Trace elements transport in western Siberia rivers across a permafrost gradient

Abstract

Abstract. Towards a better understanding of trace element transport in permafrost-affected Earth surface environments, we sampled ∼ 60 large and small rivers (< 100 to ≤ 150 000 km2 watershed area) of Western Siberia Lowland (WSL) during spring flood and summer and winter base-flow across a 1500 km latitudinal gradient covering continuous, discontinuous, sporadic and permafrost-free zones. Analysis of ∼ 40 major and trace elements in dissolved (< 0.45 μm) fraction allowed establishing main environmental factors controlling the transport of metals and trace elements in rivers of this environmentally important region. No statistically significant effect of the basin size on most TE concentration was evidenced. Three category of trace elements were distinguished according to their concentration – latitude pattern: (i) increasing northward in spring and winter (Fe, Al, Ga (only winter), Ti (only winter), REEs, Pb, Zr, Hf, Th (only winter)), linked to leaching from peat and/or redox processes and transport in the form of Fe-rich colloids, (ii) decreasing northward during all seasons (Sr, Mo, U, As, Sb) marking the underground water influence of river feeding and (iii) elements without distinct trend from S to N whose variations within each latitude range were higher than the difference between latitudinal ranges (B, Li, Ti (except summer), Cr, V, Mn, Zn, Cd, Cs, Hf, Th). In addition to these general features, specific, northward increase during spring period was mostly pronounced for Fe, Mn, Co, Zn and Ba and may stem from a combination of enhanced leaching from the topsoil and vegetation and bottom waters of the lakes (spring overturn). A spring time northward decrease was observed for Ni, Cu, Zr, Rb. The southward increase in summer was strongly visible for Fe, Ni, Ba, Rb and V, probably due to peat/moss release (Ni, Ba, Rb) or groundwater feeding (Fe, V). The Principal Component Analysis demonstrated two main factors potentially controlling the ensemble of TE concentration variation. The first factor, responsible for 16–20 % of overall variation, included trivalent and tetravalent hydrolysates, Cr, V, and DOC and presumably reflected the presence of organo-mineral colloids, as also confirmed by previous studies in Siberian rivers. The 2nd factor (8–14 % variation) was linked to the latitude of the watershed and acted on elements affected by the groundwater feeding (DIC, Sr, Mo, As, Sb, U), whose concentration decreased significantly northward during all seasons. Overall, the rank of environmental factors on TE concentration in western Siberian rivers was latitude (3 permafrost zones) > season > watershed size. The effect of the latitude was minimal in spring for most TE but highly visible for Sr, Mo, Sb and U. The main factors controlling the shift of river feeding from surface and subsurface flow to deep underground flow in the permafrost-bearing zone were the depth of the active (unfrozen) seasonal layer and its position in organic or mineral horizons of the soil profile. In the permafrost-free zone, the relative role of carbonate mineral-bearing base rock feeding vs. bog water feeding determined the pattern of trace element concentration and fluxes in rivers of various size as a function of season. Comparison of obtained TE fluxes in WSL rivers with those of other subarctic rivers demonstrated reasonable agreement for most trace elements; the lithology of base rocks was the major factor controlling the magnitude of TE fluxes. The climate change in western Siberia and permafrost boundary migration will affect essentially the elements controlled by underground water feeding (DIC, alkaline-earth elements (Ca, Sr), oxyanions (Mo, Sb, As) and U). The thickening of the active layer may increase the export of trivalent and tetravalent hydrolysates in the form of organo-ferric colloids. Plant litter-originated divalent metals present as organic complexes may be retained via adsorption on mineral horizon. However, due to various counterbalanced processes controlling element source and sinks in plants – peat – mineral soil – river systems, the overall impact of the permafrost thaw on TE export from the land to the ocean may be smaller than that foreseen by merely active layer thickening and permafrost boundary shift.

Biogeochemistry of dissolved carbon, major, and trace elements during spring flood periods on the Ob River

“…All sampling, storage, and analytical procedures were identical to those described previously for the Western Siberian Lowland (WSL) rivers (Pokrovsky, Manasypov, Loiko, Krickov, et al, ; Vorobyev et al, ). Briefly, water samples were collected from 0.5 m depth in precleaned polypropylene bottles and were immediately filtered through sterile, single use Minisart ® filter units (Sartorius, acetate cellulose filter) with a pore size of 0.45 μm.…”

Size Distribution, Surface Coverage, Water, Carbon, and Metal Storage of Thermokarst Lakes in the Permafrost Zone of the Western Siberia Lowland

“…Three groups of elements can be distinguished: (i) major and trace elements, whose storage in lakes is less than 10% of that in rivers (DIC, Mg, Ca, Sr, Ba, K, Si, B, Fe, Co, Ni, Mn, and Ce); (ii) elements presenting non-negligible storage in lakes (20% ± 10% of that in rivers): DOC, Rb, Zn, Cu, V, Mo, Zr, As, Nd, and Th; and finally (iii) elements having significant, 30 to 70% storage in lakes, relative to rivers: Cd, Pb, La, Cr, and Al. It can be seen from this classification that major cations, DIC, B, Si, and metals subjected to significant redox transformations (Mn, Fe, and Ce), are essentially present in the rivers because they are delivered by groundwater feeding or shallow subsurface flux [79,80]. The groundwater and subsoil feeding are very low in lakes which have frozen peat on the border and frozen sediments at the bottom, throughout the year [54].…”

“…The thermokarst lakes are typically 1-2 pH units more acidic than the surrounding rivers [50,80], and this may enhance the solubility and mobility of many low-soluble trivalent hydrolysates (Al, Fe 3+ , rare earth elements), from the lake sediment to the water column. Another important source of solutes to the lakes is surface flow from surrounding peat bogs covered by mosses and lichens, consistent with the essentially allochthonous source of DOM in thermokarst lakes [10,50].…”

“…The stock of dissolved components in lakes on the permafrost-affected WSL territory can be compared to that delivered by all rivers of the WSL from the same territory to the Arctic Ocean. For this, watershed size-averaged, year-round fluxes of carbon, and major and trace elements assessed in previous works [79,80], can be used. A diagram of element stock in thermokarst lakes, relative to that in rivers of the WSL, is presented in Figure 6.…”

“…The filtered samples were stored at 4-5 • C in the dark, before analysis. The concentrations of dissolved organic and inorganic carbon (DOC and DIC, respectively), cations, and trace elements (TEs), were measured using routine methods for analyzing boreal water samples in the GET laboratory (Toulouse) [79,80]. The DOC and DIC were measured using a Shimadzu Total Organic Carbon Analyzer TOC 6000 with an uncertainty of 5%.…”