Question

Carbon-14 dating works for fossils up to about 75,000 years old; fossils older than that contain too little ¹⁴C to be detected. Most dinosaurs went extinct 65.5 million years ago. (a) Can ¹⁴C be used to date dinosaur bones? Explain. (b) Radioactive uranium-235 has a half-life of 704 million years. If it was incorporated into dinosaur bones, could it be used to date the dinosaur fossils? Explain.

Answer 1

Step 1: Explanation for (a)

Carbon-14 dating is used for fossils that are about 75,000 years old. However, dinosaurs became extinct about 65.5 million years ago. This suggests that carbon-14 dating cannot be used to date the bones of dinosaurs.

¹⁴C present in the dinosaur would have decayed too much when the dinosaurs died about 65.5 million years ago. As a result, too little ¹⁴C would have been left to measure.

Step 2: Explanation for (b)

Uranium-235 has a half-life of 704 million years. This suggests that after 65.5 million years, only one-tenth of the uranium-235 would have been decayed. As a result, a lot of U-235 would be present in the fossils to measure.

Therefore, U-235 can be used to date dinosaur fossils because when they became extinct (65.5 million years), plenty of U-235 would be left for measurement.

(a) No, too little ¹⁴C (carbon-14) would be left to measure after 65.5 million years.

(b) Yes, because uranium-235 has a half-life of 704 million years, and dinosaurs became extinct 65.5 million years ago.