Isn't it interesting to note the following about the strength of the Earth's magnetic fiel

---
Master Index Current Directory Index Go to SkepticTank Go to Human Rights activist Keith Henson Go to Scientology cult

Skeptic Tank!

> Isn't it interesting to note the following about the strength of the > Earth's magnetic field: > 8000 bC 98 Gauss > 6000 bC 35 " > 5000 bC 20 " > 4000 bC 12 " > 1970 AD 0.62 " (I) IS THERE SOMETHING WRONG WITH THESE NUMBERS? These numbers are identical to those proposed by creationist Thomas Barnes for his "magnetic field decay" idea, as published in an Institute for Creation Research Technical Monograph called _The Origin and Destiny of the Earth's Magnetic Field_. Barnes's assumptions are, um, rather unique. It is safe to expect that Mr. Kiledal is working from either Barnes directly or from a source which presents Barnes's data uncritically. It is highly unlikely that someone else independently arrived at identical numbers. Kiledal presents these numbers as if they were factual, but that is not even close to the truth. The only number in Kiledal's list which is even remotely connected with any physical measurement is the number presented for 1970 AD. Barnes's data extends back to about 1820 AD, and every value previous to that -- i.e., every other data point that Kiledal mentions -- is based on an unjustified exponential extrapolation from the recent values. (Erik may not have been aware of this if his source doesn't present how Barnes arrived at his numbers.) Barnes's data rests on the following untenable assumptions: 1) Exponential extrapolation is justified far beyond his data (x50) 2) A uniform decay of a process known to not change in a uniform manner 3) An out-of-favor model for the source of the earth's magnetic field 4) There is no force that "drives" the field (i.e., no energy input) 5) The non-dipole component of the field can be ignored 6) A better straight-line fit to the data can be ignored If Mr. Kiledal is curious about these topics in more detail, he is encouraged to read (Dalrymple, 1983) or (Dalrymple, 1986, pp. 54-57). We get this topic regularly in talk.origins as an alleged "proof" of a young earth and I'm sure most people here are tired of hearing about the small details. References: Dalrymple, G. Brent, 1983, "Can the Earth be Dated from Decay of Its Magnetic Field?", Journal of Geological Education, 1983, Vol. 31, pp. 124-133. Dalrymple, G. Brent, 1986, "USGS Open-File Report 86-110: Radiometric Dating, Geologic Time, and the Age of the Earth: A Reply to ``Scientific Creationism''", U.S. Geological Survey, 76 pp. The latter paper (Dalrymple, 1986) is an excellent introduction to radiometric dating and creationist criticisms thereof. It is available from the Government Printing Office for $14, but I have the author's permission to distribute it and will send a copy to anyone who is willing to cover postage (~$2.20 for anywhere in the United States). This topic is also covered well in Strahler's _Science and Earth History_. There is also a Canadian anti-creationist publication which graphs Barnes's data and his extrapolation. It is quite an eye-opener. Ask me via Email if you want an address to order the issue. ============================================================================= (II) CAN THE RECENT HISTORY OF THE EARTH'S MAGNETIC FIELD BE MEASURED? It's trivial to shoot Barnes's propositions full of holes, but it is even better to try to test them by measuring the strength of the earth's magnetic field back beyond Barnes's limited data. This can be done by measuring the remnant magnetism in lava flows and archaeological objects (e.g., clay pots). If Barnes were correct, then the data would confirm his exponential extrapolation. However, the data refutes Barnes. The diagram I've seen appears originally in (Chapman, 1980) and is reproduced in (Dalrymple, 1983) and (Dalrymple, 1986, Figure 15). It shows a decreasing trend over the last 2500 years (or so), preceeded by an increasing (on average) trend that covers 2000 years. The data indicates that the strength of the magnetic field from about 8000 years ago to about 4000 years ago was slightly less than its present strength. (Barnes's "model" predicts that the strength of the field would have been approximately 20 to 50 times its present value, during that same interval.) There is no support from the measurement of remnant magnetism for a continuous field decay, linear or exponential. This is not too surprising, as there is ample evidence that the entire field has reversed itself several times through history -- meaning that any unidirectional extrapolation would be senseless. References: Champion, D. E., "Holocene geomagnetic secular variation in the earth's magnetic field in the western United States: Implications for the global geomagnetic field", Ph.D. Thesis, California Institute of Technology, 1980. Dalrymple, G. Brent, 1983. See Section (I). Dalrymple, G. Brent, 1986. See Section (I). ============================================================================= > Lifeforms who died about 8000 years ago then may have had very little > C-14 in them. As a result, they would appear to be much older than they > really are if you hang on to Benjamin's 2nd assumption. (III) IS THE [14]C CONTENT OF THE ATMOSPHERE ASSUMED CONSTANT FOR "CARBON DATING"? Young-earth creationists, who know full well that carbon dating works in general, have to compress 40,000-year-old dates into their 4,000-year post-flood history. In doing so, they propose the same thing which Mr. Kiledal proposes here (though Kiledal appears to allow twice the creationists' time-scale). Neither Mr. Benjamin nor any lab performing carbon dating assumes a constant level of concentration of [14]C in the atmosphere over time. The carbon dating method is calibrated against the dendrochronological scale (a continuous sequence of tree rings which goes back about 8,000 years). Here is how that is done: The same equation used to calculate an "age"... age = half-life * log-base-2 (c-14-original / c-14-now) ... can also be solved for another variable and so used to calculate [14]C level at the time of formation, when the age of the object is known by another means ... c-14-original = c-14-now * 2 ^ ( age / half-life ) We can carbon date the tree rings from the dendrochronological scale, and then construct a plot of "dendrochronological age" (i.e., ring count from present) versus carbon "date" assuming today's [14]C level. That plot would indicate how much the [14]C level of the atmosphere has changed over time (by indicating disagreements between the two dating methods -- dendrochronology and uncorrected [14]C "dating"). If Mr. Kiledal and the young-earth creationists were correct, the plot would show that these [14]C "dates" are badly out of whack with respect to the dendrochronological scale as we get back into the past. A level of half of the present [14]C level would result in an error of about 5500 years in the resulting "date." This plot appears in (Dalrymple, 1986, Figure 10) and also in (Ralph and Michaels, 1974). It shows that [14]C "dates" from 1000 AD to 500 BC generally are from 0 to 200 years "younger" than the dendro- chronological dates. Beyond that, to about 5500 BC, [14]C "dates" are about 10% "older" than dendrochronological dates (with 5% variation in either direction). None of the [14]C "dates" are more than 900 years off. [NOTE: I'm putting "scare quotes" around "dates" because these are not actual [14]C ages. They are uncorrected for changes in the [14]C content of the atmosphere, which is necessary in order to COMPUTE those changes in [14]C level with time. Once the computation is done and a correction is arrived at, we can compute actual [14]C ages of things whose age is not known by any other means.] Derived from such a table is a graph of the "correction" to be applied to a [14]C date. This is the calibration which takes into account the varying level of [14]C in the atmosphere over time. A sample diagram of these corrections can be found in (Faure, 1986, p. 391). When a [14]C age is corrected in this manner, as is standard procedure, the resulting age is not dependent on the assumption of a constant [14]C level of the atmosphere. Thus, Kiledal's proposal fails on two counts. First, there is no assumption of a constant level of [14]C in the atmosphere in carbon dating. Second, there is positive evidence that the [14]C level of the atmosphere has been within 10% or so of its present value for the past 8,000 years. (I understand that other calibrations, e.g. ocean sediments, have been used to calculate the [14]C content of the atmosphere back to about 40,000 years BP. But I do not have a reference handy for that.) References: Dalrymple, 1986, See section (I) Faure, G., 1986, _Principles of Isotope Geology_ Second Edition, John Wiley & Sons. Ralph, E. K., and H. N. Michaels, "Twenty-five years of Radiocarbon dating", _Amer. Sci._, Vol. 62, pp. 553-560. ============================================================================= > Erik Kiledal It is interesting to note that Mr. Kiledal reappeared, removing the "non-creationist" label from his signature in the same article where he advances a standard (bogus) young-earth creationist argument. Is it a coincidence or is it that a facade is being dropped? I strongly recommend Mr. Kiledal get a copy of the Dalrymple paper and read it so that he will get a basic understanding of radiometric dating. He jumped on a couple of people pretty hard in regard to mammoths, but he's as much in the dark on this topic as anyone else was on that other one. Are you up to the challenge, Erik?

---

E-Mail Fredric L. Rice / The Skeptic Tank