Which statement about ice cores is true?

• A. They show unconformities in deposition.
• B. They hold index fossils, which are used to date the cores.
• C. They reveal the color of ancient skies.
• D. They contain evidence showing changes in atmospheric composition over time.

Answer: (D)

Ice cores hold a record of the past atmosphere because each layer of snow that becomes ice traps tiny pockets of air from the time it formed. Those trapped air bubbles preserve the actual composition of the atmosphere, so scientists can measure past levels of gases like carbon dioxide and methane and see how the atmosphere changed over thousands of years. That’s why the statement about ice cores containing evidence of changes in atmospheric composition over time is the true one.

Index fossils belong to living organisms preserved in sediment rocks, not in ice cores, so that idea doesn’t apply here. Unconformities are gaps in deposition in sedimentary records, which isn’t how ice cores are typically described. And while dust or volcanic material can appear in ice layers, ice cores don’t directly reveal the color of ancient skies.