Faunal-based sea surface temperature reconstruction of the mid-Pliocene equatorial Pacific Ocean

The modern equatorial Pacific Ocean is marked by a strong east-west temperature gradient; the cyclic relaxation of this gradient characterizes El Nino conditions. Published Mg/Ca paleothermometry and alkenone- derived temperature estimates document Pliocene warming in the eastern equatorial Pacific (EEP) relative to modern and static temperatures in the western equatorial Pacific (WEP). These results infer that El Nino-like conditions may have been the norm during the Pliocene. We present a new sea surface temperature (SST) reconstruction based on planktic foraminiferal assemblages for the mid-Pliocene equatorial Pacific. Our reconstruction suggests relaxation of upwelling and concomitant surface water warming in the EEP with little change in the WEP. The faunal assemblage structure suggests the WEP remains warm and stable during late Pleistocene glacial-interglacial cycles and was unchanged during the mid-Pliocene. These new data corroborate a much-reduced east-west temperature gradient during the mid-Pliocene.