erickson, 2008). One of the most impressive types of earthworks found here is raised fields. Raised fields were built by excavating canals/ditches and using the extracted earth to build elevated platforms for cultivation. Several types of raised fields, differing in shape and dimension, are found in the LM (Denevan, 1970; Lombardo et al., 2011a), but all of them are characterised by the alternation of elevated beds and canals, from which sediments were taken to raise the level of the fields. Since the discovery of raised fields in 1908 by Erland Nordenskiöld (Denevan, 1983), scholars have been studying raised fields due to their importance for the development and existence of pre-Columbian complex societies in Amazonia (Darch, 1988; Denevan, ⁎ Corresponding author. Tel.: +41 31 631 8890. E-mail address: [email protected] (L. Rodrigues).

2001; Erickson, 1995; Lombardo et al., 2011a, 2013a; Rostain, 2010; Walker, 2004). However, very few field-based studies have been carried out in the region (Erickson, 1995; Lombardo et al., 2011a; McKey et al., 2010; Walker, 2004). Recent research, using carbon isotopes, pollen and phytoliths (Iriarte et al., 2010; Renard et al., 2012a; Whitney et al., 2014) suggest that C4 plants, including maize, were important cultivars grown on raised fields. The currently most accepted model regarding the functioning and management of raised fields was inspired by the Chinampas wetland agriculture method reviewed in (Lombardo et al., 2011a; Renard et al., 2012b). Based on this model, the fields were fertilised through continuous addition of green manure and/or muck from the canals to the fields. In order to be able to produce green manure it is assumed that the canals retained water during the dry season. This kind of fertilisation is thought to have allowed continuous production without the need of fallow periods (Barba, 2003; Erickson, 2008; Lee, 1997; Saavedra, 2006) and the development of large sedentary populations. This model has been challenged by recent studies that stress that there is limited evidence to support some of its premises (Baveye, 2013; Lombardo et al., 2011a). Lombardo et al. (2011a) have shown that the topography of the areas where fields are found and the particle size composition of the sediments are not compatible with the presence of permanent water in the canals during the dry season. Up to now,

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Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021

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there is insufficient data regarding the management of the fields, agricultural productivity, cultivated crops and the time-frame of their utilisation, greatly limiting our capacity to estimate pre-Columbian population density and livelihoods. Raised fields have shown to be poor in artefacts and are for this reason difficult to date (Erickson, 1995; Walker, 2004). Nevertheless, raised fields located in the LM have been estimated to be as old as 2000 years (Erickson, 1995; Erickson, 2006). Radiocarbon ages of settlements located in the northern part of the LM, near the town of Santa Ana de Yacuma, suggest that nearby raised fields were in use during two particular time periods separated by approximately 900 years. These are the San Juan site, AD 446-613, and the Cerro site, AD 1311-1446 (Walker, 2004). The period between AD 100 and AD 1500 was characterised by unstable climatic conditions, when severe floods and droughts caused by ENSO anomalies were more frequent than today (Meggers, 1994; Moy et al., 2002; Rein et al., 2005). Based on this palaeo-climatic data, Lombardo et al. (2011a) have suggested that raised fields were introduced as a tool to mitigate the risks deriving from a period of more severe and frequent flooding events. The present study has two main objectives: firstly, to provide a chronology of the building, utilisation and abandonment of pre-Columbian raised fields in the indigenous community of Bermeo, in the LM, and secondly, to gain a better understanding of their management. The chronology of the fields is based on the stratigraphic analysis of the fields in combination with radiocarbon and optically stimulated luminescence (OSL) dating. To understand field management in ancient times, the chronological sequence is combined with standard soil laboratory methods, including micromorphological analysis of soil thin sections and magnetic susceptibility. 2. Study area The study site is located near the indigenous community of Bermeo, situated in the area of San Ignacio de Moxos in the south-western part of the LM (Fig. 1). The LM is one of the largest seasonally inundated wetlands in South America, inhabited since the early Holocene (Lombardo et al., 2013b). During the rainy season, from November to April, the flooded area can cover up to 80,000 km2 (Hamilton et al., 2004). According to Köppen's climate classification system, climate in the central LM is Aw (periodical dry savannah climate with dry winters). The mean annual precipitation in Bermeo is 2000 mm (Hijmanns et al., 2005). The

site is covered by dense forest growing on elevated fluvial deposits of palaeo-rivers; savannah vegetation grows on the lower-lying areas. The forest-savannah ecotone is controlled by seasonal floods (Mayle et al., 2007). The community of Bermeo is located on the northern bank of a palaeo-river channel and the raised fields studied are on its southern bank (Fig. 1). According to the inhabitants, the area of the fields is subject to flooding during the rainy season, whereas their community is rarely flooded. 3. Material and methods At the time of the study, the fields were covered by dense forest and trees had to be cleared to make the study site accessible. The local topography was measured using a digital level Sokkia D50. In total 4218 elevation points were recorded. A digital elevation model (DEM) describing the shape and dimension of the fields was generated, covering an area of 0, 77 hectares, using the 3D analyst extension of ArcGis with natural neighbour interpolation (Fig. 2). The location for the excavation of the fields and canals was chosen following a first field survey. The fields chosen were those that seemed best preserved. A trench (1000 × 130 cm) was excavated from the elevated bed FA to the elevated bed FB, passing through the lowest point in the canal (C) (Fig. 3). From the highest part of the bed, the soil profile FA was dug to a depth of 210 cm, where a dark brown palaeosol appeared. Since FB is 20 cm higher, it was dug to a depth of 230 cm. Two additional pits were dug: one on the embankment of the fields (Fig. 2, E) and the second one on the other side of the river levee, at a distance of 450 m from the raised fields, in order to have a reference profile (Fig. 1, REF). The reference profile is located in a site without fields that has the same kind of sediments as FA and FB, originating from the same palaeoriver levee. The reference profile and the fields are both located on an outer bank of the palaeo-river channel, ensuring that they had a similar depositional environment. In total five profiles were prepared for sampling: two from the adjoining fields FA and FB (Fig. 3), one from the canal (C) (Fig. 3), one from the embankment (E) (Fig. 2), and the fifth (REF) at a distance of 450 m from the raised fields (Fig. 1), on the other side of the river levee. In the reference area, a sampling pit was excavated to a depth of 100 cm. In order to perform a visual analysis of the stratigraphy a 300 cm core was extracted with a Wacker vibracorer (Fig. 6). The different profile units identified in the visual analysis

Fig. 1. Right: MODIS image of the Llanos de Moxos with the small continent map of South America. Main rivers are presented as blue lines. Red dot indicates the Bermeo study site. Left: The Bermeo study area. Red areas indicate the location of the raised fields studied and the reference profile REF; the blue line is the palaeo-river. Image source: Google Earth. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021

L. Rodrigues et al. / Catena xxx (2014) xxx–xxx

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Fig. 2. a.) Digital elevation model of the present-day morphology of the raised fields studied. Rectangles show the location of the trench excavated from FA to FB, the small rectangle indicates Profile E on the embankment and the topographic transect is indicated with a line from 1 to 2. b.) Digital elevation model showing the reconstruction of the original arrangement of the fields, without embankment.

were sampled. Descriptions of the horizons/layers were carried out following the guidelines for soil description of FAO (2006). For each sample, about 50 g of loose sediment was collected, stored in a plastic bag and air-dried afterwards. For the preparation of soil thin sections a

total of six undisturbed samples were collected from the identified stratigraphic units of the raised field FB (Fig. 3). The blocks were dehydrated in a freeze dryer for two days at − 50 °C, impregnated with hardening resin (100 g NSA, 40 g ERL 4221, 32 g DER 736, 1.2 g

Fig. 3. Schematic drawing of the trench going from field FA to FB. Sampling points for dating are indicated with circles and squares and calibrated ages. The horizontal dashed line separates Sequence 1 from Sequence 2. Ancient field boundaries are marked by lines showing the different phases of construction, 1/2, 3/4 and 5.

Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021

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DMAE*) and stored in a vacuum for 2 h to remove air bubbles. The blocks were cut, polished to 30 μm thick slices and adhered to a glass plate by Geoprep at the Department of Earth Sciences, University of Basel. Pedogenic features were identified under a Leica DMLP microscope in plain polarised and crossed polarised light (PPL, XPL) following the guidelines of Stoops et al. (2010) and Stoops and Vepraskas (2003). For soil pH, elemental and C/N analyses, 20 g of each sample were sieved through a 2 mm sieve and pulverised afterwards. In order to measure the pH, the first homogenised sample was mixed with a 0.01 M CaCl2 solution. Element analysis was performed using X-ray fluorescence spectroscopy (XRF). For each sample, 4 g (± 0.0009 g) of milled sample and 0.9 g (±0.0009 g) of Licowax C Micropowder were homogenised in an agate mortar for 10 min and pressed into pills at a pressure of about 234 bar. Concentrations of C and N were measured by dry combustion and gas chromatographic separation with a CNS analyser. For the determination of organic carbon content two separate samples were prepared. One was measured for total C and the other one for inorganic C after loss of ignition at 550 °C (LOI). Organic C is obtained by subtraction of inorganic C. To study particle size distribution, organic matter was removed with 30% H2O2 and then measured with a Malvern Mastersizer Hydro 2000S after dilution in 15 ml dispersing solution (3.3 g sodium hexametaphosphate + 0.7 g sodium carbonate per 1000 ml). Magnetic susceptibility was measured in the laboratory by means of a Barrington high sensitivity MS2E scanning sensor on homogenised samples. AMS 14C analysis on three charcoal samples and one soil sample was conducted at the Poznan Radiocarbon Laboratory and calibrated using Calib 7.0 (Stuiver and Reimer, 1993) and the SHcal13 calibration curve for the southern hemisphere (Hogg et al., 2013). The 14C-ages are all given in AD and BC. OSL dating was carried out on quartz (2 mm spray mask aliquots), using the Single-Aliquot Regenerative Dose (SAR) protocol (Murray and Wintle, 2000), with preheating at 230 °C for 10 s prior to all OSL measurements. More than 50 repeated measurements were carried out for each of the samples and these show symmetric Gaussian-like distributions with only very few outliers at the upper end of the distribution of samples Bermeo-1 and Bermeo-3. These are interpreted to represent incomplete bleaching of the OSL signal prior to deposition. The Finite Mixture Model (FMM) of Galbraith et al. (1999) was used to isolate the well-bleached population of DE values applying a sigma-b value of 0.17, as found for the well-bleached sample Bermeo-2. We identified two populations in the two samples, the lower of which contains 94% (Bermeo-1) and 98% (Bermeo-3) of all DE values. This indicates that the effect of partial bleaching is marginal in these samples, as also indicated by the small difference between the FFM age and that calculated using the Central Age Model (Galbraith et al., 1999), which includes all data. 4. Results 4.1. Morphology of the fields The DEM shows that the fields are all elongated, with a maximum length of 100 m, and a gradient of more than 50% between the top of the field and the bottom of the original canal (Figs. 2a and 3). In general the fields are better preserved in the northern part. This is particularly evident in the case of the largest field, which is heavily eroded in its southern side. The profile excavated from points 1 to 2 (Fig. 2a), cutting across the fields and canal, shows that the width of the fields decreases from west to east, going from a maximum of 100 cm to a minimum of 50 cm. The maximum field height, measured as the difference in elevation between the highest point of the field and the lowest point in the canal, is of 250 cm. According to Denevan's definition (1970), the fields can be classified as ridged fields. The lowest point in the DEM is a palaeo-channel in the north. Fig. 2a shows that, nowadays, the fields are more or less parallel and mostly embanked. However, Profile E, located on the embankment between Field FA and FB, revealed a reworked upper part, with pieces of pottery, and a thin yellowish

organic layer at a depth of 84 cm; the radiocarbon age of this layer is modern 109.02 ± 0.35 pMC (percent modern carbon), indicating that at least this field embankment is from recent times (Table 4). 4.2. Macrostructure and stratigraphy The field identification of stratigraphic units in Profiles FA and FB is based on the observed differences in density, texture and colour of the sediments. A detailed description is given in Table 1. The profiles were divided into numbered units, each comprising the same sedimentological and pedological properties. A well expressed palaeo-topsoil (bAh), with a dark brown colour, is found at the base of the Profiles FA and FB. The bAh indicates a period of stable conditions preceding the deposition of fluvial sediments. Two similar consecutive sequences with units of alternating bright-light sandy sediments and yellow–brown loamy sediments were identified on top of the bAh (in Figs. 3 and 4 the sequences are identified as 1 and 2). Sequence 1, extending from the bottom up to 100–110 cm, is composed of five distinct units in both profiles (FA:5A–9A and FB:6B–9B); whereas Sequence 2 includes four units in Profile FA (1A–4A) and five in Profile FB (1B–5B). Both sequences consist of units relatively rich in clay and sesquioxides (5A and 6B), sandwiched between two sandy units. There are two important differences between Sequences 1 and 2. In the first place, Sequence 1 has clearly defined boundaries between units and the sediment's density is higher than that of the overlying Sequence 2. In Sequence 2 the units are less well defined, with diffuse boundaries. Secondly, in Profiles FA and FB, the clayey unit present in Sequence 1 (at depths of 110–130 (5A) and 140–160 cm (6B)) is more conspicuous, with an intense brown–orange colour and a clear boundary (Figs. 3 and 4). Clay coatings were recognised by the lining of the ped surfaces and their shiny surface. These horizontal units (5A and 6B) are located 70–100 cm above the bAh. These observations suggest that Sequence 1 is the natural undisturbed soil on which the fields were built. Hydromorphic features are present in the lower part of the profiles, manifested in orange soft mottles and small iron and manganese concretions indicating repeated fluctuations in the water table. Charcoal was found at depths of 43 cm (Unit 3B) and 105 cm (Unit 5B) and as a cluster in the infilling of the canal (see sampling points for radiocarbon dating in Fig. 3). In contrast to the layering of the fields separated by units, the sediments in the canal appear homogenous; they are grey and present abundant hydromorphic features. The original boundary of the fields is easily visible as a transition zone separating the grey infilling of the canal from the stratigraphic units of the fields (Fig. 4). The canal was cut through the dark brown palaeosol bAh by removing it entirely. The centre of the canal is located at a distance of 150 cm from the point in which the canal cuts bAh below Field FA and at a distance of 250 cm from the point in which it cuts bAh below Field FB. This suggests that that the original position of the canal was closer to Field FB and that FB had a steeper slope than FA (Fig. 3). Similar stratigraphic units were observable in the REF profile, with a dark brown palaeo-topsoil at a depth of 210 cm, comparable with Sequence 1 from the Fields FA and FB (Fig. 5). 4.3. Soil physical and chemical properties 4.3.1. Grain size The analysis of particle size distribution reinforces the field observations and confirms the differences in the texture of the units of both field profiles and the REF profile. The results of the field observations and the particle size distribution are summarised and illustrated in Table 2 and Fig. 3. The dominant fraction for all profiles is silt. The units of Profile FA are composed of a silty loam, with a mean percentage of 67.3 silt, 22.6 clay and 19.7 sand. Up to a depth of 60 cm, including Units 1A–2A, there is no change in texture. Further down the profile there are alternating units of relatively coarser sediments (4A and 6A) and finer sediment (3A and 5A). The finer sediments in Units 3A and

Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021

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Table 1 Description of soil profiles FA, FB, Canal C and reference profile REF; following FAO (2006) guidelines. Depth (cm)

Units Description

Profile FA 0–10 10–50 50–70 70–100 110–130 130–150

1A 2A 3A 4A 5A 6A

150–180 180–190

7A 8A

200–210

9A

Canal 0–30 30–50 50–100

1C 2C 3C

Silty loam, water saturated, reduced grey matrix, small yellow–red mottles and diffuse boundary. Silty loam, water saturated, reduced light grey matrix (5YR 7/1), soft reddish yellow (7.5YR 6/6) iron mottles, diffuse boundary. Reduced reddish grey matrix (10R 6/1) abundant soft red Iron mottles (10R 5/6) and concretions up to 2 cm diameter, silty loam, water saturated, charcoal pieces.

Profile FB 0–10 10–40 40–80

1B 2B 3B

80–100 100–135 140–160 160–190

4B 5B 6B 7B

190–200 200–210

8B 9B

210–230

10B

Pale brown 10YR 6/3, very thin topsoil with little organic matter, diffuse boundary. Brownish yellow 10YR 6/6 silty loose granular structure, fresh roots, diffuse boundary. Very pale brown 10YR 8/3, dusty layer (more clay) just present on the highest part of the field, disappears towards the margins of the field, diffuse boundary. Light brown 7.5YR 6/4, similar to layer 5 but less dense silty loam, blocky structure, fresh roots, diffuse boundary. Light yellowish brown 10YR 6/4, silty loam, fine root channels Clear boundary, burrow holes, gradual boundary not as clear as in Profile FA. Strong brown (7.5YR 5/6) very defined dense layer, compressed blocky structure. Silty loam, sticky, clay coatings with shiny surfaces, clear boundary. Light yellowish brown 10YR 6/4, sandy loam, channels filled with more clayey sediments, burrow holes (0.5–1 cm wide) due to bioturbation, fine old roots channels but no roots, gradual boundary. Light yellowish brown 10YR 6/4, same as layer 7, but less sandy. Yellowish red 5YR 5/8, silty loam, transition zone with hydromorphic, features, yellowish red mottling 75% and soft manganese and iron concretions (3–5%), gradual smooth boundary. Paleosoil f A, Dark greyish brown 10YR 3/2, clay loam, dense-very low porosity. Fine white soft salts and red Iron concretions (3–5%).

Profile REF 0–10 20–30 50–40 60–70 70–80 90–100

1 2 3 4 5 6

10YR 4/3 (Brown–dark brown), silty top soil with a gradual boundary, fresh fine roots and major roots diameter of 2 cm 10YR 8/6 Yellow silty layer, gradual boundary 10YR 6/6 brownish yellow silty loam, hydromorphic orange stained, gradual boundary 5YR 6/8 reddish yellow, transition zone silty layer, lighter colour, diffuse boundary Matrix 10YR 8/6 Yellow mottles 5YR 5/8 yellowish red hydromorphic features, iron mottles, dense layer with gradual boundary 10YR 8/6 Yellow, silty layer lighter colour, less stained by iron oxidation, no mottles

Pale brown 10YR 6/3, very thin topsoil with little organic matter, diffuse boundary. Brownish yellow 10YR 6/6 silty loose granular structure, fresh roots, diffuse boundary. Brown 7.5YR 5/4, similar to layer 5 but less dense silty loam (more clay), blocky structure, fresh roots, diffuse boundary. Light yellowish brown 10YR 6/4, silty loam, fine root channels Clear boundary, burrow holes, clear boundary. Strong brown (7.5YR 5/6) very defined dense layer, compressed blocky structure. Silty loam, sticky, clay coatings with shiny surfaces, clear boundary. Light yellowish brown 10YR 6/4, sandy loam, channels filled with more clayey sediments, burrow holes (0.5–1 cm wide) due to bioturbation, fine old roots channels but no roots, gradual boundary. Light yellowish brown 10YR 6/4, same as layer 6, but less sandy. Yellowish red 5YR 5/8, silty loam, transition zone with hydromorphic, features, yellowish red mottling 75% and soft manganese and iron concretions (3–5%), gradual smooth boundary. Paleosoil f A, Dark greyish brown 10YR 3/2, clay loam, dense-very low porosity. Fine white soft salts and red iron concertinos (3–5%).

5A consist of only 9.1% and 8.4% sand respectively, whereas the coarser sediments in Units 4A and 6A consist of 29% and 37% sand respectively. This pattern is consistent with the changes described of alternating yellowish and brownish Munsell soil colours (Table 1 and 2). Profile FB reveals a similar granulometry, with 63.9% silt followed by 21.4% clay and 19.2% sand. Up to a depth of 80 cm, including Units 1B–5B, there is no significant change in texture composition. A marked change is noticed at a depth of 150 cm between Units 6B and 7B, with only 9% sand in

Unit 6B versus 46% in Unit 7B. Units 6B and 7B are analogous to Units 5A and 6A, comprising the same texture. The grain size of the dark brown bAh at the bottom of the profiles (Units 9A and 10B) is characterised by a far higher percentage of clay, indicating a different sedimentary depositional environment. The REF profile shows a similar stratigraphy. The dominant fraction is also silt, with a mean of 62.9% silt followed by 22.7% clay and 13.5% sand. A unit comprising considerably less sand (7.9–9.6%) is repeated

Fig. 4. Excavated trench, showing Profile FA and the canal. Left: Profile FA showing Sequence 1 and 2 with palaeo-topsoil bAh at the bottom. Dashed lines indicate the boundaries of units with elevated clay content. Right: Trench from Profile FA to the canal. Narrow dashed line indicates the transition zone separating the infilled canal from the original field; note that the layering of the fields differs from the hydromorphic and homogenous grey infilling of the canal.

Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021

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Fig. 5. Pictures of the Reference Profile (REF). Left: Dashed lines indicate the boundaries of units with elevated clay content. Right: Extracted invasive core, arrow indicating palaeosol.

twice in the profile (depth 40–50 and 110–115 cm). The texture composition of these two REF units coincides with 3A, 5A and 6B. At a depth of 120 cm there is a distinct change towards silty clay, probably due to a less energetic depositional environment.

4.3.2. Magnetic susceptibility The results are given by the dimensionless value (k) which is directly related to the magnetic susceptibility of the materials (Table 2). In Profile FA, Units 1A to 3A have much higher magnetic susceptibility values

Table 2 Physical properties of the selected profiles. Sample Name Profile FA (units) 1A 2A 2A 3A 4A 5A 6A 7A 8A 9A

Profile FB (units) 1B 2B 3B 4B 5B 5B 6B 7B 8B 9B 10B

REF profile (units) 1 2 3 4 5 6 7 8 9

Depth:

Sand

Silt

Clay

Magnetic susceptibility

10–20 20–30 40–50 60–70 90–100 120–130 140–150 170–180 190–200 200–210 Mean:

63.00 μm–2000.00 μm 12.638 16.556 15.408 9.183 29.042 8.427 37.478 24.076 17.207 7.988 19.778

2.00 μm–63.00 μm 62.848 62.17 61.089 65.93 53.994 67.759 50.853 58.525 62.375 60.206 67.305

b4.00 μm 22.943 19.24 23.503 24.887 15.065 23.314 10.597 16.248 18.114 31.806 22.857

(κ) 17 33 10 22 7 6 4 4 4 x

10–15 20–30 43–48 80–87 100–105 128–133 150–155 180–190 190–200 200–210 220–230 Mean:

63.00 μm–2000.00 μm 17.854 16.711 15.408 16.774 18.753 17.331 9.098 46.633 21.446 18.18 7.474 18.697

2.00 μm–63.00 μm 60.164 61.659 58.635 65.132 66.11 62.621 69.62 45.126 60.946 60.213 55.847 60.552

b4.00 μm 20.342 21.63 23.503 18.094 15.073 21.127 22.919 8.241 16.643 21.607 32.061 20.113

(κ) 14.00 26.00 14.00 22.00 13.00 8.00 7.00 4.00 4.00 4.00 4.00

0–10 20–30 50–40 60–70 70–80 90–100 110–115 120–125 175–180 Mean:

63.00 μm–2000.00 μm 15.621 17.187 9.583 22.003 12.044 32.496 7.944 0.643 4.091 13.512

2.00 μm–63.00 μm 68.712 67.313 67.265 60.460 67.494 51.809 61.381 45.651 73.672 62.640

b4.00 μm 15.667 15.500 23.152 17.537 20.462 15.695 20.098 53.706 22.237 22.673

(κ) 8 4 7 5 6 3 x x x x

Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021

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than the Units 5A–9A below. Overall values decrease with depth from 17 to 4 k; however, two peaks were identified: one in the upper part of Unit 2A, at a depth of 20–30 cm with 33 k, and the other in Unit 3A, at a depth of 60–70 cm with 22 k. A similar pattern is evident in Profile FB. The intensity of the magnetic susceptibility in Units 1B to 4B is much higher than in Units 5B–10B. Values decrease with depth from 14 to 4 k, with two peaks, in Unit 2A, at a depth of 20–30 cm, and in Unit 4B, at a depth of 80–87 cm. For both field profiles, the values of the units in Sequence 1 double those of the underlying units in Sequence 2. For the REF profile an overall decrease in magnetic susceptibility is also observed with depth, with values between 8 and 3 k.

lipid clay coatings can be characterised as typic with approximately regular thickness and a fine layering indicating processes of clay illuviation (Fig. 7a) Stoops et al. (2010). Hydromorphic features can be identified in the thin sections from the moderately iron-impregnated domains of the matrix and iron nodules (Fig. 7e). The intensity of hydromorphism increases in the lower part of the profile, which is more affected by redox changes due to year-round fluctuating groundwater conditions (Scheffer et al., 2002). Charcoal was identified in the upper Units 2B– 5B; this is most clear in the thin sections from Units 4B and 5B taken at a depth of 85 cm and 105 cm (Fig. 7g and h). Apart from charcoal no other anthropogenic amendments could be identified.

4.3.3. Geochemistry In all the profiles the amounts of organic carbon (Corg) and nitrogen (N) are low and decrease with depth (Tab.3). The highest value for Corg, 1.27%, is found in the REF profile at a depth of 0–10 cm. Inorganic C is present in extremely low concentrations and the highest measured value is 0.03%. The amount of N varies in the range of 0.12 to 0.01%. These low N values are not surprising given the high mineralisation rates in tropical climates (Scheffer et al., 2002). As a whole, values of Corg concentration in Profile FA decrease with depth to a minimum of 0.1%. However, there is a relatively high value of 0.34% in Unit 4A, at a depth of 90–100 cm. Thereafter, there is a sharp decrease from 0.13 to 0.1%. A similar pattern is identified in Profile FB. In this profile, two units with slightly elevated Corg contents were identified. The two peaks are found at depths of 80 cm and 150 cm. The Corg values in the REF profile also decrease with depth, from 1.27% at the top to 0.25% at the bottom, but, contrary to what happens in the raised fields, in the REF profile there are no peaks of relatively high Corg values. The soil pH values of the raised fields (FA and FB) are in the range of 4.1–4.6, increasing towards the bottom. The pH values in Profile REF are slightly higher, ranging from pH 4.89 to 5.4.Percentage contents for major and trace elements are given in Table 3 and displayed in Fig. 6. All the profiles show a similar composition, which is dominated by SiO2, with a mean value of 68%. Other important elements include Al2O3, with a mean value of 16.37%, Fe2O3, with a mean value of 4.56% and MgO and K2O, with a mean value of 1.42% and 2.75% respectively. Element concentrations show no clear trend throughout the profiles. However, Profile FA (Units 5A–9A) and Profile FB (Units 6B–9B) show that the concentrations of the major oxides SiO2, Al2O3, TiO2, and Fe2O3 and trace elements Mn, Zn, CU, and Pb, correlate directly with the amount of clay (Fig. 6). This is due to the fact that several oxides and elements (Al2O3, Mn, TiO2, Fe2O3, Cu, Zn, Mg) are more abundant in the finer fractions of the sediments, whereas other oxides, such as SiO2, are more concentrated in the coarser fraction of the sediments (Moore et al., 1989; Zhang et al., 2002). This effect of grain size on the concentration of specific oxides and elements is not observed in Units 1A–4A nor in Units 1B–4B. In Profile REF the correlation between grain size and element composition is clearly present in all units. The element composition and grain size of Units 3 and 5 in Profile REF are similar to Unit 5A in Profile FA and Unit 6B in Profile FB.

4.5. Chronology

4.4. Microstructure Pedological features and their micromorphological characteristics were identified by analysing thin sections (Fig. 7). The grains all appear orange–brown, stained by the presence of iron oxides. The thin section analysis shows evidence of anthropogenic reworking between the top and Unit 6B. The groundmass from top until Unit 6B is characterised by irregular voids (Fig. 7a, b, d and f) and aggregates with different grain size compared to the matrix (Fig. 7c, d and f). Most of the voids have been filled in with sediments that are finer than the matrix (Fig. 7b and d). Dusty clay coatings (Fig. 7c, d and f) as well as early stages of limpid clay coating formation along the voids walls can be observed in thin sections taken from Units 2B to 6B (Fig. 7a and b). The

A total of three OSL ages and six radiocarbon datings have been obtained. Three OSL and two radiocarbon ages are from the infilling of the canal, three radiocarbon ages from Field FB and one radiocarbon age is from the embankment E (Fig. 3). The OSL ages increase with depth. The oldest age, AD 570–770, was detected at the bottom of the canal at a depth of 135 cm; the youngest age, AD 1629–1679, was found at a depth of 45 cm. In addition, radiocarbon ages for two charcoal pieces taken from a charcoal accumulation in the canal at a depth of 100 cm were dated AD 1334–1391 and AD 1388–1438. Two pieces of charcoal from Field FB were also dated by radiocarbon; the two ages are AD 1144–1232, at a depth of 43 cm, and AD 958–1046, at a depth of 105 cm. The dark brown palaeosol, bAh, at the bottom of the fields has been dated BC 5515–5304. This palaeosol predates the deposition of the alluvial sediments on which the fields were built. Similar palaeosols have been described in the region by Lombardo et al (2012). As a whole, the data shows an increase in age with depth in the profiles. This age– depth correlation is common for undisturbed “natural” soils (Wang et al., 1996). Reworking due to agriculture often hampers the possibility of obtaining a consistent chronology in raised fields (Erickson, 1995). Furthermore, natural causes of reworking, like bioturbation and argilliturbation, can cause the displacement of charcoal fragments. (Noller et al., 2000). However, in the case here reported, radiocarbon ages seem to be consistent with their stratigraphic location, suggesting that the charcoal has not been significantly affected by agriculture or bioturbation. 5. Discussion The DEM shows that, nowadays, the fields are more or less parallel and are embanked in the southern side and also in the northern side in the direction of the palaeo-river. However, the radiocarbon age from Profile E (Table 4) shows that the embankment of the fields is relatively recent. It is likely that when the community of Bermeo was established, people modified the original fields, building a causeway across them. It is important to note that this causeway is still used today by the inhabitants of Bermeo. The material used to build the causeway was taken from Field FB (Fig. 2). Taking this into account, the DEM has been modified in order to show the original shape of the fields (Fig. 2b). The reconstruction of the original shape of the fields shows that the canals on both sides of the largest field were not embanked and, therefore, allowed the water to drain into the palaeochannel. The original depth of the canal and the shape of the fields are evident in the stratigraphy, as the hydromorphic characteristics of the sediments are markedly different. The original position of the canal, at the time of the first construction event, was closer to Field FB. This is shown by the cut through the bAh in Field FA and FB and the transition zone between the infilling of the canal and the stratigraphy of the field (Figs. 3 and 4). As already described by Lombardo et al. (2011a) the original depth of the canal, at the time of its construction, was at least 1 m deeper than today. The reconstruction of the original shape shows that the

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Table 3 Geochemical properties of the selected profiles. Sample name Profile FA 1A 2A 3A 4A 5A 6A 7A 8A 9A

Profile FB 1B 2B 3B 4B 5B 6B 7B 8B 9B 10B

Profile REF 1 2 3 4 5 6

Depth:

pH

Corg (%)

N(%)

Na2O%

MgO%

Al2O3%

SiO2%

Mn%

Fe2O3

TiO2%

K%

Ca%

Px%

Pb%

Zn%

Cu%

10–20 20–30 40–50 60–70 90–100 120–130 140–150 170–180 190–200 Mean:

4.13 4.19 4.16 4.32 4.55 4.61 4.53 4.52 4.56 4.40

0.51 0.41 0.25 0.25 0.34 0.13 0.20 0.08 0.10 0.25

0.07 0.05 0.04 0.05 0.02 0.03 0.01 0.02 0.03 0.04

0.32 0.36 0.35 0.38 0.35 0.36 0.36 0.34 0.40 0.36

1.41 1.26 1.47 1.29 1.24 1.59 1.23 1.28 1.54 1.37

16.71 15.26 16.83 15.71 14.16 18.14 14.19 15.03 17.14 15.91

66.72 70.14 66.15 70.06 73.06 63.20 73.90 71.09 66.44 68.97

0.05 0.05 0.04 0.02 0.04 0.05 0.03 0.04 0.05 0.04

5.16 4.46 5.65 3.18 4.04 6.30 3.84 4.49 5.51 4.74

0.374 0.369 0.391 0.338 0.309 0.396 0.328 0.361 0.388 0.362

2.86 2.72 2.90 2.62 2.56 2.97 2.64 2.68 2.91 2.76

0.20 0.19 0.15 0.22 0.18 0.21 0.16 0.17 0.24 0.19

0.0752 0.0641 0.0691 0.0237 0.0774 0.0662 0.0648 0.0587 0.0624 0.0624

0.0034 0.0036 0.0035 0.0036 0.0041 0.0045 0.0034 0.0032 0.0032 0.0036

0.0063 0.0066 0.0076 0.0063 0.0052 0.0080 0.0044 0.0060 0.0079 0.0065

0.0008 0.0014 0.0021 0.0023 0.0015 0.0013 0.0009 0.0014 0.0016 0.0015

10–15 43–48 80–87 100–105 128–133 150–155 180–190 190–200 200–210 220–230 Mean:

4.18 4.30 4.56 4.54 4.48 4.68 4.77 4.73 4.72 5.10 4.61

1.20 0.34 0.41 0.33 0.18 0.24 0.12 0.10 0.32 0.65 0.39

0.12 0.08 0.07 0.07 0.05 0.06 0.04 0.02 0.04 0.09 0.06

0.39 0.36 0.51 0.44 0.43 0.45 0.49 0.36 0.36 0.46 0.42

1.24 1.57 1.30 1.82 1.49 1.74 1.27 1.17 1.45 1.67 1.47

15.13 18.03 14.40 19.16 15.91 18.82 14.46 15.70 16.89 19.45 16.80

70.34 65.04 71.92 60.93 69.80 62.39 73.05 72.10 67.74 61.93 67.52

0.05 0.05 0.04 0.06 0.03 0.05 0.03 0.01 0.04 0.03 0.04

3.84 5.12 3.66 6.02 4.19 5.82 3.71 2.52 5.31 3.79 4.40

0.351 0.367 0.333 0.381 0.345 0.383 0.325 0.361 0.388 0.338 0.357

2.49 2.75 2.45 3.00 2.60 2.88 2.47 2.67 2.87 2.85 2.70

0.15 0.14 0.21 0.29 0.20 0.20 0.10 0.17 0.26 0.33 0.20

0.0854 0.0618 0.0622 0.0731 0.0817 0.0638 0.0587 0.0624 0.0610 0.0248 0.0635

0.0032 0.0033 0.0032 0.0039 0.0035 0.0040 0.0032 0.0032 0.0036 0.0032 0.0034

0.0083 0.0097 0.0087 0.0109 0.0085 0.0108 0.0060 0.0079 0.0074 0.0114 0.0090

0.0010 0.0015 0.0011 0.0022 0.0010 0.0022 0.0014 0.0016 0.0010 0.0024 0.0015

0–10 30–20 50–40 60–70 70–80 90–100 Mean:

4.89 5.40 5.01 5.13 5.10 4.93 5.08

1.27 0.34 0.30 0.30 0.29 0.25 0.46

0.13 0.05 0.05 0.05 0.04 0.03 0.06

0.36 0.47 0.41 x 0.40 0.33 0.39

1.29 1.10 1.60 x 1.52 1.27 1.36

15.02 13.78 18.67 x 17.45 16.26 16.24

70.95 72.91 62.10 x 63.80 70.19 67.99

0.02 0.04 0.05

3.86 3.86 6.42 x 6.03 4.14 4.86

0.316 0.348 0.400 x 0.407 0.366 0.306

2.76 2.48 3.04 x 3.02 2.79 2.82

0.25 0.33 0.32 x 0.31 0.19 0.28

0.0748 0.0339 0.0467 x 0.0600 0.1360 0.0586

0.0031 0.0025 0.0040 x 0.0038 0.0034 0.0028

0.0045 0.0047 0.0089 x 0.0076 0.0073 0.0055

0.0011 0.0014 0.0030 x 0.0013 0.0000 0.0014

Field FA was elevated 250 cm above the level of the bottom of the original canal and the slope was much steeper than today, leaving only 40% of the initial surface available for agriculture, the rest being occupied by canals or slope (Lombardo et al., 2011a). According to the local people of Bermeo, the raised fields never become flooded today (personal communication, July 2012). This is confirmed by the species of trees and palms that grow on the edge of the raised field. These species, such as

0.04 0.01 0.03

the motacú palm (Attlea phalerata), do not tolerate prolonged periods of waterlogging. According to the Hooghoudt (1940) drainage equation as demonstrated in Youngs (1985), the water table draw-down is a function of water table depth, saturated hydraulic conductivity of the sediments and depth and spacing of the canals (van der Ploeg et al., 1999). Hence, the deeper the canals are excavated the wider the raised fields can be built. The depth of the canals when they were first

Fig. 6. Affinity of elements to grain size. Units with elevated clay content are indicated with thick grey lines.

Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021

L. Rodrigues et al. / Catena xxx (2014) xxx–xxx

9

Fig. 7. Thin section from different depths: a.) Initial stages of clay coatings (CC) with sharp boundaries along a void (V), taken at a depth of 85 cm (XPL light). b.) Infilling of a void with finer groundmass and clay coatings, taken at a depth of 105 cm (XPL light). c.) Fine aggregates incorporated in coarser matrix taken at a depth of 64 cm (PPL light). d.) Infilled voids with fine sediments and dusty coatings taken at a depth of 64 cm (PPL light). e.) Hydromorphic feature: large Iron nodules, taken at a depth of 235 cm (PPL light). f.) Deformed voids and dusty clay coatings; evidence of reworked material, taken at a depth of 60 cm (XLP light). g.) Pieces of charcoal, taken at a depth of 85 cm (PPL light). h.) Fragment of charcoal, taken at a depth of 105 cm (PPL light).

excavated and the narrow spacing between the canals suggests that the risk of flooding, which had to be mitigated, was extremely high. If this had not been the case, it would have not been necessary to elevate the

fields to this level, losing so much cultivable land. This suggests that the main function of the canals was not to retain water but to improve the drainage by lowering the local water table.

Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021

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L. Rodrigues et al. / Catena xxx (2014) xxx–xxx

Table 4 AMS radiocarbon ages of charcoal samples, given both as 14C age BP and calibrated radiocarbon age in AD/BC format at two-sigma level. OSL ages are given as years before sampling (rounded to the next 5 years) and converted to AD/BC format to allow direct comparison with radiocarbon ages. pMC (percent modern carbon). Radiocarbon ages 14C Profile Depth (cm) Material 14 C age 95.4% (2σ) cal age ranges RAUPD Lab number Cal date

Field FB 43 Charcoal 905 ± 30 BP AD 1144– 1232 0.915 Poz-36131 10.06.2014

Field FB 105 Charcoal 1085 ± 35 BP AD 958–1046 0.854 Poz-36133 10.06.2014

Field FB 230 6480 ± 60 BP BC 5515–5304 1 Poz-39569 10.06.2014

Profile

Canal C

Canal C

Embankment E

Depth (cm) Material 14 C age 95.4% (2σ) cal age ranges RAUPD Lab number Cal date

100 Charcoal 695 ± 30 BP AD 1334–1391 0.571 Poz-36130 10.06.2014

105 Charcoal 585 ± 30 BP AD 1388– 1438 0.878 Poz-39568 10.06.2014

84 Plant remains 109.02 ± 0.35 pMC Modern

Profile

Canal C

Canal C

Canal C

Depth (cm) OSL age Converted to A.D.

135 1340 ± 100 (FFM a) AD 570–770

70 465 ± 35 (CAM a) AD 1509–1579

45 355 ± 25 (FFM a) AD 1629–1679

Poz-36134

OSL ages

RAUPD: relative area under probability distribution.

Geochemical changes in the sediments, which could have derived from intensive agriculture, were not identified. Studies of anthropogenic soils have revealed enrichment in certain elements, such as P, Ca, Mg, Zn, Cu and Mn, as a result of past human occupation, due to kitchen and garden waste accumulation (Costa et al., 2013; Kern et al., 2003; Woods and McCann, 1999). Phosphorous has shown to be persistent due to its high potential of fixation by soil elements like Fe, Al and Ca (Holliday and Gartner, 2007). However, such enrichment was not detected in the case of the raised fields under study. Consequently, there is no evidence that manure was applied on the fields on a continuous basis, as suggested by several authors (Barba, 2003; Erickson, 2008; Lee, 1997; Saavedra, 2006). The combined results from the analysis of grain size, stratigraphy, pH and the geochemistry of the sediments indicate that there are strong similarities between the REF profile and the lower part of Sequence 1 (5A–9A and 6B–10B) of Profiles FA and FB. This suggests that these sediments predate the construction of the fields and show that natural soil properties have been preserved in Sequence 1. The pattern of alternating coarser and finer sediments is a relic of the parent material caused by changes in the river stream power at the time of the deposition of the alluvial sediments (Bridge, 1992). In contrast, Sequence 2 (1A–4A and 1B–5B) shows evidence of having been reworked. The thin sections provide clear signs of human disturbance, such as dusty clay coatings and voids which are partially infilled with fine sediments. Initial stages of limpid clay coating formation can also be observed. Interestingly, these clay coatings were found in all thin sections, but they are most pronounced in the upper reworked Sequence 2. The major soil forming factors for the illuviation of clay and in-situ formation of clay coatings are time, good drainage and pH values in the range of 5–7 (Velde and Meunier, 2008). The study of Bockheim and Hartemink (2013) describing a variety of soils with clay enriched horizons by illuviation, showed that a time interval of more than 2000 years is needed for pedological formation of argillic horizons. If we consider the short time period since the construction of the fields and the low pH values of the fields, we would more likely expect that the formation of clay coatings occurred before the construction of the fields. However, the fact that the clay coatings are more pronounced in Sequence 2 and are always found along the voids with sharp

boundaries, indicates in-situ pedogenic clay coatings formation. Since in well drained soils and in areas with intense precipitation pH is very variable, the pH could have been different in the past (Scheffer et al., 2002). Recent studies on rates of pedogenic processes, such as formation of clay coatings by illuviation in Luvisols, are reviewed in Alexandrovskiy (2007). This study points out that in pre-weathered soils, with textural differentiations, pedogenic processes proceed much faster and Bt horizons can already start to form after only 70 years. Young soils which have been ploughed for agriculture may quite possibly comprise dispersed topsoils (Jongerius, 1970). Clay infillings due to agriculture have also been reported for pre-Columbian raised fields in Ecuador (Wilson et al., 2002). Such coatings have been described as dusty coatings mixed with silt as a result of intensive agriculture. It is possible that the more dispersed material in Sequence 1 was translocated to lower horizons by rain water. The fact that most of the clay coatings found in the thin sections are limpid suggests the existence of periods of undisturbed pedogenic conditions (i.e. absence of agriculture), such as periods of abandonment. The association between specific oxides and elements and grain size, noted in Sequence 1, does not occur in the upper parts of the fields. Therefore, the stratigraphy of the fields can be divided into two parts: the basal first metre (Sequence 1), above the bAh, which is undisturbed and subject to natural forming conditions, and the following part (Sequence 2), which results from human activity (Figs. 4 and 7). An interesting difference between Field FA and FB was identified; in Field FA, the texture of the units of Sequence 2 mirrors that of the units in Sequence 1. This repeated sequence does not occur in Field FB. In Field FB the texture of Sequence 2 is almost homogenous. This can be explained by the fact that Field FA is wider than FB and therefore less susceptible to erosion. It is therefore possible that the erosion of Field FB was more severe and the reworking needed for its maintenance caused the mixing of the sediments. No distinct fossil A horizon could be identified in the field between Sequence 1 and Sequence 2. However, the high peak of organic carbon identified in Unit 5A and 6B indicates the earlier presence of a fossil A horizon above these Units. It has been shown that the vertical distribution of Corg with depth can vary significantly depending on vegetation and climate; nevertheless, as a rule, it tends to decrease exponentially

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L. Rodrigues et al. / Catena xxx (2014) xxx–xxx

with depth (Jobbagy and Jackson, 2000). The higher value in Unit 4B is probably due to the presence of charcoal. The results of magnetic susceptibility show clear peaks in Units 2A and 3A in Profile FA and Units 2B and 4B in Profile FB. Values here are more than two orders of magnitude higher than in the units above and below. The analysis of magnetic susceptibility has been proven to be a valuable method to reconstruct past human occupation and identify the use of fire in agricultural practice (Arroyo-Kalin, 2012; Bellomo, 1993; Ketterings et al., 2000; Reyes et al., 2013). It has been shown that weakly susceptible minerals, like iron oxides, after burning increase their magnetic susceptibility (Tite and Mullins, 1971). However it has to be acknowledged that several factors, such as parent material, soil formation processes, water regime, topography and climate, also have important effects on the magnetic susceptibility of minerals within soil profiles (Hanesch and Scholger, 2005). Generally, magnetic susceptibility increases when soil matures (Hanesch and Scholger, 2005; Singer et al., 1996). Waterlogging has been reported as an important factor that weakens the magnetic susceptibility of soils (Grimley et al., 2004; Maher, 1998). The water regime of the field site has been modified extensively by raising and draining the soils through the construction of the canals. As a result, Sequence 1 is not subject to prolonged waterlogging anymore and this could very possibly explain why the values for the units in Sequence 1 are more than double those in Sequence 2. Geochemical changes derived from anthropogenic amendments were not detected, suggesting that anthropogenic influences can be ruled out. It has to be considered that the undisturbed sediments in the REF profile have been exposed to pedogenesis for the same period of time as the sediments in the field site. For this reason it is likely that drainage and better aeration are responsible for the overall higher values in Sequence 1. The charcoal pieces found in the thin sections in Units 4B and 5B (Fig. 7g and h) suggest that fire was used to prepare the fields for cultivation. The use of fire could therefore be a possible explanation for the two pronounced peaks of organic carbon in the upper horizons of the fields. It is very likely that the site was forested before the construction of the fields. Additionally, the use of fire could have been a way to overcome the occurrence of diseases and weed invasion in the fields. The study, by Stab and Arce (2000), of agricultural field experiments carried out in the Bolivian lowlands, has shown that weed invasion and diseases are a major problem. Several studies have reported that burning and fallow periods are an efficient way to control weeds and diseases with relatively low labour cost (de Rouw, 1995; Pleasant et al., 1992; Roder et al., 1995). In addition, the two peaks of magnetic susceptibility in Sequence 1 could be the results of two stable periods without deposition of new sediments, allowing therefore natural soil formation and causing an increase in magnetic susceptibility. The two peaks in magnetic susceptibility can indicate soil forming conditions, the use of fire or a combination of both. In any case, they suggest the presence of two distinct fossil surfaces chronologically and stratigraphically separated by a construction event that increased the elevation of the fields. The study of the physical properties, geochemistry, and micromorphology of the fields of Bermeo shows that the Chinampas model is not applicable in this case. As reviewed in Lombardo et al. (2011a), the Chinampas model suggests that continuous use of the fields was possibly based on the assumption that fields were fertilised with nutrient rich sediments and manure from the canals; this required the canals to retain water during the dry season. The capacity of the canals to retain water has been challenged by Lombardo et al. (2011a), who argued that most fields, including the site of Bermeo, were built on fluvial deposits, which are the best drained spots. Considering the long time period of about 600 years between the first detected use and abandonment of the fields, a repeated accumulation of sediments and manure transported from the canals to the fields would have resulted in a lamination of the sediments and a more homogeneous distribution and accumulation of organic substance on top of the fields. This could not be detected in the stratigraphy of the fields or the geochemistry of Sequence 1, which is not significantly different from Sequence 2 or the REF profile.

11

The thin sections do not show any feature of lamination. Except for a few charcoal pieces, neither anthropogenic amendments nor signs of intensive fertilisation, such as dark staining of clay minerals, could be identified. The application of manure has been reported in preColumbian raised fields in Ecuador, where thin sections of buried agricultural soils were characterised by a dark brown micromass containing brown dusty clay coatings resulting from manuring (Wilson et al., 2002). The lack of pronounced pedogenic changes suggests that the fields in Bermeo were managed more extensively and with very long fallow periods. OSL dating has shown that the raised fields have been in use since as early as AD 570–770, when the canal was first excavated and the fields were raised. The latest detectable evidence of human activity comes from two charcoal pieces, dated AD 1334–1391 and AD 1388–1438, originating from a dark brown accumulation in the canal at a depth of 100 cm. This age probably marks the last period of field management, which might have involved the use of fire. The chronology and spacing of the OSL ages suggest that the infilling of the canal, which probably started with the abandonment of the field, is the result of a constant and undisturbed deposition of sediments. The time of abandonment can be estimated, therefore, to be between AD 1388–1438 and the beginning of the continuous infilling, AD 1509–1579. Since AD 1509–1579, 70 cm of sediment has been deposited in the canal from the erosion of the adjunct fields and the overflow of the palaeo-river, which during the rainy season can be reactivated. It can be safely assumed that the infilling was faster at the beginning, when the canal was deeper and the erosion from the fields greater, and that it slowed down with time, as the slope between fields and canal became less steep. However, between the sediments dated AD 570–770 at the bottom of the canal and the time of abandonment, at AD 1509–1579, there is only 50 cm of sediment. This is far less than one would expect to have been deposited during almost 900 years. We interpret this as evidence that the canal has been re-excavated to a depth of 100 cm, where we can find the accumulation of charcoal. The results suggest that the fields were raised during a few major construction events, which were then followed by relatively short periods of use and very long fallow periods. The history of Bermeo's raised fields can be divided in five phases (Fig. 3). 5.1. Phase of construction 1 and 2 The area was first cleared in AD 570–770 by burning and this was followed by the excavation of the canal to a depth of at least 130 cm. Taking the present canal as a reference, the original position of the canal was more towards Field FB. The canal was about 1 m wider than it is today. The sediments were piled on top of Sequence 1, resulting in a mirrorinverted succession that maintained the original granulometric characteristics of the bottom part of Sequence 1. A similar construction has been described in the case of raised fields studied in French Guiana, where layers of soil were raised up-side down and supplemented with organic rich top soil from elsewhere (McKey et al., 2010). The exact extent to which the field was elevated during Phase 1 cannot be reconstructed because it is not clear how much of it was eroded. Nevertheless, the charcoal accumulation and the higher magnetic susceptibility indicate a former surface at about 80–90 cm below the current field. After a period of abandonment, Phase 2 started with the clearing of the forest; this is associated with particularly well-marked units (3A and 4B) showing clear signs of burning and containing abundant charcoal dated back to AD 958–1046. This points to a period of field use, where the surface was cleared with fire. 5.2. Phase 3 and 4 of construction and Phase 5 of abandonment Phase 3 and 4 are characterised by a construction stage between AD 958–1046 and AD 1144–1232, when the fields were raised at least up to the present level (250 cm). This implies that the surface of these two

Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021

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fields was elevated an impressive 100 cm above the ancient surface (Sequence 1). As well as elevating the fields, the canal was dug further to a depth of at least 200 cm below the raised fields, as shown by the modern ages from the in-filling of the canal. The fact that the fields were built so high suggests that, in this period, drainage was the most important requirement for the design of the fields' shape and size. The time of abandonment lies between the last fire management AD 1388–1438 and the beginning of the continuous infilling from AD 1509 to 1579. The time when fields were in use coincides with a period of strong climatic fluctuations. ENSO activity has been widely recognised to be an important driver of millennial variability (Cole et al., 2000; Espinoza Villar et al., 2009; Moy et al., 2002; Rein et al., 2005), responsible for extreme drought and floods during the last 2500 years in South America (Díaz and Markgraf, 2000; Meggers, 1994; Moy et al., 2002; Rein et al., 2005). Studies comprising the south-western Amazon Basin have shown a correlation between ENSO activity and precipitation; this is most pronounced during negative ENSO (La Niña) years (Aalto et al., 2003; Bookhagen and Strecker, 2010; Ronchail et al., 2005). Higher frequencies of ENSO related extreme events occurred between 1000 and 2000 years ago (Moy et al., 2002), reaching a maximum at around AD 800 and, from this moment on, decreasing to present levels. During this interval several extreme flood events have been reported in Peru, (Craig and Shimada, 1986; Magilligan and Goldstein, 2001). The dimension and the time of the second raising of the fields bear evidence of an adaptation to the described climate fluctuations. Nevertheless, the spatio-temporal pattern of rainfall in the Amazon Basin is complex and not yet fully understood (Bookhagen and Strecker, 2010). For an accurate assessment, more high resolution palaeo-climate records of local conditions are needed, as well as further research on raised fields in the Llanos de Moxos and other areas. 6. Conclusion OSL dating has shown that the raised fields of Bermeo, in the Llanos de Moxos, were constructed around AD 570–770 and thereafter were modified at least two times; the first time between AD 958–1046 and AD 1144–1232, when the fields were elevated up to 250 cm above the level of the adjacent canals and, more recently, when the modern indigenous population constructed a causeway across the fields. The time of the initial construction and of the later modification of the fields seem to be related to strong climatic fluctuations, most probably driven by increasing variability of ENSO activity. The impressive height of the fields suggests that an important objective was to improve the drainage. The geometric characteristics of the fields and increased frequency of past strong precipitation suggest that the Bermeo fields were an adaptation to periods of increased risk of floods. Contrary to what has been suggested by some authors (Barba, 2003; Erickson, 2008; Lee, 1997; Saavedra, 2006), our data suggests that the raised fields in Bermeo were in use for limited periods of time and not intensively cultivated. We suggest that raised fields were constructed primarily to overcome periods of more intense flooding, with the first priority being drainage to protect the crops from inundations. Acknowledgements The present study has been funded by the Swiss National Science Foundation (SNSF), grant no SNF 200020-141277/1, and performed under authorisation N_ 017/2012 issued by the Unidad de Arqueología y Museos (UDAM) del Estado Plurinacionalde Bolivia. We thank Dr. M.R. Michel López from the Ministerio de Culturas and our Bolivian counterpart Dr. J.M. Capriles for their support. We acknowledge the community of Bermeo for their collaboration in the field and for allowing us free access to their land. Fieldwork assistance by B. Vogt, A. Plotzki and B. Roesti is gratefully acknowledged. We thank D. Fischer for technical support in the laboratory. Gamma spectrometric measurements have been carried out by S. Szidat, Department of Chemistry &

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Please cite this article as: Rodrigues, L., et al., Pre-Columbian agriculture in the Bolivian Lowlands: Construction history and management of raised fields in Bermeo, Catena (2014), http://dx.doi.org/10.1016/j.catena.2014.08.021