A 40K-40Ca approach to tracing silicate and carbonate weathering in the Himalayan erosional system

Tuesday, 16 December 2014
Jesse Davenport, Guillaume Caro and Christian France-Lanord, CRPG Centre de Recherches Pétrographiques et Géochimiques, Vandoeuvre-Les-Nancy, France
Understanding the effects of chemical erosion on the geologic CO2 cycle and seawater chemistry requires the ability to differentiate between the relative contributions of silicate and carbonate weathering to the dissolved load of rivers. Previous approaches (i.e. major elemental budgets or Sr isotopic compositions) do not always produce a straightforward explanation to the question at hand. This is especially the case in the Himalaya, where Sr isotopic compositions are extreme even in carbonate phases owing to metamorphic redistribution of radiogenic 87Sr. The aim of this study is to investigate a new isotopic approach using the 40K-40Ca system to better quantify the contributions of silicate and carbonate lithologies in the dissolved load of major Himalayan rivers. The silicate upper crust, with a K/Ca ratio of 1, is expected to have developed a radiogenic ε40Ca of ca. +2 ε–units, while marine carbonates are characterized by a homogeneous ε 40Ca=0 [1]. The 40K-40Ca system was therefore expected to produce robust constraints on the relative contribution of silicate vs. carbonate lithologies in dissolved river loads. To this end, we present high precision 40Ca data on river sediments, dissolved river loads and bedrock representative of the main Himalayan formations. Our results show that 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 ε40Ca ranging between +1 in the Tethyan Sedimentary Series (TSS) to +4 ε-units in the LH. Results obtained from a series of 27 Himalayan rivers show that ε40Ca in the dissolved load is significantly influenced by silicate lithologies, with ε40Ca ranging from +0.1 in carbonate dominated catchments to +1.6 ε-units in rivers draining predominantly gneissic catchments of the High Himalaya. Coherent, two end-member mixing trends between ε40Ca and major elements 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.

[1] Caro G., Papanastassiou D.A., Wasserburg G.J. 40K-40Ca isotopic constraints on the oceanic calcium cycle. Earth and Planetary Science Letters 2010;296: 124-132.