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

Low-temperature synthesis experiments on ferric oxide–water systems have resulted in disparate oxygen isotope fractionation–temperature (a-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 (dO) of the water from which ferric oxides precipitated can be independently measured or estimated. This allows comparison of experimental a-T relationships with data from natural systems. Selective dissolution methods were refined for the pretreatment of fine-grained minerals in order to obtain reliable dO values for pure and crystalline ferric oxides. The difference (DdO) between the dO value of goethite and that of local mean meteoric water ranges from 21.5 to 16.3‰ 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 dO of formation water and that of local mean meteoric water, induced probably by O-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 dO between goethite and formation water is ;21.5‰ 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 dO 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 © 2000 Elsevier Science Ltd