Temporal variations of geochemical fluxes in boreal rivers in permafrost context: trace element and Sr and U isotope data (Nizhnaya Tunguska Watershed - Central Siberia)

In this work, we propose to characterize the temporal variability of dissolved chemical fluxes carried by boreal rivers under permafrost conditions. This is a significant issue as high latitude regions present specific hydrological systems likely to be strongly affected by global warming. For this study, two rivers draining the South of the basaltic plateau of Putorana in Central Siberia (Kochechumo and Nizhnaya Tunguska) were sampled regularly over two years and the dissolved loads of the water samples were analysed for major and trace element concentrations as well as for strontium and uranium isotopic compositions. Our results show that chemical variations along the year follow hydrological variations and define three contrasted periods : (1) a very low water period from October to May, during which soluble elements are affected by concentration processes and chemical fractionation processes, (2) a spring flood in May/June characterized by an important water input and also by the mobilization of organic and inorganic colloids together with traditionally insoluble element, (3) an intermediate high water period from June to the end of September. Important strontium and uranium isotopic variations are observed, implying the contribution of several sources to the stream water over the year. In particular, deep undergroud reservoirs and suprapermafrost flow could be major contributors depending on the time of the year. These temporal variations of chemical fluxes have not been taken into account by geochemical studies yet. However, they are critical to understand which sources contribute to the chemical flux as well as to establish chemical budgets and to evaluate weathering rates in permafrost regions