Possible Implications of Variations of Carbon Dioxide Concentration and 13C/12C In a Mid-Cretaceous Paleosol as Estimated from the Fe(CO3)OH Component of Goethite

The concentrations and δ¹³C 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 δ¹³C 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 δ¹³C 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 D_S (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 D_S 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.