HR: 0800h
AN: B31A-0956    [Abstracts]
TI: Unexpected Arsenate/Arsenite Gradients in Pore-Water Profiles From a Shallow-Water Hydrothermal System
AU: Price, R E
EM: reprice@mail.usf.edu
AF: University of South Florida, Department of Geology, Tampa, FL 33620 United States
AU: * Pichler, T
EM: pichler@shell.cas.usf.edu
AF: University of South Florida, Department of Geology, Tampa, FL 33620 United States
AU: Amend, J P
EM: amend@levee.wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, St. Louis, MO 63130 United States
AB: The shallow-water submarine hot-springs near Ambitle Island in eastern Papua New Guinea provide us with the exceptional opportunity to study the biogeochemistry of arsenic (As) along a horizontal and a vertical gradient. Hydrothermal venting occurs as discharge of a clear, two-phase fluid from discrete orifices, 10-15 cm in diameter, with minor phase separation (boiling) at the sea floor. In addition diffuse seepage of the same fluid, but without phase separation occurs throughout the area. Fluid temperatures for individual springs range from 89 to 98/degC, while diffuse seepage temperatures are generally lower. The hydrothermal fluids contain up to 1000 ęg/L As, which is exclusively present as the trivalent species arsenite (As(III)). Stepping away to a distance of 300 m from the area of focused venting, we collected 10 pore-water profiles down to a depth of 1 m in 10-cm intervals. The profiles were collected with a 10-port sampler by simultaneously filling 10 syringes to reduce vertical flow. In those samples 10 cm below the seawater-sediment interface, total As concentrations decreased from 900 ęg/L closest to the vents to 6 ęg/L at 300 m away. The 6 ęg/L at 300 m is still 3-times the expected value for seawater, thus indicating the potential extension of hydrothermal influence. As(V)/As(III) ratios were determined to investigate the transport and fate of As(III) and to evaluate horizontal and vertical redox gradients. Surprisingly, As in the vertical pore-water profiles occurs predominantly as the oxidized form, As(V), with As(V)/As(III) ratios ranging from 0.05 to 0.1, whereas vertical pore-water profiles from a "non-hydrothermal" control site show a redox gradient with a ratio ranging from 0.5 to 1.0. Along the horizontal gradient the 10 cm pore-water samples show an increase in the As(V)/As(III) ratio, from 0.07 near the vents to 1.75 at 300 m away. These unexpected ratios suggest that microbes may be catalyzing the oxidation of hydrothermal As(III) to As(V). Oxidation of As due to mixing with seawater is not likely due to the sluggish reaction kinetics. Microbial counts of three samples, 7.5, 12 and 30 m away from the vents revealed an inverse relationship between As concentration and calculated cell density, as well as varying cell morphology between samples.
UR: http://www.biocomplexity.usf.edu
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0456 Life in extreme environments
DE: 0460 Marine systems (4800)
SC: Biogeosciences [B]
MN: Fall Meeting 2005