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

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Low-temperature synthesis experiments on ferric oxide-water systems have resulted in disparate oxygen isotope fractionation-temperature (α-T) curves. In this study, recent ferric oxides, mostly goethites of Holocene age, were collected and analyzed from a variety of modern soil, stream, and marine environments, where formation temperature and the oxygen isotopic composition (δ18O) of the water from which ferric oxides precipitated can be independently measured or estimated. This allows comparison of experimental α-T relationships with data from natural systems. Selective dissolution methods were refined for the pretreatment of fine-grained minerals in order to obtain reliable δ18O values for pure and crystalline ferric oxides. The difference (Δδ18O) between the δ18O value of goethite and that of local mean meteoric water ranges from -1.5 to +6.3‰ 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 δ18O of formation water and that of local mean meteoric water, induced probably by 18O-enrichment of soil waters by evaporation or other processes in soil horizons where 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 evaporative processes, provide crucial natural evidence that the difference in δ18O between goethite and formation water is ~-1.5‰ at ~10°C. This result is consistent with our prior laboratory synthesis results (Bao and Koch, 1999), but in conflict with other experimental calibrations. Given the highly variable δ18O value of soil or other surface water, as well as the potential of initially formed ferric oxides for reequilibration with subsurface burial fluids during maturation to crystalline phases, an understanding of formational and diagenetic 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.

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Geochimica et Cosmochimica Acta

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