Decay of uranium in the earth's crust releases the gas radon which produces ²¹⁰Pb by decay in the atmosphere. The lead isotope enters the earth’s crust or the water through precipitation. In the water phase 210Pb is adsorbed to particulate matter and together they are deposited in the sediment. ²¹⁰Pb decays with a half-life of about 22 years. The remaining amount of ²¹⁰Pb at a certain depth will reveal the age of the given sediment layer within the last 75-100 years. The diagram illustrates an age depth curve based on ²¹⁰Pb analysis.