Decoupling of temperature and pH gradients along the equatorial Pacific during the Pliocene

Equatorial Pacific dynamics drive tropical climate patterns such as the El Niño-Southern Oscillation and provide nutrients for one of the world’s most productive marine ecosystems. How this region will respond to global warming remains an important area of study with profound implications for both human wellbeing and economic and ecosystem stability. In light of this, numerous studies have investigated equatorial Pacific dynamics during the Pliocene epoch (5.3-2.6 million years ago) as an analogue for future behavior of the region under global warming (1–12). Current paleoceanographic records from the Pliocene tropical Pacific present an apparent paradox, with proxy evidence of a reduced east-west sea surface temperature gradient along the equator(1,5,6,11)– indicative of reduced wind-driven upwelling – conflicting with evidence of enhanced biological productivity in the region (13–15) which is typically driven by upwelling. Here we reconcile these observations by providing new evidence for older, more acidic, and nutrient-rich water reaching the equatorial Pacific by way of a Pacific meridional overturning cell during the Pliocene (16). This provides a mechanism by which enhanced productivity could have existed alongside a reduced east-west sea surface temperature gradient in the Pliocene equatorial Pacific. Furthermore, these results challenge the current paradigm of a decline in biological productivity in warmer worlds due to enhanced thermal stratification (17). Our findings shed a new light on equatorial Pacific dynamics and help constrain potential changes to them in the near-future, given that the Earth is expected to reach Pliocene-like temperatures by the end of the century. The equatorial Pacific is a region of great significance as it hosts one of the most important climate phenomena on the planet, the El Niño Southern Oscillation (18), and supports massively productive fisheries that provide key ecosystem services to numerous communities (19,20), and our results provide novel insight on how it might change as the oceans adjust to a warming world.