U-series isotopic signatures of soils and headwater streams in a semi-arid complex volcanic terrain
U-series isotopic signatures of soils and headwater streams in a semi-arid complex volcanic terrain
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DOI：
10.1016/j.chemgeo.2016.04.003
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发表时间：
2016-12
期刊：
影响因子：
3.9
通讯作者：
David Huckle;Lin Ma;J. McIntosh;Angélica Vázquez-Ortega;C. Rasmussen;J. Chorover
中科院分区：
文献类型：
--
作者：
David Huckle;Lin Ma;J. McIntosh;Angélica Vázquez-Ortega;C. Rasmussen;J. Chorover
铀系同位素正在成为表征关键区风化和土壤形成过程的工具。本研究旨在了解美国新墨西哥州火山口火山口半干旱和岩性复杂的火山地形（流纹质火山碎屑和凝灰岩）中 U 系列同位素在化学风化和土壤形成过程中的行为。对来自杰梅斯河流域关键区观测站的一套全面的样品进行了系统研究，包括基岩、土壤、灰尘、土壤连续提取物、土壤孔隙水、泉水和溪流水。在四个土壤剖面中测量的 (234U/238U) 值范围为 0.90 至 1.56，(230Th/238U) 值范围为 0.48 至 1.39。浅层土壤剖面中显着的 230Th 富集被解释为在土壤形成过程中与富含 230Th 的火山灰混合的证据。过去火山灰在这些土壤中的间歇性混合的证据表明，在不考虑可能的大气输入的情况下使用质量平衡方法模拟土壤形成是有问题的，并且现有模型在异质火山土壤中的未来应用应谨慎应用。一种土壤剖面中显着的 234 U 富集被解释为从富含 234 U 的土壤溶液向土壤中添加 U 的证据。土壤连续提取证实，大部分 U 包含在有机金属胶体中，并且在该剖面的浅层土壤中以可交换形式存在。U 系列同位素也显示出作为水停留时间和不同水源混合的示踪剂的前景。在本研究中，溶解 U 的 (234U/238U) 比率用于追踪 Valles Caldera 内小型（3.29 平方公里）源头集水区中源水对溪流流量贡献的季节性变化。与旱季（（234U/238U）范围为 1.9 至 3.1）相比，在融雪期间泉水中和溪水中的溶解 U 值（范围为 1.7 至 2.8）系统性较低（234U/238U）值（范围为 1.7 至 2.8），同时主要元素示踪剂 Cl 和 Si 表明更深部地下水源的贡献更大。与之前的研究相比，深层地下水中较低的 (234U/238U) 值归因于随着水-岩石相互作用持续时间的增加和化学溶解的增加，易风化的 234U 逐渐消耗。使用更定量的年龄示踪剂（例如 3 H）进行进一步研究，有助于确定停留时间对溪流源水 (234U/238U) 值的影响。如果成立，(234U/238U) 可能成为流域规模的水源和溪流水中停留时间的强大示踪剂。
Uranium-series isotopes are emerging as a tool to characterize weathering and soil forming processes in the Critical Zone. This study seeks to understand the behavior of U-series isotopes during chemical weathering and soil formation in the semi-arid and lithologically complex volcanic terrain (rhyolitic volcaniclastics and tuff) of the Valles Caldera, New Mexico (USA). A comprehensive set of samples from the Jemez River Basin Critical Zone Observatory, including bedrock, soils, dust, soil sequential extracts, soil pore water, spring water, and stream water, was systematically investigated. (234U/238U) values measured in four soil profiles ranged from 0.90 to 1.56 and (230Th/238U) values ranged from 0.48 to 1.39. Significant230Th enrichment in shallow soil profiles was interpreted as evidence of mixing with230Th-enriched volcanic ash during soil formation. Evidence of past episodic mixing of volcanic ash in these soils suggests modeling soil formation using a mass balance approach without considering possible atmospheric inputs is problematic, and future applications of existing models in heterogeneous volcanic soils should be applied cautiously. Significant234U enrichment in one soil profile was interpreted as evidence of addition of U to soils from234U-enriched soil solutions. Soil sequential extraction confirms that most of the U is contained in organo-metal colloid and exchangeable forms in shallow soils of this profile.U-series isotopes have also shown promise as a tracer of water residence time and mixing of different water sources. In this study, (234U/238U) ratios for dissolved U are used to trace seasonal variation in source water contributions to stream flow in a small (3.29 km2), headwater catchment within the Valles Caldera. Systematically lower (234U/238U) values (ranging from 1.7 to 2.8) were observed in dissolved U in spring and stream waters during snowmelt compared to dry seasons ((234U/238U) ranging from 1.9 to 3.1) in conjunction with greater contributions of deeper groundwater sources as suggested by major element tracers Cl and Si. The lower (234U/238U) values in deeper groundwater, in contrast to previous studies, were attributed to progressive depletion of easily-weathered234U with increasing duration of water-rock interaction and increasing chemical dissolution. Further studies with more quantitative age tracers, such as3H, could help to establish the influence of residence time on stream source waters' (234U/238U) values. If established, (234U/238U) could be a powerful tracer of water sources and residence time in stream waters at the catchment scale.