NIOMI B Nuclear Instruments and Methods in Physics Research B75 (1993) 454-457

North-Holland Beam Interactions with Materials 8 Atoms

PIXE analysis of cave sediments, prehispanic paintings and obsidian cutting tools from Baja California Sur caves

J. Miranda, A. Oliver, A. Dacal, J.L. Ruvalcaba, F. Cruz and M.E. Ortiz Institute de Fkica, Uniuersidad National Aut&oma de M&co, Apartado Postal 20-364, 01000 M&co, DF, Mexico

R. Viiias ’

Institute de Investigaciones Antropolbgicas, Universidad National Autbnoma de M&co, Ciudad Universitaria, 01000 M&co, DF, Mexico

Elemental PIXE analysis of cave sediments, minerals, pigments of the prehispanic paintings and obsidian cutting tools from

caves in Baja California Sur has been carried out with a 0.7 MeV proton beam. The elements analysed in this sample set (Al to Co)

provide an idea of the environment of the caves. The obsidian data analysis suggests that the human communities who made these

painting used more than one obsidian source to manufacture cuttings tools.

1. Introduction

In Baja California Sur, Mexico, there exist many cave paintings that constitute a very important histori- cal document for the study of and knowledge about the first inhabitants of Mexico. These “sanctuaries” repre- sent one of the most relevant centers, not only in Mexico but in the world, for the study of the pictorical expressions of the gathering-hunting groups. The iden- tity and historical reality of the artists who painted

these caves is still unknown. Recently, a group of scientists has begun to study these human groups.

The caves are located in the Sierra de San Fran- cisco at longitude 113”W and 27” 45’ N latitude. Among the nearly two hundred painted caves, La Pintada and San Gregorio are the most well known. The paintings represent human figures, deers and sea animals. The colors of the paintings are only white, black, red, and yellow. The site is very isolated, suggesting that the pigments were taken from the region. Archaeological objects are very rare and no evidence of human settle- ment has been found yet.

PIXE is a relatively new method in art and archae- ology, nevertheless much work has been done on this subject [l]. Recently, Menu and Walter [2], have anal- ysed prehistoric paints in several French caves. In this work, we present the first PIXE analysis of a sample set of objects chosen so as to give an idea of the paintings pigments, the environment inside the caves

’ On leave from: Universidad de Barcelona, Barcelona, Spain.

and the elemental composition of several obsidian cut- ting tools found in their interior.

2. Measurements

The PIXE analysis was carried out with the 0.7 MeV Van de Graaff accelerator at the Instituto de Fisica, UNAM. The measurements were made in a target chamber with a Si(Li) detector located at 160” from the beam direction and at 9 cm from the chamber center. The quantification of elements was made by means of sensitivity curves obtained with MicroMatter calibration standards.

Two kinds of samples have been chosen: those extracted from the rock of the mountain and soil of caves and the others associated with the presence of man. First, we took rock and soil from the interior of the La Pintada Cave, a dark deposited cluster on the rock from the same cave, and colored rocks taken from the region which were suspected to be the source of the pigments. Also, we analysed two rock fragments belonging to the paintings of the La Pintada Cave and obsidian cutting tools found in the interior of the San Gregorio Cave. The samples taken from the raw rock, soil, dark deposited clusters, and colored rocks were carefully ground and deposited on mylar using a solu- tion of Apiezon grease and toluol as a glue [3]. The grain size after grinding the samples was about 15 km in each case. The other samples were analysed as received.

0168-583X/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

J. Miranda et al. / Cave sediments, prehispanic paintings, obsidian cutting tools 4.55

3. Results and discussion Table 2

The cave rock analysis was made only for rock which has not been exposed to the environment. Light and dark zones were analysed from two different rock pieces: light zone from piece 1 (LZl), light zone from piece 2 (LZ2) and dark zone from piece 2 (DZ2). Three soil samples were taken. One corresponding to the surface (S3), and two others at 10 cm (S2) and 20 cm (Sl) depth. The element composition up to Co of these samples, as well as a deposited cluster (DC) is shown in table 1. The rock composition is approxi- mately the same for the light and dark zones, except for the larger concentrations of Si and Cl but less Fe in the dark zone. The soil composition is different than that of the rock. The soil samples were taken from a sheltered part of the cave, apparently without occupa- tion in recent times. In general the cave rock plays an important part in the process of sedimentation and in the type of soil produced. However, many sediments contain inclusions without a geological origin. They were brought in by either man or animal. The soil samples at some depth have a significant amount of P, S, Cl, K and Ca, while the surface samples shows lesser concentrations of these elements. This suggests that the cave was inhabited by either man or animal in past times [9,10].

Elemental composition in mg/g of rocks suggested as possible

pigment sources. Uncertainties about 20%

Element Y El R2 R3

Al 200 43.1 59.9 93.3

Si 306 322 216 357 P 5.58 3.16 4.36 3.348

s 27.9 0.513 0.333 0.412

Cl 0.603 0.515 0.171 0.422

K 12.89 7.94 2.76 8.60

Ca 31.0 61.7 47.0 60.5

Ti 11.65 6.32 5.80 10.52

Cr _ 0.224 0.17 _

Mn 0.763 1.82 0.896 1.67

Fe 104.6 74.5 59.5 82.4

co 1.34 0.597 0.284 1.42

general, forms sulfates and sulfides with the other

elements present in the rock. Some of these sulfates are colorless, lime aluminum sulfate (aluminum is pre- sent in great quantities). Important colored sulfates are cobalt sulfate which is red (cobalt is present in low quantities), and iron sulfate which is yellow. Then the yellow color can be due to the hydrated iron oxides in combination with sulfates. It is highly likely that they are used as the pigment source.

Two kinds of rocks, suspected to be the source of the pigments, were analysed: a yellow rock (Y), corre- sponding to a sandstone sedimentary type rock and three reddish pebbles of scoria “tezontle” (volcanic rock). The scoria pebbles present different reddish tones, one of them dark red (Rl) and two with a dark orange color (R2, R3). Table 2 shows the elemental compositions of these samples. In the four analysed rocks there is Fe in a significant quantity. It is well known that the reddish tone of the rocks is due to the presence of the iron oxides. When the oxides become hydrated, their color changes to yellow. The yellow rock (Y) contains a great quantity of S which, in

The composition concentrations, in mg/g, shown in tables 1 and 2 have an uncertainty estimated of about 20%. Corrections due to matrix effects in’ the grains were taken into account.

The analysis of the obsidian cutting tools was based on the chemical composition given by Ericson et al. [4]. The elemental composition of these thick targets was determined using the computer code described in ref. [5]. Oxygen is estimated assuming stoichiometric oxides adding up to 100%. The X-ray enhancement due to secondary fluorescence has been calculated. This cor- rection was of the order of 2% in Ka X-rays of the Al and Si. The elemental mass concentrations (%) of 10

Table 1

Elemental composition in mg/g of the rock, soil, and deposited cluster from La Pintada cave. The uncertainty estimated is 20%

Element LZl LZ2 DZ2 Sl s2 s3 DC

Al 66.9 82.4 94.7 133.3 101.5 81.7 190.1 Si 282.3 293.5 394.5 222.1 185.1 167.3 142.9 P 1.844 2.137 1.766 11.04 10.56 6.56 11.07 S 0.675 2.71 1.020 6.60 6.07 4.11 21.89 Cl 1.459 1.123 3.21 19.7 17.1 15.5 26.5 K 9.29 9.40 9.89 43.0 33.4 26.3 66.3 Ca 29.0 32.6 27.7 112.7 100.4 61.4 46.0 Ti 3.59 3.84 3.26 2.26 1.71 1.85 1.277 Mn 1.118 0.53 1.4 1.111 0.838 0.349 0.473 Fe 36.2 37.8 20.7 28.7 21.6 17.2 15.00 co 0.291 0.167 0.263 0.081 0.061 0.45 0.068

VI. GEOLOGICAL SAMPLES

456 J. Miranda et al. / Cave sediments, prehispanic paintings, obsidian cutting tools

Table 3

Elemental mass concentration (%) of obsidian cutting tools. Estimated uncertainties are 20%

Element s.l s.2 s.3 s.4 s.5 S.6 S.7 S.8 s.9 s.10

0 50.3 51.5 50.5 50.3 50.4 49.8 50.2 50.0 50.1 49.5

Al 8.96 2.00 8.81 9.13 8.87 9.26 10.4 8.20 9.50 10.7

Si 36.1 42.8 36.3 35.9 36.1 35.1 34.8 36.0 35.3 33.7

P 0.024 0.629 0.051 0.042 0.057 0.048 0.034 0.040 0.224 0.018

S 0.013 0.242 0.036 0.031 0.034 0.041 0.038 0.034 0.022 0.027

Cl 0.104 0,326 0.124 0.123 0.142 0.155 0.088 0.131 0.152 0.130

K 2.49 0.45 2.50 2.69 2.56 3.36 2.78 3.41 2.87 4.04

Ca 0.526 1.62 0.584 0.667 0.537 0.792 0.633 0.732 0.643 0.771

Ti 0.087 0.013 0.093 0.088 0.097 0.114 0.093 0.111 0.094 0.089

Mn 0.018 0.105 0.033 0.018 0.016 0.049 0.019 0.029 0.017 0.030 Fe 0.89 0.335 0.94 0.94 1.19 1.25 0.89 1.28 1.10 0.900 co 0.0096 0.010 0.0092 0.013 0.004 0.0084 0.018 0.011 0.0074 0.024 Zn _ 0.016 - _ _ _ _ Sr 0.473 - _

obsidian objects is shown in table 3, with an estimated uncertainty of 20%, considering stopping power, ion- ization cross section, mass attenuation coefficients, ef- ficiency, surface roughness and angular variations [6-81.

It has been established that the obsidians of a single volcano or of a group of nearby volcanoes contain trace elements that are very similar. An extensive study of the Pacific region obsidians was made by Duerden et al. [8]. Their results for K, Ca, Ti, Fe, Sr, Rb and Zr are particularly useful for source identification. A dis- criminant diagram for K, Ca and Fe is shown in fig. 1 for these data. It is clear that those three elements can be used to identify a source. Fig. 2 shows a similar diagram using these same elements for the obsidian cuttings tools found in the San Gregorio Cave. It shows that the obsidian used for these tools came from the same source, except for one of them. The only obsidian source in the region is the Tres Virgenes Volcano. At this moment, a sample from this source is not avail- able.

Fig. 1. Discriminant diagram using K, Ca and Fe elements to

identify obsidian sources of the South West Pacific region.

Table 4

Elemental composition of two rock fragments containing red and black pigments. The data shown are ratios of peak areas

normalized to Ca. Uncertainties about 10%

Element RR1 RR2 RBl RB2 RE2 RLl RL2 RDl DC

Al 0.003 0.003 0.004 0.003 0.004 0.010 0.005 0.012 0.038

Si 0.024 0.477 0.185 0.054 0.062 0.865 0.266 0.549 0.228

P 0.025 0.001 0.019 0.023 0.030 0.018 0.019 0.012 0.083

S 0.498 0.033 0.290 0.467 0.538 0.044 0.096 0.030 0.824

Cl 0.004 0.072 0.015 0.004 0.008 0.127 0.050 0.066 2.32

K 0.024 0.529 0.174 0.045 0.068 0.480 0.207 0.612 4.64

Ca 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

Ti 0.002 0.124 0.024 0.004 0.006 0.059 0.030 0.084 0.010

Mn 0.00053 0.002 0.005 0.00040 0.00066 0.009 0.003 0.007 0.005

Fe 0.023 0.411 0.130 0.105 0.093 0.148 0.144 0.738 0.086

Co 0.00032 0.006 0.002 0.00057 0.00092 0.004 0.00079 0.005 0.00023

J. Miranda et al. / Cave sediments, prehispanic paintings, obsidian cutting tools 457

BO,” C.l,fo.n,o

K CO

Fig. 2. Discriminate diagram using K, Ca and Fe elements for

the obsidian cutting tools found in San Gregorio Cave.

Two rock pieces containing painting fragments were also analysed. The analysis was carried out for these two fragments in a zone with black pigment (RBl, RB2), and in a zone with red pigment (RRl, RR2). These two rock fragments were analysed also in zones without pigments in order to compare with those zones with pigments. The rock fragment 2 was bombarded in a zone exposed to the environment (RE2). The unex- posed zone was analysed for both fragments: the light tone of the rock (RLl, RL2) and the dark tone (RDI). In this case the quantification was not possible; how- ever, the ratio of peak areas normalized to Ca are shown in table 4. A direct analysis of the dark de- posited cluster (DC), bombarded as it was received, is included for comparison. The estimated uncertainty is about 10%. The elemental composition of the red and black pigments in two rock fragments of a painting did not give sufficient information about the pigments used. There is a great dispersion of the data, probably due to the non-uniformity of the pigment thickness. As we pointed out above, it is very likely that the pigments for

these great paintings were taken from the minerals found in the region, The dark deposited clusters have similar elemental concentrations to those of the soil, except for the significant amounts of Al, S, and K found in the clusters.

Several conclusions can be made. The analysis of rock and soil from within the caves gives an idea of the environment of the painted caves. The painting pig- ments could have been taken from minerals in rocks of the region. The obsidian material to manufacture cut- ting tools could have been taken from more that one source, or, perhaps the human communities who made these paintings were not so isolated as the anthropolo- gists suppose. The analysis of the deposited clusters could be of help to painting restoration project.

Acknowledgement

The authors are indebted to Mr. K. Lopez for accelerator operation.

References

[l] Proc. Int. Workshop Ion Beam Analysis in the Arts and

Archaeology, Nucl. Instr. and Meth. B14 (1986) 1-167.

121 M. Menu and P. Walter, Nucl. Instr. and Meth. B64

(1992) 547.

[3] T. Gill, Sticky mylar as a sample holder for PIXE analy-

sis, Internal Report, Physics-Geology, University of Cali-

fornia, Davis (1989).

[4] J.E Ericson, A. Makishima, SD. Mackenzie and R.

Berger, J. Non-Cryst. Solids 17 (1975) 129.

[5] J. Rickards, A. Oliver, J. Miranda and E.P. Zironi, Appl.

Surf. Sci. 45 (1990) 155.

[61 W.R. Ambrose, P. Duerden and J.R. Bird, Nucl. Instr.

and Meth. 191 (1981) 397.

171 J.L. Campbell, R.D. Lamb, R.G. Leigh, B.G. Nickel and

J.A. Cookson, Nucl. Instr. and Meth. B12 (1985) 402.

181 P. Duerden, D.D. Cohen, E. Clayton, S.R. Bird, W.R.

Ambrose and B.F. Leach, Anal. Chem. 51 (1979) 2350.

[9] E. Bornemisza and K. Igue, Soil Sci. 103 (1976) 347.

[lo] J.E. Gieseking, Soil Components, vol. 1 (Springer, New

York, 1975) pp. 305-341.

VI. GEOLOGICAL SAMPLES