Multi-coral reconstructions give new perspectives on Southwest Pacific oceanographic variability under anthropogenic forcing

The Southwest Pacific region, integral to understanding global climate dynamics due to its pronounced interannual to interdecadal variability, has been under-monitored by instrumental observations before the satellite era. To address this gap, proxy data from long-lived tropical corals are essential as they compensate for the late start of instrumental observations and provide insights into past oceanographic variability. Despite advancements from the current proxy-based state of the art, gaps remain related to regions such as the Western Pacific Warm Pool and understanding of hydrological and interannual variability in the South Pacific Convergence Zone (SPCZ). This thesis aims to further the knowledge of interannual to century-scale changes in the South Pacific Convergence Zone (SPCZ) position and climatology and how they may respond to global climate change. By utilizing coral proxies, it seeks to elucidate past sea surface temperature (SST), sea surface salinity (SSS), ocean acidification trends, and coral responses to changing carbonate chemistry for a multifaceted approach in describing the SPCZ region. New, monthly-resolved coral-based time series from Rotuma, Fiji, and Nomuka Iki, Kingdom of Tonga, extending back to pre-industrial baselines to 1821 and 1848, respectively, extend the geographical coverage of the South Pacific coral network.
Monthly-resolved Sr/Ca and reconstructed δ18Oseawater (sw) from paired Sr/Ca and δ18O measurements from previously undocumented island of Rotuma indicate a significant warming trend of up to 1.45 °C in the Western Pacific Warm Pool since the beginning of the 20th century. Reconstructed δ18Oseawater (sw) from paired Sr/Ca and δ18O show that the low salinity water (“fresh”) pool of the SPCZ has been extending over the last century as the Tonga coral records a freshening trend. These findings highlight the robustness of coral-based reconstructions in capturing reef to regional conditions, despite challenges posed by thermal stress events in very warm environments such as the Warm Pool that can disrupt the Sr/Ca-SST relationship. The newly generated coral-based time series in this work serve to join other SPCZ coral records in constructing of composite indices (SPCZcoral indices) to describe the regional trends and reconstruct past interannual variability. SPCZcoral indices were developed from coral geochemical time-series across multiple locations (Rotuma and Savusavu islands (Fiji), Nomuka Iki Island (Tonga), and Rarotonga). These indices extend instrumental SSS records by approximately 100 years and improve the resolution of past composite SST and SSS reconstructions of the region. They document a consistent warming trend of 1 °C and a freshening of 0.42 practical salinity units (SP) since the 1880s SPCZ wide. They offer new insights into the impact of interannual variability on the South Pacific Convergence Zone by showing differential responses between Eastern and Central Pacific El Niño episodes and reconstructing past SPCZ zonal events that bring droughts to island nations. As this thesis aims to provide a comprehensive overview of potential anthropogenic influences on the oceanographic variability of the SPCZ region, impacts of ocean acidification (“the other CO2 problem”) were explored through δ11B records as a proxy for seawater and coral calcifying fluid pH. Coral records from Rotuma and Tonga resolved annually to approximately biennially, dating back to 1821 and 1777, respectively, document a clear decline in pH since the late 19th century. Coral adaptive responses to increasing acidity were examined through calcifying fluid carbonate chemistry reconstructions from paired B/Ca and δ11B measurements. Despite the unfavorable conditions as reconstructed from the coral from Tonga, the coral has displayed resilience, and maintained growth rate, thus suggesting potential mechanisms for adaptation to ongoing environmental stressors. Rotuma shows a reversal of the falling pH trend since mid-20th century, which shows potential reversal of the region from a sink into a source of CO2 to the atmosphere due to observed warming of the Western Pacific Warm Pool region in the past few decades. The results warrant caution in using coral δ11B for studying open ocean acidification as local drivers add to the secular seawater pH trends and can have a varying magnitude of influence over the δ11B isotopic signature preserved in coral skeletons.
Collectively, the findings presented in this thesis converge to describe the multiple facets of changing conditions in the SPCZ in the past approximately 200 years. From observing local changes in coral locations, to the SPCZ wide changes with compositing approach, they underscore the critical role of coral proxies in climate reconstructions, particularly in regions with sparse instrumental data. Certain caveats and challenges remain in coral-based geochemistry and reconstructions, especially relating to coral metabolic imprints on proxies and the richness of local environmental conditions they might be recording, not captured by regional gridded instrumental datasets or in-situ measurements of any kind. This body of work contributes to the field of paleoclimatology by providing new coral-based records from the SPCZ, and specifically the WPWP area, extending historical climate records of the SPCZ and improving our understanding of the natural variability and anthropogenic impacts on the Earth's climate system.