Measurement of 236U at the GEOTRACES East Pacific zonal transect

Abstract

During October-December 2013 U.S. GEOTRACES sampled the East Pacific Zonal Transect (EPZT). The transect went from Peru (12°S, 77°W) to Tahiti (17°S, 149°W) and included the in-depth analysis of the Peru Margin upwelling and oxygen minimum zone (OMZ), and the large hydrothermal plume (HP) originating from the southern East Pacific Rise. Respectively St. 11 (12°S,94°W) and St. 18 (15°S,113°W). That area might have been influenced by the US and French nuclear weapon testing sites at the Pacific Ocean.Several water profiles were collected for the analysis of 236U. Surface samples, i.e. above 400 m, were measured on the1 MV AMS system at the Centro Nacional de Aceleradores (CNA, Sevilla, Spain) because the 236U/238U atom ratio (AR) was at the order of 10-10, and the maximum sensitivity achieved on this system is 7x10-11. In deeper samples the expected AR was at the 10-12-10-11 level, and for that reason they were measured on the 600 KV Tandy facility at the ETH Laboratory of Ion Beam Physics (Zürich,Switzerland), with a lower background (at the order of 10-14).

We present here the results for stations 11 and 18. Our results show that: i) Samples above 1000 m presented 236U concentrations from 0.02·106 to 7·106 at kg-1, lower than the activity concentrations at the Equatorial North Atlantic (NA), from 2·106 to 6·106 at kg-1, above 1000 m. The data indicated the presence of anthropogenic 236U, but no additional sources than global fallout were identified here. In contrast,deep samples in the North Atlantic showed the presence of 236U from European nuclear reprocessing plants. ii) Most of the samples between 600 and 2000 m presented very low values, close to the lithogenic/natural ratios (i.e. at the 10-12 level). To the best of our knowledge, these ratios are the lowest ones measured so far in seawater. Below 2000 m it might detected a slight increase in the 236Uconcentration in both OMZ and HP stations.iii) Regarding the biogeochemical behaviour, similar 236U/238U profiles were obtained in the upper 1300 m in both OMZ and HP. The analysis of other radionuclides sampled during the cruise, e.g. 237Np and 129I, and of additional EPZT profiles will provide insights of the origin of 236U fluctuations in depth.