Tracing Silicate Weathering in the Himalaya Using the 40K-40Ca System: A Reconnaissance Study

International audienceThis study investigates the potential of the 40K-40Ca decay scheme as a tracer of silicate vs. carbonate weathering in the Himalayan erosional system. To this end, we present high precision 40Ca data on river sediments, dissolved river loads and whole-rock carbonates representative of the main tectonic units of the Himalayas. Our results show that metamorphosed dolomites from the Lesser Himalaya (LH) exhibit no radiogenic 40Ca excess despite highly variable 87Sr/86Sr signatures (0.73-0.85). In contrast, silicate material is radiogenic, with ɛ40Ca ranging between +1 in the TSS to +4 ɛ-units in the LH. Preliminary results obtained from a series of 27 Himalayan rivers show that ɛ40Ca in the dissolved load is significantly influenced by silicate lithologies, with ɛ40Ca ranging from +0.1 to +1.6 ɛ-units. Our results suggest that the 40Ca signature of Himalayan rivers primarily reflects the lithological nature of their erosional source, and highlights the potential of the 40K-40Ca decay scheme as a tracer of silicate weathering