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^(238)U, ^(234)U and ^(232)Th in seawater

Chen, J. H. and Edwards, R. Lawrence and Wasserburg, G. J. (1986) ^(238)U, ^(234)U and ^(232)Th in seawater. Earth and Planetary Science Letters, 80 (3-4). pp. 241-251. ISSN 0012-821X. doi:10.1016/0012-821X(86)90108-1.

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We have developed techniques to determine ^(238)U,^(234)U and ^(232)Th concentrations in seawater by isotope dilution mass spectrometry. U measurements are made using a ^(233)U-^(236)U double spike to correct for instrumental fractionation. Measurements on uranium standards demonstrate that ^(234)U/^(238)U ratios can be measured accurately and reproducibly. ^(234)U/^(238)U can be measured routinely to ± 5‰ (2σ) for a sample of 5 × 10^9 atoms of ^(234)U (3 × 10^(−8) g of total U, 10 ml of seawater). Data acquisition time is ∼ 1 hour. The small sample size, high precision and short data acquisition time are superior toα-counting techniques. ^(238)U is measured to ± 2‰ (2σ) for a sample of 8 × 10^(12) atoms of ^(238)U (∼ 3 × 10^(−9) g of U, 1 ml of seawater). ^(232)Th is measured to ± 20‰ with 3 × 10^(11) ^(232)Th atoms (10^(−10) g ^(232)Th, 1 1 of seawater). This small sample size will greatly facilitate investigation of the ^(232)Th concentration in the oceans. Using these techniques, we have measured ^(238)U, ^(234)U and ^(232)Th in vertical profiles of unfiltered, acidified seawater from the Atlantic and ^(238)U and ^(234)U in vertical profiles from the Pacific. Determinations of ^(234)U/^(238)U at depths ranging from 0 to 4900 m in the Atlantic (7°44′N, 40°43′W) and the Pacific (14°41′N, 160°01′W) Oceans are the same within experimental error (± 5‰,2σ). The average of these ^(234)U/^(238)U measurements is 144 ± 2‰ (2σ) higher than the equilibrium ratio of 5.472 × 10−5. U concentrations, normalized to 35‰ salinity, range from 3.162 to 3.281 ng/g, a range of 3.8%. The average concentration of the Pacific samples (31°4′N, 159°1′W) is ∼ 1% higher than that of the Atlantic (7°44′N, 40°43′W and 31°49′N, 64°6′W). ^(232)Th concentrations from an Atlantic profile range from 0.092 to 0.145 pg/g. The observed constancy of the ^(234)U/^(238)U ratio is consistent with the predicted range of ^(234)U/^(238)U using a simple two-box model and the residence time of deep water in the ocean determined from ^(14)C. The variation in salinity-normalized U concentrations suggests that U may be much more reactive in the marine environment than previously thought.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1986 Elsevier Science. Received May 21, 1986, revised version received August 26, 1986. We appreciate invaluable scientific and technical advice from D.A. Papanastassiou. We would like to thank P.M. Williams, C. Reimers, K.L. Smith, Jr., and the captain and crew of the R/V "Melville" for assistance in collecting and obtaining samples. D. Piepgras, and M. Stordal graciously provided some of the samples. Ship time on the R/V "Melville" was supported in part by NSF grants OCE 8315306 to C. Rermers, OCE 8417913 and OCE 8409075 to K.L. Smith, Jr. and OCE 8417102 to P.M. Williams. We are particularly grateful to K. Turekian and an anonymous reviewer for suggesting different formulations of the box model equations. This work has been supported by NSF grant OCE 8320516. Division Contribution No. 4338 (496)
Funding AgencyGrant Number
NSFOCE 8315306
NSFOCE 8417913
NSFOCE 8409075
NSFOCE 8417102
NSFOCE 8320516
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Caltech Division of Geological and Planetary Sciences4338
Lunatic Asylum Lab496
Issue or Number:3-4
Record Number:CaltechAUTHORS:20131023-111048720
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Official Citation:J.H. Chen, R. Lawrence Edwards, G.J. Wasserburg, 238U,234U and232Th in seawater, Earth and Planetary Science Letters, Volume 80, Issues 3–4, November 1986, Pages 241-251, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42025
Deposited By: Ruth Sustaita
Deposited On:23 Oct 2013 20:46
Last Modified:10 Nov 2021 04:37

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