Weathering Processes: The Uranium Paradox of Nepales Rivers

Potential interactions between silicate erosion and the CO₂ cycle and therefore global climate are controversial. The chemistry of the rivers provides an insight into weathering processes of both carbonate and silicate sources. Quantification of the inputs from groundwater and hydrothermal sources into the river is important for determining weathering fluxes from different sources. This study focuses on one of the large Nepalese river catchments, the Marsyandi which drains all of the main structural units of the Nepalese Himalaya. Here we use the major element, Sr and Sr isotope composition and U-series isotopic ratios of the dissolved load, bedload, suspended load and rock samples to quantify weathering inputs. Uranium series isotopic ratios provide powerful tracers of both sources and temporal variations of inputs to chemical weathering fluxes. Water/rock interaction normally elevates ²³⁴U/²³⁸U activity ratios due to the relative vulnerability of the ²³⁴U within the silicate mineral lattices leaving weathered rocks with activity ratios below unity. Surprisingly the dissolved load of the Nepalese rivers flowing through these bedrocks has ²³⁴U/²³⁸U ratios below one which confirms earlier work by Chabaux et al. (2001). Preliminary results on 'fresh' rocks collected at the surface have ²³⁴U/²³⁸U ratios significantly less than 1. A likely explanation for this paradox is that the rocks were affected by groundwater circulation prior being weathered at the surface. We use the major element, Sr and U contents of spring waters, the riverine dissolved and particulate load to evaluate surface water and groundwater inputs to the riverine flux. We speculate on how past and present temporal variations in weathering processes may result in the extraordinary U-series isotopic compositions of the Marsyandi waters and rocks. Chabaux et al. (2001) Isotopic tracing of the dissolved U fluxes of Himalayan rivers: Implications for present and past U budgets of the Ganges-Brahmaputra system, GCA, V. 69, pp.3201-3217