Stable isotope composition of precipitation across the southern Patagonian Andes

Abstract

A surface water and precipitation transect across the southern Patagonian Andes at 47°–48°S was conducted to assess how mountains affect the isotopic composition of precipitation. This westerly wind region derives its moisture that falls as precipitation from the Pacific. Orographic uplift of air over the Andes causes pseudoadiabatic cooling and orographic precipitation on the western side and a strong rain shadow on the eastern side of the mountains. These processes also produce a profound isotopic rain shadow, with δ18O values ∼4‰ lower in the east compared to the west. On the windward western side of the Andes, the isotope values of precipitation and surface waters show weak, although systematic, trends with elevation, but a pronounced correlation with distance from the main source of moisture (Pacific Ocean). A Rayleigh simulation of precipitation δ18O values as a function of condensation altitude matches our data well, suggesting that other processes, such as mixing of water sources and postcondensation evaporation, are negligible, and that topography is the dominant control on the isotope ratio of precipitation. In contrast, isotope ratios of waters on the leeward eastern side of the southern Patagonian Andes vary neither with elevation nor with distance from the mountains, consistent with much of the precipitation on the leeward side being derived from precipitation carried over the Andes by winds. Thus, paleoprecipitation isotopic composition on the leeward side of orogens in similar climatic settings (single moisture source and cool climate) yield useful estimates of the elevation of these orogens, but not local elevation.