63 Soil Respiration: From Human to Geologic Time Scales

Tuesday, 7 October 2008
8:00 AM - 12:00 PM

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

Paper number: 63-6
Program: JOINT SESSIONS
Time: 9:25 AM – 9:45 AM

Authors

Abstract

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: CS = CA*(δ13CA(CC)-δ13CO(CC))/(δ13Cm(CC)-δ13CO(CC)), where CS and CAare values (ppmV) of CO2in the soil and troposphere, respectively, δ13CA(CC) and δ13CO(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 δ13Cm(CC) is the δ13C value of paleosol calcite. δ13Cm(CC) is measured directly; δ13CA(CC) and δ13CO(CC) may be estimated by δ13 values of contemporaneous marine calcite and in-situ paleosol organic matter, respectively. If CA is known through independent proxies, then CS may be reported in terms of quantitative values. Alternatively, CS 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.