New TIMS constraints on the uranium-238 and uranium-234 in seawaters from the main ocean basins and the Mediterranean Sea 1 1Throughout the paper we use the δ234U notation which represents the deviation of the measured 234U/238U atomic ratio from the 234U/238U atomic ratio at secular equilibrium: δ234U=1000×[(234U/238U)/(234U/238U)eq−1], where (234U/238U)eq is the ratio of the two decay constants: λ238=1.5513×10−10 year−1 (Jaffey et al., 1971) and λ234=2.826×10−6 year−1 recently revised by Cheng et al. (200 ...

Doriane Delanghe, Edouard Bard, Bruno Hamelin
2002 Marine Chemistry  
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
more » ... 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 a-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 d 234 U is homogeneous in the ocean at an average of 149.6x(1SD of 1xin large samples, n = 9 and of 3xon all samples analyzed, n = 23), greater than previous estimation by TIMS (mean of 139.5x , 1SD of 3.5x , 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 d 234 U ratio. Finally, d 234 U values measured in modern corals from the Atlantic and the Pacific Oceans average, 146.6x(1SD of 1.4x , n = 10), are very close but slightly lower than the seawater value. D (D. Delanghe). 1 Throughout the paper we use the d 234 U notation which represents the deviation of the measured 234 U/ 238 U atomic ratio from the 234 U/ 238 U atomic ratio at secular equilibrium: d 234 U = 1000 Â [( 234 U/ 238 U)/( 234 U/ 238 U) eq À 1], where ( 234 U/ 238 U) eq is the ratio of the two decay constants: k 238 = 1.5513 Â 10 À 10 year À 1 (Jaffey et al., 1971 ) and k 234 = 2.826 Â 10 À 6 year À 1 recently revised by Cheng et al. (2000) . Therefore, an activity ratio of 1 equals ( 234 U/ 238 U) eq = k 238 /k 234 = 54.89 Â 10 À 6 in atomic ratio. This latter value is f 3xdifferent than the one previously used and thus data from the literature have been recalculated accordingly. All U concentrations are expressed in pmol/g (to be multiplied by 0.2380274 for comparison with some previous works which used ng/g; e.g. Chen et al., 1986 ). www.elsevier.com/locate/marchem Marine Chemistry 80 (2002) 79 -93
doi:10.1016/s0304-4203(02)00100-7 fatcat:te7lisydyneyxhldzxc2ya3qtm