From A Storehouse of Knowledge

Radioactive dating is the procedure of calculating an age for an artifact by determining how much of the radioactive material has decayed and calculating how long that would take given the half-life (how long it takes for half the material to decay) of the material being tested. Several dozen methods exist, using different radioactive isotopes and decay products, with varied dating ranges and precision. The procedure, however, is difficult, and many tests have shown that it can be inaccurate, and it is at times not even considered reliable by mainstream scientists.

Contents 1 Principles 1.1 Terms 1.2 Prerequisites 1.3 Decay Rates 2 Methods 2.1 Uranium-lead dating 2.2 Potassium–argon dating 2.3 Radiocarbon dating 3 Accuracy 3.1 Radiocarbon dating 3.1.1 Commentary 3.1.2 Calibration curves 3.1.3 Carbon-14 in deeply buried carbon 3.2 Dating of historic lava flows 3.2.1 Potassium-argon method 3.2.2 Argon-argon method 3.3 Consistency between methods and between repeated measurements 4 Variability of decay rates with environment 4.1 Electron capture and internal conversion 4.2 Bound-state beta decay 4.3 Influence of the Sun 5 Variability of decay rates with time 5.1 Biblical creationists 5.2 Physics 5.3 Nuclear processes and parameters in the geologic past 5.4 Comparison with independent dating methods 5.4.1 Astronomical dating 5.4.2 Helium diffusion 5.4.3 Heat diffusion 6 Note 7 References

Principles

Terms

• isotopes are different types of atoms of the same chemical element, each having a different number of neutrons
• decay product - the material(s) which is(are) produced by the decay of an isotope (the parent), also known as the daughter product(s)
• half-life - the time taken for half of the isotope to decay

Prerequisites

It is impossible to measure the age of something, except to time it as it actually occurs, so radioactive dating methods calculate the age, based on (i) measurements of quantities of specified materials, (ii) measurements of decay rates, and (iii) assumptions about the history of the sample.

Suppose we have a tank partly filled with water, and a hole in the bottom through which the water is leaking out of the tank. We wonder how long the hole has been there, that is, how old the hole is. We could measure (a) how much water the tank holds, (b) how much is still in the tank, and (c) the rate at which it is leaking out. We can calculate the age of the hole by subtracting (b) from (a) to find out how much water has left the tank, and then dividing this by (c), the rate at which it is leaking out.

However, in doing so, we have, consciously or subconsciously, made a number of assumptions about other factors that could have affected the calculations.

• We have assumed that the tank was full to start with.
• We have assumed that none of the water has escaped the tank in any other way, such as evaporation.
• We have assumed that no water has been added to the tank. Perhaps it rained yesterday and topped up the tank after the hole developed.
• We have assumed that the hole has always been that size, and was not for example, temporarily or partially blocked or enlarged. Perhaps water was leaking out at a faster rate before it was partly blocked.

Our calculation of the age of the hole will not be accurate unless all these assumptions are true. Similar factors apply to radioactive dating methods.

• We can't always know for certain how much of the original isotope was in the sample.
• We can't always know for certain that there was not already some of the daughter material present to start with.
• It is possible in some cases for either or both parent and daughter materials to leach into or out of the sample over time.
• It is possible that decay rates have changed.

Unless these factors are known, the calculated dates will not be reliable. Sometimes a hypothesis must be made that may be plausible but has not been proven. At other times an additional measurement can eliminate the need for one assumption, although no science can be done without assumptions at some level. For example, isochron methods do not assume any particular concentration of the daughter isotope in the original sample, but calculate that concentration based on other measurements. The reliability of that calculation will in turn depend on other conditions.

Given the complexity of radioactive dating, confirmation bias can also be a problem. This arises when the person performing the analysis has a strong expectation of what the result should be. For example, the geological formations and dates from surrounding features may suggest that the "true" date can only lie within a certain range. As another example, a date that is obviously wrong would confirm a strong belief in the fundamental unreliability of radioactive dating. In either case, there will be a subjective tendency to accept the result, rather than performing additional checks that might reveal unsuspected problems. Anyone using these methods should be well aware of the conditions for validity, the known confounding factors, and the sources of error. In critical cases it is best to maintain a healthy skepticism until all possible checks of consistency have been made and two or more independent methods have produced the same result.^[1]

Decay Rates

Different isotopes have different decay rates. Radioactive dating is usually considered most accurate if the age of the sample is not too much different than the half-life of the isotope used, although in favorable cases the age of the sample could be as much as 100 time less than the half-life or 10 times more. The appropriateness of a particular isotope also depends on the mineralogy and history of the sample, so not all samples can be dated.

Some isotopes used in dating

parent	daughter	half-life
147Sm	143Nd	106 billion years
87Rb	87Sr	50 billion years
238U	206Pb	4.47 billion years
40K	40Ar	1.3 billion years
235U	207Pb	704 million years
234U	230Th	80,000 years
235U	231Pa	34,300 years
14C	14N	5730 years
3H	3He	12.3 years

Carbon-14 for example is considered accurate by its supporters on ages from 2 to 50 thousand years. Modern methods can detect essentially any Carbon-14, and therefore produce dates up to about 100,000 years.^[2][3] Beyond this, there is essentially no Carbon-14 left, so any age larger than that can not be determined.

Methods

A sample is taken and prepared by removing any extraneous material, and removing any inclusions from the sample. The sample is then crushed and dissolved. The sample is then placed in a mass-spectrometer and a chart is produced showing the quantities of each element or isotope. That result is compared to decay curves to get a time interval. That time interval is compared to calibration information and corrections made for known variations. This provides a calendar date with an error margin.

Uranium-lead dating

This form of dating measures the decay of uranium within igneous zircon over a scale of tens of millions to billions of years. As uranium decays to two different isotopes of lead at different rates of decay, two clocks are inherently built into the system.

If the two agree with each other, the confidence in the date will be high. If they disagree, it may be because lead has been lost at some point in the history of the sample, for example, if there was an episode of heating above 1000 degrees C. (The mineralogy of zircon makes it highly unlikely that either any uranium is lost from the crystal or that any lead was in the crystal to begin with.) To deal with this possibility, an independent measurement is made from several points in the sample. Usually, different amounts of lead will have been lost, for example more from the surface of the crystal than from the interior, so there will be a range of isotope ratios. If the differences are due to one episode of loss of lead, then these points will lie on a straight line, as is often observed. Given this confirmation of the confounding factor, the line of observations may be extended until it intersects the curve consisting of the possible values without loss of lead. This value is taken as the true age of the sample.

A variation of this method is also known as lead-lead dating. It does not determine the age of a single sample directly, but the time at which different samples (with differing amount of uranium and lead) were separated from a common pool. This method has particular significance because it is the only method that purports to give a value for the age of the Earth (that is, the time at which the Earth and asteroids condensed out of the planetary nebula) rather than only a lower limit on the age of the Earth (that is, the age of the oldest surviving rocks on the Earth). There are a number of assumptions involved, but if they all hold, when the ratio of ²⁰⁷Pb/²⁰⁴Pb is plotted against the ratio ²⁰⁶Pb/²⁰⁴Pb for the various samples, the points should lie on a straight line. If any of the assumptions do not hold, then there is no reason to expect this to happen. In fact, when points are plotted for each of five meteorites that contain varying levels of uranium, a single data point for all meteorites that do not, and one data point for modern terrestrial sediments, all seven points lie on a single line. The slope of that line corresponds to an age of 4.5 billion years.^[4][5][6]

Potassium–argon dating

Whereas uranium-lead dating relies on the properties of the minerals to expel lead atoms on formation, potassium-argon dating relies on the fact that noble gases, in this case argon, easily diffuse out of molten rock, but are trapped within the crystal if they are produced within the solidified rock. The material being dated must contain potassium, but that is relatively common. The nearly ideal application for potassium-argon dating is lava that has been quickly cooled (otherwise some of the radiogenic ⁴⁰Ar may escape). One of the biggest concerns is that the sample may be contaminated by atmospheric argon. Minor contamination can be taken into account by measuring the amount of ³⁶Ar in the sample. (The atmosphere contains 99.6% ⁴⁰Ar, 0.34% ³⁶Ar, and 0.06% ³⁸Ar.) This method is applicable to the oldest rocks on Earth, and recent advances have pushed the range of precise dating down to a few thousand years, under ideal conditions.

Radiocarbon dating

For Carbon-14 (radiocarbon) dating, the sample is the remnant of a living organism which was in equilibrium with the atmosphere while it was alive, but when it died it ceased to absorb carbon-14 and this isotope began to decay with a half-life of about 6,000 years. As the sample ages, more and more Carbon-14 converts to Nitrogen-14.

Carbon 14 dating is a little different than other methods. With the other methods, the quantities of the parent and daughter products are added to determine the original amount of the parent material. With Carbon 14 dating, the daughter material, N¹⁴, escapes, so the original quantity cannot be calculated. However, when the creature was alive, it was taking in both C¹² and C¹⁴ in a ratio which was based on the ratio of those two elements in the atmosphere (in the form of carbon dioxide). So by measuring the ratio of C¹² to C¹⁴ and comparing it to the ratio that would have existed when the creature was alive, the age of the sample since the time it died can, in principle, be calculated.

Carbon dating can be, and has been, calibrated by dating samples of known antiquity. In some cases, timber is used, and the age of the timber is found by counting its tree rings.

Accuracy

Dating methods are often presented as accurate, objective, measures of the age of an item, but the public perception of their accuracy does not match the reality of the dating methods being inaccurate in many cases.

Mungo Man

In 1969 over 175 bone fragments were found on the edge of dry Lake Mungo in New South Wales. They were the remains of a woman, and were carbon dated to between 24,500 and 26,500 years old. Five years later more remains, this time of a man, were found 300 metres0.3 km
0.186 miles
14.913 chains away. It has not been possible to use carbon dating on these remains, but the initial publication by their discoverer, Jim Bowler, and Alan Thorne estimated the age to be 28,000 to 32,000 years on the basis of geomorphological criteria and stratigraphic association with the previous find. In 1999 Thorne argued for an age of 62,000 ± 6,000 years based on new results using three different techniques (uranium series, electron spin resonance, and optically stimulated luminescence). That study was criticized in the professional literature on a variety of grounds. In 2003 Bowler presented a review of the dating attempts, including OSL data from 25 new samples. His conclusion was an age of 40 ± 2 kyr for both finds, which seems to have become a durable concensus.

This story provides a case study of the difficulties and limitations of dating human remains, especially near the “event horizon”, which is now about 50,000 years with the best available technology. In retrospect, some of the controversy arose because various effects (like contamination of the bones with recent carbon) added systematic error to the dating results, and some arose because the age of related objects (like the layer of sand in which the body was buried) could not be directly transfered to the find itself. The intensity of the controversy was doubtless fueled by the significance the older date would have on the field of human origins, particularly in relation to the theory that had been proposed by Thorne. The ages proposed at various times differed by up to a factor of two, compared to the ±5% error estimate of the current consensus. The extreme value propagated by Thorne was in the end about three of his error bars removed from the value later accepted.

Java Man

The homo erectus fossils from Indonesia were dated in 1996 at 30,000 to 50,000 years of age, using analysis of radioactive elements in fossil-bearing sediment. However, new dates published in 2010 based on argon decay in rocks above and below the fossils gave their age as about 550,000 years.

It all depends which radiometric method you use to assess the fossils’ age, New York University anthropologist Susan Antón reported at the annual meeting of the American Association of Physical Anthropologists.^[7]

More examples of anomalous dates can be found on the research page.

Radiocarbon dating

Commentary

Carbon dating is arguably the most accurate radiometric dating method, given its ability to be correlated with historically-known dates, but it is not entirely reliable. The assistant editor of the Anthropological Journal of Canada marveled that so much use is made of it:

In the light of what is known about the radiocarbon method and the way it is used, it is truly astonishing that many authors will cite agreeable determinations as "proof" for their beliefs. ...
Radiocarbon dating has somehow avoided collapse onto its own battered foundation, and now lurches onward with feigned consistency. The implications of pervasive contamination and ancient variations in carbon-14 levels are steadfastly ignored by those who base their argument upon the dates.
The early authorities began the charade by stressing that they were "not aware of a single significant disagreement" on any sample that had been dated at different labs. Such enthusiasts continue to claim, incredible though it may seem, that "no gross discrepancies are apparent", Surely 15,000 years of difference on a single block of soil is indeed a gross discrepancy! And how could the excessive disagreement between the labs be called insignificant, when it has been the basis for the reappraisal of the standard error associated with each and every date in existence?
Why do geologists and archaeologists still spend their scarce money on costly radiocarbon determinations? They do so because occasional dates appear to be useful. While the method cannot be counted on to give good, unequivocal results, the numbers do impress people, and save them the trouble of thinking excessively. Expressed in what look like precise calendar years, figures seem somehow better — both to layman and professional not versed in statistics — than complex stratigraphic or cultural correlations, and are more easily retained in one's memory. "Absolute" dates determined by a laboratory carry a lot of weight, and are extremely helpful in bolstering weak arguments.' ...
No matter how "useful" it is, though, the radiocarbon method is still not capable of yielding accurate and reliable results. There are gross discrepancies, the chronology is uneven and relative, and the accepted dates are actually selected dates. "This whole blessed thing is nothing but 13th-century alchemy, and it all depends upon which funny paper you read."^[8]

Scientists sometimes disregard radioactive dates when the dates don't produce the results they believe are correct.

If a C14 date supports our theories, we put it in the main text. If it does not entirely contradict them, we put it in a footnote. And if it is completely 'out of date', we just drop it.— Säve-Söderbergh and Olsson^[9]

Frankly, among archaeologists, carbon dating is a big joke. They send samples to the laboratories to be dated. If it comes back and agrees with the dates they’ve already decided from the style of pottery, they will say, “Carbon-14 dating of this sample confirms our conclusions.” But if it doesn’t agree, they just think the laboratory has got it wrong, and that’s the end of it. It’s only a showcase. Archaeologists never (let me emphasize this) never date their finds by carbon-14. They only quote it if it agrees with their conclusions.— David Down^[10]

Calibration curves

Because the level of ¹⁴C in the atmosphere varies over time, confidence in the accuracy of radiocarbon dating is not possible without calibrating the method by comparison with samples of known age. For ages less than a few thousand years, this can be done with historically dated material. For example, one of the first papers describing the radiocarbon method^[11] applied it to Egyptian wood samples taken from the tomb of Sneferu of Meydum, determined historically to be (in 1949) 4575 ±75 years old, and from the tomb of Zoser at Sakkara, determined historically to be 4650 ±75 years old. (Note, though, that this particular example is according to Egyptian chronology, which is itself a matter of dispute.)

For dates older than this, and to fill in the calibration curve where no historically datable artifacts are available, the radiocarbon dates can only be compared with other dating methods. The preferred standard, at least for dates up to several thousand years, is wood that has been dated by dendrochronology, i.e., counting the annual tree rings. Single trees can be dated by their rings back nearly 5,000 years, while matching the overlapping ring patterns of a series of trees is claimed to be able to extend the chronology back to 12,594 years before the present. However, dendrochronology is not itself an exact dating method, and for older dates carbon dating is used to help determine dendrochronological dates, which is a circular exercise.

Recently, by including a variety of other evidence such as varves and corals, a new standard for the radiocarbon calibration curve was agreed on, which extends to 50,000 years before the present.^[12] This study concludes that an object with an (uncorrected) radiocarbon age of 50,000 years will actually be 3,000 to 4,000 (6-8%) older. To the extent that these "incremental" dating methods are valid, they remove the otherwise necessary assumption that the ratio of ¹⁴C to ¹²C in the atmosphere has not changed.

Nevertheless, such attempts have a degree of circularity, as they assume an evolutionary history of the planet. The global flood would have significantly changed the atmospheric carbon ratio, causing artefacts from near the time of the flood to appear to be a lot older than they really are, but because the existence of the flood is not accepted by secular scientists, carbon dating calibration curves make no allowance for it

Carbon-14 in deeply buried carbon

Carbon-14 makes up about 1 part in 10¹² of the carbon in the atmosphere. Levels on the order of 1 part in 10¹⁵ can be precisely measured with state-of-the art mass spectrometer systems, although at that level it is difficult to rule out contamination during the processing and measurement. Nevertheless, it appears that the purest sources of carbon occurring in the crust of the Earth, including diamonds^{[13] [14]} and deeply buried coal,^[15] have levels corresponding to ages of 40,000 to 100,000 years, assuming that the concentration was originally near that found in the modern atmosphere, and that the carbon-14 has decayed continuously with the current 5730 year half-life. There have also been reports of apparent radiocarbon ages between 30,000 and 45,000 years for pieces of wood found embedded in basalt and sandstone, although in those cases contamination during geologically recent contact with the biosphere or handling of the samples could easily be more severe than in the cases of diamonds and coal.^[16]

Potential sources of this carbon-14 contemplated by secular geologists include contamination by bacteria and production in situ from nitrogen by background radiation. Creation geologists tend to believe that this residual carbon-14 reflects some combination of low concentration before the Flood, sequestration by burial during the Flood, and radioactive decay since the Flood. This would require that carbon-14 be exempt from the general acceleration of decay rates believed to have occured during the Flood.

Dating of historic lava flows

In general, dates in the "correct ball park" are assumed to be correct and are published, but those in disagreement with other data are seldom published nor are discrepancies fully explained.— Richard Mauger^[17]

• Lava flows from the 20th century AD have been dated between 0.27 and 3.5 million years old by potassium argon dating. ^[18]

Potassium-argon method

Just as there is a maximum age for each method of radiometric dating, so is there a minimum age for which meaningful results can be obtained.^[Fact?] This varies with the details of the method and the currently available analytical equipment, but tends to be on the order of one hundred to one thousand times less than the half-life. For classical potassium-argon dating, with a half-life of 1.3 billion years, this would be in the range of 1 to 10 million years, far too old to allow a check of the method to ages known historically or even through incremental dating methods. Nevertheless, a check on the consistency of the method is possible by dating lava flows known to have occurred in the last few thousand years, or even within the last few decades. If the method is reliable, then these calculations should return an age indistinguishable from zero. If they don't, then the magnitude, frequency, and causes of the false ages must be carefully examined, and the method improved, restricted, or abandoned, accordingly

As outlined above, the potassium-argon method relies on the fact that diffusivity of argon is very high in molten rock and very low in solidified rock. There are three situations, however, where this simple principle becomes more complicated.

1. The sample may become contaminated with atmospheric argon in the field or during the analysis. This effect is relatively easy to correct because the isotopic composition of atmospheric argon is precisely known and stable. All that is needed is to measure the amount of ³⁶Ar. The amount of ⁴⁰Ar attributable to atmospheric contamination will be 295.5 times greater than this.
2. The sample may suffer some loss of radiogenic argon due to heating or weathering. Weathering is the smaller problem because a trained geologist can recognize the signs of this and choose undisturbed samples.^[19] An episode of heating, but not complete melting, at some time in the past may result in a partial loss of the radiogenic argon. This will make the sample appear to have an age somewhere between the time of original solidification and the heating. This should be kept in mind if there is any reason to suspect later heating, such as metamorphic changes,^[20] but at any rate the loss of radiogenic argon is not able to produce an age greater than the age of solidification.
3. The sample may contain "excess argon" due to inclusions that are not melted with the lava. Magma, like other types of rocks, is usually a complex mixture of minerals in various states, and may contain solid particles ranging in size from microscopic crystals up to boulders. When it is transported to the surface as lava, any argon present can escape from the liquid matrix, but not necessarily from the solid inclusions. If the potassium-argon age is included by analysis of the isotopes in the whole rock, then the argon measured will be the sum of the radiogenic argon in the matrix and the argon in the inclusions, both initial and radiogenic. The calculated age will thus be greater than the time since the eruption.

"Excess argon" has long been recognized as a potential problem in the interpretation of the potassium-argon age of lava flows as determined by whole rock analysis. In 1969 Dalrymple^[21] published a study of 26 distinct flows. Of these, no argon anomaly could be detected in 18 cases, 70% of the total. Three cases showed an excess of ³⁶Ar, yielding apparent negative ages up to 0.20 million years. The other five cases, about 20% of the total, showed an excess of ⁴⁰Ar, yielding apparent ages as large as 0.22 million years, except in one case, the Hualalai flow in Hawaii, which showed an apparent age of approximately 1.1 million years. This study was followed in 1970 by work of Krummenacher^[22] of another 19 flows. More recently Snelling^[23][24] reported the potassium-argon dates obtained for 13 samples from a single lava flow from the 20th century AD. Five of these showed, as one would expect, an age indistinguishable for zero, but seven showed calculated ages between 0.8 and 1.5 million years, and one sample returned 3.5 million years.

In summary, there are several factors that can lead to the method providing an incorrect age.

Argon-argon method

In the 40 years since the seminal studies of Dalrymple and Krummenacher, a number of very important improvements have been made to the potassium-argon method, most notably extending it to the argon-argon method, which allows

• much more precise measurement of the isotope ratios,
• easier exclusion of contaminating crystals, and
• the measurement of an "isochron", which in turn provides
  • a measurement of the excess argon in the sample,
  • the possibility to detect - and in some cases correct for - argon loss, and
  • a consistency check of the data.

When the argon-argon method is applied to the matrix of recent lava flows, avoiding any inclusions, and considering only samples which yield a clean isochron, the million-year anomalies of the K-Ar method are claimed to be no longer found.^[25][26][27] There is, however, some evidence that the K-Ar ages are systematically too young by 5-7%.^[28]

The advantages of the argon-argon method allow dating of much younger lava flows. A good example of the accuracy of this method when the conditions are ideal is the dating of the famous eruption of Vesuvius in 79 A.D. that destroyed Pompeii. The radiometric age was calculated 1918 years after the eruption was 1925 ± 94 years.^[29][30] This establishes a direct connection between historical dates and a method applicable to the earliest rocks. However, an argon-argon dating of recent lava flows in Hawaii gave extremely old dates.

Creation “science” attacks new phenomena in science in an attempt to support religious ideas. For example Argon-Argon dating of young Hawaiian volcanic rocks gives an age that is older than the age measured and calculated for the Earth. Rather than show that all radioactive age measurements are incorrect, these data throw light onto a new process—the adsorption of excess argon by mica minerals.— Ian Plimer^[31]

Consistency between methods and between repeated measurements

For methods, such as U-Pb, that are not able to date samples as young as 50,000 years, the only recourse—other than Astronomical dating discussed above—is to compare the calculated dates with dates calculated from other radiometric methods. In the best case, a chain of comparisons back to historical dates can be established, but in any case a general check on consistency is also very valuable. Another way to check the consistency of a dating method is to verify that the dates provided by multiple tests of a single object, or tests on a number of objects that should be the same age, yield consistent results.

One difficulty is that many samples rocks have a complex history. Since different methods have different "closure temperatures", an episode of heating may reset one radiometric clock but not another. Some events will change the rock in such a way, for example through partial loss of the daughter isotope, that an isochron is no longer produced. These effects limit the applicability of radiometric dating and complicate its interpretation without necessarily calling the fundamental principle into question. Therefore, to test the fundamental principle by looking at the consistency between different methods, it is important to choose samples that are unlikely to have been modified since their formation. The ideal objects in this respect are meteorites. These have been in space since their formation near the beginning of the solar system, and have not been subjected to the thermal, mechanical, and chemical processes on Earth that might compromise the assumptions of the method. Dozens of meteorites have been dated, many of them several times, many of them with multiple techniques. The majority of the ages fall between 4.4 and 4.6 billion years.^[32][33] For the purpose of establishing the consistency of radioactive data, the reproducibility of a result using different methods on a single object is of interest. Commenting on the work done on one of these meteorites, Dalrymple writes^[34]

In the case of St Severin, for example, we have 4 different natural clocks (actually 5, for the Pb-Pb method involves 2 different radioactive uranium isotopes), each running at a different rate and each using elements that respond to chemical and physical conditions in much different ways. And yet, they all give the same result to within a few percent.

Another good test of consistency under ideal conditions is the Cretaceous-Tertiary (K-T) boundary, which exists world-wide and was identified by geologists in the early 1800s, long before radiometric dating was invented. Dalrymple summarizes^[35] 187 radiometric age measurements made on 3 different minerals and on glass, by 3 distinctly different dating methods, each involving different elements with different half-lives. The dating was done in 6 different laboratories and the materials were collected from 5 different locations in the Western Hemisphere. The calculated ages are the same within analytical error (0.2 to 2%).

Although radiometric dating has been shown to be consistent under ideal conditions, it is harder to answer the question of how good it is under more typical conditions. It is widely recognized that there are many factors which can lead to a date which is inconsistent with other information about the sample. Some of these can be corrected or at least recognized using various advanced techniques, but others will never be discovered. Woodmorappe^[36] searched the literature for reports of radiometric dates that were at least 20% different from the values expected, and found 350 instances, which in most cases were rejected in favour of the expected dates.^[37] His search was reportedly extensive, including "54 reputable geochronology and geology journals",^[38] so it is possible that his collection represents a good fraction of the discordant dates reported in the literature, although this is hard to quantify.

Variability of decay rates with environment

The belief in constancy of decay rates of many nuclear processes has been based on a presumed insensitivity to the environment of the atom, a not-unreasonable presumption considering the small size and high energy levels of a nucleus compared to atomic scales. There are, however, exceptions. The known effects are either very small or occur under extreme conditions, so that they, by themselves, can not significantly alter dates calculated with radiometric methods. However, the fact that rates can vary opens the possibility of other, as yet unknown, ways in which rates could change.

Electron capture and internal conversion

These decay processes can be affected in a few light atoms because they contain electrons in orbitals that interact with the nucleus as well as with the electrons of adjacent atoms. An especially large effect of this type is the increase of nearly 1% in the half-life of ⁷Be in metals as opposed to insulators.^[39]

Bound-state beta decay

More extreme conditions can result in more extreme effects. Under conditions found in very hot stars, heavy nuclei can be stripped of most or all of their electrons, enabling a decay mode known as bound-state beta decay. This effect was first observed in 1992.^[40][41] In this experiment, dysprosium atoms, which are normally stable when neutral, were found to decay with a half-life of 47 days when completely ionized. (See also Woodmorappe^[42].)

Influence of the Sun

Recently some researchers have found systematic variations in decay rates of several elements on the order of 0.1%.^[43][44] Correlations have been reported with the Earth-Sun distance, with the rotation of the core of the Sun, and with solar flares. The results have met with interest but skepticism in the scientific community. At least one follow-up study has returned a negative finding,^[45] while others appear to have confirmed the effect.^[46][47][48]

Variability of decay rates with time

Biblical creationists

Historically Biblical creationists have emphasized the difficulties involved in radiometric dating and the discrepancies between calculated dates and other radiometric or geological information about the samples, with the implication that the methods are so unreliable that they cannot be used as evidence against a young Earth, even if the decay rates have always been what they are now. This position is epitomized by Woodmorappe's compilation of hundreds of discrepant dates. Secular scientists were never very embarrassed by these discrepancies, dismissing the creationist research as pseudoscience and believing that the evidence for "deep time" was overwhelming.

This situation began to change around 2000, when the RATE project began. RATE (Radioisotopes and the Age of The Earth) was a concerted effort by credentialed scientists to comprehend radiometric dates in the context of the Biblical account of the Earth's history. Although keenly aware of the shortcomings of radiometric methods, the participants clearly stated from the outset that the evidence that a huge amount of radioactive decay has occurred was overwhelming. One of them, D. Russell Humphreys, stated

Thus we observe a wide spectrum of nuclear decay effects: (1) daughter isotopes along the whole decay chain, (2) visible scars (halos) from α-decay, (3) the α-particles themselves (He nuclei), (4) visible tracks from decay by fission, and (5) the heat produced by nuclear decay. The most reasonable hypothesis is that all these products of nuclear decay were indeed produced by nuclear decay! But the amounts of those products we observe are much greater than thousands of years could produce—at today’s rates.^[49]

And later

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 [Humphreys, 2000, p. 335-337]. We emphasize this point because many creationists have assumed that "old" radioisotope 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.^[50]

The conclusion drawn by the RATE participants was that the rates of decay must have been at least billions of times higher at some time in the past. Candidate periods are creation week (particularly the first two or three days, before life was created), the time after the Fall of Man, and the year of Noah's flood. The project was oriented toward providing experimental evidence and theoretical support for this position. These periods were chosen because two difficulties with this proposal are the extreme heat produced by such intensive decay and the destructive effects of radiation on living beings, especially humans.

Physics

Radioactive decay rates as determined by high-precision measurements over several decades have shown no sign of changing over time, although the period of observation is miniscule compared to the ages over which they are expected to operate. A common view among Young Earth Creationists is that the present day decay rates are stable, but that there were one or more periods of accelerated decay in the past, for example during the Great Flood. This case would be compatible with the constancy in laboratory measurements. Another reason young Earth creationists, including the RATE project, support this scenario is that compressing the geological timescale from billions to thousands of years would require a million-fold increase in environmental radiation, which would be deleterious to many forms of life including humans. The excess radiation must somehow be "hidden" from human history, either temporally, by occurring early in the creation week, before life was formed, or spatially, by being buried deep in the Earth (or perhaps under the waters of the Flood). In particular, if a large acceleration occurred during the Flood, then the percentage of potassium in the form of the radioactive isotope potassium-40 in the bodies of the people and animals on the Ark must have been much lower than the present value. The advantage of hypothesizing that most of the acceleration occurred during the Flood is that it provides a natural explanation for the gradation of radiological ages in the geologic column, that is, deeper rocks show more decay than rocks farther up. Baumgardner nevertheless proposes, based mostly on the present correlation between radioactive elements in the ground and the heat flow to the surface, that only about 200,000 years worth of decay occurred during the flood and the rest—4.5 billion years worth—occurred before the end of day 3 of creation. In either case, the shorter the period of acceleration, the more intensive it must have been.

Many physicists see radioactive decay rates as a consequence of the laws of nature and a few fundamental constants. Even if those fundamental constants changed over time, there is no known way that could happen to accelerate all the dozens of decay rates used in radiometric dating by the same amount. In particular, it would require special conditions, if it is possible at all, to change the alpha decay of the uranium-lead method by the same amount as the electron capture decay process of the potassium-argon, since they are governed by very different nuclear forces ("strong" vs. "weak"). If the decay rates were accelerated by very different amounts, then consistent, systematic differences between various methods would be expected, e.g. that rocks dated to 3 billion years with U-Pb would consistently be dated to 2 billion years with K-Ar. No such systematic differences have been reported beyond those compatible with known mineralogical differences such as the "closure temperatures" of the system. Most Young Earth Creationists see, at least to some extent, the direct action of God in the Flood, which could, of course, include the uniform acceleration of all radioactive decay rates, as well as limiting the heat released by those decays so it does not melt the Earth. The purpose that would be served by such a miracle is unknown, but would have to have a motivation other than misleading radiometric dating.

Nuclear processes and parameters in the geologic past

The possibility that the constants of nature might have been different in the past has long fascinated physicists. They have therefore looked for and found a number of means of testing past values of physical constants.^[51][52] One famous method is the analysis of the products of the "Oklo reactor", which shows only miniscule detectable difference^{[note 1]} in the fine structure constant and neutron capture cross sections between then and now. If the decay rates had been accelerated while the Oklo reactor was active, so that an actual age of a few thousand resulted in a radiometric age of two billions years, then most scientists would expect to see evidence of this dramatic change when examining the reactor products.

Another line of evidence, commonly cited as supporting the constancy of the laws of nature from the beginning of the universe until now, is observations of astronomical objects that are calculated to be hundreds of thousands or even billions of light-years away. These should reflect the laws of physics as they were when the light we now see was emitted, but no difference is observed to the present laws of physics. Two observations of this type are the characteristics of supernovae, which depend on the decay rates of a large number of isotopes, and the fine structure constant, whose value in these objects is within 1 part in 100,000 of the present value.

Comparison with independent dating methods

There are other methods to estimate the age of rocks that are independent of nuclear processes. In most cases, there are many uncertainties in the input data and theoretical models so that a high accuracry cannot be expected, but the accuracy should be sufficient to serve as a test of the accelerated decay hypothesis.

Astronomical dating

Many sedimentary deposits exibit a large number of layers, identifiable by appearance or by a number of mechanical, chemical, or isotopic characteristics. In the last decade, it has been claimed that the spatial pattern of these layers corresponds closely to the temporal variation in orbital parameters of the Earth. Since the values the orbital parameters would have had over the past several million years can be reconstructed using uniformitarian assumptions, it is an obvious hypothesis, supported by plausible mechanisms, that the variations in the layers are caused by the variations in the orbital parameters, and therefore the layers were laid down at the corresponding times. This would indicate that the deposition occurred over millions of years, in contrast to the young Earth model of deposition during the year of the Great Flood. However, the research itself was based on a number of unprovable assumptions and only estimated the accuracy, not calculating it exactly.^[54]

The ages calculated for the layers using radiometric methods are found to agree within a few percent with the astronomical dates. The mainstream view takes this as strong confirmation of the accuracy of radiometric dating. In particular, if the astronomical dating is correct, the radiometric ages of rocks, at least up to a few tens of millions of years, cannot be explained simply by an acceleration of the decay rates.

For more details, see Research:Radioactive dating#Astronomical dating

Helium diffusion

One physical process that potentially takes place on geological time scales is helium diffusion out of mineral crystals. In particular, when uranium and thorium nucleii in zircon crystals decay, they produce both lead isotopes and helium. The helium will diffuse out of the crystal at a rate that in principle can be calculated. Based on the observation of how much helium is left in the crystal, one can calculate how long ago the decay occurred. If the result is a few thousand years, that would be evidence for an accelerated decay rate and the Biblical timescale. If the result is a few billion years, that would be evidence for an essentially constant decay rate and the evolutionist time scale.

In practice, solid state diffusion is not a simple process. The diffusivity depends sensitively on the temperature, pressure, and crystalline state of the mineral. If the concentration of helium in the surrounding rocks is high enough, helium will enter the crystal rather than diffuse out of it. Finally, there may be several different types of sites within the crystal with radically different diffusion constants.

The best-known attempt to apply this method is the investigation sponsored by RATE of zircons recovered from a borehole at Fenton Hill, New Mexico.^[55] The age reported was 6000 +/- 2000 years, fitting neatly in the Biblical timescale. This study has been criticized on a number of grounds, beginning with misidentifying the type of rock involved and misunderstanding published results that were used, to using models that neglect very important effects.^[56]

• The geological history of the region is complex, and the authors may not have properly accounted for potential changes in the temperature and helium concentration over time. The temperature is important because the diffusivity depends very sensitively on it. The helium found in the crystals may not be due to radioactive decay but due to contamination from nearby sources.
• The diffusivity measurements were made in vacuum, but there is evidence that the tremendous pressure in situ can radically decrease the diffusivity.
• Perhaps the more severe problem is interpreting the diffusivity measurements in terms of a simple, single-domain model. If there are two or more diffusivity domains in the crystals, as there is reason to believe, the laboratory measurement will give a value much higher than the geologically effective rate.

One study^[57] attempted to remedy as many shortcomings as possible, especially by applying a multi-domain diffusion model. The conclusion was that zircons in young Earth models, properly applied, are not able to get rid of helium fast enough and can thus be ruled out. The study concludes,

The old-earth model matches the revised measurements much better than any of the young- earth models considered. The RATE team claimed that essentially no helium would be left in these zircons if they were much more than a few thousand years old. However, direct computation has demonstrated otherwise – The helium content and the ~1.5 billion year radiometric age of these zircons are in very good agreement. Since no anomaly exists, there is no scientific need to postulate the existence of exotic physics, like accelerated nuclear decay, to explain the phenomenon.
Not only does this result deprive the accelerated nuclear decay hypothesis of its best case, it actually counts as evidence against accelerated nuclear decay. Two independent clocks (nuclear decay and helium diffusion) are now in agreement on the billion year age of these rocks. Now the accelerated nuclear decay hypothesis requires accelerated diffusion as well. At some point accelerating natural processes becomes an untenable scientific position, and one must start reading nature’s “clocks” at face value.

Another study^[58] took a different approach. Instead of trying to properly account for all the uncertainties at the New Mexico site, rocks in India with a simple history were investigated. This study as well reports results incompatible with the Biblical timescale:

This leaves the question of how the zircons that have certainly been at low temperatures during a time span that exceeds the creationist estimate of the age of the earth could have lost most of their helium. There simply has not been enough time for the helium to have diffused out.

Heat diffusion

Diffusion of a very different type can also be used as a geological clock. The diffusion of heat in rocks is well understood (and much simpler than the diffusion of helium). The size of a pluton can easily be determined by field measurements, and therefore how long it should take to cool down from one temperature to another. The temperature in turn affects the "closure" of radioactive mineral systems. Above the closure temperature, products of radioactive decay are lost, below the closure temperature they are retained. In a pluton where several radiometric systems are available with different closure temperatures, the cooling curve can be used as a clock, and it can be determined how much radioactive decay took place in the time between two closures. Meert^[59][60] describes such a system.

Mineral Used--Isotopic System	Closure Temperature +/- Error	Age +/- error
Zircon--U-Pb (SHRIMP)	850 +/- 50 C	532.1 +/- 5.2 Ma
Hornblende--40Ar/39Ar	500 +/- 50 C	512.7 +/- 1.3 Ma
Biotite--40Ar/39Ar	350 +/- 50 C	478.9 +/- 1.0 Ma
K-spar--40Ar/39Ar	200 +/- 25 C	435.0 +/- 10 Ma
K-spar--40Ar/39Ar	100 +/- 25 C	410 +/- 10 Ma

The mere existence of these temperature in the pluton is evidence for cooling that, with measured thermal constants, would be expected to require on the order of 100 million years. The fact that the radiometric dates are consistent with this time scale is evidence that the decays rates when the pluton formed were similar to those observed today.

Note

References

• http://earthsci.org/fossils/geotime/radate/radate.html
• http://facstaff.gpc.edu/~pgore/geology/geo102/radio.htm