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. 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, 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. 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.

The historical record of daily light cycle in tropical and subtropical regions is short. moreover, it remains difficult to extract this cycle in the past from natural archives such as biogenic marine carbonates. Here we describe the precise analysis of sr/Ca, mg/Ca, and Ba/Ca ratios in a cultivated giant clam shell, using a laterally high-resolution secondary ion mass spectrometer with 2 µm resolution. The sr/Ca ratio exhibits striking diurnal variations, reflecting the daily light cycle. A clear seasonal variation in sr/Ca is also observed in another longer set of measurements with 50 µm resolution. Light-enhanced calcification and elemental transportation processes, in giant clam and symbiotic algae, may explain these diurnal and annual variations. This opens the possibility to develop the sr/Ca ratio from a giant clam shell as an effective proxy for parameters of the daily light cycle.

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The response of Acropora digitifera to ocean acidification is determined using geochemical proxy measurements of the skeletal composition of A. digitifera cultured under a range of pH levels. We show that the chemical composition (d 11 B, Sr/Ca, Mg/Ca, and Ba/Ca) of the coral skeletons can provide quantitative constraints on the effects of seawater pH on the pH in the calcification fluid (pH CF ) and the mechanisms controlling the incorporation of trace elements into coral aragonite. With the decline of seawater pH, the skeletal d 11 B value decreased, while the Sr/Ca ratio showed an increasing trend. The relationship between Mg/Ca and Ba/Ca versus seawater pH was not significant. Inter-colony variation of d 11 B was insignificant, although inter-colony variation was observed for Ba/Ca. The decreasing trend of pH CF calculated from d 11 B was from *8.5, 8.4, and 8.3 for seawater pH of *8.1, 7.8, and 7.4, respectively. Model calculations based on Sr/Ca and pH CF suggest that upregulation of pH CF occurs via exchange of H ? with Ca 2? with kinetic effects (Rayleigh fractionation), reducing Sr/Ca relative to inorganic deposition of aragonite from seawater. We show that it is possible to constrain the overall carbonate chemistry of the calcifying fluid with estimates of the carbonate saturation of the calcifying fluid (X CF ) being derived from skeletal Sr/Ca and pH CF (from d 11 B). These estimates suggest that the aragonite saturation state of the calcifying fluid X CF is elevated by a factor of 5-10 relative to ambient seawater under all treatment conditions.