Reconstruction of 137Cs and 239+240Pu baseline inventories in the Southern Hemisphere and Equatorial Soils

Artificial fallout radionuclides (FRNs), such as ¹³⁷Cs and ^239+240Pu, released during nuclear weapon testing from the 1950s to 1980, have been widely used over the past three decades to quantify land degradation processes. The spatial distribution of global fallout generally aligns with latitudinal bands in areas with similar precipitation regimes. Despite nearly 80 years since these tests, no comprehensive reference map exists for FRN inventories across the Southern Hemisphere.

Therefore, this study aims to create the first complete reference maps for ¹³⁷Cs and ^239+240Pu along with their spatial uncertainties over the entire Southern Hemisphere and Equatorial band. To accomplish this objective, we employ the random forest algorithm to predict the concentrations of FRNs in soils of this part of the world. This is achieved by utilizing a compiled data set of soil ¹³⁷Cs (n=429) and ^239+240Pu (n=102) inventories (Bq/m²) (Dicen et al., 2024; under review) under the AVATAR Project (“A reVised dATing framework for quantifying geomorphological processes during the Anthropocene”). The training phase of modeling relied on environmental covariates, including monthly mean precipitation (following test months), elevation (m.), euclidean distance to test sites (km.), terrain wetness index, flow accumulation, and soil texture.

Preliminary results indicate that precipitation distribution strongly influences FRN inventories, with the highest values observed between 20°S to 30°S due to atmospheric circulation patterns driving fallout dispersion. Consistent with previous studies, southern America emerges as a major depositional region, while Australia exhibits the lowest inventories. These differences primarily reflect the proximity to nuclear test sites, such as French Polynesia, a significant source of fallout radionuclides. The euclidean distance and westerly wind patterns are key factors shaping FRN inventories.

In equatorial regions, recent land disturbances highlight the need for additional sampling, particularly for ^239+240Pu, to enhance prediction accuracy and reduce spatial uncertainties. This study addresses critical gaps in the spatial distribution of FRN inventories, providing a robust foundation for geomorphological reconstructions in the Southern Hemisphere and Equatorial regions.