A Review of Global Bedrock (234U/238U) Disequilibrium and its Controlling factors on Earth's Surface

The ²³⁴U-²³⁸U disequilibrium system offers a broad spectrum of applications to study various Earth-surface processes, such as the evolution of weathering profile and erosion through time and sediment transport processes across the river catchments. Many of these applications require a critical assumption regarding the initial bedrock (²³⁴U/²³⁸U) activity ratios ( ), typically assumed to be in secular equilibrium. However, it was until recently that this assumption was gradually challenged. To address this uncertainty and the controlling factors responsible for disequilibrium, we measured of the uncomminuted bedrock retrieved from Qinghai, China, in which one of the samples (ZC2F) exhibits severe ²³⁴U deficiency, with measured to be 0.691 ± 0.004 (2σ; n = 3). To complement our new data, we compiled the published global values ranging from 0.700 to 1.280, with an average of 0.981 ± 0.070 (2σ; n = 160), indicating that visibly unweathered rocks worldwide could have activity ratios higher or lower than 1. This widely observed ²³⁴U depletion among global bedrock (especially in the near-surface region, < 10 m) cannot result from the direct α-recoil of ²³⁴U alone but is attributed to the preferential release of ²³⁴U from microfissures created during rock diagenesis and probably from hydrothermal alteration processes. Moreover, the bedrock ²³⁴U deficiency corroborates the local lithology, primarily defined by the mineral crystal sizes and abundance of specific minerals (biotite and quartz in granites) and the local precipitation amount where the bedrocks were recovered. In addition, bedrock samples can likewise be enriched with ²³⁴U, as evidenced by the nine chemically treated bedrocks’ compiled in this study, ranging from 1.002 to 1.020 (n = 9), attributed to alpha recoil gain from neighboring grains or surface deposits enriched with uranium. As bedrock can be modified by geologic factors, such as tectonism, lithology, and climate, notably in the near-surface region, should be constrained with an optimal sampling strategy to provide a holistic perspective of evaluating catchment-scale Earth-surface processes.