Variable C14/C12 ratio invalidates C14 dating

[edit] Claim

Carbon dating is based on the atmospheric C14/C12 ratio, but that ratio varies. Thus the carbon dating method is not valid.

Morris, Henry M., 1974. Scientific Creationism, Master Books, Arkansas, pp. 162-166.

This claim presumes that past carbon-isotope ratios cannot be determined. However, there are objects whose age is known independently of radiocarbon readings, including trees and sections of stony corals (whose age can be determined by careful examination of the growth rings of the wood and coral they are made of). Likewise, there are air samples of known age that have not been contaminated by later fluctuations in carbon-isotope ratios -- these include hermetically-sealed human artifacts, and also air bubbles trapped within polar ice caps. Thus, we can directly measure what the carbon-isotope ratios are for objects of known age, and we can use those measurements to calibrate radiocarbon readings taken from objects whose age was not previously known.
add more responses

Hasty Generalization (the fact that a method is unreliable when misapplied does not mean that it is always unreliable.)

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 (17 Oct). A stable-isotope tree-ring timescale of the Late Glacial/Holocene boundary. Nature 353: 647-649.
Dickin, A. P. (1995), Radiogenic Isotope Geology, Cambridge University Press, pp. 364-366.
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 (14 May). A large drop in atmospheric 14-C/12-C and reduced melting in the Younger Dryas*, documented with 230-Th ages in corals. Science 260: 962-967.
Kitagawa, H., and van der Plicht, J., 1998 (20 Feb). Atmospheric radiocarbon calibration to 45,000 yr B.P.: Late glacial fluctuations and cosmogenic isotope production. Science 279: 1187-1190.

Dalrymple, G. Brent, 1991. The Age of the Earth. Stanford University Press, California.
Lowe, J. John, ed., 1991. Radiocarbon Dating: Recent Applications and Future Potential. Quaternary Proceedings Number 1, 1991, Published for the Quaternary Research Association, Wiley.