The chemical analysis of ice cores provides crucial proxy data for reconstructing past climates. What is the fundamental principle that allows scientists to infer past atmospheric temperatures from the ice itself?
- The annual thickness of the ice layers is directly proportional to the average global temperature in that year.
- The concentration of trapped dust particles in the ice correlates with global volcanic activity, which controls temperature.
- Air bubbles trapped in the ice contain samples of ancient air whose pressure is a reliable indicator of past temperature.
- The ratio of heavy to light isotopes of oxygen and hydrogen in the ice's water molecules is dependent on the temperature at the time of snowfall. (correct answer)
Explanation: The correct answer is D. Water molecules (H2O) can contain heavier isotopes of oxygen (Oxygen-18) and hydrogen (Deuterium). The process of evaporation and condensation is temperature-dependent, causing water vapor that travels to the poles to become progressively more depleted in the heavier isotopes as temperatures drop. This results in a strong, quantifiable relationship between the isotopic ratio (e.g., δ¹⁸O) found in the layers of ice and the local temperature when the snow originally fell. Layer thickness (A) is more related to precipitation rate. Dust (B) relates to atmospheric circulation and aridity. Air bubbles (C) are invaluable for determining past greenhouse gas concentrations, not temperature via pressure.