Index to Creationist Claims,  edited by Mark Isaak,    Copyright © 2005

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Claim CD031:

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Response:

1. The KBS Tuff controversy illustrates many of the problems with radiometric dating, but it equally illustrates that the problems are not insurmountable.
   
   The KBS Tuff (for "Kay Behrensmeyer Site," after the geologist who first described it) is a layer of redeposited volcanic ash, so it contains a mixture of older sediments, too. It is still possible to date the layer, but care must be taken to choose only the youngest rocks, else one would be dating the age of older sediments washed into the layer, not the age of the layer itself. This is what happened with the first ages reported from the tuff. In a study to test the feasibility of dating samples from the tuff, the samples were contaminated with non-juvenile components which could not be separated out, giving ages over 200 million years. It was recommended that new samples be collected from which suitable individual crystals could be separated (Fitch and Miller 1970). These new samples were dated at 2.61 +/- 0.26 million years, based on the ⁴⁰Ar/³⁹Ar dating method (Fitch and Miller 1970).
   
   Discrepancies with this date soon turned up, though. Work with animal fossils, particularly of pigs, showed that the strata in question matched younger strata in the nearby Omo Valley. In its early stages, this fossil work was imprecise enough that the 2.61 Myr date could still be justified (Maglio 1972). However, the fossils continued to point to a younger date as the quality of the work on them improved (White and Harris 1977). And in 1975, another lab, using K-Ar dating, reported dates of 1.82 and 1.60 Myr (Curtis et al. 1975).
   
   Fitch and Miller turned to an independent method to resolve the discrepancy, fission-track dating. Initial results gave an age of 2.44 +/- 0.08 Myr (Hurford et al. 1976). This fit well with the age of 2.42 Myr, which Fitch et al. (1976) recalculated from their original results. Subsequent ⁴⁰Ar/³⁹Ar measurements they took gave a scattering of ages from 0.52 +/- 0.33 to 2.6 +/- 0.3 Myr. They attributed the spread to reheating of the crystals after deposition. Paleomagnetic studies gave ambiguous results (Brock and Isaac 1974; Hillhouse et al. 1977).
   
   The weight of evidence soon began to converge on an age near 1.9 Myr, though. A study of trace elements in the minerals showed that the KBS Tuff correlates with the H2 tuff in the Shungura Formation, uncontroversially dated about 1.8 Myr (Cerling et al. 1979). The 1.60 Myr age reported by Curtis et al. (1975) was found to be an error due to a faulty balance (Drake et al. 1980). A later fission-track study which took pains to eliminate possible errors gave an age of 1.87 +/- 0.04 Myr (Gleadow 1980). Because the controversy had become quite heated, another expert, Ian McDougall, was called in to do independent dating. He came up with an age of 1.89 +/- 0.01 using K-Ar dating and 1.88 +/- 0.02 using ⁴⁰Ar/³⁹Ar dating (McDougall et al. 1980; McDougall 1981, 1985). Geological evidence and the consistency of dates derived from various sources indicates that reheating after deposition is unlikely.
   
   The lessons to be learned from the KBS Tuff dating controversy are not that radiometric dating does not work, but that it works with some caveats.
   • Some formations are easier to date than others. The KBS Tuff was particularly difficult to date because it included volcanic sediments of several different ages. Furthermore, it looked the same as other tuffs, so care was needed to make sure the same layer was being referred to in different areas. All of this requires careful work from knowledgeable geologists. Were it not for its importance to determining the ages of important hominid fossils, geologists probably would not have bothered with dating it at all.
   • Some dating techniques are simply inappropriate in some circumstances. As noted above, paleomagnetic study is not particularly useful at this site.
   • Discrepant dates are not dismissed out of hand. In addition to trying to resolve the issue with further dating, the discrepancies caused people to look for the sources of error. The original erroneous date by Fitch and Miller could be an accurate date of a roughly 2.5 Myr ash layer, present in neighboring areas but apparently eroded from the Koobi Fora Formation. Apparently, some pumice from that volcanic event had been incorporated into the KBS Tuff. Samples sent to an independent lab for "blind" dating confirmed its older age (Fitch et al. 1996). Alternatively, this and other discrepant ages may be due to contamination with older material. Such contamination caused ages in the 2.0 - 6.2 Myr range in the analysis of Curtis et al. (1975) until they revised their sample purification procedures. A high atmospheric argon contamination in their samples and analytical errors may have contributed, too (McDougall et al. 1980).
     
     The fission-track study which gave the 2.44 Myr age was the first such study to date zircons so young. The reanalysis by Gleadow (1980) noted problems with the standard methods and contributed new methodology for dealing with zircons with low track densities.
   • People's preconceptions and personalities can get in the way of evaluating the data objectively. In the KBS Tuff controversy, personality conflicts may have contributed to delay in the resolution and certainly contributed to the drama. But in the end, the objective evidence is a constraint that every scientist must meet. Replication, free access to information, and awareness of conflicts of interest help assure that personal foibles do not determine outcomes. Because such mechanisms were in place, all of the scientists who initially supported the older 2.6 Myr date for the KBS Tuff later came to accept the 1.88 Myr age (Lewin 1987).
   
   Note that different methods give the same results when known sources of error are removed. K-Ar, ⁴⁰Ar/³⁹Ar, and fission-track methods ultimately all gave the same results. These results were correlated with strata of the same age at other locations on the basis of fossil and trace element analysis.
   
   
2. The different ages which were seriously debated for the KBS Tuff, from 1.6 to 2.6 million years, were never close to ages required by young-earth creationism.

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