For dating, eggshells are better
But ash is not always current. In Africa, nonetheless вЂ” and before the Holocene, through the center East and Asia вЂ” ostrich eggshells are common. Some web sites also contain ostrich eggshell ornaments produced by early Homo sapiens.
Elizabeth Niespolo, during the time a graduate student in UC BerkeleyвЂ™s Department of world and Planetary Science, splitting uranium from thorium in a lab at the Berkeley Geochronology Center. (Picture by Warren Sharp, BGC)
A graduate student in UC BerkeleyвЂ™s Department of Earth and Planetary Science, conducted a thorough study of ostrich eggshells, including analysis of modern eggshells obtained from an ostrich farm in Solvang, California, and developed a systematic way to avoid the uncertainties of earlier analyses over the last four years, Sharp and Niespolo, at the time. One key observation ended up being that pets, including ostriches, don't use up and keep uranium, although it is common at parts-per-billion levels in most water. They demonstrated that newly set shells that are ostrich no uranium, but that it's absorbed after burial in the ground.
The exact same will also apply to seashells, however their calcium carbonate structure вЂ” a mineral called aragonite вЂ” is not as stable whenever hidden in soil once the form that is calcite of carbonate discovered in eggshell. Because of this, eggshells retain better the uranium taken up through the very first century or more that they are buried. Bone, consisting mostly of calcium phosphate, features a mineral framework that also doesn't remain stable in soil environments that are most nor reliably retains absorbed uranium.
Uranium is great for dating because it decays at a rate that is constant time for you to an isotope of thorium that can be measured in moment quantities by mass spectrometry. The ratio of the thorium isotope to the uranium still present tells geochronologists just how long the uranium has been sitting within the eggshell.