U-Th dating of carbonate platform and slope sediments

Absolute chronology of marine sediment beyond the C-14 age range provides a test for models of climate change and has many other applications. U-Th te chniques have been used for such chronology by dating corals, but extending these techniques to marine sediment is complicated by the presence of sign ificant initial Th-230-both in detrital material and scavenged from seawate r. In this study, we investigate four methods of solving the initial Th-230 problem for a particular type of marine sediment-the aragonite-rich sedime nts of carbonate platforms and slopes. Bulk sediment U-Th analyses can be c orrected for initial Th to yield ages with approximate to2 to 3 kyr precisi on for highstand periods when sediment aragonite contents are particularly high. Uncertainty on the corrections causes inadequate precision for sedime nt from other periods, however. Removal of scavenged Th before analysis wou ld enable a dramatic increase in this precision but has not proved successf ul despite a range of chemical leach approaches. Using heavy liquids to sep arate the various carbonate minerals found in Bahamas sediment enables an i sochron approach to correct for initial Th, but the presence of initial Th from two sources requires correction or removal of one source of initial Th before the other is deconvolved by the isochron. Quantitative removal of d etrital material before isochron analysis proves a successful approach. Suc h isochron data demonstrate that, although sediment remains closed to U-Th on a centimetre scale, nuclides are moved from grain to grain by alpha -rec oil. Such intergrain exchange is expected to be observed in all sediments c ontaining mineral grains with different U concentrations. Measured U-234/U- 238 allows the recoil movement to be corrected and results in isochron ages with precision sometimes as low as 3 kyr. The accuracy of this approach ha s been proved by dating samples within the 14C age range. Sediments spannin g the penultimate deglaciation have also been dated. After a small correcti on for bioturbation, the age for this event is found to be 135.2 +/- 3.5 ka . TWs date is approximate to8 kyr before the peak in northern hemisphere in solation and suggests that deglaciation is initiated by a mechanism in the southern hemisphere or tropics. This isochron approach shows considerable p romise for dating of sediments older than this event, which will provide fu rther information about the timing and mechanisms of global climate change. Copyright (C) 2001 Elsevier Science Ltd.