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Then as now for El Niño Coarse resolution palaeoclimate proxy evidence has suggested that the Pliocene warm period (PWP) between 3 million and 5 million years ago was characterized by permanent El Niño conditions in which the equatorial Pacific was uniformly warm, instead of having the modern-day 'cold tongue' extending westward from South America. New high-resolution climate proxy data from fossil corals raise doubts over this assertion. Well-preserved PWP-era fossil corals with clear skeletal annual bands, discovered in the Philippines, show that ocean conditions in the western Pacific during the PWP were characterized by El Niño variations that are similar to those we see today. Coarse-resolution palaeoclimate proxy evidence has suggested that the Pliocene warm period (∼3–5 million years ago) was characterized by permanent El Niño conditions in which the equatorial Pacific was uniformly warm, instead of having the modern-day 'cold tongue' extending westward from South America. This study uses high-resolution climate proxy information from fossil corals to challenge this assertion and shows that ocean conditions in the western Pacific during the Pliocene warm period were characterized by El Niño variations similar to modern-day variations. 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 Ocean 1 . This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for near-term climate change 2 , 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 controversial 1 , 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 δ 18 O 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, δ 18 O anomalies from a nearby live Porites coral are correlated with modern records of ENSO variability. Second, negative-δ 18 O events in the fossil corals closely resemble the decreases in δ 18 O 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.

Fragments of branching Pocillopora damicornis coral colonies were grown in experimental flumes under two water flow regimes. Colony size and buoyant weight increased most rapidly in the fast-flow regime. Branch tips from the upper and outer parts of the colonies showed the lowest and most consistent skeletal oxygen isotope ratios. Flow regime had little influence on the lowest oxygen isotope ratios, which were at least 3.5‰ lighter than the calculated oxygen isotopic equilibrium. These “kinetic” isotope effects are comparable to those observed in Porites corals. Relatively more branch tips showed extreme 18 O depletions under low-flow conditions, and among small coral colonies. Isotopic variability was greater among branch tips from the lower and inner parts of the colonies and at high flow. Skeletal oxygen and carbon isotope ratios generally showed positive correlations. Despite the particularly large offsets from isotopic equilibrium, the isotopically lightest branches showed the greatest isotopic consistency and therefore would make the best isotopic thermometers. Isotopic variability within the colony may provide an indication of flow regime.