Distribution and sources of fallout ¹³⁷Cs and ^239+240Pu in Equatorial and Southern Hemisphere reference soils

Abstract. Past nuclear weapons testing (NWT) and nuclear power plant (NPP) accidents have resulted in the ubiquitous deposition of radionuclides in the environment. While radionuclide contamination of the environment is associated with concerning health risks, these fallout radionuclides (FRNs) are considered the privileged markers (“golden spikes”) of the Anthropocene stratigraphic layers. Their deposition in the 1950s coincided with the “Great Acceleration”, which is characterized by large-scale shifts in the Earth’s systems, including increased land-use change and soil degradation. Among the FRNs deposited globally, ¹³⁷Cs has been the most commonly used to assess soil erosion and/or the chronology of sediment deposition, and ^239+240Pu is an emerging soil erosion tracer and chronological marker increasingly used due to a number of advantages.

We compiled ¹³⁷Cs and ^239+240Pu data published from undisturbed (so called “reference”) soils in the Equatorial and Southern Hemisphere regions to build a database under the AVATAR Project (“A reVised dATing framework for quantifying geomorphological processes during the Anthropocene”). Using this database, named the AVATAR-Soils Database, we determined the distributions of ¹³⁷Cs and ^239+240Pu inventories in Equatorial and Southern Hemisphere soils, along with the relative contributions of different fallout nuclear weapon sources by analysing their isotopic ratios. Additionally, we demonstrated how the database can be used to identify the environmental factors that influence the distributions of ¹³⁷Cs and ^239+240Pu in reference soils by applying a machine-learning algorithm.

Our metanalysis revealed that high ¹³⁷Cs and ^239+240Pu inventories were recorded near the equator and within the 20–40° S latitudinal bands, which coincide with the location of multiple NWT. The ²⁴⁰Pu/²³⁹Pu atomic ratios suggest that sources other than the global fallout (primarily from US and USSR weapon testing with a ²⁴⁰Pu/²³⁹Pu atomic ratio of ~ 0.18) contributed to the reference inventories in the Southern Hemisphere. These additional sources have been relatively neglected so far. On average, the French fallout contributed ~20 % to the reference soil ^239+240Pu inventories in South America and up to 70 % in French Polynesia. In contrast, the British fallout contributed ~27 % to the reference soil ^239+240Pu inventories in the rest of Oceania. Our machine-learning algorithm identified precipitation of the coldest quarter, longitude, and latitude as the strongest predictors of ¹³⁷Cs inventory. For ^239+240Pu inventory, mean diurnal temperature range, temperature annual range, and precipitation of the driest quarter were the strongest predictors. Altogether, these findings demonstrate the potential of the AVATAR-Soils Database as resource for improving our understanding of the distribution and sources of ¹³⁷Cs and ^239+240Pu in Equatorial and Southern Hemisphere soils and refining their application as tools in various Earth Science research. The AVATAR-Soils Database may be accessed at https://doi.org/10.5281/zenodo.14008220 (Dicen et al., 2024).