The method used for carbon dating was developed by W. Libby in about 1946.
The basis for Carbon 14 dating is that all living things constantly take in carbon - all the isotopes equally for what is present in the area. When an organism dies, it stops taking in carbon. One isotope of carbon is radioactive, that of carbon 14 (often called radiocarbon).
Carbon has three isotopes (and amount present in nature):
- Carbon 12: 98.89%
- Carbon 13: 1.11%
- Carbon 14: 0.00000000010%
Carbon 14 is formed when a neutron is ejected from an atom after being struck by a cosmic ray releaseing a free neutron (this is a
secondary cosmic ray). This free neutron then is captured by a a
nitrogen atom (7 protons, 7 neutrons) and becomes carbon 14 (6 protons, 8 neutrons) and a proton (hydrogen atom - 1 proton, 0 neutrons). This most often happens in the upper
atmosphere at which point the carbon 14 atom is
oxidized and to become
carbon dioxide.
Plants, constantly taking in carbon dioxide remain in balance with the ratios of carbon isotopes found in nature. Things that eat plants likewise remain in balance, as do those that eat the things that eat the plants - given that no one eats organic material that has been dead for a significant period of time (a couple of decades there about).
As carbon 14 exists, it slowly decays. The half life of carbon 14 is 5568 years. That is, if you have 100 grams of carbon 14 today, in 5568 years you will have 50 grams of carbon 14. Carbon 14 decays by emitting a weak beta particle (electron) turning it back to Nitrogen 14.
To date something, compare of the amount of existing carbon 14 that is in a sample to the amount of carbon 12. The first step in this process is to measure the amount of each isotope of carbon in the material. This can be done as either as Mass Spectrometry (very big and expensive though works with very small samples) or the more common measuring of the activity of a sample compared to a standard.
Once a the ratio of carbon 14 to carbon 12 and 13 is known in a sample it is compared to the activity in a modern sample. This is most often done against Oxalic Acid I or Oxalic Acid II.
The modern calibration is based upon a sample of Oxalic Acid I which was made from a batch in 1955 and equal to the measured activity of a 1890 sample of wood as it would be in 1950. 1950 was chosen to honor the publication of the first radiocarbon dates calculated in December 1949. This batch was actually prepared from 1955 sugar beet crop.
Unfortunately, only 1000 pounds of the original Oxalic Acid I stock was made. In the 1970s this supply began to become scarce (today, labs are very careful with the Oxalic Acid I that remains). The Oxalic Acid II batch was made from a 1977 French beet molasses. The Oxalic Acid II sample is 1.2933 +/- 0.001 times as active as the Oxalic Acid I sample.
Other standards exist such as the ANU (Australian National University) sucrose standard. Many laboratories have their own standards, however, all reports are given against NBS Oxalic acid or a sub standard related to it.
When to measure a a sample by radioactivity three items are tested.
- The unknown
- The standard
- A geologic sample
The geologic sample is some ancient sample of carbon, most often coal,
limestone, or marble. Measuring the 'activity' of this sample gives
an indication of the background activity. This amount is deducted from
the standard and the unknown.
The age of a sample (termed conventional radiocarbon age) is based off
of several facts, conventions and assumptions:
- carbon 14 has a half life of 5568 years
- A proper standard is measured and used
- The preference for carbon 13 over carbon 14 by photosynthesis and
other forms of fractionation
- The assumption that all C14 reservoirs have remained constant over time
So, over a period of time, the number of beta decay events is recorded.
The time from the present is (conventional radiocarbon age):
t = -8033 ln(Asn/Aon)
Where t is in years, -8033 represents the mean lifetime of a carbon
14 sample, Aon is the activity in counts per minute of the modern
standard,
and Asn is the counts per minute in the sample.
When reporting age, if the sample is younger than 200 years it is
"modern" and if it is more
recent than 1950 it is ">modern". If the sample is older than 50,000
years it is indistinguishable from the background radiation.
Age range date rounded to Error (+/-)
--------------- --------------- -------------
0 - 1,000 nearest 10 nearest 5 up
1,000 - 10,000 nearest 10 nearest 10 up
10,000 - 25,000 nearest 50 nearest 10 up
25,000 - 50,000 nearest 100 nearest 50 up
Granted, this is the "simple" error reporting. The methods for counting beta decay have different error rates, there is various contributing errors that add to this. It is
impossible in this scope of this writeup to list all of the laboratories, their methods and the errors associated with them.
Common things that are carbon-14 dated are:
Humans inadvertently have modified the ratios of carbon 14 to the stable
carbon isotopes. There are two main sources of this:
- Industrial effect
- With the use of fossil fuels large amounts of carbon dioxide has been emitted into the atmosphere. The carbon in these fuels are of "geologic age" and have almost no carbon 14 in them. Thus, the amount of carbon 14 has been reduced by about 2%. This was discovered by Hans Suess in 1955 and is often named after him.
- Atom bomb
-
Another "glitch" that has occurred in the past 100 years is that of thermonuclear bombs. With the large number of neutrons that these bombs produce it has almost doubled the amount of carbon 14. This was discovered by De Vries in 1958. The amount of "artifical" carbon 14 in the northern hemisphere peaked in 1963 at twice normal levels. Since then, it has slowly dropped as it has been absorbed into the biosphere.
This peak has also allowed scientists to track the distribution and movement of carbon in the biosphere - the sliver lining of nuclear tests. One program to use this is GEOSECS (geochemical Ocean Section Study) which has used the bomb carbon 14 to track the ocean currents.
http://www.c14dating.com/
Brief time working in a radiocarbon lab