Radioactive dating

Radioactive dating uses the known physics of "half-life" as a way to attempt to tell the "age" of either:

• Rocks
• Remains of living things

While the fundamental concepts used for these two kinds of dating are the same, the actual radioactive methods are very different and are subject to different factors which might affect their accuracy.

Myths

Below are common myths. These are not things about radioactive dating which are argued by evolutionists vs. creationists. These are myths about the accepted science!

• Carbon-14 dating is used to tell the age of the earth. No, C-14 dating can only be used to date the remains of living things. The maximum limit is to date something which died 50,000 years ago.
• Radioactive dating is precise to within a few years. Depending on the kind of dating, and the assumed age of the sample, all methods have acknowledge margins of error of 1% or more. In addition, there is scientific debate about much larger possible errors. Dates are often reported with a margin of error like "791+-20 million" years.
  Note: The "plus or minus 20" is due to the limitations of measuring amounts of radioactive elements.
• Newer methods like K/Ar can be used date the remains of dinosaurs. Only C-14 dating is used to date remains of living things. Other fossils of living things may be dated by attempting to date the rock in which they are found.

A Simple Example

The following example is over-simplified, but demonstrates the basic principles of radioactive dating. For an example, we will say that we have the remains of a living creature. The remains were apparently buried at death (remains which stay in the air totally decompose!)

The atmosphere contains a small amount of CO2. That CO2 contains two forms of carbon. Most of it is C-12, but a small percentage (as little as one atom out of a trillion) is C-14, a radioactive form of carbon. In the atmosphere, C-14 is constantly being formed by the action of the sun's radiation on Nitrogen. But C-14 also decays back to Nitrogen. This results in a balance (actually a changing balance but more on that later!).

Living things constantly interchange CO2 with the environment either by eating plants (which animals do) or by using CO2 from the air (which plants do). Thus while alive, living things contain about the same proportions of C-14 and C-12 as the atmosphere.

When a living thing dies, it stops exchanging CO2 with the environment. The C-14 slowly decays back to Nitrogen. Therefore the ratio of C-14 to C-12 goes down over time.

As an example, we know that the Carbon in the atmosphere is a certain percentage C-14. The half-life of C-14 is 5730 years. This means that over time the ratio of C-14 changes: Notice that the amount C-12 does not change, only the amount of C-14 (which decays into Nitrogen). The Nitrogen escapes into the air and is not measured.
The following chart shows what happens if we start with a sample with 1000 grams of carbon.

Years	grams C-12	grams C-14
0	1000	10-9 (one billionth)
5730	1000	.5 * 10-9 (a half billionth)
11460	1000	.25 * 10-9

Now to find out how old the remains of a living thing are, we just measure the proportion of the carbon which is C-14 and we know how long ago it died! Using calculus, we can actually derive the formula:

Notice that because so little of the total carbon is C-14 that either:

1. We must use a large sample, which may make it necessary to destroy most or all of the original material, or:
2. Have laboratory techniques which can detect very small numbers of C-14 atoms.

Recent advances in laboratory techniques and equipment have helped to allow C-14 dating to be used on smaller samples.

Assumptions

If we think about the above simple example, we can see that it relies on several assumptions. Using assumptions does not mean that our estimates are wrong! But we must be careful to check those assumptions against the real world to see if they seem to be correct. If one of the assumptions is incorrect, then the age estimate is incorrect!

The assumptions include:

1. The most obvious assumption is that the 5730 year half-life of C-14 has been the same ever since the living thing died.
2. The second assumption is that since the time the living thing died, no C-14 has been added to the dead material. This could have happened if the material was exposed to the air and exchanged carbon with the air. Or some process might have added C-12 to the material.
3. The third assumption is that we know the ratio of C-14 to C-12 in the living thing at the time it died. For living things, we assume that this is the same as the ratio of C-14 to C-12 as we find in the atmosphere today. When we look at rocks, the assumption will be more generally stated as knowing the amounts of the various isotopes when the rock was new.
   Note: In fact, the ratio of C-14 to C-12 is changing somewhat. Dating using the C-14 method tries to account for this. We will return to this complication later.

What if the assumptions are wrong?

If any of the assumptions is incorrect, then the radioactive dating process may yield dates which are off by millions of years. We noted earlier that dates are reported with a "+-" factor to indicate the limitations of the laboratory equipment used to measure amounts of radioactive elements. Such "+-" factors do not make any allowance for errors in the assumptions!

Things About Carbon Dating

There are a few very important things to note about Carbon-14 dating:

• It is used only for dating the remains of living things, not rocks. Other methods are used for rocks.
• It can be used only for fairly "recent" dates, up to 10s of thousands of years. With a half-life of only 5730 years, C-14 would be virtually totally decayed in a sample millions of years old.

Verifying Carbon Dates

We can try to verify the assumptions of carbon dating by using the method to date the remains of living things which died in historical times and places (i.e. we know when it died). Remember, in science, new data can never "prove" a theory right, it can only help us gain confidence in the theory. And, of course, the data can only prove that the theory is wrong!

Because C-14 is used for dating fairly recent dates, there is opportunity to verify dates using samples with known historical dates. There are known conflicts between historical dates and C-14 dates. This conflicts have led to discussions about the amount of C-14 in the atmosphere in times past. Furthermore, there are scientific debates about whether some living things selectively take in more or less C-14 than the ratio in the environment.

Dating Rocks

Rocks are dated using the same concept as carbon dating, but with different radioactive elements. Key facts about dating rocks are:

• Rocks are dated using decay sequences of radioactive elements with very long half-lives. For example, the Uranium sequence (decaying to lead) we saw with nuclear energy has a half-life of billions of years.
• Because of the long time periods involved with dating rocks, the three assumptions become far more difficult to verify.
• Especially with rocks, the third assumption is subject to extensive scientific debate.
  For example, lava flows at Sunset Crater in Arizona are known to be quite recent due to the Indian artifacts in the rocks formed by the eruption. It appears the Indians moved quickly to a safer location about 900 years ago. Tree ring dating places the eruption at 1065AD. Using K/Ar dating (potassium to argon), lava flows have been dated at over 200,000 years. The official explanation for the discrepancy is that there was excess argon in the rock before the eruption. But could this be true of other rocks of unknown age? Other examples lead to this problem:

  When we date rocks of Known Age         When we date rocks of Unknown Age
           |                                       |
  	 |                                       |
  	 V                                       V
  Radioisotope dating does not work       Radioisotope dating assumed to work
  

• At the Grand Canyon, extensive radioisotope dating has been used over the years by different scientists. Different kinds of dating (i.e. checking for different radioactive elements in different minerals) have been used with successive refinements in the mathematics used to compute ages. The mathematical methods are beyond the scope of these notes, but they have caused different dates to be accepted as methods are advanced. The problem is that there are seriously discordant dates. Specific problems include:
  • Some canyon rim basalts with dates of the same mineral ranging from 10 thousand to 117 million years.
  • Lava flows on the rim (know to be in historical times) with dates of 1.34 billion years, older than the rocks buried far below.

  As one scientist said, "...something is going on which is not yet fully understood by creationists or evolutionists."

• With dating rocks, there becomes an implicit fourth assumption:
  4. The earth is at least old enough for the present amount of daughter product (e.g. lead in the Uranium sequence) to have been produced by present decay rates. Thus, radioactive dating depends on an assumption that the earth is old. This means that radioactive dating can be used to help determine how old a particular sample is, but not whether the earth is young or old.

What is Day Zero?

Something interesting to study is the definition of when a rock is "new". Using C-14 dating, the event we are studying, the death of the living thing, is easily understood. With rocks, the zero time is more complicated. The zero time is generally defined as the time that the rock cooled from liquid and thus stopped exchanging various minerals and atoms with the rest of the environment. Scientists say that the when the rock cooled, it became a closed system. Note that this definition contains an assumption that the earth cooled from liquid rock!

The Assumptions

1. While most scientists cautiously accept that half-lives have been constant over time, evidence is emerging that this might not be so. The new evidence is not conclusive, but does open new avenues of research.
2. A number of factors are known to cause daughter products to leak from samples. For example, with Potassium-Argon dating (radioactive potassium decays into argon gas), high temperatures are known to cause the argon to leak out of the rocks. Thus, it is necessary to know the history of the temperatures the rock has been exposed to - a daunting challenge.
3. Other specific minerals are subject to leaching by water.
4. With some decay sequences, there may be large amounts of what is called "common" -- the same atom as the daughter product but not caused by decay -- it was just there at the beginning. Scientists attempt to separate common daughter products from those made from radioactive decay by noting that the two kinds might be more likely to occur in different minerals (mixed with different chemicals). However, this science is complex and far from certain.

Other Issues with Carbon-14

Equilibrium Time

Assuming that at creation (either divine Creation or when the earth cooled after the big bang) there was no C-14, C-14 would gradually build up as it is made by the action of cosmic rays on atmospheric nitrogen. At some point, the amount of C-14 which decays back to nitrogen would equal the amount being made. Initially, evolutionary scientists assumed that the earth had long ago reached equilibrium. However, measurements of samples of known historical dates have indicated that the earth has not reached equilibrium. This means that either the earth is younger than 30,000 years or that other factors have affected the amount of C-14.

Modern Changes in C-14

Several modern phenomena have measurably disrupted the ratio of C-14 in the atmosphere. These include:

• Volcanos have added a large amount of CO2 which is virtually 100% C-12 with no C-14. This lowers the ratio of C-14.
• Large scale burning of fossil fuels adds CO2 from coal and oil which has nearly zero C-14.
• Nuclear testing (mostly from 1945 - 1970) added a lot of C-14 to the atmosphere.

This raises many questions about what might have affected the C-14 to C-12 ratio in the past!

Tree Ring Confirmation

The science of dendrochronology, or studying tree-rings, has been used to try to confirm the C-14 ratio in the past. This is done by examining a certain ring of a tree. We assume that we understand tree rings well enough to know how many years ago that ring formed and then measure the C-14/C-12 ratio in that ring. This technique has been used to learn how the C-14/C-12 ratio has changed over time. However, there are questions about the validity of the technique because of known cases of tree rings not following the usual annual patterns due to extreme environmental factors such as drought or cold.