Riverine particulate C and N generated at the permafrost thaw front: case study of western Siberian rivers across a 1700 km latitudinal transect

Abstract

Abstract. In contrast to numerous studies on the dynamics of dissolved (<0.45 µm) elements in permafrost-affected high-latitude rivers, very little is known of the behavior of river suspended (>0.45 µm) matter (RSM) in these regions. In order to test the effect of climate, permafrost and physio-geographical landscape parameters (bogs, forest and lake coverage of the watershed) on RSM and particulate C, N and P concentrations in river water, we sampled 33 small and medium-sized rivers (10–100 000 km2 watershed) along a 1700 km N–S transect including both permafrost-affected and permafrost-free zones of the Western Siberian Lowland (WSL). The concentrations of C and N in RSM decreased with the increase in river watershed size, illustrating (i) the importance of organic debris in small rivers which drain peatlands and (ii) the role of mineral matter from bank abrasion in larger rivers. The presence of lakes in the watershed increased C and N but decreased P concentrations in the RSM. The C:N ratio in the RSM reflected the source from the deep soil horizon rather than surface soil horizon, similar to that of other Arctic rivers. This suggests the export of peat and mineral particles through suprapermafrost flow occurring at the base of the active layer. There was a maximum of both particulate C and N concentrations and export fluxes at the beginning of permafrost appearance, in the sporadic and discontinuous zone (62–64∘ N). This presumably reflected the organic matter mobilization from newly thawed organic horizons in soils at the active latitudinal thawing front. The results suggest that a northward shift of permafrost boundaries and an increase in active layer thickness may increase particulate C and N export by WSL rivers to the Arctic Ocean by a factor of 2, while P export may remain unchanged. In contrast, within a long-term climate warming scenario, the disappearance of permafrost in the north, the drainage of lakes and transformation of bogs to forest may decrease C and N concentrations in RSM by 2 to 3 times.

Major and trace elements in suspended matter of western Siberian rivers: First assessment across permafrost zones and landscape parameters of watersheds

“…This can be linked to Cu and Ni coprecipitation with/ adsorption onto Mn oxy(hydr)oxides. Additional point to consider is that this effect was consistent withan increase in Corg (RSM) in small rivers (Krickov et al, 2018) thereby demonstrating a close link between C and Cu. In autumn, there was an increase in particulate Cu share northward (Fig.…”

“…Stronger depletion (RSMWSL/RSMWorld < 0.2) has been observed for Zr and Mo in WSL rivers. We interpret this result as being due to the sizeable proportion of organic matter in RSM in western Siberia (Krickov et al, 2018), and the corresponding impoverishment of suspended matter in elements hosted by silicate (clay) materials. Note however that the world average value is essentially based on data from big rivers.…”

“…The WSL rivers are sizebly enriched relative to the world average only with respect to P and Mn (a factor of 2.5 and 4.8, respectively). Along with elevated C concentration in suspended matter of WSL rivers (ca., 15%, Krickov et al, 2018), this reflects presence of peat particles and vegetation debris as important components of RSM.…”

“…In the permafrost-affected territories, this transport is essentially controlled by suprapermafrost flow over the frozen layer via organic or mineral horizons. Although the mineral particles dominate the suspended load of all rivers regardless of size, the very small ones (Swatershed < 100 km²) are strongly influenced by plant litter and organic detritus (Krickov et al, 2018). Irregular spatial and temporal delivery of these C-rich particles from soil to river produces much higher scattering in the particulate fraction value.…”

“…We could not find any systematic difference in mineralogical composition of RSM related to season (spring flood versus summer baseflow). Note that the RSM of all WSL rivers has unusually high concentration of organic carbon (15.3±9.7 wt.%) as averaged over seasons (Krickov et al, 2018). Small rivers (Swatershed < 100 km²) have 2 to 3 times higher Corg (ca., 20-30%) compared to large ones (5-10% Corg in rivers with Swatershed > 10,000 km²).…”