Changes in sea-surface conditions in the Equatorial Pacific during the middle Miocene–Pliocene as inferred from coccolith geochemistry

This study presents a new alkenone-derived Sea Surface Temperature (SST) record and δ18ONoelaerhabdaceae data of the 2–5 μm carbonate fractions from the IODP site U1338 located in the Eastern Equatorial Pacific (EEP), over the Miocene–Pliocene. Our data and those available from other sites of the same area show the establishment of a cold tongue during the early Pliocene (4.4–3.6 Ma). SST and δ18ONoelaerhabdaceae time-series indicate periods of significant salinity variations. Comparison with the δ18Obenthic curve from sediment cores of the Equatorial Pacific Ocean allows us to distinguish between global changes and local surface salinity variations in the EEP. Ice sheet growth and evaporation–precipitation are then discussed as possible drivers of such changes, along with the role of Central American and the Indonesian seaway restriction. Our data suggest a shallowing of the thermocline in the EEP, between 6.8 and 6 Ma, and its shoaling between 4.8 and 4.0 Ma, suggesting the appearance of the cold tongue (Steph et al., 2010). The Pliocene climate transition would therefore not be primarily driven by Northern hemisphere glaciation.

► Alkenone-derived temperature curve in the Equatorial Pacific over the last 16 Myr.
► Directly compares alkenones (SST) and δ18ONoelaerhabdaceae: a single signal carrier.
► Differentiates the influence of temperature, salinity and glacial effect on the δ18O.
► Shoaling of the thermocline around 4 Ma: earlier than previously suggested.