Plutonium isotopes as time-markers in South American lake sediments: identifying pre- and post-1959 and French test periods

Abstract. In regions where rapid and extensive environmental changes have occurred, particularly in South America, reliable time markers are essential for dating sediment sequences and quantifying environmental degradation. In this respect, the use of ²³⁹Pu and ²⁴⁰Pu isotopes in South American sediments may help to identify distinct sources of fallout radionuclides, including radioactive fallout from French atmospheric nuclear weapons tests (NWTs) conducted between 1966 and 1974 at the Mururoa and Fangataufa atolls (French Polynesia). Here, we present post-1900 continuous records of ²⁴⁰Pu/²³⁹Pu isotope ratios in sediments cores from sites located between 32° and 52°S latitude: Lakes Natri and Laja in Chile; Lakes La Barrancosa, Melincué, Ñe Luan, and Roca in Argentina; and the Rincón del Bonete Reservoir and La Estanzuela Pond in Uruguay. Depth profiles revealed two ^240+239Pu activity peaks, from which the more recent is not concomitant to the ¹³⁷Cs maximum activity peak dated back to 1964-1965. The low ²⁴⁰Pu/²³⁹Pu atom ratio (< 0.08) associated with this more recent Pu peak confirms a contribution from French fallout, dated to the late 1960s to early 1970s. The investigated lakes exhibited similar patterns in Pu isotope ratios: (i) an initial phase dominated by the U.S. NWTs signature (²⁴⁰Pu/²³⁹Pu > 0.20, also often referred to pre-moratorium), followed by (ii) increasing Pu activities characterized by a Pu isotopic signature consistent with global fallout (²⁴⁰Pu/²³⁹Pu ~ 0.18) and, finally, (iii) a period of increased Pu activities from French NWTs fallout (0.03 <²⁴⁰Pu/²³⁹Pu < 0.08). The ²³⁹Pu and ²⁴⁰Pu isotopes revealed a consistent nuclear source pattern with a distinct French fallout contribution, confirming their suitability as an additional time marker for environmental reconstruction in South America.