. Multiproxy Reduced-Dimension Reconstruction of Pliocene Equatorial Pacific Sea Surface Temperatures

Abstract
The Pliocene (5.3-2.6 Ma) is the most recent time interval in Earth history when global climate was significantly warmer than the present, and as such is considered a potential analog to future climate conditions. A controversial aspect of the Pliocene climate system is a sea-surface temperature (SST) distribution resembling that during El Nino events, which is largely inferred from SSTs reconstructed from several sites in the equatorial Pacific. We utilize a reduced-dimension methodology on a compilation of previously published multiproxy (Mg/Ca, Uk'37, TEX86, and foraminifer assemblage) Pliocene SST records from the equatorial Pacific to reconstruct spatial and temporal snapshots of SST anomalies and a time series of Nino Indices from 5 to 1 Ma. We find that the Early Pliocene equatorial Pacific was characterized by a reduced zonal SST gradient due to extreme warmth in the eastern equatorial Pacific which peaked at 4.3 Ma. The intensity of this mean El Nino-like SST state then gradually diminished toward modern conditions. We also use the Pliocene Nino 4 time series to quantify the past strength of Indian Summer Monsoon (ISM) given the modern Nino 4-ISM correlation. Results indicate ISM was weaker throughout the study interval with weakest conditions (49% less rainfall than modern) occurring at 4.32 Ma, congruent with regional proxy records. In summation, this reduced-dimension approach spatially and temporally resolves the warm, mean state of the Pliocene equatorial Pacific and has numerous applications to infer paleoclimate conditions in distal regions teleconnected to El Nino today.