Paleoceanography of the eastern equatorial Pacific over the past 4 million years and the geologic origins of modern Galápagos upwelling

An isolated, volcanic archipelago at the confluence of several major ocean currents, the Galápagos Archipelago (GA) is among the most biologically diverse places on Earth. There remain many open questions concerning evolution and speciation in the GA, with the details of the geologic formation of the islands over the past millions of years representing a key source of uncertainty. Paleoceanographic sea surface temperature (SST) proxy records from the far eastern equatorial Pacific (EEP) indicate that the modern gradient of SST across the GA (the cross-island SST gradient, or CIΔT) emerged relatively abruptly ∼1.6 Ma. As the modern CIΔT is the result of a blockage and subsequent upwelling of the Equatorial Undercurrent (EUC) by the GA, we infer from these paleoceanographic data that the modern period during which the GA is arranged such that the islands constitute a significant topographic barrier to the EUC began ∼1.6 Ma. An extensive suite of ocean circulation model experiments-new and previously published-confirms that the sign and magnitude of the change in CIΔT captured in paleoceanographic records can be explained by the islands impinging upon the EUC. Implications for the geologic history of the Galápagos and related biogeographical questions are discussed. Additionally, these results suggest that investigations of the Pan-Pacific SST gradient (PPΔT) should use one of the available proxy sites in the EEP that is not influenced by regional, geologically forced oceanographic changes; such an analysis supports recent suggestions of a more gradual development of the modern PPΔT over the Plio-Pleistocene.