Plutonium in tree rings from France and Japan

Plutonium, along with other radionuclide concentrations, was measured in evergreen tree rings from two different locations. This was used as an information source for the past two centuries. Tree rings are a product of annual layers and thus chronological information is clearly visible. Three trees were harvested in 1988–1990: a French white fir (137 years old) and a spruce tree (177 years old) from the France-Germany border near Nancy, France and a sugi (78 years old) from Nagasaki, Japan. The uniform branchless part of the trunks from the harvested trees were immediately separated into a set of tree ring samples each of which contained 3–20 years of growth. The separated samples were mechanically powdered, dried at 105°C to obtain the dry weight, ashed at 350°C to measure 40K, 134Cs and 137Cs and ashed again at 600°C to determine 239+240Pu. The highest 239+240 Pu concentration of 30.0 mBq/kg of dry wood was obtained from the tree rings from Nagasaki, located at the centre of the local fallout of the Pu A-bomb detonated in 1945. This concentration peak was, however, observed in tree rings of 1965–1967. The concentration was only 2.9 mBq/kg for the tree rings of 1944–1946. The contribution of the local fallout on the surface soils from the A-bomb was 181 mBq/cm2 at the harvested area of the tree, while the contribution of global fallout by many weapons testing was 5.9 mBq/cm2 (or 3.3% total fallout in the region). The reason for the over 20 year time lag of 239+240Pu uptake by the tree rings is unknown because many factors influence the routes of Pu into the tree rings. Also the chemical form of Pu in surface soils may have been changed by the surrounding environment. The highest concentration in the tree rings from France was 9.4 mBq/kg which is about 31% of Nagasaki 239 + 240Pu concentration. The harvested area did not have any recorded Pu sources other than global fallout. An interesting result was that that distribution of 134Cs and 137Cs concentrations in the French white fir was different from Nagasaki. Data suggested that these new radionuclide inputs were from the Chernobyl accident. The mobility (or diffusion coefficient) of cesium is 2–8 cm2/yr in the portion of heart-wood tree rings (1870–1955). Although tree rings can record chronological inputs of various trace elements, some elements cannot be used. These exceptions would be elements that: (1) are mobile within tree rings; (2) have little understood entry routes to the tree rings (via roots, leaves or barks); and (3) have unknown biogeochemical behaviour in the surrounding environment. Further investigation is warranted to use tree rings as a tool to record past environmental history.