Useries Dating

The disequilibrium of 238U and its long-lived radioactive decay products 234U and 230Th can be used to date precipitated calcite such as flowstones and stalagmites, back to about 500 ky bp (e.g. Richards and Dorale 2003). A radioactive disequilibrium occurs in aqueous precipitates because of the relative insolubility of Th, whereby U is co-precipitated (e.g. with calcite) in great excess over Th. The date since formation of the calcite can be derived from in-growth of radiogenic Th as the radioactive equilibrium is slowly re-established, although disequilibrium between 234U and 238U needs to be measured and accounted for. An additional problem is the incorporation of detrital material (e.g. sediments) into the precipitating calcite. Detritus brings with it U and 230Th, usually leading to over-estimated U-series dates. However, measurements of common thorium (232Th) are used to detect the presence of detritus. Where the detrital 230Th/232Th can be well characterized, usually from the insoluble residues from leached calcite or sediments, a correction can be applied (e.g. Schwarcz and Latham 1989). Alternatively, the measured U-series isotopic ratios on isochronous samples that are contaminated to different degrees can yield a corrected age (e.g. Ludwig 2003).

The interstratification of rock art and flowstone has been used to verify the authenticity of Palaeolithic cave art, for example in the Grande Grotte and Grotte du Cheval at Arcy-sur-Cure, France (Liger 1995). Actual verification of the age of the art by dating of flowstone remains rare, however. One example of such, using U-series dating of calcite, has provided minimum dates for cave paintings at Covalanas, Cantabria, Spain (Bischoff et al. 1999). It is imperative that the stratigraphic relationship between the bed-rock, the art, and the calcite deposit is secure for the resulting U-series dates to be meaningful. At Church Hole and Robin Hood Cave there are several motifs that are clearly incised into the bed-rock, with subsequent thin flowstone formation partially covering the engravings. Three areas where this stratigraphic relationship is unambiguously represented were sampled for U-series dating (Fig. 3.1): the vertical 'Notches' below the Stag (Panel III) and the 'bird/female' forms (Panel VII) in Church Hole, and the 'vulva' in Robin Hood cave. Calcite

Fig. 3.1. Sketches showing sample locations for the 'notches' and 'birds/females' in Church Hole and for the 'vulva' in Robin Hood Cave

samples were removed by manually scraping the thin (0.5-2 mm) layers of calcite in shallow spits. The calcite-bed-rock junction was clearly represented by their contrast in colours, and the last aliquot, which might contain a mixture of the bed-rock with calcite, was discarded. Thicker samples (>2 mm) and small stalagmites were removed using a coring drill.

U-series measurements were made using a standard total dissolution procedure for carbonates (e.g. Edwards et al. 1987). Insoluble residue, usually representing silicate detritus, was decanted and dissolved in hydrofluoric acid before being recombined with the leachate. Thus, the results represent the combined U-series isotopes from the precipitated calcite and exogenous detritus.

All the samples have 230Th/232Th < 20, indicating relatively high levels of detrital thorium, which is not uncommon in young samples. High levels of detrital contamination such as this will result in apparently older ages unless a correction is applied. Isochronous samples could not be guaranteed in such thin calcite, so a detrital correction was applied using the measured 232Th and 238U concentration in leached sediments collected from the cave. We correct using a molar Th/ U value of 3.8 + 0.5 calculated from the 2-sigma variation in four separate samples from the cave (Table 3.1). This value encompasses the Th/U ratio of the Earth's upper continental crust which has a range of 3.6-3.8 (Taylor and McLennan 1995; Wedepohl 1995). There is a large variation between the four detritus samples, but the errors that arise from this variation are propagated to the corrected dates.

The detritally corrected and uncorrected U-series results are shown in Table 3.2. It can be seen that the effect of detrital correction is very marked when the 230Th/232Th is low, indicating more detrital contamination. There is considerable variation in the corrected U-series dates between different samples from each location (Table 3.2, Fig. 3.2). This reflects a multi-phase development of the flowstones. In general the thicker flowstones and stalagmites (e.g. samples CHC-B,C1,C7, E3, RHC-F1, F2) gave younger dates which would be consistent with an increase in calcite precipitation in the last few

Table 3.1. U-series results from leached insoluble detritus collected from Church Hole

Detritus

234 U/238U (activity ratio)

232 Th/238 U (molar ratio)

Was this article helpful?

0 0

Post a comment