Hydrogen isotope composition of dry season atmospheric water vapor on Quelccaya Ice Cap, Peru

In-situ measurements of modern meteorological conditions at Quelccaya Ice Cap's summit, including the isotopic composition of atmospheric water vapor, may aid in the interpretation of the 1500-year, annually resolved ice-core record available from the site (Thompson et al., 2003). Betweeen July 7 and July 9, 2011, we collected 11 samples of atmospheric water vapor from the summit of Quelccaya and analyzed the hydrogen isotopic composition on a Finnegan MAT-252 mass spectrometer using the method of Strong et al 2007. δD values ranged from -134% to -168%, and specific humidity ranged from 1.5 to 3 g/kg. The isotopic composition of tropical Andean ice cores has been variously interpreted in terms of simple Rayleigh distillation models, in which water evaporates from the tropical Atlantic and condenses as it moves upslope (Grootes et al., 1989; Pierrehumbert, 1999), or in terms of the condensation temperature (Thompson et al., 2003). The joint distribution of water vapor concentrations and δD values in our dataset cannot be explained by a simple upslope Rayleigh distillation model. Such a model predicts higher water-vapor concentrations and lower δD values than those measured during the sampling period. We hypothesize that the joint distribution of water vapor mixing ratio and isotopic composition can be explained by large-scale mixing of air parcels that were last saturated in the upper tropical troposphere. Such mixing necessarily leads to parcels that have higher delta values than would be expected for the simple Rayleigh distillation to the observed mixing ratio. Local effects of snow sublimation may exert additional controls over the water-vapor mixing ratio and delta values. Further monitoring during both the wet and dry seasons may clarify the relationship between large-scale water-vapor transport and the snow and ice preserved on Quelccaya. References Friedman, I. (1953) Deuterium content of natural waters and other substances, Geoch. et Cosmochim. Acta, 4, 89-103. Grootes, P.M., Stuiver, M., Thompson, L.G., Mosley-Thompson, E. (1989) Oxygen isotope changes in tropical ice, Quelccaya, Peru, J. Geophys. Res., 94, 1187-1194. Pierrehumbert, R.T. (1999) Huascaran δ18O as an indicator of tropical climate during the last glacial maximum, Geophys. Res. Lett., 26, 1345-1348. Strong, M., Sharp, Z.D., Gutzler, D.S. (2007) Diagnosing moisture transport using D/H of water vapor, Geophys. Res. Lett., 34, L03404, doi:10.1029/2006GL028307. Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Lin, P.N., Henderson, K., Mashiotta, T.A. (2003) Tropical glacier and ice core evidence of climate change on annual to millennial timescales, Clim. Change, 59, 137-155.