Tuesday, 7 October 2008: 10:05 AM
George R. Brown Convention Center, General Assembly Theater Hall C
The concentrations and δ13
C of the Fe(CO3
)OH component in pisolitic goethite at different depths in a mid-Cretaceous (~94 Ma) paleosol in southwestern Minnesota were measured using the CO2
extracted by incremental vacuum dehydration. Characteristics of the thermal evolution of CO2
and a plot of δ13
C vs. 1/X (X = mole fraction) seem to suggest two generations of pedogenic goethite that break down at different dehydration temperatures. The working hypothesis is that CO2
evolved at 220°C derives from relict small amounts of goethite that may retain information on the Cenomanian environment. The δ13
C vs. 1/X data array from this goethite has a positive slope (0.0064), which could represent mixing in the soil of two CO2
end-members (atmospheric CO2
and organic matter-derived CO2
). This slope implies an atmospheric CO2
concentration of 1900 ppmV.
The most negative δ13C values and highest values of X for increments of CO2 extracted at 220°C were assumed to have derived from goethite formed at depths that significantly exceeded the characteristic depth of the soil (z*). To facilitate discussion, it was assumed that the soil CO2 was transported by steady state diffusion under conditions of approximately constant DS (soil CO2 diffusion coefficient). For this circumstance, in combination with: 1) a formation temperature of 24°C estimated from the hydrogen and oxygen isotope composition of co-existing kaolinite; 2) the inferred atmospheric CO2 concentration of 1900 ppmV; and 3) specified values for z* of 30 cm, and DS of 0.002 cm2/s, a soil respiration rate of 0.5 gCO2 /m2 /h was calculated. This value is within the range observed for modern soils.