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Try out PMC Labs and tell us what you think. Learn More. We investigated rock varnish, a thin, manganese- and iron-rich, dark surface crust, on basaltic lava flows and petroglyphs in the Owens and Rose Valleys California by portable X-ray fluorescence pXRF and femtosecond laser-ablation inductively-coupled-plasma mass spectrometry fs-LA-ICPMS.

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Desert varnish, also called rock varnish, is a thin biogenic layer of Mn-oxides, Fe-oxides, and clays that coats rock surfaces in arid and semi-arid regions. The mass of these metals in the varnish registers cumulative biologic activity over time and presents a possible dating mechanism, subject to appropriate assumptions and restrictions. We have used a portable x-ray fluorescence PXRF unit to measure Mn and Fe in numerous desert varnishes, both in the field and laboratory; the anticipated relationship between age and mass emerges from these data.

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Our attempts to refine the PXRF technique for absolute dating of desert varnish are confounded by the limited of "dated" varnishes available to calibrate and test the method. Such rocks are useful in the time range of perhapsto 10, years; surface spalling with loss of varnish presents an upper time limit and difficulty in dating Holocene basalts presents a lower limit.

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Basalt flows typically provide horizontal surfaces that are ideal for PXRF measurements because, as a biogenic process, varnish development even at a single site varies with solar orientation. This process restarts varnish development and the pecked surface gradually repatinates over time.

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At some locales, certain figures, symbols, and stylistic elements can be associated with an archaeological culture of known antiquity and duration, thus providing an age range for such glyphs. Many of these petroglyphs are developed on vertical surfaces, which complicates dating both within and between sites.

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Such ages may in turn be associated with the initial exposure of fresh surfaces on cobbles and boulders on these geomorphic surfaces. We have compared the Mn and Fe accumulation in desert varnish from a variety of sites where the age of the fresh surface is constrained by one of the three approaches described above.

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thus far suggest that the current PXRF technique can provide absolute ages useful in situations where a ificant uncertainty range is tolerable. Obvious sources of error in calibration and deployment of the PXRF technique that need to be addressed include errors in the dates of materials in the training sets, time lag between exposure of rock surface and varnish development, variations in surface orientation and micro-climate, and climate change over the course of varnish development.

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Volume Content. Export Citation. Abstract Desert varnish, also called rock varnish, is a thin biogenic layer of Mn-oxides, Fe-oxides, and clays that coats rock surfaces in arid and semi-arid regions.

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