Calculation of site-specific carbon-isotope fractionation in pedogenic oxide minerals

Ab initio molecular dynamics and quantum chemistry techniques are used to calculate the structure, vibrational frequencies, and carbon-isotope fractionation factors of the carbon dioxide component [CO₂(m)] of soil (oxy)hydroxide minerals goethite, diaspore, and gibbsite. We have identified two possible pathways of incorporation of CO₂(m) into (oxy)hydroxide crystal structures: one in which the C⁴⁺ substitutes for four H⁺ [CO₂(m)_A] and another in which C⁴⁺ substitutes for (Al³⁺,Fe³⁺) + H⁺ [CO₂(m)_B]. Calculations of isotope fractionation factors give large differences between the two structures, with the CO₂(m)_A being isotopically lighter than CO₂(m)_B by ≈10 per mil in the case of gibbsite and nearly 20 per mil in the case of goethite. The reduced partition function ratio of CO₂(m)_B structure in goethite differs from CO₂(g) by <1 per mil. The predicted fractionation for gibbsite is >10 per mil higher, close to those measured for calcite and aragonite. The surprisingly large difference in the carbon-isotope fractionation factor between the CO₂(m)_A and CO₂(m)_B structures within a given mineral suggests that the isotopic signatures of soil (oxy)hydroxide could be heterogeneous.