RA-226 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 fro m 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 Souther n Ocean. Similarities between the radium, barium and calcium concentra tions 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 36 1 mg g(-1)) shells indicate that these divalent cations were lattice-b ound 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 m u g(-1)) despite the discrimination factors [DFCaelement = (element/Ca )(sample)/(element/Ca)(seawater)] of 0.244 to 0.252 for Ra and 0.214 t o 0.226 for Ba. Together, these preliminary data suggest that it shoul d be viable to develop a Ra-226/Ba chronometer which complements and i s independent of C-14/C-12 for dating calcareous fossils from the Anta rctic marine ecosystem up to several thousand years old. (C) 1998 Else evier Science B.V.