Stable carbon and oxygen isotopes of pedogenic carbonates, Ajo Mountains, southern Arizona: implications for paleoenvironmental change

In this study we compare the stable-isotope composition of late Holocene pedogenic carbonate from a late Holocene soil developed on the pediment below the Ajo Mountains in southern Arizona (USA) with measurements of the δ18O of the soil water throughout the seasonal cycle and with δ18O and δ13C measurements on older, dated soils. The comparison with the soil-water composition shows that the pedogenic carbonate is forming in equilibrium with highly evaporated soil water at the highest summer temperatures. The δ13C of the carbonate apparently reflects equilibrium with CO2 of an isotopic composition consistent with the present mixture of C3 desert shrubs and CAM cactus. Comparison with glacial-period soil carbonates shows that there has been little change in the δ18O, but that the δ13C was about 4.5‰ heavier. Independent evidence indicates that glacial-period summer temperatures were probably much cooler, and that the δ18O of precipitation was also lighter. The increase in the carbonate-water fractionation factor with decreasing temperature probably counteracted the decrease in the δ18O of precipitation, producing little net change in the δ18O of the soil carbonate. We attribute the decrease in the δ13C to replacement of a glacial-period C4 grassland by Holocene C3/CAM desert shrubs and succulents. This replacement was probably in response to an increase in temperature and reduction in summer precipitation at the end of the last glacial period. The δ13C of older soil carbonates indicates that landscape was dominated by grassland for most of the past 700 ka and that the present vegetation likely represents a relatively brief anomaly.