Abstract

Radioactive disequilibrium relationships among Th230, U234, and U238 can be used to date pedogenic carbonates formed in regions of arid to semiarid climate. Samples suitable for dating consist of dense carbonate rinds around pebbles from the Cca soil horizon. Analytically, the method involves leaching the samples with dilute hydrochloric acid and measuring U238, U234, Th230, and Th232 in both the leachate and residue fractions. As the soil carbonate commonly incorporates silicate mineral-bearing detritus, corrections are made to account for possible introduction of detrital Th230 and U234 into the acid leachate. The corrections are based on the assumptions that (1) the carbonate initially contains negligible amounts of Th232 and Th230, or has a Th230/Th232 ratio similar to that in detrital minerals, (2) U238, U234, and Th230 in the detrital silicate phase are in secular equilibrium with each other, and (3) the thorium isotopes in the detrital phase are not fractionated by the acid leaching. Application of the method to calcareous soils developed on upper Quaternary alluvial deposits of the eastern Mojave Desert in southern California gives ages that are internally consistent and that agree with the geomorphic and stratigraphic relative age relationships. Fourteen samples from an upper Pleistocene geomorphic surface, Q2b, yielded an average age of 83,000 ± 10,000 yr. This age is confirmed by a different assessment of the data independent of the aforementioned assumptions. On one specimen two different layers of a pebble coating were dated, and a carbonate accumulation rate of about 1 mm/8,000 yr was obtained. It is hoped that this study will serve as a basis for further research into the absolute dating of various types of impure carbonates of late Quaternary origin.

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