Uranium–thorium dating
Uranium–thorium dating estimates when calcium-carbonate deposits formed by measuring radioactive uranium isotopes and their thorium decay products.
Uranium-234: A radioactive uranium isotope that decays to thorium-230 and occurs naturally in water and minerals. Its decay supplies one of the isotope ratios used to calculate carbonate age.
Calcium carbonate: A chemical compound, CaCO₃, found in limestone, shells, corals, and cave deposits. The dated material is commonly carbonate that incorporates uranium from water.
Speleothem: A mineral deposit, usually calcite or aragonite, formed in a cave by dripping or flowing water. Cave formations are common targets for dating past rainfall and climate.
Radiocarbon dating: A method that estimates the age of once-living material from its carbon-14 content. It dates organic remains rather than carbonate deposits and reaches much younger ages.
Thorium-230: A radioactive isotope produced in the decay chain of uranium-234, with a half-life of about 75,700 years. Its accumulation after carbonate forms provides the method’s principal clock.
Isotope: An atom of a chemical element with a particular number of neutrons in its nucleus. Dating distinguishes uranium and thorium isotopes by their nuclear masses.
Coral reef: A marine ecosystem built largely from calcium-carbonate structures secreted by corals. Fossil corals can constrain past sea levels and ocean conditions.
Potassium–argon dating: A radiometric method that dates rocks by measuring potassium-40 and radiogenic argon-40. It is suited to volcanic rocks, not the young carbonate archives targeted here.
Uranium-238 decay chain: The sequence of radioactive decays beginning with uranium-238 and ending in stable lead-206. Uranium-234 and thorium-230 belong to this chain, whose disequilibrium the method measures.
Half-life: The time required for half the radioactive nuclei in a sample to decay. Known half-lives translate measured isotope proportions into elapsed time.