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Old 19th November 2008, 12:40 AM   #1
contingent
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Carbon Dating, Uranium Dating

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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Old 19th November 2008, 12:58 AM   #2
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I have to go now - I'll be back a little later, but for now I'll suggest that you get your roomate to look up "Isochron Dating".
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Old 19th November 2008, 02:08 AM   #3
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Carbon dating has been calibrated against other methods (dendrochronology, artifacts of known provenance, annually layered sediments and so on). It has been complicated by the use of fossil fuels and nuclear testing, as well as known problematic periods, for example one associated (perhaps) with the end of the last ice age.
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Old 19th November 2008, 02:40 AM   #4
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Originally Posted by contingent View Post
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.
I'd like to hear how he can back up the two bolded statements. Specifically, I'd like to know how lead can end up in the same mineral as uranium if it didn't come from radioactive decay and whether he can produce examples. Also, there's the fact that the technique involves comparing the 206 and 207 isotopes of lead, which would fall apart if what he said was true.
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Old 19th November 2008, 02:55 AM   #5
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Originally Posted by contingent View Post
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."
The half-life of Uranium-238 is very well known and pretty straightforward to measure.
As pointed out previously, carbon-14 levels are cross-checked with tree-ring data and such.
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Old 19th November 2008, 02:55 AM   #6
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http://en.wikipedia.org/wiki/Uranium_lead_dating

Originally Posted by Scazon View Post
Carbon dating has been calibrated against other methods (dendrochronology, artifacts of known provenance, annually layered sediments and so on). It has been complicated by the use of fossil fuels and nuclear testing, as well as known problematic periods, for example one associated (perhaps) with the end of the last ice age.
The two layered sediments with the best information are varves and ice cores. The ice cores are especially important for determining Carbon isotope ratios in the atmosphere in a given year.
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Old 19th November 2008, 04:38 AM   #7
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Originally Posted by Spud1k View Post
Specifically, I'd like to know how lead can end up in the same mineral as uranium if it didn't come from radioactive decay and whether he can produce examples.
And in fact how one can tell whether any specific sample of lead that is mixed in with uranium came from radiological or non-radiological sources. Is there an easily-discerned difference?
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Old 19th November 2008, 04:40 AM   #8
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Here's the skinny on isochron dating, by the way:
http://www.talkorigins.org/faqs/isochron-dating.html
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Old 19th November 2008, 08:42 AM   #9
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Originally Posted by arthwollipot View Post
And in fact how one can tell whether any specific sample of lead that is mixed in with uranium came from radiological or non-radiological sources. Is there an easily-discerned difference?
Yes. You won't find any lead in Zircon from non-radiological sources.
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Old 19th November 2008, 09:55 AM   #10
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Originally Posted by arthwollipot View Post
And in fact how one can tell whether any specific sample of lead that is mixed in with uranium came from radiological or non-radiological sources. Is there an easily-discerned difference?
Uranium decays to one specific lead isotope, 206Pb; non-radiological lead is a particular fairly-constant mixture of about 50% 208Pb, 25% 207Pb, and 25% 206Pb.
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Old 19th November 2008, 09:56 AM   #11
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Contingent, here is a good general reference on all things radiometric dating...

Radiometric Dating and the Geological Time Scale

That site also contains many common creationist objections to radiometric dating, such as your roommate has mentioned, plus the rebuttals to those arguments.

When in doubt, search Talk Origins
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Old 19th November 2008, 10:05 AM   #12
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The short explanation is that the "problem" that's described in the original post is exactly why radiometric dating is only used in specific circumstances where the original values are known (usually because one of them started at zero). Many rock formations can't be measured this way because they don't happen to have any minerals with known convenient starting points like that, but the ones that do can.
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Old 19th November 2008, 05:09 PM   #13
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Your roommate might get something out of this site:

Radiometric Dating: A Christian Perspective
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Old 19th November 2008, 10:38 PM   #14
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Your tax dollars at work.
USGS publication on Geologic Time.
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Old 19th November 2008, 10:41 PM   #15
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Originally Posted by geni View Post
Yes. You won't find any lead in Zircon from non-radiological sources.
Originally Posted by ben m View Post
Uranium decays to one specific lead isotope, 206Pb; non-radiological lead is a particular fairly-constant mixture of about 50% 208Pb, 25% 207Pb, and 25% 206Pb.
I did not know that. Thanks.
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