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

Abstract

In contrast to good understanding of chemical composition of the river suspended matter (RSM) of large rivers, small rivers remain strongly understudied, despite the fact that they can provide valuable information on mechanisms of RSM generation and transport depending on key environmental parameters of the watershed. This is especially true for permafrost-affected boreal and subarctic territories, subjected to strong modification due to permafrost thaw and landscape changes under climate warming. We selected Earth’s largest frozen peatland zone, the western Siberia Lowland (WSL) in order to test an impact of climate warming, permafrost thaw and landscape zone changes on riverine transport of particulate material from mainland to the Arctic Ocean. We sampled 33 small and medium size WSL rivers during spring flood, summer baseflow and autumn flood over a 1700 km gradient of climate and permafrost. Major and trace elements in particulate (>0.45 µm) and dissolved (<0.45 µm) fraction were analyzed. We hypothesize that future increase in active layer thickness and the change of dominant landscape from bogs and lakes to forest can be predicted via analyzing the actual pattern of RSM chemical composition across various permafrost zones and landscape parameters of WSL river watershed. We observed a minimum concentration of Li, Mg, Na, K, Rb, V, Cr, Zn, Cu, Co, Ni, Al, Ga, Y, REEs, Nb, W, Ti, Zr, Hf, Th and U in RSM collected from isolated and sporadic permafrost zones. Considering all seasons together, the presence of forest in the permafrost-bearing zone increased particulate concentrations of all alkalis and alkaline-earth elements, B, As, Nb, Mn, Co, Al, Ga, REEs, Ti, Zr, Hf, Th. This is consistent with element mobilization from mineral horizons that become available for interacting with soil fluids under forested regions. Lakes retained particulate alkaline-earths, Fe, Mn, Co, trivalent and tetravalent hydrolysates (TE3+, TE4+). The concentration of lithogenic low-soluble elements (TE3+, TE4+) in the RSM strongly increased with the river size (watershed area).Compared to the world RSM average, the WSL rivers exhibited lower concentrations of all elements except Mn and P and a low share of suspended elements relative to total (suspended + dissolved) forms of trace metals and of low-mobility (lithogenic) elements. Likely reasons for these features are: (i) low runoff and low RSM concentration as there is no rock and mineral substrate exposed to physical weathering in WSL peatland; (ii) organic, rather than mineral, nature of surrounding “solid” substrates and as a result, organic rather than silicate nature of RSM, and (iii) high DOC and Fe concentration leading to high concentrations of typically low-solubilty elements in the dissolved (<0.45 µm) fraction due to colloids.From a climate warming perspective, the increase in active layer thickness and involvement of mineral horizons into soil fluid migration in discontinuous to continuous permafrost zone will likely increase the share of particulate fraction in total element transport for many soluble (labile) elements and also lithogenic elements in WSL rivers. At the same time, permafrost boundary shift northward may decrease particulate concentrations of most major and TE in rivers of discontinuous permafrost zone. The lake drainage and forest colonization of tundra and bogs in the permafrost-affected part of WSL may increase the concentration of alkali and alkaline-earth elements, divalent metals and trivalent and tetravalent hydrolysates. As a result, export of particulate metal micronutrients and toxicants from the WSL territory to the Arctic Ocean may increase.