However, the open lattice structure of the hydroxyapatite makes it highly contaminated with carbonates from ground water. Removal of carbonate contaminants through dilute acid washing is also not applicable because hydroxyapatite is acid soluble. Laboratories use the protein component of bone samples in AMS dating because it is relatively acid insoluble and, therefore, can be easily isolated from the hydroxyapatite component and other carbonates. In cases when the protein portion of the bone sample is not well preserved and have already degraded due to warm conditions and fungal or bacterial attack, AMS dating labs carbon date individual amino acids to check if several of them give the same radiocarbon age.
This process is doable in AMS dating labs because only small samples are required. However, this process is costly and time consuming. Radiocarbon dating individual amino acids is not recommended unless necessary as in the case of old bone samples where the presence of even small levels of contaminants produce a large error. The time-width of any given sample reflects the total growth of the original organism and the span of time that organism interacted with the biosphere. For most organisms that have bones, the time of their death is contemporaneous with their cessation of exchange with the biosphere.
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Radiocarbon dating results on bones need not be subjected to an age offset but bone samples have time-width. Literature suggests that a bone does not cease to assimilate carbon from the biosphere until death; there is a turnover time of about 30 years for human bone and a shorter period for animal bone. Time-width data is necessary because they affect calibration of radiocarbon results and, consequently, the way radiocarbon age is converted to calendar years.
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Any carbon-containing material that may affect the carbon 14 content of bones is considered a contaminant. Considering that bones are often found surrounded by different kinds of organic matter, bones are arguably one of the most highly contaminated samples submitted to AMS labs for radiocarbon dating. The common contaminants are humic and fulvic acids, which are organic acids present in soil that are produced by the microbial degradation of plant or animal tissues.
According to literature, other organic compounds that can contaminate bone samples are polyphenols, polysaccharides, lignins, and degraded collagen.
Radiocarbon Dating Bones
Depending on the location of the excavation, bones can also be contaminated by limestone. These contaminants are considered natural because they came in contact with the bones due to natural occurrences. Artificial contaminants, on the other hand, are those that were introduced by man during the collection, conservation, or packaging of the bone samples.
When bones are applied with animal glue during labeling, a contaminant has already been introduced to the sample. This is because animal glue is chemically identical to the bone sample. AMS lab results with this sample will be inaccurate. Other potential contaminants that can be introduced to bone samples after excavation include biocides, polyvinyl acetate and polyethylene glycol conservation chemicals , cigarette ash, and labels or wrappers that are made of paper.
The effect of contamination on bone samples that were subjected to AMS dating is dependent on these factors: Limestone is of geological origin and will therefore be much older than any archaeological samples. The presence of humic and fulvic acids during AMS radiocarbon dating will lead to inaccurate results as well.
Dating history
Bones can also be exposed to modern sources of carbon due to plant rootlet intrusions. Modern sources of carbon can make the AMS carbon dating result of a bone younger than its true age. In general, infinite-age contaminants add considerable number of years to the true age of a bone sample, making it older than it is. Modern carbon, on the other hand, makes the bone sample significantly younger than its true age.
The diminishing levels via decay means that the effective limit for using c14 to estimate time is about 50, years. After this time, there is little if any c14 left.
However, to avoid confusion all radiocarbon laboratories continue to use the half-life calculated by Libby, sometimes rounding it to years. Any organic material that is available in sufficient quantity can be prepared for radiocarbon dating. Modern AMS accelerator mass spectroscopy methods require tiny amounts, about 50 mg. AMS technology has allowed us to date very small samples such as seeds that were previously undatable. Since there are practical limits to the age range of the method, most samples must be younger than 50, years and older than years.
Most samples require chemical pre-treatment to ensure their purity or to recover particular components of the material. The objective of pre-treatment is to ensure that the carbon being analyzed is native to the sample submitted for dating. Pre-treatment seeks to remove from the sample any contaminating carbon that could yield an inaccurate date. Acids may be used to eliminate contaminating carbonates. Bases may be used to remove contaminating humic acids. Some types of samples require more extensive pre-treatment than others, and these methods have evolved over the first 50 years of radiocarbon dating.
For example, it was once standard practice to simply burn whole bones, but the results were eventually seen to be unreliable.
Chemical methods for separating the organic collagen from the inorganic apatite components of bone created the opportunity to date both components and compare the results. The collagen fraction usually yields more reliable dates than the apatite fraction see Dates on bones. In addition to various pre-treatments, the sample must be burned and converted to a form suitable for the counter. The sample must be destroyed in order to measure its c14 content. The first measurements of radiocarbon were made in screen-walled Geiger counters with the sample prepared for measurement in a solid form.
These so-called "solid-carbon" dates were soon found to yield ages somewhat younger than expected, and there were many other technical problems associated with sample preparation and the operation of the counters.
C14 Dating Techniques :: Coastal Systems Group
Gas proportional counters soon replaced the solid-carbon method in all laboratories, with the samples being converted to gases such as carbon dioxide, carbon disulfide, methane, or acetylene. Many laboratories now use liquid scintillation counters with the samples being converted to benzene. All of these counter types measure the C content by monitering the rate of decay per unit time. A more recent innovation is the direct counting of c14 atoms by accelerator mass spectrometers AMS. The sample is converted to graphite and mounted in an ion source from which it is sputtered and accelerated through a magnetic field.
Targets tuned to different atomic weights count the number of c12, c13, and c 14 atoms in a sample. Many samples reported as "modern" have levels of radioactivity that are indistinguishable from modern standards such as oxalic acid. Due to contamination from bomb testing, some samples are even more radioactive than the modern standards. Other very young samples may be given maximum limits, such as 40, years. The very old samples have such low radioactivity that they cannot be distinguished reliably from the background radiation. Very few laboratories are able to measure ages of more than 40, years.
Several aspects of radiocarbon measurement have built-in uncertainties. Every laboratory must factor out background radiation that varies geographically and through time. The variation in background radiation is monitered by routinely measuring standards such as anthracite coal , oxalic acid, and certain materials of well-known age. The standards offer a basis for interpreting the radioactivity of the unknown sample, but there is always a degree of uncertainty in any measurement. Since decay-counting records random events per unit time, uncertainty is an inherent aspect of the method.
Most laboratories consider only the counting statistics, i.
However, some laboratories factor in other variables such as the uncertainty in the measurement of the half-life. Some laboratories impose a minimum value on their error terms. Most laboratories use a 2-sigma criterion to establish minimum and maximum ages.
How Does Radiocarbon-14 Dating Work?
In keeping with its practice of quoting 2-sigma errors for so-called finite dates, the Geological Survey of Canada uses a 4-sigma criterion for non-finite dates. The first radiocarbon dates reported had their ages calculated to the nearest year, expressed in years before present BP. It was soon apparent that the meaning of BP would change every year and that one would need to know the date of the analysis in order to understand the age of the sample.