SYSTEMATIC VARIATION OF STABLE CARBON AND OXYGEN ISOTOPES WITH ELEVATION IN SOIL CARBONATE FROM THE ATACAMA DESERT, CHILE

Stable carbon and oxygen isotopic measurements were determined for soil carbonate along an elevational gradient across the arid Pacific slope of the central Andes, Atacama Desert, Chile. We analyzed both young (stage 1) soil carbonate and modern vegetation to characterize the current relationship between vegetation and isoptopic values of soil carbonate, and old (stage 2-4) soil carbonate in an attempt to track past vegetation incursions into what is now barren desert. Soil carbonate is present in this region between ~2500m-4000m. Modern vegetation belts include: 1) Andean Steppe (3800-4500m), consisting of C3 perennial grasses and some C4 annual grasses, 2) Tolar (3200-3800m), containing C3 shrubs, some Atriplex (C4), and some C4 annual grasses, 3) Pre-Puna (2600-3200m), comprised of Atriplex, cushion cacti (CAM), and C3 annuals. Below ~2500 m there are no vascular plants and soils salts are predominately gypsum. Important results from the modern vegetation/soil carbonate isotopic system include: 1) enriched d13C values of C3 vegetation in the Andean Puna grasslands, with an average value of -22.35‰, 2) enriched d13C values of soil carbonate from depth (> 50 cm) with respect to vegetation (e.g. values of -4‰ for Andean steppe carbonate and from -1 to +2‰ for Tolar and Pre-Puna carbonate), suggesting a strong atmospheric CO2 component to soil carbonate as a result of low respiration rates, and 3) the strong influence of local microclimatic features such as slope aspecton isotopic values of soil carbonate. Isotopic values of Stage 3 soil carbonate were depleted by 1-3‰ over Stage 1 carbonates at the same location, suggesting that older soil carbonates formed in equilibrium with vegetation belts that today are at higher elevations. These results have broad implications for the use of stable isotopes in soil carbonate for reconstruction simple C3/C4 vegetation abundances and for the use of soil carbonate for tracking past vegetation migrations.