234U/238U ratios and 230Th ages for Hateruma Atoll corals: implications for coral diagenesis and seawater 234U/238U ratios

Details 234U/238U ratios and 230Th ages for Hateruma Atoll corals: implications for coral diagenesis and seawater 234U/238U ratios 234U/238U ratios and 230Th ages for Hateruma Atoll corals: implications for coral diagenesis and seawater 234U/238U ratios

Mar 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.115...65h&link_type=abstract

Earth and Planetary Science Letters, Volume 115, Issue 1-4, p. 65-73.

Physics

21

Scientific paper

Published results for corals that are more than ca. 50 ka old from, for example, Barbados and Vanuata, often yield initial (234U/238U) activity ratios that are significantly greater than 1.144 (+/-0.004), the value for modern seawater. This study reports results for corals from Hateruma Atoll, Japan, and investigates the effects of diagenesis on the initial (234U/238U) 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 (234U/238U) ratios, the scope for increasing the (234U/238U) 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 (234U/238U) ratios. Of the six `pristine' corals, three have anomalous (230Th/238U) ratios requiring U loss and/or 230Th 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 (234U/238U) ratios in past seawater from this period has been found: the best estimates for (234U/238U) ratios are close to the current seawater value.

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