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

American Geophysical Union (AGU) - Tập 11 Số 12 - 2010
Morten B. Andersen1,2, Claudine H. Stirling1,3, C. Sciacca4, Alex N. Halliday4,5
1Institute for Isotope Geochemistry and Mineral Resources ETH Zurich Zurich CH‐8092 Switzerland
2Now at Department of Earth Sciences, University of Bristol, Queens Road, Bristol BS8 1RJ, UK.
3Now at Department of Chemistry, University of Otago, PO Box 56, Union Place, Dunedin, New Zealand.
4Institute for Isotope Geochemistry and Mineral Resources; ETH Zurich; Zurich; CH-8092; Switzerland
5Now at Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK.

Tóm tắt

Uranium has a long residence time in the open oceans, and therefore, its salinity‐normalized U concentration and 234U/238U activity ratio (expressed herein as δ234U, the ‰ deviation from secular equilibrium) are assumed to be uniform. The marine 234U/238U activity ratio is currently in radioactive disequilibrium and shows a ∼15% excess of 234U with respect to the secular equilibrium value due to continuous input from riverine sources. Knowledge of the marine δ234U, 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 δ234U are technically difficult. Thus, existing compilations of the open ocean δ234U value vary by up to ∼10‰, and the assumed uniformity in the oceanic δ234U 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‰ (2σ) is achieved for δ234U measurements. Applying these very high precision techniques to open ocean seawater samples, an average δ234U of 146.8 ± 0.1‰ (2σm, n = 19) is obtained. These high‐precision seawater measurements yield an external reproducibility of better than ±0.4‰ (2σ) and show that the open oceans have a uniform δ234U on the sub‐‰ level. These new data constrain the vertical mixing time of the open oceans to less than 1000 years.

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American Geophysical Union (AGU)

Tập 11 Số 12

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