Bedrock (234U/238U) disequilibrium and its impact on inferring sediment comminution age in Taiwan Island

Abstract

Among many 234U–238U disequilibrium applications, the “comminution age” approach enabled quantitative constraints of sediment transport timescale since the genesis of detrital sediment. This method requires a critical assumption regarding the initial bedrock (234U/238U) activity ratios (A0), typically assumed to be in secular equilibrium (=1). However, this supposition needs further validation as more authors proposed preferential leaching as a vital mechanism that produces U-series disequilibrium among bedrocks. To address this uncertainty and to examine the extent of impacts responsible for the bedrock (234U/238U) disequilibrium, we compiled the published A0 covering a wide range of lithologies. In this study, the global A0 values ranged from 0.700 to 1.280, with an average of only 0.981 ± 0.070 (2σ; n = 160). This widely observed 234U deficiency among global bedrocks cannot be resulted from the direct α-recoil of 234U alone but is attributed to the preferential release of 234U from the microfissures created in rock diagenetic history. To further evaluate the impact of A0 on comminution age calculation, the Zhuoshui River (Taiwan Island) sediment comminution age was re-calculated using the newly measured A0 from the Zhuoshui River basin, which is 0.978 on average. The newly derived comminution age was nearly three-fifths shorter than the values estimated using A0=1 in the previous study. Our case study demonstrated that assuming bedrock secular equilibrium could lead to overestimated comminution age, inferring a significantly lengthier sediment transport timescale across the river basin; therefore should be carefully constrained to provide a holistic perspective of catchment-scale earth-surface processes.