Stable isotope geochemistry of corals from Costa Rica as proxy indicator of the EL Niño/southern Oscillation (ENSO)

We analyzed the δ18O and δ13C time-series contained in coral skeletons collected from Isla del Caño, Costa Rica, that survived the 1982–1983 El Niño warming event. Coral δ18O give a record of thermal histories with a precision of ~0.5°C. For this locality, we have determined that the average Δδw effect in the coral skeletons is equivalent to 33% of the skeletal δ18O range. Therefore, if δw effects are not compensated for, the annual skeletal-δ18O range displays a temperature range of 1 °C lower than actual values.The isotopic record of Porites lobata skeletons shows simultaneous depletions in 18O and 13C at skeletal levels corresponding to 1983, coincident with the El Niño/Southern Oscillation (ENSO) event. Therefore, the El Niño event is not only recorded as negative δ18O anomalies in the skeleton, suggesting the warming of ambient waters, but also in the δ13C signal as negative anomalies, indicating coral bleaching.Contrary to the predictions of the “13C-insolation model” that the annual carbon isotope variation should be attenuated with depth in proportion to the decrease in light-intensity variation with depth, we found a clear trend where Δδ13C increases with depth. Coral bioenergetics, which depends on both coral physiology and ecology, may adequately explain the unexpected increase in δ13C range with depth, without contradicting the seasonal character of δ13C variability with the solar irradiance cycle.Although some authors have determined the presence of hiatuses in the skeletal record due to severe stress and growth cessation, in this study it has been found that skeletal growth was not seriously diminished during the El Niño year of 1983. We attribute the good agreement of the δ18O record with the timing and magnitude of the El Niño 1982–1983 warming event to the nature of the warming trend in the area, as well as to some synergistic implications of coral tolerance to thermal stress.