NOAA/WDS Paleoclimatology - Equatorial Pacific Foram-Bound N Isotopes from the Pliocene to the Pleistocene

Upwelling generates a nutrient-rich "cold tongue" in the eastern equatorial Pacific Ocean (EEP), with impacts on global climate, oceanic biological productivity, and the carbon cycle. The cold tongue was reduced during the Pliocene Epoch, a feature attributed to weaker upwelling and an associated deepening of the surface mixed layer in the EEP. Here we report nitrogen isotope evidence that modern-like upwelling occurred in the EEP during the Pliocene and persisted over the last 5 million years. We explain the reduced Pliocene cold tongue as an expression of the reduced temperature difference between surface and subsurface waters in the tropical Pacific. The attendant reduction in the vertical density gradient may have maintained EEP upwelling despite the expected slackening of the trade winds under Pliocene warmth.