Archaeomagnetic paleointensity in the American Southwest during the past 2000 years

Physics of the Earth and Planetary Interiors, 56 (1989) 1—17 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

1

Archaeomagnetic paleointensity in the American Southwest during the past 2000 years Robert S. Stemberg

*

Department ofGeosciences, University ofArizona~Tucson, AZ 85721 (U.S.A.) (Received February 1, 1988; revision accepted April 19, 1988)

Sternberg, R.S., 1989. Archaeomagnetic paleointensity in the American Southwest during the past 2000 years. Phys. Earth Planet. Inter., 56: 1—17. An archaeomagnetic paleointensity study was carried out on pottery samples from the Hohokam, Anasazi, and Mogollon cultures of the American Southwest. The Thellier—Thellier paleointensity experiment was used on 187 specimens fom 77 different sherds derived from 23 archaeological sites. Interpretations were made for 127 specimens from 54 sherds, about a two-thirds success rate. The median value for the quality factor was 12. The results were corrected for magnetic anisotropy owing to a fabric in the pottery, using an easy-plane model of magnetization. The anisotropy of the hard axis relative to the easy plane of magnetization averaged 31%. The correction factor for the paleointensity was typically 5.4%, and systematically increased the paleointensities because the laboratory heatings were applied within the easy plane. The sample-average paleointensities were compiled along with other data from North America. The Hohokain data were kept as a separate set because of uncertainties concerning the Hohokam chronology. Curves of the virtual axial dipole moment (VADM) variation for the Hohokam and non-Hohokam data sets were derived using a moving-window smoothing technique. The Hohokam and non-Hohokam curves show good agreement, regardless of which Hohokam chronology is used. Relative paleointensity records from North American lake sediments are also congruent with the archaeointensity results. Despite the scatter in the archaeomagnetic data, the correlation of these records suggests that the secular variation pattern is being recovered. The field strength at 300 BC is 30—40% higher than at present; a low from 250 to 1250 AD is 10—15% weaker than at present; and a peak at 1500 AD is about 30% higher than at present. The low from 250 to 1250 AD in comparison with global paleointensity compilations suggests a substantial non-dipole field in North America at that time. The rate of change of the archaeomagnetic VADM is similar to rates observed today.

1. Introduction

Much of the detailed knowledge on geomagnetic secular variation comes from direct observations of the field. These data become increasingly sparse further back in time, especially before the twentieth century. Archaeomagnetic data, along with paleomagnetic data from igneous rocks and rapidly deposited sediments, provide a means of

*

Present address: Department of Geology, Franklin and Marshall College, Lancaster, PA 17604-3003, U.S.A.

extending our knowledge of the magnetic field into prehistory. Because archaeomagnetic materials carry a thermoremanent magnetization (TRM), it is possible to use them to examine secular variation of paleointensity as well as direction. Recent reviews by Creer et al. (1983) and Wolfman (1984) summarized the extant archaeomagnetic database for both directions and intensities. In this study, an archaeomagnetic paleointensity record spanning the last 2000 yr has been developed for the American Southwest. This record complements a secular variation record of direction for the same region (Sternberg, 1989).

2 115W

105W

__________

*

40N

* — * * * *

— *

*

—

_____________

30N

Fig. 1. Site locations for pottery samples. More than one sherd was used for many of the sites.

firing are given in Sternberg (1982, Appendix D). All sherds were independently dated as they came from projects that were complete or well under way. Typical potsherds (samples) used this study 2 andinthicknesses had surface areas of 25—100 cm of 2.5—7.5 mm. Cylindrical specimens were cored from the sherds using a diamond-tipped drill core mounted in a drill press. Diameters of the specimens were 1.25 cm, so the typical specimen volume was 6.1 cm3. A grid was laid out on each sherd so that multiple specimens from the same sherd would be oriented relative to one another. Axes were set up such that x- and y-axes were in the plane of the pottery and the z-axis was perpendicular to this plane. The x-axis was drawn towards the opening (top) of the vessel when this direction could be determined from curvature or the presence of a lip. The specimens could be oriented to no better than 100 because of the curvature of the sherds and the roughness of the grid. Specimens were taken from areas of the sherd that had the most uniform color in cross-section. Surface smudge marks were avoided when possible. A total of 187 specimens from the 77 sherds were used for the paleointensity experiment. —

The archaeointensity record was developed using pottery samples from the three major cultures in later Southwestern prehistory: the Hohokam, Anasazi, and Mogollon (Lipe, 1983).

3. Methods 2. Materials 3.1. Paleointensity experimental procedure The most prevalent artefacts and features found at Southwestern archaeological sites that carry a TRM are ceramics and hearths. Pottery was used in this study because abundant amounts are recovered during archaeological excavations, and the sherds are relatively accessible thereafter. The samples used represented a compromise between what was desired and what could be obtained. A total of 77 sherds from 23 different archaeological sites (Fig. 1) were used for paleointensity analysis. These sherds were obtamed from archaeologists and museum collections: 26 samples were from the Snaketown site, 15 from Walpi, six each from Arroyo Hondo and Unkar Delta, four from Chaco Canyon, and one or two from each of the other sites. Information on provenance, typology, and estimated dates of

The paleointensity experiment used in this study followed the Thellier—Thellier double-heating paradigm (Theffier and Thellier, 1959; Thellier, 1977). Measurements were made on a commercial 2-axis cryogenic magnetometer, or occasionally on a 15-Hz spinner magnetometer (non-commercial). The furnace (non-commercial) used for heatings was able to replicate temperatures to 2°C, and was accurate to within 5°C.For a laboratory field of 50 ~sT, the magnetic field varied by <0.5% within the useful furnace space. Seven groups of specimens were run. For six of these runs, the laboratory field was applied along the x-axes of the specimens during both of the paired heating steps to the same temperature, with the specimens reversed by 180°( + x to x) rela—

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tive to the applied magnetic field in the furnace between the two heatings. A laboratory field of 50 ~T was used and left on during the entire heating—cooling cycle (Levi, 1975). The first ternperature step above room temperature was 100 or 150°C, with subsequent steps at intervals of 50°C or occasionally 25°C. Heatings were continued until temperatures of 600 or 725°Cwere reached, or until the NRM—TRM diagrams (Arai, 1963; Nagata et al., 1963) were obviously non-linear, For the seventh run, the laboratory field was off during the first heating and on during the second heating (Coe, 1967). A weak vacuum of 100 Pa (1 Torr) was applied in the furnace during one run (Khodair and Coe, 1975). The high porosities of the pottery specimens made lower pressures impractical. For the other six runs the heatings were done in air. Heating of samples in the furnace was at a rate of 30°Cmm 1 and cooling at 10°C mm ~‘. These rates are similar to those that occur during the firing of primitive pottery by contemporary Native Americans (Colton, 1951; Shepard, 1965, pp. 81—91), and are presumably also similar to the rates used in antiquity. Thus there was no need to make a correction to the paleointensities for differences between the heating—cooling rates during acquisition of the NRM and the laboratory TRM (Fox and Aitken, 1980). Several of the reliability tests suggested by Thellier and Thellier (1959), Thellier (1977) and Fucugauchi (1980) were used to minimize the possibility of interpreting spurious paleointensities (Barbetti et al., 1977; Walton, 1987). First, directions of the NRM and TRM vectors were momtored. NRM direction drift is an important indicator of secondary components and/or mineralogical change, even when the NRM—TRM diagram remains linear (Kitazawa and Kobayashi, 1968). Observation of TRM directions provides a check on sample orientation within the furnace. Second, repeat PTRM tests were used. After higher-ternperature steps were completed, a PTRM at 300°C was induced to assess any change in PTRM capacity owing to thermal alteration. Third, multiple specimens from a sherd and multiple coeval sherds were used whenever possible. —

—

—

3.2. Paleointensity interpretation

When the results from the Thellier—Thellier experiment are plotted on the NRM—TRM diagram, and the appropriate temperature interval is selected, the paleointensity is calculated as the magnitude of the slope of the regression line through the data multiplied by the strength of the laboratory magnetic field. The reduced major axis was used for the linear regression (Kermack and Haldane, 1950; York, 1966; Coe et a!., 1978). One problematic aspect of interpreting paleointensity data is selection of the data (temperature interval) from which the paleointensity will be calculated. Many specimens show departures from the ideal linearity of the NRM—TRM diagram at low and/or high temperatures. There may also be a general curvature to the NRM—TRM diagram, or two or more linear segments with different slopes. Coe (1967) discussed some theoretical rcasons for these deviations from linearity. Selection of the best interval was primarily subjective. Factors considered in selecting the interval and their general order of importance were: overall linearity of the NRM—TRM diagram; secondary inflections in this diagram; comparison of diagrams from multiple specimens of the same sherd; constancy of NRM directions with increasing temperature; objective indicators of goodness-offit—correlation coefficient, Z-transformation of correlation coefficient (Fisher, 1958, pp. 175—210), relative error of the slope, and quality factor (Coe et al., 1978); repeat PTRM results; color changes of the specimen; agreement of inferred paleointensity with that of other contemporaneous samples. The second and third criteria, in particular, can be evaluated by eye but are difficult to quantify. No minimum criteria were established for interpreting a paleomntensity. 3.3. Magnetic anisotropy

Archaeologists who study ceramics have noted a fabric anisotropy in ceramics (Shepard, 1965, pp. 183—184; Rye, 1981, pp. 58—95). The net alignment of platy and elongated grains can occur as a result of pottery construction. Archaeomag-

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netists have recognized that pottery is magnetically amsotropic (Burlatskaya and Petrova, 1961; Rogers et a!., 1979). Rogers et a!. (1979) ruled out demagnetizing effects due to shape anisotropy as the cause because their cylindrical specimens were nearly equidimensional and not strongly magnetized. Burlatskaya and Petrova (1961) also rejected the possibility of shape anisotropy because of weak magnetization. Rogers et al. (1979) coneluded that magnetic anisotropy in pottery is a result of a fabric. Preliminary tests showed that the primitive non-wheeled pottery used in this study did have a significant magnetic anisotropy, presumably owing to a magnetic fabric amsotropy. To correct the paleointensities for this a.nisotropy, a general treatment of TRM acquisition by anisotropic materials was followed (McElhinny, 1973, pp. 63—67; Stacey and Banerjee, 1974, pp. 118—120). The easy-plane model of magnetization that was used assumes that the TRM susceptibility was isotropic in the plane of the pottery (x—y-plane), and higher in this plane than perpendicular to it. The higher susceptibility makes this the easy plane of magnetization, and the z-axis is the hard direction of magnetization. This model is consistent with the quasi-planar fabric imparted during pottery manufacture, and is similar to the approach used by Veitch et a!. (1984). Aitken et a!. (1981) also discussed the use of the easy-plane model for correcting archaeomagnetic paleointensities, a!though they primarily used it as a reconnaisance technique. The amsotropy is given as P = Cm~/C~ = tan 9/tan q, where cm~and c~,are the TRM susceptibilities in the easy plane and hard threetion, respectively. In this study, the laboratory TRMs were all applied along the x-axis, an easy direction of magnetization. As a result, the amsotropy-corrected paleointensity is given as

the Appendix of Aitken et a!. (1981) reduce to these under the restriction that Q = 0, r~ = r~ = a, and p = 0 (in their equations); in other words, the generalized easy plane of magnetization is in the plane of the pottery, as assumed here. The correction f is zero when the magnetization is along the easy direction; it equals the anisotropy when magnetization is along the hard direction. The relative error in the anisotropy-corrected paleointensity is the sum of the relative error of the uncorrecte4 paleointensity and the relative error of the correction factor f. One anisotropy value was determined for each sherd using additional specimens cut specifically for that purpose. The same anisotropy value was then used for all specimens from that sherd that had been used for the paleointensity experiment. It would, in general, be possible to incorporate the anisotropy determination into the paleointensity experiment, allowing for the testing of a temperature-dependent anisotropy (Aitken et al., 1981). For the determination of F, PTRMs were applied between room temperature and either 300 or 400°C, depending on whether lower- or highertemperature intervals had been used for the paleointensity interpretation for that sample. Total TRMs were avoided to minimize the possibility of mineralogical change during even this single heating. The laboratory field was 50 itT. PTRMs were applied once each along the x, y- and z-axes. To calculate the contribution of the anisotropy error to the paleointensity error, d P was found from P using the empirical relation dP/P = 0.1, based on replicate anisotropy runs with sherds SNO2O—SN026. The angles 4 and d4 were calculated for each specimen using the NRM directions for each step within the temperature interval used for paleointensity interpretation. For the sherd coordinate system used (z-axis as the hard direction), 4 is equivalent to the magnetic inclination

BP =

in sample coordinates. Weighted statistics were used to calculate the average Fisher inclination or 4 for the appropriate temperature interval. The weights were the NRM values for the correspond-

=

~

P

cos ~ (p2 tan2q

+ 1~h/’2 /

where B

=

B

0 (NRM/TRM) is the usual paleointensity for an isotropic sample, and f is the anisotropy correction factor. The equations in

ing temperature steps. The 953 angle (Irving, 1964, p. 60), the circular standard deviation, was used for d~.

5

4. Rock magnetism Rock magnetic aspects of the pottery samples were examined using coercivity analysis. For the 77 sherds used in this study, 75 specimens from 74 sherds were progressively demagnetized in an altemating field. After the demagnetization was complete, 44 of these samples were used for progressive IRM acquisition. Eight samples which had not been previously demagnetized were also used for IRM acquisition. The peak fields used for IRM acquisition were between 0.75 and 0.85 T. For the 75 specimens that were AF-demagnetized, the NRMs range from 0.0183 to 10.2 A m2 (magnetic moment), with a median value of 0.732 A m2 and an interquartile range of 0.252—1.60 A m2. The maximum IRMs show a similarly shaped distribution with values from 2.22 to 302 A m2, a median of 27.0 A m2 and an interquartile range of 10.5—80.0 A m2. The NRMs and maximum IRMs for these specimens are linear!y correlated, The coercivity of remanence can be represented by the median destructive field (MDF). The MDFs range from 17 mT to > 100 mT. The median MDF is 36 mT and the interquartile range is 26—64 mT. These values are considerably higher than those found in Southwestern hearths (Sternberg, 1982), and probably indicate that hematite or titanohematite carries more of the remanence in pottery than in hearths. The coercivity spectra of IRM acquired in the laboratory can be described by the ratios of IRM acquired at 0.1 and 0.3 T to the IRM acquired for the maximum field. Only three specimens had values of IRM(0.1 T)/IRMm~> 95%. At 0.3 T, where magnetite would already be saturated, 15 of 52 specimens (30%) still had ratios of IRM(0.3 T)/IRMm~< 95%. Thus the IRM coercivity spectra suggest that hematite is a significant bulk ferromagnetic mineral for some specimens. This is probably the result of the oxidizing environment used to fire certain pottery types. The magnetic mineralogy of the ceramics probably includes both titanomagnetite and titanohematite. Anisotropies were determined for 71 specimens from 62 sherds. The mean anisotropy within the easy plane, y-axis relative to x-axis, is 1.068 ±

0.017 (1 S.E.), whereas the anisotropy of the hard axis relative to the easy plane is 1.312 ±0.030. The latter is significantly greater than the former, so the easy-plane model again appears to be a good first-order approximation.

5. Results 5.1. Specimen results Paleointensities were interpreted for 127 specimens (68% of those tried) from 54 different sampies (70%). Specimen results, along with the reiability indicators and anisotropies, are given in Sternberg (1982, Appendix E); a table of these results is available from the author. Results from Sternberg and Butler (1978) were re-interpreted so that all interpretations would be consistently based on the same criteria. Anisotropy corrections have also been applied to those earlier results. The results reported here for samples SNOO1—SNOO9 supersede those for sherds numbered 2,’ 4, 5, 6, 7, 10, 13, 14, 17 in Sternberg and Butler (1978). The values of the quality factor q range from 0.4 to 77, with a median of 12, and an interquartile range of 6—28. These quality factors are lower than Champion’s (1980) for Holocene igneous rocks of the western United States, which range from 0.7 to 224, with a median of 29 and an interquartile range of 14—56. The lower values here probably reflect the complex mineralogy of the pottery and consequent difficulties with the paleointensity experiment. Also, the temperature interval selected for the paleointensity interpretation for a given specimen was not necessarily the one with the highest value of q. All paleointensity results were corrected for anisotropy. The median value for f is 1.054, with an interquartile range of 1.013—1.140. The median correction factor of 5.4% is much less than the median anisotropy P of 31.2% because of the dependence of f on ~ as well as on P. This correction was still significant, especially as the anisotropy correction for this study was not random, but systematically increased the paleointensities. Five Hohokam sherds (5N004, SNOO6, SNOO8, SNOO9 and SNO12) were too small for

6 hJ H

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anisotropies to be determined from additional specimens. For these cases, the anisotropy was taken as the average anisotropy of other sherds from the same Hohokam ceramic phase. A typical set of paleointensity data are shown for sample SNO19 (Fig. 2). Specimens A and D have parallel NRM—TRM diagrams for the midtemperature range. They show different behaviors at low temperatures, probably as a result of lowtemperature reheating or unstable secondary components of magnetization. The curves are thus offset from one another, but the paleointensity results are still similar because the linear segments of the diagrams are parallel. The NRM—TRM diagram for specimen C is generally non-linear, This specimen was taken from a part of the sherd that was smudged with carbon, hence the curvature of the diagram can be attributed to the oxidation of this area as the laboratory heatings progressed. The repeat PTRMs after 400°Care higher than the original PTRMs, and those after 600°C have increased still further. This indicates a progressive alteration of the b!ocking temperature spectra c~wngto the laboratory heatings. The changes in PTRMs are greatest for the smudged specimen C, which has probably undergone the greatest mineralogical change. The NRM directions for specimens A and D are moderately stable over the interpreted range, even though the directions for D are unstable below 200°C, where the NRM—TRM diagram is also non-linear. NRM directions are unstable for specimen C, presumably owing to the smudge. Despite the problems with specimen C, for which no interpretation was made, the relative error for the sample average, 0.004, was one of the best from this study, indicating the close agreement of the interpreted paleointensities from specimens A (q = 33) and D (q = 9.1). Examples of NRM—TRM diagrams and their interpretation for several other samples were given by Sternberg (1982). The reddish, well-oxidized pottery from the Hohokam site of Snaketown gave generally good paleointensity results with high quality factors. It is not surprising that well-oxidized pottery would give good paleointensity results for laboratory heatings in air. In general, this study did not demonstrate that heating under partial vacuum

was particularly useful, but this might depend on the original firing atmospheres for the pottery (Colton, 1939) and the magnetic mineralogy. Paleointensities were inferred for some of the vacuum heatings. However, when specimens from the same sample were heated in both air and vacuum, the air heatings generally (but not a!ways) gave better results. 5.2. Sample average paleointensities and VADMs Paleointensities interpreted for multiple specimens from the same sherd (sample) were averaged. Various permutations of weighted statistics have been used to average specimen paleointensities (Sternberg and Butler, 1978; Coe et a!., 1978; Shaw, 1979; Champion, 1980). Weighted statistics should be used with caution if the statistical variances used as weights are only estimates of the true population variances (van der Waerden, 1969, p. 111). It was decided here that weighting of the data was not warranted. Sample average paleointensities are listed in Table I. More complete provenance information is given in Sternberg (1982, Appendix D). The virtual axial dipole moment (Barton et al., 1979; Champion, 1980), or VADM, and the standard error s v is also tabulated for each sample. When only one specimen from a sherd was used, the sample average was set equal to the specimen value. An arbitrary value of 1 x 1022 A m2 was then assigned to si,. This error is large enough to limit the influence of these single-specimen samples in the computation of weighted-average VADMs discussed below. Age ranges for the estimated dates of pottery firing were as low as 1 yr for historic material, 7 yr for protohistoric samples, and 12 yr for prehistoric sherds. The assigned age ranges for non-Hohokam samples had a fairly uniform distribution with a maximum range of 105 yr. Sherds from the Hohokam culture are not as precisely dated as those from the Anasazi and Mogollon cultures because of the scarcity of tree-ring dating at the desert Hohokam sites. Ages were initially assigned to the Hohokam samples (SN, GR, and RI) using the ‘long-count’ ceramic phase chronology of Haury (1976, pp. 325—340), according to which

8 TABLE I Sample average paleointensities Name

Site

ANOO1 CCOO1 CCOO2 CCOO3 CCOO4 CNOO1 GPOO1 GROO1 GROO2 LAOO1 LAOO2 LAOO3 MVOO1 PAOO1 PAOO2 RI001 SNOO1 SNOO2 SNOO4 SNOO5 SNOO6 5N007 SNOO8 SNOO9 SNO1O SNO11 SNO13 SNO14 SNOI5 SNO16 SNO17 SNO18 SNO19 SNO2O 5N021 SN022 SN023 SN025 TMOO1 UNOO4 VKOO1 WAOO1 WAOO2 WAOO3 WAOO4 WAOO7 WAOO8 WAOO9 WAO1O WAO11 WAO12 WAO15

Antelope Hse. Chaco Canyon Chaco Canyon Chaco Canyon Chaco Canyon Carnue Escalante Las Colinas Sidewinder P. Encierro P. Encierro LA 5407 Mesa Verde Hickiwan AZAA:12:14 P. de Agua Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Snaketown Tarahumara Unkar Delta Reward Mine Walpi Walpi Walpi Walpi Walpi Walpi Walpi Walpi Walpi Walpi Walpi

Latitude (deg.) 36.16 36.08 36.03 36.07 36.07 35.06 33.05 33.47 33.05 35.64 35.64 34.20 37.17 32.36 32.30 32.06 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 33.19 27.75 36.06 32.60 35.83 35.83 35.83 35.83 35.83 35.83 35.83 35.83 35.83 35.83 35.83

Longitude (deg.) 250.56 252.01 252.10 252.04 252.04 253.54 248.60 247.89 248.60 253.67 253.67 251.45 251.46 247.50 249.98 249.01 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 248.08 252.37 247.91 247.96 249.60 249.60 249.60 249.60 249.60 249.60 249.60 249.60 249.60 249.60 249.60

N 2 2 2 2 2 2 2 2 1 2 2 2 2 3 2 2 3 6 3 3 3 3 2 2 3 3 3 3 3 3 3 3 2 2 2 2 1 2 4 2 2 1 4 1 2 2 2 2 1 2 2 3

B~ (itT) 73.5 52.7 58.4 52.1 70.5 90.7 113.8 51.1 39.5 74.5 80.3 72.5 58.4 35.7 78.0 53.8 77.5 62.1 49.6 46.5 43.0 38.5 45.3 50.2 66.8 57.4 46.4 66.2 44.7 50.7 44.0 45.6 48.6 88.9 51.6 39.9 33.9 48.2 49.1 112.3 57.3 48.8 60.4 51.6 70.9 56.8 81.1 44.7 41.3 62.7 88.0 50.7

sB~,, (ST) 14.85 16.85 4.75 4.60 5.95 0.85 6.50 3.40 ~

0.30 2.20 7.70 2.10 1.37 13.80 0.80 9.05 2.87 0.84 2.38 1.05 1.43 9.20 2.90 5.06 0.17 0.15 2.49 0.62 2.29 5.86 3.09 0.20 2.05 6.35 2.40 ****

3.00 2.28 12.85 0.50 ****

1.37 *** *

11.55 2.05 4.50 4.15 *~ ~ *

2.05 20.00 1.71

VADM 22A (x10 13.28 9.53 10.59 9.43 12.77 16.64 121.39 9.59 743 13.56 14.62 13.43 10.43 6.78 14.80 10.10 14.54 11.65 9.31 8.73 8.07 7.23 8.50 9.42 12.54 10.77 8.71 12.42 8.39 9.51 8.26 8.56 9.12 16.68 9.68 7.49 6.36 9.05 9.88 120.33 10.83 8.86 10.97 937 12.88 10.31 14.73 8.11 7.50 11.38 15.98 9.20

S~ m2)

2.686 3.051 0.861 0.833 1.077 0.156 1.222 0.638 ****

0.055 0.400 1.427 0.375 0.260 2.619 0.150 1.698 0.539 0.158 0.447 0.197 0.268 1.727 0.544 0.950 0.032 0.028 0.467 0.116 0.430 1.100 0.580 0.038 0.385 1.192 0.450 ~

0.563 0.460 2.327 0.094 ~“~‘“

0.249 ****

2.097 0.372 0.817 0.754 ** ~ ~ 0.372 3.632 0.310

Age range (BC/AD) 700 500 500 1045 1050 1763 1350 1100 1100 1479 1520 472 1200 1956 1875 1100 1300 900 700 700 550 350 200 0 1300 900 200 350 350 500 550 650 750 —300 —300 —300 0 200 1977 1100 1882 1700 1700 1700 1800 1850 1850 1850 1885 1885 1885 1900

750 600 600 1100 1150 1770 1450 1400 1400 1520 1625 484 1285 1957 1910 1250 1450 1100 900 900 700 550 350 200 1450 1100 350 550 550 650 700 850 950 0 0 0 200 350 1978 1150 1902 1800 1800 1800 1880 1880 1880 1880 1900 1900 1900 1955

Alternate (BC/AD)

1175 1175

1400 1400

1175 1300 1000 875 875 800 750 700 650 1300 1000 700 750 750 800 800 850 950 500 500 500 650 700

1275 1450 1175 1000 1000 875 800 750 700 1450 1175 750 800 800 850 875 950 1125 650 650 650 700 750

9 TABLE I (continued) Name

Site

ZI001 Z1002

Zia Zia

Latitude (deg.) 35.50 35.50

Longitude (deg.) 253.29 253.29

N 3 2

B,, (MT) 47.1 55.9

sB~ (MT) 1.80 0.50

VADM Sj, 22A m2) (x10 8.59 0.328 10.19 0.091

Age range (BC/AD) 1910 1910

Alternate (BC/AD) 1950 1950

Name, sample designation; Site, archaeological site; Lat., site latitude; Long., site longitude; N, number of specimens; B,,, sample average paleointensity; sB~,standard error; VADM, virtual axial dipole moment; Sj,, standard error of VADM; Age range, date of firing (using long-count chronology for Hohokam samples); Alternate, date of firing using short-count chronology for Hohokam samples. Asterisks indicate that standard errors could not be calculated because sample had only one specimen.

age ranges of 150, 200 and 300 yr were assigned to 10, 14 and 5 sherds, respectively. According to this long-count chronology, the earliest Hohokam ceramic phase, or Vahki, began in 300 BC, whereas the most recent phase (Civano) ended in 1450 AD. There has been considerable debate concerning the chronology of the Hohokam culture (Schiffer, 1982), so the Hohokam data were also analyzed using the ‘short-count’ chronology of Schiffer (1982). According to this chronology, the Vahki phase began in 500 AD, although the Civano phase still ended in 1450 AD. Thus the early part of the chronology has been compressed. Other ‘intermediate-count’ chronologies are also discussed in Schiffer (1982), but were not considered in this study. The sample-average VADMs from this study are plotted in Fig. 3. Error bars represent uncertainty in age and the standard error of the mean VADM. The non-Hohokam results (Fig. 3a) and the Hohokam results (Fig. 3b) are plotted separately because of the uncertainty over the Hohokam chronology, The precisions of the paleointensity results are indicated by their relative errors. The relative error of a specimen result includes contributions from the nonlinearity of the NRM—TRM diagram and from the uncertainty of the anisotropy correction. The median total relative error of 9.0% is three times the median contribution from the non-linearity term alone. The relative errror for the sample-average paleointensities results from the dispersion between specimen results. Most of these relative errors are <15%; the median is 5.0% and the interquartile range is 2.6—10.%. The sample relative errors are less than the total relative errors on the individual specimens, so preci-

sion of the sample results is increased by using multiple specimens. The median specimen standard error of 5.8% (not including anisotropy correction) and the median sample standard error of 5% observed in this investigation are typical for pa!eointensity studies (Senanayake et a!., 1982).

6. Discussion 6.1. Smoothing the VADM data Moving windows (Tarkhov, 1964; Burlatskaya, 1972) similar to those used in the analysis of archaeomagnetic directions by Sternberg (1989) were used to smooth the VADMs. The general concept is the same, only now the quantity to be smoothed is a scalar paleointensity rather than a vector direction. For each sample, the result is weighted for the uncertainty in the age of the pottery firing. Thus for the jth window, each VADM is assigned a weight w =f1j which is a result only of its fractional age overlap with that window. The VADMs were not weighted according to their uncertainties, although this option was explored in Sternberg (1982). The data were sparse enough and the sample precisions were variable enough (by two orders of magnitude) for weighted means to be highly influenced by results with high precisions. The use of precision weighting by Sternberg (1982) did not alter the general patterns of paleointensity variation, although the amplitudes of the peaks were affected. The data density for a given windowing scheme is indicated by the effective number of points per interval, N~,equal to the sum of the fractional overlaps for all sampies with that window of time.

10 A

A

20

25

20

0 Is

I

—250

.4

(1~

____

0

250

500

750

0

+

~

.4..

.~

1000 1250 1500 1750 2000

—250

0

250

YEARS BC/AD

B

B

20

—0—

1000 1250 1500 17502000

L /~

1

~

+4~jt~ ~ -4--

750

IS

12

~

to

500

YEARS BC/AD

25

~is

+ 4~..

I

154±

7

J-~-~ 500

1Q00

6 1500

2000

YEARS BC/AD

Fig. 3. Virtual axial dipole moments (VADMs) corresponding to sample average paleointensities from this study. VADMs in 2. Error bars show age ranges and standard units of 1022 A m errors of the mean VADM. (A) Non-Hohokam set. (B) Hohokam data set.

Outlier tests were used to identify anomalous sample results. The outlier tests suggested by Grubbs (1950, 1969) were used to detect single high or low outliers, double high or low outliers, or a pair of one each high and low outliers. The

—250

0

250

500

750

1000 1250 1500 1750 20~

YEARS BC/AD

Fig. 4. Composite non-Hohokam data set. (A) Sample average paleointensities. Circles—this study; diamonds—Champion (1980); stars—Arbour and Schwarz (1982); squares—Lee (1975); crosses—Parker (1976). One diamond plots off figure to the left; one star plots below figure at 1350 AD (error bars just visible). (B) Smoothed curve showing interval means and standard errors of the means.

5% significance level was used for all tests. Each VADM was tested as an outlier for all windows of time for which its fractional overlap was more

11

than 20%. Pairs of VADMs were tested as joint ouliers if the product of their fractional overlaps was > 5%. Some of the outliers had large effects on the interval means, so the data set was tested for outliers a second time. 6.2. Compilation of archaeointensity results from North America Non-Hohokam Paleointensity results from Holocene igneous rocks (Champion, 1980), Southwestern archaeointensities from Lee (1975) and Parker (1976), and Canadian archaeointensities from Arbour and Schwarz (1982) were compiled. Some of the sample paleointensities were re-averaged, and dates revised, as described by Sternberg (1982). The archaeointensities and transformed VADMs from these other studies were tabulated by Sternberg 6.2.1.

(1982, Appendix G). The non-Hohokam VADMs, including those from this study, are plotted in Fig. 4a. The non-Hohokam data were smoothed, using window lengths of 200 yr, and overlap of 100 yr between successive windows. Window lengths of 50 and 100 yr were also tried, but did not elicit additional detail in the resulting curves. This window length is also consistent with the general precision with which the ages of the samples are known (Tarkhov, 1964). As part of the smoothing process, the following outliers were deleted: UNOO4, GPOO1, and CNOO1 on the first pass over the data; LA003, CH004 (sample from Champion, 1980), SAOO3 (Arbour and Schwarz, 1982) and SCO12 (Schwarz and Christie, 1967) on the second pass. The smoothed VADM curve is tabulated in Table II and plotted in Fig. 4b. The Canadian VADMs are generally lower than the other results after 750 AD, which could be owing to the 25°

TABLE II Smoothed VADMs for composite southwestern data set Age range (BC/AD) —500 —300 —400 —200 —300 —100 —200 0 —100 100 0 200 100 300 200 400 300 500 400 600 500 700 600 800 700 900 800 1000 900 1100 1000 1200 1100 1300 1200 1400 1300 1500 1400 1600 1500 1700 1600 1800 1700 1900

N 6 4 5 6 5 4 5 4 5 7 6 3 7 7 8 8 7 7 6 3 4 7 6

3.43 3.07 2.24 2.16 2.14 2.01 1.90 1.91 1.86 4.57 4.62 2.35 4.69 5.96 5.81 5.44 5.15 6.22 3.86 1.79 2.99 6.24 5.50

VADM 22A m2) (X10 12.75 14.01 12.12 11.58 10.30 9.83 9.49 9.13 9.17 9.24 9.33 10.31 8.59 8.63 9.68 10.92 11.34 10.38 11.03 12.87 11.44 10.32 9.55

S~

sv/VADM

0.877 0.529 1.348 1.068 0.896 0.986 0.839 0.979 0.760 0.468 0.501 1.505 0.915 0.946 1.065 0.861 0.543 0.412 0.687 1.256 1.612 0.836 0.506

0.0688 0.0378 0.1113 0.0922 0.0869 0.1003 0.0885 0.1073 0.0829 0.0507 0.0537 0.1460 0.1064 0.1096 0.1101 0.0788 0.0479 0.0397 0.0623 0.0976 0.1408 0.0811 0.0529

VAbM (x1022A m2 a~) 0.01257 —0.01895 —0.00539 —0.01275 —0.00467 —0.00348 —0.00360 0.00042 0.00069 0.00091 0.00980 —0.01715 0.00036 0.01049 0.01241 0.00417 —0.00958 0.00649 0.01845 —0.01427 —0.01128 —0.00761

Age range, window interval; N, number of samples overlapping that interval; N~,effective number of points per interval; VADM, virtual axial dipole moment for window; .s.-, standard error of the VADM; sv/VADM, relative error of the VADM; VADM, rate of change of the VADM.

12

geographic separation between Ontario/Quebec and the Southwest. Data from Mesoamerica are also discussed in Sternberg (1982), but are omitted here because of the geographic distance between this area and the Southwest.

A

25

20

_. I

~,_4_-.4-~

> ~

T

~

-...~L~ ~ J ~-1’-~ —~

-250

0

250 600

were tabulated by Sternberg (1982, Appendix H), and plotted along with the results from this study inFig.5a. The Hohokam data from Bucha et al. (1970), Lee (1975), and the present study were combined into a composite data set and smoothed. The 200-yr windows with 100-yr overlaps were used. The following results were identified as outliers

__~-

T

4_

.~_

750

250

1750 2000

YEARS BC/AD

B I

I 12

I I

I

9

I

e 7

8

result 250

and deleted from the data set: SNO14, SN023, BU052 and 0R002 on the first pass; and BU044, BU045 and BU046 on the second pass (BU prefixes indicate data from Bucha et al., 1970). The Hohokam data were also analyzed using the short-count chronology of Schiffer (1982). The ages for the Hohokam samples according to this chronology are tabulated in Table I for the results of this study, and in Sternberg (1982, Appendix H) for the results from the other studies. The same

::;~:0:f;.deleted as outliers from this data set

II

to

6.2.2. Hohokam Two previous studies besides that of Sternberg and Butler (1978) have used Hohokam ceramics from the Snaketown site (Haury, 1976) for paleointensity analyses. Bucha et al. (1970) reported results for 10 samples. Two of these (their samples 44 and 442) seem to represent specimens from the same sherd, but are not averaged because the paleointensities were quite different. These results were averaged together here. Lee’s (1975) single-year ages for his two Snaketown results were re-assigned ceramic phase ages according to the Haury (1976) chronology. These Hohokam paleointensities and the corresponding VADMs

0

260

500

760

I 000 1260 I 500 I 750 2000

The smoothed interval VADMs are listed in Table III, and the ‘long-count’ curve is plotted in Fig. Sb. The large error bars for the early part of the curve are the result of the higher paleointensity interpreted for sample SNO2O. This result was not tagged as an outlier using the tests described above, but Fig. Sb shows that the deletion of this would have a substantial effect on the paleointensity curves.

YEARS BC/AD

Fig. 5. Composite Hohokam data set. (A) Sample average paleointensities. Circles—this study; squares—Lee (1975); tnangles—Bucha et al. (1970). (B) Smoothed curve showing interval means and standard errors of the means; triangles show effect of deleting SNO2O.

6.3. Comparison of VADM curves The smoothed VADM curves for the nonHohokam and Hohokam data using the long-count

13 TABLE III Smoothed VADMs for composite Hohokain data set Age range (BC/AD) Long-count chronology —300 —100 —200 0 —100 100 0 200 100 300 200 400 300 500 400 600 500 700 600 800 700 900 800 1000 900 1100 1000 1200 1100 1300 1200 1400 1300 1500 1400 1600 Short-count chronology 500 700 600 800 700 900 800 1000 900 1100 1000 1200 1100 1300 1200 1400 1300 1500 1400 1600

N

A~,

VADM 22A m2) (x10

s~/VADM

VAbM (x1022A m2 a’)

4 4 5 1 4 6 6 7 8 10 6 10 5 6 2 4 3 2

2.67 2.67 1.83 1.00 2.50 3.75 3.25 3.92 5.00 5.33 5.50 5.00 4.25 3.00 1.67 2.33 2.33 0.67

11.26 11.26 10.89 9.42 8.92 8.68 8.68 8.81 8.78 9.09 9.19 10.34 11.47 11.32 9.84 11.69 12.22 13.54

1.961 1.961 1.563 1.563 0.201 0.399 0.399 0.419 0.364 0.212 0.207 0.532 0.753 0.608 0.254 1.148 1.439 1.004

0.1741 0.1741 0.1435 0.1659 0.0226 0.0459 0.0459 0.0476 0.0415 0.0233 0.0225 0.0515 0.0656 0.0537 0.0258 0.0982 0.1177 0.0741

0.00000 —0.00368 —0.01471 —0.00502 —0.00236 0.00000 0.00124 —0.00031 0.00319 0.00095 0.01149 0.01135 —0.00151 —0.01479 0.01849 0.00532 0.01317

5 11 15 10 10 7 7 4 3 2

5.00 8.33 11.30 9.29 6.84 5.08 3.41 2.97 2.44 0.67

10.89 9.69 8.93 9.09 10.34 11.01 11.01 11.69 12.22 13.54

1.563 0.780 0.211 0.212 0.532 0.603 0.603 1.148 1.439 1.004

0.1435 0.0805 0.0237 0.0233 0.0515 0.0547 0.0547 0.0982 0.1177 0.0741

—0.01205 —0.00759 0.00166 0.01244 0.00669 0.00000 0.00684 0.00532 0.01317

Column headings same as for Table II.

chronology are compared in Fig. 6a. The data comprising these two curves are generally from the same geographic area (with the exception of the Canadian sherds and those igneous rock samples from the Northwest), so one would expect the curves to be similar. They are, in fact, very much alike. They are both high near 1 AD, low at 500 AD, and have maxima near 1100 and 1500 AD. Although the shapes of the curves are similar after 750 AD, the Hohokam curve lags the Southwest curve by about 200 yr. This is probably within the uncertainty of the Hohokam chronology. The Hohokam curve after deletion of sample SNO2O is also shown. The agreement with the Southwest curve is better when SNO2O is included.

In Fig. 6b, the smoothed VADM curves for the Southwest and for the Hohokam data using the short-count chronology are plotted together. The overall agreement is just as good as for Fig. 6a. Here, similarity to the Southwest curve is about the same whether SNO2O is included or not. It is not possible at present to use archaeointensity dating to arbitrate among the various Hohokam chronologies based on comparison of Hohokam archaeointensity results with other archaeointensities from the Southwest (Sternberg and McGuire, 1981). 6.3. .L Relative paleointensitiesfrom sediments King et al. (1983) used a new rock magnetic

14 A

curve which shows the same paleointensity pattern

IS

as shown in Champion (1980), based on archaeointensities from Sternberg and Butler (1978) and igneous paleointensities from Champion (1980). The sedimentary record also is congruent with the Southwestern curve developed here. Verosub and Mehringer (1987) reported on a relative paleointensity record from the sediments of Fish Lake, Oregon, which is also similar to both the

0 9

LeBoeuf and archaeomagnetic curves. 6.4. Implications for secular variation

8 7

Despite the scatter in the sample averages, I believe that the patterns revealed in the smoothed

6

curves

—260

0

250

500

750

1000 1250 1500 17602000

YEARS BC/AD

B

~

~

I:

8 7

6

Champion, -250

0

250

500

750

1000 1260 1500 17602000

YEARS BC/AD

Fig. 6. Comparison of non-Hohokam and Hohokam smoothed VADM curves. (A) Circies—non-Hohokam curve; triangles— Hohokam curve using long-count chronology; crosses—same as triangles with sample SNO2O deleted; solid line—VADMs calculated from direct measurements of field intensity in southem Arizona. (B) Same as (A), only using the short-count Hohokam chronology,

approach for the identification of sediments from which reliable, relative paleointensities can be derived. They applied this technique to sediments from LeBoeuf Lake, Pennsylvania, and obtained a

do represent true behavior of the Earth’s magnetic field. This is supported by the general consistency between the smoothed curves. The non-Hohokam curve is congruent with the Hohokam curve, which would be expected from two data sets representing the same geographic region. The similarity between the archaeomagnetic and limnomagnetic paleointensities is impressive because the magnetic field strength is being recorded by very different processes. The Southwestern archaeomagnetic VADM curve shows a field strength at 300 BC that is 30—40% higher than at present; a low from 250 to 1250 AD that is 10—15% weaker than at present; and a peak at 1500 AD about 30% higher than at present. All curves show high field strengths near the BC/AD transition, consistent with a peak in the global dipole field determined from compilations of paleointensity data (Barton et al., 1979; 1980; McElliinny and Senanayake, 1982). The lows in the North American curves at 250—1250 AD occur during a time when the global VADM compilations show a gradually decreasing field strength, suggesting the existence of —

a significant non-dipole field component over this region at that time. The rates of change for the paleointensities are consistent with present-day behavior of the magnetic field. Using the Southwestern paleointensity data set, the median rate of change for the corresponding VADMs is 0.0044 A m2 a1, with an average and standard deviation of 0.0067 ±0.0062 A m2 a Using the compilation of Yukutake et - ~.

15

al. (1979), a rate of change was calculated for the VADM from each of 38 magnetic observatories over the period 1940—1973. The median rate of these 38 values was 0.0042 A m2 a’~,with an average and standard deviation of 0.0044 ±0.0032 A m2 a’1. Champion’s (1980) globally averaged archacomagnetic VADMs for the period 1—1800 AD have a median VADM rate of change of 0.0044 A m2 a1, and an average and standard deviation of 0.0040 ±0.0026A m2 a ‘.The rates of change in these three data sets are quite similar. The rate of secular variation is important when considering the scatter of the data. Dating and paleointensity errors will have a greater effect on scatter during times of rapid secular variation (Shaw, 1979).

26

20

____________

____

~o

4

~

I

_______

1’ 1840

leO

1880

1900

1920

1940

1960

1980

YEARS AD.

6.5. Historic samples One way to estimate the accuracy of the

paleointensity method is to use historic pottery made and fired using primitive techniques, but during a time when the field intensity was known from direct observation. In the Southwest, direct readings of the magnetic field were made in Nogales, Arizona, in 1855 and 1892 AD and in Tucson in 1902 (Hazard, 1917). Annual means from the U.S. Geological Survey Tucson magnetic observatory are available beginning in 1910 (Yukutake et al., 1979). Fourteen of the samples listed in Table I were fired since 1840. The paleointensities for these samples along with the modern trend are plotted in Fig. 7. The results are scattered about the modern trend. Although the more precise paleointensities are generally closer to the historic curve, it is obvious that individual samples can give anomalous results. This is not surprising in view of the scatter observed above for supposedly contemporaneous results. However, even this apparently foolproof test of the paleointensity experiment cannot be taken at face value. Verbal and photographic accounts of how modern primitive pottery is made by Native Americans frequently indicate the presence of tin (i.e., tin-plated steel), scrap iron, or other unidentified metals (probably steel) used as platforms on which to support the pots over the fire, as separators between pots, or sometimes as rudimen-

Fig. 7. VADMs from paleointensities on historic samples. Error bars represent age ranges and standard errors of the mean VADMs. Dashed line is the historic VADM curve calculated from direct measurements of field intensity in southern

tary kilns (Boggs, 1936, Plate XXII, figure 1; Wormington and Neal, 1951, pp. 41—42; Fontana et al., 1962, figures 62—67, 70). The presence of these objects during firing can distort the direction and intensity of the ambient field (Burlatskaya and Petrova, 1961) so that the field intensity that the pot experienced during firing would not be known, and the comparison of the archaeointensities with historic field changes would not be a valid test of the paleointensity experiment. 7. Conclusions Paleointensities were interpreted for about two-thirds of the samples and specimens used in this study. Although this is a respectable ‘success’ ratio for paleointensity work, there is considerable scatter among the data. The causes of this scatter cannot be unambiguously identified, but it is probably a result of some combination of inaccurately dated samples, rapid secular variation, and possibly spurious paleointensities. Anisotropy due to a magnetic fabric significantly affects archaeointensities, even for primitive pottery, but can be

16

corrected for using an easy-plane model of magnetization. The moving-window method is effective for extracting the geomagnetic signal from noisy data. Prehistoric rates of secular variation of field strength in the American Southwest are consistent with rates of the present-day field. Comparison with global compilations of dipole moment behavior suggest a significant non-dipole field feature existed over the Southwest from 250 to 1250 AD.

Acknowledgments My initial interest in the problem of magnetic field strength variations was stimulated by my work with Paul Damon on radiocarbon geophysics. Robert Butler provided wise and patient guidance during the course of this study. Emil Haury supplied many of the samples, along with encouragement concerning the potential archaeological significance of work such as this. Randall McGuire helped me sift through the chronological information, and taught me some basic archaeology along the way. Other archaeologists and curators who kindly provided the samples that made this study possible were Michael Jacobs, Jane Kepp, Keith Anderson, Alan Ferg, Thomas Windes, James Rodgers, David Doyel, Marsha Gallagher, Stewart Peckham, Earl Ingmanson, George Teague, E. Charles Adams, and Marian Rodee. Laboratory assistance was ably given by Joe Hart, Chuck Sheldon, Pat Debroux, Mark Bieniulis, and Jim Holmlund. Duane Champion provided a much-needed preprint and useful advice. Marykirk Hull did the word processing. Timothy Loomis provided the use of a plotter on which many of the graphs were drawn. My thanks to Duane Champion, David Dunlop and Kenneth Verosub for their very helpful reviews of the manuscript. Financial support was provided by NSF grants EAR 77-22340, EAR 79-19726, and EAR 81-16196. References Aitken, Mi., Alcock, P.A., Bussell, GD. and Shaw, C.J., 1981. Archaeomagnetic determination of the past geomagnetic

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