Pliocene Weakening of Gradients in Temperature but Not in Productivity in the Eastern Equatorial Pacific

The modern eastern equatorial Pacific Ocean (EEP) exhibits strong upwelling, producing pronounced gradients in sea surface temperature (SST), nutrient concentration, and biological productivity between 80 degrees and 140 degrees W. During the globally warmer late Pliocene (3.0-3.6 Ma), the EEP may have experienced permanent El Nino-like conditions, supported by a reduced SST gradient across the equatorial Pacific. However, the weakened east-west SST gradient has been controversial, with disparate results depending on the proxy used to monitor Western Warm Pool SSTs. We present new Pliocene alkenone-based SST and paleoproductivity records from four Ocean Drilling Program (ODP) cores spanning an east-west transect across the EEP, which present an internally consistent picture of SST and productivity gradients in the modern cold tongue, resolved at orbital-scale variability. Strong agreement between core top reconstructions and satellite estimates indicates that alkenone paleotemperature and paleoproductivity proxies are appropriate for reconstructing Pliocene EEP conditions. The average SST gradient between 90 degrees and 120 degrees W was reduced from the modern 1.8 degrees C gradient to 0.9 degrees C in the late Pliocene. Despite the weakened SST gradient, the surface productivity gradient was stronger during the late Pliocene compared to modern, based on calibrated X-ray fluorescence biogenic opal and alkenone average accumulation rates. Contrary to modern El Nino SST and productivity patterns, reduced Pliocene surface productivity did not accompany the weakened SST gradient. Instead, strong Pliocene biogenic opal and alkenone concentration accumulation gradients in the eastern EEP suggest that subsurface tilting of the nutricline and thermocline persisted to supply vigorous upwelling of warm but nutrient-rich subsurface waters in a warmer climate. Ocean surface temperature is often related to productivity, or the intensity of biological activity from algae that live in surface waters and use sunlight energy to grow. Colder waters rich in nutrients important to these algae are brought from the deeper ocean to the sunlit zone at specific locations in the ocean, including the eastern tropical Pacific Ocean. Therefore, colder waters are often associated with higher biological productivity. This research assesses the relationship between temperature and productivity in eastern Pacific waters during a period in Earth's history around 3 million years ago when global temperatures were warmer than modern. We used geochemical tools to reconstruct temperature and productivity from four locations in the eastern Pacific. We found that 3 million years ago, the easternmost site was significantly warmer than modern, but biological productivity remained similar to today, which contradicts the pattern we observe in the modern Pacific Ocean. As we expect ocean and global temperatures to warm in the coming decades, relationships between temperature and productivity are important to understand. Determining how organisms at the base of the food web respond to warmer waters has implications for fishing, shipping, and other societal and economic activities along the eastern Pacific. East-west sea surface temperature gradients in the eastern tropical Pacific "cold tongue" were smaller in the Pliocene compared to modern High easternmost equatorial Pacific biogenic opal and alkenone accumulation rates imply stronger productivity gradients than modern Taken together, these observations suggest vigorous upwelling of subsurface waters that were significantly warmer than today