Changes in the evaporation-precipitation balance are a dominant control on changes in sea water oxygen isotopic composition (and of salinity). Periods of excess rainfall are reflected in lighter (more negative) oxygen isotopic composition of the surface ocean (and lower salinity), and this signature is recorded in the skeletons of corals growing in that water. In addition to this water composition effect, there is a temperatur dependence for oxygen isotopic fractionation from water into aragonite which results in approximately 0.2‰ decrease in skeletal δ18O for everv 1 °C increase in SST. In oceanic regions of abundant convective rainfall, the strong positive correlation between SST and rainfall results in an amplified climatic signal in coral δ18O (due to the fact that high SST and high rainfall both drive coralline δ18O towards more negative values). This is the situation which prevails in the field area of the present study on the north coast of Papua New Guinea.
The present short paper focuses exclusively on the use of coral skeletal δ18O as a combined SST and SSS indicator. However, it is appropriate to note here that there are several other trace and minor elements in coral skeletons which provide climatic information. In particular, the incorporation of Sr and Mg into coral skeletons has been shown to be temperature dependent. Consequently, coral Sr/Ca and Mg/Ca have been developed as proxies for past SST with a possible temperature resolution of ±0.5℃ (Beck et al., 1992; Mitsugushi et al., 1996). This opens up the exciting prospect of using combined coral δ18O, Sr/Ca and Mg/Ca analyses to quantify both SST and SSS changes (e.g., Gagan et al., 1998).
