supermagnetic enhancement, superparamagnetism, and archaeological soils

Geoarchaeology: An International Journal, Vol. 14, No. 5, 401–413 (1999)q 1999 John Wiley & Sons, Inc. CCC 0883-6353/99/050401-13

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Supermagnetic Enhancement,

Superparamagnetism, and

Archaeological Soils

Clare Peters and Roy Thompson

Department of Geology and Geophysics, University of Edinburgh, West Mains

Road, Edinburgh, EH9 3JW, Scotland, United Kingdom

A range of mineral magnetic measurements have been carried out on archaeological sedi-ments from Orkney and Cyprus. In a soil profile from Orkney, a magnetic enhancement factorof over 200 is observed in susceptibility data between the bedrock and a Norse sediment. Themagnetic enhancement is associated with an increase in superparamagnetic grains probablycaused by burning. Until now it has proved difficult to confirm the presence of superpara-magnetic grains in natural samples using room temperature magnetic measurements. How-ever, clear differences are to be found between the hysteresis loops of various magneticdomain states, including superparamagnetism. An algorithm has been developed to unmixhysteresis loops in terms of constituent domain states of ferrimagnetic iron oxides. Unmixing128 hysteresis loops of archaeological sediments has shown that the dominant domain statein all sediments is superparamagnetic. Remarkably uniform superparamagnetic grain sizes ofbetween 80 and 95 A were found for all 128 sediments. q 1999 John Wiley & Sons, Inc.

INTRODUCTION

Archaeologically developed soils are magnetically very complex. Various ar-chaeological events can modify the magnetic properties of soils; in particular, burn-ing is believed to produce very fine grained superparamagnetic particles. However,the magnetic identification of superparamagnetic grains has previously provedquite difficult. Although superparamagnetic grains do not carry a remanence manyof their magnetic properties are similar to those of multidomain grains. For ex-ample, in the classic biplot for determining domain states by Day et al. (1977)(Mr/Ms vs. (B0)cr/(B0)c) the presence of multidomain and superparamagnetic grainshave similar effects. They both reduce the magnetization ratio Mr/Ms and increasethe coercivity ratio (B0)cr/(B0)c. Temperature-dependent magnetization and suscep-tibility measurements can help detect the presence of superparamagnetic particles.However, a new technique using the room temperature magnetic measurements ofhysteresis loops has been developed. Our technique not only identifies the presenceof superparamagnetic grains, but also estimates the grain size of the superpara-magnetic grains within any given sample and quantifies their mass.

Mineral magnetic measurements have been carried out on soils from two ar-chaeological sites. The first site is located close to St. Boniface Kirk on the island

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of Papa Westray, Orkney, Scotland. Eighty-five samples of till, naturally developedsoils, Iron Age and Norse Age sediments were measured. The second archaeolog-ical site is located at Kissonerga on the west coast of Cyprus. Thirty-five samplesfrom laterally extensive, but archaeologically barren sedimentary units, of un-known origin, were measured. Additionally eight soil samples collected from closeto the Kissonerga site were measured.

METHODS

Initial and low temperature susceptibility measurements were made using Bar-tington susceptibility bridges. Hysteresis loops were measured using a Molspinvibrating sample magnetometer. Magnetisations were measured at 81 fields cyclingfrom and back. A Curie-Weiss balance was used to monitor variations1 T to 2 1 Tin magnetisation from room temperature up to 6007C. Anhysteretic remanent mag-netisations were induced in alternating fields of superimposed on a direct99 mTfield of using an adapted Molspin AC demagnetizer. Isothermal remanent0.1 mTmagnetisations were grown in fields of using electromagnets. The remanent1 Tmagnetisations were measured using a Molspin fluxgate magnetometer. All mea-surements were made on bulk samples.

RESULTS

Magnetic Enhancement

Natural soil development can produce an increase in magnetic concentration inthe upper soil horizons especially the topsoil. For example, the imperfect antifer-romagnetic mineral haematite often found in subsoils can be altered to the mag-netically stronger ferrimagnetic minerals in topsoils. Magnetic enhancement in soilprofiles can be further increased due to burning of the soils in which superpara-magnetic particles enhance the magnetic signal.

Initial magnetic susceptibility (x) measurements are a quick and easy way tolook at magnetic enhancement within soil profiles. The measurements can be car-ried out in the field or laboratory. In Figure 1, laboratory-measured x for each ofthe sediment types from the Orkney profile is plotted. An overall increase in x isobserved in the development of natural soils from the till. Further enhancement isobserved in the development of Iron Age and Norse Age sediments. Comparing thex values of the Upper Norse sediments with the till, we observe a magnetic en-hancement factor of over 200. The range of x values are from 30.1 to 22.2 mmkg21, with an average of . The anhysteretic remanent magnetizations3 215.0 mm kgrange from , with an average of , and the2 21 2 210.1 to 8.2 m A m kg 3.0 m A m kgisothermal remanent magnetizations, grown in a field of , are from 0.7 to1T

, with an average of .2 21 2 21240 m A m kg 49 m A m kg

Identification of Superparamagnetic Grains

Monitoring variations in magnetic properties with temperature can indicate thepresence of superparamagnetic grains. Figure 2 shows the variation in magnetiza-

SUPERMAGNETIC ENHANCEMENT, SUPERMAGNETISM, AND ARCHAEOLOGICAL SOILS

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL 403

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Figure 1. Initial magnetic susceptibilities for 85 samples grouped according to their sediment type. Thesamples are from a soil profile on Orkney. The units of susceptibility are .3 21mm kg

tion of an Iron Age sediment as it is heated up to 6007C in air. The linear decreasein magnetization with heating to approximately 5807C and the reversibility of thethermomagnetic curve indicate the presence of superparamagnetic iron oxidegrains. A curve similar to that shown in Figure 2 was obtained by Fassbinder (1992)for a magnetic separate from an archaeological soil, except the thermomagneticcurve is not reversible between 250 and 4507C due to the presence of maghaemite.Low temperature measurements can also indicate the presence of superparamag-netic grains. Figure 3 shows the variation of susceptibility from liquid nitrogen toroom temperature. The linear nature of the low temperature susceptibility curveindicates that the magnetic grains are superparamagnetic at room temperature(Radhakrishnamurty and Deutsch, 1974; Radhakrishnamurty et al., 1978). The fallin susceptibility observed with decreasing temperature in Figure 3 could be due tothe grains exhibiting a range of blocking temperatures at which they become stablesingle domain.

Monitoring of samples with either high or low temperatures is time consuming

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Figure 2. Variation of induced magnetisation with temperature for an Iron Age sediment from Orkney.

and the high temperature work can lead to chemical alteration. A relatively quickmethod of identifying superparamagnetic grains is to carry out room temperaturemeasurements of hysteresis loops. Superparamagnetic grains have thermal ener-gies similar to their magnetic energies at room temperature. As a result, the direc-tion of their magnetization is continually changing, and hysteresis is not observed.However, the magnetization (M) of superparamagnetic grains varies with field andis given by

M/M 5 coth a 2 1/a, (1)s

where

a 5 vM H/kT (2)s

and v is the volume, Ms is the saturation magnetisation, H is the field, k is Boltz-mann’s constant, and T is the absolute temperature (Chikazumi, 1964).

Using Eqs. (1) and (2) hysteresis loops of superparamagnetic grains of varying



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Figure 3. Low temperature susceptibility curve for a Norse sediment from Orkney.

sizes can be calculated. Figure 4 shows superparamagnetic hysteresis loops of grainsizes from 40 to 300 A (volumes from ). Particles226 3 223 36.4 3 10 m to 2.7 3 10 mof 40 A are 87% saturated at 1 T. The upper grain size limit for superparamagneticgrains of magnetite is 300 A (Dunlop, 1973). Additionally, the measured hysteresisloop of a natural multidomain magnetite crystal from Shetland is included3 mmin Figure 4. The low field gradients of the superparamagnetic loops vary as a func-tion of grain size. The largest grains exhibit very steep curves with distinct knee-shaped approach to saturation. We observe in Figure 4 that 60 A superparamagneticgrains exhibit similar low field gradients to the multidomain magnetite hysteresisloop. However, the curved approach to saturation of the superparamagnetic curvein contrast to the knee-shaped approach of the multidomain magnetite loop allowsus to distinguish between superparamagnetic and multidomain grains.

Unmixing Calculations

Natural samples are made up from many components. It is important not onlyto identify the different components, but also to quantify them. The unmixing pro-cedure described here is a general approach and could be easily adapted to applyto a wide range of data types or magnetic parameters. The unmixing procedurewill, however, be described here solely in terms of hysteresis loop data.

We want to “unmix” a measured hysteresis loop in terms of different end-mem-bers, that is, we want to find out how much of each end-member is present in thesample. We represent the measured hysteresis loop by the vector a (x), i 5 1,mi

where a is the magnetization corresponding to the field value x. There are m

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Figure 4. Hysteresis loops of a natural multi-domain magnetite and four theoretically calculated su-perparamagnetic curves, with grain sizes given in angstroms. All loops are normalized to their magne-tisation in a field of 1T.

field values. We want to unmix the measured hysteresis loop in terms of end-mem-bers. We can calculate a model hysteresis loop by adding together different ratiosof the hysteresis loops of the end-members. We can represent the model hysteresisloop by

n

b (x) 5 r c (x), i 5 1, . . . ,m, (3)i O j ijj51

where b is the magnetization of the model hysteresis loop corresponding to a par-ticular field value x, of which there are m values, cj represents the magnetizationsof the n end-members, and rj represents the mass of the jth end-member. We wantthe difference between the measured hysteresis loop and the model hysteresis loopto be as small as possible, that is, we want to minimize ^(x) in



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Table I. Magnetic properties of a till and an enhanced soil from Orkney and a Cyprus soil.

Ms

SaturationMagnetization(m A m2 kg21)

MSaturation

Magnetization(Ferrimagnetic)(m A m2 kg21)

Mrs

SaturationRemanence

(m A m2 kg21)

(B0)c

CoerciveForce(mT)

xSusceptibility

(mm3 kg21)

ARMAnhystereticRemanence

(m A m2 kg21)

Orkney (till) 36 10 1 6.1 0.3 0.2Orkney

(enhanced)129 124 19 7.2 22.2 8.2

Cyprus 75 35 4 6.7 7.9 0.5

m2^(x) 5 [a (x) 2 b (x)] . (4)O i i

i51

We are trying to minimize a least-squares function. Various methods are availablefor carrying out the minimization shown in Eq. (4), depending on the nature of theproblem. Here we use the NAG Fortran Library subroutine E04FCF, which is basedon the modified Gauss–Newton method (Gill and Murray, 1978).

As an estimate of how good a fit the final modelled hysteresis loop is to thesample hysteresis loop, a correlation coefficient R is calculated. The R value isdefined by Eq. (5). Values of indicate a perfect fit between the sample andR 5 1.0model hysteresis loops. Values of R of less than 0.9990 indicate a poor fit betweenthe sample and model hysteresis loops:

^(x)R 5 1 2 . (5)S Dm 2o a (x)i 5 1 i

Figure 4 shows that we can distinguish between hysteresis loops of superpara-magnetic and multidomain grains. Single-domain grains clearly exhibit hysteresis(they have open hysteresis loops in comparison to the multidomain loops). Thus,using mass specific magnetization data of different domain states, we can modelthe components of natural samples. We can also include para- and diamagnetismin the calculations. In our calculations we assume that the ferrimagnetic grains aremagnetite with a saturation magnetization of . If maghaemite grains2 2192 A m kgare present instead, then the calculated concentrations will be higher by a factorof 1.24 (the ratio of the saturation magnetizations of magnetite and maghaemite).

Two examples of applying our new unmixing algorithm to archaeological sedi-ments (Table I) are shown in Figures 5 and 6. One sample is from an archaeologicalsite on Orkney, the other from Cyprus. The higher maximum magnetization of theOrkney sediment in Figure 5, , compared to for the2 21 2 210.129 A m kg 0.075 A m kgCyprus sediment in Figure 6 is reflected in the unmixing results. The Orkney

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Figure 5. Unmixing an enhanced Upper Norse sediment from Orkney in terms of different domainstates. The R value of 0.9993 indicates a good fit between the sample and model loops. The estimatedsuperparamagnetic grain size is 85 A. Note the R values of over 0.999, here and in Figure 6, mean thatthe model and sample hysteresis loops plot almost on top of each other.

sediment has a higher total magnetite component than the Cyprus sediment. Bothhysteresis loops have high values for the low field gradients. Neither hysteresisloop is open above so that high coercivity minerals such as goethite/hae-200 mTmatite are not present. Also as the Ms-T curves are largely reversible (e.g., Figure2) iron sulphides, ferrihydrites, and pure maghaemite are also not of importancein these samples. The most likely magnetic mineralogy is thus impure maghaemiteor magnetite with Curie temperatures just below 6007C. The unmixing algorithmhas found superparamagnetism (of grain sizes greater than 60 A) to be the dominantdomain state. If the magnetizations of the two samples were due entirely to super-paramagnetic grains, hysteresis would not be observed. However, the opening ofboth loops indicates the presence of single-domain grains, confirmed by the un-mixing results. The positive high field gradients of the hysteresis loops reflect theparamagnetic components. The Cyprus hysteresis loop (Figure 6) has a larger highfield gradient than the Orkney sediment (Figure 5), and thus a higher paramagneticcomponent, confirmed by the unmixing results.



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Figure 6. Unmixing the hysteresis loop of a sediment from Kissonerga, Cyprus, in terms of differentdomain states. An excellent fit ( ) is observed between the sample and model hysteresis loops.R 5 0.9999The superparamagnetic grain size is estimated to be 82 A.

DISCUSSION

The concentration of magnetic minerals in soils can be increased in four mainways: (1) Enhancement of susceptibility can result from the “natural” developmentof soils. The average enhancement of the B-horizon of soils in temperate climatesis around (Maher and Thompson, 1995). This degree of soil26 3 210.3 3 10 m kgenhancement is very common. The maximum “natural” magnetic enhancement ofsoils is probably around (Maher and Thompson, 1995). (2) The26 3 214 3 10 m kgmagnetic susceptibilities of soils from industrial areas are commonly greatly in-creased by pollution. For example, top soil enhancement of upto 5 3

have been found in the industrial zones of Poland (Strzyszcz, 1991).26 3 2110 m kgThis magnetic enhancement is mainly caused by fly ash particles from power plants.The particles have diameters of up to a few hundred microns. Such magnetic ma-terials have soft multidomain properties (Thompson and Oldfield, 1986) and so canoften be identified magnetically. Individual cultivated fields can be strongly en-hanced due to the addition of industrial waste as a fertilizer. (3) Enhancement canresult from the production of unusually high concentrations of bacterial magne-tosomes. Fassbinder and Stanjek (1993) report magnetic susceptibility enhance-

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ment, most probably of bacterial origin, of a factor of 6 in a very localized envi-ronment, namely, the center of an old post. This particularly productive organicsetting gave rise to a magnetic enhancement of . Here the magnetic26 3 217 3 10 m kggrains were magnetically moderately hard with SIRM/x ratios of about 215 k A mand ARM/SIRM ratios of about 0.05 and so distinct from superparamagnetic dom-inated assemblages. (4) Burning is well established as a further process which leadsto magnetic enhancement. McClean and Kean (1993) have found an enhancementfactor of 22 in an ash layer in a modern day fire pit that has been repeatedly burnt.The total enhancement was . The importance of superparamag-26 3 2116 3 10 m kgnetic grains in their samples is indicated by an SIRM/x ratio of under .210.1 k A mAverage enhancement factors of up to 4 of archaeological soils in Italy have beenreported by Tite and Linington (1986). The total enhancement, above that of agri-cultural soils from the same areas, was typically . The extra26 3 213 3 10 m kgenhancement of these archaeological soils was presumably primarily caused byburning.

The magnetic enhancement factors found here, for the Upper Norse soil, areseveral hundred times higher than for natural soils and higher than previouslyreported on archaeological sites. Our enhancements of , due26 3 2114–20 3 10 m kgto superparamangetic additions, are almost identical to those of the modern dayfire pits of McClean and Kean (1993).

There are many approaches to analyzing hysteresis loops. An early example isthe work of Bean (1955). Recent examples include the work of Roberts et al. (1995),Peters (1995), von Dobeneck (1996), and Tauxe et al. (1996). The ability of thehysteresis loop unmixing algorithm described here to estimate the concentrationand grain size of superparamagnetic components is very pleasing. Figure 7(a)shows the range of superparamagnetic volumes calculated for 128 sediments andsoils from the archaeological sites on Orkney and Cyprus. In comparison, Figure7(b) shows the range of superparamagnetic volumes for 194 naturally (i.e., non-archaeologically) developed soils from the catchments of Jackmoor Brook, south-west England, and Lake Bussjo, southern Sweden. An additional 32 natural samplesfrom Jackmoor Brook and Lake Bussjo plotted outwith the range of Figure 7(b).Note the tight distribution of superparamagnetic volumes of the archaeologicalsamples. The unmixing algorithm consistently selected grain sizes between 80 and95 A for all 128 archaeological samples. In comparison, the superparamagneticvolumes selected for the natural samples from Jackmoor and Lake Bussjo show amuch larger distribution, from 59 A to grain sizes greater than the 300 A superpar-amagnetic/single-domain limit of magnetite. Unmixing of the samples from all thestudied sites indicates that the hysteresis loop unmixing algorithm can successfullydistinguish between superparamagnetic grains and multidomain grains using onlyroom temperature magnetic measurements.

One puzzling aspect of the magnetic properties of the enhanced Orkney soilsconcerns the combination of their magnetization and remanence properties. Whilethe magnetization data of their hysteresis loops can be interpreted well in terms ofa combination of superparamagnetic and stable single domain ferrites, as can the



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Figure 7. Superparamagnetic volume distribution for (a) Cyprus and Orkney and (b) Jackmoor Brookand Lake Bussjo. In (a) each bar represents a volume increase of and in (b),223 30.005 3 10 m 0.05 3

. Note that all of diagram (a) falls in the range of one of the columns of (b).223 310 m

low temperature susceptibility and thermomagnetic data, the remanence/a.c. sus-ceptibility data is more difficult to explain. Such a combination of superparamag-netic plus stable single domain ferrites would be expected to produce SIRM/x ratiosmuch lower than the observed .218 k A m

The magnetic remanence properties of magnetite grains close to the superpar-amagnetic/stable single-domain boundary are very sensitive to small changes in

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grain size. For example, unusually high ARMs are to be found (Ozdemir and Ba-nerjee, 1982; Maher, 1988). We speculate that the highly enhanced Orkney samplescontain grains close to the superparamagnetic/stable single domain boundary thathave unusually high SIRM/x ratios but low coercivities (i.e., extremely narrow butvery square hysteresis loops). Such grains have to our knowledge not yet beenobserved in synthetic magnetite or haematite samples. A combination of such soft,square looped grains, plus high ARM grains, plus superparamagnetic grains wouldaccount for the magnetic enhancement in the Orkney soils.

CONCLUSIONS

1. A magnetic enhancement factor of over 200 is observed in susceptibility datafrom an archaeologically developed soil profile in Orkney.

2. Linear low temperature susceptibility and high temperature magnetisationcurves indicate the presence of superparamagnetic grains.

3. Room temperature hysteresis loop measurements can distinguish betweensuperparamagnetic and multidomain components, in addition to single-do-main minerals.

4. The hysteresis loop unmixing technique can quantify the concentrations ofdifferent domain states, including superparamagnetic.

5. Unmixing of 128 hysteresis loops of archaeological sediments from Orkneyand Cyprus show superparamagnetic grains of 80–95 A to be the predominantmagnetic component in all samples.

The work was supported by a NERC studentship to CP. We are very grateful to Dr. S. Carter, AOC, andDr. R. Tipping for providing the archaeological samples from Orkney and Cyprus, respectively. We thankProfessor F. Oldfield for use of the low temperature susceptibility equipment. Helpful reviews by O.Ozdemir and an anonymous referee are gratefully acknowledged.

REFERENCES

Bean, C.P. (1995). Hysteresis loops of mixtures of ferromagnetic micropowders. Journal of AppliedPhysics, 26, 1381–1383.

Chikazumi, S. (1964). Physics of magnetism, New York: Wiley.Day, R., Fuller, M., & Schmidt, V.A. (1977). Hysteresis properties of titanomagnetites grain-size and

compositional dependence. Physics of Earth and Planetary Interiors, 13, 260–267.Dunlop, D.J. (1973). ‘Superparamagnetic and single-domain threshold sizes in magnetite. Journal of

Geophysical Research, 78, 1780–1793.Fassbinder, J.W.E. (1992). Die magnetischen Eigenschaften und die Genese ferrimagnetischer Minerale

in Boden, Ph.D. Thesis, Munich: Der Ludwig-Maximilians-Universitat.Fassbinder, J.W.E., & Stanjek, H. (1993). Occurrence of bacterial magnetite in soils from archaeological

sites. Archaeologia Polona, 31, 117–128.Gill, P.E., & Murray, W. (1978). Algorithms for the solution of the nonlinear least-squares problem. SIAM

Journal of Numerical Analysis, 15, 977–992.Maher, B.A. (1988). Magnetic properties of some synthetic sub-micron magnetites. Geophysical Journal,

94, 83–96.Maher, B.A., & Thompson, R. (1995). Paleorainfall reconstructions from pedogenic magnetic suscepti-

bility variations in the Chinese loess and paleosols. Quaternary Research, 44, 383–391.



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McClean, R.G., & Kean, W.F. (1993). Contributions of wood ash magnetism to archaeomagnetic prop-erties of fire pits and hearths. Earth and Planetary Science Letters, 119, 387–394.

Ozdemir, O., & Banerjee, S.K. (1982). A preliminary magnetic study of soil samples from west-centralMinnesota. Earth and Planetary Science Letters, 59, 393–403.

Peters, C. (1995). Unravelling magnetic mixtures in sediments, soils and rocks, Ph.D. Thesis, Edinburgh:University of Edinburgh.

Radhakrishnamurty, C., & Deutsch, E.R. (1974). Magnetic techniques for ascertaining the nature of ironoxide grains in basalts. Journal of Geodynamics, 40, 453–465.

Radhakrishnamurty, C., Likhite, S., Deutsch, E.R., & Murthy, G. (1978). Nature of magnetic grains inbasalts and implications for palaeomagnetism. Proceedings of the Indian Academy of Science, 87,235–243.

Roberts, A., Cui, Y., & Verosub, K. (1995). Wasp-waisted hysteresis loops: Mineral magnetic character-istics and discrimination of components in mixed magnetic systems. Journal of Geophysical Re-search, 100, 17909–17924.

Strzyszcz, Z. (1991). Ferromagnetism of soils in some Polish national parks. Mitteilungen der DeutscheBodenkundl. Gesellschaft, 66, 1119–1122.

Tauxe, L., Mullender, T.A.T., & Pick, T. (1996). Potbellies, wasp-waists, and superparamagnetism inmagnetic hysteresis. Journal of Geophysical Research, 101(B1), 571–583.

Thompson, R., & Oldfield, F. (1986). Environmental magnetism, Allen and Unwin, London.Tite, M.S., & Linington, R.E. (1986). The magnetic susceptibility of soils from central and southern Italy.

Esttrato da Prospezioni Archeologiche, 10, 25–36.von Dobeneck, T. (1996). A systematical analysis of natural magnetic mineral assemblages based on

modelling hysteresis loops with coercivity-related hyperbolic basis functions. Geophysical JournalInternational, 124, 675–694.

Received January 1, 1997

Accepted for publication March 3, 1997

supermagnetic enhancement, superparamagnetism, and archaeological soils

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