Dating from the Adams and Eves Threads

Jazzns,Strictly speaking, most paleontologists agree that the term fossil means any evidence of past life. As such 10k year old dry organic giant sloth dung is a fossil, so is a frozen mammoth, as well as an insect trapped in amber.What you are describing is a permineralised fossil, which for all intents & purposes is synonymous with fossil in this discussion.MarkThis message has been edited by mark24, 12-28-2005 06:43 PM

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There are 10 kinds of people in this world; those that understand binary, & those that don't

mark24 (or anyone else that is knowledgable about dating and fossils and such),I have a question. I am assuming that for fossils that are millions of years old we date the surrounding rock/sediment to determine the age of the fossil. Obviously we would have problems knowing if this is accurate or not for individual fossils (assuming we can not directly date the fossil).Do we know that dating is accurate because we always find the same fossils in the same layers? For example, if the age of the surrounding sediment and/or rock in the area of a T-Rex find approximately matches that of the vast majority of other T-Rex finds (hopefully in disparate locations) it would seem to me that we now know with good certainty the approximate age of the T-Rex fossils. Is this a decent layman's explanation of how this dating works?

Mini_Dikta, No. We can have confidence in dating methods because different labs & methods arrive at similar conclusions.You are getting confused with the concept of index fossils I think. Index fossils are numerous & widespread fossil species that are found in a narrow age range. The range is determined via radiometric dating. If the species actually does turn out to have a narrow age range after many tests, then we can with some confidence assert that any rocks we find them in are of that age. Yes, a reasonable explanation, but T-Rex would make a bad index fossil. Too few examples of this species.Mark

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There are 10 kinds of people in this world; those that understand binary, & those that don't

Very cool, I do understand that we have several different dating methods (from reading the age of the earth threads on this site) and that we pretty certain that they are accurate due to the correlation of the results. (If I use 10 independent methods to date something and they all basically agree I can assume my data/results are good).Can we directly date most fossils or do we use the surrounding rock to date most fossils? (For the sake of argument let's talk fossils older than 50,000 years). If we can not directly date fossils the method of using index fossils you described makes perfect sense to me. (thanks for indulging me..... I am an engineer and haven't taken any pure science since my college days)Cheers

Mark answered your question quite nicely.As far as I've read, the Lake Suigetsu macrofossils (used for dating) are in near original condition, but of course it would be nice to actually see this in print someplace or hear it directly from the scientists directly involved in these studies.

Easily done, Rox: Science 20 February 1998:
Vol. 279. no. 5354, pp. 1187 - 1190

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.

Thanks a lot... that's just what I was looking for. So really, this is all a big exercise in interpolation and statistical correlation. I am assuming that as we gather more and more data points our dating process can become more and more accurate. Very interesting!

I don't think you have an appreciation for how carefully radiocarbon dates are acquired. Serious scientists go to great lenght to determine if there is alteration or contamination of the sample. The fact that consistent dates can be obtained is testament to the fact that it can work. Yes, and there is a reason for that. Stubbornness is not always a virtue, G. Probably not even necessary. If all of these were problems, the radiocarbon record should be a disaster of nonconcordance. It isn't. Irrelevant, for the reasons stated.

Robin, you were puzzling, on your thread with Faith, about how half-lives of things like potassium 40 are determined. "Half-life" is just one of the alternate ways to express the rate at which an isotope decays. An example experiment would be to put a kilogram of potassium next to a Geiger counter and count the "blips" that the counter makes at various times over a year or two. The rate would be some number on Day 1, due to the 0.117 grams of potassium-40 present (and measured by independent means) in the kilogram, and the rate would slowly decline, as less and less potassium atoms are there to be able to decay each passing day.From the change in rate over time, it's just a very simple mathematical manipulation to express the "rate constant" as a half-life - it's an alternate way to say exactly the same thing: "How fast does this stuff decay?"I'm going off to play with
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where you can run your own experiment to determine one. Note that you don't have to run your experiment for a whole 1.7 billion years, though. Longer means more accuracy, but isn't at all required if you measure rates closely enough.This message has been edited by Coragyps, 12-29-2005 12:27 PM

Another half-life link:
Half_Life

So I guess the idea is to record a tiny amount of radioactive decay and then extrapolate that out for a half-life.

That sounds a little crude.In fact, radioactive decay has been extensively observed and investigated, and has been should to alway be an exponential decay. That is, the rate of decay is proportional to the amount of the material present. The decay is quite accurately describably with an exponential probability distribution. This makes it possible to make quite accurate predictions, albeit probabilistic predictions.When a new dating method is devised, that method is independently tested by comparing dates measured with the new method with dates obtained by other known reliable dating methods.

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

quote:

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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)

Thanks, JonF. That's good information.