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Bones are one of the most common materials sent to accelerator mass spectrometry (AMS) labs for radiocarbon dating. This is because bones of animals or humans are often subjects of archaeological studies.
A lot about the prehistoric era has been learned due to archaeological studies and radiocarbon dating of bones. More in-depth information about old civilizations is also available due to radiocarbon dating results on bones.
A bone is 30% organic and 70% inorganic. The organic portion is protein; the inorganic portion is the mineral hydroxyapatite, which is a combination of calcium phosphate, calcium carbonate, calcium fluoride, calcium hydroxide, and citrate. The protein, which is mostly collagen, provides strength and flexibility to the bone whereas the hydroxyapatite gives the bone its rigidity and solid structure.
In theory, both organic and inorganic components can be dated. 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. Hence, these organisms’ radiocarbon age at death is zero.
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.
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. 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: type of contaminant, degree of contamination, and the relative age of the bones and the contaminant.
If limestone has not been removed prior to AMS carbon dating, the radiocarbon age will be much older than the sample’s true age. 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. Depending on the age of the organism that produced the organic acids, the AMS lab’s result might reflect a radiocarbon age younger or older than the bone sample’s true age.
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.
To prevent these inaccuracies, AMS labs perform pretreatment on all bone samples before subjecting them to AMS radiocarbon dating.
Physical pretreatment refers to processes done on the bone samples for carbon dating without using chemicals. Examples of physical pretreatment done on bones in AMS labs are removal of plant rootlets and reduction of sample size by crushing.
AMS lab personnel visually examine bone sample submissions for obvious contaminants.
Rootlets are removed using a pair of tweezers or forceps. A surgical scalpel or a dental grill is used to scrape off contaminated exterior layers of bone samples.
AMS labs would also check the bone’s hardness. Softness indicates the potential absence of collagen, which is needed for AMS carbon 14 dating.
After initial removal of visible contaminants, AMS lab personnel crush bone samples in a mortar and pestle. Size reduction is done to increase the surface area of the sample during succeeding pretreatment methods.
Different AMS labs may have slight variations in the procedure of chemical pretreatment, but they often use the same chemicals in treating bone samples.
The crushed bone sample is washed with dilute, cold hydrochloric acid (HCl) repeatedly until hydroxyapatite is eliminated and the collagen is isolated. Rootlets, if present, are further removed from the collagen.
To ensure the complete removal of organic acids, collagen is washed with an alkali solution, usually sodium hydroxide (NaOH). AMS labs, however, skip alkali washing when the collagen sample is not well preserved and the washing may remove the remaining organic materials that can still be dated.