DEVELOPING THE USE OF THE MASSIVE CORAL GENUS DIPLOASTREA FOR PALEOCLIMATE RECONSTRUCTION

The widespread coral genus Diploastrea forms massive colonies in the western and central Pacific several meters in diameter and only vertically accretes skeleton at ~3 to 5 mm per year. This growth rate is less than half that of the genus Porites, which is the common coral currently used for paleoclimatic studies in the Pacific. Thus long cores from Diploastrea colonies may potentially contain temporally longer multi-century records of sea surface temperature (SST) and salinity preserved in the geochemical composition of their skeletons. In addition, unlike Porites, Diploastrea appears to be minimally disturbed from boring organisms and grazing fish. However, no published studies exist which have assessed the paleoclimatic utility of Diploastrea skeletons.

Coral cores from colonies of both Diploastrea and Porites have recently been recovered from Nasavusavu Bay in Fiji (17.5° S, 178.5° E). To test different sampling strategies, we have analyzed d ¹⁸O and d ¹³C on samples of Porites and Diploastrea spanning ten years (1997-1987). For Porites we sampled in 1mm increments along the maximum growth axis. For Diploastrea we used two methods; 0.5 mm increments along the exothecal portion of one corallite, and 0.5 mm increments across an entire corallite. Comparison between these temporally synchronous coral data and satellite data allows for the calibration of our chronology and sampling procedures. Initial results indicate that sampling of exothecal material results in annual variations most similar to Porites, and may more accurately record ocean conditions than a mixture of skeletal material across an entire corallite, which shows reduced seasonal amplitude, likely due to time averaging. Most importantly, both Porites and Diploastrea have recorded the same interannual d¹⁸O signal in this ten year period.