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°38′N, 144°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 ∼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 (δ18O) of a decrease (increase) of 0.94 ± 0.3 m/year for every 1‰ increase (decrease) in rainfall δ18O, 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 δ18O and Mg/Ca ratios show annual cyclicity at some, but not all sites, accentuating the complex nature of cave hydrology. A stalagmite δ18O record for the last ∼160 years indicates the existence of droughts of decadal length, when rainfall is estimated to be ∼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. Key Points Computed a transfer function bet rainfall amount and oxygen isotopic composition Dripwater delta18O and Mg/Ca ratios show annual cyclicity at some cave sites Guam stalagmite delta18O record (past 160 years) shows decadal‐scale droughts