Reliability of Geologic Dating
in fact, all external forces. The basis for all radioactive dating methods is this constancy of decay rate. "for his method to use Carbon for age determinations in archaeology, geology, geophysics, and other Image removed due to copyright restrictions. Each result was within the statistical range of the true historic date. presently employed in the Space & Atmospheric Sciences Group at the Los Alamos Radiometric dating--the process of determining the age of rocks from the universe is so large, images from distant events take a long time to get to us. He is presently employed in the Space & Atmospheric Sciences Group at the Los Radiometric dating--the process of determining the age of rocks from the decay Because God's universe is so large, images from distant events take a long.
In an appendix to this article, Wiens addresses and responds to a number of specific creationist criticisms. Here is a condensed summary of these items, quoted from Wiens' article [ Wiens ]: Radiometric dating is based on index fossils whose dates were assigned long before radioactivity was discovered. This is not at all true, though it is implied by some young-Earth literature. Radiometric dating is based on the half-lives of the radioactive isotopes.
These half-lives have been measured over the last years. They are not calibrated by fossils. No one has measured the decay rates directly; we only know them from inference. Decay rates have been directly measured over the last years.
In some cases a batch of the pure parent material is weighed and then set aside for a long time and then the resulting daughter material is weighed. In many cases it is easier to detect radioactive decays by the energy burst that each decay gives off. For this a batch of the pure parent material is carefully weighed and then put in front of a Geiger counter or gamma-ray detector.
These instruments count the number of decays over a long time. If the half-lives are billions of years, it is impossible to determine them from measuring over just a few years or decades. The example given in the section [in Wiens' article] titled, "The Radiometric Clocks" shows that an accurate determination of the half-life is easily achieved by direct counting of decays over a decade or shorter. Additionally, lavas of historically known ages have been correctly dated even using methods with long half-lives.
The decay rates are poorly known, so the dates are inaccurate. Most of the decay rates used for dating rocks are known to within two percent. Such small uncertainties are no reason to dismiss radiometric dating. Whether a rock is million years or million years old does not make a great deal of difference. To date a rock one must know the original amount of the parent element. But there is no way to measure how much parent element was originally there.
It is very easy to calculate the original parent abundance, but that information is not needed to date the rock. All of the dating schemes work from knowing the present abundances of the parent and daughter isotopes.
There is little or no way to tell how much of the decay product, that is, the daughter isotope, was originally in the rock, leading to anomalously old ages. A good part of [Wiens' article] is devoted to explaining how one can tell how much of a given element or isotope was originally present.
Usually it involves using more than one sample from a given rock. It is done by comparing the ratios of parent and daughter isotopes relative to a stable isotope for samples with different relative amounts of the parent isotope. From this one can determine how much of the daughter isotope would be present if there had been no parent isotope. This is the same as the initial amount it would not change if there were no parent isotope to decay.
Figures 4 and 5 [in Wiens' article], and the accompanying explanation, tell how this is done most of the time. There are only a few different dating methods. There are actually many more methods out there. Well over forty different radiometric dating methods are in use, and a number of non-radiogenic methods not even mentioned here. A young-Earth research group reported that they sent a rock erupted in from Mount Saint Helens volcano to a dating lab and got back a potassium-argon age of several million years.
This shows we should not trust radiometric dating. There are indeed ways to "trick" radiometric dating if a single dating method is improperly used on a sample. Anyone can move the hands on a clock and get the wrong time.
Likewise, people actively looking for incorrect radiometric dates can in fact get them. Geologists have known for over forty years that the potassium-argon method cannot be used on rocks only twenty to thirty years old. Publicizing this incorrect age as a completely new finding was inappropriate.
The reasons are discussed in the Potassium-Argon Dating section [of Wiens' article]. The other three, Carbon, beryllium, and chlorine are produced by cosmic rays--high energy particles and photons in space--as they hit the Earth's upper atmosphere.
Very small amounts of each of these isotopes are present in the air we breathe and the water we drink. As a result, living things, both plants and animals, ingest very small amounts of carbon, and lake and sea sediments take up small amounts of beryllium and chlorine The cosmogenic dating clocks work somewhat differently than the others. Carbon in particular is used to date material such as bones, wood, cloth, paper, and other dead tissue from either plants or animals.
To a rough approximation, the ratio of carbon to the stable isotopes, carbon and carbon, is relatively constant in the atmosphere and living organisms, and has been well calibrated. Once a living thing dies, it no longer takes in carbon from food or air, and the amount of carbon starts to drop with time. Since the half-life of carbon is less than 6, years, it can only be used for dating material less than about 45, years old.
Dinosaur bones do not have carbon unless contaminatedas the dinosaurs became extinct over 60 million years ago. But some other animals that are now extinct, such as North American mammoths, can be dated by carbon Also, some materials from prehistoric times, as well as Biblical events, can be dated by carbon The carbon dates have been carefully cross-checked with non-radiometric age indicators.
For example growth rings in trees, if counted carefully, are a reliable way to determine the age of a tree. Each growth ring only collects carbon from the air and nutrients during the year it is made.
Clocks in the Rocks
To calibrate carbon, one can analyze carbon from the center several rings of a tree, and then count the rings inward from the living portion to determine the actual age. This has been done for the "Methuselah of trees", the bristlecone pine trees, which grow very slowly and live up to 6, years. Scientists have extended this calibration even further. These trees grow in a very dry region near the California-Nevada border. Dead trees in this dry climate take many thousands of years to decay.
Growth ring patterns based on wet and dry years can be correlated between living and long dead trees, extending the continuous ring count back to 11, years ago.
An effort is presently underway to bridge the gaps so as to have a reliable, continuous record significantly farther back in time. The study of tree rings and the ages they give is called "dendrochronology". Tree rings do not provide continuous chronologies beyond 11, years ago because a rather abrupt change in climate took place at that time, which was the end of the last ice age.
During the ice age, long-lived trees grew in different areas than they do now. There are many indicators, some to be mentioned below, that show exactly how the climate changed at the end of the last ice age.
It is difficult to find continuous tree ring records through this period of rapid climate change. Dendrochronology will probably eventually find reliable tree records that bridge this time period, but in the meantime, the carbon ages have been calibrated farther back in time by other means. Calibration of carbon back to almost 50, years ago has been done in several ways.
One way is to find yearly layers that are produced over longer periods of time than tree rings. In some lakes or bays where underwater sedimentation occurs at a relatively rapid rate, the sediments have seasonal patterns, so each year produces a distinct layer.
Such sediment layers are called "varves", and are described in more detail below. Varve layers can be counted just like tree rings. If layers contain dead plant material, they can be used to calibrate the carbon ages. Another way to calibrate carbon farther back in time is to find recently-formed carbonate deposits and cross-calibrate the carbon in them with another short-lived radioactive isotope. Where do we find recently-formed carbonate deposits?
If you have ever taken a tour of a cave and seen water dripping from stalactites on the ceiling to stalagmites on the floor of the cave, you have seen carbonate deposits being formed. Since most cave formations have formed relatively recently, formations such as stalactites and stalagmites have been quite useful in cross-calibrating the carbon record.
If one predicts a carbon age assuming that the ratio of carbon to carbon in the air has stayed constant, there is a slight error because this ratio has changed slightly. Figure 9 shows that the carbon fraction in the air has decreased over the last 40, years by about a factor of two. This is attributed to a strengthening of the Earth's magnetic field during this time.
A stronger magnetic field shields the upper atmosphere better from charged cosmic rays, resulting in less carbon production now than in the past.
Changes in the Earth's magnetic field are well documented. Complete reversals of the north and south magnetic poles have occurred many times over geologic history. A small amount of data beyond 40, years not shown in Fig. What change does this have on uncalibrated carbon ages? The bottom panel of Figure 9 shows the amount Figure 9.
Clocks in the Rocks
Ratio of atmospheric carbon to carbon, relative to the present-day value top panel. The bottom panel shows the offset in uncalibrated ages caused by this change in atmospheric composition. Tree-ring data are from Stuiver et al. The offset is generally less than years over the last 10, years, but grows to about 6, years at 40, years before present.
Uncalibrated radiocarbon ages underestimate the actual ages. Note that a factor of two difference in the atmospheric carbon ratio, as shown in the top panel of Figure 9, does not translate to a factor of two offset in the age.
Rather, the offset is equal to one half-life, or 5, years for carbon The initial portion of the calibration curve in Figure 9 has been widely available and well accepted for some time, so reported radiocarbon dates for ages up to 11, years generally give the calibrated ages unless otherwise stated.
The calibration curve over the portions extending to 40, years is relatively recent, but should become widely adopted as well. These methods may work on young samples, for example, if there is a relatively high concentration of the parent isotope in the sample. In that case, sufficient daughter isotope amounts are produced in a relatively short time. As an example, an article in Science magazine vol.
There are other ways to date some geologically young samples. Besides the cosmogenic radionuclides discussed above, there is one other class of short-lived radionuclides on Earth. These are ones produced by decay of the long-lived radionuclides given in the upper part of Table 1. As mentioned in the Uranium-Lead section, uranium does not decay immediately to a stable isotope, but decays through a number of shorter-lived radioisotopes until it ends up as lead.
While the uranium-lead system can measure intervals in the millions of years generally without problems from the intermediate isotopes, those intermediate isotopes with the longest half-lives span long enough time intervals for dating events less than several hundred thousand years ago.
Note that these intervals are well under a tenth of a percent of the half-lives of the long-lived parent uranium and thorium isotopes discussed earlier. Two of the most frequently-used of these "uranium-series" systems are uranium and thorium These are listed as the last two entries in Table 1, and are illustrated in Figure A schematic representation of the uranium decay chain, showing the longest-lived nuclides.
Half-lives are given in each box. The radiocarbon dating method is based on the rate of decay of the radioactive or unstable 14C which is formed in the upper atmosphere through the effect of cosmic ray neutrons upon nitrogen The reaction is as follows: The carbon dioxide mixes throughout the atmosphere, dissolves in the oceans, and via photosynthesis enters the food chain to become part of all plants and animals.
In principle the uptake rate of 14C by animals is in equilibrium with the atmosphere. As soon as a plant or animal dies, they stop the metabolic function of carbon uptake and with no replenishment of radioactive carbon, the amount of 14C in their tissues starts to reduce as the 14C atoms decay. The half-life of 14C Libby and his colleagues first discovered that this decay occurs at a constant rate.
They found that after years, half the 14C in the original sample will have decayed and after another years, half of that remaining material will have decayed, and so on. This became known as the Libby half-life.
After 10 half-lives, there is a very small amount of radioactive carbon present in a sample.