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. •There is elevated detrital 230Th/232Th in calcite and soil from central Texas as compared to bulk earth assumptions.•Detrital 230Th/232Th is likely sourced from soil above the caves in central Texas.•Detrital 230Th/232Th estimates from zero-age calcite are verified by modeling from a central Texas stalagmite.