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|Title: ||Coral U-series ages: U in corals and in the oceans.|
|Authors: ||Esat, TM|
|Issue Date: ||Jul-2007|
|Citation: ||Esat, T. M., & Yokoyama, Y. (2007). Coral U-series ages: U in corals and in the oceans. International Union for Quaternary Research XVII Congress (INQUA) – “The Tropics: Heat Engine of the Quaternary”, 28th July – 3rd August 2007. Cairns, Australia: Cairns Convention Centre. In Quaternary International, 167-168, 110.|
|Abstract: ||A by product of U-series dating of corals is the 234U/238U ratio in the oceans at the time the coral was growing. The expectation is that this ratio remains invariant in the oceans over the U-series time scale, of at least 500,000 years, and hence it can be used to detect itinerant U or Th through the sample over time. Often, a suite of corals from a reef section show an intriguing, approximate 1:1, correlation between the 234U/238U ratio and age. This pattern can arise by the addition of almost equal amounts of external 230Th and 234U into the coral over an extended period. Currently, the favoured explanation postulates addition of equal amounts of 230Th and 234Th. The latter has a very short half life of about 25 days but the advantage that during aqueous transport it should not fractionate from 230Th. There are at least three prescriptions for removing the extraneous added components, based on the difference between the measured and expected 234U/238U ratio, to derive a corrected or “true” age for the sample. However, there is growing evidence that 234U/238U may not have been invariant in the oceans through Glacial-Interglacial transitions. In addition, some of the correlated data sets cross over to values of 234U/238U lower than in the present ocean thus requiring both the removal and addition of 230Th and 234Th, in neighbouring corals, which is not easy to explain. We will present data from Huon Peninsula coral reefs that show much more complex behaviour than the so far recognised 1:1 linear trend. Firstly, during major sea-level transitions the 234U/238U ratio shifts from low to high values in step with rising sea-levels. There are several well documented examples of this type of behaviour. Secondly, the variability in 234U/238U is amplified during rapid changes in sea-level. Candidates for this type of behaviour occur during rapid sea-level changes associated with Heinrich-events and also during the OIS6 to OIS5 transition. At these times, there is a direct correlation between sea-level change and U as the 234U/238U ratio shifts between high and low values in step with sea-level. This type of behaviour cannot be explained by extraneous 230Th-234Th infusion or through any other plausible diagenetic process. Conversely, these results cast doubts on the 230Th-234Th addition explanation for the 1:1 234U/238U-age trend in the data and more so on the so-called corrected ages calculated from them.|
|Appears in Collections:||Conference Publications|
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