Erosion rates of the Bhutanese Himalaya determined using in situ-produced ¹⁰Be

Western Bhutan provides an ideal setting to understand the interplay between uplift, erosion, and fluvial sediment transport in an active mountain environment. Using in situ-produced ¹⁰Be (49 samples) and ²⁶Al (5 samples) in fluvial sediment from nested catchments throughout the Puna Tsang Chhu drainage basin, we examine erosion rates in different geomorphic environments including two high-relief regions – a glacierized zone in the north and a high-rainfall zone in the south – as well as remnants of an uplifted, lower-relief paleosurface between them. The erosion rates roughly mirror this north–south zonation: lower rates (avg. 388 ± 32 m My⁻¹, n = 16) prevail in the low-relief zone, roughly coinciding with lower-relief terrain where mean annual precipitation is ~ 1500 mm yr⁻¹; the highest rates (avg. 956 ± 160 m My⁻¹, n = 13) are in the south (27.10°–27.35°N), where rainfall is > 4000 mm yr⁻¹; high rates (avg. 700 ± 62 m My⁻¹, n = 15) also occur in the northern, glacierized region (27.70°–28.10°N). All 49 purified mineral separates used in this study contain measurable amounts of native ⁹Be (up to 900 μg), violating the assumption of negligible ⁹Be that is commonly made in the isotope dilution method used to quantify ¹⁰Be. To correct for this native ⁹Be, we use high precision, replicate measurements of ⁹Be in each sample to calculate ¹⁰Be concentrations from measured isotopic ratios. Neglecting native ⁹Be would have led to erosion rate overestimates from <20% to >400%. The pervasive nature of ⁹Be in these samples underscores the importance of quantifying the native ⁹Be concentration in mineral separates used for cosmogenic ¹⁰Be analysis.