Marvin W. Rowe

Release of uranium from granitic rocks during in situ weathering and initial erosion (central Texas)

Abstract The abundance, distribution and nature of occurrence of uranium in granitic rocks in central Texas, and that in soil profiles and local stream sediments derived from these rocks have been determined by fission-track mapping, delayed-neutron counting and gamma-ray spectrometry. Selected samples were also analyzed for Th and K. In the granites, U occurs primarily in weathering-resistant accessory minerals (here called resistate U) and along grain boundaries of major minerals, particularly biotite (here called intergranular U). During in situ weathering and initial erosion of the granite, changes in U concentration are controlled by the chemical mobility of intergranular U and dispersal of the resistate U. No distinct trend is detected in soil profiles except that, in general, a slight depletion of U is noted at the top of the profiles, which may be followed by a slightly enriched zone and another zone of depletion before approaching the original U content of the granite at depth. Neither organic matter nor clays (mostly kaolinite) appear to be effective fixing agents of U in this area. However, in briefly-transported granitic sediment, mostly sand and gravel, significant loss of both U and Th has occurred. In the 1-km distance studied, U abundance reflects almost entirely the concentration of resistate U. These results indicate that most, if not all, of the mobile, intergranular U is released from granites during in situ weathering and initial transport; in granitic sediments resistate U is likely the predominant form. The mineralogical occurrence of Th has not been determined, but the rate of Th loss in stream sediments is even greater than that of U. Th, leached from granite during weathering, may have become adsorbed on clays and dispersed with them.

Radiocarbon Dating of Ancient Rock Paintings

A technique based on cold argon and oxygen plasmas permits radiocarbon dates to be obtained on paintings that contain inorganic pigments. (To listen to a podcast about this feature, please go to the Analytical Chemistry website at http://pubs.acs.org/journal/ancham.).

Direct Radiocarbon Dating of Rock Art

In 14C dating of pictographs, we use a low-temperature oxygen plasma coupled with high-vacuum techniques to selectively remove carbon-containing material in the paint without contamination from the rock substrate, even if limestone (CaCO3). In addition to one previously published measurement, we analyzed two more pictograph samples, which are in accord with archaeological inference. A sample of known age charcoal, also processed by our method, matched the control. This technique produces little mass fractionation, the maximum 813C being 0.16%o from the untreated sample. Limestone decomposition does not occur during our procedure. Although the technique development is in its infancy, these new results demonstrate that our non-destructive technique has great potential for producing accurate 14C ages.

Mass Spectrometric Radiocarbon Dates from Three Rock Paintings of Known Age

Radiocarbon age determinations are presented on three hieroglyphic texts from Naj Tunich cave in Guatemala containing Maya calendar dates. The ages obtained are on average 110–140 years older than the calendar dates. Several possible reasons are discussed for this discrepancy: one that is applicable to all radiocarbon dates on charcoal, one that applies to rock paintings, and one that is specific for the tropics. Possible problems with the ages ascribed to the Maya calendar dates are also discussed. Even with the potential problems that may exist, these dates still fall within 110–140 years of the ascribed calendar dates. Caution is urged in the interpretation of dates on charcoal pigments from rock paintings; consideration of the “old wood” and “old charcoal” factors is important.

Comment on some evidence of a date of first humans to arrive in Brazil

J Archaeol Sci 30 (2003) 351 reported extremely interesting dates on a calcite layer covering a pictograph at the Toca da Bastiana rock shelter within the Serra da Capivara National Park, Piaui, Brazil. Thermoluminescence and electron paramagnetic resonance ages indicate that humans were present in Brazil prior to 35 ky ago. We report radiocarbon dates for rock paintings at the same rock shelter and other nearby shelters that contradict Watanabe et al.s results.

Composition and chirality of amino acids in aerosol/dust from laboratory and residential enclosures

Initial results from the analyses of geological and anthropological samples for amino acids were difficult to accept because of the high enantiomeric purities of the analytes (i.e., predominantly L-amino acids). Consequently, sources of contamination had to be considered. All sources were eliminated except for direct atmospheric contamination. Essentially invisible, microscopic, aerosol/dust was found to rapidly contaminate the surface of samples and sample containers even after brief exposure times in clean laboratories. Contamination increased with exposure time. The aerosol/dust amino acids were contained predominantly in a proteinaceous material. Aerosol/dust from different locations can contain different percentages of proteinoid/amino acid material. However, the relative concentrations of the amino acids were similar for both laboratory and residential samples. The enantiomeric purity of the L-amino acids studied in aerosol/dust appears to be 99% or greater for the samples examined. Thus, even slight contamination of any sample with microscopic dust or aerosol particles can skew the results of trace amino acid analyses and amino acid e.e. determinations.

Radiocarbon dates for pictographs in Ignatievskaya Cave, Russia: Holocene age for supposed Pleistocene fauna

Samples from three charcoal pictographs at Ignatievskaya Cave, in the southern Ural Mountains of Russia, have been radiocarbon dated. An advanced antiquity was expected, with some paintings thought to be more than 10,000 years old, as suggested by the imagery. One charcoal painting, for example, resembles a mammoth. The radiocarbon date of that motif, however, dates only to 7370±50 BP. If that motif actually represents a live mammoth, it places mammoth extinction in the Urals nearer to the present than is currently accepted. A charcoal pigment sample, a drawing of lines radiating from a central focus, has also been dated; its age was a few hundred years older than the ‘mammoth’: 7920±60 BP. A charcoal line has been dated with an age of 6030±110 BP. Although radiocarbon analysis was attempted on a red-pigmented painting of a woman, there was not enough organic material in the paint sample to obtain a viable date. Radiocarbon dates on pictographs in Ignatievskaya Cave obtained so far suggest that the paintings may be more recent than has been supposed.

14C dating of ancient rock art: A new application of plasma chemistry

We have developed a new statically operated oxygen plasma method that allows direct14C dates to be obtained from ancient rock paintings. The method is applicable even to paintings on limestone (CaCO3) walls. A sample of a pictograph which had naturally spalled offa shelter wall in the Lower Pecos region of Texas was subjected to a low temperature (∼ 150°C) oxygen plasma to selectively remove the organic carbon-containing material used in the paint as CO2, without contamination from the limestone substrate. The Zürich EHT accelerator mass spectrometer was then used to determine the radiocarbon age of this prehistoric rock painting. It was successfully dated at 3865 ± 100 years BP, in good accord with the archaeological context which has set the onset of this Pecos River style of pictograph between 4100 and 3200 years BP. The method appears feasible and is applicable to rock art in which organic materials were used in the paint.

Nondestructive radiocarbon dating: Naturally mummified infant bundle from SW Texas

Plasma oxidation was used to obtain radiocarbon dates on six different materials from a naturally mummified baby bundle from the Lower Pecos River region of southwest Texas. This bundle was selected because it was thought to represent a single event and would illustrate the accuracy and precision of the plasma oxidation method. Five of the materials were clearly components of the original bundle with 13 dates combined to yield a weighted average of 2135 ±11 B.P. Six dates from a wooden stick of Desert Ash averaged 939 ± 14 B.P, indicating that this artifact was not part of the original burial. Plasma oxidation is shown to be a virtually nondestructive alternative to combustion. Because only sub-milligram amounts of material are removed from an artifact over its exposed surface, no visible change in fragile materials has been observed, even under magnification. The method is best applied when natural organic contamination is unlikely and serious consideration of this issue is needed in all cases. If organic contamination is present, it will have to be removed before plasma oxidation to obtain accurate radiocarbon dates.

Rock-art image in Fern Cave, Lava Beds National Monument, California: not the AD 1054 (Crab Nebula) supernova

The visual manifestation of the recent Hale-Bopp comet reminds us how telling are those rare objects which suddenly flare in the sky. One can suppose ancient people living by natural light were more compellingly struck by the sight of comets and supernovae, and understandably researchers seek images of them in the shapes of rock-art motifs. An absolute dating contradicts that supposition in respect of a presumed image of the visible supernova of AD 1054.