Carbon Isotope Geochemistry of Co-Existing Paleosol Calcite and Organic Matter: Indicator of Soil CO2 Concentration, Actual Evapotranspiration, and the Driest Landscapes on Earth

We present a model to resolve differences in soil-respired CO₂ and productivity, actual evapotranspiration (AET), and aridity across penecontemporaneous Middle and Late Permian landscapes by using the δ¹³C values of co-existing pedogenic calcite and organic matter. Soil-respired CO₂ may be estimated by the following equation: C_S = C_A*(δ¹³C_A(CC)-δ¹³C_O(CC))/(δ¹³C_m(CC)-δ¹³C_O(CC)), where C_S and C_Aare values (ppmV) of CO₂in the soil and troposphere, respectively, δ¹³C_A(CC) and δ¹³C_O(CC) are δ¹³C values of calcite formed only from CO₂ derived from oxidation of in-situ oxidation of organic matter and tropospheric CO₂ respectively, and δ¹³C_m(CC) is the δ¹³C value of paleosol calcite. δ¹³C_m(CC) is measured directly; δ¹³C_A(CC) and δ¹³C_O(CC) may be estimated by δ¹³ values of contemporaneous marine calcite and in-situ paleosol organic matter, respectively. If C_A is known through independent proxies, then C_S may be reported in terms of quantitative values. Alternatively, C_S may alternatively be reported as a proportionality among populations of penecontemporaneous paleosol profiles. Middle and Late Permian paleosol calcite δ¹³C values collected thus far range from -3±1‰ to -12±1‰, whereas organic matter δ¹³C values exhibit relatively little variability. These results indicate a broad range of soil PCO₂ across the Middle and Late Permian landscapes.

Soil-respired CO₂ is strongly correlated with actual evapotranspiration (AET) from the soil. Soils with low AET values occur in dry (or very cold) climates with low biological productivity. Thus, soil PCO₂ estimates from co-existing paleosol calcite and organic matter permit determination of which landscapes were most arid, and characterized by limited biological productivity. Results indicate that lowest soil PCO₂ occurred in subtropical landscapes, continental interiors of Pangea. These results indicate those sites were among the most arid landscapes upon the supercontinent. These isotope-based results agree well with AET and Biome estimates derived from Global Circulation Models.