Influence of environment on soil carbonate clumped isotope records, Andean piedmont of Central Argentina (33-34⁰S)

Abstract:

Clumped isotope thermometry measures the formation temperature (T(Δ ₄₇)) of authigenic carbonates at or near the Earth’s surface. Pedogenic carbonates are a common target for clumped isotope studies of paleoaltimetry and paleoclimate. However, the interpretation of T(Δ₄₇) data from soil carbonates is limited by uncertainties in the timing and conditions responsible for carbonate formation. This study examines the potential for variability in soil temperatures at five soil pits in the summer-precipitation dominated eastern Andean piedmont. Four of the pits were excavated at ~1 km elevation over a ~ 40 km N-S transect between the Divisadero Largo Nature Reserve and the Villavicencio Nature Reserve in Argentina’s Mendoza Province (~33⁰S). The substrate for each soil pit is coarse-grained conglomerate. Vegetation at each of these sites is dominated by C3 plants including genus Larrea shrubs (creosote bush), cactus and grasses. Three of the pits were instrumented with soil temperature and soil moisture sensors to a depth of 1 m. The fifth pit, at an elevation of 0.6 km and 34°S in the Nacuñan Nature Reserve, lies in a mixed C3/C4 vegetation environment that is dominated by C4 grass biomass. This site was sampled to 1 m depth and instrumented with both soil (moisture, temperature and CO₂ concentration) and atmospheric sensors (temperature, relative humidity, insolation and rainfall amount).

Measured carbonate δ¹⁸O values at all sites exhibit a decrease with depth, indicating undisturbed soil profiles. Consistent with a previous study on the eastern flank of the Andes, average T(Δ₄₇) values for all sampled pits were found to vary little with depth. The mean T(Δ₄₇) value for the N-S transect bordering the mountains is 32°C ± 3°C (± 1SE). T(Δ₄₇) for the Nacuñan pit is 30°C ± 3°C. These temperatures are 10°C higher than those previously reported from a 1 m pit at 1.1 km in the nearby Río Mendoza valley. This suggests the potential for substantial variability in T(Δ₄₇) at the same elevation for a region dominated by a single precipitation and vegetation regime and elevation. The invariance of temperatures with depth suggests that carbonate formation occurs in the spring or fall when the soil profile is near isothermal, or that some other temperature dependent process controls carbonate formation in semi-arid desert soils.