The Contrasting Behaviour of Arsenic and Germanium Species in Seawater

The vertical profiles of inorganic arsenic [As(III)+As(V)], monomethylarsonic acid (MMAA), dimethylarsinic acid (DMAA), inorganic germanium and monomethylgermanium (MMGe) were investigated at three sampling stations in the Pacific Ocean. In addition, the concentrations of these species in various surface waters have also been determined. The vertical profile of both inorganic arsenic and germanium displayed low concentrations, 1100 to 1450 ng dm^-3 for inorganic arsenic and <0.7 to 2 ng dm^-3 for inorganic germanium, in the surface zone. The concentrations of inorganic arsenic increased with depth to maximum concentrations that varied from 1500 to 2200 ng dm^-3 at a depth of 2000 m and then slowly decreased to concentrations that varied from 1300 to 1900 ng dm^-3 at a depth of 5000 m. On the other hand, the vertical profiles of inorganic germanium displayed a relatively constant concentration (4 to 8 ng dm^-3) from a depth of 2000 m to 5000 m. These vertical profiles of inorganic germanium were linearly correlated with those of silicate with a Ge/Si molar ratio of 0.715×10^-6. Both MMAA and DMAA displayed maximum concentrations in surface water and abruptly dropped with depth from 0 to 200 m. The concentration in surface water was 12 ng dm^-3 for MMAA and varied from 48 to 185 ng dm^-3 for DMAA. At depths >200 m, MMAA and DMAA were generally at comparable concentrations of about 3 ng dm^-3. In the case of MMGe, it was uniformly distributed throughout the water column at a concentration of approximately 16 ng dm^-3, indicating that MMGe was not involved in the biogeochemical cycling of inorganic germanium. In deep waters (>200 m), the concentrations of both inorganic arsenic and germanium increased from the southern Tasman Sea to the north. The increase in inorganic arsenic concentration was linearly correlated with that of phosphate and the increase in inorganic germanium concentration was linearly correlated with that of silicate, with apparent ΔAs/ΔP and ΔGe/ΔSi molar ratios of 4.53×10^-3 and 0.73×10^-6, respectively.