Assessing the reproducibility of coral oxygen and carbon isotope time series from Fiji and Tonga and their application to the reconstruction of South Pacific Convergence Zone movements since the mid-1800s

A major feature affecting the hydrology of the southern hemisphere is the South Pacific Convergence Zone (SPCZ); a band of high rainfall extending southeastward from the Western Pacific Warm Pool (WPWP). It is a key source of atmospheric water vapor and latent heating. While it is clear that the SPCZ plays a fundamental role in Earth's climate, little is known about the patterns and mechanisms responsible for interannual to century-scale changes in its position and how it may respond to global climate change.;This research was focused on developing precisely dated, coral-derived geochemical records of SPCZ variability over the last 200 years. A unique regional coral network consisting of five coral time-series from Fiji (cores 1F, AB, FVB1, FVB2 and 16F) and three from Tonga (cores TH1, TNI2 and TF1) was used to investigate the reproducibility of coral delta18O and delta13C records to create Fiji and Tonga coral delta 18O and delta13C composite records for paleoclimatic reconstruction. Using at least three coral delta18O records from the same area is sufficient to obtain a statistically reliable signal for climate reconstruction. Fiji and Tonga coral delta18O composites at annual resolution are a suitable proxy for past sea surface salinity (SSS) variability. The Fiji coral core delta13C composite can be used to reconstruct the oceanic delta13C variability of the dissolved inorganic carbon (DIC) reservoir. The long-term decreasing trend in delta13C Fiji-DIC could be considered a representation of the well-known Suess effect. This study discusses interannual, decadal/interdecadal and long-term trends in the SPCZ salinity front movements, which appear to reflect mostly changes in water mass advection, rather than changes in precipitation linked with SPCZ displacement. Fiji and Tonga long term freshening trends suggest that the SPCZ is intensifying and/or expanding. The difference in salinity between Fiji and Tonga indicates that Fiji is freshening at a more rapid rate than Tonga. This study concludes that an intensification of the eastward flowing Subtropical Counter Current during the 20th century appears to be bringing relatively colder and saltier waters to Tonga; those conditions counterbalance the effects of the warmer and fresher conditions induced by the SPCZ expansion.