Multidecadal rainfall variability in the South Pacific convergence zone using the geochemistry of stalagmites from the Solomon Islands

The instrumental record of hydroclimate variability in the South Pacific is sparse and of short duration; rainfall observations span on average 60-70 years, albeit, non-continuously. Therefore, proxy records of rainfall variability are needed to extend the hydroclimate record into the pre-instrumental period. In this study, we investigate the multidecadal timescale variability in rainfall using stable isotopic variations in three absolutely dated stalagmites (cave deposits) from the Central Province (CP) and Western Province (WP) of the Solomon Islands (SB) (~9.5°S, ~160°E). Hydroclimate variability in the SB is associated with the position and intensity of rainfall of the South Pacific Convergence Zone (SPCZ). The relatively high temporal resolution of the three stalagmites directs the focus on decadal-scale variability using oxygen isotope (δ18O) variations in the stalagmites, which are a proxy for rainfall variability in the tropics. We compare the three Solomon Island (SB) stalagmite δ18O records with previously published stalagmite δ18O records from Santo, Vanuatu and Guadalcanal, Central Province, Solomon Islands (CPSB) to assess the regional coherency amongst the various proxy rainfall records. The three new SB stalagmite δ18O records from this study are dominated by abrupt and large amplitude changes (~1-2‰) on multidecadal timescales. Similar patterns of multidecadal stalagmite δ18O variability have previously been identified in the stalagmite δ18O from Guadalcanal (CPSB) and Vanuatu and implies that multidecadal rainfall variability occurred across the South Pacific Ocean over the last millennium. However, the multidecadal changes do not occur in phase with time, which is in part due to age errors, but is also greatly influenced by the different seasonality of rainfall across the sites that bias which months of rainfall change will be recorded by a stalagmite. Our results also show that large changes in δ18O may not necessarily translate to equally large changes in rainfall amount as the modern rainfall - δ18O relationships at the various sites do not have equivalent slopes on multidecadal timescales. After deriving rainfall- δ18O transfer functions for the different sites, we observe that South Pacific stalagmite δ18O records translate to roughly similar amounts of rainfall change on multidecadal time scales for Guadalcanal, CPSB, Suku, CPSB and Santo, Vanuatu. However, Western Province, Solomon Island (WPSB) site appears to have the largest rainfall amount change (~2.0m/year) on multidecadal timescale. We also demonstrate that it is challenging to precisely date decadal and multidecadal scale variability in young stalagmites (past 1000 years) that have very low 238U concentrations and are very short in length. These characteristics lead to larger temporal uncertainty than solely analytical error. This will be of importance to future studies that may overlook the difficulties with young and short stalagmites records with low 238U concentrations.