77. Which one of the following statements is TRUE when determining the age of a fossil using
carbon dating?
(A) Carbon dating is based on carbon-13 to carbon-12 ratio in fossils
(B) Carbon dating is useful for determining the age of only fossils older than 100,000 years
(C) Older the fossil, lesser the carbon-14 to carbon-12 ratio
(D) Older the fossil, lesser the carbon-12 to carbon-14 ratio
Understanding Carbon Dating: Principles, Limitations
Carbon dating determines fossil age by measuring the decay of radioactive carbon-14. The correct statement is option (C): Older the fossil, lesser the carbon-14 to carbon-12 ratio.
How Carbon Dating Works
Carbon dating relies on the radioactive isotope carbon-14 (¹⁴C), which living organisms absorb from the atmosphere in a fixed ratio with stable carbon-12 (¹²C). After death, ¹⁴C decays to nitrogen-14 with a half-life of about 5,730 years, while ¹²C remains constant. Scientists measure the declining ¹⁴C/¹²C ratio to calculate time since death, effective up to roughly 50,000-60,000 years.
Correct Answer: Option (C)
Option (C) states: “Older the fossil, lesser the carbon-14 to carbon-12 ratio.” This is true because ¹⁴C decays exponentially over time, reducing the ratio as the fossil ages, while ¹²C stays stable. This principle directly enables age estimation through the known decay rate.
Why Other Options Are Incorrect
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Option (A): Carbon dating uses carbon-14 to carbon-12 ratio, not carbon-13 to carbon-12. Carbon-13 serves other isotopic studies, like paleodiet analysis.
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Option (B): Carbon dating suits fossils younger than 60,000 years; beyond that, too little ¹⁴C remains detectable. Older fossils require methods like uranium-lead dating.
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Option (D): This reverses the principle. Older fossils have less ¹⁴C relative to ¹²C, so the ¹²C/¹⁴C ratio increases, not decreases.
| Option | Statement | Correct? | Reason |
|---|---|---|---|
| A | Carbon-13 to carbon-12 ratio | No | Uses ¹⁴C/¹²C ratio |
| B | Only for fossils >100,000 years | No | Limited to <60,000 years |
| C | Older fossil: less ¹⁴C/¹²C | Yes | Matches decay principle |
| D | Older fossil: less ¹²C/¹⁴C | No | Ratio increases |
Key Applications and Limitations
Carbon dating excels for organic fossils like wood, bone, or shells from recent geological periods. Limitations include contamination risks and atmospheric ¹⁴C variations, calibrated via tree rings. For precise results in research, combine with stratigraphic context.
