14C dating

AMS 14C dating Abbreviation for accelerator mass spectrometry radiocarbon dating. AMU Acronym for atomic mass unit. 

A detailed description of the 14C dating method goes beyond the scope of this chapter for those interested, many resources can be found in the literature.[5,6] In the following, only the basics that are fundamental to the understanding of the present topic are illustrated. 

In this way, it is possible to reach an extremely high selective sensitivity down to 1 part in 1015, which in 14C dating corresponds to being able to date samples about 50 000 years old. Moreover, modern systems can measure isotopic ratios in modern carbon, both C/ C and C/ C, with an ultimate precision as good as 2%o and l%o, respectively. The former value corresponds to determining the conventional radiocarbon age with an absolute error, smaller than in the past, better than 20 years, while the l%o precision for the 13C/12C allows an adequate correction for isotopic fractionation effects. Even in routine measurements, at least in the case of historical samples, a precision of 5%o in the 14C/12C measured value is standard, corresponding to an uncertainty in the radiocarbon age of 40 years.[27]... 

For more recent dating processes carbon dating is used. With a half-life of 5717 years 14C dating relies on the replenishment of the 14C parent species, which occurs by neutron capture. The process occurs in the atmosphere as a result of bombardment of the Earth with neutrons ... 

In 1947 W. F. Libby and collaborators [17-19] measured for the first time 14C produced by cosmic radiation in the atmosphere. He then proposed the use of this radioisotope for dating of organic material. A unique constellation of factors makes the 14C dating technique a most fascinating and powerful instrument for studies of the last 50,000 y ... 

For information on the 14C dating method see also Badio-aarbon 1980 [20]. 

Because of the ease of making such measurements, many descriptions of the technique have tended to emphasize its impact as a new 14C dating method. We believe that this emphasis on radiocarbon dating has been somewhat premature, as there is an enormous practical difference between the detection of 14C atoms in samples at natural concentrations using existing equipment and the measurement of 12C/13C/14C isotopic ratios with the necessary precision for dating to better than one percent accuracy. In this... 

Despite the vast amount of work on 14C dating which has already been accomplished and despite the fact that it is the best developed method available today, numerous difficulties still exist with its application. First, carbonate geochemistry which helped control 14C concentrations in the past is not simple to reconstruct. Carbonate minerals are commonly in a state of near equilibrium with groundwater, and only slight changes in water temperature or chemistry will promote either dissolution or precipitation of carbonate ions. In this way, the proportion of modern carbon in the water can be changed and some isotope... 

Theoretical studies [25,42] have shown that significant amounts of a number of radionuclides usually assumed to be derived only from the atmosphere may actually be produced in the subsurface, largely through interactions with secondary neutrons produced by alpha capture reactions. The alpha particles are derived mostly from normal decay of natural U and Th. Whether or not subsurface production of radionuclides can indeed influence dating has yet to be demonstrated by field and laboratory tests. The matter needs further study, particularly in relation to 14C dating of water which is more than 40,000 years old. 

The sedimentation history of the upper Hudson Canyon has been measured at the 430 m depth level by Drake et al., [10]. The average sedimentation rate over the past 6,300 years, as determined by 14C dating of a piston core, has been an average of 70-80 cm/103 yrs the past 2,000 years appear to have a sedimentation rate of about 150 cm/103 years, Drake et al., [10]. 

Benthic mixing in deep sea cores as determined by 14C dating and its implications regarding climate stratigraphy and fate of fossil fuel C02, In Anderson, W. R., Malahoff, A., eds., Fate of Fossil Fuel C02 in the Ocean, Mar. Sci. Ser, 6,... 

Conventional 14C date, t = 5730 years, no calibration for Bristlecone pine. fFirst number, sample, second number, oxalic acid standard. 

Olsson, I. U., Some Problems in Connection with the Evaluation of 14C Dates, Geol. Foren Stockholm Forh, 1974, 96, 311-320. 

C-dating of travertines is possible in some instances the initial 14C/12C ratio of speleothem or travertine will generally be less than that of the atmosphere at the time of deposition, due to admixture of dead carbon from the limestone from which the Ca and part of carbonate have been obtained. But, where active... 

Why then, is such a complicated and expensive set up necessary AMS combines mass spectrometric features with efficient discrimination of isobaric and molecular interferences. Therefore, it can detect and quantify atomic species of very low abundance. In the case of 14C dating, before AMS was utilized, about 1 g of carbon was needed to date an archaeological item. One gram of fresh carbon contains about 6 x 1010 14C atoms, of which 14 decay per minute. To get 0.5% statistical precision using decay counting, a 48 h acquisition time is necessary. The same result can be obtained with AMS in about 10 min and with only 1 mg of carbon. 

---