About C14 dating

Nov 2016
550
Germany
#1
Here is an overview of the basics and problems of C14 dating, which required additional calibration using dendrochronological methods from the 1960s.

The three isotopes of carbon, C12, C13 and C14, have an equal number of protons (6) in the atomic nucleus, but different numbers of neutrons, which are recognizable in the isotope name. Unlike the stable isotopes C12 and C13, the C14 atom is unstable (radioactive, therefore "radiocarbon"), it disintegrates into a nitrogen atom, an electron and an antineutrino in a random process. The half-life of C14 is approx. 5730 years, i.e. a large quantity of C14 atoms - without external supply of new C14 atoms - is reduced to 50% after approx. 5730 years, to 25% after a further 5730 years and to 0.1% of the original quantity after approx. 5730 years, i.e. 10 half-lives. Therefore, 60,000 years are the absolute time horizon for performing a C14 dating.

At the beginning of the C14 formation process is cosmic radiation. Most of it originates in the sun, the higher-energy remainder comes from other stars, supernovae, quasars, etc. When this radiation hits the upper layers of the Earth's atmosphere, neutrons are released at an altitude of 15 kilometres and mainly in the polar regions by fission, which, when they hit the most common atmospheric isotope, nitrogen N14 (7 protons in the nucleus), result in the transformation of a proton into a neutron, turning the nitrogen into a carbon atom with 6 protons and 8 neutrons. This radioactive C14 atom is also called ´cosmogenic radionuclide´.

At any given time, all earthly carbon reservoirs contain a total of approx. 62 tons of C14.

The share of C14 in the total carbon balance of the atmosphere is about 10 high-minus 10 %, i.e. only the billionth part of the C12 content. The compound of C14 with oxygen O2 produces carbon dioxide CO2, which mixes with the carbon dioxide produced from the compounds of C12 and C13 with O2. The carbon dioxide containing C12/C14 (and C13) is absorbed by plants that process C in the photosynthetic process to build up carbohydrates and excrete O2 again. In this way, C14 enters the biological cycle via the food chain - in addition to inhalation - and becomes a chemical component of all earthly organisms, including marine organisms, since carbon dioxide is also absorbed by the water reservoirs, whereby salt water (oceans) has a 10 times higher absorption capacity for CO2 than fresh water (rivers) and therefore contains more C14.

In the context of further argumentation, the carbon ratios in the oceans are of utmost importance. The oceans contain about 85% of the total earthly carbon (= 42 x 10 to the power of 15 kg), of which 82% is deep water and 3% is surface water ("oceanic surface layer", approx. 50-100 m thick). (By comparison, the atmosphere contains only about 2% of global carbon). It should be noted that deep oceanic water, by far the largest carbon reservoir on earth, has a significantly lower concentration of C14 (in relation to C12) than all other reservoirs (surface water, atmosphere, humus, biosphere).

Between atmosphere and oceanic surface layer an exchange (diffusion) of CO2 takes place continuously and globally in both directions, depending on whether the CO2 partial pressure predominates in the atmosphere or in the ocean (pressure difference). These processes differ very strongly locally in terms of direction and quantity. One reason for this is that hot water releases CO2 into the atmosphere due to a higher partial pressure of CO2, while cold water absorbs CO2 from the atmosphere due to a lower pressure. Irregularities in the CO2 circulation between atmosphere and ocean can also occur by feeding fresh water with little or no C14 into the oceans, e.g. by glacier melt.

Even more important, because it is much more frequent, are diffusion irregularities due to the complex oceanic flow behaviour, which locally transports large quantities of C14-poor or -free deep water to the surface, whereby according to C14-poor or - free CO2 quantities are diffused into the atmosphere, which leads to a local and temporary reduction of the atmospheric C14 concentration.

Volcanic eruptions also lead to local-temporary changes in the C14 concentration.

How problematic all these reservoir-related effects (= "reservoir effects") are for the reliability of C14-dating will be discussed below.

The history of C14 dating:

At the end of the 1940s, the American Willard F. Libby (University of Chicago) developed a method for dating archaeological samples by measuring their C14 activity, which earned him a Nobel Prize in 1960. To guarantee the reliability of his measurements, Libby postulated the following basic assumptions:

The production of C14 in the atmosphere is continuous and uniform.

The mixing of the atmosphere is fast and globally uniform.
Every living organism reflects the atmospheric isotope mixture (especially of C12 and C14) proportionally and accurately.

All organisms absorb C14 in the same concentration ratio (to C12) (= organic invariance).

After completion of the metabolism (death), an organism no longer absorbs C14. The C14 concentration (= quantitative ratio of C14 to stable C12) decreases continuously according to the decay law.

From the measured C12/C14 ratio of an archaeological sample, the time elapsed since the end of the metabolism - and thus the age of the sample - can be calculated according to the decay law.

Very important: The current atmospheric C12/C14 ratio (= C14 concentration) (for Libby: mid 20th century) serves as a binding yardstick for calculating the elapsed C14 decay time in a sample, i.e. the atmospheric C14 concentration is - for Libby - at any time in the past 60,000 years (C14 time horizon) the same as in the middle of the 20th century.

This last postulate is also called the "fundamental principle" (= temporal invariance of the atmospheric C14 concentration). Without its validity, the C14 decay cannot be calculated back to the end of the metabolism because the precise C12/C14 ratio (= C14 concentration) must be known at this point, which presupposes that the atmospheric C14 concentration is invariant in time and space. Without the application of this principle, one would not know to what point of a C14 concentration in the sample would have to be calculated back according to the decay law in order to determine its age.

The postulate of the spatial invariance of the atmospheric C14 concentration is called the "principle of simultaneity". According to him, the atmosphere mixes so rapidly that any local concentration fluctuations do not have a significant impact on the reliability of a C14 dating.

A distinction must be made between ´C14 age´and ´historical age´. Ideally, both are identical, i.e. if a measured C14 age is 1800 years BP (= Before Present, i.e. by default: before 1950) according to the decay law, then ideally one time of death of the organic sample in the year 150 CE corresponds to it. In this case, the C14 measurement would have produced an absolute date. However, this presupposes the validity of Libby´s fundamental principle, according to which the atmospheric C14 concentration does not change. Otherwise, the measurement would indicate a too high or too low age.

In the 1950s there were repeated conflicts between Libby and his followers on the one hand and Egyptologists on the other, because archaeological C14-dates often deviated rejuvenating from historically and astronomically recognized dates, whereby Libby insisted on the validity of his measurements and accused the Egyptologists of sitting up on erroneous dates. Until 1958 there was a stalemate due to undecidability about the priority of both dating procedures.

This changed from 1958 by the softening of Libby´s fundamental principle due to the discovery of significantly strong fluctuations of the atmospheric C14 concentration in the past, recognizable by the different C14 concentration in the rings of long tree ring sequences. For example, the 1960 Sequioa Tree Ring Series, a ring sequence dating back to around 200 CE, exhibited 14 major variations from the expected decay curve that shook the belief in the invariance of atmospheric C14 concentration. Such fluctuations are called "wiggle" in the C14 mainstream (to wiggle).
 
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