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Whewellite rock crusts in the Lower Pecos Region of Texas.

ABSTRACT. -- The Lower Pecos region of Texas contains some of the most outstanding prehistoric rock paintings found in the New World. Covering the paintings is a natural rock crust that may play an important role in preserving these artifacts. We have determined that the rock crust is composed primarily of calcium oxalate in the mineral form whewellite. The origin of the whewellite is unknown. Key words: whewellite; calcium oxalate; rock crusts; rock art.

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The Lower Pecos region of Texas, generally described as the area surrounding the confluences of the Pecos and Devils rivers with the Rio Grande (Fig. 1), contains one of the longest continuous records of human occupation in North America (Turpin, 1982). Evidence indicates that people began inhabiting rock shelters in the region about 9,000 years ago and remained until historic times (Hester, 1983). Part of the legacy of these ancient Texans is the extraordinary prehistoric rock paintings (pictographs) found in over 250 rock art sites. The most impressive and oldest pictographs are the large, polychromatic Pecos River style pictographs. These artifacts have been reported to be between 3,000 and 4,200 years old, based on radiocarbon dating of organic matter in the paint (Russ et al., 1990; Russ et al., 1992a) and population density (Turpin, 1990). The longevity of the pictographs may be due to the naturally occurring crusts that form on the back wall of the dry rock shelters and cover the paintings. Here we report that this crust is composed primarily of calcium oxalate in the monohydrate mineral form whewellite (Ca[C.sub.2][O.sub.4] * [H.sub.2]O).

Whewellite has been reported as rare in the geological environment (Graustein et al., 1977; Zak and Skala, 1993), but recently it has been identified on quartzite and sandstone rock faces in Australia (Watchman, 1990) and on marble and calcareous limestone monuments, historical buildings and natural outcrops in Italy (Del Monte et al., 1987; Seaward et al., 1989). The origins of whewellite in these environments is still unknown; proposed mechanisms for its production include metabolic activity of encrusting lichen (Del Monte et al., 1987) and organic acids in rain water reacting with calcium-rich dust (Watchman, 1990). Radiocarbon dating experiments of the oxalate-rich crusts in Australia led Watchman to suggest that the crusts may be useful as chronological and paleoenvironmental indicators (Watchman, 1991).

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In the Lower Pecos, two distinct forms of rock accretions cover the limestone rock surfaces: a dark gray to black rock varnish coats areas that are directly exposed to rain and rain run-off while a light brown crust forms in more sheltered areas. Since the majority of the pictographs are in dry rock shelters, it is the light brown crust that covers the artifacts. This material is translucent, allowing the prehistoric paint to show through, though giving them a "faded" appearance. Zolensky (1982) identified the mineralogy of various pigments used in some of the prehistoric rock paints from the Lower Pecos using x-ray diffraction, but reported only calcite and gypsum as the minerals composing the rock crusts that cover the paintings. During chemical analysis of several prehistoric paints we identified oxalate as the primary component in the crusts. The aim of the study we report here was to determine the regional occurrence of oxalates in the Lower Pecos and to identify the mineral form.

MATERIALS AND METHODS

Samples of rock crusts were collected from sixteen rock shelters throughout the Lower Pecos region. Samples were taken from the shelter wall directly exposed to sunlight as well as regions remaining shaded throughout the day. All samples were from areas that are not exposed to rain, rain run-off or ground water seepage. Paint samples were also collected from two sites. The samples were analyzed using optical microscopy, scanning electron microscopy (SEM) with an energy dispersive x-ray analyzer (EDS), Fourier transform infrared spectrometry (FTIR) and powder x-ray diffractometry (XRD).

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The SEM/EDS analyses were carried out using a JEOL 6400 SEM equipped with a Noran I-2 Integrated Imaging X-Ray Microanalysis System. Samples were prepared by thin-sectioning after immersing in epoxy or by attaching fragments of the crusts to one centimeter aluminum stubs. The samples were gold sputter coated prior to placing in the SEM.

We used a Nicolet 510P FTIR with a Nicplan infrared microscope to obtain the infrared absorbance spectra. This system allows the infrared beam to be focused through small scrapings of the crust (~0.1 mg) placed in a diamond compression cell. Multiple analyses from each sample were compared for consistency.

The powder XRD was performed using a Rigaku computer automated D/Mx IIIV BX X-ray Powder Diffractometer with a monochromator for selecting the copper K [alpha] radiation. Samples were prepared by scraping the crust from the limestone substrate using a stainless steel dental pick and then grinding it in an agate mortar and pestle.

RESULTS

The surface of the crusts have a botryoidal morphology (Fig. 2), with colors ranging from light brown (Munsell designation 5 YR 6/4) to grayish orange (Munsell designation 10 YR 7/4) (Munsell, 1951). When viewed in cross-section under an optical microscope, samples show three distinct layers: the surface rock crust, a white microcrystalline interface and the limestone substrate. Samples containing paint show an additional strata with the paint lying between the crust and interface (Russ et al., 1992b). The thickness of the crust layer averaged 170 [micro]m with a maximum thickness of 850 [micro]m. Only one sample showed stratification of the crust; in that case two layers were observed. This is in contrast to the oxalate-rich crusts in Australia and Italy where multiple layers are common.

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Using FTIR, we determined that the crusts from all 16 sites contained oxalate as the primary component. Figure 3 shows an overlay of FTIR spectra of (a) calcium oxalate standard and (b) a sample of crust collected from archaeological site 41VV576. The only significant difference in the two spectra is a broad peak between 1000 to 1100 wavenumbers in the spectrum of the crust. Because both sulphates and silicates absorb in this region, this peak is likely due to gypsum and/or quartz. Figure 4 illustrates the similarities of the FTIR spectra of (a) a sample of red paint from site 41VV576 and (b) a sample of crust also from site 41VV576, indicating that the primary component in the paint layer is oxalate.

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SEM observation showed that the oxalate is microcrystalline or amorphous, with distinct gypsum (CaS[O.sub.4] * 2[H.sub.2]O) crystals dispersed throughout the oxalate phase (Fig. 5). Spherical quartz grains were also seen in several samples. Using EDS we determined that the oxalate crusts contained higher concentrations of the elements aluminum and silicon relative to the substrate, probably from aolian clays. The presence of quartz and clays in the crusts implies that particulate matter accumulated in the crust as it formed.

Results from XRD analyses indicate that the oxalate is in the monohydrate mineral form whewellite. Weddellite, the polyhydrate form of calcium oxalate, was not evident in any of the samples. Gypsum was also identified in the crusts as was quartz and calcite. Figure 6 shows a XRD spectrum of a crust sample from Goat Bone Shelter.

[FIGURE 5 OMITTED]

DISCUSSION

Whewellite was the primary component of the rock crusts from all 16 sites sampled. This may indicate that the mineral is ubiquitous to the dry rock shelters of the Lower Pecos. The origin of this material is unknown, but we suggest it is the result of metabolic activity of lichen or fungi at the surface of the limestone substrate. The significance of oxalate-rich rock crusts in the Lower Pecos region is that it may provide a means for determining chronologies of the rock paintings without compromising the art and be used as a paleoenvironmental indicator. Furthermore, understanding the processes that form the accretions is imperative for developing viable conservation strategies for the Lower Pecos pictographs.

ACKNOWLEDGMENTS

This paper was delivered, in part, at the 1992 Texas Archaeological Society meeting. We thank J. Labadie, National Park Archaeologist, for his valuable help during our field work. This work was supported, in part, by the Welch Foundation, the Texas Archaeological Society and Sam Houston State University Research Enhancement Funds.

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LITERATURE CITED

Del Monte, M., C. Sabbioni, and G. Zappia. 1987. The origin of calcium oxalates on historical buildings, monuments and natural outcrops. The Science of the Total Environment, 67:17-39.

Graustein, W. C., K. Cromack, and P. Sollins. 1977. Calcium oxalate: occurrence in soils and effect on nutrient and geochemical cycles. Science, 198:1252-1254.

Hester, T. R. 1983. Late Paleo-Indian occupations at Baker's Cave, southwestern Texas. Bull. Texas Archaeological Soc., 53:101-120.

Munsell Rock-Color Chart. 1951. Geological Society of America, New York.

Russ, J., M. Hyman, H. J. Shafer, and M. W. Rowe. 1990. Radioocarbon dating of prehistoric rock paintings by selective oxidation of organic carbon. Nature, 348:710-711.

Russ, J., M. Hyman, and M. W. Rowe. 1992a. Direct radiocarbon dating of rock art. Radiocarbon, 34:867-872.

______. 1992b. Dating and chemical analysis of Pecos River style pictographs. Pp. 35-42, in American Indian Rock Art Volume XVIII (F. G. Bock, ed.), American Rock Art Research Association, San Miguel, 118 pp.

Seaward, M. R. D., C. Giacobini, M. R. Giuliani, and A. Roccardi. 1989. The role of lichens in the biodeterioration of ancient monuments with particular reference to central Italy. International Biodeterioration, 25:49-55.

Turpin, S. 1982. Seminole Canyon: The Art and Archeology. Texas Archeological Survey Research Report 83. The Univ. Texas at Austin, 293 pp.

______. 1990. Speculations on the age and origin of the Pecos River Style, Southwest Texas. Pp. 99-122, in American Indian Rock Art 16 (S. A. Turpin, ed.), A joint publication of the American Rock Art Research Association and The Univ. Texas at Austin, 252 pp.

Watchman, A. 1990. A summary of occurrences of oxalate-rich crusts in Australia. Rock Art Research, 7:44-50.

______. 1991. Age and composition of oxalate-rich crusts in the Northern Territory, Australia. Studies in Conservation, 36:24-32.

Zak, K., and R. Skala. 1993. Carbon isotope composition of whewellite (Ca[C.sub.2][O.sub.4]*[H.sub.2]O) from different geological environments and its significance. Chemical Geol., 106:123-131.

Zolensky, M. 1982. Analysis of pigments from prehistoric pictographs Seminole Canyon State Historical Park. Pp. 279-284, in Seminole Canyon: The Art and Archeology. Texas Archeological Survey Research Report 83. The Univ. Texas at Austin, 293 pp.

JON RUSS, RUSSELL L. PALMA, AND JAMES L. BOOKER

Department of Chemistry, Sam Houston State University, Huntsville, Texas 77341, Department of Physics, Sam Houston State University, Huntsville, Texas 77341, and Analytical Group Leader, Ralph Wilson Plastics Co., Temple, Texas 76503
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Author:Russ, Jon; Palma, Russell L.; Booker, James L.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:May 1, 1994
Words:1783
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