Assessing the paleoclimatic utility of the Indo-Pacific coral genus Diploastrea in a 225-year oxygen isotope record from Fiji

In the Western Pacific there remains, both spatially and temporally, a sparse record of surface ocean conditions and very few long paleoclimate records able to extend our understanding of this important region. The coral genus Porites is the common coral currently used for Pacific paleoclimate studies and has proven to be very useful. The massive coral Diploastrea, due to its slow growth rate and dense structure, may preserve temporally longer geochemical proxy records than Porites colonies of the same length. Its long lifespan and fossil history give this genus great potential; however no assessment has been made of the paleoclimatic utility of Diploastrea skeletons. Presented here are Diploastrea d18O time series from Savusavu, Fiji, a region sensitive to combined SST and precipitation changes due to activity of the El Niño Southern Oscillation (ENSO) and the South Pacific Convergence Zone (SPCZ). Sampling of a single skeletal element and/or a narrow sample track results in annual variations with the least amount of time averaging and greatest amplitude. Higher winter growth rates coupled with a constant sampling interval have preferentially captured winter conditions in the geochemical composition of Diploastrea\u27s skeleton. These winter-biased d18O time series illustrate that Diploastrea is as effective as Porites in recording the interannual environmental history of the region, dominated by both sea surface temperature (SST) and SPCZ-related rainfall. Examination of SST and precipitation data suggests that the trend component in Porites d18O at this site is amplified relative to observed trends in SST and expected trends in d18Oseawater, and that Diploastrea-generated d18O time series more closely reflects variability on this time scale. Utilizing Singular Spectrum Analysis (SSA) analysis, it is demonstrated that Diploastrea captures climate variability related to the Interdecadal Pacific Oscillation (IPO) as faithfully or more so than Porites. Wavelet analysis reveals a splitting of ENSO spectral power around 1880 to include lower frequencies, indicating that this interdecadal mode of variability may be fundamentally linked to ENSO, although it is observed to operate independently. The results presented here suggest that Diploastrea skeletal d18O reliably records both high and low frequency climate signals with some notable improvements over the genus Porites, and has enormous future potential for reconstructing tropical climate over the past 600-800 years