Cation-ratio dating of rock varnish: Why does it work?

Cation-ratio dating of rock varnish is an empirical surface-exposure dating method based on decreases in the cation ratio (Ca+K):Ti over time. Although these changes were attributed to the preferential leaching of Ca and K from varnish, the existence of such leaching has not been demonstrated. In varnish collected from the Cima volcanic field, California, distributions of the minor elements used in cation-ratio (CR) dating reflect varnish stratigraphy as defined by the major elements Mn, Fe, and Si. In general, K is associated with Si, Ca with Mn, and Ti with Fe. Because of these associations, variations in CRs both within samples and between samples typically reflect variations in major element composition associated with varnish stratigraphy. No systematic changes with depth or age are present in the ratios K:Si or Ca:Mn as would be expected if K and Ca have been preferentially leached from varnish. Available data suggest that, instead of a leaching process, thickness-dependent changes in the amount of substrate being incorporated into varnish analyses contribute to the empirical trend of decreasing CR with increasing surface age, although further research is required to verify this hypothesis.