Some observations on the radiocarbon and cosmogenic isotope dating of petroglyphs, Foz Coa, Portugal.

Some observations on the radiocarbon and cosmogenic isotope dating of petroglyphs, Foz Coa, Portugal. Further remarks on the age of the Foz Coa rock-engravings, Portugal,in the light of studies by absolute-science methods in the March 1997ANTIQUITY.In the March 1997 issue of ANTIQUITY, two articles by Fred Phillipset al. (1997: 100-104) and Ron Dorn (1997: 105-15) further contributedon the date of the Coa valley petroglyphs in Portugal. Using in situ In place. When something is "in situ," it is in its original location. chlorine-36, Phillips et al. found evidence that 'indicates howlong rock surfaces have been exposed', and 'conclude thatpanels were available for engraving during the Palaeolithic; they havebeen 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 trappedwithin rock patinas and varnishes where neither older nor younger carboncan reach it; accordingly radiocarbon determinations on these depositsdo not provide true measurements of date' (Chippindale 1997: 1).Unfortunately both papers have critical flaws in their assumptions, andinterpretations of analytical results fail to consider the nature of thematerials sampled in relation to the event purportedly dated.Measurements of chlorine-36 in Coa valley schists assume that thisisotope only begins to accumulate once a rock panel becomes exposed tothe atmosphere and to cosmic rays. This assumption is proved false bythe authors' own data where at a depth of 16 m below the river bedat the proposed dam wall site chlorine-36 was present in quantitiesequivalent to surface exposure ages of between 795 and 2780 years(Phillips et al. 1997: table 1). The depth of this accumulation is atleast eight times greater than that at which in situ cosmogenic cos��mo��gen��ic?adj.Produced by cosmic rays.[cosm(ic ray) + -genic.]Adj. 1. isotopesare generally produced because 'one or two metres of rock willblock most cosmic radiation' (Phillips et al. 1997: 102). Althoughmuon muon(my`ŏn), elementary particle heavier than an electron but lighter than other particles having nonzero rest mass. reactions can produce chlorine-36 beyond these generally acceptedshallow depths (Cerling & Craig 1994: 285), a fundamental problemstill exists with the Coa measurements because the authors have shownthat buried rock surfaces can also accumulate significant quantities ofchlorine-36 before they are exposed to the atmosphere.Generally, measuring the amount of chlorine-36 in a sample taken froman exposed rock surface today indicates the sum of isotopiccontributions from atmospheric, cosmogenic and radioactive sources,assuming that chlorine-36 was not introduced either by prior exposure orfrom groundwater. Under ideal circumstances of rapid exposure by acataclysmic event (fault or landslide) of a rock surface to cosmicradiation Noun 1. cosmic radiation - radiation coming from outside the solar systemCBR, CMB, CMBR, cosmic background radiation, cosmic microwave background, cosmic microwave background radiation - (cosmology) the cooled remnant of the hot big bang that fills the entire the in situ cosmogenic chlorine-36 dating process can providean age estimate for exposure of a surface where atmospheric and nuclearproductions are insignificant. However, under slow rates of erosionand/or high levels of introduced atmospheric chlorine-36 a cosmogenicisotope 'age' cannot define when a surface was exposed to theatmosphere. In the Coa valley, where river down-cutting and hill slopeerosion 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 notonly reflect exposure to the atmosphere. Given that, the calculated insitu cosmogenic ages from exposure of the engraved panels in the Coavalley are too large.The authors use a range of 0-2 mm/millennium for surface erosionrates because they assume that 'higher rates of surface erosionwould have obliterated the engravings' (Phillips et al. 1997: 102).This suggests that the authors either already knew the surfaces hadpre-Palaeolithic ages, or they conveniently ignored the possibility thateven at the maximum erosion rate petroglyphs less than 1000 years oldwould still be visible. Dorn had sampled petroglyphs 'at localeswhere the bottoms of the grooves were [approximately]2-4 mm beneath theadjacent joint face' (Dorn 1997: 106), so erosion rates of up to 4mm/millennium would still agree with much youngerestimates of petroglyph pet��ro��glyph?n.A carving or line drawing on rock, especially one made by prehistoric people.pet ages (Bednarik 1995; Watchman WATCHMAN. An officer in many cities and towns, whose duty it is to watch during the night and take care of the property of the inhabitants. 2. He possesses generally the common law authority of a constable (q.v. 1995b).Students of chemistry know that all chlorides are soluble, an aspectforgotten by cosmogenic chlorine-36 proponents. Instead of invoking muonproduction for cosmogenic chlorine at 16 m depth at the proposed damsite, a simple explanation of groundwater transport of chlorine fromhigher elevations might be considered. Siliceous siliceousrelating to or made of silica or a silicate. minerals are much moresoluble in the presence of chloride ions (Wey n. 1. Way; road; path.v. t. & i. 1. To weigh.n. 1. A certain measure of weight. & Sieffert 1961) andsilica skins have precipitated on some schist schist(shĭst), metamorphic rock having a foliated, or plated, structure called schistosity in which the component flaky minerals are visible to the naked eye. surfaces, so groundwaterhas likely transported atmospherically derived chlorine-36 into theschists. Measurements of chlorine-36 on successive joint faces atRibeira dos Piscos provide evidence confirming chlorine solubility;analysed rock from the oldest exposed joint surface has the lowestchlorine abundance suggesting that some chlorine has been leached fromminerals (Phillips et al. 1997: table 1). Firm conclusions aboutchlorine-36 mobility in samples analysed by Phillips et al. at Coacannot be made because the authors did not provide petrographic pe��trog��ra��phy?n.The description and classification of rocks.pe��trogra��pher n. descriptions to indicate the degree of mineral weathering. They also didnot reveal the precise locations from where they collected samples sothe analyses may not relate to engraved surfaces.The focus of the other article is 'whether available radiocarbonresults constrain the ages of Coa engravings' (Dorn 1997: 105), buthis results are from weathering rinds (cortex), and not rock surfacecoatings or silica glazes as he asserts. He reported only onedetermination from silica glaze within an overlying overlyingsuffocation of piglets by the sow. The piglets may be weak from illness or malnutrition, the sow may be clumsy or ill, the pen may be inadequate in size or poorly designed so that piglets cannot escape. rock coating,whereas he made 14 radiocarbon measurements on 'organicmatter' from weathering rinds and 5 from the interface betweenrinds and their overlying coatings (Dorn 1997: table 1, 108). Hisconclusion that 'analytical data indicate that silica glaze at Coadoes not form a closed system' (Dorn 1997: 106) therefore has nobasis. What he has found instead is that 'organic matter' inweathering rinds does not provide suitable source materials forestablishing the age of petroglyphs at Coa because they contain carbonof different ages.Preliminary dating results for fossilized surfaces and rock paintingshave been obtained outside the Coa valley using microexcavationextraction (Arsenault 1996; Watchman 1990; 1992; 1994; 1995a). Toconfirm that carbon-bearing substances were fossilized and trapped insilica accretions in a closed stable system I dated three micro-layersin one skin from Penascosa (one of the Coa petroglyph sites). Themicro-layers consisted of a basal layer of silica (FC-7; 3490[+ or -]90years b.p.), an upper silica layer (FC-6; 2060[+ or -]50 years b.p. andthe silty surface (FC-25, modern; Watchman 1995b; table 1). Theseoff-art results establish a chronological sequence consistent with slowaccumulation of carbon in a closed, stratified stratified/strat��i��fied/ (strat��i-fid) formed or arranged in layers. strat��i��fiedadj.Arranged in the form of layers or strata. amorphous silica system.Dorn clearly illustrates the relationship of 'organic matter inpores of weathering rinds' with the overlying silica'glaze' (Dorn 1997; [ILLUSTRATION FOR FIGURE 2 OMITTED]), andhe infers that it was carbon 'in weathering rinds within petroglyphgrooves' (p. 106) that he sampled for dating. That example, forwhich he obtained a radiocarbon age (FC-95-4) shows 'organicmatter' at depths exceeding 100 microns beneath the petroglyphsurface and almost 0.3 mm beneath the adjacent joint surface. To samplepetroglyph weathering rinds for dating 'pieces of schist werechipped from each petroglyph with a tungsten-carbide needle' (Dorn1997: 106), but he gives no information about how the 'organicmatter in pores of weathering rind' was extracted and processed fordating. He does not describe the types of 'organic matter' hefound. Uncertainty therefore exists as to the inclusion of accidentalcarbon-bearing contaminants; from graphite in the schist (Watchman1995b; 1996), from modern carbon during sampling, during processing(Beck et al. 1996), or from conversion to graphite in the datinglaboratory. If only 'organic matter' from the weathering rindunder petroglyphs was present in samples used for dating, then themeasured radiocarbon age reflects the age of the material and not carbontaken from the rock coating or 'inorganic' graphite from therind. His conclusions therefore relate to material beneath petroglyphsand not to the age of the accretions lying over them. Dorn'sdeterminations for 'organic matter' from weathering rinds aretherefore maximum ages. His samples were also probably contaminated bygraphite from the schist because he did not separate the differentfractions in all samples. Where he did separate weathering rind'organic matter' into dense and less dense fractions (FC95-5b)he may have only divided it into its two forms; dense graphite stubs andlight graphite flakes combined with modern organics. We will never knowwhat was dated because he did not identify the components, only callingthem 'organic matter'.Dorn compares my dating result from a railway quarry of known age(Watchman 1995b: FC-28) with his radiocarbon determination for silicaglaze over an unengraved panel surface (FC-95-2a), but he has no ideahow old the surface is that he sampled. The only comparison possible isthat similar rock surface accretions were sampled from schist in the Coaand that similar radiocarbon results were obtained. I do not share hisview about modern carbon exchange in these samples; there isoverwhelming evidence to support an opposite hypothesis (Watchman 1996).There is no need to use complex arguments about modern carbon exchangewhen the simple explanation of accidental inclusion of carbon-14deficient graphite from the schist underlying rock surface accretionsfits all the evidence, including field observations made by all datingparties. The radiocarbon determination of 29,990 years ago, obtainedfrom dense 'organic matter' in the rind of an 'unexposedjoint crevice' (Dorn 1997: table 1, FC95-5b), fits this scenario.(It is near the detection limit of AMS AMS - Andrew Message System radiocarbon for a mixtureconsisting mostly of graphite.)The major radiocarbon dating problem at Coa is imprecision insampling the extremely thin, compositionally variable silica skins. Aswith all rock surface accretion dating methods, extracting carbon ofknown identity and origin is very difficult; often the best that can bedone is to take a sequence of stratigraphic stra��tig��ra��phy?n.The study of rock strata, especially the distribution, deposition, and age of sedimentary rocks.strat samples to verify that achronological order of deposition exists. Techniques for doing this(Watchman 1993; Watchman & Campbell 1996) are being refined.Radiocarbon dating results from Coa do date the rock surfaces and thepetroglyphs, but not to the estimates and interpretations of Dorn andPhillips et al. Publication of these two papers without details of themethods, the precise locations of samples, the nature of rocks andaccretions analysed, the pre-treatment steps, and the weights andidentities of carbon-bearing components not only does archaeometry adisservice, but it degrades scientific debate. While the publishedresults are controversial, so are the methods and assumptions, butreaders were not provided with enough information to judge the accuracyof the results. Surfaces and materials of known ages were not used tovalidate these dating approaches adequately in the Coa valley,especially where several potential sources of isotopes were likely.ReferencesARSENAULT, D. 1996. Le projet Nisula: recherche re��cher��ch��?adj.1. Uncommon; rare.2. 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