Stable silicon isotopic compositions of the Lena River and its tributaries: Implications for silicon delivery to the Arctic Ocean

Silicon isotope values (δ30SiDSi) of dissolved silicon (DSi) have been analyzed in the Lena River and its tributaries, one of the largest Arctic watersheds in the world. The geographical and temporal variations of δ30SiDSi range from +0.39 to +1.86‰ with DSi concentrations from 34 to 121 μM. No obvious patterns of DSi concentrations and δ30SiDSi values were observed along over 200 km of the two major tributaries, the Viliui and Aldan Rivers. In summer, the variations of DSi concentrations and δ30SiDSi values in the water are either caused by biological uptake by higher plants and phytoplankton or by mixing of water masses carrying different DSi concentrations and δ30SiDSi values. DSi in tributaries from the Verkhoyansk Mountain Range seems to be associated with secondary clay formation that increased the δ30SiDSi values, while terrestrial biological production is likely more prevalent in controlling δ30SiDSi values in Central Siberian Plateau and Lena Amganski Inter-River Area. In winter, when soils were frozen, the δ30SiDSi values in the river appeared to be controlled by weathering and clay formation in deep intrapermafrost groundwater. During the spring flood, dissolved silicate materials and phytoliths were flushed from the upper thawed soils into rivers, which reset δ30SiDSi values to the values observed prior to the biological bloom in summer. The results indicate that the Si isotope values reflect the changing processes controlling Si outputs to the Lena River and to the Arctic Ocean between seasons. The annual average δ30SiDSi value of the Lena Si flux is calculated to be +0.86 ± 0.3‰ using measured δ30SiDSi values from each season. Combined with the estimate of +1.6 ± 0.25‰ for the Yenisey River, an updated δ30SiDSi value of the major river Si inputs to the Arctic Ocean is estimated to be +1.3 ± 0.3‰. This value is expected to shift towards higher values in the future because of the impacts from a variety of biological and geochemical processes and sources under global warming.