HR: 0830h
AN: B31E-0356    [PDF]
TI: The global iron budget: estimates of isotopic composition and elemental fluxes
AU: * Fantle, M S
EM: mfantle@eps.berkeley.edu
AF: Univeristy of California, Dept. of Earth and Planetary Science, 307 McCone Hall, Berkeley, CA 94720-4767
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Univeristy of California, Dept. of Earth and Planetary Science, 307 McCone Hall, Berkeley, CA 94720-4767
AB: Recent advances in the field of Fe isotope geochemistry show that isotope fractionation occurs under controlled conditions as a result of reduction, organic complexation, aqueous speciation, and/or hydroxide precipitation. These processes are integral to weathering, and are especially important for mobilizing Fe in an oxidizing environment. Our measurements of the Fe isotopic composition of soils, rivers, and marine sediments demonstrate significant isotopic variations (ca. 4$\permil $ in $^{56}$Fe/$^{54}$Fe) due to continental weathering. Based on these measurements, we hypothesize that dissolved Fe in rivers is variable but may be up to 3$\permil$ lighter than igneous rocks. The flux and isotopic composition of dissolved riverine Fe could significantly affect the $\delta$$^{56}$Fe of the ocean over geologic time, indicating changes in the productivity of the terrestrial biosphere or oxidation state at the Earth's surface. A preliminary global Fe budget is constructed for the modern oxidized surface Earth based on studies of global Fe fluxes in the literature. The total Fe flux to the modern ocean is dominated by riverine material, but the flux of dissolved Fe is roughly equally partitioned between atmospheric, riverine, and hydrothermal sources. There may be a significant diagenetic Fe flux to the water column from marine sediments, which we hypothesize has an Fe isotope composition that is substantially lighter than igneous rocks. A diagenetic Fe flux may be especially important in high productivity regions, such as the coastal ocean and enclosed near-shore basins, and during periods in Earth history when large portions of the ocean turn anoxic. The Fe isotopic signals of the diagenetic and riverine Fe fluxes, if preserved in the geologic record, may be useful for characterizing changing levels of biological productivity and oxidation state at the Earth's surface through time.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1886 Weathering (1625)
DE: 4805 Biogeochemical cycles (1615)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting