Stable isotope variability in drip water and its implications for speleothem-based Indian Summer Monsoon reconstruction

Study region Dandak Cave, located in the Core Monsoon Zone of India. Study focus Understanding the transfer of isotopic composition from precipitation through the karst system to cave drip water is essential for interpreting speleothem-based reconstructions of the Indian Summer Monsoon. Drip water was monitored at three sites in Dandak Cave between 2004 and 2006, with measurements of δ 18 O, δ 13 C, and drip rate conducted to investigate the hydrological and geochemical controls on drip water isotopic composition. The observed drip-water δ 18 O was compared with precipitation δ 18 O estimates from a Rayleigh box model and an isotope-enabled general circulation model ensemble. New hydrological insights for the region Drip water δ 18 O displayed a seasonal cycle, with lower (higher) values during the monsoon (non-monsoon) months, consistent with the seasonal evolution of the estimated precipitation δ 18 O. The relatively small amplitude of drip water δ 18 O compared to that of precipitation suggests attenuation of event-scale variability within the soil–epikarst. Although the three monitoring sites differed in mean drip rate and drip rate variability, their seasonal mean δ 18 O values varied by only 0.1–0.2‰, indicating that they record a similar integrated seasonal signal. δ 13 C varied among sites, reflecting the combined influence of drip hydrology, carbon sources, and cave processes. δ 13 C and radiocarbon analyses yielded limestone-derived carbon contributions of 26 ± 3% and 24 ± 4%, respectively, indicating substantial host-rock carbon input.