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

Abstract:

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.