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

Highlights

• Paleo records show the modern SST gradient across the Galápagos emerged ∼1.6 Ma.

• Models confirm emergence of SST gradient is due to Galápagos blocking undercurrent.

• Implications for geological and biogeographical mysteries are discussed.

• Studies of Pliocene Pan-Pacific SST gradient should consider geologic forcing.

• Results support gradual development of the modern Pan-Pacific SST gradient.

Abstract

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 CI$\mathrm{\Delta }\mathrm{T}$ 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 CI$\mathrm{\Delta }\mathrm{T}$ 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 $\mathrm{PP}\mathrm{\Delta }\mathrm{T}$ over the Plio-Pleistocene.

Introduction

The Galápagos Archipelago (GA) is comprised of over one hundred islands and islets, many of which are volcanically active. While it is generally accepted that subaerial islands have existed in the GA for at least 17 million years (Werner et al., 1999, Werner and Hoernle, 2003), the timeline of their formation, emergence, and paleogeography remains highly uncertain (Geist et al., 2014). This uncertainty has broad implications; for example, it hinders our ability to piece together the evolution and migration of species in one of Earth's most famous natural laboratories.

The unique environmental conditions on the GA are in large part driven by the interaction of the islands themselves with basin-scale oceanography. Specifically, the GA stands directly in the path of the Equatorial Undercurrent (EUC) as it flows along the equator from the western Pacific toward South America at $\sim 100\text{m}$ depth (Fig. 1). The direct result of this geologic coincidence is a very intense upwelling of cold, nutrient–rich water from the EUC along the western shores of the westernmost islands in the GA (Isabela and Fernandina). The effects of this tropical oasis of cold, highly productive surface water are felt throughout the entire GA ecosystem including on large animals such as penguins and fur seals (Boersma et al., 2013, Karnauskas et al., 2015).

The physical influence of the GA on ocean currents and sea surface temperature (SST) in the eastern equatorial Pacific (EEP) has been studied extensively through in situ observations (Stevenson and Taft, 1971, Christensen, 1971, Pak and Zaneveld, 1973, White, 1973, Leetmaa, 1982, Lukas, 1986, Steger et al., 1998, Sweet et al., 2009; Karnauskas et al., 2010, Karnauskas et al., 2014) and ocean general circulation model (OGCM) experiments (Brentnall, 1999, Eden and Timmermann, 2004, Cravatte et al., 2007; Karnauskas et al., 2007, Karnauskas et al., 2008, Karnauskas et al., 2014). The salient result, gleaned from observations and common across all model experiments, is that the presence of the GA serves to obstruct the EUC, thus cooling SST in the region immediately west of the GA (out to ∼95°W) and warming SST east of the GA (between the GA and mainland South America). In other words, the robust oceanographic fingerprint of the GA is a positive cross-island SST gradient (CI$\mathrm{\Delta }\mathrm{T}=$ SST east of the GA–SST west of the GA), whereas the absence of the GA would correspond to a negative CI$\mathrm{\Delta }\mathrm{T}$.

Over the past several decades, the paleoceanographic community has conducted extensive drilling operations throughout the world ocean, including the Pacific basin, toward reconstructing variations in Earth's climate spanning the past several million years (Fig. 1a). Quite fortuitously, the combined set of paleoceanographic SST proxy records from the far EEP (Fig. 1b) constitutes a paleo-observing network well suited for detecting potential geologic changes in the GA given its known oceanographic fingerprint. The primary goal of this study is to leverage the existing paleoceanographic SST reconstructions from the EEP along with a wide range of OGCM experiments to provide a new constraint on the geologic and biogeographic history of the GA. The paleoceanographic records and suite of model simulations are summarized in the following section, results are presented in section 3, implications for local geologic history and biogeography, as well as basin-scale climate transitions are discussed in section 4, and concluding remarks are offered in section 5.