Click here for a special offer. Click here for our free newsletter. But dating events in the first 3.5 billion years of that record, the Precambrian era, has been hit or miss, with uncertainties of a billion years not uncommon. Now researchers have found a way to date ancient sediments that could considerably shrink the uncertainty and clarify such early Earth phenomena as the evolution of animals.
Sedimentary rocks are hard to date because they're made of particles from older rocks.
And Precambrian rocks contain few if any animal fossils. Younger sedimentary rocks, which hold about 12 percent of Earth's history, contain abundant fossils that are accurate age indicators. Geologists from the University of Western Australia solved the Precambrian puzzle by analyzing the mineral xenotime, which crystallizes in the small spaces between grains in sedimentary rock shortly after deposition. Xenotime contains uranium, which slowly decays to lead. After using a beam of charged particles to free lead from tiny xenotime crystals, researcher Neal McNaughton and colleagues measured the lead's composition and used it to calculate age.