UC Santa Cruz

Many questions about the past state of ocean systems cannot be answered through direct observation, either because monitoring data were never collected or because the timescale in question predates observation entirely. This dissertation addresses that gap using the geochemistry of biogenic calcium carbonate archives, fish otoliths and coral skeletons, which grow sequentially and preserve a continuous record of the conditions under which they formed. Three chapters apply this approach across different spatial and temporal scales. Chapter 1 uses otolith stable oxygen and carbon isotopes to test whether Red Snapper (Lutjanus campechanus) and Vermilion Snapper (Rhomboplites aurorubens) differ in adult habitat use following the 2010 Deepwater Horizon oil spill, as assumed by vulnerability frameworks that classify Red Snapper as migratory and Vermilion Snapper as resident. Results show the two species partition trophic resources but share statistically indistinguishable thermal histories and adult depth habitats, challenging the premise that migratory capacity alone determines disturbance exposure and providing a post-spill baseline for future fisheries assessments. Chapter 2 uses a multi-proxy coral skeletal record of Porites lutea from Ofu, American Samoa, a remote island with little sustained instrumental monitoring, to establish a historical baseline for reef nitrogen dynamics. A historical interval of depressed density and anomalously low nitrogen isotope values is attributed to a transient shift in ocean circulation rather than local anthropogenic input, providing context for interpreting the reef's mechanistically distinct modern decline. Chapter 3 uses coral extension and density of Porites from drowned reef terraces off Hawaii (IODP Expedition 389) to evaluate how coral growth responds to past sea-level change over ~200,000 years. The volatility of sea-level rate best predicts reduced coral extension, with direct relevance to reef response under increasingly volatile, accelerating sea-level rise. Together, these chapters demonstrate that biogenic archives can resolve questions inaccessible to direct observation, offering context for reassessing species vulnerability, disentangling natural from anthropogenic drivers of reef change, and informing future conservation and management decisions.