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Science and Creationism 5. Radiometric Dating © Colin Frayn, 2008-2011 www.frayn.net
© Colin Frayn, 2008-2011 www.frayn.net Background What is radiometric dating? –A method for ascertaining the age of a sample –Uses the known decay rate of different radioactive elements Naturally present in the sample when it was formed Decay over time –Different techniques Using different elements –Applicable across a range of timescales Centuries to billions of years –Based on fundamental physics –Error-correcting
© Colin Frayn, 2008-2011 www.frayn.net Assumptions Does radio-isotope dating rely on assumptions? –Original quantity needs to be known Not true Isochron dating allows us to determine the original quantity –Decay rate doesnt change Decay rates never change This is fundamental physics –No contamination Again, not actually an assumption We can easily test for contamination
© Colin Frayn, 2008-2011 www.frayn.net Isochron Dating Measure the parent, daughter, and sister elements –Sister is a different isotope of daughter element –E.g. 87 Rb/ 86 Sr plus 87 Sr Benefits –Initial amount of daughter need not be known We can work it out! –Contamination can be spotted We know if contamination has occurred because the points will not be colinear –Loss/gain of daughter isotope can be spotted Again, we can test for this and avoid calculating dates for unreliable samples Checks –We can test dating methods by using several different tests on the same sample If 3 or 4 independent methods all agree on the same age then the chance that they were all wrong by the same enormous factor seems impossible –We can test some recent dating methods by calibration Check with dates from other sources e.g. volcanic eruptions, the Earths orbital cycles etc. C 14 C 12 N 14 N 15 Decay DaughterSister ParentStable Parent
Isochron Dating Example © Colin Frayn, 2008-2011 www.frayn.net Parent : Sister Ratio Daughter : Sister Ratio
© Colin Frayn, 2008-2011 www.frayn.net Examining Isochron Dating Possible problems: –Rock didnt form in one epoch For example, by repeated re-melting Geological samples are usually very obviously cogenetic (formed at the same time) or not Isochron methods would give the youngest age in this case –So the sample could only be older! –Rock is mixed from two sources This can be detected Other tests show that this is a very rare occurrence All possible mishaps either… 1.Have a small effect, at most a few 10s of % 2.Underestimate the true age of the sample
© Colin Frayn, 2008-2011 www.frayn.net Time ladder Are radiometric dating methods just calibrated from each other? No – they can also be calibrated using many other methods –C-14 dating can be calibrated against known archaeological remains –Dating of the Hawaiian archipelago is consistent with tectonic models –(Dating is consistent with Milankovitch cycles) –Dating is consistent with the luminescence method –Dating is consistent with geological models of deposition
© Colin Frayn, 2008-2011 www.frayn.net Errors An understanding of exponentials helps… A measurement error of 2 times does not mean a date error of 2 times –It means a date error of 1 half life C-14 dates down to 6,000 years from 40,000 years –Requires an error of roughly a factor of almost 80 in measurement! If a sample is polluted with extra parent isotope –Then ages would be an underestimate! Multiple different, independent dating methods can be applied to one sample –You can check that they agree
© Colin Frayn, 2008-2011 www.frayn.net Dating Mammoth Remains One part dated 29,500 yrs, another 44,000 yrs? –No, this is a lie The dates refer to two samples in one paper The samples are of different animals! –The paper referenced: USGS Professional Paper 862, Troy Péwé (1975) A similar dishonesty is also used to claim that a Siberian mammoth was dated unreliably –Again, these were samples from different sources
© Colin Frayn, 2008-2011 www.frayn.net Carbon-14 in Coal Baumgardner, 2003 –Claims that there is C-14 in very old coal deposits From the Carboniferous, 300 Myr ago –Proves that the deposits are young? C-14 can be produced in minute amounts by other processes –Natural decay of Uranium-Thorium isotopes –Possibly also from bacteria/fungi in some circumstances C-14 is only found in some, not all oil/coal deposits Contamination could also be a factor Research is ongoing into this discovery
© Colin Frayn, 2008-2011 www.frayn.net Anomalous Carbon-14 Keith & Anderson, 1963 –Living mollusc shells were dated as 2300 yrs old –This claim ignores two things: The snails were living in an extreme environment with poor access to atmospheric carbon Dating shells doesnt work! Molluscs get much of their carbon from dissolved limestone (which is old!) Wakefield, 1971 –A recently dead seal was dated as 1300 yrs old –This is due to the (well-known, documented) effect of lower C-14 activity around Antarctica –Old water upwells from the ocean, skewing ages –This only affects (some) ocean-dwelling creatures Riggs, 1984 –Living snails were dated to 27,000 yrs old –Snails were living in artesian springs –Yet another example of how dating shells is dodgy –Has no implications for the validity of the rest of C-14 dating
© Colin Frayn, 2008-2011 www.frayn.net K-Ar Anomalies This method has been largely replaced by more reliable 40 Ar/ 39 Ar methods Funkhouser and Naughton, 1968 Recent volcanic lava dated to 1.6Myr Not quite true – they dated xenoliths in the lava Examination of the lava showed that the Argon levels were raised So this was a clear that the K-Ar method wouldnt work This is a known flaw with K-Ar dating in some rocks Other methods date these rocks just fine Dalyrmple, 1969 Analysis of historic lava flows using 40 Ar/ 36 Ar More anomalous readings Effect on old rocks is negligible Dalrymple showed that the 40 Ar/ 39 Ar method worked fine
© Colin Frayn, Fairbanks Mammoth One part dated 29,500 yrs, another 44,000 yrs? –No, this is a lie The dates refer to two samples in.
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