A 173 YEAR STABLE-ISOTOPE RECORD FROM A TROPICAL SOUTH-PACIFIC CORAL

We have generated a 173 year-long time series (A.D. 1806 to 1979) of changes in the deltaO-18 and deltaC-13 Composition of a coral head (Platygyra lamellina) to investigate environmental change in the climatologically sensitive region of the tropical South Pacific. Little is known about decadal- and centennial-scale climate change at sea level in this region. Our coral is from near the south coast of Espiritu Santo Island (15-degrees-S, 167-degrees-E), Republic of Vanuatu. The stable isotope record from this coral is consistent with meteorological and oceanographic records during the period of overlap in the records (1964-1978): deltaO-18 values positively correlate with SST (r = 0.77) and deltaC-13 Values are highly coherent with rainfall (r = 0.82) at the annual cycle. These relations suggest that the deltaO-18 signal in this coral is a function of variations in SST and rainfall-induced changes in SSS, whereas the deltaC-13 signal may be related to rainfall and cloud cover modulation of photosynthesis in the coral. deltaO-18 and deltaC-13 values are positively correlated over the entire length of the record (r = 0.65), a relation that is even stronger between 1806 and 1866 (r = 0.81), i.e. prior to the time of possible anthropogenic influence on the deltaC-13 record. Because of the positive correlation between temperature and rainfall in this region, we interpret the long-term record of deltaO-18 in terms of joint variations of these two variables. The most significant cool/dry excursion in the Santo record occurs during the nineteenth century (approximately 1832-1866) and ends abruptly in 1866 with a change to modem values. Superimposed on this pattern is a slight (0.2 parts per thousand) cooling/drying toward the end of the twentieth century. Cross-spectral analysis of the deltaO-18 and deltaC-13 records indicates a strong concentration of variance at the quasi-biennial (approximately 2 years) and El Nino/Southern Oscillation (ENSO) (approximately 4-5 years) frequency bands, in addition to a approximately 15 year peak found also in global temperature records. Cross-spectral analysis of coral isotope records from Santo and the Philippine Sea indicate a concentration of variance at ENSO (approximately 4-5 years) frequency bands. However, comparison of the Santo coral isotope record with high-latitude northern hemisphere temperature records indicates that the major cool/dry excursion in the Santo record (approximately 1832-1866) is not seen in the northern hemisphere record. If verified in other tropical South Pacific coral records, the results have implications concerning the global scale of cooling for Little Ice Age events.