Changes in the East-West contrast of the upper equatorial Pacific Ocean over the last 10 Ma

International audienceThis study presents new data of the past 10 Ma climate in the Equatorial Pacific. Combining UK'37 and TEX86derived temperatures as well as carbon and oxygen isotope of calcifying planktonic species living in surface and subsurface waters at the IODP site U1338 (Eastern Equatorial Pacific) and 806 (Western Equatorial Pacific) we investigate the temporal evolution of the zonal gradient across the equatorial Pacific. This multi-proxy approach is used to reconstruct changes in the asymmetric pattern between the Eastern and Western Equatorial Pacific surface and thermocline depth waters. Based on the cross-analysis of our data and those available in the literature we propose a schematic view of long-term La Niña-and El Niño-like alternations from the upper Miocene in the equatorial Pacific Ocean. We suggest a general shoaling of the thermocline along the equator from about 11 Ma ago demonstrate that this shoaling is linked to the equatorial upwelling and the establishment of the Eastern Pacific Cold tongue particularly discernible during three time intervals referring to La Niña-like periods (11.5-9 Ma, 6.8-6 Ma and 4.8-1.4 Ma). Our study also reveals intervals of weakened oceanic circulation during El Niño-like periods (9-6.8 Ma and 6-4.8 Ma). The role of global ice sheet, the Indonesian seaway restriction and the Central American seaway closure as driving factors of the observed changes are discussed