The stable isotopes of water, measured in melt samples taken from snow pits and cores at locations between 1750- and 5930-m altitude on Mount Logan (5951 m) and between 2900 and 4900 m on Mount Steele (5079 m), in the Saint Elias Mountains, Yukon, show a distinctive altitudinal distribution. Several delta-O-18 and delta-D versus altitude profiles indicate the general persistence of a nearly iso-delta step, or staircase structure, separating a lower region of altitude dependent isotopic fractionation between 1750 and 3350 m from another apparent fractionation sequence appearing above about 5300 m. Both of these sequences, but especially the lower one, indicate orderly processes. On the one hand, postdepositional changes to isotope ratios in snow at different altitudes may cause distortions to an otherwise nearly monotonic isotope fractionation sequence, but the main anomaly appears to be far too large to be explained in this way. On the other hand, an explanation linked to processes occurring in the lower and midtroposphere is based on established meteorological principles as well as on upper air data. This hypothesis is proposed as the primary one to explain the gross features of the observed isotope profiles. It is compatible with the concept of secondary-source moisture arriving via the upper troposphere, and it does not exclude the effects of postdepositional stratigraphic and stable isotope ratio changes. Over interannual time scales, any vertical modulation of the observed isotope-altitude structure, from, for example, changes in wind regime, would give rise to an additional signal in any ice core delta time series. These findings identify a potential difficulty in the interpretation of stable isotope records obtained from high mountain ice core sites. It is possible that the results may have application to atmospheric circulation modeling, where the effects of extreme topography are being studied.
