Investigating Rainfall Variability in the South Pacific Convergence Zone using the Geochemistry of Stalagmites from the Solomon Islands

Abstract:

The South Pacific Convergence Zone (SPCZ), which extends southeastward from New Guinea to Tahiti, is the largest perennial rainfall feature in the Southern Hemisphere. The position of the SPCZ and its associated rainfall varies significantly on multidecadal timescales, as documented by instrumental and climate proxy data. For example, stalagmite δ¹⁸O records (rainfall proxy) from Espiritu Santo, Vanuatu (Partin et al., 2013) and Guadalcanal, Solomon Islands (Maupin et al., 2014), document large (~1‰), abrupt changes in stalagmite δ¹⁸O on multidecadal timescales over the past 600 years that arise from internal variability in the climate system. The proxy data agree with the type of rainfall changes observed in the instrumental record, such as the change across 1976/77, but the older changes are larger in relative magnitude.

We expand on these earlier studies of rainfall variability in the SPCZ system using stable isotope variations in stalagmites from two other locations in the Solomon Islands (Munda, New Georgia, 8.3°S, 157.3°E; Suku, Nggela Pile (9.8° S, 160.2° E). These stalagmites range in age from about 400 CE to 1850 CE, based on U-Th dating, and have relatively fast growth rates (60 to 300 µm/yr). Stalagmite δ¹⁸O time series were generated from sub-samples milled every 500 µm, or approximately 1 to 8 years per data point. Initial results from these two new Solomon Island stalagmites not only confirm the presence of multidecadal variability in stalagmite δ¹⁸O identified in previous studies, but suggest that the same amplitude of variability has occurred over several windows of time during the past 1600 years. When complete, these new proxy rainfall records from Munda and Suku will further constrain the pattern and mechanism of SPCZ rainfall variability in western tropical Pacific region.