These changes provide a long-term sequence of climatic events, recorded as changes in the thickness of sediment layers (known as "varve analysis"—the term "varve" means a layer or layers of sediment.Typically, varve refers to lake or glacial sediment), as temperature induced changes in the isotopic ratios for oxygen isotopes in sediments, and in the relative abundance of fossils.Because these can be calibrated reliably over a period of 40 million years this provides an alternate verification to radiometric dating in cases where sufficient record exists to provide a reliable trace.Polarity reversals in the Earth's magnetic field have also been used to determine geologic time.Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves.
To date an object, scientists measure the quantity of parent and daughter isotope in a sample, and use the atomic decay rate to determine its possible age.
The amount of Argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors.
These factors introduce error limits on the upper and lower bounds of dating, so that final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure and/or open-air.
Trees usually add growth rings on a yearly basis, with the spacing of rings being wider in high growth years and narrower in low growth years.
Patterns in tree-ring growth can be used to establish the age of old wood samples, and also give some hints to local climatic conditions.