Dating from the Adams and Eves Threads

The fact that your surce wrote this sentence means nothing. If you wish to argue that leaching is a problem, you need reference to studies and evidence which actually measure and establish that leaching is a problem. If you actually do the research necessary to establish this, you will find that leaching is almost alweays detectable, and it's impossible to explain all the results we have by leaching into young material.

Radiodating methods that are used to date inorganics (rocks and minerals) start the clock when the material solidifies. 14C dating is different. It can only be used for organic, formerly-living matter.14C is being produced all the time, mostly in the atmosphere by 14N being hit by cosmic rays. 14C is radioactive and decays back to 14N with a half-life of 573040 years. At any time there is some ratio between radioactive 14C and stable 12C in the atmosphere (and it turns out it doesn't change much). Living things such as terrestrial plants that take in their carbon from the atmosphere have the same 14C/12C ratio as the atmosphere. Living things that eat these plants also have the same ratio. Living things that eat the formerly living things that ate those plants also have the same ratio. And so on.As soon as a plant or animal dies, it stops taking in or excreting carbon. The 14C continues to decay, but the 12C doesn't. Over time the ratio of 14C to 12C (in the remains) decreases. We can measure how long it is since the organism died by measuring the 14C/12C ratio and comparing it to the 14C /12C ratio in the organism when it died. Oops, we don't know the 14C/12C ratio in the organism when it died.In the early years of 14C dating, people assumed that the 14C/12C ratio never changed, and produced dates expressed in "radiocarbon years". They knew radiocarbon years weren't exactly calendar years, but it was better than nothing.But, for some time now, we've been producing calibration curves that relate radiocarbon years to calendar years. These could be used to figure out the 14C/12C ratio in the past atmosphere, but nobody bothers; it's calendar years that we are looking for. These curves are produced by dating something which can be dated by both 14C/12C and some other absolute method. Tree ring chronologies (actually many different ones from all over the world), lake varves, and thorium-dated corals are used for this. The correction is on the order of 10% or less (see CALPAL 2004 January, which would have a slope of exactly 45° if radiocarbon years equaled calendar years), so radiocarbon years are off but not all that far off.How far back we can date things is determined by the sensitivity of our instruments and the point at which the "background noise" overwhelms the "signal". Right now we can get pretty solid dates back to 30,000-40,000 years, and we can obtain dates in the 50,000-60,000 year range which are less solid. It seems unlikely we will get much farther back than that unless somebody comes up with something really new and ingenious.Marine organisms like seals and clams get a lot of their carbon from dissolved limestone, which is older than the carbon in the atmosphere and has a lower 14C/12C ratio, so such organisms can't be dated by 14C. Limitations such as this are well known and understood among real scientists.See How does the radiocarbon dating method work? (by a creationist) and Radiocarbon web-info (more detailed and somewahat more technical). For tree-ring dating, Ultimate Tree-Ring Web Pages. Plenty of references on lake varves already in this thread.

Not much for me to add ... No evidence-based doubt whatsoever. Obviously there's some emotionally-based doubt by some parties, or this thread wouldn't be here. But note that amount of evidence presented by Golfer for his viewpoint ...Creationists love to point to an occasional double ring and claim that this invalidates the entire method, but such double rings are detectable. Dendrochronologists use some pretty sophisticated statistical techniques to match up rings between different trees. That dendrochronology site I linked to is pretty interesting. "Radioemtric" just means "measurement of radioactivity" so 14C dating is a radiometric method. As has already been pointed out, 14C dating is quite different from other radiometric methods.

It's even tougher than that; for the most part, we can't date the fossil materials nor can we date the rocks in which the fossils are found. Fossils are found in sedimentary rocks. We are interested in the time of lithification (oversimplifying, when the already-solid grains of those rocks got stuck together, not when the grains themselves formed). There are materials in many sedimentary rocks that form at lithification (e.g. xenotime), and there's been significant progress in dating rocks using those materials. But accurate radiometric dating of sedimentary rocks is not common.We're mostly stuck dating igneous and metamorphic layers above and below fossiliferous layers, and inferring that the fossiliferous layer is older than the covering layer and younger than the layer it covers. But there are lots of dateable layers, and cross-correlations between sites, and we have a pretty solid handle on the eage of the fossils.

I looked up a few things. The following emphasizes the negative, but it does illustrate how difficulties are handled:

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Three approaches have so far been followed to determine the decay constants of long-lived radioactive nuclides.

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1. Direct counting. In this technique, alpha, beta or gamma activity is counted, and divided by the total number of radioactive atoms. Among the difficulties of this approach are the self-shielding of finite-thickness solid samples, the low specific activities, imprecise knowledge of the isotopic composition of the parent element, the detection of very low-energy decays, and problems with detector efficiencies and geometry factors. Judged from the fact that many of the counting experiments have yielded results that are not compatible with one another within the stated uncertainties, it would appear that not all the difficulties are always fully realized so that many of the given uncertainties are unrealistically small, and that many experiments are plagued by unrecognized systematic errors. As the nature of these errors is obscure, it is not straightforward to decide which of the, often mutually exclusive, results of such counting experiments is closest to the true value. Furthermore, the presence of systematic biases makes any averaging dangerous. Weighted averaging using weight factors based on listed uncertainties is doubly dubious. It is well possible that reliable results of careful workers, listing realistic uncertainties, will not be given the weights they deserve-this aside from the question whether it makes sense to average numbers
that by far do not agree within the stated uncertainties.

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2. Ingrowth. This technique relies on measuring the decay products of a well-known amount of a radioactive nuclide accumulated over a well-defined period of time. Where feasible, this is the most straightforward technique. Ingrowth overcomes the problems encountered with measuring large fractions of low-energy b-particles, as in the case of 87Rb and 187Re. It also comprises the products of radiation-less decays (which otherwise cannot be measured at all) like the bound-beta decay branch of 187Re and the possible contribution to the decay of 40K by electron capture directly into the ground state of 40Ar. Among the drawbacks of this approach is that the method is not instantaneous. The experiment must be started long before the first results can be obtained because long periods of time (typically decades) are required for sufficiently large amounts of the decay products to accumulate. “Ingrowth”-experiments further require
an accurate determination of the ratio of two chemical elements (parent/daughter) as well as an accurate determination of the isotopic composition of parent and daughter element at the start of the accumulation (see below). Moreover, because of the hold-up in the chain of intermediaries, for uranium and thorium measuring the ingrowth of the stable decay products in the laboratory does not work at all.

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3. Geological comparison. This approach entails multichronometric dating of a rock and cross-calibration of different radioisotopic age systems by adjusting the decay constant of one system so as to force agreement with the age obtained via another dating system. In essence, because the half-life of 238U is the most accurately known of all relevant radionuclides, this amounts to expressing ages in units of the half-life of 238U.

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(Begemann, F., K. R. LUDWIG, G. W. LUGMAIR, K. MIN, L. E. NYQUIST, P. J. PATCHETT, P. R. RENNE, C.-Y. SHIH, I. M. VILLA, and R. J. WALKER. "Call for an improved set of decay constants for geochronological use", Geochimica et Cosmochimica Acta, Vol. 65, No. 1, pp. 111-121, 2001)

That's a very difficult question to answer briefly.The radiation started when the atoms were created, in the heart of some supernova. That's not when the radiometric clock started.There are many different radiometric methods. All of them depend on the premise that radioactive decay rates have been constant. There are many, many excellent theoretical and experimental reasons to believe that this is true. Sylas summarized some in Message 12, and there's some discussion and references at Claim CF210.Simple accumulation methods are based on the additional premises that, when the rock solidified, there was none of the daughter product (that is, the element that is produced when the parent isotope decays) present and there was no gain or loss of daugher product other than by radioactive decay (that is, "closed system"). These turn out to be true most of the time in the case when the daughter product is a gas (like argon) that easily escapes liquids, and samples are chosen with reasonable common sense (e.g. don't test the weathered outer surface of a rock). We know that these are true most of the time because so many simple accumulation measurements agree with the more sophisticated methods. Potassium-argon is about the only simple accumulation method used, because it's well understood, low cost, and almost always accurate. It's common to cross-check potassium-argon results with another method. Creationist criticism of radiometric dating is almost exclusively confined to simple accumulation methods.By far the most commonly used methods are age-diagnostic methods. These do not depend on the closed-system premise listed above, and most do not depend on the zero-initial-daughter premise listed above. Age-diagnostic methods produce an age (or fail to do so) and an indication of how reliable that age is. All of them indicate when the system has not been closed, and some of them can produce a valid age even when the system has not been closed. All of them work this "magic" by making use of extra information that is not used in simple accumulation methods. Isochron methods use the fact that, in suitable systems, there is an isotope of the daughter element that is not produced by radioactive decay. When several sample points fail to plot as a straight line on an appropriate graph, the system has been opened and no date is produced. The argon-argon method uses incremental heating to release differently-bound samples from one rock, and compares the results to a rock of known age. When the results do not plot as a horizontal line on an appropriate graph, the system has been open; but if a large portion of the results are on a horizontal line, a valid age is produced. The most widely used method, U-Pb concordia-discordia, uses two similar but different radioactive decay systems (²³⁸U and ²³⁵U) and compares the results of the two methods. The results are presented on a graph called a "concordia diagram"; if the results do not fall on a predetermined concordia curve, the system has been opened; but if the results fall on a straight line, the upper intersection of this straight line with the concordia curve is a valid age.Of the above-mentioned methods, only the concordia-discordia method presumes that none of the daughter product (lead) is present when the rock solidifies. Lucky for us there are several minerals that so strongly reject lead at solidification that it's physically impossible to get one that had a noticeable amount of lead at solidification, and one of these minerals (zircon) is found in all sorts of different rocks and has several other properties that make it convenient for radiometric dating. Even the creationist RATE group admits that, when lead is found in zircons, it is the result of radioactive decay:

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Samples 1 through 3 had helium retentions of 58, 27, and 17 percent. The fact that these percentages are high confirms that a large amount of nuclear decay did indeed occur in the zircons. Other evidence strongly supports much nuclear decay having occurred in the past [14, pp. 335-337]. We emphasize this point because many creationists have assumed that "old" radioisotopic ages are merely an artifact of analysis, not really indicating the occurrence of large amounts of nuclear decay. But according to the measured amount of lead physically present in the zircons, approximately 1.5 billion years worth ” at today’s rates ” of nuclear decay occurred. Supporting that, sample 1 still retains 58% of all the alpha particles (the helium) that would have been emitted during this decay of uranium and thorium to lead.

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(Humphreys, D.R.; S.A. Austin; J.R. Baumgardner and A.A. Snelling, "Helium Diffusion Rates Support Accelerated Nuclear Decay," Proceedings of the Fifth International Conference on Creationism, R. Ivey (ed.), Creation Science Fellowship, Pittsburgh, PA. Error | The Institute for Creation Research)All of these age-diagnostic systems can be fooled by low probability occurences, so some of our dates may be wrong (although the extensive cross-checking makes even this unlikely). But there's no possibility that all or even many of our dates are wrong.Why and how these methods work as they do is more complex than can be gone into in this medium. Radiometric Dating: A Christian Perspective is an excellent overview with more detail than I can present here. Isochron Dating is a moderately technical exposition of, duh, isochron dating. There's a seriously technical exposition of concordia-discordia dating at THE U-TH-PB SYSTEM: ZIRCON DATING.

By observing and testing over the last 110 or so years, and from theoretical considerations. See the references on constant decay rate in my previous message. If you want to get some seriously heavy material, Modern Nuclear Chemistry.