2. The table below shows radioactive isotopes that are commonly used for dating

Question

2. The table below shows radioactive isotopes that are commonly used for dating fossils including their half-life and useful time period for dating. Why can you not use Carbon- 14 to date a fossil that is older than 60,000 years old? Why can you not use Potassium- 40 to date a fossil that is less than 10 million years old? Half-life Useful dating (years) range (years) Decay Radioisotope product Carbon-14 (C) Nitrogen-14 ("N) 5,700 100-60,000 Uranium-234 U) Thorium-230 (200Th) 80,000 10,000-500,000 Uranium-235aU) Lead-207 @0Pb) 704 million 200,000-4.5 billion Potassium-40 rgon-40(A 13 bilion 10 million -4,.5 billon UFE Ne Figure 23.

Explanation / Answer

The half-life of C-14 = 5700 yrs
C-14 isotope cannot be used to date samples that are older than 40000 years old. Generally, C-14 constitute 1 part per trillion carbon atoms around us. After 40000 years, only ~0.79% of the original C-14 will be left in the sample. This magnitude of radioactivity cannot be measured using the existing methods. At this low concentrations, it gets very difficult to accurately measure the radioactivity. Environmental contamination (Cosmic rays continuously release C-14 in the environment...!). So, this would lead to a big error in the age estimation.

The same principle holds true for K-40.

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