Abstract:
The study examines the question whether speleothems from Niue Island (19°00'S, 169°50'W), a large carbonate platform located at the edge of West Pacific Warm Pool, can serve as archives of hydroclimate controlled by El-Niño/Southern Oscillation (ENSO) and of catastrophic cyclones that frequent the island. Niue Island is heavily karstified, with modern and fossil speleothems hosted by coastal and inland caves. The flank margin caves on Niue are shown to be formed by the action of corrosive groundwaters on uplifted Pleistocene-age reef carbonates in a tectonically active region. The focus of this study is an actively growing stalagmite sampled from a flank margin cave (Avaiki Cave) that contains about 146 years of deposition (2002-1856 AD). The stalagmite consists of sub-annual couplets alternating between white porous calcite laminae deposited during the austral summer and dark, compact calcite laminae deposited during the austral relatively dry winter. High resolution (sub-annual) stable isotope and trace element profiles accompanied by trace element X-ray mapping were used to test the validity of ENSO-controlled hydroclimate and tropical cyclones archived in the stalagmite. The results show that interannual variability in the stalagmite d18O and d13C time series agrees well with instrumental-derived ENSO phases (El Niño and La Niña events during 1866-2002) and the sea level pressure differential (Samoa-Fiji)-based SPCZ index (SPI) that controls the interdecadal hydroclimate variability. Severe cyclones that directly impacted Niue Island over the last century are recorded by abrupt, large increases in trace element concentration values of Mg, and S accompanied by stable isotope positive excursions bearing seawater-derived signatures. Application of selected trace elements (i.e., Mg, Na, S, P) as proxies of severe storms is a novel technique that can be successfully applied in carbonate coastal areas with flank-margin caves impacted by severe cyclones. This study also demonstrates that sub-annual geochemical cycles in trace element laminae, unresolved by analytical linear transects due their complex distribution pattern, are successfully imaged by large area X-ray mapping of the stalagmite
