Bomb pulse

The bomb pulse is the sudden increase of carbon-14 (¹⁴C) in the Earth's atmosphere due to the hundreds of above-ground nuclear bombs tests that started in 1945 and intensified after 1950 until 1963, when the Limited Test Ban Treaty was signed by the United States, the Soviet Union and the United Kingdom.^[2] These hundreds of blasts were followed by a doubling of the relative concentration of ¹⁴C in the atmosphere.^[3]

The reason for the term “relative concentration”, is because the measurements of ¹⁴C levels by mass spectrometers are most accurately made by comparison to another carbon isotope, often the common isotope ¹²C. Isotope abundance ratios are not only more easily measured, they are what ¹⁴C carbon daters want, since it is the fraction of carbon in a sample that is ¹⁴C, not the absolute concentration, that is of interest in dating measurements. The figure shows how the fraction of carbon in the atmosphere that is ¹⁴C, of order only a part per trillion, has changed over the past several decades following the bomb tests. Because ¹²C concentration has increased by about 30% over the past fifty years, the fact that “pMC”, measuring the isotope ratio, has returned (almost) to its 1955 value, means that ¹⁴C concentration in the atmosphere remains some 30% higher than it once was. Carbon-14, the radioisotope of carbon, is naturally developed in trace amounts in the atmosphere and it can be detected in all living organisms. Carbon of all types is continually used to form the molecules of the cells of organisms. Doubling of the concentration of ¹⁴C in the atmosphere is reflected in the tissues and cells of all organisms that lived around the period of nuclear testing. This property has many applications in the fields of biology and forensics.