U-TH DATING OF DEEP-SEA CORALS

Abstract

230Th, 232Th, 234U and 238U compositions of several deep-sea solitary corals, mainly the species Desmophyllum cristagalli, were determined by thermal ionization mass spectrometry (TIMS). It is possible to obtain high precision ages on modern pristine corals that have low [232Th] (5 to a few hundred ppt). However, because older deep-sea corals tend to have higher [232Th] compared to surface corals, and the initial 230Th/232Th ratio is uncertain, older deep-sea corals have larger age uncertainties (+/-several hundred years for samples with a few thousand ppt 232Th). Therefore, the key hurdle for precise U-Th dating is to remove or account for contaminants which contain elevated 232Th and associated 230Th not due to closed system decay within the coral lattice. A modification of the trace metal cleaning methods 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 precipitated annually, but this estimate has a large uncertainty. If appropriate tracer calibrations can be established, these corals are therefore suitable to record decadal or sub-decadal oceanographic changes over the course of their lifetime. The δ234U values of all modern samples from different localities and different depths are similar (mean 145.5 +/- 2.3%%) and indistinguishable from the data obtained from surface corals. At a precision of about +/-2%%, we find no structure in the oceanic profile of δ234U ratios over the top 2000 m of the water column.