The geochemistry of inorganic germanium in natural waters

Abstract

We present the initial results from a survey of the distribution of dissolved inorganic germanium in rivers, in the oceans, and in hydrothermal vents and plumes. In the ocean, germanium cycling mimics that of silica, since siliceous organisms incorporate Ge as a trace constituent in biogenic opal. The vertical and horizontal distributions of Ge concentrations in seawater are thus identical to those of silica (Ge/Si ∼ 0.7 × 10−6) and reflect uptake into and dissolution from the frustules of siliceous organisms. Germanium enters the ocean via dissolution of continental and seafloor silicates. The naturally weathered flux from continents to oceans carries a dissolved Ge/Si atom ratio in rivers of about 0.7×10−6, which reflects the ratio in average continental granites. Rivers draining industrial regions with significant coal combustion are contaminated with Ge leached from fly ash particles, resulting in Ge/Si ratios up to tenfold above the naturally weathered background. The flux to the oceans via high‐temperature alteration of seafloor basalts (hot springs) produces a Ge anomaly in hydrothermal plumes over the East Pacific Rise crest (20°S). Combining the observed Ge and 3He anomalies in plumes with 3He/Si observations in hot springs suggests that the Ge/Si hydrothermal flux to the oceans is about 4×10−6, near the ratio for average oceanic basalts (∼2.6×10−6). Vent effluents from the Galapagos hot springs measured directly display Ge/Si ratios of 16×10−6, suggesting that the ratio is variable between vent fields. Metalliferous sediments accumulating on the EPR crest (17°S) are enriched in germanium, suggesting that a fraction of the hydrothermal germanium source on the rise crest is scavenged into these rapidly accumulating iron and manganese hydroxyoxide phases.