paleoindian open-air sites in tropical settings: a case study in formation processes, dating...

Research Article

Paleoindian Open-Air Sites in Tropical Settings: A Case Study inFormation Processes, Dating Methods, and PaleoenvironmentalModels in Central BrazilAstolfo G.M. Araujo,1,* Andre M. Strauss,2 James K. Feathers,3 Julio Cesar Paisani,4 and Thomas J. Schrage5

1LEVOC—Laboratory for Interdisciplinary Research on Evolution, Culture, and Environment, Museum of Archaeology and Ethnology, University of SaoPaulo—MAE/USP, Sao Paulo, Brazil2Max Planck Institute for Evolutionary Anthropology, Department of Human Evolution, Leipzig, Germany3Laboratory of Luminescence Dating, University of Washington, Seattle, Washington, D.C.4Universidade Estadual do Oeste do Parana, UNIOESTE, Campus Francisco Beltrao, Parana, Brazil5Department of Geography, University of Sao Paulo, Sao Paulo, Brazil

Correspondence*Corresponding author;

E-mail: [email protected]

Received4 March 2012

Accepted13 December 2012

Scientific editing by LucyWilson and Howard Cyr

Published online in Wiley Online Library

(wileyonlinelibrary.com).

doi 10.1002/gea.21442

Paleoindian sites in eastern South America are generally found inside rock-shelters. Recently, we designed a program to find open-air Paleoindian sitesin the Lagoa Santa region of Brazil. Here, we present data gathered at Sumi-douro site, a multicomponent site with a Paleoindian horizon, detected on ashoreline slope of Sumidouro Lake, Minas Gerais State. We present a model forlate Pleistocene/Holocene slope evolution that suggests at least three differentepisodes of instability (erosion/sedimentation) followed by periods of stability(pedogenesis). Higher soil accretion rates during the Mid-Holocene are prob-ably not linked to a stable dry climate, but to greater climatic instability. Wealso found that the position of both archaeological materials and charcoal canbe explained by burial of ancient surfaces and are not the result of downslopemovement. The role of bioturbation in the vertical displacement of such ma-terials is much less important than previously acknowledged. C© 2013 WileyPeriodicals, Inc.

INTRODUCTION

Detection of Paleoindian sites in eastern South Americais often constrained by an approach directed toward ex-cavation of rock-shelters (Schmitz et al., 1989; Prous &Malta, 1991; Guidon et al., 1996). Lagoa Santa, in East-ern Central Brazil, is a good example. Since the pioneer-ing work of Danish naturalist Peter W. Lund, in the firsthalf of the nineteenth century, all Paleoindian sites foundin the region are located inside caves or rock-shelters(Walter, 1958; Laming-Emperaire et al., 1975). Thismeans that in 170 yr of research, the picture of Paleoin-dians from Lagoa Santa has been heavily biased towardshelter settings. Despite the richness and wealth of infor-mation that shelter sites can provide, such as human andother animal remains, bone, lithic tools, and plant mate-rial, they do not fully encompass Paleoindian variabilityin terms of land use, territorial boundaries, and resourceexploitation.

In 2000, researchers from the University of Sao Paulo,headed by W.A. Neves and colleagues, initiated a long-

term paleoanthropological project in the region (Neves& Pilo, 2008). The work involved excavation of severallimestone rock-shelters and, after 2003, efforts towardfinding and characterizing early open-air sites through anintensive subsurface prospection program. We detectedtwo Paleoindian sites, Sumidouro and Coqueirinho, onthe shores of Sumidouro Lake (Figure 1A). In this paper,we present Sumidouro site data derived from OSL and ra-diocarbon dating, geochemistry, micromorphology, andartifact spatial distribution. Aside from the archaeologi-cal contributions, our data also suggest that Paleoindianopen-air sites in tropical settings can be a valuable sourceof information regarding slope processes and paleoenvi-ronmental reconstructions.

Although several authors have dealt with the relation-ship between soils, archaeological materials, and pale-oenvironments, most approaches address soils in temper-ate (Haynes, 2008; Mandel, 2008; Cordova et al., 2011;Holliday et al., 2011) or Mediterranean climates(Angelucci et al., 2007; Mallol et al., 2011) or, whenin the tropics, in semi-arid environments (Ashley et al.,

Geoarchaeology: An International Journal 28 (2013) 195–220 Copyright C© 2013 Wiley Periodicals, Inc. 195

PALEOINDIAN OPEN-AIR SITES IN TROPICAL SETTINGS ARAUJO ET AL.

Figure 1 Location of the study area in Brazil; (A) Location of Lagoa Santa region, southeast Brazil, and topography of the Sumidouro Lake doline and its

tributaries, showing Sumidouro and Coqueirinho sites. (B) Topographic map of Sumidouro site, 0.5 m contour intervals, showing all excavation units.

2011). We therefore argue for the necessity of approach-ing the archaeological record in humid tropical soils witha geoarchaeological perspective, leading to the develop-ment of a tropical geoarchaeology, providing a fresh ap-proach toward long-held assumptions about tropical soilsmade by pedologists and geomorphologists.

GEOLOGICAL AND ARCHAEOLOGICALCONTEXT

The Lagoa Santa region is a karstic area encompassing1022 km2 near the city of Belo Horizonte, State of MinasGerais (Figure 1A). The geology of the area is composedmainly of upper Pre-Cambrian metasedimentary rocks ofthe Bambuı Group (IBAMA/CPRM, 1998). The BambuıGroup is composed of a basal metacalcareous unit, theSete Lagoas Formation, and an upper metapelitic unit,the Serra de Santa Helena Formation. Hydrothermal so-lutions cutting the Pre-Cambrian rock bodies producedsiliceous veins and geodes, many of them with hyalinequartz that were heavily used as raw materials by the lo-cal Paleoindians. Weathering of the Serra de Santa He-lena Formation allowed Oxisol soils to form (Pilo, 1998),

whereas weathering of the pure carbonates of the SeteLagoas Formation produced a karst landscape with caves,dolines, and other features. Vegetation today is comprisedby patches of savanna (“cerrado”), deciduous (“mataseca”), and semideciduous forests (IBGE, 1992).

Sumidouro Lake formed at the bottom of a “polje,”or an interior valley, formed by a coalescence of dolines(Pilo, 1998). The lake forms a semi-ellipse, with its ma-jor axis measuring 1.2 km northwest–southeast, and itsminor axis 0.7 km northeast–southwest (Figure 1A). Thelake shore has gentle slopes, with the exception of thesoutheast extremity, where a 50 m high block of resid-ual limestone acts as a natural dam. Samambaia Creek,coming from the west, is the lake’s major tributary. Asinkhole formed underneath the outcrop drains the lakewater. In the wet season (September–March) the inflowexceeds the sinkhole drainage capacity, and the lake fills.During the dry season (April–August) the lake level dropsand in very dry years the lake dries completely leav-ing a flat plain within which the Samambaia Creek hasincised.

The Sumidouro site is located near the lake shore onthe lower portion of a 380 m long, 12% inclined slope.This long slope has a very gentle topography, interrupted

196 Geoarchaeology: An International Journal 28 (2013) 195–220 Copyright C© 2013 Wiley Periodicals, Inc.

ARAUJO ET AL. PALEOINDIAN OPEN-AIR SITES IN TROPICAL SETTINGS

only by scattered termite mounds. Soils in the region arevery thick (10 m in average; Pilo, 1998) and overlay thelimestone bedrock.

Project personnel found the site in 2003 by subsurfaceprospection, using a motorized bucket auger. This equip-ment allowed recovery of 3 m-long soil columns in a fewminutes. We spaced the auger holes at 15 m intervals in ahexagonal grid system (Krakker et al., 1983) across a 120× 120 m2 quadrate. Sieving of sediment from the augerholes allowed recovery of artifacts. We initially discov-ered archaeological materials in two adjacent cores, B1and B2, and subsequently placed two 1 × 1 m2 test pitsat these locations. The coring recovered a quartz flakeat 177 cm depth in Unit B1, and the excavation un-covered three more quartz flakes at a depth of 137 cmdepth, another five flakes at 150 cm depth, and severalmore lithics at approximately 160 cm depth. Finally, at180 cm depth, more quartz flakes appeared, togetherwith a whitish, heavily patinated flint flake. From thisinitial work, we obtained two radiocarbon dates fromcharcoal samples associated with archaeological materi-als: 5020 ± 70 14C yr B.P. (5920–5600 cal. yr B.P.; Beta205350) at 137 cm depth and 8310 ± 40 14C yr B.P.(9450–9240 cal. yr B.P.; Beta 205351) at 160 cm depth.The 180 cm depth level had no charcoal associated witharchaeological materials (see Araujo & Feathers, 2008 formore details). These dates represent the first open-air Pa-leoindian ages in Lagoa Santa. Subsequently, we datedthe archaeological levels of B1 by OSL with preliminaryages reported in Araujo & Feathers (2008) and updatedhere. Some discrepancies between the OSL and radiocar-bon ages were apparent, but since we were dealing witha deep tropical soil, we did not expect good agreementand did not even expect a good fit between the verticalpositioning of pieces and their original placement.

Between 2005 and 2008, we opened another eight1 m2 excavation units (Figure 1B). Data obtained in-cluded observations about soil horizons and clastic inclu-sions, vertical positioning of artifacts and charcoal frag-ments, and radiocarbon ages. Besides a rather diffusechange in color and texture, no further stratigraphicchanges were obvious, and the soil was sampled in 10cm intervals. All charcoal fragments found in the sievesduring excavation (larger than 5 mm) were collected,dried in the laboratory, and weighted in an analyticalscale.

METHODS

Soil Geochemistry and Micromorphology

Soil nomenclature was made according to the BrazilianSystem of Soil Classification—SiBCS (EMBRAPA, 2006)

that follows the FAO 2006 World Reference Base forSoil Resources. We determined soil chemical composi-tion from different horizons and excavation units witha Phillips PW2400 XRF Unit, conducted at the Laboratoryfor X-Ray Fluorescence, Instituto de Geociencias, Univer-sity of Sao Paulo, Brazil. We analyzed soil micromorphol-ogy of undeformed soil samples at the Laboratory for Op-tical Microscopy of the Universidade Estadual do Oestedo Parana—UNIOESTE, Francisco Beltrao Campus, Brazilusing a Leica DM 2500 P trinocular microscope with acoupled Leica EC 3 camera and Leica Application Suite—LAS EZ software, version 1.4. We identified sedimentaryfeatures, source materials, and post-depositional changesvia microstructures/pedofeatures using criteria and ter-minology of Stoops (2003), and methods of Paisani andPontelli (in press).

Radiocarbon and Luminescence Ages

Dating efforts concentrated on a few excavation units inorder to monitor the behavior of charcoal and to compareradiocarbon with luminescence results. To ensure a max-imum control over the sample provenience, we collectedcharcoal fragments for radiocarbon dating only from pro-files or during excavation (Figure 2) and never from thesieve. All radiocarbon samples were processed at Beta An-alytic, Inc., the ages obtained from individual charcoalfragments. We employed luminescence dating because ofinconsistent radiocarbon ages obtained initially and be-cause single-grain luminescence dating could also provideinformation on soil turnover and history of movementof individual particles. Laboratory procedures are givenin Appendix. We also provide here a brief descriptionof luminescence dating for readers unfamiliar with theanalysis.

Luminescence dating is made possible by the long-termstorage, in the crystal lattice of some minerals, of energyabsorbed from natural radioactivity. The stored energy isin the form of charge carriers, such as electrons, trappedwithin defects in the crystal structure. This energy is re-leased by the emission of light (luminescence) followingstimulation by heat or sunlight, making possible the dat-ing of thermal or depositional events. A luminescenceage is the quotient of the equivalent dose and the doserate. The equivalent dose (De) is the laboratory estima-tion of the total dose absorbed through time since thelast release event. It is obtained by calibrating the naturalluminescence signal against applied radiation in the lab-oratory. The dose rate is determined from the radioactiv-ity of the sample and its immediate environs, assuming aconstant rate through time. We measured the dose ratesonly in the laboratory (see Appendix), as an attempt tomeasure dose rates in the field was not successful. We



Figure 2 North and east profiles of Unit S4 showing the major stratigraphic units, sample collection locations, and radiocarbon ages.

measured equivalent dose (De) on single grains of quartzusing the single-aliquot regenerative dose (SAR) method(Wintle & Murray, 2006). We initially used 150–180 μmgrains, but subsequently used 180–212 μm grains. Be-cause of the relatively low sensitivity of the grains, therewere no significant differences in results from either grainsize. Because De is measured on every grain, the outputis a distribution of De values. Even if all grains are thesame age, these values will not be identical for statisticalas well as other reasons. One advantage of single-graindating is the ability to remove from analysis those grainsthat have either no signal or signals which behave in away that do not meet the assumptions of the method(criteria are given in Appendix). This is particularly im-portant for the Sumidouro samples because, even thoughof not great age, the quartz appeared to saturate at a rel-atively low level, so that many grains had a large signalbut a De could not be determined. Overall, only about 9%of all grains measured were accepted for analysis. A sec-ond major advantage of single-grain dating, and one thatis particularly relevant at Sumidouro, is the ability to de-tect post-depositional mixing by the scatter in De valuesamong grains. This is possible only by controlling othercauses of scatter, either intrinsic (related to the lumines-cence characteristics of the grains themselves or to ma-

chine reproducibility) or extrinsic (related to the natureof the deposition).

Intrinsic sources of scatter can be evaluated by a dose-recovery test, whereas extrinsic causes are controlled. Inthis test, grains that have had their natural luminescencesignal removed are given a laboratory dose (in this caseabout 20 Gy), and then the SAR procedure is applied tosee if that dose can be obtained.

Granulometry

For granulometric analysis, we used two different meth-ods: standard pipette analysis was applied to 27 samplescoming from two excavation units, B2 and S4, in order toassess the primary soil texture, meaning that the sampleswere crushed and sieved, and therefore the grains werereduced to their smallest size. We also measured 30 sam-ples from Unit S4 by laser scattering, using a Malvern Hy-dro 2000MU particle-size analyzer. We measured thesesamples in natura, without any crushing or sieving, andonly treated them with a sodium pyrophosphate solutionfor deflocculation. Since the Oxisols present in the regionare ultimately composed of indurated, extremely resis-tant aggregates of clay that behave as sand (Pilo, 1998),episodes of sedimentation and sheetwash should produce



aggregate sorting depending on flow competence. Crush-ing the aggregates, as in standard granulometry, wouldtherefore not be informative.

THEORETICAL EXPECTATIONS

Based on the vast literature about tropical soils, biotur-bation, and slope processes (Cahen & Moyersons, 1977;Thomas, 1994, 2004; Araujo, 1995; Boulet et al., 1995;Miklos, 1992), we started our work with two initial com-peting theoretical expectations regarding slope processesand site formation processes. The first states that archae-ological materials are present in discrete levels that re-flect different episodes of human occupation, interbeddedwith colluvium deposited by means of sheetwash pro-cesses in a subhorizontal manner. The second is that thereare no discrete archaeological “layers.” Soil creep pro-cesses (Clarke et al., 1999) are responsible for the burialand, in some cases, concentration of archaeological mate-rials. This process of colluvial accretion occurs not by sub-horizontal deposition but in a convolute manner, mix-ing materials from different occupation events, includingcharcoal. If the first expectation is correct, then charcoalfragments are good indicators of soil ages and, therefore,artifact deposition events. On the other hand, if the sec-ond expectation is correct, there will be no direct relation-ship between the soil mass, artifacts, and charcoal frag-ments. In this case, direct dating of burned artifacts (stoneor ceramics) will be necessary, and luminescence datingof quartz grains should be a better indication of deposi-tional age than radiocarbon dating of charcoal fragments.

After the initial assessment of the site, and based uponour theoretical expectations, we decided to invest moreeffort in determining the ages, cultural contents, and for-mation processes operating at the site. Given the data ob-tained in Units B1 and B2, we thought the site stratig-raphy would present a rather monotonous sequence ofa reddish upper soil horizon overlying a yellowish bot-tom horizon, both following the extant slope angle, andarchaeological materials mirroring the vertical distribu-tional pattern observed earlier. However, reality wasquite different and several later observations were totallyunexpected.

RESULTS

Paleotopography, Soil Horizons, and StoneLines

We placed two additional excavation units east and westof B1 and B2, following approximately the same contourline (Figures 1B, 2, and 3A). In Unit S3, 15 m west ofB1, we encountered a gravel layer at 170 cm depth com-

posed of very poorly sorted milky quartz and quartziteclasts. The clasts are angular to rounded in shape withsizes ranging from granules to cobbles up to 25 cm diam-eter. Because of difficulties in removing the cobble layer,excavation stopped after 250 cm depth, or only 80 cminto the layer. Unit S4, 7.5 m east of unit B2, revealed avery different profile. We encountered at 230 cm deptha 20-cm thick organic deposit, probably related to an an-cient high lake stand. It was extremely well-developed,compared to weak organic development in the neighbor-ing unit B2.

We placed additional excavation units (S5, S6, S7, S9,and S10) along the slope to delimit the site and assessdifferences related to height above the lake (Figures 1Band 3B). Most striking was the differential thickness ofthe upper “reddish” soil layer (colors 5YR 5/6, 7.5YR 4/4,and 7.5YR 5/6), which measures at least 300 cm thickupslope in Unit S10 and steadily decreases in thicknessdownslope to 110 cm in Unit S6. Below the red soil is ayellowish one, only visible (within the extent of excava-tion) in the downslope units (colors 10YR 6/6, 10YR 5/8,10YR 4/6).

The presence of well-developed gravel layers in UnitsB1, B2, S3, S4, S6, and S7 and less-developed gravel hori-zons in Unit S10 is also of interest. Such gravel layers arecommon in tropical settings, under the general designa-tion of “stone lines,” the genesis of which is controver-sial (Johnson, 2002; Hiruma, 2007; Santos et al., 2010).The stone line here is discontinuous (occurring in UnitsS6 and S4, but not in the intermediate Unit S5, for ex-ample), very poorly sorted, and with variable thickness.From grain-size measurements using the standard pipettemethod in Units B2 and S4, the colluvium at the site ap-pears to be mainly clay, with a small increase in sand inthe lower horizons. Average clay content was 62% forUnit B2 and 53% for Unit S4 (Figure 4).

The compound evidence suggests that the paleotopog-raphy in the late Pleistocene was very different from theextant gentle slope. A branch of the lake appeared toreach the bottom of Unit S4 but not adjacent units, andthe several pockets of gravel accumulation forming stonelines probably relate to gully infilling.

Matrix Characterization

Overall, the chemical composition is very homogeneousacross the soil profiles (Table I). CaO and P2O5 decreasedwith depth, as expected, due to the diminution of the in-fluence of organic compounds. There was also a minordecrease in Fe2O3 with depth, probably related to porespaces and the presence of the water table. Figure 3Agives the location of the geochemistry samples for UnitsB1 and S4.



Figure 3 (A) Topographic profile of Sumidouro Site showing the relative position of Units S5, S6, S7, S9, and S10, placed along a catena. (B) Relative

location of Units S3, B1, B2, and S4. Numbers under unit name provide their height over the datum (lake level in July 2008) in millimeters. Note the wavy

pattern of the stone line, and the discontinuous nature of the organic levels and gleysols.

Figure 4 Granulometry (standard sieve and pipette method) determined for Units B2 and S4.



Table I Soil chemical composition from different horizons and excavation units obtained by XRF.

Sample: B1-F1 B1-F2 B1-F3 B1-F4 B1-F6 S4–40 S4–70 S4–230 S4–240 S6–100 S6–170 S6–270

Depth: 15 cm 60 cm 152 cm 243 cm 290 cm 45 cm 75 cm 230 cm 240 cm 100 cm 170 cm 270 cm

Number: 1512a 1513a 1514a 1515a 1516a 1131a 1128a 1018a 1031a 1541 1526 1542

SiO2 56.23 57.53 56.15 59.90 59.37 59.37 56.79 62.48 64.89 54.63 54.21 57.17

Al2O3 21.23 22.35 23.78 21.88 21.79 20.43 23.22 20.64 18.59 24.22 19.72 22.06

MnO 0.043 0.018 0.013 0.011 0.009 0.320 0.024 0.008 0.008 0.011 0.287 0.035

MgO 0.46 0.42 0.45 0.44 0.42 0.41 0.40 0.40 0.37 0.48 0.55 0.72

CaO 0.30 0.18 0.07 0.03 0.03 0.18 0.12 0.03 0.03 0.20 0.12 0.07

Na2O 0.04 0.03 0.02 0.04 0.02 0.02 0.02 0.03 0.04 0.02 0.02 0.03

K2O 0.76 0.70 0.69 0.71 0.73 0.67 0.66 0.63 0.58 0.69 1.03 1.41

TiO2 1.272 1.155 1.227 1.207 0.915 1.367 1.267 1.253 1.109 1.140 0.999 0.968

P2O5 0.285 0.207 0.163 0.145 0.172 0.209 0.230 0.141 0.129 0.219 0.273 0.226

Fe2O3 7.15 7.40 7.21 6.66 7.81 6.33 7.00 5.03 5.14 6.39 13.04 8.73

Loi 12.59 10.59 10.07 9.39 9.00 10.91 10.17 9.75 8.53 10.38 9.12 8.65

Total 100.36 100.58 99.84 100.41 100.27 100.22 99.90 100.39 99.42 98.38 99.37 100.07

Ba 259 229 230 236 226 235 229 270 204 262 345 468

Ce 116 127 120 103 82 96 136 121 101 122 142 154

Co 7 4 7 7 6 8 8 7 8 9 47 12

Cr 127 130 126 125 142 117 129 127 112 127 144 124

Cu 30 28 29 28 30 27 31 30 25 33 39 32

Ga 30 31 33 31 31 29 32 32 27 34 28 31

La 29 42 38 35 34 34 36 44 35 43 51 55

Nb 24 22 23 23 18 26 24 24 22 22 16 15

Nd 29 32 34 31 33 31 30 41 24 36 49 45

Ni 37 36 39 37 37 34 38 33 30 45 37 43

Pb 34 35 36 34 36 30 34 29 28 37 40 45

Rb 63 56 54 53 56 54 57 51 46 63 79 98

Sc 21 19 21 20 20 19 20 22 17 20 22 20

Sr 43 43 44 45 54 41 42 43 40 49 61 59

Th 18 18 19 18 15 18 18 15 15 16 15 14

U 8 6 6 6 6 7 7 5 6 5 4 5

V 154 154 176 142 162 172 157 164 149 154 192 155

Y 23 22 22 23 19 25 23 43 33 24 25 24

Zn 64 62 66 63 65 58 65 58 53 80 87 90

Zr 282 272 274 281 229 299 283 278 268 262 233 226

Cl <50 <50 <50 <50 <50 <50 <50 <50 <50 <50 <50 <50

F 602 823 714 918 930 891 799 591 779 941 1042 1480

S <300 <300 <300 <300 <300 <300 <300 <300 <300 <300 <300 <300

aSample position shown in Figure 3A.

To study matrix features, we analyzed thin sections onfive samples collected in the north profile of Unit S4, des-ignated 1158, 1159, 1166, 1160, and 1165 from top tobottom (Figure 2, Table II).

Sample 1158 (130 cm depth) shows microfeatures sug-gesting colluvial deposition under a high viscosity sedi-mentary flux (Table II, first column). Vesicular pores sug-gest air was trapped during this process (Bertran & Texier,1999). The amount of water was sufficient to liquefy thematerial, the fine fraction (“b-fabric” according to Stoops,2003) witnessing vertical mobilization of its constituents,as suggested by iron dissociated from other elements of

the plasm (Figure 5A–D). This gives the groundmass aporphyritic organization (Table II, third column), whichis common in high viscosity sedimentary fluxes. Thesource material is the Oxisol present in the upper por-tions of the slope, with a high degree of geochemical evo-lution. Subangular blocky structures and microaggrega-tion suggest post-depositional processes related to strongbioturbation by both flora and fauna. The absence ofiron dissociation and feature infilling (intrusive pedofea-tures) suggest a relatively young bioturbation process.The structural organization of the materials, the strong tomoderate pedality, and the blocky and channel/chamber



Table II Selected main micromorphological characteristics.

Post-Deposicional Transformations Microstrutures/Pedofeatures

Pedalityc Voidd Microstructuree Groundmass

c/f Related b-Fabrich Matrix

Distributiong Featuresi

Horizon/Bed

(cm)

Sample

Sedimentary

Featuresa

Source

Material

FeaturesbIntrusive

Featuresf

2Bniticb/VII mx sd, m-s, us,

ug, ve, dif

mx rb, nj s-m ck, paj, plj,

vj,vejsb-cr dp uk, csj, psj, gsj siol, hvj, qcj,

nb,ccj, mj, ccj

(130)

1158

1BAb/IV mx sd, m-s, us,

ug

mx rb, nl w-m vk, cl, paj,

plj,

v-ch dp uk, csl, psj, gsj siol, hvj, na,b cj, mj

(190)

1159

1Bb/IV mx sd, m-s, us,

ug

mx sb, nj m-s ck, pll, vl,

paj,

sb-cr dp uk, spl siol, hvl, hgj,

na,bdcil, cj, mj

(210)

1166

1Bb/IV mx sd, m-s, us,

ug

mx sb, nj m-s ck, pal, pll,

vjsb-cr dp-sp spk, ul siok, hvl, hgj,

na,bdcil, cj, mj

(225)

1160a + b

1Cg3b/III mx sd, m-s, us,

i, ug

mx db, nj w plk, cj, vj m sp-cp spk, ul, rj siok, hvl, hgl,

na,bdcil, mj

(245)

1165

amx sd = matrix-supported, m-s = mud-sandy, us = unstratificaded, i = locally inclined, ug = ungreded, dif = downward intersticial flow.bmx = matrix, rb = reddish-brown, sb = strong brown, db = dark brown collor, n = strongly impregnation nucleic nodule with subangular quartz.cs = strong, m = medium, w = weakly developed.dc = channel/chamber, pa = packing, pl = planes, v = vughs, ve = vesicles.esb = subangular blocky, cr = crumb (microaggregate), v = vugh,ch = channels, m = massive.fc = capping, m = micropan, cc = crescent coating, dci = dense complete infilling vughs planes.gcp = close, sp = single spaced, dp = double spaced porphiric.hu = undiferentiated, cs = circular stried, ps = porostried, gs = granostried, r = random striated, sp = speckled.isio = segregation of iron oxides (depletion), n = nodule stronga, mediumb, weaklyc impregnation; hv = hypocoating voids, hg = hypocoating grain,

qc = quasicoating voids.j> 2%.k<10%.l2–10%.

microstructures suggest an incipient B horizon formingin colluvium. These represent characteristics of a buriednitic horizon1 (Bnitic Ab).

Sample 1159 (190 cm depth) also showed features sug-gesting high viscosity sedimentary flux, but the absenceof vesicular pores and interstitial vertical movements sug-gests a system with less water than observed in 1158. Thesource area is also the same, a very mature soil from ups-lope, with iron oxyhydroxides and nodules; however, thepresence of greater number of nodules suggests that theprimary source was an even more mature soil horizon(Figure 6). Post-depositional transformations include dis-sociation among plasm constituents, generating cavitarymicroporosity, and bioturbation. In this case, bioturba-

1According to EMBRAPA (2006), nitic horizons are thick min-eral horizons, with low activity clay, well drained, clayey to veryclayey, subangular blocky structure, angular or prismatic moder-ate or strong, sometimes with shiny surfaces that may be relatedto coatings or compression surfaces.

tion is weak to moderate, insufficient to promote the in-dividualization of the subangular blocks. These observa-tions are characteristic of a B horizon in transition to A,developed in a colluvial deposit (BAb).

Sample 1166 (210 cm depth) shows microfeatures sug-gesting the same depositional processes observed at Sam-ple 1159. The relict material from the source area isalso similar to the previous sample, but with a greaterfrequency of nodules and differing in color, tendingto yellow. The micropores are filled (dci), suggestingthe presence of a greater amount of expansive clays(Figure 7). Post-depositional transformations are primar-ily related to dissociation among plasm constituents, gen-erating microcavitary and fissural microporosity, and bio-turbation. In this case, bioturbation is moderate to strong,sufficient to promote subangular block individualization.These characteristics point to a Bb horizon developedover a colluvial layer. Sample 1160 (225 cm depth) isvery similar to 1166, being part of the same yellowish



Figure 5 Sample 1158 thin sections. (A) Detail showing iron remobilization due to vertical migration of the interstitial water during depositional process.

Plasm segregation andmigration to vesicular pores (ve) generating hipocutans (hc) and quasi-cutans (qc). (B) Same image, XPL, showing plasmorientation

resulting from the above-mentioned processes. (C) Microstructure in subangular blocks and microaggregates resulting from bioturbation, containing

the “a” image. Note the nodule with strong iron impregnation in contrast to areas where the plasm shows iron depletion. These represent two different

features, one related to the parental material (nodule), the other to the depositional process (iron remobilization). (D) Basal portion of the thin section

showing the fragmentation of the massive structure due to bioturbation. Fissures associated with biogenic channels can be perceived in the central

portion of the image. (E) Same image, XPL.

layer, but with a greater number of microaggregates.It is the same Bb horizon developed over a colluviallayer.

Sample 1165 (245 cm depth) represents the organiclevel at the bottom of the soil profile. The groundmass isdark brown, matrix-supported, with an angular hetero-metric skeleton. Bioturbation is moderate to weak. Theabsence of pedality and the homogeneous microstruc-ture suggests very weak pedogenesis (Figure 8). Thereis iron depletion in the groundmass, especially near thebiogenic pores. Some zones show evidence of iron re-duction, from ferric (Fe+3) to ferrous (Fe+2). These fea-tures suggest an organo-mineral sediment generated out-side the lacustrine environment, probably deposited in aviscous sedimentary flux under subaerial conditions, andlater subject to hydromorphy and bioturbation. Macro-scopic field observations of this level showed the presenceof polygonal fractures, suggesting mud cracks. Hence, it ispossible that we are dealing with a lake bottom sedimentsubject to subaerial conditions, followed by burial andtemporary hydromorphic conditions, related to water-table oscillations.

In sum, the groundmass properties point to matrix-supported materials, without stratification or gradation.The coarse fraction is mainly composed of angular quartzgrains, suggesting lack of mechanical abrasion duringtransport. The overall characteristics suggest that a highviscosity sedimentary flux (earth flow) was responsiblefor the soil accretion along the whole column, althoughother minor processes were probably involved. The up-per, reddish soil horizon is characterized by microstruc-tures with strong porosity and more developed pedality,while the lower, yellowish horizon presents a massivestructure and channel-like pores. As expected, bioturba-tion is an important factor in the overall soil develop-ment, but its intensity is irregular along the profile. Aswe will see later, this difference is probably related to pa-leoenvironmental conditions.

Vertical Distribution of ArchaeologicalMaterials and Charcoal Fragments

Due to the extreme hardness of the soil, which did notallow precise excavation, a resolution no better than



Figure 6 Sample 1159 thin section. (A) Massivemicrostructure tending to vughy. Local presence of crumbs related to bioturbation. (B) Same image, XPL,

showing areas of circular striated orientation as decorrence of plasm segregation and microcavity development. (C) Detail of the nodules with strong

impregnation (pedorelicts) and biogenic channel partially filled by microaggregates. (D) Same image, XPL, showing a lack of correspondence between

the material composing the nodules and the groundmass.

10 cm levels could be obtained for recording and quan-tifying archaeological materials and charcoal fragments.In a few instances archaeological pieces were recoveredin situ and individual coordinates recorded. Table III givesthe tabulation of charcoal weight and number of lithicartifacts for nine excavation units.

The most important archaeological observations are (1)the artifacts show frequency peaks across the profiles, butthese peaks do not correlate statistically with peaks incharcoal; and (2) there is no significant statistical rela-tionship between weight of individual artifacts and depth(Figure 9). Pearson’s r was calculated for each unit, show-ing very low correlations between lithics and charcoal(Unit S3: r = 0.019, P = 0.94; Unit S4: r = 0.299, P = 0.18;Unit S5: r = 0.078, P = 0.74; Unit S6: r = 0.042, P = 0.99;Unit S7: r = −0.156, P = 0.49). This lack of correlationcan be explained in at least two ways. First, the produc-tion of charcoal might be related to natural fires. Second,charcoal fragments could be subject to differential verti-cal movement when compared to the much denser stonefragments. This last factor is probably present to some de-gree, because evidence is present of extreme charcoal ver-tical movement. However, this may not explain the bulkdistribution. The graphs in Figure 9 show that peaks ofcharcoal are always present in levels with no archaeo-

logical materials. This suggests a more predominant roleplayed by natural fires in the production of charcoal. Thelack of correlation between artifact weight and depth, alsousing Pearson’s r (Figure 10) indicates that bioturbationby soil mesofauna is not an important factor in the ver-tical positioning of the artifacts, since it is expected thatants, termites, and earthworms would produce artifactsize sorting according to their maximum capacity for clastdislocation.

Other evidence suggests discrete artifact layers. UnitB2 at 80 cm depth contained three conjoinable pieces,composed of a pebble and two detached flakes. Figure 11shows the results obtained by Malvern granulometry. Thegraphs suggest that in spite of its very high clay content,the soil behaves texturally as a sandy soil, and thereforethe percentages of sand, silt and clay plotted in Figure 4are misleading in this context. The laser scattering analy-sis shows a scenario where mean clay content comprisesonly 0.7%, silt 19.5%, and sand 79.7% in soil volume.

The visual inspection of Figure 11 suggests differentpulses of clay, silt, and sand-sized materials. However,in order to better explore such patterns, we ran a clus-ter analysis of the 30 samples, taking into account 21granulometric intervals (from 2000 μm to 1.381 μm) us-ing Ward’s method, Euclidean distance (Figure 12). The



Figure 7 Sample1166 thinsection. (A)Microstructureshowingsubangularblocks tomicroaggregatesasdecorrenceof thedismantlingof thegroundmass

associated with fissural and cavitary pores, and bioturbation. (B) Detail of the groundmass dismantling. (C) Same image, XPL, showing mottled plasm

orientation. (D) Pore hipocutan concentrating Fe and nodule with strong impregnation. (E) Same image, XPL, showing speckled plasm orientation.

graph in Figure 12 suggests six different groups of gran-ulometric pattern, named A to F. When we compare theclusters with the granulometry in Figure 11, it is worthnoting that cluster E is more “sandy.”

The placement of the groups along the profile, withthe majority of them being composed by samples thatare contiguous in the stratigraphy, suggest that they aremeaningful in terms of depositional regimes. This in turnpoints to at least nine discrete events of soil accretion andsediment deposition, summarized in Figure 2 (left side ofUnit S4, North Profile). Some clusters show a remarkablecoincidence with archaeological layers (clusters A, B, andF), while others show the opposite (clusters C and E).

In sum, the data favor the interpretation that thereare discrete archaeological layers in the site, and thatthese layers are not result of natural processes, but ratherthe outcome of different human occupation episodesnear the lake shore that took place since the Pleis-tocene/Holocene transition.

Chronology

Radiocarbon

As mentioned, the first radiocarbon ages obtained forSumidouro, from Unit B1, indicate a late Paleoindian

age. Despite the discrepancy between radiocarbon andOSL ages, the radiocarbon sequence seems consistent:one archaeological level at 137 cm depth dated between5920 and 5600 cal. yr B.P., a second at 160 cm depthdated between 9450 and 9240 cal. yr B.P., and a third at180 cm depth, without enough charcoal to date, butprobably dating from the Pleistocene/Holocene transi-tion. Additional charcoal samples muddied the picture.For example, a charcoal fragment from Unit B2, collectedat 115 cm depth and associated with lithics, produced aradiocarbon age of 230 ± 30 14C yr B.P. (315 to 145 cal.yr B.P.; Beta 191131). Consequently, we submitted 14additional charcoal samples from Unit S4 to better un-derstand the charcoal dispersion pattern. Table IV lists allradiocarbon dating results for all excavation units.

Luminescence

We collected 12 sediment samples for OSL dating fromfour of the excavation units: seven from Unit B1(Figure 2), and the other five split among Units S4, S6,and S7 (Table V). Table VI lists the concentrations of themajor contributors to the dose rates, along with the to-tal calculated dose rates. The table arranges samples bydepth below surface. There is no significant trend of doserate with depth. The largest differences are among the



Figure 8 Sample 1165 thin section. (A) Massive microstructure with plane voids. The dark brown color of the plasm is characteristic of the accumulation

of iron and OM. (B) Iron depletion zone. (C) Detail of iron depletion zone, PPL. (D) Channel filled by material coming from upper horizons. (E) Quartz grains

showing weakly developed inclined stratification.

individual units, with S6 exhibiting high dose rates andS4 unusually low dose rates.

We applied dose recovery to three hundred 180–212 μm grains from UW2234 and UW2235, of which68 passed the regular criteria for acceptance. The ratio ofderived dose to administered dose, using the central agemodel, is 1.09 ± 0.4, somewhat overestimated. Of the 68grains, 59% were within 1σ of the administered dose and88% were within 2σ , again not quite what would be ex-pected from random error. Of the eight grains that failedto recover within 2σ , six estimated the dose. The over-dispersion, which is a measure of scatter beyond whatcan be accounted for by measurement error, is 15 ± 3%.This gives an estimation of intrinsic variability expectedfor a single-aged sample.

Table VI also gives the De for each sample, whichwas calculated using a weighting method called thecentral age model (Galbraith et al., 1999) and theover-dispersion. Overdispersion values range from 34 to128%, much higher than the 15% obtained in dose re-covery. The cause of this higher variability must be some-thing other than intrinsic factors. One possibility is partialbleaching, where only some of the grains were fully resetat the time of deposition. The quartz luminescence signalis a composite resulting from the release of electrons fromseveral different traps, some of which are more suscepti-

ble to sunlight than others. If the De from slow bleachingtraps does not differ from that of fast bleaching traps, thiscan be taken as evidence for full bleaching. We assessedthis qualitatively for each grain by measuring linear-modulated OSL (LM-OSL) after a dose at the end of theSAR sequence. In conventional OSL measurements, thestimulating source is kept at constant power, but in LM-OSL the power of the source is increased linearly duringread-out. Electrons from the fast-bleaching traps are re-leased sooner than those from slow-bleaching traps. Bycomparing the ratio of signal from early and later parts ofthe LM-OSL curve, one can obtain a qualitative measureof the relative degree to which the signal from a partic-ular grain is dominated by the fast traps. A comparisonof grains with high ratios (signifying the dominance offast bleaching traps) with those with low ratios showedno significant difference in average De values, suggest-ing the samples are well-bleached. Some grains with verylow ratios (signifying dominance of slow bleaching traps)did tend to have higher De values and these (approxi-mately 10%) were removed from analysis. These samplesmight also explain the slight overestimation from doserecovery.

Another possible cause of high variability is differen-tial dose rate at the single grain level. A likely source is thedifferential distribution of K-feldspars, from which 40K



Table

IIICha

rcoa

lweigh

t,nu

mber

oflithics,and

totaloflith

icsbylevelfor

nine

excavatio

nun

its.

UnitB

1UnitB

2UnitS

3UnitS

4UnitS

5UnitS

6UnitS

7UnitS

8UnitS

9

Num

ber

Num

ber

Cha

rcoa

lNum

ber

Cha

rcoa

lNum

ber

Cha

rcoa

lNum

ber

Cha

rcoa

lNum

ber

Cha

rcoa

lNum

ber

Cha

rcoa

lNum

ber

Num

ber

Level

Lithics

Lithics

(g)

Lithics

(g)

Lithics

(g)

Lithics

(g)

Lithics

(g)

Lithics

(g)

Lithics

Lithics

10

00.00

00

0.00

00

6.20

32

0.00

00

9.54

30.00

01

0

20

00.00

00

5.70

50

25.186

00.00

00

5.98

00.00

01

0

30

04.09

10

76.508

210

.319

85.05

53

51.43

05.87

51

0

40

00.00

00

31.257

34.12

21

20.91

345

.84

02.86

90

0

50

01.62

21

6.34

11

0.53

10

14.423

128

.04

00.00

00

0

60

00.00

01

1.55

60

1.04

70

14.529

13.49

00.00

00

0

70

10.00

00

2.76

70

1.02

90

38.425

06.12

00.00

00

0

80

12.36

50

44.822

316

.378

017

.102

110

.25

013

.430

01

90

31.05

70

2.42

31

14.857

00.83

40

3.57

00.16

70

0

100

01.92

50

12.433

075

.543

04.39

01.82

12.87

21

0

110

20.62

70

4.26

10

6.76

40

50.036

04.22

02.76

40

0

120

21.92

20

24.804

071

.234

20.68

20

18.00

01.23

20

0

130

12.60

41

25.789

029

.282

2—

—12

.91

03.22

82

2

143

43.78

12

8.98

50

7.40

60

——

33.17

10.99

82

1

155

02.58

50

8.12

51

7.32

31

——

18.41

10.00

02

0

163

60.23

90

4.93

50

21.394

0—

—4.05

10.00

00

1

170

80.27

70

7.86

93

31.680

1—

—2.23

01.29

10

0

184

20.00

00

10.892

312

.150

0—

—2.51

20.10

30

1

190

70.60

65

0.62

34

2.13

90

——

0.97

1—

—1

200

00.00

02

0.28

71

0.00

00

——

0.07

1—

—0

210

1—

—0.09

71

0.04

20

——

0.08

0—

—0

220

1—

—1.75

91

——

——

0.04

0—

—1

Total

1539

23.701

1228

2.23

824

344.62

917

166.38

69

262.72

411

34.829

108



Figure 9 Relationship between artifacts and charcoal fragments found in five excavation units, tabulated by 10-cm levels, showing a lack of statistical

correlation between the two variables.



Figure 10 Relationship between artifact weight and depth found in six excavation units, tabulated by 10-cm levels, also showing a lack of statistical

correlation between the two variables.

contributes a large portion of the beta dose rate. Grainsclose to a K-feldspar will have a higher dose rate thanthose further away, and thus will produce a higher De

value even if the ages are the same. An attempt to modelthe possible effect using measured 40K proportions andbeta dose rates, following Mayya et al. (2006), showedthat the scatter was too large to be attributed to differ-ential beta dose rate. It is likely that much of the 40K is

derived from the abundant clays in the sediments, whichwould provide a more uniform dose.

The most likely source of high overdispersion is post-depositional mixing. To look at the structure of the De

distributions, we applied a finite mixture model (as de-scribed in the Appendix), using the 15% overdispersionfrom dose recovery as typical for a single-aged compo-nent. The model identified from two to four single-aged



Figure 11 Granulometric analysis of 30 samples from Unit S4, run by laser scattering, using a Malvern Hydro 2000MU particle-size analyzer. Dashed line

represents the percentage of clay as determined by the standard pipette method.

components, the three most abundant from each sam-ple and their proportion given in Table VI. The compo-nents are statistical constructs and should not necessarilybe taken as discrete groupings that have become mixed.The distributions in fact are more or less continuous. Dis-tributions for three samples are shown as radial graphs inFigure 13.

When distributions are more or less continuous, it isdifficult to determine which, if any, of the componentsrelate to the original deposition, without some knowl-edge of how grains have moved. The central age value,for example, would be appropriate if grains moving upwere more or less countered by grains moving down.To evaluate the distributions, the central ages of eachcomponent, as well as those from the entire distribution,were calculated by dividing by the dose rate determined

on the sample as a whole. Ages for each componentwere then compared with stratigraphic and radiocarboninformation.

The best stratigraphic evidence for the relative age ofthe OSL samples is from Unit B1, from which seven sam-ples were drawn. Table VI arranges the samples in strati-graphic order from youngest to oldest and lists the cen-tral model age from the whole distribution and the agefrom the most abundant component. With the excep-tion of UW2236, both sets of ages form the correct strati-graphic order, within error terms, when using the wholedistribution. The ages of the largest component also dif-ferentiate the ages of successive samples (UW1390–1389and UW1388–2238), where the whole distribution doesnot. The largest component age for UW1390 also is closerto the radiocarbon age from that layer than the whole



Figure 12 Cluster analysis of the 30 samples of Unit S4, taking into account 21 granulometric intervals (from2000μmto 1.381μm) usingWard’smethod,

Euclidean distance.

distribution age. That both sets of ages are not too differ-ent reflects the high proportion of grains (more than 60%for most samples) in the largest component. While thereis some movement up and down profile, the close fit be-tween the largest component ages and the stratigraphicand radiocarbon information suggests that the majorityof grains have not moved much and seem to representthe depositional age. UW2236 appears slightly young, butthis sample for some reason contained very insensitive

quartz grains and the sample size was very small. Sam-ples UW1388–90 were drawn from archaeological layers,confirming a late Pleistocene/early Holocene age for thesematerials.

We collected three samples from Unit S6 and one eachfrom Units S4 and S7 (Table VI). The samples from S6are fairly mixed in terms of having grains spread moreevenly among different components, neither sample witha component making up more than 60%, and the largest

Table IV Sumidouro site 14C ages analyzed by Beta Analytic, Inc.

Laboratory Conventional

Number Excavation Radiocarbon Calibrated

Sample (Beta) Unit Depth (cm) Age 14C yr BP Age (cal. yr B.P.) 13C/12C ‰

SUMIDB1–137 205350 B1 137 5020 ± 70 5920–5600 –25.6

SUMIDB1–160 205351 B1 160 8310 ± 40 9450–9240 − 25.3

SUMID-B2–115 191131 B2 115 230 ± 30 315–145 − 24.5

1101 234507 S4 225 5860 ± 50 6790–6550 − 25.7

1102 234508 S4 138 4580 ± 50 5450–5060 − 26.4

1103 234509 S4 150 5810 ± 40 6720–6500 − 25.8

1104 234510 S4 108 2210 ± 40 2340–2120 − 24.0

1105 234511 S4 112 4220 ± 40 4850–4640 − 25.7

1019 234506 S4 235 450 ± 40 540–470 − 27.5

1136 234512 S4 26 510 ± 40 620–500 − 27.1

1173 234513 S4 130 3450 ± 40 3830–3620 − 25.8

1174 234514 S4 146 4460 ± 40 5290–4910 − 24.3

1175 234515 S4 154 5130 ± 40 5940–5750 − 25.5

1176 234516 S4 135 4640 ± 40 5470–5300 − 26.5

1180 234517 S4 46 1350 ± 40 1320–1190 − 26.4

1181 234518 S4 25 340 ± 40 500–300 − 25.8

814 234520 S4 31 400 ± 40 520–320 − 26.6

S7 190–200 256391 S7 200 660 ± 40 680–550 − 27.3

S9 190–200 256402 S9 200 6410 ± 40 7420–7260 − 24.8



Table V Location of OSL samples and concentrations of the major contributors to the dose rate for each sample.

Laboratory Field Depth Provenience Total Dose

Number Designation Unit (cm) Information 238U (ppm) 232Th (ppm) K (%) Rate (Gy/ka)

UW1392 OSL5 B1 27 A-horizon, recent

colluvium

2.81 ± 0.23 12.81 ± 1.53 0.54 ± 0.02 1.90 ± 0.10

UW1391 OSL4 B1 70 Clayey horizon 3.57 ± 0.27 12.68 ± 1.54 0.52 ± 0.02 1.96 ± 0.10

UW1886 OSL6 S7 70 Reddish colluvium 3.63 ± 0.25 9.48 ± 1.32 0.59 ± 0.04 1.85 ± 0.10

UW2234 1525 S6 105 Reddish colluvium at

base of lithic level

2.90 ± 0.20 7.30 ± 1.13 1.56 ± 0.06 2.33 ± 0.11

UW1390 OSL3 B1 137 Upper limit of middle

lithic level,

transitional horizon

2.35 ± 0.20 10.60 ± 1.25 0.50 ± 0.01 1.68 ± 0.08

UW1389 OSL2 B1 160 Middle lithic level, in

reddish colluvium

3.28 ± 0.28 17.96 ± 1.73 0.50 ± 0.01 2.15 ± 0.11

UW2237 1528 S6 171 Yellow colluvium 4.17 ± 0.30 14.51 ± 1.42 0.91 ± 0.06 2.25 ± 0.11

UW1388 OSL1 B1 196 Below lower lithic level,

contact reddish/

yellow colluvium

3.72 ± 0.29 14.63 ± 1.68 0.44 ± 0.01 1.99 ± 0.11

UW2238 1539 B1 262 Contact yellow

colluvium/

stone-line

2.69 ± 0.24 14.06 ± 1.61 0.55 ± 0.03 1.79 ± 0.10

UW2235 1524 S6 275 Above stone-line 3.16 ± 0.27 16.22 ± 1.76 1.17 ± 0.06 2.34 ± 0.12

UW2236 1540 B1 288 Stone-line 3.78 ± 0.27 12.00 ± 1.42 0.69 ± 0.04 2.01 ± 0.10

UW1885 OSL7 S4 295 Below stone-line 2.39 ± 0.15 3.71 ± 0.71 0.30 ± 0.01 1.11 ± 0.06

component showing stratigraphic inversion. It is prob-ably not possible to sort out a precise depositional agefor these two samples, but it is probably safe to say thatboth are Pleistocene in age. Both predate the archaeologystratigraphically. UW2234 was taken from the bottom ofthe archaeological layer. One component with 46% ofthe grains gives an age of 12.0 ± 1.0 ka. This sampleis early Holocene, but any more precise specification isprobably not possible. UW1885 is located 50 cm belowthe archaeological material in the stratigraphy. The sam-ple came from the bottom of the excavation unit so its oldage should not be surprising. UW1886 was located highin Unit S7, 150 cm above the lowest archaeological evi-dence. Its Mid-Holocene age is probably a reasonable es-timate. Neither of these samples seems as mixed as thosefrom S6.

OSL and radiocarbon: Comparing results

Radiocarbon ages tend to be consistent in terms of depth,since the deeper charcoal fragments are generally older(r = 0.48; P = 0.036). There are, however, some im-portant exceptions. In Unit S4 there are two instanceswhere charcoal ages could be very misleading (Figure 2).The lower archaeological horizon, present between 160and 210 cm depth, could be bracketed between the setof Mid-Holocene age samples (samples 1102, 1103, 1104,and 1105) and sample 1101. However, the age obtainedfor sample 1101 (6790–6550 cal. yr B.P.) is statistically

equal to sample 1103 (6720–6500 cal. yr B.P.), located70 cm above. If this discrepancy is not related to some sortof contamination (and the dating laboratory did not re-port any contamination problem), this represents a min-imum vertical displacement of 70 cm for the charcoalfragment. Yet, there is evidence for much greater verti-cal displacements. Sample 1019, collected at a depth of235 cm depth, produced an age of only 540–470 cal. yrB.P. This means that this charcoal fragment migrated atleast 200 cm downwards in the profile.

The problem becomes more apparent when we com-pare charcoal ages with luminescence ages and the ex-tant archaeological information for the area. The lowerarchaeological horizon in Unit S4 has all character-istics of being Paleoindian (Figure 2). It shows thesame lithic technological organization found in the rock-shelters, and more important, the presence of flint flakes.Flint is only found in the oldest horizons (from 12to 10 ka) of the three rock-shelters we excavated. Ifwe used only the radiocarbon bracket between samples1101 and 1103, this horizon would be attributed to theArchaic.

Another example of problems related to charcoalmovement was found at Unit S7, where charcoal foundat 200 cm depth was dated by radiocarbon between 680and 550 cal. yr B.P. (660 ± 40 14C yr B.P.; Beta 256391;Table IV). For the same unit, we have an OSL age of5.3 ka at a depth of 70 cm depth, which seems muchmore likely (UW 1886; Table VI).



Table

VIEq

uivalent

dose(D

e)u

sing

thecentralage

mod

elan

dov

erdispersion

forea

chsample,and

ages

forUnitsB1,

S6,S7,

andS4

(kabefore20

10).

Com

pon

ent1

Com

pon

ent2

Com

pon

ent3

Dep

thCen

tral

Cen

tral

Largest

Calibrated

Sample

Unit

(cm)

Age

De(Gy)

σb(%)

De(Gy)

%De(Gy)

%De(Gy)

%Age

Com

pon

ent

14CAge

(ka)

UW13

92B1

2716

53.4

±0.2

64.1

1.2

±0.2

122.7

±0.3

455.2

±0.4

361.8

±0.2

1.4

±0.2(45%

)

UW13

91B1

7014

58.0

±0.3

37.6

6.4

±0.4

6511

.1±

1.3

3335

.2±

6.5

24.1

±0.3

3.5

±0.3(65%

)

UW18

86S7

7086

10.6

±0.7

50.3

5.7

±0.7

209.9

±0.7

6630

.2±

3.8

145.7

±0.5

5.3

±0.5(66%

)

UW22

34S6

105

7616

.2±

1.4

68.2

0.7

±0.2

412

.1±

0.7

5027

.9±

1.7

466.9

±0.7

5.2

±0.4(50%

)

UW13

90B1

137

316

16.9

±0.5

39.4

13.0

±0.4

6226

.3±

1.3

3810

.1±

0.7

7.7

±0.5(62%

)5.7–

6.0

UW13

89B1

160

229

19.9

±0.7

45.7

11.5

±1.0

2321

.8±

1.4

6239

.1±

4.1

149.3

±0.7

10.1

±0.9(62%

)9.3–

9.5

UW22

37S6

171

9435

.9±

2.6

62.4

13.6

±1.2

2131

.9±

3.3

2759

.4±

3.3

5216

.0±

1.5

26.4

±2.1(52%

)

UW13

88B1

196

196

24.6

±0.8

34.5

16.1

±1.2

3029

.5±

1.2

6966

.3±

12.9

1312

.4±

0.9

14.8

±1.1(69%

)

UW22

38B1

262

7025

.6±

2.0

57.3

5.0

±0.8

712

.4±

2.0

1333

.2±

1.5

8114

.3±

1.5

18.5

±1.5(81%

)

UW22

35S6

275

5726

.8±

2.1

48.6

10.9

±2.1

1424

.7±

2.1

5850

.5±

5.8

2711

.5±

1.1

10.6

±1.1(58%

)

UW22

36B1

288

1910

.8±

3.4

128

1.1

±0.3

2412

.8±

1.9

3231

.6±

4.4

445.4

±1.7

15.7

±2.4(44%

)

UW18

85S4

295

196

24.6

±0.8

34.5

16.1

±1.2

3029

.5±

1.2

6966

.3±

12.9

1328

.1±

2.4

33.6

±2.7(81%

)

It is important to note that this is not simply a matterof solving radiocarbon age issues by increasing the sam-ple size and using Bayesian methods. In fact, there couldbe no charcoal fragments to date at all. As can be seen inTable III, deeper soil horizons are consistently poorer incharcoal fragments. When one reaches depths of around200 cm, there are no macroscopic charcoal fragments inthe soil. This could be circumvented by separating anddating microscopic charcoal, or dating soil OM, but theproblem of the association between depth and age willnot be solved, since soil OM, as opposed to single char-coal fragments, is a pool of ages due to soil turnover. Onepossible way to overcome this is to date only the huminfraction, and not the total soil OM (Pessenda et al., 2001).

DISCUSSION

The Human Occupation at the Site

The compound evidence of the 10 excavated units sug-gest that the site witnessed at least three distinct oc-cupation episodes: the lower layer dates to the Pleis-tocene/Holocene transition, and therefore is Paleoindian,with a minimum radiocarbon age of 9240–9450 cal. yrB.P. (8310 ± 40 14C yr B.P.; Beta 205351) and a maxi-mum OSL age of 14,800 ± 1100 yr (UW1388). The mid-dle layer dates to the late Holocene, probably around2000 cal. yr B.P. if we take into consideration radiocar-bon sample 1104 (Table IV; Figure 2), and data fromLund Site, a lithic site located only 250 m north of Sum-idouro Site, with two radiocarbon ages: 2146–2336 cal.yr B.P. (Beta 170418) and 2041–2311 cal. yr B.P. (Beta170719). The upper archaeological layer contains ceramicfragments associated with the Tupiguarani tradition. Ra-diocarbon analysis of a charcoal fragment from Unit S4(Table IV, sample 1136; Figure 2) would put this occupa-tion at 500–632 cal. yr B.P. (Beta 234512).

The Archaeological Record, Tropical Soils, andSlope Processes

At first, our expectations about the integrity of the con-textual archaeological information at Sumidouro werelow. We expected that convolute downslope processes,such as soil creep (Clarke et al., 1999), coupled withheavy bioturbation, would obliterate any pattern, andthat only very general aspects related to the material cul-ture would be. To our surprise, this seems not so. Wewere able to detect at least three different archaeologi-cal layers, and in some instances lithic refitting showedthat the spatial integrity of such pieces was preserved.Some of the spatial integrity of the artifacts was lost notdue to natural factors per se, but due to the necessity of



Figure 13 Radial graphs for three samples with different overdispersion

values: UW1388 (34.5%), UW1389 (45.7%), and UW1392 (64.1%). Radial

graphs plot precision on the x-axis against a standardized De value on

the y-axis. The standardization is the difference between the measured

De for a grain and some reference value divided by the standard error on

the grain De value. The reference values, represented by the solid lines,

are the De values for the three largest components. The shaded areas

around the two largest components encompass all points consistent at

two standard errors with the reference value. A line drawn from the origin

on the standardized estimate axis through any point bisects the radial axis

on the right at the measured De value for that point.

excavating a stone-hard clayey soil in 10 cm arbitrary lev-els. Hence, we can say that discrete archaeological layersreflect different events of human occupation, interbeddedwith colluvial accretion, in a subhorizontal manner.

Another important observation regarding slope pro-cesses near a lake shore is related to the influence of

waves in the erosion and transport of materials. The to-pographic lowermost Unit S6 showed this influence inthree ways: a very mixed OSL grain suite, suggesting re-deposition of different materials in the same setting; avery large quantity of charcoal fragments, most probablyalso redeposited from upper portions of the slope; and avery thin upper “reddish” soil and corresponding archae-ological layer, also signaling an extensive and selectiveremoval of upper soil layers. Since the lake is linked toa stream and, ultimately, to a sinkhole, the eroded soilmaterial did not accumulate at the toe-slope but was, in-stead, washed away and transported as alluvium.

Dating Methods and Paleoindian Sites inTropical Soils

Our data suggest that OSL is mandatory in tropical soils.We do not mean by this that OSL is “better” than ra-diocarbon, but that radiocarbon has to be used withextreme caution, and coupled with an independent dat-ing method. The distribution of the majority of the char-coal fragments is not random. On the contrary, we havegood reasons to believe that the distinct peaks in charcoalconcentrations are indicative of sheetwash/subhorizontalburial of ancient stable surfaces. It is also true that char-coal age increases with depth. Charcoal concentrationsseem to be testimonies of paleofires over paleosurfaces.We therefore do not think that all charcoal fragments mi-grate wildly within the profile, but that isolated fragmentsof charcoal are most probably out of their original position. Treeroots can play a major role in the vertical migration of iso-lated charcoal fragments as evidenced by the clear verticalkrotovina observed in the right side of the south profileof Unit B1 (Figure 2), which is most probably a decayedtree root. Unfortunately, these isolated fragments are fre-quently all we have to chronologically assess old sites.

Another important point is that, contrary to commonwisdom, there is a growing literature showing that char-coal (or “black carbon”) in soils can suffer heavy oxida-tion (Cheng et al., 2008; Kuzyakov et al., 2009; Majoret al., 2010). From the literature on carbon isotopes intropical soils, ages on charcoal seem always younger than9000 14C yr B.P. (Boulet et al., 1995; Gouveia & Pessenda,2000; Gouveia et al., 2002; Pessenda et al., 2004). To dateolder levels, researchers often rely on radiocarbon datingof soil OM, since there is no macroscopic charcoal be-low depths of 2 m. We observed this situation at Sumi-douro. At the same time, much older charcoal fragmentsare recovered from lake bottoms, rock-shelters, and semi-arid settings. This suggests that charcoal is oxidized andcompletely disintegrated in well-drained soils formed inhigh mean temperature settings with abundant water,and that this process occurs in approximately 10,000 yr.



Hence, open-air Paleoindian sites older than 10,000 yrwill probably have no charcoal at all.

Tropical Soils and Climate Change Signals

According to a model originally developed by Knox(1972) and later applied by many researchers in tropi-cal and subtropical settings (see Thomas, 2008; Thomas& Thorpe, 1995), abrupt climate changes trigger changesto vegetation cover and, therefore, to slope stability andsediment yield. The sinuous and discontinuous stone linepresent in the deepest portion of the profiles indicates atleast one episode of very high energy erosion that re-moved the fine soil fraction, and is probably related tounstable soil cover and/or episodes of heavy rainfall. Wetherefore interpret these conglomeratic layers at Sumi-douro as a result of disruption to the climate/vegetationequilibrium (Erhart, 1956; Knox, 1972). The OSL agesobtained suggest that this episode can be bracketed be-tween 33.6 ka (Sample UW1885, Unit S4, 295 cm depth)and 18.5 ka (Sample UW2238, Unit B1, 262 cm depth),and is probably related to climate changes operating dur-ing the last glacial maximum. After this, during the lateGlacial, the regular deposition of soil and the presence ofa stable and high-standing lake level suggest milder cli-matic conditions. The colluvium deposited during the lateGlacial is yellowish (10YR 4/6; 10YR 6/6), in contrast tothe top sequence, where reddish soils (7.5YR 4/3; 7.5 YR4/6) tend to prevail. We found no archaeological mate-rials embedded in the yellowish soil, regardless of depth,leading us to propose that the difference in color is re-lated to an environmental and chronological boundary,rather than chemical processes involving the water table.The age of this boundary is addressed by the OSL age ofsample UW 1388, which brackets the oldest lithic horizonand marks the contact between the yellow and red soils:14.8 ± 1.1 ka.

Other important information comes from the archae-ologically sterile layer illustrated in Figure 2 betweenthe lower and the upper archaeological layers. As al-ready mentioned, the lower layer is Paleoindian in age.The upper layers are probably related to a much laterhunter-gatherer occupation, which is followed by makersof ceramics. Between the Paleoindian and later hunter-gatherer occupation periods there is a strong soil accre-tion episode, with abundant charcoal and coarser sed-iment input that can be bracketed between 9.5 and2.0 ka. We consider this layer as evidence of an-other episode of climatic disruption, this time the Mid-Holocene hypsithermal (sensu Deevey & Flint, 1957),where a dryer and/or very unstable climate promoted,on a regional scale, the abandonment by humans of vastareas in Central Brazil (Araujo et al., 2005, 2006). If we

Figure 14 Soil accretion rates (in mm/year) calculated for Unit B1, using

OSL data. There is a good fit between accretion rates and Knox model,

with higher rates during the late Glacial Maximum and the Hypsithermal,

separated by a period of low rates in the Pleistocene/Holocene transition.

take into account the several charcoal fragments from thislevel as proxies for natural fires, the main occurrence ofdry periods occurred between 6.7 and 4.6 ka. Figure 14shows the soil accretion rates (in mm/year) calculated forUnit B1, using OSL data. The results suggest a good fit be-tween accretion rates and Knox model, with higher ratesduring the late Glacial Maximum (LGM) and the hyp-sithermal, separated by a period of low rates in the Pleis-tocene/Holocene transition. This is also in good agree-ment with the arrival of humans in the region (about12 ka) when the climate was probably stable, and theabandonment of the region ca. 8 ka, when soil accretionrates began to increase.

CONCLUSIONS

Based on stratigraphy, soils, micromorphology, geochem-istry, archaeological data, OSL, and radiocarbon ages, wepresent a model for the slope evolution at Sumidouro site(Figure 15). This model suggests at least three differentevents of instability (erosion/sedimentation) followed bystability (pedogenesis). While acknowledging the pitfallsof considering a single slope as representative of a re-gional geomorphic response to climate, we can advancesome tentative interpretations. The very first period ofinstability, not shown in Figure 15, was the stone-linedeposition, during the LGM. After this, several periods ofsoil erosion and sediment deposition occurred during thelate Glacial. A period of stability in the early Holocene,between 12 and 8 ka, is concomitant with the first clearhuman occupation in the area. Another period of insta-bility followed during the Mid-Holocene, between ca. 8and 4 ka. The second stability phase observed in the pro-file is associated with an increase in the archaeologicalsignal, dated ca. 4 ka inside the rock-shelters. After this,



Figure 15 Evolutionary model for the slope at Sumidouro Lake, based on micromorphology, geochemistry, archaeological data, OSL, and radiocarbon

ages.

a second period of instability followed between 4 and2 ka, when the region was again depopulated, and finallya third phase of stability and pedogenesis was establishedin the last 2000 yr, marking the return of human popu-lations to the area.

Paleoindian sites in tropical settings were subject to atleast one strong climatic event, namely the Mid-Holocenehypsithermal. Soil accretion rates at the base of hill-slopes during this period were probably high, buryingsoils deeply and calling for prospection strategies devisedfor deep inspection. This probably accounts for the lack ofan open-air Paleoindian record in most portions of Brazil.

The positioning of archaeological materials and char-coal both point to the burial of ancient surfaces, and notto a chaotic or convolute downslope movement. The roleof bioturbation in the vertical displacement of such ma-terials is much less important than previously acknowl-edged, perhaps due to the rapid burial of the depositsin the Mid-Holocene. These observations are in conflictwith the idea that charcoal fragments in tropical soils areburied due to biological activity (Miklos, 1992; Bouletet al., 1995; Gouveia & Pessenda, 2000).

Higher soil accretion rates during the Mid-Holoceneare probably not linked to a stable dry climate, but toa greater climatic instability, with years (or decades) ofwetness intercalated with periods of dryness, preclud-ing the formation of a stable vegetation cover and caus-ing increased geomorphic work (Knox, 1972). This samescenario may explain the extremely low archaeologicalsignal during this time period, when humans probablymoved away from the region (Araujo et al., 2005, 2006).

This research was funded by FAPESP grants 99/00670-7,04/01321-6, 06/52188-0, and 08/51747-0, and also by CAPES,PIBIC Grant. The UNIOESTE Micromorphology Laboratory wasfunded by the UGF/SETI/Parana Government for the financialsupport, Grant no. 13/2007. The luminescence work was fundedby the National Science Foundation. We thank the followingpersons and institutions Walter A. Neves from the Laboratory forHuman Evolutionary Studies (LEEH), University of Sao Paulo,for all the scientific and financial support; Paulo Cesar FonsecaGiannini, Elaine Aparecida da Silva, and Vitor Aguiar of the Lab-oratory of Sedimentology (LABSED) permitted and helped withthe use of MALVERN; Jose Paulo Sertek helped in the prepara-tion of FRX samples and Paulo Ernesto Mori facilitated our accessto Laboratorio for X-Ray Fluorescence, both laboratories from



Instituto de Geociencias, University of Sao Paulo. Joel Sigolo forassisting in the confection of micromorphological thin sections.Luıs B. Pilo, Augusto Perez-Alberti, and Joao Herbert for thefruitful ideas and discussions in the field, and Wenner-GrenFoundation for providing travel funding to the 2011 DIG confer-ence. We also thank Vance Holliday, Gary Huckleberry and ananonymous reviewer for very helpful comments on the paper.

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Appendix: Procedures for LuminescenceAnalysis of Coarse-grained Quartz fromSediment Samples

Sample Preparation

Sample material is removed from the collection tubesin orange-red light. The ends of the tubes, which mayhave been exposed to daylight, are set aside and used for



dose rate measurements. Only the inner, unexposed por-tion is used for luminescence measurements. The upex-posed material is first wet sieved through a 90-μm screen.The greater than 90-μm fraction is dried and then treatedwith HCl and H2O2, rinsed three times with water anddried again. It is then dry sieved to retrieve the 150–180 or 180–212 μm fraction. This fraction is etched for40 minutes in HF and then rinsed with water, HCl andwater again. After drying, it is passed through the 180-μm screen to remove any degraded feldspar. The materialcaught in the screen is density separated using a lithiummetatungstate solution of 2.67 specific gravity.

Equivalent Dose

Grains are placed in specially manufactured disks forsingle-grain measurement. Luminescence is measured ona Risø TL-DA-15 reader with single-grain attachment.Stimulation is by a 532 nm laser delivering 45 W/cm2.Detection is through 7.5 mm U340 (ultraviolet) filters.Exposure is for 0.8 second on each grain at 125◦C. Thefirst 0.06 second is used for analysis and the last 0.15 sec-ond for background. Regeneration and test doses are de-livered by a 90Sr beta source that provides about 0.1 Gy/sto coarse-grained quartz. The test dose employed is about3 Gy.

Luminescence was measured at different preheats forthe 150-180 μm grains. No significant difference inequivalent dose was detected for preheats at 10 secondin the 200–240◦C range (Table A1) and a 240◦C preheatwas used for all subsequent analyses.

Equivalent dose (De) is determined using the single-aliquot regenerative (SAR) dose protocol (Murray &Wintle 2000; Wintle & Murray 2006). The SAR methodmeasures the natural signal and the signal from a seriesof regeneration doses on a single aliquot. The methoduses a small test dose to monitor and correct for sensi-tivity changes brought about by preheating, irradiation,or light stimulation. SAR consists of the following steps:(1) preheat, (2) measurement of natural signal (OSL orIRSL), L(1), (3) test dose, (4) cut heat, (5) measurementof test dose signal, T(1), (6) regeneration dose, (7) pre-

heat, (8) measurement of signal from regeneration, L(2),(9) test dose, (10) cut heat, (11) measurement of test dosesignal, T(2), (12) repeat of steps 6 through 11 for variousregeneration doses. A growth curve is constructed fromthe L(i)/T(i) ratios and the equivalent dose is found by in-terpolation of L(1)/T(1). A zero regeneration dose and arepeated regeneration dose are employed to insure theprocedure is working properly.

An advantage of single-grain dating is the opportunityto remove from analysis grains with unsuitable charac-teristics by establishing a set of criteria that grains mustmeet. Grains are eliminated from analysis if they (1)had poor signals (as judged from errors on the test dosegreater than 30% or from net natural signals not at leastthree times above the background standard deviation),(2) did not produce, within 20%, the same signal ra-tio (often called recycle ratio) from identical regenera-tion doses given at the beginning and end of the SAR se-quence, suggesting inaccurate sensitivity correction, (3)yielded natural signals that did not intersect saturatinggrowth curves, (4) had a signal larger than 10% of thenatural signal after a zero dose, (5) produced a zero De

(within one-sigma of zero), or (6) contained feldspar con-taminates (judged visually on growth curves by a reducedsignal from infrared stimulation before the OSL measure-ment; done on two doses to lend confidence the reduc-tion in signal is due to feldspar contamination). At theend of each SAR sequence, linear-modulated OSL (wherethe laser power is ramped from 0% to 90% power in 30seconds) is measured for each grain to check for dom-inance by the fast-bleaching component, as explainedin the main text. A dose recovery test is performed onsome grains. The luminescence of the grains is first re-moved by exposure to the laser (using the same pa-rameters mentioned earlier). A dose of known magni-tude is then administered. The SAR procedure is thenapplied to see if the known dose can be obtained. Suc-cessful recovery is an indication that the procedures areappropriate.

A De value is obtained for each suitable grain. Be-cause of varying precision from grain to grain, the samevalue is not obtained for each grain even if all are of the

Table A1 Luminescence measures at different preheats for the 150–180 μm grains.

200◦C 220◦C 240◦C 260◦C

Sample N De (Gy) σb (%) N De (Gy) σb (%) N De (Gy) σb (%) N De (Gy) σb (%)

UW1389 50 23.2 ± 1.1 20.1 52 19.8 ± 1.7 53.1 92 22.1 ± 1.3 45.8 85 14.8 ± 1.0 51.4

UW1390 42 18.6 ± 1.6 40.7 243 17.2 ± 5.5 38.6

UW1391 33 8.4 ± 0.5 22.6 81 8.5 ± 0.5 43.5

UW1392 53 4.0 ± 0.3 51.1 83 3.3 ± 0.3 70.5



same age. Instead a distribution is produced. The com-mon age model and central age model of Galbraith et al.(1999, 2005) are statistical tools used in evaluation of De

distributions. These models are used in reference to De

and not “age” per se, although dividing the De values bythe bulk dose rate provides an “age” for each grain (notaccounting for differential dose rates for individualgrains). The common age model controls for differen-tial precision by computing a weighted average using logDe values. The central age model is similar except ratherthan assuming a single true value it assumes a natural dis-tribution of De values, even for single-aged samples, be-cause of non-statistical sources of variation. It computesan over-dispersion parameter (σ b) interpreted as the rel-ative standard deviation (or coefficient of variance) ofthe true De values, or that deviation beyond what canbe accounted for by measurement error. Empirical ev-idence suggests that σ b of between 10% and 20% aretypical for single-aged samples (Olley et al., 2004; Jacobset al., 2006). For samples of mixed ages, a finite mixturemodel is employed for evaluation. Finite mixture model(Roberts et al., 2000) uses maximum likelihood to sepa-rate the grains into single-aged components based on theinput of a given σ b value and the assumption of a log nor-mal distribution of each component. The model estimatesthe number of components, the weighted average of eachcomponent, and the proportion of grains assigned to eachcomponent. The model provides two statistics for estimat-ing the most likely number of components, maximumlog likelihood (llik) and Bayes Information Criterion(BIC). The finite mixture model is appropriate for samples

that have been post depositionally mixed (although withlimitations).

Dose Rate

Radioactivity is measured by alpha counting in con-junction with atomic emission for 40K. Samples for al-pha counting are crushed in a mill to flour consistency,packed into plexiglass containers with ZnS:Ag screens,and sealed for 1 month before counting. The pairs tech-nique is used to separate the U and Th decay series. Foratomic emission measurements, samples are dissolved inHF and other acids and analyzed by a Jenway flamephotometer. K concentrations for each sample are deter-mined by bracketing between standards of known con-centration. Conversion to 40K is by natural atomic abun-dance. Radioactivity is also measured, as a check, bybeta counting, using a Risø low level beta GM multi-counter system. About 0.5 g of crushed sample is placedon each of four plastic sample holders. All are counted for24 hours. The average is converted to dose rate followingBøtter-Jensen and Mejdahl (1988) and compared withthe beta dose rate calculated from the alpha counting andflame photometer results.

Cosmic radiation is determined after Prescott andHutton (1988). Radioactivity concentrations aretranslated into dose rates following Adamiec andAitken (1998).

Age is calculated using a laboratory constructed spread-sheet based on Aitken (1985). All given error terms arecomputed at one-sigma.


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