Re-evaluating the 238U-salinity relationship in seawater: Implications for the 238U–234Th disequilibrium method
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The concentration of U-238 in seawater is an important parameter required for applications of uranium decay-series radionuclides used to understand particle export and cycling in marine environments. Using modern mass spectrometer techniques, we re-evaluated the relationship between U-238 and salinity in the open ocean. The new U-238-salinity relationship determined here is based on a larger sample set and a wider salinity range than previous work in the open ocean. Four samples from 500 to 1000 m in the subtropical Atlantic deviated significantly from their concentration predicted from salinity; these low concentrations are hypothesized to be the result of a remote removal process rather than analytical bias or local removal of uranium. We also bring attention to unique deficits of Th-234 in the mesopelagic zone of the subtropical Atlantic and encourage future applications of Th-234 to delve into the cause of these features. Determining the concentration of U-238 in the open ocean is critical for minimizing uncertainty in the U-238-Th-234 disequilibrium method, which is a key tool for understanding particle flux to the deep ocean. (C) 2011 Elsevier B.V. All rights reserved.