AMS Carbon Dating of Shells

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• Shells are not easy to radiocarbon date; there are many factors that could affect its true age.
• Shells are composed of the minerals aragonite and calcite.
• Age offset is necessary in radiocarbon dating results of shells due to marine effect and hard water effect.
• The most common contaminants of shells are caused by isotopic exchange and recrystallization.
• Pretreatment of shells for radiocarbon dating involves isolation of aragonite through physical and chemical methods.

Shells are often sent to accelerator mass spectrometry (AMS) labs for radiocarbon dating. A great proportion of shell materials sent to AMS labs for carbon 14 dating are mollusk shells.

Shells are not easy to radiocarbon date; there are many factors that contribute uncertainties to the results. American physical chemist Willard Libby, a pioneer of the radiocarbon dating technology, predicted shells to be the least effective materials to radiocarbon date.

Shells can be categorized as marine, estuarine, or riverine. Analysts in AMS labs need to know the type of shell they are dealing with in order to establish the potential contaminants and determine the methods to remove them.

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Sample size required: 20-50 grams (AMS), 50 grams (radiometric);
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Components of a Shell

Shellfish obtain carbon from the biosphere for shell building. According to scientific studies, shellfish obtain organic carbon from marine or terrestrial plants and inorganic carbon from ocean water bicarbonate, atmospheric carbon dioxide, or freshwater bicarbonate.

Shells are formed by the deposition of calcium carbonate crystals to an organic matrix, which is a protein called conchiolin. This protein makes up only a few percent of the shell, hence the sample needed in the radiocarbon dating process is the inorganic portion.

Although inorganic, the carbonate is still datable since its formation involves incorporation of carbon 14 from the biosphere. The carbonate present in shells is usually in the form of the mineral aragonite although some shells are mixtures of aragonite and calcite while others, like oyster shell, is mostly calcite.

The use of the shell’s carbonate component presents problems because the substance is soluble and may isotopically or chemically exchange with its environment. When a shell exchanges carbon with soil acids around it, the shell’s carbon 14 ratio, and thus radiocarbon age, is altered. This carbon exchange usually affects the shell’s exterior.

Recrystallization, on the other hand, can affect even the inner layers of a shell. This phenomenon, accompanied by the conversion of aragonite to calcite, also alters the carbon 14 ratio. Recrystallization usually occurs when the shell exchanges carbon with modern calcite.

Reservoir Effect on Shells

There are two source or reservoir effects relevant to the radiocarbon dating of shells—the marine effect and the hard water effect. Age offset evaluation must be done on radiocarbon dating results on shells because of these effects.

The marine effect is a consequence of the slow mixing between surface waters and deep waters in the oceans. The rapid exchange of carbon between the atmosphere and the biosphere via the carbon dioxide pathway is not exactly the same between the atmosphere and the oceans.

Carbon dioxide equilibrium between the atmosphere and surface waters is achieved relatively quickly. Surface waters, however, exchange carbon dioxide with deeper waters in a rate that is so slow, the carbon 14 content of the incoming carbon dioxide from the surface water and the outgoing carbon dioxide from the deep waters may already be in different stages of radiocarbon decay. Studies show that the residence time of carbon 14 in the atmosphere ranges between 6 years and 10 years while the residence time of carbon 14 in oceans could take thousands of years. 

Upwelling is another phenomenon that dilutes radiocarbon content of surface waters. In certain parts of the globe, the equatorial region in particular, deep waters move up. This phenomenon is latitude dependent and occurs as a consequence of trade winds. Coastline shape, local climate and wind, and ocean bottom topography also contribute to upwelling. The slow mixing and the upwelling of deep waters mean that the surface water of oceans already have apparent radiocarbon age relative to the atmosphere.

Freshwater shells may not be affected by the marine effect, but they are susceptible to the hard water effect—the presence of calcium ions resulting from the dissolution of infinite-age calcium carbonate. The presence of calcium ions coincides with carbon 14 depletion although the magnitude of the hard water effect is not directly correlated with the amount of calcium ions. The hard water effect can account for discrepancies of several centuries in radiocarbon dating results.

Hard water effect can also affect marine shells deposited in areas where there is an influx of carbonate-rich freshwater like in river mouths. Terrestrial shells, like snail shell, are also affected by the hard water effect in cases when the organism has been feeding on carbonate-rich areas like a chalkland.

AMS lab analysts must know the reservoir effects that could affect any given shell sample so they will know the age offsets needed. AMS labs quantify the marine and hard water reservoir effects by assuming there has been no change in radiocarbon content and by dating known-age shells of the same species from the same locality that have been collected before the nuclear weapons testing of the 1950s and 1960s.

Contamination in Shells

The local environment of an organism assimilating the carbon is one of the factors to be considered before subjecting the sample to accelerator mass spectrometry radiocarbon dating. AMS lab analysts must know the types of contaminants the shell samples could have been exposed to.

Any carbon-containing substance that can change the carbon 14 content of a shell sample upon contact is a contaminant. This means that calcium carbonate, soil humic materials, and soil carbon dioxide are potential contaminants. The most common contaminants of shell samples for radiocarbon dating are those that are caused by isotopic exchange and recrystallization.

AMS labs perform pretreatment before carbon 14 dating to remove all possible contaminants that would lead to inaccurate results.

Pretreatment of Shells in AMS Labs

Physical pretreatment of shells before carbon dating involves removal of all visible contaminants from the shells without using any chemicals as well as reduction of the sample size.

The outer layer of the shell is removed with a drill and carborundum paper to isolate aragonite—the analyte for AMS radiocarbon dating. Calcite that has recrystallized, and therefore a contaminant, is white and chalky and easily removed by drilling.

AMS lab analysts crush the shell samples in a mortar and pestle to increase the surface area before further pretreatment is done.

Chemical pretreatment employed by AMS labs involve washing the shells with dilute acid, usually hydrochloric acid (HCl), to remove a portion of the shell’s exterior and calcite components.