Deconvolving the δ 18O seawater component from subseasonal coral δ 18O and Sr/Ca at Rarotonga in the southwestern subtropical Pacific for the period 1726 to 1997

Abstract

To reconstruct oceanographic variations in the subtropical South Pacific, 271-year long subseasonal time series of Sr/Ca and δ 18O were generated from a coral growing at Rarotonga (21.5°S, 159.5°W). In this case, coral Sr/Ca appears to be an excellent proxy for sea surface temperature (SST) and coral δ 18O is a function of both SST and seawater δ 18O composition (δ 18O sw). Here, we focus on extracting the δ 18O sw signal from these proxy records. A method is presented assuming that coral Sr/Ca is solely a function of SST and that coral δ 18O is a function of both SST and δ 18O sw. This method separates the effects of δ 18O sw from SST by breaking the instantaneous changes of coral δ 18O into separate contributions by instantaneous SST and δ 18O sw changes, respectively. The results show that on average δ 18O sw at Rarotonga explains ∼39% of the variance in δ 18O and that variations in SST explains the remaining ∼61% of δ 18O variance. Reconstructed δ 18O sw shows systematic increases in summer months (December–February) consistent with the regional pattern of variations in precipitation and evaporation. The δ 18O sw also shows a positive linear correlation with satellite-derived estimated salinity for the period 1980 to 1997 (r = 0.72). This linear correlation between reconstructed δ 18O sw and salinity makes it possible to use the reconstructed δ 18O sw to estimate the past interannual and decadal salinity changes in this region. Comparisons of coral δ 18O and δ 18O sw at Rarotonga with the Pacific decadal oscillation index suggest that the decadal and interdecadal salinity and SST variability at Rarotonga appears to be related to basin-scale decadal variability in the Pacific.