The distribution of radionuclides and som.e trace metals in the water columns of the Jalpan 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 (90Sr, 137Cs and Pu isotopes), and some trace elements (V, Zn, Cd, Cu, Mn, and Ni ) in two water , columns of the Jap;an and Bonin trenches down to the bottom depths of 7585 m and 9750 m, respectively. Hydrographic properties such as tlemperature, salinity, dissolved oxygen, and nutrient content within the trench valley remain constant at the same levels as tlhose in the bottom water of the Northwest Pacific basin (typically -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 wa.s 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 239, 240Pu 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 zloPb and 231Pa 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 228Th data show scattering within the Bonin Trench that is largely ascribable to analytical errors. If, however we accept that the lscatter of 228Th data is real and the variation is caused solely by decay of its parent 228Ra we can set an upper limit of -5 years for the renewal time of the trench , water. 0 Elsevier. Paris U/Th series nuclides I artificial radionuclides I scavenging I trace metals I renewal of trench water Resume Repartition des nucleides radioactifs et de quelques metaux-traces, dans les fosses du Japon et des iles Bonin. La premiere s&e detaillee de donntes geochimiques obtenues dans deux colonnes d’eau des fosses du Japon et des iles Bonin, respectivement jusqu’a 7585 met 9750 m, est presentee ici. Elle Porte sur la serie uranium/thorium, sur les isotopes de thorium, protactinium, actinium et plomb et sur les retombees des nucleides radioactifs artificiels (strontium-90, cesium-137 et plutlonium) ainsi que sur quelques elements a l’etat de t:races (vanadium, zinc, cadmium, cuivre, manganese et nickel). Les caracteristiques hydrologiques telles que temperature, salinite, oxygene dissous et teneurs en nutriments prennent les mCmes valeurs aux m&mes profondeurs, dans les fosses comme dans l’eau de fond du bassin nord-occidental du Pacifique (typiquement, vers 6000 m). De m&me, les valeurs de l’activitt des nucleides radioactifs et les concentrations * Correspondence and reprints. Oceanologica Acta 0399 1784/98/03/O Elsevier, Paris 469