The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere.
Atmospheric nuclear weapon tests almost doubled the concentration of 14C in the Northern Hemisphere.
An error margin of 2–5 % has been achieved on younger Mesozoic rocks.
18.3 Modern Dating Methods Radiometric dating has been carried out since 1905, and since then the techniques have been greatly improved and expanded.
Dating can now be performed on samples as small as a billionth of a gram using a mass spectrometer.
Comparing the remaining 14C fraction of a sample to that expected from atmospheric 14C allows the age of the sample to be estimated.
The technique of radiocarbon dating was developed by Willard Libby and his colleagues at the University of Chicago in 1949.
Carbon-14 has a half-life of 5730 years and would have long ago vanished from Earth were it not for the unremitting cosmic ray impacts on nitrogen in the Earth’s atmosphere, which create more of the isotope.
The neutrons resulting from the cosmic ray interactions participate in the following nuclear reaction on the atoms of nitrogen molecules (N2) in the atmosphere: The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km, and at high geomagnetic latitudes, but the carbon-14 spreads evenly throughout the atmosphere and reacts with oxygen to form carbon dioxide.While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments, from which their ratios are measured.The scheme has a range of several hundred thousand years.18.3.5 Uranium-Thorium Dating Method A relatively short-range dating technique is based on the decay of uranium-238 into thorium-230, a substance with a half-life of about 80,000 years.It is accompanied by a sister process, in which uranium-235 decays into protactinium-231, which has a half-life of 34,300 years.18.3.2 Method This is based on the beta decay of rubidium-87 to strontium-87, with a half-life of 50 billion years.