TIMS U-SERIES DATING AND STABLE ISOTOPES OF THE LAST INTERGLACIAL EVENT IN PAPUA-NEW-GUINEA

The extensive flight of uplifted reef terraces which occurs along the Vitiaz strait on the northern flank of the Huon Peninsula in PNG (Papua New Guinea) contains a particularly good record of sea level changes in the last 250 ky. The Huon terraces were the target of an international expedition which took place in July-August 1988. In particular, we searched for suitable samples for U-series dating in a reef complex designated as VII, which is correlated with the last interglacial episode and high sea level stand. This complex is composed of a barrier reef (VIIb), a lagoon, and a fringing reef (VIIa). Twelve corals from these terraces and two corals from the older reef complex VIII were selected for analysis. The petrography, oxygen and carbon isotope compositions, and magnesium and strontium concentrations were determined along with the concentrations and isotopic compositions of uranium and thorium. Th-230-U-234 ages of the corals with >99% aragonite, having primary textures, and which show U/Sr ratios around 0.4 x 10(-3) and initial deltaU-234 values close to that of present seawater, appear to be reliable. The ''most reliable'' ages from complex VII corals fall in two tight groups centered at 118 ky and 134 ky. Corals with deltaU-234 (T) values higher than 160 and U/Sr ratios substantially lower than seawater are assumed to have undergone diagenetic alteration, which appears to be common in this area. The simplest model for sea level height for terrace VII is a continuous rise between 134 and 118 ky. Alternatively, there may have been two periods of rapid sea level rise. In contrast, in the Bahamas, there is evidence that sea level remained rather constant over the time interval 132 to 120 ky. The absence of ages between 132 and 120 ky in PNG could be the result of changes in the local tectonic uplift rates during that time, or erosion that disrupted the continuous record. In any event, we find no basis for accepting a single brief time for the age of the last interglacial and applying this age as a precise chronometer for worldwide correlation, or as a test of climatic models. The older ages reported here precede the Milankovitch solar insolation peak at 128 ky, and the younger ages are approximately 10 ky after this peak. If the present high-precision data are correct, then it will be necessary to reassess the validity of the Milankovitch theory of climatic changes. The fundamental issue which must now be resolved is a means of identifying coral samples that have not been disturbed by diagenetic processes.