Some Observations on the Radiocarbon and Cosmogenic Isotope Dating of Petroglyphs, Foz Coa, Portugal
Watchman, Alan, Antiquity
Further remarks on the age of the Foz Coa rock-engravings, Portugal, in the light of studies by absolute-science methods in the March 1997 ANTIQUITY.
In the March 1997 issue of ANTIQUITY, two articles by Fred Phillips et al. (1997: 100-104) and Ron Dorn (1997: 105-15) further contributed on the date of the Coa valley petroglyphs in Portugal. Using in situ chlorine-36, Phillips et al. found evidence that 'indicates how long rock surfaces have been exposed', and 'conclude that panels were available for engraving during the Palaeolithic; they have been exposed 16,000-136,000 years' (Phillips et al. 1997: 100; Chippindale 1997: 1). Dorn's (1997) results were less conclusive; he found signs 'that radiocarbon is not safely and stably trapped within rock patinas and varnishes where neither older nor younger carbon can reach it; accordingly radiocarbon determinations on these deposits do not provide true measurements of date' (Chippindale 1997: 1). Unfortunately both papers have critical flaws in their assumptions, and interpretations of analytical results fail to consider the nature of the materials sampled in relation to the event purportedly dated.
Measurements of chlorine-36 in Coa valley schists assume that this isotope only begins to accumulate once a rock panel becomes exposed to the atmosphere and to cosmic rays. This assumption is proved false by the authors' own data where at a depth of 16 m below the river bed at the proposed dam wall site chlorine-36 was present in quantities equivalent to surface exposure ages of between 795 and 2780 years (Phillips et al. 1997: table 1). The depth of this accumulation is at least eight times greater than that at which in situ cosmogenic isotopes are generally produced because 'one or two metres of rock will block most cosmic radiation' (Phillips et al. 1997: 102). Although muon reactions can produce chlorine-36 beyond these generally accepted shallow depths (Cerling & Craig 1994: 285), a fundamental problem still exists with the Coa measurements because the authors have shown that buried rock surfaces can also accumulate significant quantities of chlorine-36 before they are exposed to the atmosphere.
Generally, measuring the amount of chlorine-36 in a sample taken from an exposed rock surface today indicates the sum of isotopic contributions from atmospheric, cosmogenic and radioactive sources, assuming that chlorine-36 was not introduced either by prior exposure or from groundwater. Under ideal circumstances of rapid exposure by a cataclysmic event (fault or landslide) of a rock surface to cosmic radiation the in situ cosmogenic chlorine-36 dating process can provide an age estimate for exposure of a surface where atmospheric and nuclear productions are insignificant. However, under slow rates of erosion and/or high levels of introduced atmospheric chlorine-36 a cosmogenic isotope 'age' cannot define when a surface was exposed to the atmosphere. In the Coa valley, where river down-cutting and hill slope erosion are arguably slow and episodic, and where deep rock weathering (iron-stained schists) and groundwater movements (fracture fillings) provide evidence of probable atmospheric chlorine-36 contamination the 'exposure age' of panels dated by Phillips et al. does not only reflect exposure to the atmosphere. Given that, the calculated in situ cosmogenic ages from exposure of the engraved panels in the Coa valley are too large.
The authors use a range of 0-2 mm/millennium for surface erosion rates because they assume that 'higher rates of surface erosion would have obliterated the engravings' (Phillips et al. 1997: 102). This suggests that the authors either already knew the surfaces had pre-Palaeolithic ages, or they conveniently ignored the possibility that even at the maximum erosion rate petroglyphs less than 1000 years old would still be visible. Dorn had sampled petroglyphs 'at locales where the bottoms of the grooves were [approximately]2-4 mm beneath the adjacent joint face' (Dorn 1997: 106), so erosion rates of up to 4 mm/millennium would still agree with much younger
estimates of petroglyph ages (Bednarik 1995; Watchman 1995b). …