『Abstract
This study presents the Si isotope compositions of the dissolved
phase of twenty rivers from across Iceland. The high-resolution
Nu Plasma 1700 MC-ICP-MS was used to provide interference-free
30Si/28Si ratios with an average limiting
precision of ±0.04‰δ30Si. The Si isotope composition
ranges from -0.08‰ to 1.46‰δ30Si, averaging 0.63±0.38‰(±1σSD). This is only slightly lighter than the recently
reported mean of 0.84±0.19‰ for high precision measurements of
Swiss rivers but is significantly lighter than compositions reported
for some other rivers. The Si isotope composition varies slightly
with the amount of glacial cover, which provides evidence that
the continental Si input to the oceans may be climatically controlled
and thus likely to vary over glacial-interglacial cycles. After
correction for the dissolution of trace amounts of calcite in
one region, a correlation id\s found between δ30Si
and Ca/Si. This is consistent with the measured basaltic composition
and indicates isotopic fractionation during Si removal from the
dissolved phase. By calculating the fraction of Si that is removed
from the dissolved phase into secondary phases it is possible
to use the riverine Si flux (Si denudation rate) to calculate
the initial dissolution rate of Si within these catchments. From
this study we estimate that the total amount of Si being dissolved
by primary weathering of Iceland is 3 million tons per yr. However
only ～50％ of this Si is released into the rivers and contribute
to denudation. Using the Si isotope composition and the Si fraction
removed, it is possible to model the Si isotope fractionation
as either a Rayleigh-type fractionation or a steady-state system.
Both yield similar fractionation factors of bout -1.5‰δ30Si,
implying only one underlying fractionation process that is controlled
by external parameters, such as runoff. Furthermore, rivers that
follow a steady-state fractionation model are associated with
higher Si dissolution rates and lower δ30Si values,
indicating an riverine relationship between weathering rates and
dissolved riverine Si isotope composition.
Keywords: silicon; isotopes; weathering; Iceland; MC-ICP-MS』
1. Introduction
2. Climate and geology
3. Sampling and analytical methods
3.1. Sample preparation
3.2. Correction for wet depositional and hydrothermal inputs
3.3. Silicon isotope measurement by MC-ICP-MS
4. Results
4.1. Riverine aqueous chemistry
4.2. Isotope composition of the dissolved riverine Si
5. Discussion
5.1. Assessing the silicate derived weathering flux
5.2. Variations of the elemental load and δ30Si
5.3. Estimating Si removal
5.4. Relative weathering mobility of Si
5.5. Rayleigh-type vs. steady-state isotope fractionation
5.6. Biotic vs. abiotic fractionation
5.7. Controls on fsi and δ30Si
5.8. Glacial-interglacial variations of riverine δ30Si
6. Conclusions
Acknowledgements
References