Transport and exchange of U-series nuclides between suspended material,dissolved load and colloids in rivers draining basaltic terrains |
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| Authors: | Philip AE Pogge von Strandmann Kevin W Burton Don Porcelli Rachael H James Peter van Calsteren Sigurður R Gislason |
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| Institution: | 1. University of Arizona, Department of Hydrology and Water Resources, Tucson, AZ 85721, United States;2. University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79968, United States;3. University of Notre Dame, Department of Civil & Environmental Engineering & Earth Science, Notre Dame, IN 46556, United States;4. University of Arizona, Department of Soil, Water & Environmental Science, Tucson, AZ 85721, United States;1. Zhejiang Provincial Key Laboratory of Carbon Cycling in Forest Ecosystems and Carbon Sequestration, Zhejiang A & F University, Hangzhou 311300, China;2. Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China;3. National Center for Tea Plant Improvement, Tea Research Institute, Chinese Academy of Agricultural Sciences, Key Laboratory of Tea Biology and Resources Utilization, Ministry of Agriculture, Hangzhou 310008, China |
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| Abstract: | This study presents uranium and thorium concentrations and activity ratios for all riverine phases (bedload, suspended load, dissolved load and colloids) from basaltic terrains in Iceland and the Azores. Small basaltic islands, such as these, are thought to account for ~ 25% of CO2 consumed by global silicate weathering, and for ~ 45% of the flux of suspended material to the oceans. These data indicate that U] and Th] in the dissolved and colloidal fractions are strongly controlled by pH, and to a much lesser extent by levels of dissolved organic carbon (which are low in these environments). At high pH, basalt glass dissolution is enhanced, and secondary mineral formation (e.g. Fe-oxyhydroxides and allophane) is suppressed, resulting in high dissolved U], and low colloidal U] and Th], indicating a direct chemical weathering control on elemental abundances. When the dissolved (234U/238U) activity ratio is >~1.3 (i.e. when physical weathering, groundwater contribution or soil formation are high), there is little isotope exchange between dissolved and colloidal fractions. At lower activity ratios, the dissolved load and colloids have indistinguishable activity ratios, suggesting that when chemical weathering rates are high, secondary clay formation is also high, and colloids rapidly adsorb dissolved U. Many of the suspended sediment samples have (234U/238U) activity ratios of > 1, which suggests that uptake of U onto the suspended load is important. Identical (230Th/232Th) in suspended, dissolved and colloidal samples suggests that Th, like U, is exchanged or sorbed rapidly between all riverine phases. This particle-reactivity, combined with poorly constrained contributions from groundwater and hydrothermal water, and short-term variations in input to soils (volcanic and glacial), suggests that U-series nuclides in riverine material from such basaltic terrains are unlikely to reflect steady state erosion processes. |
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