High-resolution geochemical and petrographic study of a last glacial Cook Islands flowstone and the hydrological significance of calcite optical properties

Abstract

The South Pacific Convergence Zone (SPCZ), the largest component of tropical circulation on our planet, extends diagonally across the South Pacific and its displacement controls rainfall variability and tropical cyclone activity. Speleothems that formed during the last glacial period in the Cook Islands offer the opportunity to investigate the strength and position of the SPCZ across both the last glacial termination, and during rapid climate fluctuations (e.g. the Dansgaard-Oeschger Events). We present a high-resolution, multiproxy study of an 80 cm-thick flowstone retrieved in Atiu (Southern Cook Islands) covering the period from 10 to 80 ka. Optical and fluorescence microscopy, synchrotron X-ray fluorescence mapping, stable isotope ratios and LA-ICP-MS trace elements analyses allowed us to characterise the geochemical and optical properties of different calcite fabrics and speleothem facies. The geochemical and textural properties of the flowstone are directly controlled by prior calcite precipitation (PCP), which caused large positive shifts in δ13C and δ18O values, and synchronous peaks in Mg concentration and other hydroclimate-sensitive elements. These rapid shifts are particularly marked between 35 and 45 ka and coincide with several Dansgaard-Oeschger events recorded by the Greenland Ice Cores. The positive shifts in δ13C and δ18O values and Mg concentration are commonly recorded within compact, translucent fabric characterized by low greyscale values. By contrast, shifts to more negative δ13C and δ18O values and low Mg concentrations are associated with a porous, open fabric with a high density of fluid inclusions and high greyscale values. This confirms previous findings from modern Atiuan stalagmites and cave-farmed calcite (Faraji et al., 2022, 2023), thus opening the possibility to utilise greyscale mapping and fabric logs as novel, high-resolution hydroclimate proxies.