Rusty Relics of Earth History: Iron(III) Oxides, Isotopes, and Surficial Environments

The two most common low-temperature iron(III) oxides on Earth are goethite (α-FeOOH) and hematite (α-Fe2O3). The δ¹⁸O values of natural goethites range from −15.5‰ to +3.3‰, whereas δ¹⁸O values of low-temperature hematites range from −16.7‰ to +4.7‰. Plots of δD against δ¹⁸O for continental goethites are approximately parallel to the meteoric water line of Craig (H Craig. 1961. Science 133:1702–3). This suggests that goethite-water fractionation factors are systematic over a wide range of surficial environments and may indicate that isotopic equilibrium is commonly attained or closely approached. Several experimental or calculated mineral-water, oxygen isotope fractionation curves have been determined for both goethite and hematite. Although there is not yet a consensus on which of these curves best approximates isotopic fractionation in natural samples, oxygen isotope measurements of both goethite and hematite have provided evidence of significant continental climate change on time scales that range from thousands to millions of years. The concentration and δ¹³C values of an Fe(CO3)OH component in apparent solid solution in goethite are proxies for the partial pressure and δ¹³C values, respectively, of CO2 in the environment at the time of goethite crystallization. Biological productivity, CO2 pressures in soil or groundwater, and partial pressures of atmospheric CO2 in ancient environments have been estimated from measurements of the mole fractions and δ¹³C values of Fe(CO3)OH in goethite.