wAbstract
@It has been proposed that silicon (Si) isotopes are fractionated
during weathering and biological activity leading to heavy dissolved
riverine compositions. In this study, the first seasonal variations
of stable isotope compositions of dissolved riverine Si are reported
and compared with concomitant changes in water chemistry. Four
different rivers in Switzerland were sampled between March 2004
and July 2005. The unique high-resolution multi-collector ICP-MS
Nu1700, has been used to provide simultaneous interference-free
measurements of 28Si, 29Si and 30Si
abundances with an average limiting precision of }0.04ρ on Β30Si.
This precision facilitates the clarification of small temporal
variations in isotope composition. The average of all the data
for the 40 samples is Β30Si{0.84}0.19ρ (}1ΠSD).
Despite significant differences in catchment lithologies, biomass,
climate, total dissolved solids and weathering fluxes the averaged
isotopic composition of dissolved Si each river is strikingly
similar with means of {0.70}0.12ρ for the Birs, {0.95}0.22ρ for
the Saane, {0.93}0.12ρ for the Ticino and {0.79}0.19ρ for the
Verzasca. However, the Β30Si undergoes seasonal variations
of up to 0.6ρ. Comparisons between Β30Si and physico-chemical
parameters, such as the concentration of dissolved Si and other
cations, the discharge of the rivers, and the resulting weathering
fluxes, permits an understanding of the processes that control
the Si budget and the fate of dissolved Si within these rivers.
@The main mechanism controlling the Si isotope compositions of
the mountainous Verzasca River appears to be a two component mixing
between the seepage of soil/ground waters, with heavier Si produced
by clay formation and superficial runoff associated with lighter
Si during high discharge events. A biologically-mediated fractionation
can be excluded in this particular river system. The other rivers
display increasing complexity with increases in the proportion
of forested and cultivated landscapes as well as carbonate rocks
in the catchment. In these instances it is impossible to resolve
the extent of the isotopic fractionation and contributed flux
of Si contributed by biological processes as opposed to abiotic
weathering.
@The presence of seasonal variations in Si isotope composition
in mountainous rivers provides evidence that extreme changes in
climate affect the overall composition of dissolved Si delivered
to the oceans. The oceanic Si isotope composition is very sensitive
to even small changes in the riverine Si isotope composition and
this parameter appears to be more critical than plausible changes
in the Si flux. Therefore, concurrent changes in weathering style
may need to be considered when using the Si isotopic compositions
of diatoms, sponges and radiolaria as paleoproductivity proxies.
Keywords: Si isotopes; seasonal variability; river; weathering;
MC-ICPMSx
1. Introduction
2. Sampling and analytical methods
@2.1. Sampling sites and geological setting
@2.2. Sampling
@2.3. Source of additional data
@2.4. Correction for wet depositional input
@2.5. Chemical preparation techniques
@2.6. Mass spectrometry
3. Results
@3.1. Major element chemistry
@3.2. Silicon concentrations and isotopic compositions
4. Discussion
@4.1. Verzasca
@4.2. Ticino
@4.3. Birs and Saane
@4.4. Implications for the global silicon budget
@4.5. Glacial-interglacial variations in seawater Β30Si
5. conclusions
Acknowledgement
References