Enhanced geochemical gradients in a marine shallow-water hydrothermal system: Unusual arsenic speciation in horizontal and vertical pore water profiles

The shallow marine hydrothermal vents near Ambitle Island in eastern Papua New Guinea discharge hot, slightly acidic, As-rich, chemically reduced fluid into cool, slightly alkaline, oxygenated seawater. Gradients in temperature, pH, and total As (AsT), among others, are established as the two aqueous phases mix. The hydrothermal fluid contained ∼900 μg/L AsT, almost exclusively present as the reduced AsIII, while local seawater measured between 1.2 and 2.4 μg/L As, with approximately equal levels of AsIII and AsV. Of particular interest in this study was As speciation and abundance in pore waters as a function of sediment depth and as a function of distance from the area of focused venting. With increasing distance, AsT concentration in the pore water decreased rapidly, but remained elevated up to 300 m from the area of focused venting when compared to a non-hydrothermal control site. As a function of depth (to ∼100 cm) AsT concentration in the pore water profiles was elevated and generally increased with depth. Surprisingly, aqueous AsV far exceeded aqueous AsIII at almost all distances and depths investigated, while at the control site the AsIII concentration exceeded that of AsV. In the Tutum Bay hydrothermal system, chemical disequilibria among As species provide potential metabolic energy for arsenite oxidizing microorganisms where hydrothermal fluid mixes with seawater near the vent orifice, and for arsenate reducing microorganisms with increasing distance and depth from the hydrothermal point source.