Deuterium and oxygen 18 in precipitation: Modeling of the isotopic effects during snow formation

TLDR

The classical Rayleigh model, which assumes isotopic equilibrium, cannot explain the deuterium and oxygen‑18 contents of polar snow, predicting unrealistically high temperature‑isotope gradients, low δD‑δ18O slopes, and an excessive range of deuterium excess values. The study introduces a new model incorporating an isotopic kinetic effect during snow formation in supersaturated vapor over ice. The model’s kinetic effect is derived from microphysical considerations and validated with laboratory experiments and field observations. The new formulation successfully reconciles predicted and observed temperature‑isotope and δD‑δ18O relationships for realistic supersaturation levels over ice.

Abstract

The classical Rayleigh model assuming isotopic equilibrium fails to explain the deuterium and oxygen 18 contents of polar snow. This model leads to too high temperature‐isotope gradients (both for δD and δ 18 O), to too low δD ‐ δ 18 O slopes, and consequently to an excessively large range of deuterium excess values ( d = δD ‐ 8δ 18 O). We present a new model that takes into account the existence of an isotopic kinetic effect at snow formation as a result of the fact that vapor deposition occurs in an environment supersaturated over ice. This kinetic effect is thoroughly discussed from a microphysical point of view and tested against experimental data and field observations. This new formulation reconciles predicted and observed values both for the temperature‐isotope and δD ‐ δD 18 O relationships for reasonable values of supersaturation over ice.