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Abstract

The existence of cool sea surface temperatures (SSTs) in the Eastern Equatorial Pacific (EEP)—colloquially known as the “Cold Tongue”—and warm SST in the Western Equatorial Pacific are important components of the air-sea dynamics that ultimately produce the El Niño-Southern Oscillation (ENSO). In an attempt to predict the response of ENSO to rising greenhouse gases, the high atmospheric CO2 of the Pliocene is seen as an analog. Many studies of this ENSO-greenhouse gas sensitivity reconstruct the east-west equatorial Pacific Sea Surface Temperature (SST) gradient across this time period. But the EEP Cold Tongue is also a modern “nutrient tongue,” where studies show that nitrate consumption is sensitive to the same air-sea dynamics as the zonal SST gradient. Here, we use bulk sediment and planktic foraminifera-bound N isotopes to reconstruct the history of the EEP nutrient tongue over the past 5 million years. These measurements indicate the persistence of an east-west bulk sediment and foraminifera-bound N isotope gradient and therefore an EEP nutrient tongue over the past 5 million years—including the warm Pliocene. We explore the implications of this work and more in our discussion.