5:15 PM - 6:30 PM
[AOS04-P05] Distribution of 236U in the North Pacific Ocean
Keywords:U-236, North Pacific Ocean, AMS, Geotraces, global fallout
Seawater samples were collected from the North Pacific Ocean in GEOTRACES cruises with R/V Hakuhomaru, in 2011, 2012 and 2014 (KH-11-07, HK-12-4 and KH14-6). 1 L, 5 L and 20 L of seawater samples were collected from several depths in each site, and immediately after the sampling, the water was filtered with about 0.45 mm pore-size cartridge filters. 238U concentrations in seawater were measured with ICP-MS after acidification. As for 1 L of seawater samples, uranium was purified with UTEVA resin , and precipitated in only 100 µg of iron carrier to prepare targets for the measurement of 236U/238U by AMS. In the 5 L and 20 L samples, no column separation for uranium was done, but actinide elements were separated by a simple co-precipitation with iron hydroxide, which leaves the possibility of detecting several actinides (U, Np, Pu) from one sample.
Using the newly constructed target preparation procedure for the measurement of 236U in small sizes of seawater samples, 5-10 times higher ion currents were achieved compared to the conventional method and 236U was successfully determined on all levels of the water column. Also, measurement times could be significantly reduced, which seems promising for future applications of 236U as oceanographic tracer, when large numbers of samples from vast ocean areas need to be analysed in a timely and cost-efficient way. 236U/238U isotopic ratios were highest (7.6×10-10 to 1.4×10-9) in shallow water. From surface level to a depth of about 1000-1500 m, all depth profiles showed a steep decrease in 236U concentrations and 236U/238U ratios in deep water were in the order of 10-11-10-12. The inventories of 236U on the water column were calculated as (3.6–7.3)×1012 atoms/m2, which is significantly lower than for the Sea of Japan with (1.4-1.6)×1013 atoms/m2. These results show the lower extent of vertical transport in the Pacific Ocean and are probably an indicator for lower precipitation rates in the North Pacific Ocean. 236U distributions were in correspondence to the main water masses (as defined by physical oceanographic parameters) and 236U concentration patterns were similar to those of 137Cs, which has been conventionally used as oceanographic tracer in this area.