Abstract
The Ganges is one of the world's largest rivers and lies at the heart of a body of literature that investigates the interaction between mountain orogeny, weathering and global climate change. Three regions can be recognised in the Ganges basin, with the Himalayan orogeny to the north and the plateaus of peninsular India to the south together delimiting the Ganges alluvial plain. Despite constituting approximately 80% of the basin, weathering processes in the peninsula and alluvial plain have received little attention. Here we present an analysis of 51 water samples along a transect of the alluvial plain, including all major tributaries. We focus on the geochemistry of silicon and its isotopes. Area normalised dissolved Si yields are approximately twice as high in rivers of Himalaya origin than the plain and peninsular tributaries (82, 51 and 32 kmol SiO2 km−2 yr−1, respectively). Such dissolved Si fluxes are not widely used as weathering rate indicators because a large but variable fraction of the DSi mobilised during the initial weathering process is retained in secondary clay minerals. However, the silicon isotopic composition of dissolved Si (expressed as δ30Si) varies from +0.8‰ in the Ganges mainstem at the Himalaya front to +3.0‰ in alluvial plain streams and appears to be controlled by weathering congruency, i.e. by the degree of incorporation of Si into secondary phases. The higher δ30Si values therefore reflect decreasing weathering congruency in the lowland river catchments. This is exploited to quantify the degree of removal using a Rayleigh isotope mass balance model, and consequently derive initial silica mobilisation rates of 200, 150 and 107 kmol SiO2 km−2 yr−1, for the Himalaya, peninsular India and the alluvial plain, respectively. Because the non-Himalayan regions dominate the catchment area, the majority of initial silica mobilisation from primary minerals occurs in the alluvial plain and peninsular catchment (41% and 34%, respectively).
Highlights
On geological timescales, Earth’s climate is regulated by a balance between silicate weathering reactions that consume atmospheric CO2 and a continuous input of carbon from volcanic and metamorphic degassing (Walker et al, 1981)
There is some longitudinal variation in the Ganges mainstem, with concentrations increasing from ∼75 μM at the base of the Himalaya to a plateau of ∼135 μM around the Allahabad–Varanasi region, before decreasing to ∼80 μM at Calcutta (Fig. 3a), almost identical to that seen previously (e.g. Fontorbe et al, 2013)
The Ramganga, a partially Himalayan river that drains the Siwaliks only, has higher [dissolved Si (DSi)] (190 and 184 μM) that are closer to concentrations in samples from the rivers draining the Deccan traps (Chambal, Betwa; mean = 219 μM) and of small floodplain systems, perhaps reflecting that the majority of the Ramganga catchment lies within the GAP
Summary
Earth’s climate is regulated by a balance between silicate weathering reactions that consume atmospheric CO2 and a continuous input of carbon from volcanic and metamorphic degassing (Walker et al, 1981). Given that degassing rates vary on long timescales but climate has remained broadly stable, a negative feedback between the rate of CO2 removal and atmospheric CO2 concentrations must exist (Berner and Caldeira, 1997). The dependency of silicate weathering rates on climate is the strongest contender for such a feedback, the exact nature of this dependency remains elusive. Driven by the hypothesis that the long-term global cooling since the early Eocene, illustrated, for example, by marine sediment records (Zachos et al, 2001) can be attributed to the Himalayan orogenesis over the same period (Raymo and Ruddiman, 1992), many investigations focus on the major and trace element and isotope geochemistry of the mountainous headwaters of large rivers. There is a growing awareness that lowland regions
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