Longterm variation of chemical elements fluxes from river basins in Central Siberian Plateau, underlain by permafrost.
Subarctic rivers in Central Siberian Plateau have specified fed-characteristics due to permafrost distribution and dynamics of active layer thawing. During the winter mean water period the greatest contribution in river feeding is made by ground water discharged through taliks, during the frost-free period – by precipitation, infiltrating in soil (Prokushkin et al 2011).
On the basis of climate data analysis, which indicates that temperature, precipitation and water discharge have been increasing during last 50 years for Central Siberian rivers, our objective was to estimate elements’ fluxes from river watersheds, situated on Central Siberian Plateau – Nizhnyaya Tunguska and Tembenchi. This study could give an idea of tendency of biogeochemical processes’ intensity on river basins, underlain by permafrost. We had for an object to calculate elements’ fluxes (HCO3-, SO42-, Cl-, Ca2+, Mg2+) from the Nizhnyaya Tunguska river basin (for the period 1970-2011) and from the Tembenchi river basin (for the period 1960-2011). The first river basin has watershed area of 268000 km2 and underlain by different types of permafrost, the second river basin has an area of 21600 km2 and underlain only by continuous permafrost. We have observed biogeochemical composition in studied rivers since 2004, sampling station is located in Tura, Russia. In addition, archive data for water discharges and biogeochemical composition (1960-2011) were received from Roshydromet.
Considering that there are insufficient amount of measurements in archive data, the studied period was divided on following intervals: (for Nizhnyaya Tunguska river - 1960-1969, 1970-1979, 1980-1989, 1990-1999, 2000-2009, 2010-2011; for Tembenchi river – 1970-1979, 1980-1989, 1990-1993, 2008-2011). For calculating daily fluxes, we applied the following stages: 1) elements’ concentrations were multiplied by water discharges for all available dates; 2) then, we analyzed fluxes (F) dependence on water discharges (Q) for each anion and cation and we found that this dependence could be described by the function F = a × Q b most significantly (a and b – coefficients). Confidence levels (p) for a and b were obtained with using STATISTICA 10 and all coefficients for entire period and all anions had necessary reliability (p ≤ 0.05, or p ≥ 95%).
Studied ions concentrations have inverse relationship on water discharges. During flood time river water is diluted by melted water and their mineralization is increased (Kadamtseva 2005). During winter period concentrations of studied ions are increased as far as active layer freezes through, and taliks with composition specified by evaporate and whinstone become the main input source for rivers. As a result, within low water level in winter, when water discharges have a minimum of all year, total mineralization is increased (Kadamtseva, 2005).
Annual ions’ fluxes from the Nizhnyaya Tunguska watershed has been increased: bicarbonate from 2.92 to 12.77 Mg/km2/year, chloride from 0.95 to 11.07 Mg/km2/year, sulfate from 1.13 to 2.79 Mg/km2/year, calcium from 1.48 to Mg/km2/year, magnesium from 0.4 to 1.44 Mg/km2/year). Thus, total flux of the main ions have been risen more than 4.5 times from the Nizhnyaya Tunguska river basin. There are probable causes of this increasing: 1) increasing high latitude rivers’ water discharges (Peterson et al, 2002); 2) permafrost degradation caused by climate warming (Frey and McClelland, 2009), which may lead to changes in river feeding. Trend of changing in elements’ fluxes from the Tembenchi river basin is different from that for Nizhnyaya Tunguska. The flux growth (from 1970 to 2011) was observed for following ions: bicarbonate (from 6.96 to 11.63 Mg/km2/year), calcium (from 2.46 to 4.84 Mg/km2/year) and magnesium (from 0.57 to 0.94 Mg/km2/year). However, the sulfate flux has been decreased from 1.47 to 0.4 Mg/km2/year and chloride flux varied from 1.88 to 3.39 Mg/km2/year without any trend. Nevertheless, the total export of main ions from the Tembenchi watershed increased from 14.45 to 20.37 Mg/km2/year since 1970 to 2011.
Two different data sources were described previously and some differences in flux values were obtained. For example, sulfate flow from the Nizhnyaya Tunguska river basin from 2005 to 2011 changed within 1.82 – 2.32 Mg/km2/year according to our data, and within 2.36 – 5.43 Mg/km2/year according to ROSHYDROMET data. Also for Tembenchi sulfate flux had significant decrease from 1.54 Mg/km2/year (1993, ROSHYDROMET data) to 0.54 Mg/km2/year (2005, our data). Such fact may cause by some distinctions in sampling and water analysis.
Total flux of anions and cations from two watershed – Nizhnyaya Tunguska (1960-2011) and Tembenchi (1970-2011) – has been increased since 1960 to 2011 from 6, 38 to 32,72 Mg/km2/year and from 14,45 to 20,37 Mg/km2/year, respectively. Possibly, quantity characteristic between Nizhnyaya Tunguska and Tembenchi may be caused by differences in average annual temperatures and different types of permafrost distribution on studied watersheds. Nevertheless, the increasing of total element flux from watersheds for entire period were obtained and. Such phenomenon could be accounted to climate changing and probable permafrost degradation.