Regarding the often misquoted and maligned (by creationists) 14C dating method, here is a partial chronology of the development of the calibration of 14C, which includes consideration of sources of fluctuations in the background level of 14C.

Much of creationist commentary on the possible errors in 14C is intended to portray the scientific community as being either incompetent or dishonest regarding such issues as the consideration of potential errors in radiometric dating methods. Creationists never discuss the fact that given these analyses, scientists have found ways of identifying and mitigating the effects of these sources of error.

What follows is a summary from chapter 14.1.3, p.364-366 of Dickin [1995]:

1941: Thellier proposed that Earth’s magnetic field experiences secular variations.

1949: Arnold and Libby publish radiocarbon dates of items of known age.

1952: Libby publishes first book on radiocarbon dating.

1954: Forbush observed that the 11-year cycle of sunspot activity was inversely correlated with cosmic-ray intensity.

1955: Suess proposed dilution due to the burning of fossil fuels for the 2% depletion of 14C activity seen in 20th century wood compared to 19th century wood.

1956: Elasser, et al. predicted variations in the cosmic ray flux due to secular variations in Earth’s magnetic field.

1958: de Vries found that 17-th century wood had a 2% higher activity than 19th century wood.

1961: Stuiver used historical records of sunspot activity to calculate cosmic ray intensity, and hence 14C production for the past 1500 years, and suggesting that the observations of de Vries, correlated with a sunspot minimum.

1965: Stuiver used more detailed records to confirm the correlation of a sunspot minimum with de Vries observations.

1967: Bucha and Neustupny provided paleomagnetic intensity measurments that supported the existence of secular variations in the Earth’s magnetic field first proposed by Thellier. They were able to model the variations of 14C production, and almost exactly match the deviations between the tree-ring and radiocarbon time scales.

By 1969, enough radiocarbon dates of objects of known age, it became apparent that calibration of the 14C dating method was both possible, and required, to make radiocarbon dates useful for the determination of calendar dates. Indeed, it is often material from prior to 1969 that creationists use as ammunition against the 14C dating method.

One useful method of calibrating 14C is dendrochronology, or age dating by counting tree rings. The basic idea of calibration is simple. Plot the true date determined from counting tree rings versus the radiocarbon date. The result is a plot that can be used to both determine the actual original 14C/12C ratios and can be used to convert radiocarbon dates of other samples into calendar dates. The calibration curve also allows the original 14C/12C ratios to be determined.

The first use of dendrochronlogy to calibrate 14C over a long period of time was made by Furgeson in 1970.

1970: Furgeson used dendrochronology of bristlcode pines to calibrate radiocarbon dating back to 7484- years b.p. (before the present).

Through comparison with tree ring dates, the 14C method has been calibrated back to more than 13,000 years before the present,

1991: Becker, et al publish a stable dendrochronological calibration of 14C back to 13,000 years before the present.

In addition, 14C dating has also been calibrated back to more than 30,000 years before the present using uranium-thorium (isochron) dating of corals [Bard, et al, 1990] and [Edwards, et al, 1993]. While it is unlikely that 14C will be useful for objects older than 50,000 years, owing to the problems of background contamination [Dickin, 1995] and [Lowe, 1991], there is a recent paper by [Kitagawa, H., and van der Plicht, J., 1998] discusses calibration of 14C dating back to 45,000 b.p. using U-Th dates of glacial lake varve sediments (periodic sedimentary layers).

The bottom line is that 14C dating is quite a bit more advanced than creationist sources give it credit for being.

References:

Arnold, J. R. and Libby, W. F. (1949) Age determinations by radiocarbon content: Checks of samples with known age. Science 110, 678-680.

Bard, E., Hamelin, B., Fairbanks, R.G., and Zinder, A., (1990), Calibration of the 14C timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals, Nature, 345, 405-410.

Becker, B., Kromer, B., and Trimborn P., 1991, A stable-isotope tree-ring timescale of the Late Glacial/Holocene boundary: Nature, vol. 353 (17 Oct 1991), 647-649.

Dickin, A. P. (1995), Radiogenic Isotope Geology, Cambridge University Press.

Dalrymple, G. Brent, (1991), The Age of the Earth. California: Stanford University Press, ISBN 0-8047-1569-6.

Edwards, R. L., Beck, J. W., Burr, G. S., Donahue, D. J., Chappell, J. M. A., Bloom, E. R. M., Druffel, E. R. M., Taylor, F. W., 1993, A large drop in atmospheric 14-C/12-C and reduced melting in the Younger Dryas*, documented with 230-Th ages in corals: Science, vol. 260 (14 May 1993), 962-967

Furgeson, C. W. (1970), Dendrochronology of bristlecone pines, Pinus aristata. Establishment of a 7484-year chronology in the White Mountains of eastern-central California, USA. In: I. U. Olsson (Ed.), Radiocarbon Variations and Absolute Chronology, Proc. 12th Nobel Symp. Wiley, pp. 629-40.

Kitagawa, H., and van der Plicht, J., (1998), Atmospheric radiocarbon calibration to 45,000 yr B.P.: Late glacial fluctuations and cosmogenic isotope production, Science, v. 279, 20 Feb 1998.

Libby, W. F. (1952) Radiocarbon dating, University of Chicago Press.

Libby, W. F. (1970) Ruminations on radiocarbon dating In: I. U. Olsson (Ed.), Radiocarbon Variations and Absolute Chronology, Proc. 12th Nobel Symp. Wiley, pp. 629-40.

Lowe, J. John, ed. (1991) Radiocarbon Dating: Recent Applications and Future Potential, Quaternary Proceedings, Number 1, 1991, Published for the Quaternary Research Association, Wiley.