Question Breakdown

Samples from the floor of a rock shelter were dated by analyzing the carbon that they contained. Depending on how much carbon the samples contained, dates were assigned that associated with human activities. We are told that these dates formed a consistent series, "beginning with the present and going back in time, a series that was correlated with the depth from which the samples came from." The oldest and deepest sample was dated at 19,650. Skeptics suggested that the samples were probably contaminated by dissolved "old carbon" through the percolation of groundwater from nearby coal deposits, because they believed that date was too early and inconsistent with the accepted date of human migration into North America. We are looking for the answer choice that weakens the skeptics' suggestion. Answer choice (A) states: "No likely mechanism of contamination involving percolating groundwater would have affected the deeper samples from the site without affecting the uppermost sample." So, if there is a likely mechanism of contamination involving percolating groundwater, then it would not have affected the deeper samples from the site without affecting the uppermost sample. Well, we know that the uppermost samples were from a more recent time since all the samples displayed a progression, beginning with the present and going back in time, i.e. they were not affected by contaminated water. However, answer choice (A) says that if there was a likely mechanism of contamination involving percolating groundwater, then both the deep samples and uppermost samples would have been affected, i.e. would have shown us a much older date than what it actually was. Therefore, since we know that the uppermost were not affected, since we know there was a steady progression of samples dated earlier to later, there is no likely mechanism of contamination that involves percolating groundwater. Thus, answer choice (A) argues strongly against the skeptics' suggestion. Hope that clears things up! Please let us know if you have any other questions.

Quick question how do we know that the upper most level is not affected?

@JayDee8732 because the uppermost samples were from a more recent time since all the samples displayed a progression, beginning with the present and going back in time. This tells us that they were not affected by contaminated water. Hope that helps! Please let us know if you have any other questions.

Couldn't the samples still display a progression, but all the dates would just be shifted back further than their "real" date?

why not c?

@darbyhenry Thanks for the question. Coal deposits could still exist even if humans weren’t using it yet for fuel, so it is not necessary for humans to be using coal for fuel for the deposits to get into the water. What is relevant is that the deposits in each layer were different, preserving a glimpse of the past.