Habacuc Pérez-Tribouillier (Zürich): Radionuclides in the Ocean: From Contamination to Circulation Tracers

Anthropogenic radionuclides released since the onset of the nuclear era are now present throughout the global ocean. Controlled by their chemical speciation, these isotopes partition between dissolved and particulate phases, governing their transport and fate. While their presence raises concerns due to potential radiological impacts on marine ecosystems, their distinct sources make them uniquely powerful tracers of oceanographic processes.
This dual role is explored through the major sources of radionuclides to the marine environment: nuclear weapons testing, nuclear fuel reprocessing, and nuclear accidents. The Fukushima accident provides a well-constrained pulse input, and a time series spanning more than fifteen years is used to characterize the long-term evolution of ¹³⁷Cs, ⁹⁰Sr, and ¹²⁹I in the Pacific Ocean.
In contrast, nuclear fuel reprocessing facilities, particularly at La Hague (France) and Sellafield (UK), represent sustained point sources of long-lived radionuclides such as ¹²⁹I and actinides. These inputs enable tracing of ocean circulation over basin scales, with observations extending from the English Channel to the Arctic Ocean, providing insight into water mass pathways and transit times in a region undergoing rapid climatic change.
Reprocessing-derived radionuclides also exhibit contrasting biogeochemical behavior. While conservative species remain in the dissolved phase, particle-reactive elements such as plutonium and americium can associate with particles and accumulate in sediments and biota. Given their radiotoxicity as alpha emitters, they may represent an ecological concern. However, systematic and up-to-date studies of their distribution in source regions, particularly in the North Sea, remain limited and motivate ongoing work.
Together, these results and perspectives demonstrate how anthropogenic radionuclides have become tools for investigating ocean circulation and geochemical processes across a wide range of spatial and temporal scales, while providing new insight into the long-term evolution of coastal systems following a major nuclear accident such as Fukushima.