A first transect of 236U at the Equatorial Pacific

Abstract

The 2013 U.S. GEOTRACES East Pacific Zonal Transect (EPZT) between Peru and Tahiti chosen to encompass a range of processes that influence the supply, removal, and internal cycling of trace metals in the ocean, including from east to west a productivity gradient, a large oxygen minimum zone, and the largest known hydrothermal plume. With respect to 236U and other anthropogenic radionuclides, the major source to the Eastern Tropical South Pacific has been global stratospheric fallout. In addition to this, nuclear tests conducted in the Pacific Ocean also generated tropospheric fallout. Here we present dissolved 236U profiles collected at 4 full-depth stations along the EPZT. After pre-concentration of 4L samples (LDEO) and purification (CNA), 236U was analyzed by AMS at one of two facilities depending on collection depth. Surface samples (first 400m depth) were analyzed at the 1MV CNA facility. In this case 236U /238U atom ratios were expected to be at the 1010 level, so the parameters of the CNA system were optimized to achieve the necessary abundance sensitivity. Deeper samples were measured at the 600kV ETH facility. The most relevant conclusions are the following. a) Concentrations of 236U range from 0.02x106 to 6.5x106 atoms/kg, showing a typical distribution associated to a conservative element for which the atmospheric input to the ocean dominates. b) Inventories of 236U range from 1.45x1012 to 2x1012 at/m2 , in agreement with the expected values for global fallout. c) Below 1000 m depth, the four profiles presented 236U / 238U atom ratios at the 1012 level, which are among the lowest ratios reported so far in seawater. d) A slight enhancement of 236U at the bottom layers, near the seafloor, might be detected and most especially close to the hydrothermal plume. e) No influence of the local fallout from the French Polynesia is observed. These results will be presented and discussed.