"The Implications of Metamorphism and Weathering of the Lesser Himalayan Formation in Eastern Nepal for Climate Change"

Oliver, Lee (2003). "The Implications of Metamorphism and Weathering of the Lesser Himalayan Formation in Eastern Nepal for Climate Change". PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f73a


This study evaluates the significance of Sr-isotopes as a silicate weathering proxy in the context of Himalayan river systems. The dissolved load of the Bhote Kosi (central Nepal) displays a rapid increase in 87Sr/86Sr ratio immediately downstream of the Main Central Thrust, a feature common to other Himalayan tributaries of the Ganges. The results of statistical and mass balance analyses identify the weathering of radiogenic (87Sr/86Sr >0.8) calc-silicate lithologies from the Lesser Himalayan Formation as the predominant control upon the dissolved 87Sr/86Sr ratio.

In situ laser ablation analysis of dolomite in these calc-silicates yields radiogenic 87Sr/86Sr, indistinguishable from bulk rock ratios, confirming the presence of a highly soluble source of radiogenic strontium. The distribution of Rb-Sr isotopes suggests homogenisation of Sr-fsotopes between silicate and carbonate at both grain-size and bulk-rock scale. Similar lithologies can be traced across much of the southern Himalaya, and multiple regressions from Bhote Kosi data successfully predict dissolved 87Sr/86Sr ratios in other major Himalayan rivers, indicating that findings from the Bhote Kosi are applicable across eastern Nepal.

Flux calculations indicate that at least 50% of the impact of the Bhote Kosi river on the global marine 87Sr/86Sr ratio can be traced to weathering of Lesser Himalayan carbonates. Hence a significant proportion of the increase in marine 87Sr/86Sr ratio over the past 7-10 Ma is ascribed to the exposure of these lithologies. The widespread assumption that dissolved 87Sr/86Sr ratios are a proxy for silicate weathering rates is therefore undermined where ancient metamorphosed calc-silicate rocks are exposed in river catchments. Although this conclusion casts doubt upon the use of the marine strontium isotope record to quantify carbon dioxide drawdown resultant from present day Himalayan silicate weathering, evidence suggests that prior to uplift of the Lesser Himalaya, dissolved Sr-isotope ratios in Himalayan runoff did provide an indication of silicate weathering rates.

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