CONCENTRATION AND CARBON ISOTOPE COMPOSITION OF FE(CO3)OH IN PEDOGENIC GOETHITE OF A MID-CRETACEOUS PALEOSOL IN SOUTHWESTERN MINNESOTA

A mid-Cretaceous (~94 Ma B.P.), pisolite-bearing laterite in southwestern Minnesota, which formed at the eastern margin of the Western Interior Seaway of North America, was sampled for isotopic analysis. Pisolites were handpicked from samples collected at different depths in a single outcrop of the paleosol. X-ray diffraction (XRD) spectra indicate that the following minerals are present in the sampled pisolites: goethite, hematite, gibbsite, quartz, kaolinite, and minor diagenetic berthierine. The concentration of Al substituted for Fe in the goethite structure was determined by XRD and ranged from 14% to 28 mole %. These substituent Al concentrations are typical of pedogenic goethites formed in Al-rich soils. Goethite in each sample was concentrated by selective dissolution to remove all gibbsite and most of the kaolinite before isotopic analysis.

The concentrations and carbon isotope compositions of the Fe(CO₃)OH component in pisolitic goethite (α-FeOOH) at different depths in the soil profile were measured using the CO₂ extracted by incremental vacuum dehydration-decarbonation. Measured mole fractions (X) range from 0.0006 to 0.02, whereas measured δ¹³C values range from -5.4 permil to -22.8 permil. Characteristics of the thermal evolution of CO₂ and a plot of δ¹³C values against 1/X seem to suggest two generations of pedogenic goethite: (1) A possibly younger generation, which evolves CO₂ at lower temperature. This “low-T” CO₂ seems to reflect mixing of three distinct carbon sources in the ambient soil at the time of goethite formation (atmospheric CO₂, soil organic matter, and possibly a local carbonate); (2) An older generation evolving CO₂ at higher temperature with CO₂ concentration and δ¹³C values that appear to represent a mixing array with a positive slope defined by two-components (atmospheric CO₂ and organic matter-derived CO₂). This positive slope implies that the inferred two-component mixing array could be a source of information on ancient atmospheric CO₂ pressures during the warm climate of the mid-Cretaceous. However, there is an interesting complication to this apparent two-component mixing array. Its constituent data display a reversed relationship with depth, with samples at shallower depth having elevated CO₂ concentrations and more negative δ¹³C values.