Trace element transport in western Siberian rivers across a permafrost gradient
Towards a better understanding of trace element (TE) transport in permafrost-affected Earth surface environments, we sampled ∼ 60 large and small rivers (< 100 to ≤ 150 000 km<sup>2</sup> watershed area) of the Western Siberian Lowland (WSL) during spring flood and summer and...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-03-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/13/1877/2016/bg-13-1877-2016.pdf |
| Summary: | Towards a better understanding of trace element (TE) transport in
permafrost-affected Earth surface environments, we sampled ∼ 60 large and small rivers (< 100 to ≤ 150 000 km<sup>2</sup>
watershed area) of the Western Siberian Lowland (WSL) during
spring flood and summer and winter baseflow across a 1500 km latitudinal
gradient covering continuous, discontinuous, sporadic and permafrost-free
zones. Analysis of ∼ 40 major and TEs in the dissolved
(< 0.45 µm) fraction allowed establishing main environmental
factors controlling the transport of metals and TEs in rivers of
this environmentally important region. No statistically significant effect
of the basin size on most TE concentrations was evidenced.
Two groups of elements were distinguished: (1) elements that show the same
trend throughout the year and (2) elements that show seasonal differences.
The first group included elements decreasing northward during all seasons
(Sr, Mo, U, As, Sb) marking the underground water influence of river
feeding. The elements of the second group exhibited variable behavior in the
course of the year. A northward increase during spring period was mostly
pronounced for Fe, Al, 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 springtime northward decrease was observed for
Ni, Cu, Zr and Rb. The increase in element concentration northward was observed for Ti, Ga, Zr and Th only in
winter, whereas Fe, Al, rare earth elements (REEs), Pb, Zr, and Hf
increased northward in both spring and winter, which could be linked to
leaching from peat and transport in the form of Fe-rich colloids. A
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). Finally, B, Li, Cr, V, Mn, Zn, Cd, and Cs did not show any distinct trend
from S to N.
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The order of landscape component impact on TE concentration in rivers was
lakes > bogs > forest. The lakes decreased export of
Mn and Co in summer and Ni, Cu, and Rb in spring, presumably due to biotic
processes. The lakes enriched the rivers in insoluble lithogenic elements in
summer and winter, likely due to TE mobilization from unfrozen mineral
sediments. The rank of environmental factors on TE concentration in western
Siberian rivers was latitude (three permafrost zones) > season
> watershed size. The effect of the latitude was minimal in
spring for most TEs 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 versus bog water feeding
determined the pattern of TE concentration and fluxes in rivers
of various sizes as a function of season.
<br><br>
Comparison of obtained TE fluxes in WSL rivers with those of other subarctic
rivers demonstrated reasonable agreement for most TEs; the
lithology of base rocks was the major factor controlling the magnitude of TE
fluxes. Climate change in western Siberia and permafrost boundary
migration will essentially affect 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
plant–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 with merely active layer thickening and permafrost boundary
shift. |
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| ISSN: | 1726-4170 1726-4189 |