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Radiocarbon Dating

Careful sample preparation by StrataData is matched by the world class analysis by Beta Analytic Inc. Converting the results of the analyses to numerical ages is achieved using the INTCAL13 calibration curve. Our reports contain official radiocarbon dating certificates for each sample together with calibration curves showing how each conventional radiocarbon age for samples has been calibrated with the calendar year curve.

Application

Suitable for dating sediments up to c.45Ka old as long as they contain suitable biogenic material (molluscs, microfossils, wood). Non-marine as well as marine and terrestrial sediments can be dated using this method. Each sample is prepared in the same way as for a micropalaeontological analysis. Suitable specimens are selected by picking through the residue.

Cost and turnaround time

Price depends on the nature of the material and turnaround time required; the basic price for analysis with a turnaround time of less than 3 weeks is £395 plus the cost of sample preparation. Please contact us to discuss your requirements.

Technical Information

Method

The radiocarbon method of dating was first developed by a group led by Willard F.Libby in 1949, since when it has been used mainly as an archaeological tool. 14C is an unstable isotope of carbon which is created from 14N in the upper atmosphere. Carbon (both 14C and 12C) rapidly oxidises to CO2 and disperses in the atmosphere. It is then absorbed by all living organisms via photosynthesis (in the case of plants) or through the food chain (in the case of animals). Because 14C is an unstable isotope it is constantly decaying, but the proportion of 14C in living organisms stays relatively constant over time through continued metabolic uptake. In life the 14C/12C ratio therefore remains in equilibrium with the atmosphere. However, upon death there is no further uptake of carbon and the 14C decays to 12N with a half life of 5730 years. Measurement of the amount of 14C remaining in a dead organism will therefore give the date of its death. Using this method, the maximum detectable age is no greater than 50,000 years and over 40,000 years dates are unreliable. In addition, material from the last 300 years gives unreliable ages, mainly due to the widespread burning of fossil fuels and more recently the explosion of nuclear bombs, both of which have artificially increased the amount of 14C in the atmosphere. Thus, it is not possible to accurately date organisms which have died since 1950. Because of the small size of most industrial samples the Accerator Mass Spectrometry (AMS) method is used. This method can be used to radiocarbon date one milligram of carbon or less and has the added advantage that the small sample size permits more selective sampling. Suitable material includes molluscs, microfossils and wood from terrestrial, non-marine and marine environments.

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Comparison of the original curve calculated by Libby and the calibrated INTCAL98 curve. (From Stuiver et al. 1998)


Calendar year calibration

The results of 14C dating have traditionally been quoted in “conventional radiocarbon years”, typically in years BP (before present) where the “present” is 1950. The initial assumption that the concentration of atmospheric 14C has been constant as measured in 1950 has been shown to be incorrect. Dating annual tree growth rings of known historical age has demonstrated that in the past there have been short term variations in atmospheric 14C levels. Tree ring dating has been used to construct a probabilistic calibration curve extending back to 11857 calendar years BP (see figure below). This method is not without its own inaccuracies and has limited precision, but it does provide dates in terms of calendar years. Extension of the calibration curve further back to about 24,000 calendar years BP has been achieved using U-Th decay series dates from marine organisms (principally corals). The INTCAL98 calibration curve thus constructed has been accepted by international consent (see Stuiver et al., 1998). Two other U-Th dates provide a tentative extension to the INTCAL98 curve which has been used in this report to provide a calibration curve back to about 40k years BP. Using the INTCAL98 curve it is possible to convert radiocarbon years to calendar years by projecting the radiocarbon age onto the curve and observing the intercept on the calendar year axis. This can be done manually, but there are now computer programs in the public domain which will provide a more consistent and accurate calibration. Because the calibration curve is not a straight line it is possible to have many intercepts on the calendar year axis, each with its own probability range.

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Conversion of conventional radiocarbon years to calendar years using the INTCAL98 calibration curve (from Beta Analytic)


Reservoir Age

The average difference between a radiocarbon date of a terrestrial organism such as a tree and a marine shell of the same age is about 400 radiocarbon years. This difference which is called the reservoir age is caused both by the delay in exchange rates between atmospheric CO2 and oceanic bicarbonate, and the dilution effect caused by mixing surface waters with upwelling deep waters which are very old. A reservoir correction factor must therefore be applied to conventional radiocarbon dates based on the remains of marine organisms.

Reworking

For practical purposes 14C dating records the time of death of an organism and not the point at which it is finally entombed in sediment. Reworking of older material into younger sediments frequently occurs and can dramatically increase their 14C age. It is important, therefore, to select specimens for analysis with great care and to record any doubtful material before analysis.