Re-evaluating the 238U-salinity relationship in seawater: Implications for the 238U– 234Th disequilibrium method

The concentration of 238U 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 238U and salinity in the open ocean. The new 238U-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 234Th in the mesopelagic zone of the subtropical Atlantic and encourage future applications of 234Th to delve into the cause of these features. Determining the concentration of 238U in the open ocean is critical for minimizing uncertainty in the 238U- 234Th disequilibrium method, which is a key tool for understanding particle flux to the deep ocean. ►A new 238U-salinity relationship has been derived from a large sample set with a wide salinity range. ►Low 238U concentrations were observed in the subtropical Atlantic in 2007 between 500 and 1000 m. ►Unique deficits of 234Th relative to 238U in the mesopelagic were identified.