『Abstract
Major ion composition of waters, δ13C of its DIC (dissolved
inorganic carbon), and the clay mineral composition of bank sediments
in the Brahmaputra River System (draining India and Bangladesh)
have been measured to understand chemical weathering and erosion
and the factors controlling these processes in the eastern Himalaya.
The time-series samples, collected biweekly at Guwahati, from
the Brahmaputra mainstream, were also analyzed for the major ion
composition. Clay mineralogy and chemical index of alteration
(CIA) of sediments suggest that weathering intensity is relatively
poor in comparison to that in the Ganga basin. This is attributed
to higher runoff and associated physical erosion occurring in
the Brahmaputra basin. The results of this study show, for the
first time, spatial and temporal variations in chemical and silicate
erosion rates in the Brahmaputra basin. The subbasins of the Brahmaputra
watershed exhibit chemical erosion rates varying by about an order
of magnitude. The Eastern Syntaxis basin dominates the erosion
with a rate of 〜300 t km-2 y-1, one of the
highest among the world river basins and comparable to those reported
for some of the basaltic terrains. In contrast, the flat, cold,
and relatively more arid Tibetan basin undergoes much slower chemical
erosion (〜40 t km-2 y-1). The abundance
of total dissolved solids (TDS, 102-203 mg/L) in the time-series
samples collected over a period of one year shows variations in
accordance with the annual discharge, except one of them, cause
for which is attributable to flash floods. Na*(Na corrected
for cyclic component) shows a strong positive correlation with
Si, indicating their common source: silicate weathering. Estimates
of silicate cations (Nasil + Ksil
+ Casil + Mgsil) suggest
that about half of the dissolved cations in the Brahmaputra are
derived from silicates, a proportion higher than for the Ganga
system. The CO2 consumption rate due to silicate weathering in
the Brahmaputra watershed is 〜6×105 moles km-2
y-1; whereas that in the Eastern Syntaxis subbasins
is 〜19×105 moles km-2 y-1, similar
to the estimates for some of the basaltic terrains. This study
suggests that the Eastern Syntaxis basin of the Brahmaputra is
one of most intensely chemically eroding regions of the globe;
and that runoff and physical erosion are the controlling factors
of chemical erosion in the eastern Himalaya.』
1. Introduction
2. Brahmaputra River System: Hydrology and geology
2.1. The Brahmaputra river
2.2. Geology
3. Sampling and methodology
4. Results and discussion
4.1. Spatial and temporal variability in total dissolved
solids
4.2. Major ion composition
4.3. Chemical weathering in the Brahmaputra watershed
4.3.1. Clay composition and CIA of sediments
4.3.2. Sources of solutes in the Brahmaputra river system
4.3.3. Erosion in various subbasins
4.3.4. δ13C of dissolved inorganic carbon (DIC)
4.3.5. CO2 consumption by silicate weathering
4.4. Controls on chemical erosion in the Brahmaputra drainage
4.4.1. Runoff vs. chemical erosion
4.4.2. Physical vs. chemical erosion
4.5. Chemical fluxes from the Brahmaputra
5.Conclusions
Acknowledgments
References
Appendix