In contrast, uranium is soluble to some extent in all natura] waters, so any material that precipitates or is grown from such waters also contains trace uranium, typically at levels of between a few parts per billion and few parts per million by weight.
As time passes after the formation of such a material, uranium-234 in the sample, with a half-life of 245,000 years, decays to thorium-230.
With time, Thorium 230 accumulates in the sample through radiometric decay.
Thorium-230 is itself radioactive with a half-life of 75,000 years, so instead of accumulating indefinitely (as for instance is the case for the uranium–lead system), thorium-230 instead approaches secular equilibrium with its radioactive parent uranium-234.
At secular equilibrium, the number of thorium-230 decays per year within a sample is equal to the number of thorium-230 produced, which also equals the number of uranium-234 decays per year in the same sample.
A large drop in atmospheric 14C/12C and reduced melting in the Younger Dryas, documented with 230Th ages of corals.
The contributions of uranium/thorium and marine palynology to the dating of the Lake Wangoom pollen record, western plains of Victoria, Australia.
500 ka precipitation record from southeastern Australia: evidence for interglacial relative aridity.