Relationship between modern rainfall variability, cave dripwater, and stalagmite geochemistry in Guam, USA

Modern rainwater, cave dripwater and cave stalagmite geochemical time series from a cave in Guam (13 degrees 38'N, 144 degrees 53'E) are used to better understand how changes in cave stalagmite geochemistry relate to aboveground changes in rainfall at a tropical location. A scientific field team based in Guam collects similar to monthly samples from multiple sites for geochemical analyses at a cave and aboveground rainfall from a nearby location. We compute a transfer function between rainfall amount and oxygen isotopic composition (delta O-18) of a decrease (increase) of 0.94 +/- 0.3 m/year for every 1 parts per thousand increase (decrease) in rainfall delta O-18, based on data extracted from the International Atomic Energy Agency (IAEA) Global Networks of Isotopes in Precipitation (GNIP) database and from data generated in this study. Dripwater delta O-18 and Mg/Ca ratios show annual cyclicity at some, but not all sites, accentuating the complex nature of cave hydrology. A stalagmite delta O-18 record for the last similar to 160 years indicates the existence of droughts of decadal length, when rainfall is estimated to be similar to 0.65 +/- 0.3 m/year less than average conditions. This estimate of rainfall reduction most likely refers to wet season months, as these months preferentially contribute to groundwater recharge. The proxy-based climate record at Guam provides new evidence highlighting how a rainy site in the Western Pacific Warm Pool today can experience considerable changes in rainfall on decadal timescales.