A MULTIVARIATE RECORD OF EASTERN PACIFIC CLIMATE FROM A FOSSIL GALÁPAGOS CORAL, CE 1571-1626

In corals, the relationship between skeletal geochemistry and environmental variables is well understood, and coral-based geochemical records are well constrained for paleoceanographic reconstructions when corals are live-sampled. However, fossil corals present additional challenges for paleoclimate reconstruction, such as uncertainty in age constraints and the potential for post-mortem alteration. As a result, there is a lack of seasonally resolved records that extend >150 yr before present, which are important to understand interannual climate processes like the El Niño / Southern Oscillation (ENSO). The modern ENSO is well studied, but its historic variability is poorly understood during the Little Ice Age (LIA) (1400-1700 CE), when NH temperatures were 0.5-1°C cooler than present. To reconstruct the paleoceanography of the Eastern Equatorial Pacific (EEP) during the LIA, we generated a multivariate geochemical record from a fossil coral collected in Urvina Bay, in the western Galápagos archipelago (1°S, 91°W), where ENSO strongly affects sea surface temperature (SST). Based on a bottom U/Th date of 1571 CE ±7 yr, we used annual Sr/Ca cycles to establish that the core spans 55 years with monthly to bi-monthly resolution, ending in 1626 CE. We measured Sr/Ca (proxy for SST) and Ba/Ca ratios (proxy for upwelling) along the full length of the coral core, and collected stable isotope data from intermittent lengths. Our Sr/Ca data show an average annual SST range of ~4°C. Measured Sr/Ca and δ¹⁸Ο values are strongly correlated (r²=0.73) and suggest mean SST is 0.5°C lower than modern coral records in this region. Our record shows strong interannual SST variability, which demonstrates intermittent and highly variable occurrence of El Niño events. We observe seasonal cycles in the Ba/Ca record, which we interpret to reflect seasonal upwelling. The correlation between Sr/Ca and Ba/Ca is consistent with strong seasonal upwelling of cool, nutrient-rich water at this site that weakens during El Niño phases. As the oldest seasonally resolved SST record from the region, this record is an important contribution to the body of paleoclimate data for the EEP. In combination with other coral records from the tropical Pacific, this work provides insight into the LIA, and helps characterize Holocene and last millennium ENSO trends.