Nuclear weapons-derived radioisopes in the sediments mark the beginning of the Anthropocene Epoch

What is it about?

Many scientists are making the case that humanity is living in a new geological epoch, the Anthropocene, but there is no agreement yet as to when this epoch began. The start might be defined by a historical event, such as the beginning of the fossil-fueled Industrial Revolution or the first nuclear explosion in 1945. Standard stratigraphic practice, however, requires a more significant, globally widespread, and abrupt signature, and the fallout from nuclear weapons testing appears most suitable. The appearance of plutonium 239 (used in post-1945 above-ground nuclear weapons tests) makes a good marker: This isotope is rare in nature but a significant component of fallout. It has other features to recommend it as a stable marker in layers of sedimentary rock and soil, including: long half-life, low solubility, and high particle reactivity. It may be used in conjunction with other radioactive isotopes, such as americium 241 and carbon 14, to categorize distinct fallout signatures in sediments and ice caps. On a global scale, the first appearance of plutonium 239 in sedimentary sequences corresponds to the early 1950s. While plutonium is easily detectable over the entire Earth using modern measurement techniques, a site to define the Anthropocene (known as a "golden spike") would ideally be located between 30 and 60 degrees north of the equator, where fallout is maximal, within undisturbed marine or lake environments.

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Why is it important?

The advent of the nuclear age in itself does not merit the identification of a new geological epoch. The signature of weapons testing coincides with a range of human-driven changes that have produced stratigraphic signals that indicate a dramatic shift in the Earth system around the mid-20th century, which in total may be considered the distinctive feature of the Anthropocene. The fact that the plutonium 239 signature is coincident with other changes makes it a useful tool for defining the Anthropocene's base.