Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Contingent on 19/11/2008 08:01:13
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My Roomate claims this:
"The process of radiological dating has several intrensic flaws, the most glaring of which is that it assumes set levels of the isotopes measured between samples origionally. For example, in U238 dating, the U238 decays into lead. The only problem with dating samples based on the ratio of the two is that lead occurs natrually, and often in the company of uranium and other heavy metals. The ratio of natural lead to uranium is not constant ether, as lead can occur with little or no radiological involvement.
Basically, there is no way of predicting the actual decay time on the remaining U238, as extra natural lead is everywhere and probably with the uranium wherever it may manifest. This same inaccuracy is inherant in all other methods of radiological dating. Nothing says that the levels of carbon 14 are or were constant at any point in history, or that the levels of solar radiation that cause the isotope in the atmosphere were ever constant."
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There are many types of radioactive dating involving Uranium. The one I think your roommate is talking about involves zircon crystals, which can integrate uranium into their structure but not lead. This means that when they formed there will have been virtually no lead, hence any that is there is from radiactive decay of the uranium.
There are several other forms of dating involving ratios of different elements in the decay series of uranium. They are all difficult and there is always the possibility of contamination, but if you understand the system well then this is small.
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As Dave mentioned, the main Uranium-Lead radiometric method uses zircon crystals. Zircon readily substitutes uranium into the crystal lattice but is almost absolutely exclusive to lead, which virtually eliminates the original contamination problem. Another benefit of zircon is that it is very stable and so hard to reset the radiometric clocks by metamorphism and/or alteration.
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It is also why radiometric dates are given with an error range, which in part depends on the particular technique used to extract and measure the ratio of parent/ daughter atoms and in part on the quality of the sample.
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There are other types of uranium dating besides U/Pb dating. Uranium-series dating measures equilibrium between uranium and thorium and is used quite a bit in corals and speleothems. Uranium is soluble in water, thorium is not (at least not at natural temps and pressures) but U-238 decays to Th-234 through alpha decay at a known rate. The reaction continues through a pretty fair number of other decay steps (12 or 13 other isotopes created if I remember correctly, including a different isotope of Uranium) all at known rates.
Since Thorium is not soluble it will not be taken up by corals or incorporated into speleothems, so this decay series can be used to date quaternary age specimens with a pretty high degree of accuracy. U-238 dating has been used as a correction tool to account for varying C-14 production rates and reservoir effects.
As for C-14, your roommate is partially correct. Radioisotope production has not been constant. BUT because we are able to reconstruct solar activity using Beryllium-10 (another long lived - 1.51ma half life - cosmogenic isotope) we can begin to apply corrections to C-14 for production rates. The source of your C-14 to be analyzed also makes a big difference in the date that you get back from the lab and the type of corrections that you have to apply to your data. Plant materials that take C-14 directly from the atmosphere use one correction, while the animals that eat them get another. The animals (such as us) that eat BOTH plant and animal get a different correction, while corals and forams that live in the oceans get their own separate corrections that depend on location, depth, time period, and oceanic/atmospheric mixing rates.
The fact is, C-14 data would be pretty inaccurate if it werent for Uranium/Thorium dating, tree ring data (both of which GREATLY increase the accuracy of C-14), and a very large body of scientific evidence documenting reservoir effects and production rates. Within the given range of error, with modern technology, C-14 is pretty accurate to about 25,000 years. Theoretically, it can be used to about 70,000 years, but robust body of corrections do not exist for C-14 past about 25ka.