The effect of permafrost, vegetation, and lithology on Mg and Si isotope composition of the Yenisey River and its tributaries at the end of the spring flood

Abstract

This work focuses on the behavior of the stable Mg and Si isotope compositions of the largest Arctic river, the Yenisey River and 28 of its major and minor tributaries during the spring flood period. Samples were collected along a 1500 km latitudinal profile covering a wide range of permafrost, lithology, and vegetation. Despite significant contrasts in the main physico-geographical, climate, and lithological parameters of the watersheds, the isotope composition of both dissolved Mg and Si was found to be only weakly influenced by the degree of the permafrost coverage, type of vegetation (forest vs. tundra), and lithology (granites, basalts, carbonates or terrigenous rocks). This observation is generally consistent with the lack of chemical uptake of Mg and Si by soil mineral formation and vegetation during the early spring. The radiogenic Sr isotope composition of the Yenisey and its tributaries varied within a narrow range (0.708 ≤ 87 Sr/ 86 Sr ≤ 0.711) reflecting the dominance of Phanerozoic rock weathering and/or atmospheric deposition on these compositions. The Mg and Si isotopic compositions of riverine samples reflect two main processes with distinct isotopic signatures. First, isotopically heavier Mg (δ 26 Mg =-1.0 ± 0.2 ‰) and isotopically lighter Si (δ 30 Si = 1.0 ± 0.25 ‰) are added to the waters by river suspended matter dissolution and leaching from vegetation biomass/topsoil litter. Second, isotopically lighter Mg (δ 26 Mg =-1.5 to-1.75 ‰) and isotopically heavier Si (δ 30 Si = 1.75 to 2.0 ‰) are delivered to the Yenisey's tributaries from deep underground water feeding the rivers via taliks. This lighter Mg and heavier Si isotopic composition is interpreted to originate from Precambrian dolomite dissolution and aluminosilicate dissolution coupled with authigenic mineral precipitation, respectively, in deep underground water reservoirs. Taking account of the isotopic composition evolution over the course of the year established earlier on mono-lithological watersheds of the Yenisey basin, the average annual isotopic signatures of the Yenisey river arriving to the Arctic Ocean are estimated to be δ 26 Mg =-1.58±0.30 ‰ and δ 30 Si =+1.60±0.25 ‰. As the Yenisey is the largest river feeding the Arctic Ocean and as it

Constraining modern‐day silicon cycling in Lake Baikal

“…Despite the different geology (basalt versus dominant granitoids around Lake Baikal) values in this study are similar to those recorded in the Kulingdakan River watershed approximately 500–1000 km north of Lake Baikal, but in the same basin [ Pokrovsky et al , ]. While the winter sample from the Selenga River is similar to summer Selenga River values, suggesting no seasonal variation in river δ 30 Si DSi (+1.54‰ ± 0.14 (winter); +1.48‰ ± 0.04 (summer)), an up to +1.5‰ decrease in δ 30 Si DSi was documented in the Kulingdakan watershed during spring ice melt [ Pokrovsky et al , ] and for the wider Yenisey drainage basin during permafrost melting (up to +1.75 and +2‰ heavier) [ Mavromatis et al , ]. Seasonal decreases have also been observed in rivers in northern Sweden [ Engström et al , ].…”

“…The δ 30 Si DSi compositions of rivers flowing into Lake Baikal fall within the range of other river systems across the globe, including sites impacted by humans (−0.9‰ to +4.7‰) [ De la Rocha et al , ; Ding et al , , ; Alleman et al , ; Ziegler et al , , ; Georg et al , , ; Cardinal et al , ; Engström et al , ; Hughes et al , , , ; Opfergelt et al , , ; Cockerton et al , ; Delvaux et al , ; Fontorbe et al , ; Pokrovsky et al , ; Sun et al , ; Frings et al , ; Mavromatis et al , ]. As such, these systems encompass a wide range of land uses, geology, vegetation, and climate, which all play an active role in driving river δ 30 Si DSi compositions.…”

“…Seasonal decreases have also been observed in rivers in northern Sweden [ Engström et al , ]. In both cases the decreases have been partially attributed to an increase in suspended load and in particular the dissolution of plant derived silicon which seasonally dominates the solid phase of silicon in surface soils, both of which act as pools enriched in the lighter 28 Si [ Engström et al , ; Pokrovsky et al , ; Mavromatis et al , ]. The lack of seasonal variation in the Selenga River could reflect the absence of seasonal biotic and abotic processes affecting δ 30 Si DSi signatures, potentially related to the larger size of the Selenga River, which may buffer any seasonal signal.…”

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

“…This is especially true for high latitude boreal and arctic rivers that have experienced strong change in discharge (Peterson et al, ) and chemical composition (Drake et al, ; Walvoord & Striegl, ) over the past decades. Among all Arctic rivers investigated by terrestrial biogeochemists, the Ob River remained strongly understudied compared with North American rivers (i.e., Douglas, Blum, Guo, Keller, & Gleason, ; Guo, Cai, Belzile, & Macdonald, ; Stolpe, Guo, & Shiller, ) and other large Siberian rivers (Denfeld, Frey, Sobczak, Mann, & Holmes, ; Drake et al, ; Gladyshev et al, ; Hirst et al, ; Mavromatis et al, ; Prokushkin et al, ), with only few works devoted to dissolved element transport (Alexeeva et al, ; Dai & Martin, ; Drake et al, ; Moran & Woods, ; Shiklomanov & Skakalsky, ; Telang et al, ). However, in the context of climate warming and permafrost thaw, the Ob watershed is especially important and most vulnerable to ongoing environmental changes as it is covered by permafrost for only 30% of the basin, compared with 75%, 90%, and 100% of the Mackenzie, Yenisei, and Lena watersheds, respectively (Environment Canada, ; McClelland, Holmes, Peterson, & Stieglitz, ).…”

“…Due to the high importance of floodplain soils for terrestrial organic carbon (OC) stock and the vulnerability of the riparian soil OC (Sutfin, Wohl, & Dwire, ), the processes controlling chemical composition of large rivers exhibiting extensive floodplain zones are at the forefront of biogeochemical studies (Bradley & Cox, ; Cai, Shim, Guo, & Shiller, ; Du Laing, Rinklebe, Vandecasteele, Meers, & Tack, ; Scott, Keim, Edwards, Jones, & Kroes, ; Shim, Cai, Guo, & Shiller, ). Among factors likely to control element supply to the river main stem are plant leaching of the riparian (wetland) zone (Shim et al, ; Vorobyev et al, ) and dissolution of silicate suspended particles (Mavromatis et al, ; Pokrovsky, Reynolds, Prokushkin, Schott, & Viers, ). Fast degradation of plant litter in water (Aerts & Chapin, ; Fraysse, Pokrovsky, & Meunier, ) may provide a significant and highly labile pool for OC and a number of major (Ca, Si, and K) and trace elements, especially during the spring flood period.…”