Radiocarbon methods

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 many possible errors, as well as the difficulties inherent to the radiocarbon dating method itself, serve to emphasize the need for a close collaboration between archaeologists and natural scientists when dating archaeological samples by the radiocarbon method. 

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...

Clearly, direct techniques for measuring OH are needed that provide concentrations either at a point or over relatively restricted spatial scales. Two (absorption and laser-induced fluorescence) are direct, spectroscopic methods and two others (mass spectrometry and a radiocarbon method) rely on conversion of OH to another species that is measured. Each of these approaches and some of the intercomparisons that have been carried out are discussed briefly in the following sections. A good overview of these methods is found in a review by Eisele and Bradshaw (1993) and articles by Crosley (1994, 1995a, 1995b) and papers in a special issue of the Journal of the Atmospheric Sciences [52 (19), October 1, 1995]. 

An alternative to the above described approaches is the radiocarbon method that allows a distinction of contemporary carbon (from biogenic emissions and combustion of biomass) and carbon from combustion of fossil fuels in particulate carbonaceous matter [15, 41,42]. In contrast to fossil fuels where the 14C isotope is completely depleted, CM emitted from WB shows a contemporary radiocarbon level. Radiocarbon measurements are often combined with measurements of complementary source specific tracers (macro-tracer) for additional information of source impacts [14, 43, 44]. 

In summary, natural abundance radiocarbon is a powerful tool, because it can be used in mature and undisturbed ecosystems (as well as in younger or disturbed ones), and because it can be used to quantify turnover times across a range of time scales. In fact, it may be the only tracer for stable, or slow-cycling, C pools. The radiocarbon content, along with additional constraints regarding, for example, the relative proportions of fast- and slow-cycling SOM, can be used to model turnover times. Appendixes 1 and 2 contain more thorough explorations of radiocarbon methods and applications to SOM studies. 

Figure I. Diagrammatic outline of basic elements o/ the radiocarbon method (after Fleming, Ref. 5)...

The relatively rapid acceptance of the general validity of the radiocarbon method by most researchers is indicated by the rapid establishment of other laboratories to perform analyses. Many who attempted to duplicate the procedures required to obtain acceptable values using the solid carbon technique experienced moderate-to-severe difficulties. The result was the substitution of gas or liquid scintillation counting methods. Because of the greater efficiency of such detection systems, these developments permitted the maximum age range to be extended from about... 

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 ... 

Variability in Radiocarbon Distribution. One of the early major promises of the radiocarbon method was its potential to provide direct... 

Variability in Carbon Isotope Ratios. For the radiocarbon method, the basic physical measurement used to index time is the ratio. 

Given that soils only contain a few percent OM by mass, direct measurement of SOM mass loss is much more difficult than for litter studies. The four most common techniques for measuring soil organic carbon (SOC) turnover are (1) laboratory incubations (2) in situ soil respiration measurements (3) stable isotope measurements and (4) radiocarbon methods. We will introduce the basic methodology and highlight some of the strengths and weaknesses of each of these four techniques. For much more detailed discussions of these methods, see the references cited within. 

The control samples were pieces of ancient cloth of known date to be used to validate the results from the different labs. One of the control samples was a piece of linen from a tomb in Nubia. Islamic embroidery and Christian inscriptions on the cloth suggested a date in the eleventh or twelfth century ad. A second control sample was taken from the linen wrappings of the muimny of Cleopatra in the British Museum. This cloth had been dated using tradition radiocarbon methods between 110 bc and ad 75, around the time of the birth of Christ. The third control... 

Plant remains and mollusk shells from marine Holocene layers in the central depression (layers containing Cerastoderma, station 15) have been dated by radiocarbon methods [16]. The results provided evidence of marine sedimentation being confined to the second half of the Holocene and permitted us to date some large regressive phases in the history of the Aral. Other studies have been performed in the central basin, including oxygen isotope analysis of carbonates in mollusk shells... 

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