Stable isotope variations (delta O-18 and delta D) in modern waters across the Andean Plateau

作者：Bershaw John^*; Saylor Joel E; Garzione Carmala N; Leier Andrew; Sundell Kurt E

来源：Geochimica et Cosmochimica Acta, 2016, 194: 310-324.

DOI：10.1016/j.gca.2016.08.011

• Stable isotope geochemistry
• Meteoric water
• South America
• Andes
• Altiplano
• Climate
• Paleoaltimetry

Environmental parameters that influence the isotopic composition of meteoric water (delta O-18 and delta D) are well characterized up the windward side of mountains, where orographic precipitation results in a predictable relationship between the isotopic composition of precipitation and elevation. The topographic and climatic evolution of the Andean Plateau and surrounding regions has been studied extensively by exploiting this relationship through the use of paleowater proxies. However, interpretation on the plateau itself is challenged by a poor understanding of processes that fractionate isotopes during vapor transport and rainout, and by the relative contribution of unique moisture sources. Here, we present an extensive dataset of modern surface water samples for the northern Andean Plateau and surrounding regions to elucidate patterns and causes of isotope fractionation in this continental environment. These data show a progressive increase in delta O-18 of stream water west of the Eastern Cordillera (similar to 1 parts per thousand/70 km), almost identical to the rate observed across the Tibetan Plateau, attributed to a larger fraction of recycled water in precipitation and/or increased evaporative enrichment downwind. This may lead to underestimates of paleoelevation, particularly for sites deep into the rainshadow of the Eastern Cordilleran crest. That said, elevation is a primary control on the isotopic composition of surface waters across the entire Andean Plateau and its flanks when considering the most negative delta O-18 values, highlighting the need for sufficiently large datasets to distinguish minimally evaporated samples. There is a general increase in delta O-18 on the plateau from north to south, concomitant with an increase in aridity and decrease in convective moistening (amount effect). Lastly, stable isotope and seasonal precipitation patterns suggest easterlies provide the vast majority of moisture that falls as precipitation across the Andean Plateau and Western Cordillera, from Peru to northern Bolivia (-13 degrees to -20 degrees latitude), with Pacific-derived moisture contributing a minor amount at low elevations near the coast.

• 出版日期2016-12-1