Archaeological Chemistry - American Chemical Society

Chapter 8

Surface Analysis of a Black Deposit from Little Lost River Cave, Idaho Reshmi Perumplavil and Ruth Ann Armitage Downloaded by AUBURN UNIV on February 20, 2018 | https://pubs.acs.org Publication Date: August 16, 2007 | doi: 10.1021/bk-2007-0968.ch008

Department of Chemistry, Eastern Michigan University, Ypsilanti, MI 48197

A black coating of unknown origin obscures the pictographs in Little Lost River Cave, Idaho. We have utilized x-ray photoelectron spectroscopy (XPS) to characterize the outermost surface of the black coating, as this material has previously been used to provide a minimum age for the underlying rock paintings. Carbon and oxygen predominated in the X P spectra, whereas nitrogen was detected at varying levels in different samples of the coating. High-resolution carbon 1s X P spectra showed that carbon was present in at least three different forms: hydrocarbon C, carbonyl C, and amide/carbonate C. The Ν 1s peak was observed at ~399-400 eV binding energy, which is usually attributed to aromatic or amide N. The X P S results are consistent with identification of the coating as a water-deposited layer of humic substances from the overlying soil. As this would be a geologic deposit, the radiocarbon age determined for the coating does not likely relate to the age of the rock paintings.

152

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Glascock et al.; Archaeological Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

Downloaded by AUBURN UNIV on February 20, 2018 | https://pubs.acs.org Publication Date: August 16, 2007 | doi: 10.1021/bk-2007-0968.ch008

153 Little Lost River Cave, in south-central Idaho, is of archaeological interest because of its collection of red and yellow anthropomorphic and zoomorphic rock paintings. The paintings are obscured by a shiny black coating, which was observed in the first investigation at the site in 1954 (7), and noted again in excavations at the site in 1990 (2). In 2001, this material was sampled in the hope that by determining the radiocarbon age for the coating, a minimum age for the underlying paintings could be determined (5). Previous analyses of this material have indicated that it is rich in organic carbon (4). Steelman et al. proposed that the coating may be the result of cooking fires within the cave, and obtained for it a radiocarbon age of 2990±50 uncalibrated years B . P . (5). Pyrolysis-gas chromatography-mass spectrometry (py-GC-MS) studies (6) have shown that the cooking residue hypothesis proposed by Steelman et al. (4) is not likely because the material bears a strong resemblance to humic and falvic acids. Depending on how the coating formed - through deposition of ground water or condensation from smoke and/or cooking - the surface composition of the coating should differ. Thus this work focuses on the formation mechanism and looks at the material from a surface standpoint using X-ray photoelectron spectroscopy (XPS). Characterization of the surface of the coating is important in validating the radiocarbon date obtained by Steelman et al. (5). Plasma-chemical oxidation (PCO) was used in that case to oxidize the organic material at the surface of the coating to carbon dioxide, which was subsequently radiocarbon dated using accelerator mass spectrometry. The P C O sample preparation technique for radiocarbon dating rock paintings was pioneered by Russ et al. (7), and has since been proposed as a "nondestructive" technique for dating irreplaceable artifacts (8, 9). The low-temperature, low-pressure oxygen plasma discharge selectively oxidizes organic carbon in the presence of inorganic carbon as carbonates and oxalates (70). As the interaction with the plasma occurs primarily at the surface of the material, it is important to know that the organic material at that surface stems from an archaeologically relevant event. In this case, that means that the coating surface must not be of a primarily geologic origin i f it is to provide a minimum age for the underlying pictographs. The X P S mechanism, which can be used for quantitative and qualitative chemical analysis of surfaces, is based on the photoelectric effect. A monochromatic soft M g or A l anode X-ray source is used to irradiate the surface. The absorbed X-rays ionize the core shell, and in response, the atom creates a photoelectron that is transported to the surface and escapes. The ionization potential of a photoelectron that must be overcome to escape into vacuum is the binding energy (BE) plus the work function of the material. The emitted photoelectrons have a remaining kinetic energy (KE), which is measured by using an electron analyzer. Individual elements can be identified on the basis of their B E . The resulting X P spectrum is a characteristic set of peaks for a specific element, with B E as the abscissa and counts per unit time as



154 the ordinate. The B E of an element depends upon its oxidation state and environment. Hence, changes in the chemical state of an element give rise to shifts in peak positions. The quantification of each element depends on peak areas and sensitivity factors that correct for several instrumental parameters. The surface specificity of X P S is due to the short range of photoelectrons that are excited from the sample; photoelectrons cannot escape without energy loss during transport to the surface. Thus, X P S is sensitive to only the top 3-5 nm of a surface. It can detect all the elements in the periodic table except for H and He, which have very low ionization cross-sections. It can be used in combination with argon ion sputtering to examine the changes in a sample with depth. Ultra high vacuum (UHV) conditions are required to carry out any X P S measurements, and, thus, samples must be vacuum compatible. The surface specificity and ability to distinguish different chemical environments makes X P S a powerful technique in the analysis of thin films and coatings. X P S has not been widely used in the study of archaeological materials. Ciliberto and Spoto include a chapter on the past uses of X P S in archaeological sciences, describing applications to pottery, metallic objects, paintings, pigments, and the degradation of ancient paper (77). Lambert et al. (12) provide an extensive review of the application of X P S to archaeological materials. X P S , along with magnetic microanalytical methods, was used to differentiate organic and inorganic materials in black paints on pottery from the American Southwest (75). Using X P S and scanning Auger microscopy, Paparazzo (14) studied organic carbon-bearing species as the major source of carbon enrichment observed at the surface of a lead pipe (fistula) and a Roman bronze statue. The ability of X P S to provide chemical information from shifts in binding energy and its non-destructive nature makes it a complement to other analytical techniques used in the study of archaeological materials. XPS will aid in understanding specifically the surface of the black deposit covering pictographs in Little Lost River Cave in Idaho. This work will complement other bulk analyses carried out with pyrolysis-GC-MS and thermally assisted hydrolysis /methylation (THM)-GC-MS (75). The objectives of this project were to use X P S to qualitatively determine the surface elemental composition of the black residue; semiquantitatively characterize the surface, for comparison with other surface-related materials; and examine the relationship between the chemistry and depth by using A r sputtering. This, then, will aid in validating the radiocarbon date obtained through plasma-chemical oxidation and accelerator mass spectrometry by Steelman et al. (5). +

Methods and Materials Photoelectron spectra were recorded on a Physical Electronics, Inc. 5100 Series X P spectrometer. Samples were irradiated by use of a M g Κα


155 monochromatic x-ray source (photon energy = 1253.6 eV) operated at 15 k V and 300 W . The kinetic energies of the photoelectrons were analyzed by a hemispherical electron energy analyzer with a work function of 4.17 eV and constant pass energy. A l l experiments were performed under ultra high vacuum ( U H V ) conditions with an overall base pressure of 10- Torr. A 4-mm diameter analysis area was used. The survey and high-resolution scans were collected using pass energies of 143.05 eV and 44.75 eV respectively. For depth profiling studies, the surface was etched using an A r gun operated at 20 mA with a chamber pressure of 1 χ 10" Torr. The resulting data were then treated by use of standard X P S methods. A n asymmetric background correction using the Shirley function (described in detail by Castle and Salvi (76)), used for all spectra, and peaks were fit by using a mixed Gaussian-Lorenzian distribution. Quantitation was performed with the equation (AJS) * ZA/S where Ai is the peak area and S, is the atomic sensitivity factor for the element / being determined (77). Atomic sensitivity factors are empirical constants determined on standards of the elements (75). This yields an atom percentage (atom %) for each element at the surface of the material. 9

+

7


w

a

s

it

Several samples of the coating from Little Lost River Cave were studied using X P S . These materials were collected during three separate trips to the cave: once in 2001, and in 2004 (provided by C. Merrell, Archaeographics), and once in 2005 by one of us (RAA). Because the surface of a sample to be examined using X P S must be flat, only selected samples of the coating were appropriate. Table I describes the samples that were examined/Figure 1 shows the corresponding locations where these samples were collected in the cave. Standard materials were also investigated for comparison. These included two humic acids, one from Alfa Aesar (CAS# 1415-93-6) obtained from an unknown source, and one from the International Humic Substances Society (IHSS) obtained from standard Elliot soil. IHSS fulvic acid from Elliot soil was also used for comparison. Humic acids are ubiquitous geologic contaminants present in archaeological materials that have been exposed to soil or groundwater, and are important comparative materials for surfaces that are to be (or in this case, have been) radiocarbon dated by the P C O - A M S technique.

Results X P S spectra obtained on the uncoated dolomite sample (Figure 2) showed the presence of carbon, oxygen, and calcium, as would be expected for a calcium carbonate material. Traces of chlorine, probably present as precipitated salts were also observed by electron microprobe analysis in previous studies (4). The high-resolution C Is spectrum, however, showed that carbon was present in at least three different chemical states (Figure 2, inset). The peak occurring at a B E of 284.3 eV is consistent with the presence of adventitious hydrocarbon


156


Table I. Little Lost River Cave Coating Samples Studied Using XPS Sample and map location 1

Year collected 2001

2

2004

3 4

2004 2005

5 6 7 D

2005 2005 2005 2005

Description Dolomite fragment with thin layer of shiny black coating Shiny black coating fragment with little dolomite substrate Sooty black coating on dolomite substrate Sooty black coating with thin dolomite substrate Shiny black coating without visible substrate Shiny black coating without visible substrate Shiny black coating on the dolomite substrate Uncôated dolomite from roof spall inside cave

Figure 1. Map of Little Lost River Cave (10BT1), showing sampled locations. Numbers correspond to samples listed in Table I. Map is based on that of Gruhn and Bryan (2).


157 from the turbomolecular pump, or from hydrocarbon-like material on the surface of the dolomite. The 288.5 eV B E peak corresponds to carbonate carbon; both the carbonate and adventitious carbon peaks were also observed for a clean carbonate standard. These B E values are standard for carbonate and adventitious carbon reported in the literature as well (77). However, the third peak at 286.2 eV may correspond to carbonyl moieties at the surface, possibly due to contamination from aeolian dust, which is ubiquitous in the cave. This is consistent with literature values for soils (19).


'1s

High resolution J C 1s spectrum J\ adventitious C 1

284.3 eV

carbonate

//

I . carbonyl?

288.5 eV

//

y

286.2 eV

KLL

800

1000

600

400

200

Binding Energy, eV Figure 2. Survey spectrum of uncoated dolomite from 10BT1 (inset: highresolution Cis spectrum showing presence of at least three chemical states of C).

Survey Spectra X P survey spectra of the coating samples also showed an elemental composition for the shiny coating which was consistent with that observed by Steelman et al. (4) by electron microprobe analysis. The spectra of the shiny coating samples are dominated by carbon and oxygen, with small amounts of nitrogen and calcium and traces of chlorine. Figure 3 shows the survey spectra of the most recently sampled shiny material (Samples 5-7) and a sooty coating


158

Ο


sample

1000

Θ00

600 400 Binding energy, eV

200

0

Figure 3. XPS survey spectra for four samples of the coating. All labeled peaks correspond to Is electrons, except Ca, which is from a 2p electron.


159


(Sample 3) for comparison. Aluminum peaks were observed from the sample stubs. Nitrogen is found at varying levels in the coating, and does not appear to correspond to the appearance of the material. However, the dolomite survey spectrum also showed significant calcium signals that do not appear in all of the coating samples; when Ca is present, the concentration is less than 1 atom %. This shows that X P S is truly showing the composition of the coating's outermost surface and is not penetrating through to the dolomite substrate. This is significantly different from the earlier microprobe analysis (4). Hence the carbon signals in the X P spectra of the samples are specific to the coating, particularly when no C a signal is observed.

High-Resolution Spectra X P survey spectra are not particularly useful for identifying a surfacedeposited material, as they are purely qualitative. High-resolution spectra are more informative and can be used semiquantitatively. As in the survey spectra, the x-axis corresponds to the binding energy in eV, and the y-axis is counts. Carbon is found in different oxidation states in the coating, as can be seen in Figure 4.

sample 6

sample 5

sample 3

sample 7

Figure 4. High-resolution C spectra for four coating samples. Peak A (284 eV) corresponds to C-C, C-H (adventitious); Peak Β (287 eV) may correspond to C-O; and Peak C (289 eV) may be carbonate or amide C.



160 For the high-resolution carbon spectra, peak A corresponds to adventitious carbon, typically resulting from the ubiquitous hydrocarbon present in the highvacuum chamber from pump oil. This peak is often used as a standard by which charging effects can be determined, as it is expected at 284.3 eV (20). The coating samples exhibited little charging, exhibiting a shift of only +1-2 eV. For example, peak A was observed at 286 ± 1 eV, compared to the expected 284.3 eV. Peak B , at a charging-corrected value of 287 eV, may correspond to carbonyl or some other functionality wherein C is bonded to O. Peak C, at a corrected B E of 289 eV, is either carbon in an amide functional group, or carbonate. Identification of Peak C is dependent on the presence of either Ca or N . Sample 3 shows a significant Ν peak at a B E of -400 eV. When Ν is present in such large quantities, this could indicate that the C g9 signal in Figure 4 corresponds mostly to amide carbon rather than to carbonate (which may also be present, but to a lesser extent). For comparison, the C 89 signal observed in the dolomite standard (peak C) in Figure 5 must correspond to carbonate because there was no significant nitrogen present on the surface of that sample. 2

2

Dolomite from 10BT1 (D)

IHSS fulvic acid

Alfa Aesar humic acid Figure 5. High-resolution C spectra for three of the standards. Peak A corresponds to C - Ç C-H (284 eV adventitious); Peak Β may correspond to C=0 (288 eV); and Peak C may be carbonate or amide C (289 eV). t


161 Humic and fulvic acids show carbon peaks in Figure 5 at the same three binding energies as were observed in the coating samples. Other authors using X P S have also studied C in humic and fulvic acids from dissolved organic matter, fitting significantly more peaks to the C i region. Bubert et al. (27) fit four peaks with the aid of C N M R ; as this study used an ΑΙ Κα source without an additional monochromator, it is the most comparable to what we present here. A monochromatized x-ray source provides narrower peak shapes in highresolution X P S . Monteil-Rivera et al. (79) were able to fit eight peaks to the C region, using a monochromatized source and C magic angle spinning solidstate N M R . The high-resolution C spectra presented here were fit with only three peaks; the minimum full width of a high-resolution C peak at half maximum height ( F W H M ) under the conditions used was approximately 1.7 eV. Thus, the observed range from 284 eV to 289 eV could contain only three peaks at this width. High-resolution nitrogen Is signals of the coating samples were observed at 399-400 eV. Nitrogen signals at 399-400 eV are indicative of aromatic or amide nitrogen (22), and have been reported in typical soil humic and fulvic acids in studies of soil particles (19, 22). The N peaks for the IHSS fulvic and humic acid standards were observed at binding energies around 401-402 eV; because +1-2 eV charging was observed for the C 84 peak, the N of these materials also occured at 400 eV, as expected from the literature (19, 22). Nitrogen levels in the samples varied from trace to quantifiable amounts. It was also noticed that nitrogen levels varied irrespective of the physical appearance of the coating: the shiny black samples were not consistently high in N . The presence of nitrogen, too, was consistent with the bulk analysis by pyG C - M S and T H M - G C - M S (6, 15), which showed significant amide and aromatic nitrogen in the samples (likely derived from the proteinaceous moieties of humic and fiilvic acids). s

1 3

l s


1 3

l s

2

! s

Quantitative Results Table II shows the quantitative X P S results for all of the samples studied. Not unexpectedly, the dolomite substrate showed the least amount of carbon of all the samples. Two samples, Sample 2 (the shiny coating with little substrate) and Sample 3, had high levels of nitrogen, significantly more than would be expected for a humic or fulvic acid. The atomic compositions of the other coating samples compared well with experimental and theoretical C, N , and Ο contents for humic acids. The standard humic acid from Alfa Aesar contained a significant amount of Κ as an impurity. The origin of this particular humic acid is not known, even to Alfa Aesar. Traces of Ca, CI, Fe, and Si are likely from dissolved salts that are deposited at the cave surface.


162 Table II. Elemental Composition of Samples and Standards


Material

C, atom%

O, atom%

N, atom%

Dolomite (D)

36.9

41.8

trace

Sample Sample Sample Sample Sample

59.8 60.1 63.3 54.2 67.7

28.1 27.8 28.8 32.8 3.1

84

10.2

11.1 12.1 trace 4.5 not detected 3.6

64.0 71.3 64

25.3 23.2 32

5.8 4.1 4.4

2 3 4 5 7

Humic acid (Alfa Aesar) IHSS humic IHSS fulvic Calculated humic acid*

Other (atom%) 7.8% Ca, 10.3% Si, 1.2% CI trace Ca, CI trace Ca trace Ca trace Ca trace Ca 2.6% Κ

*for typical soil humic acid, based on data in Troeh and Thompson (23).

The high percentage of carbon in the coating, compared to that of the uncoated (but not "clean") dolomite, indicates the presence of organic carbon, as had first been proposed by Fichter et al. in the elemental analysis done in the 1950s (7). However, the origin of this organic carbon is significant. If it was deposited by human activity in the cave (e.g., cooking fires) or a brush fire, and post-dated the red and yellow pictographs, then the coating's age would provide a minimum age for the paintings. However, because the coating surface composition seems to be consistent with that of soil humic acids, either from the aeolian dust coating most of the surfaces in the cave, or deposited by penetrating ground water, the age of the coating is more likely not related to that of the paintings.

Depth Profiling Depth profiling was performed on several of the 10BT1 samples in order to determine how and i f the coating changes in composition with depth. Unfortunately, the coating sample surfaces were cracked and uneven, making sputtering of the surface difficult, and the resulting spectra were difficult to interpret. A plot of atom percent vs. sputtering time for the sooty sample


163 (Sample 3) is shown in Figure 6a. The total C decreases with sputtering, but Ca does not increase. Thus, even sputtering for 170 min cannot remove the sooty coating and reveal the dolomite underneath. Figure 6b shows that the C 84 signal, typically associated with adventitious carbon, did decrease after 10-20 min of sputtering the surface. Therefore, the aliphatic carbon observed on the surface of the coating samples was truly observed in the coating itself and was not simply an artifact of the high-vacuum system.


2

Figure 6. Result ofAr ion sputtering of Sample #1: (a) all elements, (b) carbon only, showing different chemical states present.


164


Conclusions The coating samples from different parts of the cave exhibited differences in surface atomic composition and nitrogen levels. High levels of carbon in the coating surface, and varying chemical shifts for the C signals, are indicative of the presence of organic carbon and were also observed during prior analyses of the coating (4, 5). Bulk analysis using py-GC-MS and thermally-assisted hydrolysis/methylation-GC-MS also show the coating to be a complex material (6, 15). This may indicate that there are several sources of the organic material, some of which (e.g., soot, i f it is present) might be useful in dating the underlying paintings, i f they could be selectively separated from the coating matrix. The nitrogen peaks in the high resolution X P spectra at -399-400 eV B E suggest the presence of aromatic and amide moieties in the coating surface. Hydrocarbon (C-C/C-H) at 284 ± 1 eV B E , carbonyl (C=0) at 287 eV B E and amide/carbonate C at 289 eV B E were observed at the surface of the samples. Nitrogen and carbon results were comparable to soil humic and fulvic acid standards. X P S is a useful surface technique that complements bulk analyses, and in this case provides an understanding of the organic material that would be oxidized by the plasma-chemical method. Surface and bulk analysis (6, 15) suggest that the surface bears a significant resemblance to soil humic acids. Sputtering of the samples did not show significant changes in composition, as might be expected for a coating that formed through smoke and cooking residue depositing on the surface from the air. Instead, it seems to support the idea that the coating formed through the deposition of humic acids as ground water moved through the cave and dried on the wall and ceiling surfaces. The trace elements such as Ca, Si, CI, and Fe seen in the X P spectra may arise from salts on the surface, which can occur over time through exposure to the elements. The results demonstrate that the radiocarbon date previously obtained from the black coating does not provide a reliable minimum age for the underlying pictographs.

Acknowledgements Special thanks to Simon Garrett (California State University - Channel Islands) and Todd Bryden (Henkel Technologies, Inc.) for their guidance and instrumentation help with the X P S . Carolynne Merrell, Archaeographics, provided some of the coating samples and additional information on Little Lost River Cave. R A A also thanks Richard D. Hill, Idaho B L M archaeologist, and Daniel Fraser for their aid during the site visit in 2005.


165

References 1.

2.


3.

4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17.

18. 19.

Fichter, Ε.; Hopkins, M.; Isotoff, Α.; Liljeblad, S.; Lyman, A. R.; Strawn, M.; Taylor, E. A. Exploratory Excavation in Little Lost River Cave No. 1; Idaho State College: Moscow, ID; unpublished report, 1954. Gruhn, R.; Bryan, A. Report on Test Excavation at Little Lost River Cave No. 1 (10BT1) in 1990; Report on Cultural Resource Permit ID-1-27727; Bureau of Land Management: Idaho Falls, ID, 1990. Merrell, C. L.; Hill, R. D. Pictograph Documentation from the Little Lost River Cave No.1 (10BT1); Report on Agreement No. DAA000103; Bureau of Land Management: Idaho Falls, ID, 2001. Steelman, K . L.; Rowe, M. W.; Guillemette, R. N.; Merrell, C. L.; Hill, R. D. American Indian Rock Art 2002, 28, 111-120. Steelman, K . L.; Rowe, M. W.; Boutton, T. W.; Southon, J. R.; Merrell, C. L.; Hill, R. D . J. Archaeol. Sci. 2002, 29, 1189-1198. Fezzey, S. T.; Armitage, R. A. J. Anal. Appl. Pyrolysis 2006, in press. Russ, J.; Hyman, M.; Shafer, H . J.; Rowe, M. W. Nature 1990, 348, 710711. Steelman, K . L.; Rowe, M. W. In Archaeological Chemistry: Materials, Methods, And Meaning; Jakes, K . , Ed.; A C S Symposium Series No. 831; American Chemical Society: Washington, DC, 2002; pp 8-21. Steelman, K . L.; Rowe, M. W.; Turpin, S. Α.; Guilderson, T.; Nightengale, L. Am. Antiq. 2004, 69, 741-750. Rowe, M. W. In Handbook of Rock Art Research; Whitley, D. S., Ed.; AltaMira Press: Walnut Creek, C A , 2001; pp 139-166. Modern Analytical Methods in Art and Archaeology; Ciliberto, E.; Spoto, G., Eds.; John Wiley & Sons, Inc.: New York, N Y , 2000. Lambert, J. B.; McLaughin, D. C.; Shawl, E. C.; Xue, L. Anal. Chem. 1999, 71, 614A-620A. Stewart, D . J.; Adams, R. K . ; Borradaile, J. G.; MacKenzie, J. A. J. Archaeol. Sci. 2002, 29, 1309-1316. Paparazzo, E . Archaeometry 2003, 45, 615-624. Brown, J. A. M. S. thesis, Eastern Michigan University, Ypsilanti, M I , in preparation. Castle, J. E.; Salvi, A. M. J. Vac. Sci. Technol. A. 2001, 19, 1170-1175. Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F., Eds.; In Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G . E., Ed.; Perkin-Elmer Corporation: Minnesota, 1979; ρ 189. Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. Α.; Raymond, R. H . ; Gale, L. H . Surf. Interface Anal. 1981, 3, 211-225. Monteil-Rivera, F.; Brower, E.B.; Masset, S.; Deslandes, Y . ; Dumonceau, J. Anal. Chim. Acta 2000, 424, 243-255.


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20. Johansson G . ; Hedman J.; Berndtsson Α.; Klasson M.; Nilsson R. J. Electron Spectrosc. Relat. Phenom. 1973, 2, 295-317. 21. Bubert, H . ; Lambert, J.; Burba, P. Fresenius J. Anal. Chem. 2000, 368, 274-280. 22. Abe, T.; Watanabe, A . Soil Science 2004, 169, 35-43. 23. Troeh, F. R.; Thompson, L . M. Soils and Soil Fertility; Oxford University Press: New York, 1993;p101.


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