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

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

Watanabe et al. [14] reported exciting thermoluminescence (TL) and electron paramagnetic resonance (EPR) dates on a calcite layer deposited over a red pictograph at the Toca da Bastiana rock shelter within the Serra da Capivara National Park in Piaui, Brazil. The ages determined for the calcite, if substantiated, would provide striking proof of human occupation in the area very early, before 35 ky ago. The Serra da Capivara National Park and surrounding areas in Piaui, Brazil, are of particular archaeological interest. Nie´de Guidon and her colleagues have presented considerable evidence of human occupation at Pedra Furada as far back as 48 ky ago [4,5,7]. Pigments and rock fragments with rock paintings have been found in excavated levels at Pedra Furada that date to almost 30 ky ago [3,8]. However, controversy surrounds the validity of such an early human presence in Brazil [6]. We summarize here our dating study of rock paintings within the Toca da Bastiana shelter and other nearby rock shelters ([12]; new dates presented here).

1. Organic Material

First, we dated organic material extracted from paint samples using plasma-chemical extraction and AMS radiocarbon measurement. Paint samples were collected from (1) the red painting covered by the calcite accretion dated by Watanabe et al. [14], (2) four other paintings from Toca da Bastiana shelter (each within 2 m of the calcite-dated image), and (3) four paintings from other rock shelters in the area. Some dated paintings were made with charcoal and others had red ocher pigmentation.
Secondly, we identified the presence of calcium oxalate in the calcite accretion layer using Fourier transform infrared spectroscopy and X-ray diffraction. Unlike calcite, calcium oxalate is thought to form from ambient carbon [2,10,11,15,16]. It is therefore possible to radiocarbon date the “time” of its deposition, essentially the same event dated by EPR and TL for the calcite, assuming that their formation was simultaneous. Dating oxalate provides a good comparison for EPR and TL dates because the assumptions and conditions involved in radiocarbon dating are very different from those of EPR and TL; the latter two are based on similar assumptions and conditions.

2. Results

Unfortunately, our results uniformly argue against an extreme antiquity of 35 ka or more for the pictographs at Toca da Bastiana. Fig. 1 summarizes our radiocarbon dates. Radiocarbon measurement of calcium oxalate extracted from the calcite layer dated by Watanabe et al. [14] yielded only 249030 for a minimum age of painting. A “direct” radiocarbon date for the red painting associated with the calcite layer was determined to be 373090 using plasma-chemical extraction and AMS radiocarbon dating. In addition, radiocarbon dates on paint from four other images in the same shelter were 188060, 2280110, 2970300, and 332050 . Finally, dates on four other paintings/ pigments from other nearby shelters yielded ages of only a few thousand years: at Toca do Sitio do Meio,
2700110; at Pedra Furada, 2120110 and 357050; and at Toca do Extreme, 123050 .

3. Discussion

Resolving the discrepancy between our determinations and those of Watanabe et al. [14] may not be easy. We have attempted to verify the technique utilized at the Texas A&M University laboratory for dating pictographs (e.g., see Fig. 1 and Table 1 in [13]). Although the plasma-chemical extraction method has not been verified independently by other research groups, we have no compelling reason to doubt our results. Studies in southwest Texas suggest that age uncertainty ranges for rock paintings are approximately 150–200 years . Small sample sizes for some of
these Brazilian paint samples may in fact result in an uncertainty as large as 500 years , possibly even more. But this would still not account for the order of magnitude difference between our radiocarbon results and Watanabe et al.’s [14] TL and EPR results. While TL and EPR dating have been successfully used to date calcite deposits in stalactites and stalagmites [1,9], we suggest that the use of TL and EPR for dating calcite deposited in this arid, open-air shelter may suffer from inadequate correction for the incorporation of undissolved carbonate dust into the calcite layer as it formed. Incorporation of solid carbonate would make the calcite layer appear older than its true deposition time because the shelter’s limestone rocks are millions of years old.

4. Conclusion

Regrettably, since our radiocarbon results strongly disagree with a >35 ky age of the calcite accretion obtained by EPR and TL dating [14], we question their results. The EPR and TL ages cannot be used as conclusive evidence to support very early human occupation in Brazil near Pedra Furada until this controversy is resolved. Our results do not negate the possibility of human occupation in the area as early as 35 ky ago, but those radiocarbon-dated paintings surviving on rock shelter walls are not of that extreme antiquity. Once again, and most dramatically, these
studies point to the necessity of independent studies in dating rock art.

We are grateful to Dr. Nie´de Guidon for inviting us to study this important problem. This work was supported by the Fundac¸a˜o Museu do Homen Americano, Brazil. Additional funding was supplied by a Regent’s Fellowship from the Office of the Vice-Provost for Research and a grant from the Program to Enhance Scholarly and Creative Activities, Office of the Vice-President for Research, both at Texas A&M University.


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