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 μmol/mol) than the global average ratio (0.72 μmol/mol, Mortlock and Froelich, 1996). This lower “average” ratio is driven by two distinct ΔGe:ΔSi data trends having similar slopes but different intercepts. Data from the upper ∼150 m has a Ge:Si slope of 0.74 ± 0.04 μmol/mol (2σ) and an intercept of 0.5 ± 0.4; whereas below ∼550 m the slope is 0.70 ± 0.06 μmol/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.