Radiocarbon method, assumptions

Uncertainties, Limitations, and Complications in Radiocarbon Dating. Discrepancies between a measured radiocarbon age and the otherwise verified age of certain specimens are sometime found. These discrepancies are due to deviations from the basic assumptions on which the radiocarbon method rests, which are essentially, as follows ... 

Radiocarbon Dates. Historical dates are usually expressed in calendar years, but dates determined by the radiocarbon method are expressed as radiocarbon years. This is done on the assumption that during the past the relative concentration of radiocarbon in the atmosphere has been constant ... 

The assumptions of the radiocarbon method are widely known and understood (Table 1). Each has been the subject of review and comment over more than thirty years of 14C studies [1-3]1. Included in Table 1 under the heading contextual is an assumption sometimes not emphasized in discussions of the accuracy of 14C determinations. It is essential that the field archaeologist, geologist or geomorphologist document contextual relationships of sample materials with a closely defined cultural or geological event. 

Table 1. Assumptions of the Radiocarbon Method Physical Assumptions...

The ability of the radiocarbon method to provide accurate and precise determinations of the actual or calendar age of organic samples is obviously a function of the degree to which each sample fulfills the set of basic conditions on which the validity of the method itself rests. These basic assumptions can be summarized as follows ... 

To sum up, it should be realised that both methods, O2 2md C method, have serious limitations. The C method is certainly the more sensitive one, but from data no respiration can be measured and therefore no net production can be calculated whereas the oxygen method gives a good estimate of assimilation and dissimilation (Golterman, 1975). The question, at one time actively debated, on whether uptake gives a measure of net or gross photosynthesis or some intermediate value, has never been resolved (Bunt, 1975). Mostly the assumption is made that nearly all fixed carbon comes from directly assimilated CO2 (and not partly from older respired carbon) and that the radiocarbon method more or less measures net primary production (De Vooys, 1979). 

Dating with Radiocarbon. The important information held in a sample to be dated by radiocarbon is its present radiocarbon concentration comparing this concentration to that of radiocarbon in the atmosphere, which is considered to be constant (however, see discussion below), yields the conventional radiocarbon date of the sample. All that is required to establish the age of a sample, therefore, is to determine the present-day relative amount of radiocarbon in the sample. Once this has been determined by either the conventional radiocarbon decay counting or by the AMS method (see Fig. 63), a number of internationally established conventions and assumptions are used to calculate the age of a material or object ... 

A mass of evidence seems to confirm that the mixing rate of radiocarbon in the atmosphere is rapid, and that with respect to its radiocarbon content the atmosphere can be considered as a homogeneous entirety. The contamination of samples with matter from an extraneous source can nevertheless invalidate this assumption. Two types of contamination can be differentiated physicochemical contamination and mechanical intrusion. There are two forms of physicochemical contamination. One is due to the dilution of the concentration of radiocarbon in the atmosphere by very old carbon, practically depleted of radiocarbon, released by the combustion of fossil fuel, such as coal and oil. The other is by the contamination with radiocarbon produced by nuclear bomb tests during the 1950s and later in the twentieth century. The uncertainties introduced by these forms of contamination complicate the interpretation of data obtained by the radiocarbon dating method and restrict its accuracy and the effective time range of dating. 

Radiocarbon Dating. This is a method of estimating Ihe age of carbon-containing materials by measuring the radioactivity of the carbon in them. The validity of this method rests upon certain observations and assumptions, of which the following statement is a brief summaiy. The cosmic rays entering the atmosphere undergo various transformations, one of which results in the formation of neutrons, which in turn, induce nuclear reactions in the nuclei of individual atoms of the adnosphere. The dominant reaction is... 

The primary application of is to date objects or to determine various environmental process rates. The method is based on the assumption of a constant atmospheric formation rate. Once produced, atmospheric reacts to form i COi, which participates in the global carbon cycle processes of photosynthesis and respiration as well as the physical processes of dissolution, particulate deposition, evaporation, precipitation, transport, etc. Atmospheric radiocarbon is transferred to the ocean primarily by air-sea gas exchange of Once in the ocean, is subject... 

Some of these radiocarbon atoms find their way into iiving trees and other piants in the form of carbon dioxide, as a resuit of photosynthesis. When the tree is cut down photosynthesis stops and the ratio of radiocarbon atoms to stabie carbon atoms begins to faii as the radiocarbon decays. The ratio in the specimen can be measured and enables the time that has elapsed since the tree was cut down to be calculated. The method has been shown to give consistent results for specimens up to some 40 000 years old, though its accuracy depends upon assumptions concerning the past intensity of the cosmic radiation. The technique was developed by Willard F. Libby (1908-80) and his CO workers in 1946-47. 

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