234U/238U mass spectrometry of corals: How accurate is the U-Th age of the last interglacial period?

Direct measurement of U and Th by mass spectrometry recently provided a spectacular improvement in the precision of coral dating by the U-Th disequilibrium method [1]. Mass spectrometric data also provide a much better resolution for the examination of subtle diagenetic effects revealed by small variations of the 234U//238U ratio. Such perturbations may strongly affect the accuracy of the U-Th chronometer. A compilation of all the corals analyzed to date by mass spectrometry shows that most of the corals from terraces of the last interglacial have initial 234U//238U ratios higher than present-day seawater, in contrast to modem, Holocene and last Glacial corals. Some samples that have very high 234U//238U initial ratios, up to 1.2, and high U concentrations, up to 4 ppm, were probably contaminated by continental groundwaters. However, even apparently pristine samples have 234U//238U initial ratios which are still slightly higher than present-day seawater (mean value: 1.160, compared to 1.140-1.150 [2]), with little overlap between the two distributions. This difference in the U initial ratio raises some uncertainty about the accuracy of the U-Th age determinations of these corals. In spite of the fact that the 234U-23°Th ages cluster in a narrow range between 122 and 133 kyr, the data could also be interpreted as resulting from slight contaminat ion of corals that are significantly older than 125 kyr. Two possible explanations may explain these data: (1) All these samples may have been diagenetically altered, since they all come from surface outcrops which have been directly exposed to precipitations or soil waters for 125 kyr. In this case, the true age of these corals remains uncertain, depending on the t iming of the alteration process (i.e., initial, late or continuous). (2) The second possibility is that some of the differences in 234U//238U ratios measured in 125 kyr cold corals, compared to modern seawater, may be due to a higher 234U/238U ratio in seawater 125 kyr ago. This could result from temporal variations in the weathering regimes of continental land masses.