Stalagmite δ18O Variability Records of ENSO-Controlled Rainfall From Niue Island, South Pacific

Niue Island (19°00'S, 169°50'W), a large carbonate platform located at the edge of Pacific Warm Water Pool offers an ideal opportunity to reconstruct El Nino Southern Oscillation (ENSO) history by means of speleothems. Rainfall on Niue is primarily controlled by ENSO. Previously employed layer thickness proxy revealed that ENSO phenomenon is archived in stalagmites from Niue (Rasbury et al., 2006). Here we present a high resolution (sub-annual) δ18O profile from a stalagmite (ASM1) sampled in 2002 in a active growing position from a flank margin cave on Niue Island in order to test whether oxygen isotope composition of Niue stalagmites is a reliable proxy tool for detecting rainfall and ENSO variability. The stalagmite contains approximately 150 years of deposition and consists of sub-annual couplets alternating between thick, light calcite bands deposited during the austral summer and thin, dark calcite layers deposited during the austral dry winter. Comparison of ASM1 stalagmite δ18O with Niue annual rainfall generally shows an inverse relationship between oxygen isotope ratios and the amount of precipitation, with 18O depleted values reflecting higher rainfall. Conversely, 18O enriched values usually correspond with low rainfall. Because ENSO results in severe droughts on the island, Southern Oscillation Index (SOI) was also evaluated against ASM1 stalagmite δ18O. Not surprisingly, low SOI phases (ENSO events) coincide with more positive δ18O values. Thus, stalagmite δ18O of ASM1 represents a proxy tool for the amount of rainfall on Niue and ENSO variability. Ongoing research aims to derive an additional δ18 high resolution profile from a coeval stalagmite. Power spectra analysis of the δ18O profiles will be presented at the meeting. Reference: Rasbury, M., and P. Aharon (2006), ENSO-controlled rainfall variability records archived in tropical stalagmites from the mid-ocean island of Niue, South Pacific, Geochem. Geophys. Geosyst., 7, Q07010, doi:10.1029/2005GC001232.