Antarctic ice has delta-18O values around -50 per mil, while tropical ocean water is near 0 per mil. What process creates this enormous difference?
ARadioactive decay of 18O in polar regions
BRayleigh distillation during poleward atmospheric moisture transport: each condensation event preferentially removes 18O from the vapor, progressively depleting the remaining vapor, so precipitation becomes increasingly 18O-depleted as air masses move toward colder, higher-latitude regions
CChemical reactions in polar ice alter the isotopic composition
DAntarctic ice is older and has lost 18O over time
As a moist air mass moves from the tropics toward the poles, it cools progressively. Each precipitation event removes water enriched in 18O (heavier isotope condenses preferentially), leaving the remaining vapor increasingly depleted. After multiple condensation cycles across thousands of kilometers, the vapor reaching Antarctica is extremely 18O-depleted. This Rayleigh distillation process is the fundamental control on the latitudinal and altitudinal gradients in precipitation delta-18O.
Question 2 True / False
The delta-18O of foraminifera shells in deep-sea sediment cores records only sea surface temperature.
TTrue
FFalse
Answer: False
Foram delta-18O reflects both the temperature of calcification (equilibrium fractionation between calcite and water) AND the delta-18O of the seawater, which changes with global ice volume. During ice ages, preferential removal of 16O-rich water into ice sheets leaves the ocean enriched in 18O. This ice-volume effect and the temperature effect both increase foram delta-18O during glacials, and separating them requires independent constraints -- typically Mg/Ca ratios for temperature, allowing delta-18O-seawater (ice volume) to be calculated by difference.
Question 3 Short Answer
Explain why oxygen isotope paleothermometry requires knowledge of the delta-18O of the water in which the mineral formed.
Think about your answer, then reveal below.
Model answer: The equilibrium fractionation equation for carbonate relates delta-18O-carbonate to both temperature and delta-18O-water: T = a - b*(delta-18O-carbonate - delta-18O-water) + c*(delta-18O-carbonate - delta-18O-water)^2. The mineral's isotopic composition depends on both variables. If delta-18O-water is unknown, the equation has two unknowns and temperature cannot be uniquely determined. For modern ocean carbonates, seawater delta-18O is known (~0 per mil VSMOW). For ancient samples, seawater delta-18O must be estimated from ice-volume proxies or assumed, introducing uncertainty that is often the dominant error source in paleotemperature estimates.
The mineral records the temperature and the water composition simultaneously -- disentangling them requires additional constraints.