Fractionation of the stable silicon isotopes studied using MC-ICP-MS : analytical method developments and applications in geochemistry

Sammanfattning: In spite of the high Si abundance in natural systems, expected significant mass-dependent fractionations, and the importance of the element in many areas of the Earth sciences (focusing on e.g. silicate weathering, global climate, paleoceanography, and biogeochemical Si cycling), the available information on Si isotope fractionation has remained rather limited due to the laborious and hazardous chemical purification procedures associated with the analyses. The initial focus of this thesis was therefore the development of analytical methods for the precise and accurate measurements of Si isotope ratios in a variety of matrices, which is an absolute requirement for meaningful fractionation studies. This involved detailed studies on sample preparation and refining the measurement protocol by using high resolution MC-ICP-MS. In the former stages, quantitative analyte recovery, thorough control of contamination levels and purification efficiency were the major targets, while severe spectral interferences and the need for adequate instrumental mass bias corrections challenged the latter. Efficient analyte separation, high-resolution capability of the instrument, quantitative Si recovery and accurate mass bias correction using Mg as internal standard, allowed the determination of the Si isotopic composition of Si reference materials, natural waters, plant and humus samples with long-term reproducibility, expressed as twice the standard deviation (2σ), equal to or less than 0.10‰ for δ29Si and 0.25‰ for δ30Si. Furthermore, the presence of a challenging spectral interference on 29Si originating from 28SiH+ was revealed, indicating that an instrumental resolution in excess of 3500 is required for interference-free Si isotopic analyses. However, despite complete removal of N-, O-, and C based interferences appearing on the high-mass side of the Si isotopes, it was found that exact matching of the acid matrix and Si concentration are mandatory due to tailing from the abundant 14N16O+ interference on 30Si. In addition, the condition of the high resolution slit is of the utmost importance for achieving highly accurate and precise δ30Si determinations.Methods developed were applied in geochemical studies aimed at increasing our knowledge of processes governing the terrestrial Si cycle in arctic and sub-arctic environments. This thesis includes results from the first study investigating the Si isotopic homogeneity of the major biomass component in Northern Sweden. The analyses revealed a narrow isotopically-light δ30Si range limited to (0.5 ± 0.2)‰ for bulk plant material averaging (-0.11 ± 0.18)‰, suggesting that Si isotopes might have the potential for use in the quantification of the biogenic impact on the biogeochemical cycle. Elemental analyses further revealed the presence of exogenous Si plant surface contaminations, a fact that has been neglected previously. This strongly indicates that potential surface Si contributions must be considered in biogeochemical studies.High-frequency sampling of the Kalix River system revealed detectable variations in the dissolved Si isotopic composition even on a daily basis. Hydrological modelling, elemental normalization and land cover analysis identified relative enrichments of dissolved Si originating from the forest covered areas and relative depletions from the mountainous lake areas as major processes controlling the Si budget in the system. The enrichments and depletions of dissolved Si were accompanied by decreased and increased δ30Si, suggesting that the forest areas are a source of isotopically-light Si to the system. The result of this study provides evidence that the formation and dissolution of biogenic silica has the potential to significantly affect the riverine Si budget and isotopic composition.Spatial dissolved Si isotope variations in the Lena River system, delta area and estuary suggest that processes controlling the Si budget in these systems are capable of altering the Si isotopic composition of the riverine end-member, a fact that must be considered in future studies. The isotopically-light Si isotopic composition in the Lena River, compared to data from boreal systems not underlain by permafrost, further strengthens the previous hypothesis of a significant biogenic impact in Siberia. Spatial variations in vegetation and permafrost cover, accompanied by detectable Si isotope differences, suggest that climatically induced permafrost thawing might have a significant impact on the riverine and marine Si isotope budget.