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 elementas232Th.)
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]
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