『Abstract
Exhumation of the Himalayan-Tibetan orogen is implicated in the
marked rise in seawater 87Sr/86S ratios
since 40 Ma. However both silicate and carbonate rocks in the
Himalaya have elevated 87Sr/86S ratios and
there is disagreement as to how much of the 87Sr flux
is derived from silicate weathering. Most previous studies have
used element ratios from bedrock to constrain the proportions
of silicate- and carbonate-derived Sr in river waters. Here we
use arrays of water compositions sampled from the head waters
of the Ganges in the Indian and Nepalese Himalaya to constrain
the end-member element ratios. The compositions of tributaries
draining catchments restricted to a limited range of geological
units can be described by two-component mixing of silicate and
carbonate-derived components and lie on a plane in multicomponent
composition space. Key elemental ratios of the carbonate and silicate
components are determined by the intersection of the tributary
mixing plane with the planes Na = 0 for carbonate and constant
Ca/Na for silicate. The fractions of Sr derived from silicate
and carbonate sources are then calculated by mass-balance in Sr-Ca-Mg-Na
composition space. Comparison of end-member compositions with
bedrock implies that secondary calcite deposition may be important
in some catchments and that dissolution of low-Mg trace calcite
in silicate rocks may explain discrepancies in Sr-Ca-Na-Mg covariation.
Alternatively, composition-dependent precipitation or incongruent
dissolution reactions may rotate mixing trends on cation-ratio
diagrams. However the calculations are not sensitive to transformations
of the compositions by incongruent dissolution or precipitation
processes provided that the transformed silicate and carbonate
component vectors are constrained. Silicates are calculated to
provide 〜50% of the dissolved Sr flux from the head waters of
the Ganges assuming that discrepancies between Ca-Mg-Na covariation
and the silicate rock compositions arise from addition of trace
calcite. If the Ca-Mg-Na mixing plane is rotated by composition-dependent
secondary calcite deposition, this estimate would be increased.
Moreover, when 87Sr/86S ratios of the Sr
inputs are considered, silicate Sr is responsible for 70±16% (1σ)
of the 87Sr flux forcing changes in seawater Sr-isotopic
composition. Since earlier studies predict that silicate weathering
generates as little as 20% of the total Sr flux in Himalayan river
systems, this study demonstrates that the significance of silicate
weathering can be greatly underestimated if the processes that
decouple the water cation ratios from those of the source rocks
are not properly evaluated.』
1. Introduction
2. Study area
3. Sampling and analytical methods
4. Calculation of silicate- and carbonate-derived Sr fractions
4.1. Correction for rainfall and hot-spring inputs
4.2. Modeling two-component mixing
4.2.1. Tributary chemistry: Lesser Himalayan Deopryag catchment
4.2.2. Processes that may modify water chemistry
4.2.3. Calculation of Sr inputs to the Deopryag catchment
4.2.4. Influence of secondary calcite and trace calcite on Sr
partition calculations
4.2.5. Estimates of uncertainties
4.2.6. Sr inputs in the Kanwana catchment
4.2.7. Sr inputs in the Deoban catchment
4.2.8. Calculation of Sr inputs in the High Himalayan Crystalline
catchment
4.2.9. Calculation of Sr inputs in the Tibetan Sedimentary Series
catchment
4.3. Constrains on 87Sr/86Sr ratios of
carbonate and silicate end-members
5. Calculation of Sr and 87Sr/86Sr fluxes
6. Conclusions
Acknowledgments
References