Global distribution of Pu isotopes and Np-237

Inventories and compositions of Pu isotopes and Np-237 in archived soil sam ples collected in the 1970s from 54 locations around the world were determi ned to provide regional baselines for recognizing possible future environme ntal inputs of non-fallout Pu and Np. As sample sizes used in this work wer e small (typically 1 g), inhomogeneities in Pu and Np concentrations were e asily recognizable and, as a result, we were able to determine that atypica l debris in South America, from French testing in the South Pacific, is mor e widely and uniformly distributed than previously supposed. From our resul ts we conclude that fallout Np-237/ (239)pU atom ratios are generally lower in the Southern Hemisphere (similar to 0.35) than in the Northern Hemisphe re (similar to 0.47.) Moreover, Np-237/(PU)-P-239 atom ratios are more devi ce-dependent, hence more variable, than counterpart Pu-240/ Pu-239 atom rat ios. Given predictable trends caused by sample inhomogeneities, with only t wo exceptions, the Pu results of this work are entirely consistent with (an d in several instances improve on) results previously reported for these sa me samples. However, unlike earlier interpretations used to explain these r esults, we recommend that fallout isotopic signatures be represented by mix ing lines, rather than averages, to better reflect regional variations of s tratospheric fallout inventories relative to tropospheric fallout inventori es, and provide the theoretical basis for doing so. Finally, the Np results of this work constitute one of the largest single compilations of such dat a reported to date. (C) 1999 Elsevier Science B.V. All rights reserved.