The basis of radiocarbon dating includes the assumption that there is a constant level of carbon 14 in the atmosphere and therefore in all living organisms through equilibrium. Carbon 14 is a naturally occurring isotope of the element carbon and is called radiocarbon because it is unstable and weakly radioactive.
Another characteristic of carbon 14 is that it is continually being formed in the upper atmosphere as a product of the reaction between neutrons produced by cosmic rays and nitrogen atoms. These carbon 14 atoms then instantaneously react with oxygen present in the atmosphere to form carbon dioxide. The carbon dioxide formed with carbon 14 is indistinguishable from the carbon dioxide with the other carbon isotopes; hence the pathway of carbon 14 into the ocean, plants, and other living organisms is the same as that of carbon 12 and carbon 13.
It is also assumed that there is equilibrium between carbon 14 formation and its decay, thus there is a constant level of carbon 14 in the atmosphere at any given time in the past up to the present.
The assumptions, however, do not paint the real picture. There are several factors that need to be considered because they affect the global concentration of carbon 14 and therefore that of any given sample for radiocarbon dating.
The atmosphere, oceans, and biosphere are radiocarbon reservoirs of varying concentrations. Radiocarbon formed in the atmosphere is dissolved in oceans in the form of carbon dioxide and contemporaneously assimilated by plants through photosynthesis and enters food chains. This is how terrestrial organisms take in carbon 14 in their systems.
Marine organisms and those who consume them take in carbon 14 from the exchange process of carbon 14 (in the form of carbon dioxide) in the atmosphere and the ocean or any body of water. However, carbon 14 content is not the same at the surface mixing layers and that in the deep ocean; hence, not all marine organisms have the same radiocarbon content.
There are many factors to consider when measuring the radiocarbon content of a given sample, one of which is the radiocarbon content of the plant or animal source when it was alive and its local environment.
This is especially true when comparing samples from terrestrial organisms and those that assimilated radiocarbon from the marine environment. Even if the organisms have the same age, they wouldn’t have the same carbon 14 content and would thus appear to be of different radiocarbon age.
Oceans are large carbon 14 reservoirs. Surfaces of oceans and other bodies of water have two sources of radiocarbon – atmospheric carbon dioxide and the deep ocean. Deep waters in oceans get carbon 14 from mixing with the surface waters as well as from the radioactive decay already occurring at their levels. Studies show that equilibration of carbon dioxide (with carbon 14) in surface water is of the order of 10 years. The degree of equilibration of carbon dioxide in deep water remains unknown.
Radiocarbon dates of a terrestrial and marine organism of equivalent age have a difference of about 400 radiocarbon years. Terrestrial organisms like trees primarily get carbon 14 from atmospheric carbon dioxide but marine organisms do not. Samples from marine organisms like shells, whales, and seals appear much older.
Another factor to consider is that the magnitude of the marine reservoir effect is not the same in all locations. The mixing of deep waters upward with surface waters—in a phenomenon known as upwelling—is latitude dependent and occurs predominantly in the equatorial region. Coastline shape, local climate and wind, trade winds, and ocean bottom topography also affect upwelling.
According to a study published in 1972 by J. Mangerud, global variation in marine radiocarbon reservoir effect evident in shell carbonates are due to the incomplete mixing of upwelling water of “old” inorganic carbonates from the deep ocean where long residence times of more than 1,000 years cause depletion of carbon 14 activity through radioactive decay, resulting in very old apparent carbon 14 age.
There are three methods used in determining regional differences in marine radiocarbon reservoir effect, as listed by Sean Ulm in a report dated December 2006:
Terrestrial and marine samples cannot be compared or associated without accounting for the marine radiocarbon reservoir effect. Correction factors for different oceans in the world are found in an online database, the Marine Reservoir Correction Database, funded in part by the Institute for Aegean Prehistory. Actual correction varies with location due to complexities in ocean circulation.
The database is also intended for use with radiocarbon calibration programs such as CALIB (Stuiver and Reimer, 1993) or OxCal (Bronk Ramsey 1995) using the 2013 marine calibration dataset.
The Delta±R value is only used for marine carbonates.
Depending on the age of the marine carbonate, a 200- to 500-year correction (i.e. global marine reservoir correction) is applied automatically for all marine carbonates. This automatic correction means the radiocarbon date gets more recent in time due to the fact that it takes 200-500 years for present-day carbon dioxide in the atmosphere to be incorporated and distributed (equilibrated) through the ocean water column.
A Delta±R correction is applied to the sample that has already been corrected with the global marine reservoir correction. The value that is provided by the client is subtracted or added to this already corrected age (depending if it is a Delta+R or Delta–R value). Note: A negative Delta-R will make the date older (typically presuming freshwater dilution from the global marine average).
Sample reports below show the difference between a radiocarbon date of 1000 +/-30 BP with a Delta R of 0+/-0 (i.e. with just the global marine reservoir correction) and a 1000 +/-30BP radiocarbon date with a Delta R of 222+/-35 and the global marine reservoir correction included.
Freshwater systems running through limestone or fed by old water from springs can lead to falsely old ages in carbonate AMS dates. The dissolved inorganic carbon (DIC) used by the individuals to form their shells or in the precipitation of carbonate concretions will be older than the time of formation due to old DIC from the limestone. This is termed “hard water effect” when the effect is from limestone. Aquatic systems fed by old water will have old DIC associated with that water and the same effect can be observed. Both phenomena can be classified as “reservoir effect”.
The best way to know the reservoir offset is to analyze organic materials in association with the shells which are not subject to the effect. Most commonly charcoal or seeds found in very close association with the carbonate are used to compare the Carbon-14 ages and use the difference to correct the shells.
If the researcher is not aware of any offset, the lab recommends doing a literature search and to understand the geologic systems supplying the water to the site.
Out of all shell species that have been radiocarbon dated over the years, mollusk shells have been the species tested the most. These shells both have inorganic and organic components. Conchiolin, the organic component, makes only a minute portion of the whole sample. Thus radiocarbon measurements are usually applied on the inorganic component, which is calcium carbonate.
Radiocarbon dating of shell carbonates pose many problems. Carbonates are quite soluble and chemically interact with the environment so accuracy of the carbon 14 dating results cannot be guaranteed. Results should also account for marine radiocarbon reservoir effects as well as hard water effects.