The first application of rock surface luminescence dating as erosion-meter in Jankar Valley, Lahaul Himalaya

Reconstructing the timing of glaciations and post-glacial erosion rates is essential for understanding past climate variability, the interaction between climatic systems and landforms, and for predicting future glacier behaviour under global warming. Such reconstructions also provide quantitative chronological constraints on landscape evolution. The terrestrial cosmogenic nuclide (TCN) dating technique using ¹⁰Be is among the most widely applied methods for reconstructing glacial histories by constraining the exposure ages of moraine boulders that record past glacial activity. However, the measured nuclide concentration is sensitive to post-depositional rock surface erosion, which can lead to an underestimation of exposure ages if not properly corrected. While very low erosion rates (<10^-3 mm/a) exert a negligible influence on exposure-age calculations, higher rates (>10^-2 mm/a) can significantly underestimate the apparent ages of sampled surfaces. Therefore, exposure and erosion rates are often evaluated using paired-nuclide approach or complementary dating techniques. Furthermore, the selection of different scaling models for cosmogenic nuclide production introduces additional variability and can significantly affect the derived exposure ages. This necessitated the development of a reliable complementary rock surface chronometer. Recently, rock surface luminescence dating (RSLD) has emerged as a promising technique for constraining exposure and erosion rates of rock surfaces. A few recent studies have used a combined approach integrating RSLD with ¹⁰Be dating to constrain post-exposure erosion rates and erosion corrected exposure ages. The accuracy of RSLD has been further improved through the application of General order kinetic (GOK) model, which incorporate the non-exponential decay of feldspar IRSL (Infrared Stimulated luminescence) signal within into RSLD modelling. This advanced approach has been applied in Lahaul Himalaya to determine the timing of past glacial activity and quantify post-glacial erosion rates. A total of eleven granitic gneiss boulders were sampled from five morphologically distinct moraine ridges distributed across two catchments within the Jankar Valley. Preliminary analysis revealed that the order of kinetics varied from 2.03 to 2.45, indicating the nonlinear behaviour of feldspar. Furthermore, the exposure ages derived from RSLD were significantly underestimated, suggesting the presence of relatively high surface erosion rates. The erosion rates were estimated by assuming step function erosion history and the values ranged from 64 ± 20 mm/kyr to  236 ± 30 mm/kyr, which are considerably higher than the previously reported rates of 0.8 mm/kyr. These elevated erosion rates are expected to exert a substantial influence on the apparent exposure ages obtained from TCN dating.