Seasonal variations of chemical composition of soil porewaters, streams and rivers in basaltic watersheds of central Siberia: the origin of river dissolved load in the permafrost zone

Rivers and streams draining basalts in Central Siberia allow thorough evaluation of the intensity of chemical weathering and its potential change induced by permafrost degradation. In order to get new insights to the origin of dissolved and suspended matter in this important subarctic region, we follow continuously over 4 years the chemical composition of dissolved load in two large rivers (N. Tunguska and Kochechumo), small experimental watershed (Kulingdakan) in the Yenissey River Basin and interstitial soils solutions from different landscape positions and soil horizons. The site (Tura, 64oN, 100oE) offers a unique opportunity for studying the processes occurring at high and low permafrost distribution watersheds. Indeed, at global scale we deal with two equivalent size rivers draining the northern and southern part of the Central Siberia (Kochechumo River and N. Tunguska River, respectively). At the local scale, there are north-facing and south-facing slopes of the watershed that receive equivalent precipitation but exhibit totally different heat input and consequently above-ground biomass and active layer thickness. There is a clear and strongly pronounced seasonal variability in trace and major elements concentration and organic carbon (OC) over the hydrological cycle with most of OC and trace element flux occurring during the spring flood. In summer time, the north-facing slope acts as a primary source of water and dissolved elements to the river as it exhibits the lowest thickness of the active layer. Indeed, the chemical composition of the suprapermafrost flow and that of the river water are very similar. In contrast, the south-facing slopes, although they exhibit twice higher concentrations of OC and TE in porewaters, do not deliver enough fluids to the river as these fluids are fully absorbed by thick active layer, mosses and vascular vegetation. Nevertheless, in early spring, the degradation products of plant litter from the S-facing slopes are abundant in stream waters. In winter time, when all soil fluid migration is stopped and the small rivers are fully frozen, the main source of solutes to the large rivers become groundwaters located in the thawed zone below the river channel. In collaboration with other researches, our results should allow quantitative modeling of the evolution of large boreal continental systems under global warming accompanied by the shift from the permafrost-dominating to permafrost-free environments