Carbon 14 used radiocarbon dating isotope dating fossils
This ratio would have been different before the Flood. According to these authors, the total mobile (non-carbonate) carbon in the biosphere is about 40,000-45,000 gigatons. The global carbon cycle: a test of our knowledge of Earth as a system. The earth’s sediments contain a vast amount of carbon-12 in the form of coal and oil.  Post WM, Peng TH, Emanuel WR, King AW, Dale VH, Deangelis DL. The amount of carbon in fossil fuels is estimated at 6,000 gigatons, and the amount of kerogens (organic) in sediments is about 15 million gigatons. Why do ancient samples of coal, diamonds and other carbon-containing materials consistently contain  The first such curve was published by: Arnold, JR and WF Libby. High-precision (super 14C measurement of Irish oaks to show the natural 14C variations from AD 1840 to 5210 BC. Age determinations by radiocarbon content: Checks with samples of known age. Subsequent revisions have been made, e.g., Reimer, PJ et al. It is known that such variations have occurred, but it is thought they can be corrected for by comparing a sample’s C level with the standard curve constructed using samples of known age.Another assumption is that the amounts of carbon-14 present in the geophysical reservoirs must be constant.
Surprisingly, in at least one case the date range given by There may be other examples of systematic variation in isotope decay rates.
C in coal and oil is much less than in the present environment. This gives a ratio of 300:1 for pre-Flood biosphere carbon to present biosphere carbon.
If a significant portion of the coal and oil represents organisms that were deposited in a global flood, then the pre-flood atmosphere must have contained much less C over time resulting in wobbles and deviation from expected levels in the standard calibration curve based on samples of known age?  Note the wobbles in the curves in Pearson, GW et al. Evidence for correlations between nuclear decay rates and Earth-Sun distance – et al. This differs by a factor of only two from the figure of 143:1 proposed by Brown (Brown, RH.
While the small variations in isotope decay that have been reported may not invalidate all isotopic dating, they raise questions about the assumption of completely uniform decay rates.
A second assumption is that the sample being dated has not experienced any loss or contamination of C over its history.