Mass-spectrometric U-234-Th-230 ages from the Key Largo Formation, FloridaKeys, United States: Constraints on diagenetic age disturbance

New mass-spectrometric U-234-Th-230 ages for the Key Large Formation confir m earlier, less precise alpha-counting measurements that place the unit wit hin marine oxygen isotope stage 5, Ages derived from the petrographically f reshest samples (>99% aragonite) of this study range from 125 to 138 ha, De spite careful sample selection and preparation, the initial U-234/U-238 act ivity ratios [denoted (U-234/U-238)(T)] range from 1.163 to 1.176 and so ar e all significantly higher than modern-day or inferred last-interglacial se awater. This result indicates open-system behavior of U-series nuclides, as has been observed in many other Pleistocene coral samples, and casts doubt on the accuracy of our age determinations. We have used our samples to inv estigate the processes and magnitudes of age perturbations, Repeats of the same sample dissolution (analytical reproducibility) are wit hin measured uncertainties, which are generally less than +/-1 k.y. In cont rast, different parts of the same coral, similarly screened for alteration, can show age differences of as much as similar to 3 k.y. Evidence from thi s study and others suggests that alteration generally increases U-234-Th-23 0 ages. We were able to determine, by using held relationships, that the di fferences in U-234-Th-230 ages of samples Lying on either side of an erosio n feature are at least 5 k.y. in error; the stratigraphically younger coral s above the erosion surface have U-234-Th-230 ages that are at least 5 k.y. too old. Notably, these apparently more perturbed samples have lower (U-23 4/U-238)(T) ratios than the underlying corals. This observation emphasizes that, although (U-234/U-238)(T) is a sensitive indicator of open-system beh avior, the (U-234/U-238)(T) ratio does not provide a simple index of the ma gnitude of age disturbance. Diagenetic additions of both U-234 and Th-230 to corals seem necessary to e xplain both elevated ages and (U-234/U-238)(T) ratios. We argue that these additions result from redistribution of nuclides during meteoric alteration of the reef in the first similar to 40 k.y, after formation. Variations in the amount of Th-230 added relative to U-234 can be used to model the spre ad of ages at similar (U-234/U-238) ratios, although variable amounts of re cent U loss may also have been important. The youngest age of this study (1 25 ha), just below an erosion surface, is compatible with the ages of simil ar features in the southern Bahamas, We suggest that 125 ka is the maximum age of a short-lived sea-level lowstand within the last-interglacial (5e) h ighstand.