Which best describes how ice cores are important to the study of geologic history?

• A. unconformities in deposition
• B. index fossils in ice cores
• C. evidence showing changes in atmospheric composition over time
• D. Law of Superposition

Answer: (C)

Ice cores serve as time capsules of Earth’s atmosphere. As snow piles up and compacts into ice over many years, tiny air bubbles get trapped in the layers. By digging and analyzing these layers, scientists can measure past concentrations of gases like CO2 and methane and compare isotopes that tell us about ancient temperatures. This direct record of how the atmosphere changed over time is exactly what helps us understand past climates and, in turn, geologic history.

That’s why the best description is that ice cores provide evidence showing changes in atmospheric composition over time. Other ideas—like gaps in rock deposition, fossils in ice cores, or the Rule of Superposition—don’t capture the core usefulness of ice cores, which is revealing the history of the atmosphere itself.