Permanent El Niño during the Pliocene warm period not supported by coral evidence

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Abstract

The El Niño/Southern Oscillation (ENSO) system during the Pliocene warm period (PWP; 3–5 million years ago) may have existed in a permanent El Niño state with a sharply reduced zonal sea surface temperature (SST) gradient in the equatorial Pacific Ocean1. This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for near-term climate change2, ENSO variability—and related global climate teleconnections—could have been radically different from that today. Yet, owing to a lack of observational evidence on seasonal and interannual SST variability from crucial low-latitude sites, this fundamental climate characteristic of the PWP remains controversial1,3,4,5,6,7,8,9,10. Here we show that permanent El Niño conditions did not exist during the PWP. Our spectral analysis of the δ18O SST and salinity proxy, extracted from two 35-year, monthly resolved PWP Porites corals in the Philippines, reveals variability that is similar to present ENSO variation. Although our fossil corals cannot be directly compared with modern ENSO records, two lines of evidence suggest that Philippine corals are appropriate ENSO proxies. First, δ18O anomalies from a nearby live Porites coral are correlated with modern records of ENSO variability. Second, negative-δ18O events in the fossil corals closely resemble the decreases in δ18O seen in the live coral during El Niño events. Prior research advocating a permanent El Niño state may have been limited by the coarse resolution of many SST proxies, whereas our coral-based analysis identifies climate variability at the temporal scale required to resolve ENSO structure firmly.

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Figure 1: Comparison of modern coral δ 18 O, local SST and coral δ 18 O anomalies and ENSO proxy time series.
Figure 2: Pliocene El Niño recorded in PWP oxygen isotopic records.
Figure 3: Power spectral densities.

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Acknowledgements

We would like to thank H. H. Ramos for a research permit and W. Mago and E. Azurin for their help with fieldwork. Special thanks go to F. P. Siringan for the study of modern coral; H. Nomura and K. Nakamura for preparing thin sections; R. Miyawaki for assistance with X-ray diffraction; S. Motai, Y. Seto and K. Omori for their assistance with synchrotron X-ray diffraction; C. Shimada, M. Ikeda and K. Hagino-Tomioka for their scanning electron microscopy observations; Y. Yoshinaga for microsampling; and A. Yamazaki for preparing the figures.

Author information

T.W., A.S. and T. Kase designed the research. T. Kawashima and K.M. performed isotopic analysis under the supervision of T.W., A.S. and H.K. S.M. performed statistical analysis and climatology. K.K. observed nannofossil assemblages. T.W., T. Kawashima, Y.M.A., R.W. and K.S. carried out the field survey under the supervision of T. Kase. T.W. and K.S. performed synchrotron X-ray diffraction analysis under the supervision of T.N. T.W. wrote the paper in discussion with all authors.

Correspondence to Tsuyoshi Watanabe.

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