You are an ice core researcher, and are examining a sequence of δ18o measurements obtained from an ice core from greenland. the measurements are from a section of the core that is not near the surface. the measurements show an increase of δ18o in the ice, from more negative values at a greater depth (from the older layers) to less negative values above that (from the younger layers). given what you know about the way oxygen isotopes in polar ice cores vary in response to atmospheric temperature changes, what do you think happened to the atmospheric temperature during this interval? choose the correct answer with the correct reason. a. the temperature did not change. this is known because when the climate stays the same there is not change in evaporation of the heavy isotope and thus there will be the same amount of the heavy isotope that can precipitate. the ice core will then record this constant amount of the heavy isotope. b. it got warmer. this is known because in a warmer climate there is an increase in evaporation of the heavy isotope and thus there will be more of the heavy isotope that can precipitate. the ice core will then record this increase in the amount of the heavy isotope. c. it got warmer. this is known because in a warmer climate there is an increase in evaporation of the light isotope and thus there will be more of the light isotope that can precipitate. the ice core will then record this increase in the amount of the light isotope. d. it got colder. this is known because in a colder climate there is an increase in evaporation of the heavy isotope and thus there will be more of the heavy isotope that can precipitate. the ice core will then record this increase in the amount of the heavy isotope.