thorium-230/protactinium-231 deficiency method . Calculation of an age in years for fossil coral, shell, or bone 10,000 to 250,000 years old, based on the growth of uranium daughter products from uranium isotopes that enter the carbonate or phosphate material shortly after its formation or burial. The age depends on measurement of the thorium-230/uranium-234 and protactinium-231/uranium-235 activity ratios, which vary with time. See also: uranium-series age method; ionium-deficiency method.