The Stable Carbon and Oxygen Isotopic Composition of Pedogenic Carbonate and its Relationship to Climate and Ecology in Southeastern Arizona

The stable carbon (δC) and oxygen (δO) isotopic composition of terrestrial carbonate has been used to reconstruct late Quaternary paleoecological and paleoclimatic conditions, respectively, for many different regions of the world. Quantitative reconstructions of past variability in climate and the distribution of C3/CAM/C4 vegetation from carbonate in soils and speleothems depend upon a rigorous examination of the modern soil isotopic system. To accomplish this, we examined changes in the δC and δO in relation to modern climatic and ecological conditions along an elevation gradient in southeastern Arizona. Five sites were selected for study, spanning 1,170 m of elevation. Along this gradient, δC and δO values from ≥50 cm soil depth range from -9.9 to -0.6‰ and from -9.4 to -1.3‰, respectively. Modeling results suggest that δC values were determined by soil respiration rates and the proportion of C3/CAM/C4 biomass. For sites with low respiration, δC values from >50 cm reflected an atmospheric contribution of up to 55% compared to <20% for sites with much higher respiration rates. At the lowest respiration sites, maximum observed δO values from >50 cm diverge from minimum (winter) predicted values by +4.3 to +7.1‰, reflecting the influence of evaporation. In contrast, values for the highest respiration rate site fell entirely between those predicted from winter and summer rainfall. The latter finding suggests that a significant proportion of carbonate may form during winter, and that there is a positive correlation between respiration rate and the ratio of transpiration to evaporation accounting for soil drying. Results suggest that the reconstruction of absolute changes in vegetation composition from carbonate isotopic composition will require quantification of the influence of soil respiration on δC values. In turn, this knowledge can be used to quantify the maximum extent of evaporation