Origin of chemical fluxes carried by boreal rivers : evidence from major, and trace element, U and Sr isotope data in two Siberian rivers

High latitude regions are characterized by very contrasted hydrological periods, marked by (a) a very low water flow during cold period (October-May), (b) an intense spring flood in May/June and (c) an intermediate to high water flow in summer (June– September). We propose here to quantify the intensity of geochemical fluxes associated to each of these hydrological periods and to constrain their origin. For this purpose, we analysed the temporal variations of the geochemical composition of water samples (filtered at 0.22µm) collected at the outlet of the Kochechumo and the Nizhnaya Tunguska rivers in Central Siberia (Russia). These analyses, performed over two hydrological cycles (2006-2008), were completed by a study of a smaller experimental watershed within the Kochechumo watershed. Our results combining major and trace element data together with Sr and U isotope ratios show that the melting flood in May results in the input of specific insoluble and soluble element fluxes in river waters. The mobilization of organic and inorganic colloids (from surface soil horizons) accounts for the insoluble element input during the flood period. The source of the dissolved element flux is clearly distinct from the source of winter waters and also originates from the uppermost horizons of the soil-permafrost system, with slight modification during the melting flood. Indeed, melting snow and leached litter appear to be the main chemical source at the beginning of the flood event whereas the suprapermafrost flow, more affected by water-rock interactions, dominates afterwards. This latter flux also dominates the chemical composition of summer river waters. During the winter, we observe in rivers the predominance of deep underground waters possibly affected by mineral precipitation or dissolution processes