U-Th dating of deep-sea corals

Hai Cheng, Jess Adkins, R. Lawrence Edwards, Edward A. Boyle

Research output: Contribution to journalArticlepeer-review

164 Scopus citations

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. Copyright (C) 2000 Elsevier Science Ltd.

Original languageEnglish (US)
Pages (from-to)2401-2416
Number of pages16
JournalGeochimica et Cosmochimica Acta
Volume64
Issue number14
DOIs
StatePublished - Jul 2000

Bibliographical note

Funding Information:
D. Richards and J. Doral provided stimulating discussions about many of the issues raised in this work. S. Cairns helped identify the coral samples. T. Kleindinst of WHOI produced the photograph in Figure 4 . We would like to thank G. Henderson and D. Muhs for helpful reviews of the manuscript. JFA thanks the UCAR Post-Doctoral Fellowship Program and LDEO for support during the writing of this paper.

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