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Archaeological Prospection Archaeol. Prospect. 13, 207–227 (2006) Published online 28 March 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.277

Thermally Activated Mineralogical Transformationsin Archaeological Hearths:Inversion from Maghemite Fe2O4 Phase to Haematite Fe2O4 Form DAVID MAKI1*, JEFFREYA. HOMBURG2 AND SCOTT D. BROSOWSKE3 1

Archaeo-Physics, LLC, 4150 Digit Ave, #110 Minneapolis, MN 55406, USA Statistical Research Inc., PO Box 31865,Tuscon, AZ 85751, USA 3 Courson Oil and Gas,1800 South Main Street, PerrytonTX 79070, USA 2

ABSTRACT

A series of laboratory experiments were conducted in an effort to understand why magnetic field gradient survey techniques failed to detect hearths at a prehistoric archaeological site in southern California. The study used various methods of environmental magnetism to examine the effects of exposing soil samples to a temperature of 650 C over a period 26 h. Results of the study indicate that the failure was associated with a reduction in soil magnetic susceptibility to below background levels within hearth soils. This reduction was due to high-temperature transformation of iron oxides from a highly magnetic form to a relatively non-magnetic form. The reduction in susceptibility is thought to have proceeded via the oxidation of primary (lithogenic) magnetite, Fe3O4 to maghemite, Fe2O4 followed by the inversion of maghemite to haematite, Fe2O4. The study suggests that in some instances high temperature inversion can reduce the proportion of ferrimagnetic minerals within a hearth to below initial concentrations (resulting in a negative magnetic field gradient anomaly, the opposite of what is normally expected). A field experiment was also conducted to determine what soil temperatures might be achieved 2 cm beneath a hearth.The experiment recorded temperatures ranging from approximately 400 C to 650 C, with an average temperature of about 470 C. Soil colour changes and magnetic susceptibility enhancement observed at the conclusion of the field experiment indicate that these temperatures were sufficient to activate some mineralogical changes, possibly including inversion to haematite. The implications of high temperature inversion to archaeological prospection are discussed, as is a potential archaeological application. Copyright 2006 JohnWiley & Sons,Ltd. Key words: magnetic susceptibility; oxidation; inversion; fractional conversion; archaeological prospection

Introduction This paper describes a series of laboratory experiments that were conducted on soil samples from the West Bluff Project Area, located near the * Correspondence to: D. Maki, Archaeo-Physics, LLC, 4150 Digit Ave, # 110, Minneapolis, MN 55406, USA. E-mail: [email protected]

Copyright # 2006 John Wiley & Sons, Ltd.

city of Los Angeles, in southern California. The measurements were undertaken in an effort to understand why magnetic field gradient survey failed to detect hearth features that were discovered after mechanical stripping had removed the non-cultural soil overburden from this prehistoric archaeological site. The hearths had been heavily bioturbated, but the displacement of culturally modified soil and stones from the Received 23 March 2005 Accepted 19 December 2005

208 hearths appears to have been relatively localized. Turbation of the features suggests that the thermoremanent magnetization of the hearths may have been randomized, however, many features appeared sufficiently intact to expect a positive induced magnetization anomaly resulting from culturally enhanced soil within the hearths. The paper will seek to explain why enhancement of magnetic minerals in soil associated with these hearths did not create significant and readily detectable magnetic anomalies. Controlled tests examined the initial magnetic susceptibility enhancement in soil samples upon exposure to high temperatures. This was followed by an examination of the subsequent mineralogical transformations that occurred with prolonged exposure to a temperature of 650 C. Finally, a field experiment sought to determine whether 650 C is a realistic temperature that might be expected of past conditions in soil under archaeological hearths.

Susceptibility enhancement and inversion mechanisms Several different mechanisms can cause an enhancement (increase) in the concentration of magnetic minerals in archaeological features (or archaeological soils), including fire and pedogenic production of ferrimagnetic minerals by organic and inorganic pathways. Dalan and Banerjee (1998) and Weston (2002) provide summaries of soil magnetic susceptibility enhancement pathways in archaeological contexts, and Peters et al. (2001) and McClean and Kean (1993) discuss the contribution of magnetic minerals from wood ash to archaeological features and soils. Controlled experiments conducted by Linford and Canti (2001) demonstrated that the combined effects of wood ash and thermal enhancement of soil susceptibility resulting from short-term camp fires often produce readily distinguishable magnetic anomalies. This paper will focus on just one enhancement pathway—fire (also referred to as the burning mechanism). Exposure of soil to high temperatures typically causes an enhancement (increase) in the magnetic susceptibility. This enhancement


D. Maki, J. A. Homburg and S. D. Brosowske is due to conversion of iron oxides and hydroxides in the soil from a weakly magnetic form to a strongly magnetic form; the conversion often proceeding via reduction to magnetite, followed by reoxidation to maghemite. Several factors may influence the level of enhancement that is achieved in the soil surrounding an archaeological hearth. For example, anaerobic conditions are necessary to provide a reducing environment, and oxygen is necessary if subsequent reoxidation is to occur. Hence the atmospheric conditions within the soil under and around a hearth is a variable affecting the level of enhancement. Soil atmospheric conditions are, in turn, associated with the porosity of the soil, size and tortuosity of soil pores, shape and ventilation of the hearth, thickness of ash deposits lying on the soil, and the amount of water and organic matter initially present in the soil matrix. Additional variables that influence the level of enhancement occurring at high temperature include the availability of iron minerals in the upper soil horizon, temperatures achieved during heating, and the thermal conductivity and diffusivity of the soil. Recent research suggests that the level of enhancement that may be achieved within a soil is finite, and may be partially or wholly reversible (Weston, 2002; Crowther, 2003). For example, heating experiments conducted by Weston (2002) measured an increase in soil magnetic susceptibility of up to 171 times the initial susceptibility value after ignition at 650 C, however, the magnetic susceptibility of these samples significantly decreased after reheating to 850 C. This reduction in susceptibility may have been due to high temperature oxidation and inversion. High temperature oxidation and inversion have been the subject of numerous studies in non-archaeological fields of interest (Bando et al., 1965; Tite and Linington, 1975; O’Reilly, 1983; Ozdemir and Banerjee, 1984; Ozdemir, 1987, 1990; Ozdemir and Dunlop, 1993; Dunlop and Ozdemir, 1997; Adnan and O’Reilly, 1999; Brown and O’Reilly, 1999). These studies have shown that at high temperatures maghemite, Fe2O4 is metastable, and can invert to stable haematite, Fe2O4. Inversion is a thermally activated process, with inversion temperatures ranging from 250 to 900 C. The inversion rate appears to vary with temperature, magnetic mineral grain

Archaeol. Prospect. 13, 207–227 (2006)

Thermally Activated Mineralogical Transformations in Hearths size, the degree of oxidation, pressure and lattice impurities such as substitution by aluminum or titanium. Naturally occurring maghemites often have much higher inversion temperatures than synthetic maghemites. The inversion of maghemite is structural in origin and takes place without a change in the bulk chemical composition. Maghemite is arranged in a spinel structure, which is responsible for its ferrimagnetic properties. Haematite is arranged in a rhombohedral structure and has canted-antiferromagnetic properties. This structural transformation is very significant as the magnetic susceptibility of canted-antiferromagnetic haematite is approximately 200 times weaker than that of ferrimagnetic minerals (Evans and Heller, 2003). Inversion to haematite is also associated with the reddening of soil colour often observed within archaeological hearths.

West Bluff Project Area soils and sampling strategy Archaeological investigations have been conducted since the mid-1980s at the West Bluff Project Area, which encompasses the Del Rey Site (CA-LAN-63), the Bluff Site (CA-LAN-64) and CA-LAN-206 (Van Horn, 1986, 1987; Altschul, 1997, 1999; Van Horn and White, 1997; Altschul et al., 2000). In September of 2000, a geophysical investigation of CA-LAN-63 and CA-LAN-64 was undertaken. The investigation consisted of magnetic field gradient survey over an area of 3.14 ha using a Geoscan FM36 fluxgate gradiometer operated at the 0.1 nT sensitivity level. Data were collected using a transect spacing of 0.50 m, with eight samples per metre along each transect (16 samples m2). The results of the magnetic field gradient survey are shown in Figure 1. After the geophysical investigation, portions of the sites were mechanically stripped of noncultural topsoil overburden and several archaeological features were revealed. The majority of these features consisted of concentrations and scatters of burnt stone, and associated flaked and ground stone tools and hammer stones, which were tentatively identified as hearths or hearth clean-outs (Douglass and Altschul, 2003). Copyright # 2006 John Wiley & Sons, Ltd.

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Other features include artefact concentrations and scatters, human burials, refuse deposits, a faunal bone concentration and one historicalperiod feature. Radiocarbon dates obtained thus far indicate that CA-LAN-206 dates to the Early Period (ca. 6500–7000 yr BP) and that the other two sites date to the Middle Period (ca. 1800–2800 yr BP). Research on these three sites is ongoing, but previous investigators have suggested that the sites functioned as temporary base camps (Van Horn and White, 1997) or as parts of an interdependent settlement system of sites located both on the bluff and in the wetlands below (Grenda et al., 1994). All three sites are located on the Westchester Bluffs about 2 km north of Los Angeles International Airport and about 1 km south of Ballona Creek. Soils consist of loamy sands and sandy loams formed in Quaternary aeolian sediments of the El Segundo Sand Hills. These aeolian sediments originate from beach deposits on the Pacific coast 2 km to the west that have been reworked extensively during the Holocene to form sheets of sand. The sand grains from the very fine fraction are dominated by quartz and magnetite (approximately 18% quartz and 67% magnetite), with lesser quantities of zircon, tourmaline and calcite (Mbila and Homburg, 2000). The coarser sand is dominated by quartz. The sandy soils are well drained, with slow to rapid runoff and moderate permeability. Soils in most of the project area consist of thin brown to greyish brown A horizons overlying a sandy subsoil with thin, discontinuous clay lamellae. A shell midden deposit of dark brown to dark greyish brown sandy loam approximately 1 m thick was found on the shoulder of a swale in the Del Rey Site, and cultural features were concentrated on the adjacent summits. A post-excavation analysis determined that the correlation between hearth (and hearth clean-out) features and magnetic anomalies was quite poor. Several soil samples were collected from a geomorphology test unit in an effort to understand why enhancement of magnetic minerals in soil associated with these hearths did not result in significant induced magnetic anomalies. Unfortunately no soil samples from excavated features were available for testing. The soil samples came from a profile located in


Figure1. Location ofgeophysical surveyareas and site boundaries at theWest Bluff Project Area.The magnetic field gradient surveyresults are depicted as grey-scale images. The large rectilinear magnetic lows identify the location of machine excavated trenches from previous archaeological investigations. Signal clutter created by near-surface iron debris, igneous rock and induced magnetic fields associated with plough furrows dominate the survey results.

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D. Maki, J. A. Homburg and S. D. Brosowske


Thermally Activated Mineralogical Transformations in Hearths

Figure 2. Trench1001unit profile.

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Trench 1001, located near the centre of Bluff Site (Figure 2). The soil grain size, organic content and pH from a vertical profile within Trench 1001 are provided in Figure 3, and common soil magnetism parameters are provided in Figure 4 (see next section for a brief description of these parameters). Throughout the West Bluff Project Area the top of the main occupation horizon ranged from 20 to 38 cm below surface. A soil sample (Sample A) was selected from Trench 1001 at a depth of 35 cm below surface (A1 soil horizon) for the high temperature experiments described in the following section.

Figure 3. Organic content, grain size and pH data from Trench 1001. High temperature experiments were conducted on soil samples from 35 cm below surface.



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Figure 4. Soil magnetism parameters fromTrench1001.

High temperature experiments High temperature laboratory experiments were conducted simultaneously on Sample A and additional soil samples from several archaeological sites for comparison. The additional sites are briefly described as follows: 48CA3030—the 3030 Winchester Site is a Late Prehistoric I period occupation in northeast Wyoming and soils at the site consist of sandy silts; 11S1131—the Grossmann Site is an early Mississippian period settlement located in the uplands about 18 km east of Cahokia in Illinois and soils at the site consist of silty clays; 31CD218—a prehistoric occupation located in central North Carolina


and soils at the site consist of sandy loams; 34BV100—the Odessa Yates Site is a Plains Village period settlement located in western Oklahoma and soils at the site consist of a clay loams; LA106780—a multicomponent Jornada Mogollon habitation in south-central New Mexico and soils at the site consist of silty loams. An initial high temperature experiment compared the enhancement of magnetic susceptibility in soil samples from several of these sites by applying a ‘maximum conversion’ procedure as described by Clark (1996). The procedure was slightly modified in that a temperature of 750 C was used rather than 650 C, and a 2-h oxidation period was used rather than 1 h. A Bartington



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Figure 5. A comparison of susceptibility values from five archaeological sites before and after experimental heating.

MS2 AC susceptibility meter was used to measure the low frequency (0.46 kHz) magnetic susceptibility after the experimental heating. The results of the conversion study are presented in Figure 5. Susceptibility enhancements were observed in four of the five samples subjected to the study. Enhancements ranged from 2 to 63 times the initial susceptibility values. The largest enhancement occurred in the sample with the lowest initial susceptibility (31CD218), whereas the smallest enhancement occurred in

the sample with the highest initial susceptibility (LA106780). Soil Sample A was atypical in that the final susceptibility was approximately 66% lower than the initial susceptibility. Initial susceptibility values suggest that unheated soil from the West Bluff Project Area has a relatively high concentration of magnetic minerals (Figure 5). A Day plot (Figure 6) of soil samples from five archaeological sites suggests that the ferrimagnetic mineral grain size of Sample A falls within the multidomain (MD) or

Figure 6. A comparison of the domain states of soil samples from five archaeological sites.




214 superparamagnetic (SPM) range, whereas the remaining four sites were all pseudosingledomain (PSD) (Day et al., 1977). The frequency dependence of susceptibility (fd% ¼ [(0.46 kHz 4.60 kHz)/0.46 kHz] 100) of Sample A was measured using a Bartington MS2 susceptibility meter in an effort to learn whether SPM grain sizes might be present. A relatively low average frequency dependence of susceptibility of 1.48% (n ¼ 10) suggests that SPM grains are probably not present and that MD grains are predominant. To investigate the reduction in susceptibility observed in Sample A further, some bulk magnetic properties of the sample were measured as a function of time spent at 650 C. Variations in the magnetic mineral concentration and composition with time spent at this isotherm were tracked by making room temperature measurements of the mass magnetic susceptibility (at 0.46 kHz) and a backfield parameter termed the S-ratio at regular intervals between exposures to high temperature. A detailed description of these properties and their relevance to environmental magnetism can be found elsewhere and is beyond the scope of this paper (Maher, 1986; Thompson and Oldfield, 1986; Hunt, 1991; Dunlop and Ozdemir, 1997; Evans and Heller, 2003). A brief description of these parameters, as well as the parameters presented in Figure 4, is provided below. (i)

Mass magnetic susceptibility () is the ratio of magnetization induced by a small applied field to the intensity of the magnetizing field. Susceptibility is roughly proportional to the concentration of ferrimagnetic minerals within a sample. Susceptibility is also affected by the magnetic mineral grain size. (ii) Frequency dependence of susceptibility (fd) is a measure of the variation in susceptibility with frequency. Extremely small magnetic minerals in the superparamagnetic (SPM) range contribute significantly to fd (1–20%), whereas larger grains in the single domain (SD) to multidomain (MD) range contribute very little (< 1%). The parameter is defined as follows: fd (%) ¼ [(0.46 kHz 4.60 kHz)/0.46 kHz] 100.


(iii)

Anhysteretic remanent magnetization (ARM) is produced by the combined actions of a large alternating field (AF) and a smaller constant direct current field (DC). An ARM was imparted by slowly reducing a peak AF of 100 mT to zero while at the same time applying a constant DC field of 0.05 mT. The ARM is sensitive to both magnetic mineral concentration and grain size. Small magnetic mineral grains in the SD size range are especially susceptible to ARM acquisition. (iv) Saturation isothermal remanent magnetization (SIRM) is the highest level of magnetic remanence that can be given to a sample through the application of a large DC field (1000 mT in the present case). The SIRM responds to the total concentration of magnetic minerals in a sample. The SIRM is also affected by the grain size of magnetic minerals, although to a lesser extent than ARM. (v) The interparametric ratio ARM/SIRM is a useful indicator of grain size. This ratio responds preferentially to ferrimagnetic minerals in the SD size range. (vi) The S-ratio is a backfield parameter that measures the degree of loss of remanence at selected reverse fields. It is an effective means of measuring relative changes in the proportion of ferrimagnetic minerals to antiferromagnetic minerals. Values can range from þ1 to 1, larger positive values imply a higher proportion of low coercivity ferrimagnetic minerals such as magnetite or maghemite, whereas smaller positive values (and occasional negative values) indicate an increasing concentration of high coercivity canted antiferromagnetic minerals such as haematite or goethite. Determination of the S-ratio proceeded as follows. A forward field of 1 T was applied using a Princeton Applied Research Vibrating Sample Magnetometer and the remanence imparted by this field was measured on a 2-G Superconducting Rock Magnetometer. Reverse fields of 100 mT and 300 mT were then applied and the resulting remanence measured in like manner. The S-ratio was calculated from the measured


Thermally Activated Mineralogical Transformations in Hearths remanence imparted by these forward and reverse fields. The S-ratio calculated from the two reverse fields are defined as follows: S-ratio100mT ¼ and S(IRM100mT/SIRMþ1000mT) ratio300mT ¼ (IRM300mT/SIRMþ1000mT). Initial measurements were made on the samples prior to heating (time ¼ 0). Subsequent measurements were then made at room temperature after the samples had been exposed to heating cycles. Each cycle consisted of heating approximately 50 g of soil in an open crucible for a period of 1.3 h. Eighteen minutes of each cycle were spent ramping up to 650 C, followed by 1 h at this isotherm. The samples were then rapidly cooled to room temperature and a portion was removed and packed into a plastic P-1 cube (5.28 cm3) and weighed for conversion to mass normalized values. The magnetic susceptibility was then measured on a Bartington Instruments MS2 susceptibility system. The variation in magnetic susceptibility with time spent at 650 C is presented in Figure 7. Soils from all of the five sites presented in Figure 7 experienced an initial susceptibility enhancement. Soils from three of the sites experienced a reduction in susceptibility from peak values upon extended heating. The magnetic susceptibility of Sample A dropped to below its initial susceptibility after approximately 17 h. Soil from one site continued to experience an enhancement in susceptibility for the duration of the experiment, while susceptibility from the remaining site remained relatively constant after the initial enhancement. Variations in S-ratios with time spent at 650 C are presented in Figure 8. The measured S-ratio of Sample A decreased with time spent at this isotherm. After 26 h at this temperature the ratio resulting from both a backfield of 100 mT and 300 mT were lower than the initial (unheated) ratio, although the change was significantly greater in the 100 mT backfield ratios. The S-ratio from the four remaining archaeological sites experienced an initial increase, followed by relatively constant values. In all four of the comparative sites the S-ratios remained above their initial (unheated) values. Copyright # 2006 John Wiley & Sons, Ltd.

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The results of the high temperature experiments suggested that after a small initial enhancement, the proportion of ferrimagnetic minerals in Sample A decreased with time spent at 650 C. It is suspected that this was the result of high temperature oxidation of magnetite to maghemite, followed by the subsequent inversion (or partial inversion) of maghemite to haematite. In an effort to test this hypothesis, the end product mineralogy after heating for 26 h (Sample A—heated) was compared with the mineralogy of Sample A prior to heating (Sample A—unheated).

Sample A mineralogy before and after experimental heating Sample A (heated) and Sample A (unheated) were both physically demagnetized by unpacking, mixing the soils, and repacking in P-1 cubes. The low temperature remanence behaviour of Sample A before and after experimental heating was then examined in an effort to better understand the mineralogical transformations that had occurred at high temperatures. A saturation isothermal remanence magnetization (SIRM) was applied to both samples at 10 K, after having been cooled to this temperature in the absence of a magnetic field. The variation of remanent magnetizations of the samples were then measured as they warmed to room temperature (Figure 9) using a Quantum Designs MPMS2 cryogenic susceptometer. Sample A (unheated) experienced a large drop in SIRM between 100 K and 120 K. This drop in remanence—known as the Verwey transition—is diagnostic of magnetite. The suppression of a Verwey transition in Sample A (heated) suggests magnetite minerals no longer dominate the magnetic mineral assemblage, although it should be noted that partially oxidized non-stoichiometric magnetite can also result in a suppressed Verwey transition. A Morin transition at 258 K would have provided evidence of haematite in the end product, but this was not observed. The lack of a Morin transition could indicate that haematite was not present, although this cannot be stated for certain as the transition may also be suppressed by substitution by titanium, or in haematite


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Figure 7. Variation in mass magnetic susceptibility with time spent at 650 C. All data have been normalized by the initial susceptibility value.

particles with grain sizes less 0.1 mm (Dunlop and Ozdemir, 1997). A method of unmixing magnetic mineral assemblages was next applied in an effort to further clarify what mineralogical transformations had occurred with extended heating (again, after physically demagnetizing the samples). The unmixing method is based on the quantification of magnetic coercivity components by the analysis of isothermal remanent magnetization (IRM) Copyright # 2006 John Wiley & Sons, Ltd.

acquisition curves. In this procedure, the first derivatives of the measured IRM acquisition curves are mathematically modelled using a number of separate log-normal probability density functions that are characterized by their SIRM, mean coercivity and dispersion (Kruiver et al., 2001; Heslop et al., 2002). The log-normal probability density functions that most closely approximated the observed behaviour of the IRM acquisition curves were selected after a



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Figure 8. Variation in S-ratio values with time spent at 650 C. The S-ratios represented by solids lines are defined as follows: (IRM100mT /SIRMþ1000mT). Dashed lines represent S-Ratios defined as follows: (IRM100mT /SIRMþ1000mT).

visual and statistical examination of the fit between the real and modeled data. The IRM acquisition curves were obtained for the unmixing study by exposing each sample to a stepwise-increasing uniaxial field imparted by a Princeton Applied Research Vibrating Sample Magnetometer. The IRM data were acquired in 1 mT steps from 1 to 10 mT, in 10 mT steps from 10 to 100 mT, and in 100 mT steps from 100 to 1000 mT. The resulting remanence was measured after each step using a 2-G Superconducting Rock Magnetometer. The IRM acquisition data are presented in Figure 10a. The noise characteristics of the raw Copyright # 2006 John Wiley & Sons, Ltd.

IRM acquisition data were reduced by applying a smoothing spline. Spline smoothing was desirable because the unmixing analysis utilizes the first derivatives (gradients) of the acquisition curves, which are extremely susceptible to noisy data. Spline smoothing was accomplished using the fit spline function in JMP# statistical analysis software. Values of the spline smoothing parameter () are given in Figure 10a, as are the sum of squares error (SSE). The SSE provides a measure of the ‘goodness’ of fit by summing the squared distances (residuals) from each raw data point to the fitted spline. Examination of Figure 10a reveals that Sample A (unheated)



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Figure 9. Thermal demagnetization of zero-field-cooled SIRM. Suppression of the Verwey transition is apparent in the heated samples.

continued to acquire remanence throughout the IRM acquisition procedure (implying that the sample is non-saturated), whereas Sample A (unheated) reached peak IRM values prior to the maximum applied field (implying the sample is saturated). The first step of the unmixing analysis used an utomated procedure based on the expectation-maximization algorithm (IRMUNMIX Version 2.2). The automated algorithm simply requires the user to define whether the samples have reached saturation during IRM acquisition, and the number of individual mineral phases to be modelled in the final solution (Heslop et al., 2002). The statistically optimum number of model components was estimated by this algorithm to be five. The number of model components was then systemically reduced in an effort to determine the minimum number of model components that still produced a realistic and satisfactory fit. This reduction in the number of model components was undertaken because although, ‘the fit of a finite mixture model will always improve as the number of components is increased, it is generally best to favour simplicity over complexity’ (Heslop et al., 2002). This procedure determined that a satisfactory fit was Copyright # 2006 John Wiley & Sons, Ltd.

obtained using three model components, and the initial unmixing parameters of these model components (see below) was estimated. The second step of the analysis consisted of making small incremental improvements to the fit between real and modelled data using the interactive computer program (IRM CLG). This procedure provided SSE ‘goodness’ of fit estimates after each incremental change in unmixing parameters (Kruiver et al., 2001). In this manner the SSE between real and modelled data was optimized. The results obtained from steps 1 and 2 of the study are expressed using three unmixing parameters. B1/2: the applied field at which the mineral acquires half of its saturation IRM (SIRM). This parameter provides a measure of the mean coercivity of that population. (ii) DP: a dispersion parameter expressing the distribution of each mineral phase as one standard deviation of the log-normal function. (iii) The relative contribution from each model component expressed as a percentage. (i)



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Figure10. Results ofthe three component unmixing study. (a) Raw IRMacquisition data with splined smoothed acquisition curves. (b) IRM gradient plot of the spline smoothed acquisition curve from the experimentally heated sample. (c) IRM gradient plot of the spline smoothed acquisition curve from the natural undisturbed sample.




220 Table1. Results of the three model component unmixing study Sample Sample A (unheated) Sample A (unheated) Sample A (unheated) Sample A (heated) Sample A (heated) Sample A (heated)

Component 1 2 3 1 2 3

Log (B1/2) 0.687 1.510 2.489 0.790 1.496 2.522

The results of the unmixing study are presented in Table 1. The fit between the three model components and the spline smoothed IRM acquisition curve gradients are presented graphically in Figures 10b and 10c. Examination of Table 1 shows that the contribution due to high coercivity minerals is significantly greater in Sample A (heated). The B1/2 values of the high coercivity components range from 308 to 333 mT in the unheated and heated samples, whereas the lower coercivity components were estimated to range from 5 to 32 mT. The parameter B1/2 is also often referred to as B0 cr, the remanent acquisition coercive force. Dankers (1978) determined that the B0 cr values for natural occurring magnetites, titanomagnetites and maghemites ranged from 8.5 to 67.5 mT. Naturally occurring haematites ranged from 55 to 450 mT, although values more commonly ranged from 100 to 400 mT. A comparison with Dankers (1978) data shows that the high coercivity component of Sample A (heated) falls within the normal range for haematite, providing evidence that this mineral is an end product of the high temperature phase transformations. The high coercivity component increases from approximately 11% in Sample A (unheated) to 31% in Sample A (heated). Further evidence supporting inversion to haematite is provided by a soil colour change from brown (10YR 4.5/3) in Sample A (unheated) to reddish yellow (7.5YR 6/8) in Sample A (heated). The low temperature remanence and unmixing study provided evidence that significant mineralogical transformations had occurred after exposing Sample A to a temperature of 650 C for extended periods of time. In the next section we have attempted to determine whether this temperature might realistically be expected in the soil beneath an archaeological hearth. Copyright # 2006 John Wiley & Sons, Ltd.

B1/2 (mT)

DP

5 32 308 6 31 333

0.32 0.36 0.19 0.49 0.33 0.47

Contribution (%) 21 68 11 33 36 31

Soil temperature experiment A field study was conducted to determine what soil temperatures might be expected in the soil beneath a fire hearth. The experiment was held during the 2003 University of Oklahoma field programme at the Buried City, a Plains Village Period archaeological site located in the Texas Panhandle region. A type K thermocouple was buried 2 cm beneath an experimental hearth (Figure 11). A fire was started and kept active over a period of 11.5 days according to a loosely defined schedule. The fire was stoked by field school students or volunteers in the morning before the days excavations began. The fire was stoked again at midday during lunch break, and was tended continuously after work until approximately 2200 hours to midnight. The fire had generally extinguished by morning, but still contained warm coals and ashes. Ashes were cleaned from the hearth approximately every 24 h. Mesquite (Prosopis pubescens), locust (Gleditsia triacanthos) and cottonwood (Populus deltoides var.

Figure 11. Measurement of soil temperatures beneath an experimental hearth.



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Figure12. Results of the soil temperature experiment.Temperature versus time (top) and a histogram of temperature data (bottom).

occidentalis) were the primary fuels. Temperature data from the thermocouple were recorded every 4 min on an Extech Instruments EA15 digital data logger for approximately 280 h, at which point the data logger’s batteries failed and the experiment was concluded. Results of the soil temperature experiment are presented in Figure 12. A total of 4204 temperature measurements were recorded. The average temperature over the duration of the experiment was 447 C, the maximum temperature achieved was 649.5 C. One interesting aspect of the experimental data is that temperatures generally increased over the course of the first 48 h then remained relatively constant, although significant local variations were observed. The slow ramp up to relatively stable temperatures may be due to the gradual loss of soil moisture during the first 48 h. The average temperature from 48 h to the end of the experiment was 479 C (that is, Copyright # 2006 John Wiley & Sons, Ltd.

temperature data excluding the first 48 h). The temperature also increased significantly after each ash cleaning, increases from 50 C to 150 C were common. The soil temperature exceeded 500 C for a total of 71 h during the experiment (25% of the total time). At the conclusion of the experiment the dark brown silty loam soil had changed colour to a reddish brown, indicating some inversion to haematite had occurred. Two years after the soil temperature experiment was conducted the hearth was relocated, exposed by shovel scrapping, and photographed (Figure 13). The soil colour changes resulting from the heating experiment were recorded and representative samples were taken from each colour (Figure 13). The magnetic susceptibility and frequency dependence of susceptibility of these five samples are presented in Table 2. Soil colour changes and magnetic susceptibility enhancements within the hearth were rather



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Summary and discussion

Figure 13. Planview photograph of the experimental hearth after shovel scraping in August 2005. The locations of the five samples collected for magnetic susceptibility analysis are indicated.

variable. Soil at the edge of the oxidized zone was minimally enhanced (1.1 times the natural soil), whereas the maximum enhancement occurred in the olive yellow oxidized soil (6.4 times the natural soil). Canti and Linford (2000) report that soil temperatures seldomly exceed 500 C beneath experimental fire hearths, with soil reddening occurring rarely. The somewhat higher temperatures and definite soil reddening observed during this experiment may be due to the regular removal of ash, a rather low initial soil moisture content (the experiment took place in semi-arid northwest Texas), and the use of a protective metal ring around the fire (Figure 11) that may have acted as a windbreak and a heat reflector. The relatively long duration of the experiment and natural soils with a lower inversion temperature also may be factors that contributed to the observed soil reddening.

Prolonged exposure of soil samples from the West Bluff Project Area to a temperature of 650 C resulted in a decrease in the magnetic susceptibility, falling to below the initial susceptibility value after 17 h. A reduction in S-ratio values with time spent at this isotherm suggests that the proportion of ferrimagnetic to antiferromagnetic minerals decreased with prolonged exposure to high temperatures. The reduction in these two bulk magnetic properties is thought to be the result of high temperature mineralogical transformations. Suppression of the Verwey transition and results from the unmixing study show that these transformations caused a decrease in stoichiometric magnetite and an increase in the high coercivity component of approximately 30%. This increase is thought to have proceeded via oxidation of the multidomain primary (lithogenic) magnetite to maghemite, followed by partial inversion to haematite, although further tests would be required to confirm that the high coercivity component is not composed of goethite or a goethite/haematite mixture. These tests might take the form of IRM acquisition curves using applied fields approaching 4 T, which unfortunately is beyond the range of applied fields that are possible with the equipment used during this study (France and Oldfield, 2000). Additional evidence of inversion to haematite was provided by a distinct colour change from brown to reddish yellow. Inversion may result in a ‘core’ of magnetite surrounded by maghemite and/or haematite. After 26 h at 650 C the frequency dependence of susceptibility had

Table 2. Magnetic properties of soil samples from the experimental hearth Sample exp1 exp 2 exp 3 exp 4 exp 5

Description Natural soil: greyish brown 2.5YR 5/2 Oxidized soil: olive yellow 2.5YR 6/6 Charcoal and wood ash from near centre of hearth Oxidized soil: strong brown 7.5YR 5/6 Edge of oxidized soil: very dark greyish brown 2.5YR 3/2


Mass normalized Frequency dependence magnetic susceptibility (m3 kg1) of susceptibility (%) 6.99E-07 4.46E-06 2.81E-06

4.34 7.52 4.76

1.74E-06 7.65E-07

3.80 4.94


Thermally Activated Mineralogical Transformations in Hearths increased from 1.48% to 4.32%, suggesting a small portion of the ferrimagnetic grain-size assemblage may have been reduced to SPM proportions. The soil within the experimental hearth that experienced the largest level of enhancement also experienced a relatively large increase in frequency dependence of susceptibility (see Table 2). Again, this increase in SPM minerals may be the result of a shrinking ‘core’ of ferrimagnetic minerals surrounded by a growing ‘shell’ of haematite. The results of the laboratory testing suggest that the failure to detect hearth features by magnetic field gradient survey methods at the West Bluff Project Area may have been primarily associated with two factors: (i)

a rather small initial enhancement of soil susceptibility associated with exposure to high temperatures (increase in < 20%); (ii) a drop in susceptibility to below initial values associated with prolonged exposure to high temperatures.

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Other factors affecting the survey results include randomization of the TRM component by bioturbation, and signal clutter created by bioturbation, agricultural plow furrows, disturbance from previous excavations, randomly oriented igneous and metamorphic rock at or near the ground surface, and modern ferrous iron debris. A post-excavation comparison of the location of archaeological features with the magnetic survey results was conducted in an effort to test whether inversion did result in negative magnetic field gradient anomalies. This comparison identified several hearths where a correlation between a negative magnetic anomaly and the underlying feature was found; two of these correlations are depicted in Figure 14. It should be noted that the negative field gradient anomalies identified in Figure 14 are relatively subtle, with intensities ranging from 1.9 nT m1 to 8.4 nT m1, whereas overall the magnetic field gradient data possessed a standard deviation ranging from 5 to 7 nT m1. These signalto-noise (and clutter) characteristics would have

Figure 14. Two examples where negative magnetic field gradient anomalies exist at the location of excavated hearths. Images were created from processed magnetic data. Processing included the application of a zero mean traverse algorithm, data reduction in the north^south direction and expansion in the east^west direction by linear interpolation (final display data density is 0.25 m 0.25 m). A low pass filter (one data point radius) was used to reduce the noise characteristics of the data.



224 made recognition of these anomalies rather difficult without prior knowledge of the feature locations. The inconsistent and weak nature of the negative magnetic anomalies suggests that the reduction in soil magnetic susceptibility may be partially (or wholly) offset by the contribution of high susceptibility material from wood ash. Additionally, several hearths were identified that correlate with a positive magnetic anomaly (possibly due to wood ash contributions), and many hearths created no discernable magnetic anomaly. The soil temperature experiment suggests that 650 C is at the high end of temperatures that might be expected in an archaeological hearth. As noted earlier, the average soil temperature 2 cm beneath the experimental hearth ranged from about 450 C to 480 C. Soil colour changes and susceptibility enhancement indicate that the temperatures and atmospheric conditions under the experimental hearth were sufficient to activate some mineralogical transformations, probably including high temperature inversion to haematite.

Implications to archaeological prospection The primary implication of this study is that prolonged heating of soil within a hearth may, in some cases, result in a reduction in susceptibility rather than an increase or enhancement. A localized decrease in susceptibility can result in a negative induced magnetic field gradient anomaly, the opposite of what is normally expected. The probability of a negative magnetic anomaly is greatly increased when bioturbation has randomized the thermoremanent component of the feature.

A potential application in archaeology As haematite is very stable, it is possible that fired archaeological features may retain a record of their inversion history for extended periods of time. Inversion history may eventually find practical applications in archaeology. For example, the relative concentration of haematite could be Copyright # 2006 John Wiley & Sons, Ltd.

D. Maki, J. A. Homburg and S. D. Brosowske used to estimate the ‘use life’ of archaeological hearths by the method outlined below. An expression for the rate of inversion at elevated temperatures is provided in the form of a first-order differential equation by Adnan and O’Reilly (1999). Predictions concerning the variation in magnetic susceptibility levels with time can be made using a solution to this rate expression. This solution takes the form, (t) ¼ maximum exp (t/), where (t) is the magnetic susceptibility at time t and maximum is defined as the maximum magnetic susceptibility that occurs as a result of high temperature susceptibility enhancement (e.g. the maximum susceptibility values from each site in Figure 7). In this expression, the time at which the maximum magnetic susceptibility is reached is defined as t ¼ 0, and is a rate constant related to the Arrhenius equation for a thermally activated process. During the high temperature experiment, soils from the West Bluff Project Area, 48CA3030 (Wyoming) and 31CD218 (North Carolina) all experienced an initial enhancement in magnetic susceptibility, followed by a decrease in susceptibility, apparently due to inversion. The time constant (for heating at 650 C) was determined for soils from these three sites by fitting the inversion function (above) to the experimental heating data from Figure 7. The time constant () was adjusted until a ‘best fit’ was arrived at, as determined by sum of squares error (SSE) analysis (Figure 15). Examination of Figure 15 will show a large variation in the rate constant at this isotherm. It was not possible to determine the time constant of soils from 11S1131 (Illinois) or 34BV100 (Oklahoma) because the maximum magnetic susceptibility was not achieved by the conclusion of the high temperature experiment. Experimental determination of the inversion time constant () may find a practical application in archaeology by making it possible to estimate the time a given soil sample has spent at high temperatures. By way of example, several heavily oxidized soil samples were collected from the wall of a small diameter pit hearth/oven at 48CA3030—one of the comparative sites examined during this study (Jones and Munson, 2005; Maki, 2005). Oxidized samples collected from the interior wall of the cylinder were red (2.5YR 4/6),



225

Figure15. Estimation of the rate constant for a thermally activated process.

whereas the natural soils from which the hearth/ oven was constructed were brown (10YR 5/3). The magnetic susceptibility values of oxidized soil samples were twice those of the natural soils, but only about 2% of the maximum magnetic susceptibility value recorded during the high temperature heating experiment described in this report (see Figure 7). The relatively low magnetic susceptibility of the oxidized soil samples (compared with the maximum susceptibility value) and the distinct reddening of soil colour suggests that high temperature inversion had occurred. If we assume that inversion to haematite was responsible for reducing soil susceptibility to 2% of its maximum experimental value, a prediction can be made using the experimentally determined time constant () and the inversion function introduced above. The inversion function (using ¼ 103) provides an estimate of about 400 h at 650 C to reduce the magnetic susceptibility to 2% of its maximum experimental value. In other words, the hearth’s ‘use life’ is estimated to be 400 h at this temperature. Of course such mathematical estimates must be used with extreme caution, as there are numerous sources of uncertainty, not the least of which is the estimated temperature of the soil Copyright # 2006 John Wiley & Sons, Ltd.

surrounding the hearth. The heating experiment described in this report, and previous experiments described by Canti and Linford (2000), reveal significant variation in soil temperatures achieved beneath experimental hearths. Given the uncertainty associated with soil temperatures, a more rigorous and useful approach to modelling the inversion process is recommended. This approach should proceed by utilizing the method developed by Adnan and O’Reilly (1999) for determining the two components of , the activation energy and a frequency factor. Determination of these two components allows one to use the inversion function to model changes in magnetic susceptibility at any isotherm. Unfortunately the equipment necessary to determine by this method were not available during this research.

Conclusions The experiments described in this article have documented a reduction in the concentration of ferrimagnetic minerals—and an increase in the concentration of high coercivity minerals—with prolonged exposure to high temperatures. These mineralogical changes appear to be due to high


226 temperature inversion to haematite. In the case of soils from the West Bluff Project Area, the final experimental susceptibility values were less than initial values. This implies that under some soil conditions high temperature inversion can result in a local decrease in susceptibility, which would create a negative induced magnetic field gradient anomaly (the opposite of what is normally expected). The temperature and environmental conditions necessary for inversion to proceed appear to be highly variable. At present the proportion of archaeological sites at which inversion may be a factor to consider when interpreting magnetic survey results is unknown. For now care should be taken when interpreting magnetic data from sites with soils similar to those found in the West Bluff Project Area; that is, sites with relatively high initial concentrations of multidomain primary lithogenic magnetite (magnetite is widespread in parent rocks of the Los Angeles Basin, including the granitic rocks that dominate most of the surrounding mountains). In a more general sense, high temperature inversion may be a factor to consider in soils formed in sandy deposits associated with dunes, alluvium and marine sediments that are now terrestrial.

Acknowledgements The authors would like to thank Rinita Dalan for providing a thorough and insightful review of an early draft of this manuscript. The manuscript also greatly benefited from the comments of Chris Gaffney and two anonymous reviewers at Archaeological Prospection. We would also like to thank Harold and Kirk Courson (of Courson Oil and Gas) Perryton, Texas for continuing support of this research, and the personnel of the Institute for Rock Magnetism, University of Minnesota for valuable guidance and advice throughout the project. The Institute for Rock Magnetism is supported by the USNSF and the W.M. Keck Foundation (Los Angeles). The unmixing algorithms used during this research were obtained from the paleomagnetic laboratory ‘Fort Hoofddijk’ via their home page at (www.geo.uu.nl/ forth/).



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