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 226Ra/Ba ratios in their shells reflect those of ambient seawater and can be used for dating (t1/2 226Ra=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 μg g−1, and 355 mg g−1) and mechanically treated (0.041 dpm g−1, 2.07 μg 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, 226Ra/Ba ratios in Southern Ocean seawater (0.0168 dpm μg−1) were conserved in Adamussium shells (0.0181 to 0.0198 dpm μg−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 226Ra/Ba chronometer which complements and is independent of 14C/12C for dating calcareous fossils from the Antarctic marine ecosystem up to several thousand years old.