Diagenetic Ge-Si fractionation in continental margin environments: further evidence for a nonopal Ge sink

We present data for dissolved germanic and silicic acids from several settings: sediment pore water profiles collected from the Peru-Chile continental margin, fluxes measured with in situ benthic flux chambers and shipboard whole-core incubations, and water column profiles from the California continental margin. Collectively, these data show that Ge and Si are fractionated in these continental margin sedimentary environments during diagenesis with ~50% of the Ge released by opal dissolution being sequestered within the sediments. The areal extent of this diagenetic fractionation covers station depths from ~100 m to >1000 m. Sediments from these sites typically have high pore water Fe2+ present in the upper ~2 cm. At sites with low Fe2+ concentrations in the upper pore waters, the Ge:Si benthic regeneration ratio indicates little or no fractionation during diatom dissolution. Consistent with the sedimentary fractionation, water column dissolved Ge:Si ratios along the continental margin are on average lower (0.66 (mu)mol/mol) than the global average ratio (0.72 (mu)mol/mol, [King et al., 2000]). This lower "average" ratio is driven by two distinct Ge:Si data trends having similar slopes but different intercepts. Data from the upper ~150 m has a Ge:Si slope of 0.74 ± 0.04 (mu)mol/mol (2(sigma)) and an intercept of 0.5 ± 0.4; whereas below ~550 m the slope is 0.70 ± 0.06 (mu)mol/mol, but the intercept is -5.0 ± 8.0. Assuming that the sediments sampled here are representative of all reducing marine environments, an assumption requiring further testing, our calculations indicate that sequestration of Ge into a nonopal phase throughout the global ocean in the depth range 0.2–1 km is sufficient to balance the Ge budget. Thus, we tentatively conclude that sequestering of Ge in reducing continental margin sediments is the "missing" Ge sink.