A sample of an ancient sedimentary rock contains slightly less of the isotope carbon-13 than found in inorganic material. One possible explanation for this is that
A) it was a meteorite that fell from the sky
B) some kind of radioactive decay occurred in it in the past
C) it used to be at the bottom of an ancient ocean
D) some kind of biological activity occurred in it in the past

Question

Quizplus · Accepted Answer

The slight decrease in carbon-13 is likely due to biological activity, such as photosynthesis, that preferentially uses the lighter carbon-12 isotope. This is a known phenomenon in organic-rich sedimentary rocks. A meteorite or radioactive decay would not selectively decrease the carbon-13 isotope. Being at the bottom of an ancient ocean may be possible, but it does not explain the carbon-13 depletion.

A Sample of an Ancient Sedimentary Rock Contains Slightly Less