U-234/U-238 RATIOS AND TH-230 AGES FOR HATERUMA ATOLL CORALS - IMPLICATIONS FOR CORAL DIAGENESIS AND SEAWATER U-234/U-238 RATIOS

Published results for corals that are more than ca. 50 ka old from, for example, Barbados and Vanuata, often yield initial (U-234/U-238) activity ratios that are significantly greater than 1.144 (+/-0.004), the value for modem seawater. This study reports results for corals from Hateruma Atoll, Japan, and investigates the effects of diagenesis on the initial (U-234/U-238) ratios of the corals. Hateruma consists of over 95% reef material of less than 500 ka in age. Because groundwaters associated with recent carbonates tend not to have elevated (U-234/U-238) ratios, the scope for increasing the (U-234/U-238) ratios of Hateruma corals through groundwater interaction should be minimal. The corals examined range from those which show no petrographic or X-ray evidence for diagenetic alteration and are 'pristine' to those containing secondary cements of either aragonite or high- or low-Mg calcite. These various phases have been mechanically separated and analysed. In agreement with previous assertions the analyses of these separates demonstrates that the presence of calcite should be a cause for rejection of coral samples. It is also shown that aragonite cement can form later than coral death and may bias estimates of age and initial (U-234/U-238) ratios. Of the six 'pristine' corals, three have anomalous (Th-230/U-238) ratios requiring U loss and/or Th-230 addition. The remaining fifteen of the eighteen total analyses reported here yield ages from 1.6 ka to 260 ka. No evidence for an increase in (U-234/U-238) ratios in past seawater from this period has been found: the best estimates for (U-234/U-238) ratios are close to the current seawater value.