『Abstract
This paper presents the first systematic survey of lithium and
its isotopes in the dissolved load and suspended and bed sediments
of tributaries of the Ganges, both before and after the monsoon.
Locations were chosen in order to cover catchments draining both
silicates and carbonates, at high (2000-4000 m) and low (550-1300
m) altitudes. Modelling of the dissolved composition shows that
the Li/Ca ratio of the silicate endmember in Himalayan rivers
is at least an order-of-magnitude higher than that of the carbonate
endmember. Most of the dissolved Li (＞90％) is derived from silicates
even in carbonate-dominated catchments. While the Sr-isotope composition
of the dissolved load reflects that of the bedrock, the main control
on its Li-isotope composition is fractionation during weathering.
Fractionation between the dissolved and suspended load in silicate-dominated
catchments is greatest at high altitude and lower at low altitude
where weathering is more intense. Tributaries draining silicates
have lower dissolved δ7Li values (by 2.3‰ to 4.2‰)
following the monsoon when weathering is more intense because
of higher runoff and elevated temperatures. Our data suggest that
riverine Li fluxes largely reflect silicate weathering rates,
while riverine δ7Li varies with weathering intensity.
As rivers presently contribute 〜50％ of the Li input to the oceans,
seawater Li concentrations and δ7Li show potential
as proxies for global silicate weathering processes.
Keywords: Li isotopes; silicate weathering; river chemistry; Sr
isotopes; Himalaya』
1. Introduction
2. Geological setting
3. Sampling and analyses
3.1. Sample collection and preparation
3.2. Cation analysis
3.3. Anion analysis
3.4. Li-isotope analysis
3.5. Sr-isotope analysis
4. Results
4.1. Dissolved load
4.2. Li isotopes
4.3. Sr isotopes
5. Discussion
5.1. Relative contribution of carbonate versus silicate to
the dissolved Li load
5.2. Li-isotope systematics
5.3. Implications for seawater Li and δ7Li
6. Conclusions
Acknowledgements
References