HISTORY OF ENSO IN THE GALAPAGOS IN THE 16TH CENTURY USING FOSSIL CORAL GEOCHEMISTRY

Abstract

The El Niño Southern Oscillation (ENSO) is an interannual ocean-atmosphere cycle associated with upwelling in the Eastern Equatorial Pacific (EEP), which has strong effects on local and global climate. While the modern ENSO is well-studied, its historic variability is poorly known during the Little Ice Age (LIA; 1400-1800 CE), when NH temperatures were 2-3°C cooler (Mann et al., 2009). To address the issue of ENSO variability in the LIA, we reconstructed past sea surface temperatures (SST) in the EEP by generating a multivariate geochemical record from a fossil Galápagos coral that lived during the LIA. The coral specimen was collected in Urvina Bay on Isla Isabela in the western Galápagos archipelago (1°S, 91°W). Here, the Galápagos platform sits directly in the path of the Equatorial Undercurrent, and strong upwelling affects 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, terminating in 1626. We measured Sr/Ca ratios (a proxy for SST) and Ba/Ca ratios (a proxy for upwelling) along the full core length. Our data show an average annual temperature range of approximately 4°C. Measured Sr/Ca suggest temperatures 0.5 °C lower than 20th century coral records. Annual SST maxima (Sr/Ca minima) are highly variable, and demonstrate intermittent loss of cold season SST, showing the occurrence of El Niño. The seasonal cycles in the Ba/Ca record suggest a link to seasonal upwelling. The correlation of Sr/Ca and Ba/Ca suggest that upwelling could contribute to seasonal and interannual SST change and reflect ENSO variability. As the oldest seasonally resolved SST record from the region, this record is an important contribution to the body of published data for the eastern tropical Pacific.