An assessment of isotopic equilibrium in goethites from a bog iron deposit and a lateritic regolith

Abstract

Hydrogen and oxygen isotope ratios have been measured in goethites from a lateritic regolith in the eastern Amazon Basin of Brazil and a young bog iron deposit in New Jersey, USA. The presence of exchangeable hydrogen and admixed minerals required the use of material-balance calculations to determine relevant δD and δ 18O values. For the Brazilian goethite, δD is −121‰ and δ 18O of −1.2‰ (adjusted for Al content). These values yield a calculated temperature of formation of 24 ± 3°C, which is in good agreement with the modern average annual temperature in that locale. Goethite from the New Jersey bog has a δD of −134‰ and δ 18O of −0.9‰. Its calculated temperature of formation (23 ± 3°C) resembles summer temperatures in southeastern New Jersey. Bacterially mediated rates of precipitation of ferric hydroxide appear to be higher in the bog during the summer months (Crerar et al., 1979). Thus, the temperature calculated for the New Jersey goethite could indicate that the conversion of ferric hydroxide to goethite also occurs predominantly in the summer. δ 13C values of the Fe(CO 3)OH component in goethite (−21.7‰ for Brazil and −19.3‰ for New Jersey) reflect the δ 13C values of the associated organic carbon (−28.4‰ for Brazil and −26.3‰ for the New Jersey). Values of P CO 2 in the regolith (0.063 bar) and bog (0.039 bar) were calculated from the measured abundances of the Fe(CO 3)OH component. The values of δ 13C and P CO 2 obtained from the goethites suggest that the ambient CO 2 in these systems originated primarily from oxidized organic matter. In the low-pH, high- P CO 2 waters characteristic of bogs and laterites, isotopic equilibrium may be closely approached during the formation of goethite from ferric hydroxide (ferrihydrite), even if the initial precipitation of the ferric hydroxide precursor is bacterially mediated. Therefore, meaningful paleotemperatures might commonly be calculated from stable isotope ratios of nonexchangeable oxygen and hydrogen in geothites from such environments.