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

Balaji, Rajagopalan; Marchitto, Thomas; Wycech, Jody; Molnar, Peter Hale; Gill, Emily

Citeable URL: https://scholar.colorado.edu/concern/articles/9k41zf74b

Abstract

A controversial aspect of the Pliocene climate system is a posited permanent sea surface temperature (SST) distribution resembling that during El Niño events, which is largely inferred from sea surface temperatures 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 assemblages) Pliocene SST records from the equatorial Pacific to reconstruct spatial and temporal snapshots of SST anomalies and a time series of Niño indices from 5 to 1 Ma. The use of multiple proxies increases the number of study sites and thereby improves the robustness of the reconstruction. We find that the early Pliocene equatorial Pacific was characterized by a reduced zonal SST difference due to minimal change in the west and extreme warmth in the east which peaked at 4.3 Ma. The intensity of this mean El Niño‐like SST state then gradually diminished toward modern conditions. We also use the Pliocene Niño 4 time series to estimate the past strength of Indian Summer Monsoon given the modern correlation of it to the Niño 4 index. Results indicate the monsoon was weaker throughout the study interval with weakest conditions (~37% less rainfall than modern) occurring at 4.3 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 inferences of paleoclimate conditions in distal regions teleconnected to El Niño today. Plain Language Summary The Pliocene Epoch (5.3–2.6 million years ago) is the most recent time interval in Earth history when atmospheric carbon dioxide concentrations may have been similar to today and the continents were in their current configuration. For these reasons, Pliocene paleoclimate reconstructions are considered to be a useful indicator of conditions expected by the end of the 21st century. Several marine‐derived Pliocene reconstructions from the equatorial Pacific suggest a sea surface temperature (SST) distribution that resembles SSTs during El Niño events today. El Niño events have widespread impacts including reduced marine productivity in the eastern equatorial Pacific and weakened Indian Summer Monsoon. Insights into Pliocene El Niño‐like SSTs are typically based on paleoclimate reconstructions from a few sites that are then used to infer regional conditions. In this study, we apply a statistical method to a compilation of nine Pliocene SST records across the equatorial Pacific to fill in the spatial gaps in the paleotemperature reconstructions. Our maps of reconstructed Pliocene SSTs reveal that the eastern equatorial Pacific was 3–6 °C warmer than today, which is consistent with a mean El Niño‐like state. Given the modern El Niño‐Indian Summer Monsoon relationship, we estimate that Pliocene monsoon was ~20–40% weaker than today. Key Points Pliocene equatorial Pacific sea surface temperatures were warmer than modern everywhere, with largest anomalies in the east A mean El Niño‐like state, characterized by a reduced zonal sea surface temperature difference, existed in the Pliocene equatorial Pacific Pliocene Indian Summer Monsoon is estimated to have been ~20–40% weaker than modern

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