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Human influence is such a dominant force in modern earth systems that some scholars have proposed designating the “Anthropocene” as a distinct geologic epoch (Zalasiewicz et al., 2011). Understanding the magnitude and implications of modern environmental perturbations requires paleoenvironmental reconstructions to set baselines of natural variability and predict response to forcings (Lunt et al., 2024, Burls and Sagoo, 2022). Coral skeletons are widely used as high-resolution (sub-annual scale) proxies of environmental change because the composition of their aragonite (CaCO3) skeletons is influenced by sea surface temperature and seawater composition at the time of accretion (Saha et al., 2016, Schrag 1999).
However, many factors can complicate interpretation of coral skeletal archives, including vital effects (Sadler et al, 2014), diagenesis (Weerabaddana et al., 2024; Lazareth et al., 2016), and uncertainties in the relative ages of skeletal components (Sadler et al, 2014, Nothdurft and Webb 2007). Vital effects are changes to the skeletal geochemistry that occur while the coral is alive as a result of factors such as growth rate variability that influence element incorporation independent of environmental changes (Sinclair and Risk, 2006). Diagenesis is alteration to skeletal geochemistry that occurs from mineral dissolution and re-crystallization.
With regard to coral dating, although the absolute age of a core can be estimated from radioisotopes, constructing a geochemical chronology within the core requires developing an understanding of the relative ages of skeletal components. This dating may be accomplished by identifying seasonal density bands within a skeletal x-ray (similar to identification of tree rings) (Leonard et al., 2016). Where density bands are not clear, temperature proxy elements may be used to look for a seasonal temperature signal along an assumed growth axis that can indicate when a skeletal layer accreted within a solar year. To sum up, an ideal candidate coral for paleoenvironmental analysis will have minimal geochemical influence from vital effects and diagenesis as well as clear seasonality markers expressed as annual banding or sinusoidal temperature proxy fluctuation.
This thesis explores environmental reconstructions in modern (1900s) and Holocene (~5 kya) corals. The specimens analyzed include the commonly used mounding coral Porites(Holocene) as well as a less frequently investigated branching coral, Acropora (modern). Various analytical techniques were employed — including inductively coupled optical emission spectroscopy (ICP-OES), laser ablation mass spectroscopy (LA-ICP-MS), x-ray fluorescence (XRF), and x-ray diffraction (XRD) — to extract environmental information from elemental proxies and to determine how non-environmental signals may obscure interpretations. The results underscore some of the challenges involved in studying coral proxies, including the influence of diagenetic alteration and the difficulty of reliably identifying error from vital effects.