Dating Old Carbon
Written by Studio AM.
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Reactions caused by cosmic rays in the atmosphere produce carbon-14, an unstable form of carbon. It enters carbon dioxide, then plants take it in as they grow. Animals acquire carbon through food, so carbon-14 becomes part of living tissues.
After a sample stops taking in new carbon, its carbon-14 continues to decay. About half remains after 5,730 years, and half of that remainder survives another 5,730 years. The decline is continuous; a half-life describes the time for half the atoms in a large sample to decay. Measurements can help date materials such as charcoal, bone and plant-fibre cloth. Beyond roughly fifty thousand years, the remaining amount becomes very difficult to distinguish from background contamination.
The starting proportion matters too. Atmospheric carbon-14 has not stayed constant, so a measurement cannot be converted directly into a precise calendar date. Researchers use calibration records, including tree rings of known age, to estimate a range of possible dates.
They also consider what the sample dates. Wood from an old tree can be much older than the building made from it. Dating the material is one step toward dating the event.
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Questions
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Question 1 of 4
What is the passage mainly about?
The answer is B: How carbon-14 dating works, and why its results must be corrected.
The first two paragraphs explain the method; the third explains the complication and the correction that follows from it.
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Question 2 of 4
Why does radiocarbon dating become difficult for very old samples?
The answer is C: The small remaining amount is hard to separate from background contamination.
Very little carbon-14 remains after many half-lives. Contamination then becomes especially important to the measurement.
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Question 3 of 4
Using the decay rule alone, how long does it take for one quarter of the original carbon-14 to remain?
The answer is D: About 11,500 years
One quarter remains after two half-lives: about 11,460 years. Turning an actual measurement into calendar dates also requires calibration.
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Question 4 of 4
In this passage, 'constant' means:
The answer is A: staying the same over time
The atmospheric proportion has changed over time. 'Constant' would mean it stayed the same.
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