bowles et al., 2002

Archaeomagnetic intensity results from California andEcuador: evaluation of regional data

Julie Bowles , Je Gee, John Hildebrand, Lisa TauxeScripps Institution of Oceanography, 9500 Gilman Drive, MC 0208, La Jolla, CA 92093, USA

Received 5 April 2002; received in revised form 19 August 2002; accepted 19 August 2002

Abstract

We present new archaeointensity data for southeastern California (V33N, V115W, 50^1500 yr BP) andnorthwestern South America (Ecuador, 2.4S, 80.7W, 4000^5000 yr BP). These results represent the only data fromCalifornia, as well as the oldest archaeointensity data now available in northwestern South America. In comparingour results to previously published data for the southwestern United States and northwestern South America, we notethat significant scatter in the existing data makes comparisons and interpretations difficult. We undertake an analysisof the sources of data scatter (including age uncertainty, experimental errors, cooling rate differences, magneticanisotropy, and field distortion) and evaluate the effects of scatter and error on the smoothed archaeointensity record.By making corrections where possible and eliminating questionable data, scatter is significantly reduced, especially inSouth America, but is far from eliminated. However, we believe the long-period fluctuations in intensity can beresolved, and differences between the Southwestern and South American records can be identified. The Southwestdata are distinguished from the South American data by much higher virtual axial dipole moment values fromV0^600 yr BP and by a broad low between V1000^1500 yr BP. Comparisons to global paleofield models revealdisagreements between the models and the archaeointensity data in these two regions, underscoring the need foradditional intensity data to constrain the models in much of the world.= 2002 Elsevier Science B.V. All rights reserved.

Keywords: archaeointensity; paleomagnetism; California; United States; Ecuador; South America

1. Introduction

Archaeological materials have long been recog-nized as a possible means of recovering a closelyspaced record of paleointensity variations over the

last 5000^10 000 yr (see [1], and references there-in]. They can potentially provide greater temporaland spatial resolution than volcanic materials.While extensive archaeointensity data now exist,coverage is far from uniform. In particular, largeparts of the western and southern hemisphereslack intensity data. Furthermore, much of the ex-isting data are several decades old, of question-able reliability by todays standards, and exhibitsignicant scatter.This dearth of intensity data in particular re-

0012-821X / 02 / $ ^ see front matter = 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 9 2 7 - 5

* Corresponding author. Tel. : +1-858-822-4879.E-mail address: [email protected] (J. Bowles).

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Earth and Planetary Science Letters 203 (2002) 967^981

www.elsevier.com/locate/epsl

gions can have important consequences in the de-velopment of any global geomagnetic model. Be-cause the virtual axial dipole moment (VADM) ata given time varies dramatically with location,good spatial coverage is essential in these globalmodels. For example, in approximating the aver-age global dipole moment, as in McElhinny adSenanayake [1] or Yang et al. [2], the resultingvalue will be biased towards regions with largequantities of data.In this paper, we examine archaeointensity data

from the southwestern United States and north-western South America, two regions that couldbenet from additional intensity data. Studiesfrom both regions show that signicant dieren-ces exist between the European paleointensitycurve and the Western Hemisphere data. How-ever, considerable scatter among the data setshas made comparisons and conclusions dicult.We rst examine the possible sources of scatter

in archaeomagnetic data. We make correctionswhere possible and apply minimum reliability cri-teria to extract a more reliable data set. We thencompare this data set, along with our new datafrom southwestern California and southern Ecua-dor, to global paleointensity models.

2. Examination of existing data

We have assembled all published archaeointen-sity data from the southwestern United States andnorthwestern South America, plus the volcanicpaleointensities of Champion [3] (Table 1; seealso Background Data Set1). The materials usedfor the archaeointensity studies are mostly ce-ramics, with some baked clays (Table 1). The ex-isting southwestern United States data span a re-gion approximately 1000 km across, coveringparts of Arizona, New Mexico, Colorado andUtah (Fig. 1). The rst archaeomagnetic studyin this region was undertaken by Bucha et al.[4], and signicant amounts of data were addedby Lee [5], Hsue [6], and Sternberg [7]. The SouthAmerican data span a region roughly 2000 km

across, covering parts of Ecuador, Peru and Bo-livia (Fig. 1). Data are contributed by Nagata etal. [8], Kitazawa and Kobayashi [9], Gunn andMurray [10], Kono et al. [11], and Yang et al.[12].Data from the southwestern United States are

concentrated in the last 2000 yr (Fig. 2), becauseearlier materials are rare in this region. The lonepoint at 4800 yr BP is a volcanic sample. Themajority of the South American data are alsoconcentrated in the last 2000 yr, but a signicantamount of data covers the period from 2000 to4000 yr BP.Both sets of data exhibit substantial scatter

(Fig. 2) that may obscure any real trends. Poten-tial sources of scatter and uncertainty in the datainclude dating errors, experimental inaccuracy,cooling rate dierences, magnetic anisotropy,and distortion of the ancient eld during ring.We examine these age and experimental uncer-tainties below, using data from the southwesternUnited States and northwestern South America toillustrate the magnitude of the errors.

2.1. Sources of scatter

2.1.1. Age uncertaintyAge uncertainty is perhaps the biggest concern

in archaeointensity data sets and the most dicultto eliminate. Dates are assigned to archaeologicalmaterials through a variety of methods, includingstratigraphic association, dendrochronology, pot-tery style, archaeomagnetic dating, associationwith radiocarbon dates, and thermoluminescencedating. Each of these methods has its drawbacks,and the best dates are usually derived from somecombination of methods.With the exception of thermoluminescence ^

and occasionally radiocarbon ^ dating the ageof archaeomagnetic materials invariably involvesassociation with other independently dated mate-rials. The most common method of associationrelies on the stratigraphy of excavations. Strati-graphic association can provide accurate dates ifthe stratigraphy is properly identied, includingnon-horizontal layering, cultural deposits, intru-sions and erosional channels that cut throughstratigraphy [13]. Unfortunately, in instances1 http://www.elsevier.com/locate/epsl.

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J. Bowles et al. / Earth and Planetary Science Letters 203 (2002) 967^981968

where soil layering and stratigraphy are not visi-ble (and even in some cases where they are), ex-cavation by metric level is used instead. This in-troduces an articial stratigraphy by arbitrarilyassigning a common age to all artifacts at a givendepth [13].Pottery style and archaeomagnetic dating pro-

vide indirect age constraints through comparisonwith existing records of temporal changes in pot-tery style or the regional magnetic eld. Associa-tion of a unique pottery style with a particularcultural period may yield excellent dates (withintens of years) if the cultural period is short and itsage is well-constrained by other methods.In some instances, artifacts can be sorted into

proper age order, but the details of the absolutecultural chronology may be in dispute. For exam-ple, many of the southwestern samples excavatednear Snaketown, Arizona, were produced by theHohokam culture. Until recently, conictingchronologies for this culture resulted in age dier-ences of up to 700 yr. In 1989, Sternberg [7] ac-knowledged this problem and discussed both thelong-count chronology of Haury [15], as well asthe short-count chronology of Schier [16],which is much compressed and results in consis-tently younger dates. Further work suggests that ashort-count chronology is more likely [17^19]. We

therefore use a recent chronology [18] to adjust allHohokam data in our nal compilation (Fig. 3).It should be pointed out, however, that details ofthe Hohokam chronology before approximately1300 yr BP remain in dispute.As with dating by pottery style, the accuracy of

archaeomagnetic dating depends directly on thequality of existing records used for comparison.Two early studies which provide a signicantamount of data for the Southwest [5,6] cite ar-chaeomagnetic dating for some samples. In thecase of oriented samples, archaeomagnetic datinginvolves comparing the remanence with a refer-ence curve of directional secular variation. How-ever, all of the samples from Hsue [6] and manyof the samples from Lee [5] are unoriented, mean-ing the sherds must be compared to a referencepaleointensity curve. Because these were the rsttwo extensive paleointensity studies in the South-west, it is dicult to accept that even an approx-imate date could be assigned in this manner.In addition to possible uncertainties from the

indirect associations outlined above, the absoluteages of datable material may also have signicanterrors. For example, radiocarbon dating has nu-merous pitfalls that are important to recognize[20]. Laboratory errors, resulting primarily fromerrors in counting the radioactive disintegrations,

Table 1Previous paleomagnetic studies of the southwestern United States and northwestern South America

Author Location Material used Method pTRMchecks?

Anisotropycorrection?

Datingmethod

Champion [3] American West lava and ash ows Thellier yes n/a C, or D andAM

Bucha et al. [4] American Southwest ceramics and bakedclay

Thellier no no C, AM, PS,D, S or M

Lee [5] American Southwest,Peru, Bolivia

ceramics and bakedclay

Thellier no no A, C or AM

Hsue [6] American Southwest ceramics and bakedclay

Thellier no no A, C, or AM

Sternberg [7] American Southwest ceramics Thellier yes yes A, PS, D, HDor M

Nagata et al. [8] Peru, Bolivia ceramics Thellier no no AKitazawa and Kobayashi [9] Ecuador, Bolivia ceramics Thellier no no A or CGunn and Murray [10] Peru ceramics Shaw n/a no A, TKono et al. [11] Peru ceramics Thellier no no C, S, PSYang et al. [12] Peru ceramics Thellier no not needed A, T

C= 14C, S= stratigraphy, PS= ceramic (pottery) style evolution, T= thermoluminescence, M=modern, AM=archaeomagnetic,D=dendrochronology, HD=historic documents and material culture, A=other archaeological and historical methods.

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J. Bowles et al. / Earth and Planetary Science Letters 203 (2002) 967^981 969

can become greatly amplied in the calibrationfrom radiocarbon years before present to calendardates. Even if the laboratorys reported standarddeviation is small (20 yr) and the data fall on asteep part of the calibration curve, the estimatedcalendar age range can be as great as 100 yr(68.3% condence level) [20]. Calibration acrossa relatively at portion of the curve can lead toestimated calendar date ranges of 250 yr or more.Unfortunately, many of the 14C dates used in ourcompilation studies have much larger laboratorystandard deviations, leading to much larger calen-dar date ranges (500 or even 700 yr at the 68.3%condence level). Finally, because the calibrationcurve is not monotonic, it is possible for a singleradiocarbon date to have more than one possiblecalendar date.Where original references were provided, we

have calibrated or recalibrated 14C dates withthe most recent CALIB v4.3 program of Stuiverand Reimer [21] and associated data sets [22,23].Except in a few instances, this recalibration hadvery little eect on placement of age mid-points,although it generally increased the age uncer-tainty.Thermoluminescence (TL) dating, used in two

studies in our compilation, was developed in the1960s specically as a means to directly date pot-tery (see e.g. [24]). Since then, it has come to beaccepted as a reliable method of dating bakedarchaeological materials. The technique is basedon the accumulation of trapped electrons, primar-ily in quartz and feldspar, from natural radioac-tive decay. These trapped electrons are releasedwhen the pot is red, resetting the luminescenceclock [24]. While TL dating will never approachthe precision of radiocarbon dating, it does havecertain advantages over that method. Becausepotsherds can be directly dated, errors related toartifact association are avoided. Calibration er-rors also do not come into play. Uncertaintiesin TL dating are usually about V 5^10% of theestimated age and result from errors in properlyestimating the natural level of radiation to whichthe sample was exposed [24].Perhaps the most accurate ages may be ob-

tained from the stratigraphic association of wood-en artifacts and ceramics. Dendrochronology may

Fig. 1. Map of study areas. Stars indicate sampling locationsfor present study. Circles and diamonds indicate sampling lo-cations for other studies; diamonds show sites remainingafter applying data selection criteria (see text). Note dierentscales on the two enlarged maps.

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yield dates accurate to a calendar year [14]. Whilea time lag between the death of the tree and itsassociation with other cultural deposits must beconsidered, this method has the potential to pro-vide very tight age constraints.This brief review of dating methods highlights

the uncertainties inherent in the techniques. Anideal date might come from a recently measured14C date (last 20 yr), combined with one or twoother methods, reducing uncertainty to less thanV 50 yr. However, this kind of information is gen-erally not available, and the fact remains that age

Fig. 2. All previously published data for (a) the southwestern United States and (b) northwestern South America. Vertical errorbars represent one standard deviation of repeat measurements on the same sample. Where none are shown, only one measure-ment was made. Horizontal error bars represent age uncertainties as given in original sources (see Table 1). Where none areshown, no age uncertainty was reported.

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control on most of the data in our compilation ispoor. The average age uncertainty in the compi-lation is nearly V 140 yr (1c for radiocarbon andthermoluminescence, or the length of the assigned

ceramic phase or other archaeological context).Thus, short-period (6 200 yr) uctuations in geo-magnetic intensity are unlikely to be reliably de-termined.

Fig. 3. Data selected from the literature based on the selection criteria described in the text. New data from this study are shownas squares. The light solid line is a cubic spline t to the binned and averaged data. For comparison, the Hongre et al. [47] mod-el (dotted line) and the gufm1 model of Jackson et al. [48] (bold line) have been evaluated for these two regions. The inset in (b)is a close-up of the last 700 years showing the discrepancy between gufm1 and the archaeomagnetic data.

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2.1.2. Experimental uncertaintiesTwo main techniques are currently in use for

determination of paleointensity: Thellier^Thellier[25] and Shaw [26]. The stepwise double-heatingmethod of Thellier and Thellier [25], modied byCoe [27], is commonly believed to be the mostreliable method of determining paleointensity,and most of the studies in the compilation use avariation of this method. The method involvesheating samples twice at each temperature step ^once in zero eld to remove a portion of the nat-ural remanent magnetization (NRM) and once ina controlled eld to determine the partial ther-moremanence (pTRM) gained. The ratio ofNRM lost to pTRM gained is proportional tothe ancient eld. Errors associated with the Thel-lier method include uncertainty in oven temper-ature and lab eld, errors in the measurement ofsample intensity, and sample thermal alteration.Measurement errors should average out if multi-ple specimens are taken from each sherd. It isimportant that temperature between in-eld andzero-eld steps is accurately reproduced. Thermalalteration of the sample can also lead to signi-cant error. The so-called pTRM checks are de-signed to reveal such alteration by back-trackingto repeat a lower-temperature in-eld step. Mostof the early studies in the compilation do not usethese checks.The Shaw method [26], modied by Kono [28]

and Rolph and Shaw [29], has been used in onestudy in the compilation. Designed to avoid orcorrect for thermal alteration, the Shaw methoduses AF demagnetization, but still requires heat-ing the sample above the Curie temperature toimpart a total TRM. In theory, some thermalalteration from this heating can be corrected for[29,30]. In practice, results generally tend to agreewith those of the Thellier method, but in severalcases have been reported to show more scatter[31,32].

2.1.3. Anisotropy of TRMRogers et al. [33] report strong magnetic aniso-

tropy in pottery samples that could lead to errorsof 30^40% (up to 60% for wheel thrown pottery)if left uncorrected. They suggest that this aniso-tropy stems from preferential alignment of mag-

netic grains during shaping of the pot which re-sults in an easy plane of magnetization in theplane of the pot. Scatter from remanence aniso-tropy can be corrected by measuring the TRManisotropy tensor [36].

2.1.4. Cooling rateAnother potential source of data scatter is in-

troduced through dierences between coolingrates in antiquity and in the lab [34,35]. A slowercooling rate produces a larger TRM, as magneticmoments have more time to come into equilibri-um with the eld. In general, lab cooling rates arefaster than original cooling rates, leading to anoverestimation of the ancient eld, the degree ofwhich depends on the actual cooling rate dier-ence, as well as the blocking temperature. Labcooling times reported in the compilation studiesranged from 10 min to 2 h.For ceramics and bricks red in brick kilns,

original cooling can take over 24 h, leading toan overestimation of the eld by approximately10% (based on a lab cooling time of 25 min)[36]. Most of the ceramic samples from the com-pilation studies were likely red instead in theopen or in small pit kilns, with cooling times onthe order of 1^12 h, based on observations ofNative American potters [37,38]. Pots may be re-moved from the re while still at high temperature[37], or they may be left over the re until cool[38]. How long it takes pots left over the re tocool depends on the kind and amount of fuelused, as well as to what degree the fuel surroundsthe pots and how well they are insulated [38].Actual lab overestimation of ancient eld couldtherefore range from V2 to 8%.Given the variation among modern potters, it is

dicult to comment on precise techniques in agiven location thousands of years ago. It is likelythat the maximum cooling rate error is 10%, andin many cases it is probably less than 5%. If nocorrection is made, paleointensity will always beoverestimated, leading to an upward bias ofthe entire averaged curve. For this reason, wechoose to make a 5% correction to all the data.While some of the resulting paleointensity esti-mates will now be too low and others still toohigh, this blanket correction should have the ef-

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fect of centering the scatter closer about the truevalue.

2.1.5. Field distortion during ringA possibility exists that the eld seen by the

pots during ring was not equivalent to the an-cient ambient eld. Field distortion from a self-demagnetizing eld and/or proximity to otherpots, sherds or metal during ring can also in-crease scatter in paleointensity data. A self-de-magnetizing eld will always act to subtractfrom the main eld, but little can be done toreproduce or correct for this, although Aitken etal. [39] attempted to measure the possible degreeof self-demagnetization. Distortion from externalsources is a distinct possibility, as open pit kilnconstruction can include placing broken sherdsboth under and over the pots to be red [37].After introduction of iron by Europeans roughly500 years ago, there is also the possibility thatiron objects were used to support or shield potsduring ring [39], although this is probably morecommon only in modern times [38,40]. Aitken etal. [39] suggest that one way to partially mitigateeld distortion eects is simply to take multiplespecimens from dierent parts of the pot and mul-tiple pots from each time period.

2.1.6. Temporal and spatial eld variationTwo nal sources of scatter in the data may be

geomagnetic in origin. Temporal variations in in-tensity may appear as scatter if samples cannot bewell dated or if sampling density is insucient.Spatial variation in the eld may also introduceapparent scatter. Based on the present eld, re-gions the size of our study areas (V1500 km)may have intensities that dier by as much as15%. As more data become available worldwide,it will be possible to average over smaller regions,eliminating some of this spatial scatter.

2.2. Data selection

While some of the potential sources of scattermentioned above cannot be eliminated, many canbe compensated for or minimized. By selectingonly the most reliable data, we should producea much less scattered record that more accurately

represents the true paleoeld behavior. We canuse updated ceramic chronologies, calibrate or re-calibrate 14C dates, and make cooling-rate correc-tions to existing data. Corrections for other fac-tors cannot be made in retrospect, however, andsome data must be excluded. Because data repro-ducibility is essential, we eliminate over 40% ofthe data from the compilation because only onespecimen per sample was tested. We further ex-clude any samples that show excessive intra-sam-ple scatter as measured by the standard deviationof the paleointensity estimates divided by themean (cB/Bavg).Ideal data points would have at least two sub-

samples, cB/Bavg9 0.10, use the method of Thel-lier and Thellier with pTRM checks, have an ani-sotropy correction, and an age uncertainty of lessthan V 100 yr (1c). Sadly, this would leave onlyone study from each region. Instead, we applysomewhat less stringent acceptance criteria: sam-ples must have at least two specimens; the Thel-lier technique should be utilized; and cB/Bavgshould be less than 0.20 for each sample to ensureat least a reasonable degree of within-sample scat-ter. The Lee [5] and Hsue [6] studies are excludedbecause of extreme uncertainty in dating. Theystate that they use radiocarbon dating, historical,or archaeomagnetic dating. As discussed above,archaeomagnetic dating in this context is highlyquestionable. They also provide no informationon their radiocarbon dates so the calibration sta-tus is uncertain. Because neither study species ona sherd by sherd basis the method of dating, wereject all of these samples from the compilation.We have also not included a study on Peruvianceramics [41] because site locations and individualresults were not presented.After applying these selection criteria, the scat-

ter is indeed reduced (Fig. 3), especially in SouthAmerica. However, very few points remain to de-ne changes in the geomagnetic eld throughtime, and we must recognize that even thesepoints are not ideal. Many of these points arefrom studies which did not use pTRM checks ormade no anisotropy correction. It is clear thatnew data adhering to strict reliability standardswill go far in resolving a more dependable paleo-intensity record.

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3. New results

In an attempt to increase the amount of high-quality data available for the southwestern UnitedStates and northwestern South America, we car-ried out paleointensity experiments on six pot-sherds from the southern California desert(V33.3N, V115.5W) and 14 from Ecuador(Table 2). The surface-collected California sherdsare ceramics of the Lowland Patayan tradition[42] and were obtained from the San Diego Mu-seum of Man where they were typed by MichaelWaters. These sherds have been classied intothree time periods spanning the past 1500 yearsbased on pottery style. Unfortunately, more pre-cise dating in southern California is not possibledue to very plain ceramic styles and the lack ofdendrochronology data. The South Americansherds were excavated from the Real Alto (Valdi-via) site at 2.37S, 80.72W and span the periodof roughly 4000^5000 yr BP. These sherds havealso been classied based on style, and the datesof these styles are based on extensive radiocarbonassays as well as some TL dating [43]. Severalindividual sherds have also been directly associ-ated with radiocarbon-dated material. The RealAlto sherds were obtained from the Universityof Illinois Laboratory of Anthropology collec-tions and were typed by James Zeidler.We trimmed four specimens from each sherd

and pressed them into salt pellets for ease of han-dling and to ensure consistent orientationthroughout the experiments. We used the stepwisedouble-heating method of Thellier and Thellier[25], modied by Coe [27], to recover an estimateof the ancient eld. Thermal alteration of thesample was monitored by pTRM checks after ap-proximately every third temperature step. Twosalt pellet blanks were also included in the Thel-lier experiments and showed no remanence acqui-sition.The reliability of the ancient eld determina-

tions was assessed using the criteria of Selkinand Tauxe [44], which are much more stringentthan the selection criteria applied above. Thesecriteria are: (1) The temperature interval selectedfor paleointensity interpretation should corre-spond to the nal remanence component of mag-netization ^ what we hope is the samples originalthermoremanence. This is illustrated by the decayof the zero-eld steps in the selected interval tothe origin of a vector endpoint diagram (e.g. Fig.4); the angle (K) between the principal componentof the selected interval and the vector average ofthe data should be less than 15. (2) The maxi-mum angular deviation of this principal compo-nent must also be less than 15. (3) The slopecalculated from the NRM^pTRM pairs (e.g.Fig. 4) must have the ratio (L) of the standarderror of the slope to the absolute value of the

Table 2Sherd locations, ages and types

Sample Lat. Lon. Site Sherd provenance Style Historical period Age range(N) (W) (yr BP)

sic182 33.10 114.87 C85, trail Surface collected Black Mesa Bu Patayan I 1000^1300sic188 32.76 114.72 C86N, trail Surface collected Colorado Red Patayan I 1000^1300sic194 33.12 115.36 C14, village Surface collected Salton Bu Patayan II 500^1000sic195 33.10 114.87 C85, trail Surface collected Tumco Bu Patayan II 500^1000sic198 33.02 116.28 C160, village Surface collected Colorado Bu Patayan III 50^500sic201 32.26 115.79 LC54, village Surface collected Colorado Bu Patayan III 50^500sio001 32.37 80.72 Real Alto Trench A, Structure 1,

oor deposit, Unit 59/52,Level 1

Valdivia Polished Red Valdivia Phase 3 4400^4800

sio004 32.37 80.72 Real Alto Trench A, Structure 13,wall trench, 0^10 cm

Valdivia Nicked Rim Valdivia Phase 6 3950^4100

sio013 32.37 80.72 Real Alto unprovenanceda Valdivia Red Engraved Valdivia Phase 2 4800^5300sio014 32.37 80.72 Real Alto unprovenanceda Valdivia Red Engraved Valdivia Phase 2 4800^5300a Assigned to Valdivia Phase 2 based on ceramic style.

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slope less than 0.1. (4) To ensure sucient repro-ducibility between two in-eld measurements at agiven temperature, the dierence between repeatin-eld steps normalized by the length of the se-lected NRM^pTRM segment must be less than0.10. (5) q, the quality factor as dened by Coeet al. [46], must be greater than 1. (6) For eachsample, the standard deviation of the eld esti-mates divided by the mean (cB/Bavg) must be6 0.20. Application of these criteria produced ac-ceptable paleointensity interpretations for 39specimens representing 10 dierent sherds. Repre-sentative data are shown in Fig. 4, and data fromall interpreted specimens are presented in Table 3,along with sample averages.The California sherds all showed either unidir-

ectional demagnetization behavior (for the zero-eld steps) or the presence of a small, secondary,low-temperature component that was removed by250C (Fig. 4a). Paleointensity interpretationswere made for all of the California specimens,but one sample (sic182) was rejected because of

excessive intra-sherd scatter. The Ecuadoriansamples yielded fewer successful ancient eld esti-mates. Many sub-samples exhibited vector end-point plots that did not decay to the origin, didnot pass pTRM checks or showed other non-idealbehavior. We believe this is caused by incompleteoxidation during ancient ring or ring under re-ducing conditions. As observed by others (e.g.[4,7]) the well-oxidized sherds that display a redto orange color in cross-section typically providebetter results than sherds that are gray to black incolor. Of the Ecuadorian samples that producedinterpretable results, most showed two or morecomponents, but the characteristic componentwas usually isolated by 350C (Fig. 4b). We spec-ulate that the lower-temperature components re-sult from a second ring at a lower temperature,removal of the pots from the re mid-waythrough cooling [37], or use of the pots as cookingvessels. Of the four Valdivia sherds that gavegood results, one is rejected because of excessiveintra-sherd scatter (sio014).

Fig. 4. Typical results from Thellier^Thellier experiments. (a) Vector endpoint plot of zero-eld NRM steps (left) and NRM^pTRM (right) plots for California sample sic188-1a. (b) Same for Ecuador sample sio1-1a. x^y projection on the vector endpointplot is in circles; x^z projection is in squares. Closed circles on the NRM^pTRM plot indicate temperature steps chosen forpaleointensity determination; open triangles are pTRM checks. Numbers on both plots refer to temperature steps.

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Finally, we made a correction to the paleointen-sity values for TRM anisotropy and cooling rateas described by Selkin et al. [36]. The anisotropytensor for each specimen was determined by heat-ing above Tc and cooling in lab eld in six dier-

ent positions ( V x, V y, V z). The degree of aniso-tropy as measured by the ratio of the maximumto minimum eigenvalues [45] ranges from 1.09 to1.35. Thirty out of 39 specimens show weakly tomoderately developed oblate anisotropy ellipses,

Table 3Results of Thellier^Thellier experiments on Californian and Ecuadorian ceramic pot sherds

Specimen c B B* B*avg cB cB/B*avg f g q VADM VADM2

(U1022 A m2) (U1022 A m2)

sic182-1 0.0689 83 92 70.6 14.83 0.21 0.53 0.73 13.0 13.30 12.63sic182-2a 0.0312 66 70 0.64 0.81 29.3sic182-3a 0.0157 58 59 0.71 0.81 54.5sic182-3b 0.0214 59 61 0.71 0.81 41.4sic188-1a 0.0548 66 61 64.3 2.75 0.04 0.57 0.78 12.4 12.17 11.56sic188-2a 0.0224 69 67 0.64 0.81 39.1sic188-3a 0.0186 58 63 0.68 0.83 47.0sic188-4a 0.0640 64 66 0.61 0.78 12.3sic194-1a 0.0199 63 64 62.7 2.64 0.04 0.49 0.72 28.6 11.82 11.23sic194-2a 0.0458 66 64 0.52 0.78 14.2sic194-3a 0.0256 53 59 0.66 0.84 31.5sic194-4a 0.0260 62 64 0.61 0.81 30.7sic195-1a 0.0242 45 46 44.2 6.81 0.15 0.53 0.73 18.0 8.33 7.91sic195-2a 0.0271 52 53 0.38 0.70 12.8sic195-3a 0.0589 40 43 0.36 0.71 4.6sic195-4a 0.0311 34 36 0.35 0.71 7.1sic198-1 0.0284 70 60 63.6 3.02 0.05 0.40 0.74 15.6 12.00 11.40sic198-2a 0.0195 64 67 0.69 0.87 51.1sic198-3a 0.0211 65 65 0.69 0.86 45.9sic198-4a 0.0150 66 63 0.70 0.86 63.1sic201-1 0.0600 64 61 60.7 1.75 0.03 0.58 0.77 11.4 11.57 10.99sic201-2 0.0680 65 63 0.59 0.79 10.7sic201-3a 0.0439 55 60 0.57 0.80 15.7sic201-4a 0.0397 54 59 0.60 0.80 17.8sio001-1a 0.0234 22 24 19.7 2.64 0.13 0.47 0.74 9.1 6.81 6.47sio001-2a 0.0236 25 26 0.49 0.78 10.4sio001-3a 0.0304 26 29 0.43 0.76 7.8sio004-1a 0.0161 30 31 33.3 1.90 0.06 0.43 0.74 15.6 8.60 8.17sio004-2a 0.0462 36 36 0.49 0.75 7.2sio004-3a 0.0063 26 33 0.68 0.81 71.0sio004-4a 0.0211 28 33 0.67 0.80 20.9sio013-1a 0.0194 36 36 39.3 2.58 0.07 0.74 0.77 26.2 10.17 9.66sio013-2a 0.0205 38 40 0.77 0.83 31.0sio013-3a 0.0103 41 42 0.76 0.83 64.2sio013-3b 0.0164 39 40 0.73 0.84 37.0sio014-1 0.1020 51 59 40.3 14.95 0.37 0.57 0.42 3.5 10.43 9.91sio014-2a 0.0694 27 32 0.20 0.65 1.5sio014-3a 0.0807 36 44 0.12 0.71 1.2sio014-4a 0.0440 21 26 0.18 0.73 1.9

Specimens beginning with sio represent sherds from Ecuador, while sic denotes Californian sherds. c, standard error of slope;B, ancient eld (WT); B*, ancient eld (WT) corrected for magnetic anisotropy of sample; B*avg, site mean; cB, standard devia-tion of site values; f (NRM fraction), g (gap factor), and q (quality index) as dened by Coe et al. [46] ; VADM, virtual axial di-pole moment (U1022 A m2); VADM2, cooling rate-adjusted VADM (5%). Note that samples sic182 and sio014 were rejectedbased on excessive intra-sample scatter.

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consistent with an easy plane of magnetizationwithin the plane of the sherd, as predicted byRogers et al. [33]. This resulted in corrections tothe preliminary ancient eld values of up to 30%.A further 5% reduction to intensity values wasapplied to correct for cooling rate.

4. Discussion

Our new data from California generally agreewith earlier data from the southwestern UnitedStates that meet the minimum reliability criteriaoutlined above (Fig. 3). While the dates of theCalifornian sherds examined in this study remaintoo poorly constrained to provide clear distinc-tions between new and existing data, the lowerof the two points at 750 yr BP suggests that thedecrease in eld intensity between 500 and 1000 yrBP was possibly lower than previous data imply.In general, the combined southwestern compila-tion and new data show signicant dierencesfrom both South American and global data. InFig. 5 we show the new and compilation dataaveraged into 500 yr bins to compare with theglobal archaeointensity curve of Yang et al. [2].The southwestern data show a distinct low be-

tween V1000 and 1500 yr BP that is not presentin either the South American or the global curves.Our new paleointensity results from Ecuador

are signicant in that they extend the reliableSouth American chronology back over 1500 yr(Fig. 3b). The well-dated point at approximately4000 yr BP is especially useful in providing anaccurate paleointensity estimate for a well-con-strained time. These data roughly agree withwhat is expected on a global basis, falling on ei-ther side of the global curve (Fig. 5). Like thesouthwestern compilation data, the South Ameri-can data show a much narrower peak than thebroad, 2000^3000 yr high of the global curve,which is dominated by European data. However,after removing the European data from the curve,we see that the remaining world data (bold,dashed line in Fig. 5) provide a closer match tothe long-period variations in South Americandata.To examine shorter-period trends represented

by the southwestern and South American datasets, we bin and average the selected compilationdata along with our new data in 200 yr bins overthe last 2500 yr (Fig. 3). The arithmetic meanVADM of each bin is given an age equal to thearithmetic mean of the sample ages within the bin.We connect the averaged data points (representedby asterisks in Fig. 3) with a cubic spline, merelyto guide the eye rather than to suggest a truepaleointensity curve. We must recognize that thedata scatter (especially age uncertainty) and themethod of averaging degrade any higher-fre-quency signal that may be present in the data.Certainly earlier than 2000 yr BP, both curvesare poorly dened and we cannot expect thatthe true curve is adequately represented by con-necting the few dots.Nonetheless, results of this averaging process

highlight potentially signicant dierences be-tween the two regions, including a higherVADM in the southwestern United States fromV0 to 600 yr BP and the broad low fromV1000^1500 yr BP mentioned above. These dif-ferences illustrate the importance of having spa-tially well-distributed data sets for any kind ofglobal geomagnetic model. It is equally impor-tant, however, not to use data of questionable

Fig. 5. Global archaeointensity curve of Yang et al. [2] (boldsolid line), and the global curve minus European data (bolddashed line). The southwestern United States (thin solid lineand circles) and South American (thin dotted line and trian-gles) data of Fig. 3 are shown here averaged in 500 yr binsto match the bins used by Yang et al. Symbols shown withno connecting line represent single data points rather thanbinned averages.

EPSL 6408 21-10-02


quality merely because they exist where little elsedoes.The lack of sucient quality data in certain

areas of the world can place limitations on globalmodels of eld behavior. This becomes evident inthe global archaeointensity model of Hongre et al.[47]. In South America, where the Hongre et al.model uses much of the same data as we show inFig. 3, an evaluation of their model does providea rough t to the data, at least back to about 1700yr BP. However, in the southwestern UnitedStates, where Hongre et al. provide no intensitycontrol, the model only roughly approximates thedata and overestimates intensity (Fig. 3a). Thisillustrates the danger of drawing any conclusionsfrom evaluations of such models in poorly con-strained areas.Models based on direct observations provide a

closer match to the archaeointensity data,although even these models show some discrepan-cies with the data. Since 1832, when Gauss devel-oped a method to measure absolute intensity, onemight expect the eld to be quite well-dened. Butevaluation of the gufm1 model of Jackson et al.[48], which is based on historical observations,shows a discrepancy of V10% in South America(Fig. 3b, inset). Whether this discrepancy is due toa bias in the archaeointensity data or to the in-adequate constraints on the gufm1 model in thisregion, it is important to recognize the potentiallimitations of such models.

5. Conclusions

Existing archaeointensity data covering the past5000 years in the southwestern United States andnorthwestern South America exhibit signicantscatter and contain more uncertainty than is gen-erally recognized. Experimental inaccuracies,cooling rate dierences, magnetic anisotropy, elddistortion during ring, and especially dating un-certainty serve to create scatter in the record andobscure short-period variations in eld intensity.While we can approximate corrections for someof the sources of scatter, only a fraction of theexisting data can be used with any condence.Much work remains to increase the quantity of

high-quality paleointensity data available andprovide more uniform spatial coverage for thepast 5000 years. While our new data start to llin the gaps, there is still a long way to go. Com-plete uniform global coverage may never beachieved, but signicant improvements on presentdata will result in global models that are muchbetter constrained.

Acknowledgements

We would like to thank Jim Zeidler for select-ing the Valdivia potsherds for our use and forproviding dates and other helpful information.Thanks are also due to Angela Neller for facilitat-ing the loan of the Valdivia sherds from the Uni-versity of Illinois. Thanks to Ken Hedges andGrace Johnson of the San Diego Museum ofMan for permission to study the California ce-ramics. We are also grateful for the helpful com-ments provided by Jerey Eighmy and RobertSternberg. This work was partially supported byNSF grant EAR0099294 to L.T.[RV]

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Archaeomagnetic intensity results from California and Ecuador: evaluation of regional dataIntroductionExamination of existing dataSources of scatterAge uncertaintyExperimental uncertaintiesAnisotropy of TRMCooling rateField distortion during firingTemporal and spatial field variation

Data selection

New resultsDiscussionConclusionsAcknowledgementsReferences

bowles et al., 2002

Documents

existing data

published data

questionable data

data fromcalifornia

south american data

oldest archaeointensity

sources of data scatter

dearth of intensity