Extended Cave Drip Water Time Series Captures the 2015-2016 El Nino in Northern Borneo

Time series of cave drip water oxygen isotopes (delta O-18) provide site-specific assessments of the contributions of climate and karst processes to stalagmite delta O-18 records employed for hydroclimate reconstructions. We present similar to 12-year-long time series of biweekly cave drip water delta O-18 variations from three sites as well as a daily resolved local rainfall delta O-18 record from Gunung Mulu National Park in northern Borneo. Drip water delta O-18 variations closely match rainfall delta O-18 variations averaged over the preceding 3-18 months. We observe coherent interannual drip water delta O-18 variability of similar to 3% to 5 parts per thousand related to the El Nino-Southern Oscillation (ENSO), with sustained positive rainfall and drip water delta O-18 anomalies observed during the 2015/2016 El Nino. Evidence of nonlinear behavior at one of three drip water monitoring sites implies a time-varying contribution from a longer-term reservoir. Our results suggest that well-replicated, high-resolution stalagmite delta O-18 reconstructions from Mulu could characterize past ENSO-related variability in regional hydroclimate. Plain Language Summary Cave stalagmites allow for the reconstruction of past regional rainfall variability over the last hundreds of thousands of years with robust age control. Such reconstructions rely on the fact that differences in the isotopic composition of rainwater set by regional rainfall patterns is preserved as the rainwater travels through cave bedrock to feed the cave drip waters forming stalagmites. Long-term monitoring of rainwater and cave drip water isotopes ground truth the climate to stalagmite relationship across modern-day changes in regional rainfall. Twelve years of monitoring data presented in this study identify individual El Nino-Southern Oscillation events in rainfall and cave drip water isotopic composition, providing a strong foundation for stalagmite-based climate reconstructions from this site.