HR: 0800h
AN: B51D-0244    [Abstracts]
TI: The use of Ge/Si ratios to quantify Si transformations in grassland ecosystems
AU: * Blecker, S W
EM: sblecker@nrel.colostate.edu
AF: Colorado State University, Soil and Crop Sciences Campus Delivery 1170, Fort Collins, CO 80523 United States
AU: Derry, L A
EM: lad9@cornell.edu
AF: Cornell University, Earth and Atmospheric Sciences, Ithaca, NY 14853 United States
AU: Chadwick, O A
EM: oac@geog.ucsb.edu
AF: Univ. of California - Santa Barbara, Geography, Santa Barbara, CA 93106 United States
AU: Kelly, E F
EM: pedoiso@lamar.colostate.edu
AF: Colorado State University, Soil and Crop Sciences Campus Delivery 1170, Fort Collins, CO 80523 United States
AB: Germanium (Ge) has been shown to behave as a heavy isotope of silicon (Si), enabling the use of Ge/Si ratios as a weathering tracer in terrestrial environments. The two major mechanisms of Ge/Si fractionation in soils result from mineral weathering reactions and biogenic silica formation by plants. The role of plants in Ge fractionation has been deduced from relatively few field studies, and geochemical Ge fractionation data in temperate systems are lacking. The objectives of this research were to quantify biologic Ge fractionation, and to utilize differences in Ge/Si values among the major biogeochemical pools across a grassland bioclimosequence to examine stream water silica provenance. Quantification of biological Ge fractionation was carried out under controlled experimental conditions. Plant phytoliths grown in hydroponic solutions fractionated against Ge (comparing Ge/Si_solution with Ge/Si_phytolith) by an average of 82%. Differences in Ge/Si values between roots, stems, and leaves indicate fractionation likely occurs at the root/solution interface. Phytoliths from plants grown in two different soil mediums fractionated against Ge, averaging 44% to 63%, with no clear trends among the species. From the field study, the greater fractionation factor (Kw, where Kw = (Ge/Si_clay)/(Ge/Si_bedrock)) of the tallgrass (Kw =2.8) vs. shortgrass sites (Kw =1.4) results from the increased weathering intensity across the bioclimosequence. Plant phytoliths exhibit relatively low Ge/Si values (0.15-0.44; x =0.29; n=15), compared to those of the corresponding surface soil water Ge/Si (0.22-0.94; x =0.66; n=6). Stream water Ge/Si values along the grassland climosequence (0.07-1.29, x = 0.34; n = 20) are typical of natural water Ge/Si values. Higher groundwater Ge values (0.42-3.4; x = 1.3; n=16) may represent an increased residence time or contact with minerals of higher Ge/Si ratios. The lack of Ge/Si separation among the major terrestrial pools confounds stream Si provenance. The data suggest that stream water Si is not derived entirely from ground water and that biogenic sources are plausible.
DE: 0400 BIOGEOSCIENCES
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0486 Soils/pedology (1865)
DE: 1879 Watershed
SC: Biogeosciences [B]
MN: Fall Meeting 2005