Precise determination of the open ocean 234U/238U composition

Uranium has a long residence time in the open oceans, and therefore, its salinity-normalized U concentration and U-234/U-238 activity ratio (expressed herein as delta U-234, the parts per thousand deviation from secular equilibrium) are assumed to be uniform. The marine U-234/U-238 activity ratio is currently in radioactive disequilibrium and shows a similar to 15% excess of U-234 with respect to the secular equilibrium value due to continuous input from riverine sources. Knowledge of the marine delta U-234, and how it has evolved through the Quaternary, is important for validating age accuracy in the U series dating of marine carbonates, which is increasingly relied upon for providing a chronological basis in paleoclimate research. However, accurate and precise measurements of delta U-234 are technically difficult. Thus, existing compilations of the open ocean delta U-234 value vary by up to similar to 10 parts per thousand, and the assumed uniformity in the oceanic delta U-234 remains to be confirmed. Using MC-ICPMS techniques and a suite of multiple Faraday cups instead of the typical configurations based on a combined Faraday cup-multiplier array, a long-term reproducibility of better than +/- 0.3 parts per thousand (2 sigma) is achieved for delta U-234 measurements. Applying these very high precision techniques to open ocean seawater samples, an average delta U-234 of 146.8 +/- 0.1 parts per thousand (2 sigma(m), n = 19) is obtained. These high-precision seawater measurements yield an external reproducibility of better than +/- 0.4 parts per thousand (2 sigma) and show that the open oceans have a uniform delta U-234 on the sub-parts per thousand level. These new data constrain the vertical mixing time of the open oceans to less than 1000 years.