If only 'organic matter' from the weathering rind under petroglyphs was present in samples used for dating, then the measured radiocarbon age reflects the age of the material and not carbon taken from the rock coating or 'inorganic' graphite from the rind.
The focus of the other article is 'whether available radiocarbon results constrain the ages of Coa engravings' (Dorn 1997: 105), but his results are from weathering rinds (cortex), and not rock surface coatings or silica glazes as he asserts.
Reasonable mechanisms exist for periodic insertion of younger carbon when siliceous coatings spall or dissolve, or epilithic or endolithic organisms add organic matter to the coating [ILLUSTRATION FOR FIGURE 3 OMITTED] or to the weathering rind.
are almost twice as old as organics within the underlying weathering rind (9180[+ or -]60 b.
i in pore spaces of rock weathering rinds on exposed panel faces;
iii in weathering rinds collected from unexposed rock crevices;
McKay 1986) forces 14C ages towards a long-term equilibrium found in 'control' panel weathering rinds.
Perhaps the best way to understand Coa carbon is to present a general framework of three sources: host rock; weathering rinds; and on top of weathering rinds.
Organics can also be deposited in weathering rinds before a rock coating forms.