Zoom link: https://umich.zoom.us/j/99268139971

Circulation patterns in the eastern equatorial Pacific (EEP) have an outsized impact on the global climate system. Southeasterly trade winds force upwelling along the equator and the South American coastline, maintaining a regional ‘cold tongue’ that outgasses large fluxes of carbon dioxide into the atmosphere. Shifts in the intensity of EEP upwelling can moderate the development and decay of the El Niño–Southern Oscillation (ENSO), the largest source of interannual climate variability on Earth. Leading Earth system models predict ENSO impacts to intensify under anthropogenic greenhouse warming, but rely on scarce observations of its historical variability.

In this dissertation, I leverage oxygen isotopic (δ18O) and trace elemental measurements of aragonitic coral skeletons from the Galápagos archipelago to investigate EEP oceanographic conditions in the past and present. Geochemical tracers in scleractinian coral skeletons are powerful archives of past temperature and circulation patterns. At Galápagos, a globally significant biodiversity hotspot in the EEP, these environmental signals are closely linked to ENSO variability.

The relative abundance of strontium (Sr/Ca) within Porites lobata corals varies as an inverse function of seawater temperature, as does their δ18O composition. In Chapter 2, I generate Sr/Ca and δ18O records from subfossil Galápagos P. lobata to reconstruct ENSO variability at 4000 years before present (BP). Whereas δ18O records from the central equatorial Pacific document weakened ENSO between 5000-3000 BP, my results demonstrate robust, negatively skewed ENSO variance, amplified relative to that over the preindustrial last millennium. This contrast illuminates a shift in the spatial profile of ENSO at 4000 BP, with cold La Niña events intensifying and developing farther east in response to orbital forcing.

Dramatic shifts in ENSO variability often manifest due to changes in equatorial upwelling patterns. In Chapter 3, I investigate the sensitivity of barium (Ba/Ca), cadmium (Cd/Ca), and phosphorus (P/Ca) concentrations in P. lobata corals to upwelled water supply in the Galápagos archipelago. I statistically decompose vertical velocity data from an ocean physics reanalysis product to reveal two independent modes of regional upwelling associated with the shoaling Equatorial Undercurrent and the southeasterly trade winds, respectively. The coral geochemical tracers document contrasting variance patterns at separate island sites, consistent with distinct regional expressions of these overlapping upwelling patterns. Although coral Ba/Ca and Cd/Ca generally covary within a record, implying a shared environmental driver, P/Ca results are less informative.

Prior analyses have suggested that physiological artefacts can overprint environmental signals in coral Ba/Ca records, limiting proxy fidelity. In Chapter 4, I evaluate the influence of skeletal density and linear extension rate on Ba/Ca ratios from a large assemblage of living and subfossil Galápagos corals. 83% of the Ba/Ca records analyzed demonstrate no annual covariance with these physiological parameters, linked to coral calcification rate. However, mean Ba/Ca ratios are typically reduced in faster growing records, consistent with the Rayleigh fractionation model of element partitioning. Similarly, annual Ba/Ca values in one record correlate inversely with extension, driving periodic offsets from an overlapping record. These results indicate that Ba/Ca records should be screened for physiological artefacts ahead of their application.

Altogether, this dissertation provides a robust assessment of coral geochemical proxies and their utility in reconstructing oceanographic conditions in the EEP. Whereas temperature-sensitive tracers can demonstrably be leveraged to reconstruct the variance and asymmetry of historical ENSO, upwelling proxies are best paired with regional circulation data for faithful interpretation.