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. 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, while 239+240Pu is an alternative soil erosion tracer and chronological marker that has been 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 within the framework of the AVATAR (A reVised dATing framework for quantifying geomorphological processes during the Anthropocene) project. Using this database, named the AVATAR-Soils Database, we determined the distribution of 137Cs and 239+240Pu inventories in equatorial and Southern Hemisphere soils, along with the relative contributions of different fallout nuclear weapon sources by analyzing their isotopic ratios. Additionally, we demonstrated how the database can be used to identify the environmental factors that influence the distribution of 137Cs and 239+240Pu in reference soils by applying a machine learning algorithm. Our meta-analysis 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 locations. 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. Based on the 240Pu/239Pu atomic ratios, we have estimated that 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 the precipitation of the coldest quarter, longitude, and latitude as the strongest predictors of the 137Cs inventory. For the 239+240Pu inventory, the mean diurnal temperature range, the annual temperature range, and the precipitation of the driest quarter were the strongest predictors. Altogether, these findings demonstrate the potential of the AVATAR-Soils Database as a 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 10.5281/zenodo.14008221 (Dicen et al., 2024).