Geochemical and isotopic study (Sr, Ca, U- and Th-series) of Siberian watersheds Sources of dissolved chemical fluxes and characterization of soil-water-plant chemical exchanges

The study of chemical and isotopic (U, Sr) fluxes carried by two Siberian Rivers (Nizhnyaya Tunguska and Kochechum Rivers, two tributaries of Yenisey River) permit to characterize different element fluxes which predominate successively over the hydrological year. Deep underground waters are the main chemical contributors to large rivers during winter, while surface waters significantly impact the budget of chemical elements during spring flood and summer periods. At the spring flood, the surface flux comes from the leaching of shallow organic soil levels and in summer it comes mainly from subsurface layers where the chemical flux is controlled by water-rock interactions within mineral soil. These surface fluxes carry a significant colloidal component that affects the mobilization of insoluble elements. Moreover, the high spatial resolution study of the Kochechum watershed suggests that the relative contributions of deep and surface fluxes strongly depends on the thickness and continuity of the permafrost under the watershed. The chemical and isotopic (Sr, Ca) characterization of the different compartments of a little subwatershed of the Kochechum watershed, the Kulingdakan watershed, highlighted the role of atmospheric inputs on isotopic compositions at the soil profile scale and the major impact of vegetation on chemical element distribution within soils and on soil evolution. These features are confirmed by the study of ²³⁸U and ²³²Th decay series that also reveal recent modifications (< 10 years) of soil dynamics and an increase of the depth of the active layer of the permafrost in the Kulingdakan region. (author)