New TIMS constraints on the uranium-238 and uranium-234 in seawaters from the main ocean basins and the Mediterranean Sea

An accurate knowledge of U isotopes in seawater is important for a number of applications in Earth Sciences including geochronology, paleothermometry and pollution budgets. The distribution of uranium isotopes in seawater and modern corals has been revisited by Thermal Ionisation Mass Spectrometry (TIMS) in order to refine the limited availability of precise data (mostly the work by Chen et al. [Earth Planet. Sci. Lett. 80 (1986) 241] based on 20 seawater samples from 2 oceans). Our results for samples collected in the three main oceans allow us calculation of an average U concentration of 13.56 pmol/g normalized to 35 salinity (1SD of 0.28 pmol/g, n=20) in good agreement with the two previous estimates by α-spectroscopy [Ku et al. Deep-Sea Res. 24 (1977) 1005] and by TIMS [Earth Planet. Sci. Lett. 80 (1986) 241]. Similarly, we show that the distribution of δ234U is homogeneous in the ocean at an average of 149.6‰ (1SD of 1‰ in large samples, n=9 and of 3‰ on all samples analyzed, n=23), greater than previous estimation by TIMS (mean of 139.5‰, 1SD of 3.5‰, n=9 [Earth Planet. Sci. Lett. 80 (1986) 241]). The first data obtained by TIMS for Mediterranean Sea samples show no significant departure from conservative behavior for both U concentration and δ234U ratio. Finally, δ234U values measured in modern corals from the Atlantic and the Pacific Oceans average, 146.6‰ (1SD of 1.4‰, n=10), are very close but slightly lower than the seawater value.