『Abstract
The major ion chemistry of the Marsyandi basin and six of its
tributaries in the Nepalese Himalaya have been investigated during
the monsoon months of 2002. Weekly water samples taken at 10 river
monitoring stations in the Annapurna watershed over the course
of 4 months provide chemical weathering data for the region at
an unprecedented temporal and spatial resolution. The river chemistry
of all but one basin is heavily dominated by carbonate weathering
which, compared to silicate weathering, contributes 80 to 97%
of the total solute load. This prevalence is due to a combination
of (a) intrinsically faster dissolution kinetics of carbonates,
(b) relatively high runoff and (c) glacial meltwater and low temperatures
at high altitudes resulting in enhanced carbonate solubilities.
Monitoring stations with headwaters in the Tethyan Sedimentary
Series (TSS) are particularly carbonate-rich and slightly supersaturated
with respect to calcite through half of the monsoon season. Silicate
weathering in the TSS is driven largely by sulfuric acid and therefore
does not contribute significantly to the drawdown of atmospheric
CO2. With respect to the tributaries in the
Greater Himalayan Sequence (GHS), carbonate weathering is practically
as predominant as for the TSS, in spite of the largely felsic
lithology of the GHS. Relative to the TSS, the primary proton
source in the GHS has shifted, with at least 80% of the protons
derived from carbonic acid. Averaged over the whole field area,
the CO2 fluxes, based on silicate-derived
Ca and Mg, are considerably lower than the global average. Assuming
that this study area is representative of the entire range, we
conclude that in situ weathering of the High Himalayas does not
represent a significant sink of atmospheric carbon dioxide, despite
the presence of a watershed south of the GHS that is basins appear
to be climate controlled, displaying a tight correlation with
runoff and temperature. Given the extremely low chemical weathering
under transport-limited conditions in high-altitude crystalline
terrains outside of the monsoon season, this would result in virtually
no chemical exhumation for 2/3 of the year in such a cold and
arid climate, north of the rain shadow cast by the High Himalayas.』
1. Introduction
2. Study area
3. Data acquisition and processing
3.1. Sampling and analysis
3.2. Corrections and errors
4. Results and discussion
4.1. Carbonate weathering in the catchments
4.2. Proton source in the TSS
4.3. Silicate weathering in the catchments
4.4. Temperature dependence of silicate weathering
4.5. Silicate weathering rates
4.6. CO2 drawdown in the catchments
4.7. Controls on chemical weathering
5. Conclusions
Acknowledgments
References