A multi-proxy SST and surface seawater carbonate chemistry reconstruction of the post-Industrial Revolution Southwest Pacific

Oceanic uptake of anthropogenic CO2 emissions induced global seawater pH decrease by 0.1 since the Industrial Revolution by altering ocean chemistry with the reduction of carbonate ion concentrations and the saturation states of aragonite. Massive tropical corals are ideal palaeoceanographic archives providing high-resolution records of the most recent few hundred years and offer a valuable extension to instrumental measurements. The South Pacific Convergence Zone (SPCZ), the largest persistent precipitation band in the Southern Hemisphere with an associated salinity front modulated by large-scale ocean-atmospheric interactions (El Niño/Southern Oscillation, Interdecadal Pacific Oscillation) may influence regional seawater CO2 absorption and pH variability. Here we present coral-based paleoclimatic reconstructions from two Porites sp. corals of the Southwest Pacific region from Tonga and Rotuma. The corals are analyzed using a multi-proxy approach (δ18O, Sr/Ca, Li/Mg, U/Ca, Sr-U) to assess optimal sea surface temperature reconstruction. Preliminary δ18O results from both corals suggest similar freshening and/or warming of the surface water for the last 30 years of the 20th century (Tonga: -0.0038‰ δ18O per year; Rotuma: -0.0033‰ δ18O per year). Coral B/Ca and δ11B results for the reconstruction of carbonate chemistry changes and to establish the longer-term variability of seawater pH were completed. Tonga Porites sp. δ11B signature indicate a significant decreasing trend since 1779, with a pronounced depletion in δ11B since the 1950s of -0.0626 per year. Ultimately, this study will explore the regional-scale oceanic response to increasing pCO2 and temperature, as well as the influence of interannual and decadal-interdecadal climatic fluctuations.