U concentration and ²³⁴U/²³⁸U of seawater from the Okinawa Trough and Indian Ocean using MC-ICPMS with SEM protocols

The Okinawa Trough, a back-arc basin formed by extension within the continental lithosphere, is one of the most significant ocean basins between the East China Sea and the Pacific Ocean. In an attempt to constrain the U content and the ²³⁴U/²³⁸U of seawater, five depths vertical seawater profiles from the Okinawa Trough, two from the Indian Ocean and one from the South China Sea were investigated by MC-ICPMS. The method allows us to contribute most reliable uranium isotopic measurement in seawater and greatly reduces sample size. Results showed that the U contents of seawater profiles for Okinawa Trough were from 3.153 to 3.235 ppb, with an average value of 3.186 ± 0.040 (2σ, n = 37), Indian Ocean were from 3.255 to 3.283 ppb with a mean value of 3.272 ± 0.018 (2σ, n = 19), and South China Sea were from 3.253 to 3.271 averaging 3.263 ± 0.018 (2σ, n = 3) ppb. Similarly, we obtained δ²³⁴U values of 146.1 ± 1.2, 145.1 ± 1.4, and 145.8 ± 0.5‰ for Okinawa Trough, Indian Ocean and South China Sea, with a mean of 145.7 ± 1.0‰, in good agreement with previous reports within the error limits. With two-ml samples in the Okinawa Trough, it gives a mean δ²³⁴U of 145.8 ± 1.0‰ which is identical to that observed for 7-ml seawater within the 95% confidence level. These results suggest that the distribution of δ²³⁴U is homogeneous in the Okinawa Trough and followed U conservative behavior. We also obtained average δ²³⁴U values of 146.9 ± 2.1‰ (2σ, n = 6) for Acropora coral from the Xisha Islands in the South China Sea. All results show no marked differences with the open ocean and provide significant geochemical information for the study of environmental change in the East China Sea.