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

Uranium has a long residence time in the open oceans, and therefore, its salinity‐normalized U concentration and 234 U/ 238 U activity ratio (expressed herein as δ 234 U, the ‰ deviation from secular equilibrium) are assumed to be uniform. The marine 234 U/ 238 U activity ratio is currently in radioactive disequilibrium and shows a ∼15% excess of 234 U with respect to the secular equilibrium value due to continuous input from riverine sources. Knowledge of the marine δ 234 U, 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 δ 234 U are technically difficult. Thus, existing compilations of the open ocean δ 234 U value vary by up to ∼10‰, and the assumed uniformity in the oceanic δ 234 U 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‰ (2 σ ) is achieved for δ 234 U measurements. Applying these very high precision techniques to open ocean seawater samples, an average δ 234 U of 146.8 ± 0.1‰ (2 σ m , n = 19) is obtained. These high‐precision seawater measurements yield an external reproducibility of better than ±0.4‰ (2 σ ) and show that the open oceans have a uniform δ 234 U on the sub‐‰ level. These new data constrain the vertical mixing time of the open oceans to less than 1000 years.