U-Th dating of deep-sea corals

Th-230, Th-232, U-234 and U-234 compositions of several deep-sea solitary c orals, mainly the species Desmophyllum cristagalli, were determined by ther mal ionization mass spectrometry (TIMS). It is possible to obtain high prec ision ages on modern pristine corals that have low [Th-232] (5 to a few hun dred ppt). However, because older deep-sea corals tend to have higher [232T h] compared to surface corals, and the initial Th-230/Th-232 ratio is uncer tain, older deep-sea corals have larger age uncertainties (+/-several hundr ed years for samples with a few thousand ppt Th-232). Therefore, the key hu rdle for precise U-Th dating is to remove or account for contaminants which contain elevated Th-232 and associated Th-230 not due to closed system dec ay within the coral lattice. A modification of the trace metal cleaning met hods used for foraminifera and surface corals can significantly reduce this contamination. By counting the visible growth bands and measuring the mean age of a single septum, the extension rate of D. cristagalli was estimated to be between 0.1 and 3.1 mm/year. In a mean sense, bands appear to be pre cipitated annually, but this estimate has a large uncertainty. If appropria te tracer calibrations can be established, these corals are therefore suita ble to record decadal or sub-decadal oceanographic changes over the course of their lifetime. The delta(234)U values of all modern samples from differ ent localities and different depths are similar (mean 145.5 +/- 2.3 parts p er thousand) and indistinguishable from the data obtained from surface cora ls. At a precision of about +/-2 parts per thousand. we find no structure i n the oceanic profile of delta(234)U ratios over the top 2000 m of the wate r column. Copyright (C) 2000 Elsevier Science Ltd.