Beginning in about 1952, large-scale nuclear weapons testing released detectable amounts of ¹³⁷Cs to the atmosphere. Concentrations of ¹³⁷Cs peaked between 1963 and 1964 as a result of the atmospheric nuclear explosions and later by the Chernobyl accident in 1986. These events can be identified in sediments where they form distinctive time stratigraphic marker horizons. The diagram illustrates an age depth curve based on ¹³⁷Cs and shows the nuclear weapons fallout peaks in the 1960s and the much smaller Chernobyl peak in 1986.