230Th/ 234U dating of Holocene tufas: possibilities and problems

Abstract

High-resolution chronologies in continental carbonate deposits such as tufas are required for detailed palaeoclimatic and environmental studies. This work set out to establish if high-resolution U-series dating of detritus-rich Holocene tufas is routinely possible. The study centres on a paludal Holocene tufa from southern England that already has an existing Holocene chronology, based on 14C and supported by biostratigraphy, against which to compare U-series dates. The results show that significant detrital contamination combined with low initial U concentrations, and short time for ingrowth of radiogenic 230Th make high-resolution U-series dating of Holocene tufa very difficult. Moreover, a single ( 230Th/ 232Th) initial value to correct for the presence of detrital 230Th is not appropriate at the study site, a finding that may apply to most Holocene tufas. Total sample dissolution of coeval samples demonstrates considerable variability in the isotopic composition of the detritus. The total sample dissolution data are too scattered to constrain chronologies at the required resolution and may indicate the incorporation of a 230Th-rich component in the detritus.