40K-40Ca systematics as a tracer of silicate weathering in the Himalayas

Abstract:

Increased weathering resulting from the uplift of the Himalayan Mountains has been cited as one cause (among others) of late Cenozoic cooling. Orogenic uplift generally results in the development of high relief and enhanced erosion, which in turn induces rapid chemical weathering and uptake of atmospheric CO₂. Tracing these processes can be accomplished using techniques such as the budget of major element and/or Sr isotopes of the dissolved load. However, in the Himalaya, Sr isotopic compositions are extreme even in carbonate phases, making it difficult to produce realistic estimates of silicate and carbonate inputs to Himalayan rivers. In this study, we explore the potential of the ⁴⁰K-⁴⁰Ca system to quantify the relative contributions of silicate and carbonate weathering to river dissolved loads. To this end, we analysed a suite of 40 rivers draining the main litho-tectonic units of the Himalaya, sediments, bedrock, mineral separates and soil and gravel. Our results show that Himalayan carbonates exhibit no radiogenic ⁴⁰Ca excess despite highly variable ⁸⁷Sr/⁸⁶Sr signatures (0.73-0.85). The silicate fraction of bedload sediments is variably radiogenic (+1 to +4), with ε⁴⁰Ca correlating with the ⁸⁷Sr/⁸⁶Sr ratio. Analyses of plagioclase, biotite and muscovite separates from a HHC gneiss and greywacke show homogeneous ε⁴⁰Ca despite highly variable K/Ca ratios, indicating recent metamorphic equilibration on a mineral scale. Thus, incongruent weathering of high K/Ca minerals should not represent a significant factor when considering the transfer of radiogenic calcium to the dissolved load. The ε⁴⁰Ca signatures of dissolved river material are significantly influenced by silicate lithologies, and range from +0.1 in carbonate dominated catchments to >+2 ε-units in rivers draining predominantly silicate catchments. The silicate contributions estimated from our ⁴⁰Ca results are in line with major element budgets when considering small catchments but largely exceeds these estimates for the major tributaries. This could indicate the presence of a yet unidentified source of radiogenic ⁴⁰Ca, or major element budgets that generally overestimate the carbonate contribution to Himalayan rivers.