The distribution of radionuclides and some trace metals in the water columns of the Japan and Bonin trenches

Presented here is the first detailed geochemical data set on the U/Th series Th, Pa, Ac, and Pb isotopes and artificial fallout radionuclides (Sr-90, Cs-137, and Pu isotopes), and some trace elements (V, Zn, Cd, Cu, Mn, and Ni) in two water columns of the Japan and Bonin trenches down to the bottom depths of 7585 m and 9750 m, respectively. Hydrographic properties such as temperature, salinity, dissolved oxygen, and nutrient content within the trench valley remain constant at the same levels as those in the bottom water of the Northwest Pacific basin (typically similar to 6000 m in depth). The radionuclide activities and most trace metal concentrations are also not very different from those in the overlying water at depths of around 5000-6000 m. This means that any chemical alteration which sea water undergoes during its residence within the trench was not obviously detected by the techniques used here. The suggestion follows that the trench water is rather freely communicating by isopycnal mixing with the bottom water overlying the Northwest Pacific abyssal plain. The trench waters contain high Pu-239,Pu-240 activities throughout, indicating that Pu is actively regenerating from rapidly sinking, large particles at the bottom interface, probably due to a change in the oxidation state. On the other hand, the vertical profiles of Pb-210 and Pa-231 show lower activities within the trench than those in the overlying deep waters, suggesting that the effect of boundary and bottom scavenging is significant in controlling their oceanic distributions. However, none of the trace metals studied here obviously follows the behaviour of the above nuclides. The Th-228 data show scattering within the Bonin Trench that is largely ascribable to analytical errors. If, however we accept that the scatter of Th-228 data is real and the variation is caused solely by decay of its parent Ra-228, We,,, set an upper limit of similar to 5 years for the renewal time of the trench water.