In contrast, methane made from petroleum products had no measurable radioactivity.
This discovery meant that there are three naturally occurring isotopes of carbon: Whereas carbon-12 and carbon-13 are stable isotopes, carbon-14 is unstable or radioactive.
Carbon dating error range
With our current kit 40-50K years is about the limit.
Since carbon is fundamental to life, occurring along with hydrogen in all organic compounds, the detection of such an isotope might form the basis for a method to establish the age of ancient materials.
The trouble is that after 40,000 years there is under 1% of the original C-14 left, and it becomes too hard to measure it accurately.
This isn't a fundamental limit as more accurate measurements could go further back, but at some point you'd simply run out of C-14 atoms.
For historical reasons, uncalibrated radiocarbon measurements are often referred to a half-life of 5568 years.
However, this inconsistency is corrected during calibration [the reason for using the (Willard F.) Libby half-life of 5568 years instead of the correct one of 5730 years has to do with the finding in about 1962 that the true half-life was 573030 years.
However, when the organism dies, the amount of c14 declines such that the longer the time since death the lower the levels of c14 in organic tissue.
Radiocarbon measurements are always reported in terms of years `before present' (BP).
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For an example, when they tried to get the carbon dating for presence of Aboriginal people in Australia they get to the number 40,000. Why is that 40,000 years limit for carbon dating methods?
This figure is directly based on the proportion of radiocarbon found in the sample.