uranium-234 (250,000 years) thorium-230 (75,200 years)
radium-226 (1,620 years)
radon-222 (3.83 days) lead-210 (22 years)
polonium-210 (138 days)
lead-206 (stable). The latter series decays as: uranium-235
(713 million years) protactinium-231 (32,400 years)
thorium-227 (18.6 days)
radium-223 (11.1 days) lead-207 (stable).
Some decay products with extremely short half-lives have been omitted
from these series. The most significant isotopes for dating purposes
are uranium-238, uranium-235 and thorium-230.
In a natural system where all the decay processes are undisturbed a dynamic equilibrium will prevail in which the ratio of one isotope to another will remain essentially the same. A disturbance of any part of the decay chain will instigate a new dynamic equilibrium with different isotopic ratios. A comparison of the new to the old ratios allows an elapsed time to be assessed. The most important disturbance process is the virtual insolubility of thorium-230 and protactinium-231 in water. This leads to these isotopes being precipitated from the water column and collected and buried in sediment deposits. As these sediments are buried under successive sediment layers and isolated from their parent isotopes, they continue to decay at their natural rates. This results in concentrations of both that decay exponentially with depth in the sediment layer, which can be related to the time since the deposition of the sediment using the thorium-230/uranium-234 and protactinium-231/uranium-235 activity ratios. The former has a useful dating range of 10,000 to 350,000 years BP and the latter 5,000 to 150,000 years BP.
A similar procedure is used to date carbonate shells and coral using the uranium-234/uranium-238 activity ratio. Both are deposited in equilibrium with the uranium isotope composition of the ocean, and if they are post-depositionally isolated from the ocean will decay such that a different isotope ratio will evolve over time. These isotopic ratio variations can provide useful dating information from 40,000 to 1 million years BP. Another method for dating carbonate materials involves the co-precipitation of uranium with aragonite or calcite from natural waters free of thorium and protactinium. Once isolated from the ocean the uranium-234 will decay to thorium-230 and the uranium-235 to protactinium-231, with the ratios of both a function of time and the original uranium content. The former decay process is more commonly used due to higher relative abundances, and is widely used to date raised coral terraces and provide an accurate chronology of glacio-eustatic sea level changes.
The major problems of uranium series dating include the assumptions of initial isotopic ratios and a closed system after deposition. The former assumption of long-term isotopic ratio stability is thought tenable in the oceans although much less so on land. Obtaining activity ratios for different isotopes from the same sample and cross-checking dates can provide a check on the latter assumption of a closed system since any post-depositional contamination processes are not likely to equally affect different isotopes. Another problem lies in the calculation of a mean sedimentation rate from a graph of activity ratios versus depth and then using that mean rate to interpolate the ages of events intermediate between sample levels. Estimates of the tenability of this process vary widely. See Bradley (1985).
longitude. This has also been known
as Greenwich Meat Time (GMT), Z Time or Zulu Time. It is the
time of convenction for weather reports.