Making Lemonade: Using Graffiti to date Petroglyphs

Transcription

1 Making Lemonade: Using Graffiti to date Petroglyphs J. A. McNeil Physics Department Colorado School of Mines I. II. III. IV. V. Desert Varnish and XRF Description of the experiment Data calibration and analysis How old is it? Absolute? Relative? Future work

2 Desert Varnish and Rock Art Dating Desert varnish accumulates with time through a process that binds Mn-Fe rich microbial casts to clay minerals. (Dorn, 1981, 2007) CR/AMS analysis of DV samples removed from rock art has been used to estimate dates (Francis & Loendorf, 2002) Manganese (Mn) is a significant elemental component of desert varnish that is often at trace levels in the substrate rock. As such, under certain conditions, it may be possible to estimate the age of rock art using Mn levels measured with x-ray fluorescence. (Lytle, 2004 and previous talk!) XRF non-destructive - no physical samples. Dorn, 2007

3 XRF Process x-ray

4 XRF Process e

5 XRF Process x-ray

6 Characteristic Radiation

7 The XRF Device: Schematic

8 The XRF Head Unit

9 Specifications X-ray source: kv µa 1.58 mm diameter beam spot Platform: 12' tripod with 2.5' boom 0.1 mm x-y translation Detector: 0.7 mm2 Si PIN Peltier cooled ~0.2 kev resolution

10 XRF Unit Deployed at Dancing Bear site near Dewey Bridge, Ut

11 Spectrum from Mn thin-film Calibration Target Mn Kα Ca Kα Mn Kβ

12 Graffiti with Rock Art

13 Graffiti with Rock Art: Dancing Bear Panel US128 near Dewey Bridge, Ut Sample DB-1964a-b

14 Graffiti with Rock Art: River House Ruin Comb Wash, Ut Sample RH-1885

15 Graffiti with Rock Art: Sand Island, Ut Sample SI-1963a Sample SI-1963b

16 Spectrum from Sand Island Base rock Fe Kα K Kα Fe Kβ Si Kα Cl Kα Sm Lα Ti Kα V Kα Ni Kα Zn Kα

17 Spectrum from SI-1963a: Big Horn Glyph Fe Kα Mn Kα Si Kα Fe Kβ K Kα Ti Kα

18 Spectrum and Peak Fit for the Mn-Fe region from Sample SI-1963a Fe Kα Sm Mn Fe Total Mn Kα Fe Kβ

19 Mn-region for Graffito For Sample SI-1963a Fe Kα Sm Mn Mn Kα Sm Kα Fe Total

20 Combined Spectrum and Peak Fit for the Mn-Fe region from Sample SI-1963a Quadruped Sm 3 Mn Fe Total

21 Preliminary Results Sand Island, Utah graffiti calibration = years/(µg/cm2) Mn (µg/cm2) χ2/df "Age" (ybp) (linear model) SI-1963a Big-horn sheep / /- 43 (12%) SI-1963a "3" graffito / SI-1963b Lizard glyph / SI-1963b "1" graffito / Sample 389 +/- 75 (16%) 90% confidence level SI-1963a SI-1963b

22 Preliminary Results Sand Island, Utah Using Lytle, et al. Calibration Sample SI-1963a Big-horn sheep SI-1963a "3" graffito SI-1963b Lizard glyph SI-1963b "1" graffito 90% confidence level Mn (µg/cm2) / / / / χ2/df "Age" (ybp) 130 y/(µg/cm2) / /- 880

23 Preliminary Results Dancing Bear Site - Dewey Bridge, Utah Sample DB-1964a Lobe Head DB-1964a "Bear paw" DB-1964b "Solar Deity" DB-1964a 1964 Mn (µg/cm2) / / / /-.59 "Age" (ybp) Graffiti-based 931 +/ / /- 245 c=18.3 y/(µg/cm2) "Age" (ybp) 130 y/(µg/cm2) 6,660 +/- 1,660 4,340 +/- 1,085 4,340 +/- 1, /- 83

24 Relative dating Manganese measurements of all the Petroglyphs at a site of multiple habitations could enable one to create a time-lapse animation. Simulated example Dancing Bear site

25 Simulated time-lapse

26 Summary, Conclusions and Future Directions Constructed and deployed a portable XRF device capable of measuring Mn in desert varnish at ~0.3 µg/cm2 level Measured Mn in graffiti and adjacent rock art at several sites in eastern Utah. Relative dating at single site, e.g. Dancing Bear glyphs time-lapse potential Absolute dating via linear calibration from graffiti: ~14-18 years/(µg/cm2). (much smaller than Lytle, et al.'s 130 years/(µg/cm2)) More research is needed to reconcile the measurements before the full promise of non-destructive XRF measurements of Mn is realized. Future work: 1. cross-correlate with each other and other methods (AMS/CR) 2. need to expand vastly the experimental data base 3. understand better desert varnish accumulation alternative models

27 Acknowledgements Thanks to Lynda McNeil (CU), Ed Cecil (CSM), Joe Beach (CSM), Leigh Grench (BLM), and Winston Hurst (Abajo Arch.) Farrel Lytle (calibration) Comb Ridge Heritage Project, PI - C. Cameron (CU)

28 References Cole, S Legacy in Stone, Johnson Books, Boulder, Co. Dorn, R Microbial Origin of Desert Varnish. Science 213: Rock Varnish, Chapter 8 in Geochemical Sediments and Landscapes, edited by D. J. Nash and S. J. McLaren, Blackwell, London. Francis, J. E. and Lowndorf, L. L Ancient Visions: Petroglyphs and Pictographs of the Wind River and Bighorn Country, Wyoming and Montana, University of Utah Press, Salt Lake, Ut. Tertian, R. and Claisse, F Principles of Quantitative X-ray Fluorescence Analysis, Heyden and Son, Ltd., London.