Which proxy is commonly used to infer past temperatures in paleoclimatology by analyzing isotopes?

Which proxy is commonly used to infer past temperatures in paleoclimatology by analyzing isotopes?

• A. The ratio of 16O to 18O
• B. Pollen assemblages
• C. Tree ring widths
• D. Radiocarbon dating of fossils

Answer: (A)

Oxygen isotope ratios provide a temperature signal in paleoclimatology. Oxygen has two stable isotopes, 16O and 18O, and the way these isotopes are partitioned during evaporation, condensation, and mineral formation depends on temperature. When scientists measure the ratio of 16O to 18O in materials such as ice or carbonate minerals, they get a δ18O value that records the temperature at the time that material formed (or the global ice volume that influenced seawater composition). In colder times more 16O is trapped in ice, leaving seawater—and thus carbonate shells that form from it—with relatively higher 18O; conversely, warmer conditions produce lower δ18O values. This isotope-based signal is a direct proxy for past temperatures, which is why the ratio of 16O to 18O is commonly used. Pollen assemblages reflect past vegetation, tree ring widths show growth response to climate, and radiocarbon dating is used for age rather than temperature—so they don’t provide the isotopic temperature proxy in the same sense.

Oxygen isotope ratios provide a temperature signal in paleoclimatology. Oxygen has two stable isotopes, 16O and 18O, and the way these isotopes are partitioned during evaporation, condensation, and mineral formation depends on temperature. When scientists measure the ratio of 16O to 18O in materials such as ice or carbonate minerals, they get a δ18O value that records the temperature at the time that material formed (or the global ice volume that influenced seawater composition). In colder times more 16O is trapped in ice, leaving seawater—and thus carbonate shells that form from it—with relatively higher 18O; conversely, warmer conditions produce lower δ18O values. This isotope-based signal is a direct proxy for past temperatures, which is why the ratio of 16O to 18O is commonly used.

Pollen assemblages reflect past vegetation, tree ring widths show growth response to climate, and radiocarbon dating is used for age rather than temperature—so they don’t provide the isotopic temperature proxy in the same sense.