Radiocarbon dating (14C)
Radiocarbon (14C) is the radioactive isotope of the common element carbon. It is formed in the upper levels of the atmosphere
following the interaction of cosmic rays with nitrogen (N2). Then radiocarbon is oxidized to carbon dioxide (CO2) and is
diffused in the atmosphere. As carbon dioxide is used for photosynthesis, radiocarbon is integrated into all organisms. When a
plant or an animal dies, radiocarbon decays with a half-life of 5730 years. At the radiocarbon dating laboratory the amount of
remaining radiocarbon relative to the stable element is measured. Since radiocarbon’s half-life is known, the age of the sample
can be estimated.
Tritium (3H) dating
Tritium (3H), the heaviest isotope of hydrogen, is radioactive and has a half-life of 12.4 years. Water in contact with the atmosphere
will have some tritium in it, and this tritium will be decaying to a stable, inert isotope called helium-3. Tritium can be
effectively used to investigate hydrologic mixing and transport processes. Due to its short half-life and steady state
concentration, tritium is the ideal tracer for studies requiring a time resolution to the nearest month over the past 150 to 200
years. In using tritium as a hydrological tracer, the analysis should be conducted at the lowest practical detection levels because
of the inherently low tritium levels found in natural waters (5 to 15 Tritium Units, or TUs (0.12 Bq/L)). Through electrolytic
enrichment of tritium in water samples prior to measurement, these low detection levels may be reached, affording greater
accuracy and precision.
Tritium dating can be used in Environmental Sciences, Hydro-geology, Geology, Climatology and other disciplines of science.
Uranium series dating
Uranium series dating is based on the radioactive decay of uranium in calcium carbonate and other minerals that precipitate
from solution. Natural 238U decays into 234Th, while the other isotope of U, 234U, decays into 230Th. Because uranium is soluble
in water and thorium is not, minerals that precipitate from solution often contain U but very little Th. Through time Th is
formed in the mineral as the U decays. The Th is itself a radioactive element and it decays into daughter products (i.e. 234Th
decays into 234U and 230Th decays into 226Ra). Ra is also radioactive, and it decays into Rn in a very short time.
If the mineral is of sufficient age, it contains the entire chain of U decay products, from 238U through to 206Pb. The critical
elements in this series are 238U, 234U and 230Th because of their half-lives. For any mineral that initially contained only U, the
time since its formation can be calculated from the 238U/ 234U and 234U/ 230 Th ratios.