Isotopic characterization of mountain precipitation along the easternflank of the andes between 32.5 and 35°S

HOKE G. D.; ARANIBAR J. N.; VIALE M.; ARANEO D. C.; LLANO C.

Mendoza

Congreso; CONGREMET XI; 2012

Centro Argentino de Meteorólogos

Constraining the influence of different moisture sources within and on the flanks of mountain ranges is important to validate circulation models, and understand the dynamics of water resources on human timescales. Stable isotopes in stream waters and precipitation are an ideal tool for this task. This study aims to characterize the isotope composition of precipitation and river water in the Andes, between 32.5 and 35 °S, in order to identify moisture sources of precipitation along the eastern flank of the Andes. North of 33°S, the 2500 m average elevation Cordillera Frontal forms a steep front orographic barrier and the 2000 m elevation Uspallata Valley separates precordillera from the high Andes. South of 33°S precordillera ends and the Andes return to a simple linear morphology. Synoptic and mesoscale models indicate that the eastern side of the Andes receives predominately summer precipitation from convective storms, usually triggered by daytime upslope flow or synoptic-scale easterly flow over the Precordillera. Moving westward from the low-leeward side to the range crest, the influence of the easterly summer precipitation wanes and winter spillover precipitation from mid-latitude westerly storms begins to dominate. We expected the isotope composition of precipitation to reflect these models, showing different values from East to West. This study reports the results of a two-year quasi-monthly precipitation sampling campaign, from September 2008 to September 2010, in the Mendoza Province of Argentina. A total of 104 precipitation samples from 9 sites were analyzed for dD and d18O. In addition, 81 samples from Andean rivers collected on both sides of the range in 2002 and 2007 were analyzed and utilized in this study. Our results show the local meteoric water line is slightly steeper (~0.5) than the global meteoric water line, with a y-intercept of 14. The most negative d18O values of precipitation vary as much as 15 per mil seasonally, while averages weighted by precipitation amount show a strong cross-barrier (elevation) dependence. At these latitudes, the weighted average precipitation d18O values show a significant deviation from river water near the range crest. We employed a Rayleigh isotope fractionation modeling approach to explore different plausible scenarios to explain our precipitation and river water results. The results indicate that precipitation on the eastern slopes of the Andes at ~33°S can be characterized by a westerly, Pacific-source component at elevations above 2000 m and a mixture of easterly and westerly sources below 2000 m. River water d18O values are best explained by Rayleigh fraction models representing average annual conditions for Valparaiso, Chile or Mendoza, Argentina. Further South at ~35°S, river water samples strongly reflect a winter seasonality. Our data show that the overall trend of more negative values with increasing elevation is robust despite complicated moisture source trajectories at these latitudes in the Andes.