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 Coral calibration
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Mass spectrometric determinations of uranium and thorium isotope abundances now permit precise dating of coral in the time range of 50 yr to about 500 ka BP. Ages pf coral determined by the U-Th method are not affected by factors that produce secular variations in 14C. Thus, U-Th dating can provide an independent time scale to which radiocarbon measurements can be compared.
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Carbonates precipitate from water to form, for example corals or speleothems. The carbonates will contain 234U and 238U in the ratio
in which they are found in the water, approximately in secular equilibrium. Since thorium is insoluble in seawater, newly formed coral is assume to contain no thorium.
As the carbonate ages, the Uranium-234 in the carbonate decays into Thorium 230:
If 238U and 234U in seawater are in secular equalibrium which means
Then at time t, after formation of coral
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