APPLICATION OF SEDIMENT PROVENANCE TECHNIQUES IN THE HIMALAYAN RAIN SHADOW: FOCUSED DENUDATION AND GLACIAL CONTRIBUTIONS IN THE ZANSKAR RIVER, NORTHWEST INDIA

Erosion, and ultimately the rate at which sediment is transported from source to sink, is a function of tectonic uplift and surface processes. We must isolate the effect each driver produces if we are to fully characterize erosion and make stratigraphic connections between marine delta systems and their mountain sources. Here, we aim to assess modern erosion in a tributary of the upper Indus River system, the Zanskar River, by applying several sediment provenance methods. We use bulk sediment petrography, geomorphological analysis, U-Pb detrital zircon dating and apatite fission track dating, and in-situ ¹⁰Be cosmogenic radionuclide techniques to establish regions of focused erosion and of maximum sediment production. From bulk petrography and U-Pb detrital zircon spectra of 15 modern river sediments we note a strong climatic impact on erosion in the Zanskar River and focused erosion along the High Himalaya. We find that the wettest and most glaciated subcatchment dominates the bulk sediment provenance signal, with only moderate input from other tributaries, and that other basin parameters cannot explain our observations. Catchment-averaged in-situ ¹⁰Be cosmogenic nuclide ages indicate erosion rates up to ~1.2 mm y^-1 but show strong dilution attributed to glacial sediment into the modern river, suggesting rates nearer 0.4–0.6 mm∙y^-1. These rates are consistent with long-term rates of incision (0.3–0.7 mm∙y^-1) calculated from detrital apatite fission track ages, and with incision rates inferred from Late Glacial and Holocene terraces near the Zanskar-Indus confluence. Our findings suggest that erosion in Himalayan rain shadow environments like Zanskar may be largely controlled by internal glacial fluctuations although it is not yet clear how long-term summer monsoon and winter westerlies strength impact this process.