Five centuries of climate dynamics from a South Pacific coral core
Keywords: Earth System Science.

Coral paleoclimatology uses climate proxies preserved in the exoskeletons of long-lived corals to reconstruct and analyze recent past climate. The most commonly used proxies, δ18O and the strontium/calcium (Sr/Ca) ratio, are used to reconstruct sea surface temperature (SST) and salinity at annual and subannual resolution. They provide an essential bridge between the modern instrumental era, with its vast quantities of high-resolution data, and geologic eras, which we understand at much lower temporal resolution. Coral paleoclimatology helps us understand the relatively stable climate of the Holocene and the transition to the Anthropocene at fine spatial and temporal scales. This dissertation focuses on the analysis of a 492-year-long, continuous δ18O time series from a massive Porites coral colony in Ta'u, American Samoa. Chapter 1 is a brief introduction to coral paleoclimatology and its tools, the climate dynamical modes that are often the subject of inquiry, and the scientific questions addressed in this dissertation. Chapter 2 establishes the chronology of the Ta'u coral core and relates the δ18O record to the environmental drivers of interest: SST and salinity. We then extract the interannual component of the δ18O record and compare it with instrumental records. Ta'u currently experiences contrasting impacts from different types of El Niño-Southern Oscillation (ENSO) events: conventional El Niño (La Niña) events generate cold and salty (warm and fresh) anomalies at Ta'u, while Modoki El Niño (La Niña) events warm (cool) the waters at Ta'u. A polarity switch in the δ18O-SST relationship over the twentieth century is evidence for a contracting "footprint" of conventional ENSO events. This changing sensitivity to ENSO at a single location complicates the interpretation of ENSO records in the past, and significant changes in ENSO-band variability at Ta'u may be due to changes in the areal extent or intensity of ENSO, or both. Chapter 3 examines low-frequency components of the Ta'u δ18O record. The low-pass filtered δ18O time series is a high-fidelity record of basinwide Pacific Decadal Variability (PDV), suggesting a dominant role for the tropics. The record shows dramatically reduced variability in the twentieth century and a major shift during the Great El Niño of 1789. We highlight the importance of salinity as a useful carrier of PDV signals and note contrasting salinity responses to PDV and ENSO despite the overall similarity in spatial patterns. A reconstruction of change points between phases of PDV finds a few same-polarity changes. This suggests that the Interdecadal Pacific Oscillation is a multiphase system driven primarily by stochastic integration rather than a two-phase system driven by nonlinear dynamics. Chapter 4 reports on work undertaken to develop a new laboratory technique to measure the Sr/Ca SST proxy, utilizing the Ta'u and several other coral cores. Established methods for recovering elemental concentrations in coral cores are time-consuming, laborious, and destructive of the sample. X-ray fluorescence (XRF) is a well-established technique for measuring trace elemental concentrations in a variety of substances. A series of experiments at the SLAC National Accelerator Facility synchrotron establishes both the promise and the significant challenges to XRF analysis of coral cores. These challenges include low signal-to-noise ratios, variable escape depths, and rough, heterogenous surfaces. While density data can be rapidly recovered through XRF, elemental concentrations remain problematic. This dissertation contributes an important new coral record to the growing library of high-resolution, preinstrumental proxy records. It reveals a new aspect of spatial changes in the ENSO system and presents the highest-fidelity reconstruction of PDV currently available. It also documents progress made towards developing XRF as a new analytical technique for coral paleothermometry.