Oxygen isotopic composition of ferric oxides from recent soil, hydrologic,and marine environments

Low-temperature synthesis experiments on ferric oxide-water systems have re sulted in disparate oxygen isotope fractionation-temperature (alpha -T) cur ves. In this study, recent ferric oxides, mostly goethites of Holocene age, were collected and analyzed from a variety of modern soil, stream, and mar ine environments, where formation temperature and the oxygen isotopic compo sition (delta O-18) of the water from which ferric oxides precipitated can be independently measured or estimated. This allows comparison of experimen tal alpha -T relationships with data from natural systems. Selective dissolution methods were refined for the pretreatment of fine-gra ined minerals in order to obtain reliable delta O-18 values for pure and cr ystalline ferric oxides. The difference (Delta delta O-18) between the delt a O-18 value of goethite and that of local mean meteoric water ranges from -1.5 to +6.3 parts per thousand for soil goethites from New Jersey, Indiana , Michigan, Iowa, South Dakota, and Taiwan. We argue that these variations are largely the result of, differences between the delta O-18 of formation water and that of local mean meteoric water, induced probably by O-18-enric hment of soil waters by evaporation or other processes in soil horizons whe re ferric oxides are forming. A marine goethite sample from Scotland and a subaqueous bog iron sample from New Jersey, which can not be biased by evap orative processes, provide crucial natural evidence that the difference in delta O-18 between goethite and formation water is similar to -1.5 parts pe r thousand at similar to 10 degreesC. This result is consistent with our pr ior laboratory synthesis results (Bao and Koch, 1999), but in conflict with other experimental calibrations. Given the highly variable delta O-18 valu e of soil or other surface water, as well as the potential of initially for med ferric oxides for reequilibration with subsurface burial fluids during maturation to crystalline phases, an understanding of formational and diage netic conditions is absolutely essential when attempting to use the oxygen isotope composition of ferric oxides as a paleoclimatic proxy. Copyright (C ) 2000 Elsevier Science Ltd.