Ionium–thorium dating

Ionium-thorium dating is a technique for determining the age of marine sediments based upon the quantities present of nearly stable thorium-232 and more radioactive thorium-230. (²³⁰Th was once known as ionium, before it was realised it was the same element as ²³²Th.)

Uranium (in nature, predominantly uranium-238) is soluble in water. However, when it decays into thorium, the latter element is insoluble and so precipitates out to become part of the sediment.^[1] Thorium-232 has a half-life of 14.5 billion years, but thorium-230 has a half-life of only 75,200^[2] years, so the ratio is useful for dating sediments up to 400,000 years old.^[1] Conversely, this technique can be used to determine the rate of ocean sedimentation over time.^[2]

The ionium/thorium method of dating assumes that the proportion of thorium-230 to thorium-232 is a constant during the time period that the sediment layer was formed. Likewise, both thorium-230 and thorium-232 are assumed to precipitate out in a constant ratio; no chemical process favors one form over the other. It must also be assumed that the sediment does not contain any pre-existing particles of eroded rock, known as detritus, that already contain thorium isotopes. Finally, there must not be a process that causes the thorium to shift its position within the sediment. If these assumptions are correct, this dating technique can produce accurate results.^[1][2]

References

1. Rafferty, John P. (2010), Geochronology, Dating, and Precambrian Time: The Beginning of the World As We Know It, The Geologic History of Earth, The Rosen Publishing Group, p. 150, ISBN 978-1615301256.
2. Vértes, Attila (2010), Nagy, Sándor; Klencsár, Zoltán; Lovas, Rezso György; et al. (eds.), Handbook of Nuclear Chemistry, vol. 5 (2nd ed.), Springer, p. 800, ISBN 978-1441907196.