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The tropical thermocline is a key feature in the coupled ocean-atmosphere system of the equatorial Pacific. In the eastern equatorial Pacific (Fig. F1), a shallow thermocline and upwelling favorable winds bring cold, nutrient-rich water to the surface (Fiedler and Talley, 2006). In contrast, the Western Pacific Warm Pool is characterized by warm sea-surface temperatures and a deep thermocline. During the Pliocene warm period (~3 to 5 Ma), subsurface temperatures were warm across the equatorial Pacific, suggesting the tropical thermocline was warm and/or deep (Ford et al., 2012, 2015; Steph et al., 2006, 2010).

There are key differences in the subsurface temperature evolution between the eastern and western Pacific. In the eastern equatorial Pacific, subsurface temperatures steeply cooled by ~3°C between 4.0 and 4.8 Ma and gradually cooled an additional ~2°C toward the present day (Ford et al., 2012; Steph et al., 2006, 2010). In comparison, subsurface temperatures gradually cooled ~3°C from the Pliocene to the present day (Ford et al., 2015). Integrated Ocean Drilling Program (IODP) Site U1338 (2°30.469′N, 117°58.178′W; 4200 m water depth) is ideally situated to examine the spatial pattern of east vs. west subsurface temperature evolution because it is located just west of the eastern equatorial Pacific cold tongue region (Fig. F1).

Here we reconstruct subsurface temperatures over the last 5 My at Site U1338. We use the Mg/Ca values for Globorotalia tumida to reconstruct thermocline conditions because its calcification depth is approximately 100 m, independent of thermocline depth (Rincón-Martínez et al., 2011). We find subsurface temperatures were warm during the early Pliocene (~5 Ma) and cooled toward the present day, consistent with other records from the eastern equatorial Pacific.