Diurnal to interannual rainfall δ18O variations in northern Borneo driven by regional hydrology

The relationship between climate variability and rainfall oxygen isotopic (delta O-18) variability is poorly constrained, especially in the tropics, where many key paleoclimate records rely on past rainfall isotopes as proxies for hydroclimate. Here we present a daily-resolved, 5-yr-long timeseries of rainfall delta O-18 from Gunung Mulu National Park, located in northern Borneo (4 degrees N, 114 degrees E) in the heart of the West Pacific Warm Pool, and compare it to local and regional climatic variables. Daily rainfall delta O-18 values range from +0.7 parts per thousand to -18.5 parts per thousand and exhibit a weak but significant inverse relationship with daily local precipitation amount (R=-0.19, p<0.05), consistent with the tropical amount effect. Day-to-day delta O-18 variability at Mulu is best correlated to regional precipitation amount averaged over the preceding week (R=-0.64, p < 0.01). The inverse relationship between Mulu rainfall delta O-18 and local (regional) precipitation amount increases with increased temporal averaging, reaching R=-0.56 (R=-0.72) on monthly timescales. Large, negative, multi-day rainfall delta O-18 anomalies of up to 16 parts per thousand occur every 30-90 days and are closely associated with wet phases of the intraseasonal Madden-Julian Oscillation. A weak, semi-annual seasonal cycle in rainfall delta O-18 of 2-3 parts per thousand bears little resemblance to seasonal precipitation variability, pointing to a complex sequence of moisture sources and/or trajectories over the course of the year. Interannual rainfall delta O-18 variations of 6-8 parts per thousand, are significantly correlated with indices of the El Nino Southern Oscillation, with increased rainfall delta O-18 during relatively dry El Nino conditions, and vice versa during La Nina events. We find that Mulu rainfall delta O-18 outperforms Mulu precipitation amount as a tracer of basin-scale climate variability, highlighting the time- and space-integrative nature of rainfall delta O-18. Taken together, our results suggest that rainfall delta O-18 variability at Mulu is significantly influenced by the strength of regional convective activity. As such, our study provides further empirical support for the interpretation of delta O-18-based paleo-reconstructions from northern Borneo stalagmites as robust indicators of regional-scale hydroclimate variability, where higher delta O-18 reflects regional drying. (C) 2013 Elsevier B.V. All rights reserved.