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

Abstract

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.