Controls on rainfall variability in the tropical South Pacific for the last 350 years reconstructed from oxygen isotopes in stalagmites from the Cook Islands.

The hydroclimate of the South Pacific is highly variable on a range of timescales, however, the brevity of instrumental data in the region complicates the detection and attribution of this variability. Annually laminated speleothems may fill this knowledge gap by providing paleo-hydroclimate proxy data extending beyond the instrumental period. In this study, replicated oxygen isotope (δ18O) time series from two stalagmites from Pouatea (Pu17) and Nurau (Nu16) caves in the southern Cook Islands were used to reconstruct the variability of effective infiltration over the past 350 years (1672–2019 CE) based on observational rainfall data and cave monitoring and calcite farming experiments. The reconstructed infiltration was then compared to available reconstructions of known climate drivers of the South Pacific. Significant dry and wet periods over the last 350 years were identified based on the δ18O signal of compact/porous calcite fabric and their δ13C signals. The hydroclimate significance of these proxies was validated through comparison with the instrumental records. We could not reach an annual resolution of reconstructed infiltration, due to a combination of stable isotope sampling limitations and mixing effects within the aquifer. Nevertheless, the combined δ18O record of Pu17 and Nu16 identifies that the main driver of infiltration and, hence rainfall variability, in the southern Cook Islands is the location of the South Pacific Convergence Zone (SPCZ), which in turn is modulated by the El Niño Southern Oscillation (ENSO). This study demonstrates the hydroclimate significance of speleothem fabrics when combined with O and C stable isotopes, and adds paleoclimate information to a region that has a paucity of data. • ENSO and movements of the SPCZ are the main drivers of rainfall in the southern Cook Islands. • Joint use of speleothem fabrics, δ18O and δ13C serves as a proxy of effective infiltration. • Reconstruction of effective infiltration variability over the past 350 years (1672–2019 CE). • Insights into large-scale controls on rain and speleothem δ18O variability. [ABSTRACT FROM AUTHOR]