Application of cave monitoring to constrain the value and source of detrital ²³⁰Th/²³²Th in speleothem calcite: Implications for U-series geochronology of speleothems

Cave calcite deposits (“speleothems”) are widely-used archives of terrestrial paleoclimate signals, in large part because they can be accurately and precisely dated using ²³⁸U-series disequilibria (U-series dating) measured by mass spectrometry. Difficulties arise when growth layers in stalagmites incorporate detrital material during calcite deposition that contains significant amounts of ²³⁰Th (“detrital ²³⁰Th”). U-series ages must be corrected for detrital ²³⁰Th either by 1) assuming a detrital ²³⁰Th content (expressed as the ratio ²³⁰Th/²³²Th), or by 2) developing isochron models for each stalagmite. We examine two alternative correction approaches for detrital ²³⁰Th/²³²Th: analysis of calcite grown on artificial substrates in central Texas caves (i.e., present-day or “zero-age” calcite) and analysis of soil leachates from the recharge zones of the caves. Samples collected over three years yield elevated detrital atomic ²³⁰Th/²³²Th values, ranging from 5.4 to 28.1 parts per million (ppm) ± 0.5 ppm (1σ uncertainty for a typical measurement), compared with the commonly used “bulk earth” value of 4.4 ppm. Agreement between soil leachate and calcite ²³⁰Th/²³²Th values suggests that soil material is a source of detrital Th in the stalagmites in central Texas. Correlations between Fe, Mn, and Th concentrations in soil leachates from above caves suggests that Th sorbs on to Fe and Mn colloidal material. Independent estimates of detrital ²³⁰Th/²³²Th in a young (< 100 years) stalagmite from central Texas reveal higher ²³⁰Th/²³²Th (~4 to 11 ppm) than bulk earth, consistent with measurements from zero-age calcite and soil (~5 to 20 ppm). These results offer a new method for improving U-series chronologies of stalagmites.