- Email: [email protected]
Ba ratios for dating Holocene biogenic carbonates in the Southern Ocean: preliminary evidence from Antarctic coastal mollusc shells
Chemical Geology 144 Ž1998. 331–334
Short communication 226
RarBa ratios for dating Holocene biogenic carbonates in the Southern Ocean: preliminary evidence from Antarctic coastal mollusc shells Paul Arthur Berkman a
a,)
, Teh-Lung Ku
b
Byrd Polar Research Center, The Ohio State UniÕersity, 108 Scott Hall, 1090 Carmack Road, Columbus, OH 43210-1002, USA b Department of Earth Sciences, UniÕersity of Southern California, Los Angeles, CA 90089-0740, USA Received 10 March 1997; accepted 6 October 1997
Abstract Marine bivalve molluscs Ž Adamussium colbecki . were collected alive from the Antarctic coastal zone to determine whether the 226 RarBa ratios in their shells reflect those of ambient seawater and can be used for dating Ž t 1r2 226 Ra s 1600 years. Holocene fossils from the Southern Ocean. Similarities between the radium, barium and calcium concentrations in acid-treated Ž0.039 dpm gy1 , 2.15 mg gy1 , and 355 mg gy1 . and mechanically treated Ž0.041 dpm gy1, 2.07 mg gy1 and 361 mg gy1 . shells indicate that these divalent cations were lattice-bound and not adsorbed between the unit cells of the Adamussium calcite. In addition, 226 RarBa ratios in Southern Ocean seawater Ž0.0168 dpm mgy1 . were conserved in element Adamussium shells Ž 0.0181 to 0.0198 dpm m g y1 . despite the discrimination factors wDFCa s ŽelementrCa. samplerŽelementrCa. seawater x 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 226 RarBa chronometer which complements and is independent of 14Cr 12 C for dating calcareous fossils from the Antarctic marine ecosystem up to several thousand years old. q 1998 Elsevier Science B.V.
1. Introduction The purpose of this note is to assess the viability of 226 RarBa for dating calcareous marine fossils which have circumpolar distributions in emerged Holocene beaches around Antarctica ŽBerkman, 1992.. Radium-226 and barium have chemical affinities and similar vertical distributions in the ocean which allow Ba to be treated as a ‘stable’ isotope for assessing the decay of 226 Ra ŽBroecker and Peng, 1982.. Combined with 14 Cr 12 C, 226 RarBa may en)
Corresponding author. Tel.: q1 Ž614. 292 3670; Fax: q1 Ž614. 292-4697; E-mail: [email protected]
hance the resolution of Holocene environmental events in the Antarctic coastal zone which currently are blurred by uncertainties associated with the radiocarbon reservoir in the Southern Ocean ŽGordon and Harkness, 1992; Berkman and Forman, 1996.. Radium-226 Ž t 1r2 s 1600 years. enters the ocean mostly via diffusion across the sediment–water interface in the deep sea where a large concentration gradient of 226 Ra exists due to the deposition of its radioactive precursor, the particle-borne 230 Th ŽKu and Luo, 1994.. Concentrations of 226 Ra in deep waters of the Southern Ocean fall in the vicinity of 0.20 dpm kgy1 . Because of the intense upwelling at the Antarctic Divergence, surface-water 226 Ra con-
0009-2541r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 5 4 1 Ž 9 7 . 0 0 1 3 7 - X
332
P.A. Berkman, T.-L. Ku r Chemical Geology 144 (1998) 331–334
centrations in the Southern Ocean are nearly as high as those in the Circumpolar Deep Water ŽKu et al., 1970; Ku and Lin, 1976.. Minimal vertical concentration gradients in the circumpolar Southern Ocean also are shown for Ba ŽOstlund et al., 1987. which has a similar geochemistry to 226 Ra ŽChow and Goldberg, 1960; Chan et al., 1976; Ku and Luo, 1994.. The vertical distribution of these elements in the Southern Ocean is in contrast with the low-latitude oligotrophic oceans, where surface and deep concentrations of 226 Ra and Ba may differ by a factor as large as six ŽKu and Luo, 1994.. In the Southern Ocean, the distribution of 226 Ra and Ba may be reflected by the composition of coastal marine mollusc shells which incorporate these elements in equilibrium with surface seawater. Previous studies, however, indicate that these 226 RarBa ratios in mollusc shells may be strongly influenced by the species and their carbonate mineralogy ŽBlanchard and Oakes, 1965.. For these reasons, only the circumpolar Antarctic scallop Ž Adamussium colbecki . was used in this study. Adamussium also has the advantage of being among the most common coastal marine molluscs around Antarctica, with extant assemblages that can serve as experimental analogs for interpreting the geochemistry of adjacent Holocene fossils in raised beaches ŽBerkman, 1997.. Calcitic shells of living Adamussium have unitcell dimensions which are larger than those in pure calcite crystals ŽBerkman et al., 1992. and reflect isomorphic substitutions in the carbonate matrix by divalent cations which have larger ionic radii than calcium ŽBerry et al., 1983., namely: barium, strontium or radium. If radium is occupying the unit-cell lattice sites, then radium would be in a thermodynamically stable position ŽSwalin, 1962.. In addition, the polar desert environment will minimize the postmortem migration and contamination of 226 Ra in the Adamussium fossils after they have emerged from the marine reservoir ŽIvanovich et al., 1983.. Therefore, Adamussium shells may behave as a closed system for 226 Ra ŽBerkman et al., 1992..
2. Materials and methods In this study, preliminary 226 RarBa analyses were conducted with Adamussium shells that were col-
lected alive with SCUBA in Explorers Cove Ž77834X S, 163831X E., Antarctica, during the 1986–87 austral summer. Similar size Adamussium shells Ž79.7 " 4.8 mm in height. were analyzed for 226 Ra to eliminate any geochemical variations that may be associated with their ontogeny. However, because the Adamussium shells are wafer thin Ž3–4 g per valve., several shells were combined to obtain a large enough sample volume Ž25–35 g. for the 226 Ra measurements which were made by alpha-scintillation counting with the Rn-emanation method at the University of Southern California ŽKu et al., 1970.. Each shell was individually cleaned with a brush in distilled water to remove any surface debris before being combined into several batches for the Rnemanation analyses. In the first batch ŽBatch 1., shells additionally were cleaned by dipping them sequentially in dilute acid Ž0.1 M HCl., dilute base Ž0.1 M NaOH. and distilled water to remove any adsorbed 226 Ra and Ba. This acid-cleaned batch was analyzed twice to evaluate the reproducibility of the Rn-emanation method. A second batch of shells was prepared in nearly equal aliquots, one of which was mechanically cleaned only ŽBatch 2a. and the other which was cleaned with the dilute-acid treatment ŽBatch 2b.. The final preparation procedure for all of the 226 Ra analyses involved roasting the shell batches at 5008C to decompose any organic materials Žless than 0.3% of the shell weight is organic carbon: Berkman, 1991. before dissolving them in HNO 3 . Complementary analyses of calcium and barium were conducted by Inductively Coupled Plasma ŽICP. spectrometry at the Ohio State University on Batches 2a and 2b. These ICP analyses were compared to those from a previous study which measured the calcium and barium concentrations in twelve homogenized Adamussium shells Žeach aliquot was less than 100 mg. that had been cleaned with dilute acid ŽBerkman, 1994..
3. Results and discussion Close agreement between the two 226 Ra measurements of Batch 1 Ž0.035 and 0.040 dpm gy1 . indicates that the 226 Ra analyses were reproducible within the error limits Ž0.002 dpm gy1 . of the Rn method ŽTable 1.. Nearly equal elemental concentrations also
P.A. Berkman, T.-L. Ku r Chemical Geology 144 (1998) 331–334
333
Table 1 Radium-226, barium and calcium concentrations in modern scallop Ž Adamussium colbecki . shells from Explorers Cove, Antarctica Ž77834X S, 163831X E. Adamussium preparation
Weight a Žg.
226 Ra b Ždpm gy1 .
Ba b Žmg gy1 .
Ca b Žmg gy1 .
Ra c,d DFCa
Ba c,d DFCa
226 RarBa Ždpm mgy1 .
Acid ŽBatch 1.
32.1 Ž2 runs. 25.7 24.3
0.035 " 0.002 0.040 " 0.002 0.039 " 0.002 0.041 " 0.002
– – 2.15 " 0.15 2.07 " 0.98
– – 355 " 1.6 361 " 2.8
– – 0.244 0.252
– – 0.226 0.214
– – 0.0181 0.0198
Acid ŽBatch 2a. Mechanical ŽBatch 2b. a
Weights are based on shell mixtures. " is the analytical precision Žone standard deviation.. c element Discrimination Factors wDFCa s ŽelementrCa. samplerŽelementrCa. seawater x. d Calculated relative to surface seawater in the Southern Ocean which has 226 Ra concentrations of 0.18 dpm kgy1 , Ba concentrations of 10.7 mg kgy1 and Ca concentrations of 400 mg kgy1 Žsee text.. b
are found in the acid and no-acid fractions of Batch 2 ŽTable 1.. Moreover, the Ba concentrations of 2.28 " 0.40 mg gy1 in acid-cleaned Adamussium shells from a previous study ŽBerkman, 1994. overlapped with those found in this study. These results indicate that the divalent cations were lattice-bound and not adsorbed within the intercrystalline spaces between the unit cells of the Adamussium calcite. We cannot rule out the possibility, however, that some 226 Ra and Ba atoms may reside inside the organic matrix which would not be significantly altered by our mild acid treatment ŽG.M. Henderson, pers. commun., 1997.. Based on the above results, comparisons of the 226 Ra and Ba concentrations in the Adamussium shells and Antarctic surface seawater were made to determine whether these elements were being incorporated into the shells from the seawater at constant molar ratios relative to Ca. The ‘equilibrium’ concentration of 226 Ra can be predicted in the Adamussium shells in relation to calcium which has a seawater concentration of approximately 400 mg kgy1 . The 226 Ra concentration in the ambient Antarctic surface Ž0–100 m. seawater is 0.18 dpm kgy1 ŽGEOSECS Station 287 at 69818X S, 173830X W; Ostlund et al., 1987.. If 226 Ra and Ca were incorporated from seawater into the Adamussium shells without fractionation, then 1 g of shell Ž400 mg Ca q 120 mg C q 480 mg O. should have 0.18 dpm of 226 Ra. Similarly, the concentration of barium in Antarctic surface seawater at GEOSECS Station 287 is approximately 10.7 mg kgy1 ŽOstlund et al., 1987.. If no fractionation between Ba and Ca occurred during Adamussium shell formation, then 1 g of shell material should have 10.7 mg of Ba. The
data in Table 1 show that both 226 Ra and Ba are discriminated against relative to Ca during shell formation. Based on the elemental concentrations of the Adamussium shells from Batch 2 ŽTable 1., discrimination factors were determined according to Eq. 1 ŽKu, 1972.: element DFCa s
Ž elementrCa. sample Ž elementrCa. seawater
Ž 1.
Ra Ba where DFCa varied from 0.244 to 0.252 and DFCa varied from 0.214 to 0.226 ŽTable 1.. This uniform discrimination against 226 Ra and Ba relative to Ca during shell formation reflects the conservative relationship between the 226 RarBa ratios in Southern Ocean seawater Ž0.0168 dpm mgy1 . and Adamussium shells Ž0.0181–0.0198 dpm mgy1 .. Together, these data suggest that Adamussium shells may be treated as a closed system for 226 Ra and Ba with an initial 226 RarBa ratio close to that of the ambient seawater. Because of the extremely low 230 Thr 226 Ra ratios in seawater, virtually all of the 226 Ra in living shells should be unsupported. Gamma-spectrometry measurements of the Adamussium samples also indicate that shell concentrations of 238 U Žwhich decays to 226 Ra: Broecker and Peng, 1982. were less than 0.1 ppm and similar to those reported for the majority of living mollusc shells ŽKaufman et al., 1971.. Assuming a 234 Ur 238 U ratio of close to one, any 230 Thsupported 226 Ra still would account for less than 10% of the 226 Ra in a 3000-year-old Adamussium shell.
334
P.A. Berkman, T.-L. Ku r Chemical Geology 144 (1998) 331–334
In conclusion, this preliminary study indicates that it should be viable to develop a 226 RarBa chronometer for dating calcareous marine fossils in the Southern Ocean up to several thousand years in age. In the future, high-precision measurements of 226 Ra and Ba by thermal ionization mass spectrometry ŽChan et al., 1976; Cohen and O’Nions, 1991; Volpe et al., 1991. should be conducted with individual shells of calcareous marine species Žboth modern and radiocarbon-dated fossils. to improve the interpretation of Holocene environmental variability around Antarctica.
Acknowledgements This study was supported by a grant from the National Science Foundation ŽOPP 9221784. to P.A.B. We would like to thank Mr. Yuhong Tang for his assistance with the 226 Ra analyses and Drs. Gideon M. Henderson, Willard S. Moore and G.E. Mortimer for their constructive reviews of the manuscript. This paper is Contribution No. 1046 from the Byrd Polar Research Center at The Ohio State University.
References Berkman, P.A., 1991. Holocene meltwater variations recorded in Antarctic coastal marine benthic assemblages. In: Weller, G., Wilson, C.L., Severin, B.A.B. ŽEds.., Proceedings of the International Conference on the Role of the Polar Regions in Global Change. University of Alaska, Fairbanks, pp. 440–449. Berkman, P.A., 1992. Circumpolar distribution of Holocene marine fossils in Antarctic beaches. Quat. Res. 37, 256–260. Berkman, P.A., 1994. Geochemical signatures of meltwater in mollusc shells from Antarctic coastal areas during the Holocene. In: Berkman, P.A., Yoshida, Y. ŽEds.., Holocene Environmental Changes in Antarctic Coastal Areas. Memoirs of the National Institute of Polar Research, Special Issue, No. 50, National Institute of Polar Research, Tokyo, pp. 11–33. Berkman, P.A., 1997. Ecological variability in Antarctic coastal environments: past and present. In: Battaglia, B., Valencia, J., Walton, D.W.H. ŽEds.., Antarctic Environmental Change and Conservation. Cambridge University Press, London, pp. 349– 357. Berkman, P.A., Forman, S.L., 1996. Pre-bomb radiocarbon and the reservoir correction for calcareous marine species in the Southern Ocean. Geophys. Res. Lett. 23, 363–366.
Berkman, P.A., Foreman, D.W., Mitchell, J.C., Liptak, R.J., 1992. Scallop shell mineralogy and crystallographic characteristics: proxy records for interpreting Antarctic nearshore marine hydrochemical variability. Antarct. Res. Ser. 57, 27–38. Berry, L.G., Mason, B., Dietrich, R.V., 1983. Mineralogy Concepts, Descriptions and Determinations. W.H. Freeman, New York, 2nd ed. Blanchard, R.L., Oakes, D., 1965. Relationships between uranium and radium in coastal marine shells and their environment. J. Geophys. Res. 70, 2911–2921. Broecker, W.S., Peng, T.-H., 1982. Tracers in the Sea. Eldigio Press, Palisades, New York. Chan, L.H., Edmond, J.M., Stallard, R.F., Broecker, W.S., Chung, Y.C., Weiss, R.W., Ku, T.L., 1976. Radium and barium at GEOSECS stations in the Atlantic and Pacific. Earth Planet. Sci. Lett. 32, 258–267. Chow, T.J., Goldberg, E.D., 1960. On the marine geochemistry of barium. Geochim. Cosmochim. Acta 20, 192–198. Cohen, A.S., O’Nions, R.K., 1991. Precise determination of femtogram quantities of radium by thermal ionization mass spectrometry. Anal. Chem. 63, 2705–2708. Gordon, J.E., Harkness, D.D., 1992. Magnitude and geographic variation of the radiocarbon content in Antarctic marine life: implications for reservoir corrections in radiocarbon dating. Quat. Sci. Rev. 11, 697–708. Ivanovich, M., Vita-Finzi, C., Henning, G.J., 1983. Uranium-series dating of molluscs from uplifted Holocene beaches in the Persian Gulf. Nature 302, 408–410. Kaufman, A., Broecker, W.S., Ku, T.-L., Thurber, D.L., 1971. The status of U-series methods of mollusk dating. Geochim. Cosmochim. Acta 35, 1155–1183. Ku, T.-L., 1972. Radium in the oceans. In: Fairbridge, R.W. ŽEd.., The Encyclopedia of Geochemistry and Environmental Sciences. Encyclopedia of Earth Sciences Series, Vol. IVA, Van Nostrand Reinhold, New York, pp. 1008–1014. Ku, T.-L., Lin, M.C., 1976. Radium-226 distribution in the Antarctic Ocean. Earth Planet. Sci. Lett. 32, 236–248. Ku, T.-L., Luo, S., 1994. New appraisal of radium-226 as a large-scale oceanic mixing tracer. J. Geophys. Res. 99, 10255–10273. Ku, T.-L., Li, Y.H., Mathieu, G.G., Wong, H.K., 1970. Radium in the Indian–Antarctic Ocean south of Australia. J. Geophys. Res. 75, 5286–5292. Ostlund, H.G., Craig, H., Broecker, W.S., Spencer, D., 1987. GEOSECS Atlantic, Pacific, and Indian Ocean Expeditions: Shorebased Data and Graphics, Vol. 7. National Science Foundation, Washington, D.C. Swalin, R.A., 1962. Thermodynamics of Solids. John Wiley, New York. Volpe, A.M., Olivares, J.A., Murrell, M.T., 1991. Determination of radium isotope ratios and abundances in geologic samples by thermal ionization mass spectrometry. Anal. Chem. 63, 913–916.
Short communication 226
RarBa ratios for dating Holocene biogenic carbonates in the Southern Ocean: preliminary evidence from Antarctic coastal mollusc shells Paul Arthur Berkman a
a,)
, Teh-Lung Ku
b
Byrd Polar Research Center, The Ohio State UniÕersity, 108 Scott Hall, 1090 Carmack Road, Columbus, OH 43210-1002, USA b Department of Earth Sciences, UniÕersity of Southern California, Los Angeles, CA 90089-0740, USA Received 10 March 1997; accepted 6 October 1997
Abstract Marine bivalve molluscs Ž Adamussium colbecki . were collected alive from the Antarctic coastal zone to determine whether the 226 RarBa ratios in their shells reflect those of ambient seawater and can be used for dating Ž t 1r2 226 Ra s 1600 years. Holocene fossils from the Southern Ocean. Similarities between the radium, barium and calcium concentrations in acid-treated Ž0.039 dpm gy1 , 2.15 mg gy1 , and 355 mg gy1 . and mechanically treated Ž0.041 dpm gy1, 2.07 mg gy1 and 361 mg gy1 . shells indicate that these divalent cations were lattice-bound and not adsorbed between the unit cells of the Adamussium calcite. In addition, 226 RarBa ratios in Southern Ocean seawater Ž0.0168 dpm mgy1 . were conserved in element Adamussium shells Ž 0.0181 to 0.0198 dpm m g y1 . despite the discrimination factors wDFCa s ŽelementrCa. samplerŽelementrCa. seawater x 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 226 RarBa chronometer which complements and is independent of 14Cr 12 C for dating calcareous fossils from the Antarctic marine ecosystem up to several thousand years old. q 1998 Elsevier Science B.V.
1. Introduction The purpose of this note is to assess the viability of 226 RarBa for dating calcareous marine fossils which have circumpolar distributions in emerged Holocene beaches around Antarctica ŽBerkman, 1992.. Radium-226 and barium have chemical affinities and similar vertical distributions in the ocean which allow Ba to be treated as a ‘stable’ isotope for assessing the decay of 226 Ra ŽBroecker and Peng, 1982.. Combined with 14 Cr 12 C, 226 RarBa may en)
Corresponding author. Tel.: q1 Ž614. 292 3670; Fax: q1 Ž614. 292-4697; E-mail: [email protected]
hance the resolution of Holocene environmental events in the Antarctic coastal zone which currently are blurred by uncertainties associated with the radiocarbon reservoir in the Southern Ocean ŽGordon and Harkness, 1992; Berkman and Forman, 1996.. Radium-226 Ž t 1r2 s 1600 years. enters the ocean mostly via diffusion across the sediment–water interface in the deep sea where a large concentration gradient of 226 Ra exists due to the deposition of its radioactive precursor, the particle-borne 230 Th ŽKu and Luo, 1994.. Concentrations of 226 Ra in deep waters of the Southern Ocean fall in the vicinity of 0.20 dpm kgy1 . Because of the intense upwelling at the Antarctic Divergence, surface-water 226 Ra con-
0009-2541r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 5 4 1 Ž 9 7 . 0 0 1 3 7 - X
332
P.A. Berkman, T.-L. Ku r Chemical Geology 144 (1998) 331–334
centrations in the Southern Ocean are nearly as high as those in the Circumpolar Deep Water ŽKu et al., 1970; Ku and Lin, 1976.. Minimal vertical concentration gradients in the circumpolar Southern Ocean also are shown for Ba ŽOstlund et al., 1987. which has a similar geochemistry to 226 Ra ŽChow and Goldberg, 1960; Chan et al., 1976; Ku and Luo, 1994.. The vertical distribution of these elements in the Southern Ocean is in contrast with the low-latitude oligotrophic oceans, where surface and deep concentrations of 226 Ra and Ba may differ by a factor as large as six ŽKu and Luo, 1994.. In the Southern Ocean, the distribution of 226 Ra and Ba may be reflected by the composition of coastal marine mollusc shells which incorporate these elements in equilibrium with surface seawater. Previous studies, however, indicate that these 226 RarBa ratios in mollusc shells may be strongly influenced by the species and their carbonate mineralogy ŽBlanchard and Oakes, 1965.. For these reasons, only the circumpolar Antarctic scallop Ž Adamussium colbecki . was used in this study. Adamussium also has the advantage of being among the most common coastal marine molluscs around Antarctica, with extant assemblages that can serve as experimental analogs for interpreting the geochemistry of adjacent Holocene fossils in raised beaches ŽBerkman, 1997.. Calcitic shells of living Adamussium have unitcell dimensions which are larger than those in pure calcite crystals ŽBerkman et al., 1992. and reflect isomorphic substitutions in the carbonate matrix by divalent cations which have larger ionic radii than calcium ŽBerry et al., 1983., namely: barium, strontium or radium. If radium is occupying the unit-cell lattice sites, then radium would be in a thermodynamically stable position ŽSwalin, 1962.. In addition, the polar desert environment will minimize the postmortem migration and contamination of 226 Ra in the Adamussium fossils after they have emerged from the marine reservoir ŽIvanovich et al., 1983.. Therefore, Adamussium shells may behave as a closed system for 226 Ra ŽBerkman et al., 1992..
2. Materials and methods In this study, preliminary 226 RarBa analyses were conducted with Adamussium shells that were col-
lected alive with SCUBA in Explorers Cove Ž77834X S, 163831X E., Antarctica, during the 1986–87 austral summer. Similar size Adamussium shells Ž79.7 " 4.8 mm in height. were analyzed for 226 Ra to eliminate any geochemical variations that may be associated with their ontogeny. However, because the Adamussium shells are wafer thin Ž3–4 g per valve., several shells were combined to obtain a large enough sample volume Ž25–35 g. for the 226 Ra measurements which were made by alpha-scintillation counting with the Rn-emanation method at the University of Southern California ŽKu et al., 1970.. Each shell was individually cleaned with a brush in distilled water to remove any surface debris before being combined into several batches for the Rnemanation analyses. In the first batch ŽBatch 1., shells additionally were cleaned by dipping them sequentially in dilute acid Ž0.1 M HCl., dilute base Ž0.1 M NaOH. and distilled water to remove any adsorbed 226 Ra and Ba. This acid-cleaned batch was analyzed twice to evaluate the reproducibility of the Rn-emanation method. A second batch of shells was prepared in nearly equal aliquots, one of which was mechanically cleaned only ŽBatch 2a. and the other which was cleaned with the dilute-acid treatment ŽBatch 2b.. The final preparation procedure for all of the 226 Ra analyses involved roasting the shell batches at 5008C to decompose any organic materials Žless than 0.3% of the shell weight is organic carbon: Berkman, 1991. before dissolving them in HNO 3 . Complementary analyses of calcium and barium were conducted by Inductively Coupled Plasma ŽICP. spectrometry at the Ohio State University on Batches 2a and 2b. These ICP analyses were compared to those from a previous study which measured the calcium and barium concentrations in twelve homogenized Adamussium shells Žeach aliquot was less than 100 mg. that had been cleaned with dilute acid ŽBerkman, 1994..
3. Results and discussion Close agreement between the two 226 Ra measurements of Batch 1 Ž0.035 and 0.040 dpm gy1 . indicates that the 226 Ra analyses were reproducible within the error limits Ž0.002 dpm gy1 . of the Rn method ŽTable 1.. Nearly equal elemental concentrations also
P.A. Berkman, T.-L. Ku r Chemical Geology 144 (1998) 331–334
333
Table 1 Radium-226, barium and calcium concentrations in modern scallop Ž Adamussium colbecki . shells from Explorers Cove, Antarctica Ž77834X S, 163831X E. Adamussium preparation
Weight a Žg.
226 Ra b Ždpm gy1 .
Ba b Žmg gy1 .
Ca b Žmg gy1 .
Ra c,d DFCa
Ba c,d DFCa
226 RarBa Ždpm mgy1 .
Acid ŽBatch 1.
32.1 Ž2 runs. 25.7 24.3
0.035 " 0.002 0.040 " 0.002 0.039 " 0.002 0.041 " 0.002
– – 2.15 " 0.15 2.07 " 0.98
– – 355 " 1.6 361 " 2.8
– – 0.244 0.252
– – 0.226 0.214
– – 0.0181 0.0198
Acid ŽBatch 2a. Mechanical ŽBatch 2b. a
Weights are based on shell mixtures. " is the analytical precision Žone standard deviation.. c element Discrimination Factors wDFCa s ŽelementrCa. samplerŽelementrCa. seawater x. d Calculated relative to surface seawater in the Southern Ocean which has 226 Ra concentrations of 0.18 dpm kgy1 , Ba concentrations of 10.7 mg kgy1 and Ca concentrations of 400 mg kgy1 Žsee text.. b
are found in the acid and no-acid fractions of Batch 2 ŽTable 1.. Moreover, the Ba concentrations of 2.28 " 0.40 mg gy1 in acid-cleaned Adamussium shells from a previous study ŽBerkman, 1994. overlapped with those found in this study. These results indicate that the divalent cations were lattice-bound and not adsorbed within the intercrystalline spaces between the unit cells of the Adamussium calcite. We cannot rule out the possibility, however, that some 226 Ra and Ba atoms may reside inside the organic matrix which would not be significantly altered by our mild acid treatment ŽG.M. Henderson, pers. commun., 1997.. Based on the above results, comparisons of the 226 Ra and Ba concentrations in the Adamussium shells and Antarctic surface seawater were made to determine whether these elements were being incorporated into the shells from the seawater at constant molar ratios relative to Ca. The ‘equilibrium’ concentration of 226 Ra can be predicted in the Adamussium shells in relation to calcium which has a seawater concentration of approximately 400 mg kgy1 . The 226 Ra concentration in the ambient Antarctic surface Ž0–100 m. seawater is 0.18 dpm kgy1 ŽGEOSECS Station 287 at 69818X S, 173830X W; Ostlund et al., 1987.. If 226 Ra and Ca were incorporated from seawater into the Adamussium shells without fractionation, then 1 g of shell Ž400 mg Ca q 120 mg C q 480 mg O. should have 0.18 dpm of 226 Ra. Similarly, the concentration of barium in Antarctic surface seawater at GEOSECS Station 287 is approximately 10.7 mg kgy1 ŽOstlund et al., 1987.. If no fractionation between Ba and Ca occurred during Adamussium shell formation, then 1 g of shell material should have 10.7 mg of Ba. The
data in Table 1 show that both 226 Ra and Ba are discriminated against relative to Ca during shell formation. Based on the elemental concentrations of the Adamussium shells from Batch 2 ŽTable 1., discrimination factors were determined according to Eq. 1 ŽKu, 1972.: element DFCa s
Ž elementrCa. sample Ž elementrCa. seawater
Ž 1.
Ra Ba where DFCa varied from 0.244 to 0.252 and DFCa varied from 0.214 to 0.226 ŽTable 1.. This uniform discrimination against 226 Ra and Ba relative to Ca during shell formation reflects the conservative relationship between the 226 RarBa ratios in Southern Ocean seawater Ž0.0168 dpm mgy1 . and Adamussium shells Ž0.0181–0.0198 dpm mgy1 .. Together, these data suggest that Adamussium shells may be treated as a closed system for 226 Ra and Ba with an initial 226 RarBa ratio close to that of the ambient seawater. Because of the extremely low 230 Thr 226 Ra ratios in seawater, virtually all of the 226 Ra in living shells should be unsupported. Gamma-spectrometry measurements of the Adamussium samples also indicate that shell concentrations of 238 U Žwhich decays to 226 Ra: Broecker and Peng, 1982. were less than 0.1 ppm and similar to those reported for the majority of living mollusc shells ŽKaufman et al., 1971.. Assuming a 234 Ur 238 U ratio of close to one, any 230 Thsupported 226 Ra still would account for less than 10% of the 226 Ra in a 3000-year-old Adamussium shell.
334
P.A. Berkman, T.-L. Ku r Chemical Geology 144 (1998) 331–334
In conclusion, this preliminary study indicates that it should be viable to develop a 226 RarBa chronometer for dating calcareous marine fossils in the Southern Ocean up to several thousand years in age. In the future, high-precision measurements of 226 Ra and Ba by thermal ionization mass spectrometry ŽChan et al., 1976; Cohen and O’Nions, 1991; Volpe et al., 1991. should be conducted with individual shells of calcareous marine species Žboth modern and radiocarbon-dated fossils. to improve the interpretation of Holocene environmental variability around Antarctica.
Acknowledgements This study was supported by a grant from the National Science Foundation ŽOPP 9221784. to P.A.B. We would like to thank Mr. Yuhong Tang for his assistance with the 226 Ra analyses and Drs. Gideon M. Henderson, Willard S. Moore and G.E. Mortimer for their constructive reviews of the manuscript. This paper is Contribution No. 1046 from the Byrd Polar Research Center at The Ohio State University.
References Berkman, P.A., 1991. Holocene meltwater variations recorded in Antarctic coastal marine benthic assemblages. In: Weller, G., Wilson, C.L., Severin, B.A.B. ŽEds.., Proceedings of the International Conference on the Role of the Polar Regions in Global Change. University of Alaska, Fairbanks, pp. 440–449. Berkman, P.A., 1992. Circumpolar distribution of Holocene marine fossils in Antarctic beaches. Quat. Res. 37, 256–260. Berkman, P.A., 1994. Geochemical signatures of meltwater in mollusc shells from Antarctic coastal areas during the Holocene. In: Berkman, P.A., Yoshida, Y. ŽEds.., Holocene Environmental Changes in Antarctic Coastal Areas. Memoirs of the National Institute of Polar Research, Special Issue, No. 50, National Institute of Polar Research, Tokyo, pp. 11–33. Berkman, P.A., 1997. Ecological variability in Antarctic coastal environments: past and present. In: Battaglia, B., Valencia, J., Walton, D.W.H. ŽEds.., Antarctic Environmental Change and Conservation. Cambridge University Press, London, pp. 349– 357. Berkman, P.A., Forman, S.L., 1996. Pre-bomb radiocarbon and the reservoir correction for calcareous marine species in the Southern Ocean. Geophys. Res. Lett. 23, 363–366.
Berkman, P.A., Foreman, D.W., Mitchell, J.C., Liptak, R.J., 1992. Scallop shell mineralogy and crystallographic characteristics: proxy records for interpreting Antarctic nearshore marine hydrochemical variability. Antarct. Res. Ser. 57, 27–38. Berry, L.G., Mason, B., Dietrich, R.V., 1983. Mineralogy Concepts, Descriptions and Determinations. W.H. Freeman, New York, 2nd ed. Blanchard, R.L., Oakes, D., 1965. Relationships between uranium and radium in coastal marine shells and their environment. J. Geophys. Res. 70, 2911–2921. Broecker, W.S., Peng, T.-H., 1982. Tracers in the Sea. Eldigio Press, Palisades, New York. Chan, L.H., Edmond, J.M., Stallard, R.F., Broecker, W.S., Chung, Y.C., Weiss, R.W., Ku, T.L., 1976. Radium and barium at GEOSECS stations in the Atlantic and Pacific. Earth Planet. Sci. Lett. 32, 258–267. Chow, T.J., Goldberg, E.D., 1960. On the marine geochemistry of barium. Geochim. Cosmochim. Acta 20, 192–198. Cohen, A.S., O’Nions, R.K., 1991. Precise determination of femtogram quantities of radium by thermal ionization mass spectrometry. Anal. Chem. 63, 2705–2708. Gordon, J.E., Harkness, D.D., 1992. Magnitude and geographic variation of the radiocarbon content in Antarctic marine life: implications for reservoir corrections in radiocarbon dating. Quat. Sci. Rev. 11, 697–708. Ivanovich, M., Vita-Finzi, C., Henning, G.J., 1983. Uranium-series dating of molluscs from uplifted Holocene beaches in the Persian Gulf. Nature 302, 408–410. Kaufman, A., Broecker, W.S., Ku, T.-L., Thurber, D.L., 1971. The status of U-series methods of mollusk dating. Geochim. Cosmochim. Acta 35, 1155–1183. Ku, T.-L., 1972. Radium in the oceans. In: Fairbridge, R.W. ŽEd.., The Encyclopedia of Geochemistry and Environmental Sciences. Encyclopedia of Earth Sciences Series, Vol. IVA, Van Nostrand Reinhold, New York, pp. 1008–1014. Ku, T.-L., Lin, M.C., 1976. Radium-226 distribution in the Antarctic Ocean. Earth Planet. Sci. Lett. 32, 236–248. Ku, T.-L., Luo, S., 1994. New appraisal of radium-226 as a large-scale oceanic mixing tracer. J. Geophys. Res. 99, 10255–10273. Ku, T.-L., Li, Y.H., Mathieu, G.G., Wong, H.K., 1970. Radium in the Indian–Antarctic Ocean south of Australia. J. Geophys. Res. 75, 5286–5292. Ostlund, H.G., Craig, H., Broecker, W.S., Spencer, D., 1987. GEOSECS Atlantic, Pacific, and Indian Ocean Expeditions: Shorebased Data and Graphics, Vol. 7. National Science Foundation, Washington, D.C. Swalin, R.A., 1962. Thermodynamics of Solids. John Wiley, New York. Volpe, A.M., Olivares, J.A., Murrell, M.T., 1991. Determination of radium isotope ratios and abundances in geologic samples by thermal ionization mass spectrometry. Anal. Chem. 63, 913–916.