Stream-water stable isotopes, moisture fluxes, complex topography and isotopic fractionation along the South Central Andes
Abstract
Stable-isotope data and modeling studies have focused on establishing a relationship between elevation and the δ18O and δD of precipitation in order to interpret paleo-isotope data in terms of past moisture sources and changes in the distribution of rainfall. Far less is known about how local topographic features such as the height and relief of orographic barriers and/or valley geometry influence stable isotope fractionation by modulating moisture pathways. We establish regional characteristics of stable-isotopes in rainfall and stream water from NW Argentina, and quantify the impact of topography from different morphotectonic regions (e.g., broken foreland, fold and thrust belt, orogenic plateau) on the degree of isotopic fractionation. The Central Andes constitute a major orographic barrier to atmospheric circulation. Moisture-bearing air masses rise along their eastern flanks, leading to heavy convective rainfall during austral summer. We present results of the δ18O and δD composition of water from 219 streams along three E-W transects between 22°S and 28°S, crossing several orographic barriers, intermontane basins and the Puna Plateau. Stream-water δ18O for the different segments ranges between - 3.6 to -6.7 ‰ (VSMOW) in the southern broken foreland, between -5.3 to -11.4 ‰ in the transition zone between the Subandes foreland-fold-and-thrust belt and the broken foreland, and between -6.1 to -11.6 ‰ in Subandes. In combination with TRMM-satellite derived rainfall and a thermodynamic model for the isotopic evolution of precipitation, our data reveal: (1) a strong N-S gradient in the effectiveness of isotopic fractionation in relation to different topographic configurations of the orographic front and plateau flank, resulting in a two- to three-fold difference in the efficiency of rainwater isotopic fractionation ; (2) a strong relationship between the isotopic signal and elevation along the wedge topography of the Subandes, contrasting with a weak relationship in the complex topography of the broken foreland system, due to the characteristics of moisture pathways through the intermontane basin zone; (3) a "critical" orographic barrier that forces an additional 50% rain-out of the remaining moisture behind the first orographic barrier to the east of the Puna and an orographic barrier threshold elevation between 2 to 2.5 km; and (4) an unequal distribution of catchment rainfall, resulting in a deviation from expected catchment isotopic values. Thus, in addition to different moisture sources, complex topography significantly modifies modern stable-isotope values. The influence of the topographic pathway is most pronounced in the broken-foreland province, which provides multiple pathways for moisture to penetrate into the orogen, resulting in poor correlations between isotopic fractionation and elevation. In contrast, the laterally-continuous orographic ramp of the Subandes yields strong correlations between isotopic fractionation and elevation. Therefore, isotopic records derived from soil-carbonates, plant-biomarkers, volcanic glass, and teeth in areas with similar complex topography may be problematic in reflecting elevation changes and tracking the tectonic build up of orography.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T21E2606R
- Keywords:
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- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 1854 HYDROLOGY / Precipitation;
- 8102 TECTONOPHYSICS / Continental contractional orogenic belts and inversion tectonics;
- 8177 TECTONOPHYSICS / Tectonics and climatic interactions