HR: 0830h
AN: B21B-0716    [PDF]
TI: Influence of Pedogenic Iron-oxyhydroxides on the Ge/Si Weathering Tracer
AU: * Scribner, A M
EM: ascrib@bu.edu
AF: Boston University, Dept. of Earth Sciences, Boston, MA 02215
AU: Kurtz, A
EM: kurtz@bu.edu
AF: Boston University, Dept. of Earth Sciences, Boston, MA 02215
AU: Chadwick, O
EM: oac@geog.ucsb.edu
AF: University of California-Santa Barbara, Dept. of Geography, Santa Barbara, CA 93106
AB: Germanium/silicon ratios are a promising tracer of the silica cycle in both marine and terrestrial environments. Prior work shows that Ge/Si ratios are fractionated by at least two distinct mechanisms in the weathering environment: sequestration of Ge by soil clay minerals and biological fractionation during plant phytolith formation. Ge may also be sequestered by pedogenic iron oxyhydroxides, resulting in a decoupling of the Ge and Si geochemical cycles. Ge/Si ratios in Hawaiian soils are high (3.5-25 $\mu$mol/mol) relative to the ratio in the basaltic parent rock (2.5$\mu$mol/mol). We are examining Ge behavior in a well-characterized climate gradient on the Hawaiian island of Maui to elucidate the relationship between Fe geochemistry and Ge/Si ratios. The Maui precipitation gradient is made up of 7 sites on the flank of Haleakala Volcano ranging in mean annual precipitation from 875 mm rain/year to 5066 mm rain/year (Schurr et al., 2001). All sites are developed on $\sim$400 ka parent material and are highly weathered ($>$80% Si loss relative to immobile Nb). Sites experiencing less than 3338 mm rain/year display characteristics typical of oxic, highly weathered basaltic soils. In these soils, Fe weathered from parent material is not mobile and accumulates as secondary Fe(III) minerals (ferrihydrite). The wetter sites ($>$3338 mm/yr) show evidence of extensive Fe reduction, resulting in mobilization and loss. Ge/Si ratios in relatively dry sites ($<$3338 mm rain/year) are as high as 21 $\mu$mol/mol, nearly 10 times higher than parent rock ratios. Fe-depleted horizons in the wettest sites have Ge/Si ratios as low as 3.5 $\mu$mol/mol. Reductive loss of Fe-oxyhydroxides results in a dramatic drop in Ge/Si ratios, suggesting that Ge is in part accumulating in the Fe oxyhydroxide component of soil. However, nearly Fe-free soils maintain Ge/Si ratios elevated with respect to parent material, consistent with partitioning of Ge into secondary soil alumosilicates. Regardless, the influence of pedogenic Fe-oxyhydroxides must be taken into account when using the Ge/Si system as a weathering tracer.
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 1065 Trace elements (3670)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting