『Abstract
We present an extensive river sediment dataset covering the Ganga
basin from the Himalayan front downstream to the Ganga mainstream
in Bangladesh. These sediments were mainly collected over several
monsoon seasons and include depth profiles of suspended particles
in the river water column. Mineral sorting is the first order
control on the chemical composition of river sediments. Taking
into account this variability we show that sediments become significantly
depleted in mobile elements during their transit through the floodplain.
By comparing sediments sampled at the Himalayan front with sediments
from the Ganga mainstream in Bangladesh it is possible to budget
weathering in the floodplain. Assuming a steady state weathering
regime in the floodplain, the weathering of Himalayan sediments
in the Gangetic floodplain releases ca. (189±92)×109
and (69±22)×109 mol/yr of carbonate bound Ca and Mg
to the dissolved load, respectively. Silicate weathering releases
(53±18)×109 and (42±13)×109 mol/yr of Na
and K while the release of silicate Mg and Ca is substantially
lower, between ca. 0 and 20×109 mol/yr. Additionally,
we show that sediment hydration, [H2O+],
is a sensitive tracer of silicate weathering that can be used
in continental detrital environments, such as the Ganga basin.
Both [H2O+] content and the D/H
isotopic composition of sediments increases during floodplain
transfer in response to mineral hydrolysis and neoformations associated
to weathering reactions. By comparing the chemical composition
of river sediments across the floodplain with the composition
of the eroded Himalayan source rocks, we suggest that the floodplain
is the dominant location of silicate weathering for Na, K and
[H2O+]. Overall this work emphasizes
the role of the Gangetic floodplain in weathering Himalayan sediments.
It also demonstrates how detrital sediments can be used as weathering
tracers if mineralogical and chemical sorting effects are properly
taken into account.』
1. Introduction
2. Study setting
2.1. hydrological and erosional setting
2.2. Geological setting
3. Sampling strategies and analytical procedures
3.1. River sediment sampling
3.2. Sample analysis
3.2.1. Major elements
3.2.2. Carbonate content determination
3.2.3. Hydration and D/H stable isotopic composition in sediments
4. Results
4.1. Mineral sorting and weathering systematic
4.2. Spatial and temporal variation in chemical composition
4.2.1. Major elements
4.2.2. D/H isotopic composition of sediments
4.2.3. Carbonates content and isotopic composition
4.2.4. Mgs and Cas
5. Discussion
5.1. Source effects and temporal variability
5.1.1. Himalayan tributaries
5.1.2. Southern tributaries
5.2. Locus of continental weathering
5.2.1. Quantifying weathering intensity from river sediments
5.2.2. Floodplain weathering
5.2.3. Comparison with Himalayan weathering
5.2.4. Possible weathering mechanisms
5.3. Floodplain weathering budget
6. Conclusions
Acknowledgments
Appendix A
Appendix B
Appendix C. Supplementary data
References