226Ra/Ba ratios for dating Holocene biogenic carbonates in the Southern Ocean:: preliminary evidence from Antarctic coastal mollusc shells

Marine bivalve molluscs (Adamussium colbecki) were collected alive from the Antarctic coastal zone to determine whether the Ra-236/Ba ratios in their shells reflect those of ambient seawater and can be used for dating (t(1/2) Ra-226 = 1600 years) Holocene fossils from the Southern Ocean. Similarities between the radium, barium and calcium concentrations in acid-treated (0.039 dpm g(-1), 2.15 mu g g(-1), and 355 mg g(-1)) and mechanically treated (0.041 dpm g(-1), 2.07 mu g g(-1) and 361 mg g(-1)) shells indicate that these divalent cations were lattice-bound and not adsorbed between the unit cells of the Adamussium calcite. In addition, Ra-226/Ba ratios in Southern Ocean seawater (0.0168 dpm mu g(-1)) were conserved in Adamussium shells (0.0181 to 0.0198 dpm mu g(-1)) despite the discrimination factors [DFCaelement = (element/Ca)(sample)/(element/Ca)(seawater)] of 0.244 to 0.252 for Ra and 0.214 to 0.226 for Ba. Together, these preliminary data suggest that it should be viable to develop a Ra-226/Ba chronometer which complements and is independent of C-14/C-12 for dating calcareous fossils from the Antarctic marine ecosystem up to several thousand years old. (C) 1998 Elseevier Science B.V.