『Abstract
　It has previously been concluded that the stable Si isotopes are fractionated during geochemical and biogeochemical elemental transfers, such as weathering and precipitation of clays and biogenic Si, which has opened up the possibility of using Si as a tracer in natural terrestrial ecosystems. Furthermore, quantification of the biogenic impact on the biogeochemical Si cycle has attracted significant scientific interest since biological control has been suggested. Previous observations of seasonal variations in the dissolved Si isotopic pattern further imply that high-frequency riverine sampling during main hydrological events might provide important information about natural processes governing the fluvial biogeochemical Si cycle.
　Therefore, temporal variations in the isotopic composition of riverine dissolved Si were investigated for the Kalix River, Northern Sweden, the largest pristine river system in Europe, based on high-frequency sampling during a period of 25 weeks from early April to early October 2006. Temporal variations spanning 0.4‰ for δ²⁹Si and 0.8‰ for δ³⁰Si of dissolved Si in the Kalix River were observed during the period, suggesting that the riverine dissolved Si input to the oceans cannot be considered to have a constant Si isotopic composition on a short time scale.
　The results implicate biogeochemical Si-cycling via formation and dissolution of biogenic silica as processes significantly affecting the dissolved Si transport in boreal systems during April to early October. The Si budget in the river system appeared to be controlled by relative Si enrichment during high discharge events and relative Si depletions in the subarctic mountainous and lake dominated areas. The Si enrichments and depletions were accompanied by decreasing and increasing riverine δ²⁹Si and 0.8‰ for δ³⁰Si, respectively. These isotope variations can be explained by release of plant derived silica, depleted in heavier Si isotopes, during the spring snowmelt. Further, increased volumetric contribution from the headwater and losses of dissolved Si due to biogenic silica formation by diatoms in the subarctic lakes at a later period are expected to be responsible for the preferential losses of lighter isotopes, as further verified by land cover analysis.

Keywords: Silicon isotopic composition; Dissolved Si; Biogenic silica; Subarctic river system; Biogeochemical Si cycle; MC-ICPMS』

1. Introduction
2. Sampling and methodology
　2.1. Study area
　2.2. Quantification of area-specific water contributions and land cover types in the catchment
　2.3. Sampling
　2.4. Instrumentation
　2.5. Determination of chemical and physical characteristics
　2.6. Chemical purification prior to Si isotope analyses
　2.7. Silicon isotopic analyses using MC-ICP-MS
3. Results and discussion
　3.1. Temporal variations in the discharge and percentage water contribution from different areas
　3.2. Major element chemistry
　3.3. Normalized dissolved elemental enrichment or depletion
　3.4. Silicon isotopic composition
　　3.4.1. Silicon isotopic variations during April to early June
　　3.4.2. Silicon isotopic variations during June, July and August
　　3.4.3. Silicon isotopic variations during September and early October
　3.5. Land cover analysis
4. Conclusions
Acknowledgements
References