STABLE-ISOTOPE GEOCHEMISTRY OF CORALS FROM COSTA-RICA AS PROXY INDICATOR OF THE EL-NINO SOUTHERN OSCILLATION (ENSO)

We analyzed the deltaO-18 and deltaC-13 time-series contained in coral skeletons collected from Isla del Cano, Costa Rica, that survived the 1982-198 3 El Nino warming event. Coral deltaO-18 give a record of th ermal histories with a precision of approximately 0.5-degrees-C. For t his locality, we have determined that the average DELTAdelta(w) effect in the coral skeletons is equivalent to 33% of the skeletal deltaO-18 range. Therefore, if delta(w) effects are not compensated for, the an nual skeletal-deltaO-18 range displays a temperature range of 1-degree s-C lower than actual values. The isotopic record of Porites lobata sk eletons shows simultaneous depletions in O-18 and C-13 at skeletal lev els corresponding to 1983, coincident with the El Nino/Southern Oscill ation (ENSO) event. Therefore, the El Nino event is not only recorded as negative delta O-18 anomalies in the skeleton, suggesting the warmi ng of ambient waters, but also in the delta C-13 signal as negative an omalies, indicating coral bleaching. Contrary to the predictions of th e C-13-insolation model'' that the annual carbon isotope variation sho uld be attenuated with depth in proportion to the decrease in light-in tensity variation with depth, we found a clear trend where DELTAdeltaC -13 increases with depth. Coral bioenergetics, which depends on both c oral physiology and ecology, may adequately explain the unexpected inc rease in deltaC-13 range with depth, without contradicting the seasona l character of deltaC-13 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 stu dy it has been found that skeletal growth was not seriously diminished during the El Nino year of 1983. We attribute the good agreement of t he deltaO-18 record with the timing and magnitude of the El Nino 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.