Distribution of 236U in the US GEOTRACES Eastern Pacific Zonal Transect and its use as a water mass tracer

We report dissolved concentrations of the long-lived radioisotope U-236 measured in the water column along the 2013 US GEOTRACES Eastern Pacific Zonal Transect (GP16). This transect followed a 10-15 degrees S line from Manta, Ecuador, to Papeete, Haiti, French Polynesia, crossing the southern East Pacific Rise, intercepting one of the largest hydrothermal plumes as well as a productivity gradient, which includes the upwelling zone and associated low oxygen waters offshore from Peru and the oligotrophic sub-tropical gyre. Accelerator Mass Spectrometry was used to measure dissolved seawater U-236 concentrations as low as 1 x 10(3) atom kg(-1), which are among the lowest levels reported to date. Differences in upper water column U-236 distributions from east to west are a result of variable contributions from different surface and intermediate waters encountered along the transect. The distribution of U-236, both in depth and geographically, provides complementary information to that obtained from Delta C-14 and helium isotopes, demonstrating that U-236 concentrations are diagnostic in the identification of and contributions from the different deep and bottom water masses encountered along the EPZT (Jenkins a al., 2017). For example, we observe minimum U-236 concentrations along the EPZT between 2000 and 3000 m that are consistent with contributions attributed to Pacific Deep Water. We also observe increases in U-236 below 3000 m at the eastern and western termini of the EPZT. This is consistent with contributions associated with Antarctic Bottom Water and Lower Circumpolar Deep Water. Our results indicate that U-236 may be used in conjunction with Delta C-14 and He-3 isotopes as an additional tool with which to identify and resolve contributions from different water masses in the Pacific Ocean. This article is part of a special issue entitled: Cycles of trace elements and isotopes in the ocean - GEOTRACES and beyond - edited by Tim M. Conway, Tristan Homer, Yves Plancherel, and Aridane G. Gonzalez.