THE ISOTOPIC RAIN SHADOW OF THE SOUTHERN PATAGONIAN ANDES: IMPLICATIONS FOR STABLE ISOTOPE PALEOALTIMETRY

The southern Patagonian Andes at 48°S are a narrow range mountain range of modest elevation <4000m oriented perpendicular to strong westerly winds of the region.  Because of this persistent wind direction, and the proximity of the Andes to the Pacific Ocean, this region derives virtually all of its precipitation from Pacific moisture.  The wind patterns and topography generate an orographic rain shadow 3000 mm of annual precipitation to the W, and only 300 mm to the E of the mountains.  This geographic and climatic configuration is optimal for application of stable isotope paleoaltimetry, because of: 1) a single moisture source; 2) simple climatology; 3) modest elevation, preventing divergence of winds around the mountains and 4) a cool climate limiting below cloud base evaporation in lee of the orogen.  

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.  There is a systematic decrease in the d¹⁸O values of 4‰ over ~100km from west to east on the windward, western side of the Andes, corresponding to airmasses rising over the mountains, causing pseudoadiabatic cooling, and generating precipitation.  In contrast, there is almost no variation in isotope ratio of waters on the leeward, east, side of the Andes.   This "isotopic rain shadow", with lower isotope ratios on the leeward eastern side of mountains,  spatially corresponds to the previously documented orographic rain shadow.

Expression of an isotopic rain shadow tends to occur in mountains near to a coast in the zone of westerly wind with relatively cool climates.  This is likely due to negligible below or within cloud evaporation on the leeward side in cool climates.  Whereas, in hot climates, at low latitudes, a more symmetric pattern of isotope ratio with altitude commonly occurs. The main implication of the pattern of decreasing d¹⁸O values on the windward side of the mountains and invariant values on the leeward side is that the isotopic composition of precipitation on the leeward side is controlled by the height of the mountains, but independent from surface elevation of the site.  Thus, in regions with isotopic rain shadows, surficially-formed minerals in the leeward foreland basin may be used to infer changes in the orogen surface elevation.