How can isotopes be used in dating archaeological
C is created in the atmosphere by cosmic radiation and is taken up by plants and animals as long as they live.Upon death, the isotope begins to decay and after 5730±40 years half of it is gone.
Radiometric dating and certain other approaches are used to provide absolute chronologies in terms of years before the present.Without absolute ages, investigators could only determine which fossil organisms lived at the same time and the relative order of their appearance in the correlated sedimentary rock record.Unlike ages derived from fossils, which occur only in sedimentary rocks, absolute ages are obtained from minerals that grow as liquid rock bodies cool at or below the surface.Episodes of global volcanic activity, rifting of continents, folding, and metamorphism are defined by absolute ages.The results suggest that the present-day global tectonic scheme was operative in the distant past as well.The two approaches are often complementary, as when a sequence of occurrences in one context can be correlated with an absolute chronlogy elsewhere.
Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.
Each radiocarbon date has a statistical probability shown by the ± number.
This number is called a standard deviation and is a measure of the spread of measurements around the mean (average).
Using this established record, geologists have been able to piece together events over the past 635 million years, or about one-eighth of Earth history, during which time useful fossils have been abundant.
The need to correlate over the rest of geologic time, to correlate nonfossiliferous units, and to calibrate the fossil time scale has led to the development of a specialized field that makes use of natural radioactive isotopes in order to calculate absolute isotopes has been improved to the point that for rocks 3 billion years old geologically meaningful errors of less than ±1 million years can be obtained.
When rocks are subjected to high temperatures and pressures in mountain roots formed where continents collide, certain datable minerals grow and even regrow to record the timing of such geologic events.