Altitudinal variation of the stable isotopes of snow in regions of high relief

Abstract

A major discontinuity in the variation of δ18 (δD) with altitude in high mountains was first seen in data from Mount Logan, Yukon Territory, Canada (Holdsworth and others, 1991). The profile of δ vs altitude revealed three well-defined regions: (1) a lower, monotonic, fractionation sequence below ∼3 km; (2) a middle layer, typically 1-2 km thick, within which δ values are nearly constant or stepped with altitude, and (3) part of another fractionation sequence in the "quasi-geostrophic flow region" above ∼5.3 km. The middle region was inferred to be a "mixed layer", combining moisture from regions (1) and (3). This type of structure is now seen to occur on other high-altitude mountains, including Cerro Aconcagua, Argentina, where observations reach almost 7 km. The new observations confirm the general occurrence of a multi-layered atmosphere during precipitation at high-altitude glacier sites. This structure is linked to synoptic-scale polar cyclones, where the middle layer is identified as being the signature of the warm-front zone. These results have implications for the common practice of using a specific, spatially derived, isotopic thermometer in the time domain for the paleoclimatic interpretation of high-altitude ice-core δ records.