Bedrock (234U/238U) disequilibrium and its impact on inferring sediment comminution age in Taiwan Island. (February 2023)

• Record Type: Journal Article
• Authors: Ng, Nicholas Chia Wei; Li, Chao; Wang, Chenyu; Guo, Yulong; Duan, Zhifei; Su, Ni; Yang, Shouye
• Is Part Of: Applied geochemistry. Volume 149(2023)
• Publication Date: 2023-02
• Subjects: Global bedrock -- (234U/238U) disequilibrium -- Preferential leaching -- Microfracture; Environmental geochemistry -- Periodicals; Water chemistry -- Periodicals; Geochemistry -- Social aspects -- Periodicals; Geochemistry -- Periodicals; 551.9
• DOI: 10.1016/j.apgeochem.2022.105550
• Languages: English
• ISSNs: 0883-2927

Abstract

Abstract: Among many 234 U– 238 U 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 ( 234 U/ 238 U) activity ratios ( A 0 ), 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 ( 234 U/ 238 U) disequilibrium, we compiled the published A 0 covering a wide range of lithologies. In this study, the global A 0 values ranged from 0.700 to 1.280, with an average of only 0.981 ± 0.070 (2σ; n = 160). This widely observed 234 U deficiency among global bedrocks cannot be resulted from the direct α-recoil of 234 U alone but is attributed to the preferential release of 234 U from the microfissures created in rock diagenetic history. To further evaluate the impact of A 0 on comminution age calculation, the Zhuoshui River (Taiwan Island) sediment comminution age was re-calculated using the newly measured A 0 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 A 0 = 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. Highlights: Bedrocks are not always in a "closed system" due to the micro-fissures formation. An "open system" through preferential leaching led to bedrock U-disequilibrium. Zhuoshui River bedrock ( 234 U/ 238 U) is much more depleted than expected. Comminution recalculation inferred nearly 3/5 shorter ages than assuming unity.