Depth profiles of dissolved sulfide and sulfate in the pore water of hydrocarbon seep sediments offshore Pakistan

The interaction between fluid seepage, bottom water redox, and chemosynthetic communities was studied at cold seeps across one of the world's largest oxygen minimum zones (OMZ) located at the Makran convergent continental margin. Push cores were obtained from seeps within and below the core-OMZ with a remotely operated vehicle. Extracted sediment pore water was analyzed for sulfide and sulfate concentrations. Depending on oxygen availability in the bottom water, seeps were either colonized by microbial mats or by mats and macrofauna. The latter, including ampharetid polychaetes and vesicomyid clams, occurred in distinct benthic habitats, which were arranged in a concentric fashion around gas orifices. At most sites colonized by microbial mats, hydrogen sulfide was exported into the bottom water. Where macrofauna was widely abundant, hydrogen sulfide was retained within the sediment. Numerical modeling of pore water profiles was performed in order to assess rates of fluid advection and bioirrigation. While the magnitude of upward fluid flow decreased from 11 cm yr**-1 to <1 cm yr**-1 and the sulfate/methane transition (SMT) deepened with increasing distance from the central gas orifice, the fluxes of sulfate into the SMT did not significantly differ (6.6-9.3 mol m**-2 yr**-1). Depth-integrated rates of bioirrigation increased from 120 cm yr**-1 in the central habitat, characterized by microbial mats and sparse macrofauna, to 297 cm yr**-1 in the habitat of large and few small vesicomyid clams. These results reveal that chemosynthetic macrofauna inhabiting the outer seep habitats below the core-OMZ efficiently bioirrigate and thus transport sulfate down into the upper 10 to 15 cm of the sediment. In this way the animals deal with the lower upward flux of methane in outer habitats by stimulating rates of anaerobic oxidation of methane (AOM) with sulfate high enough to provide hydrogen sulfide for chemosynthesis. Through bioirrigation, macrofauna engineer their geochemical environment and fuel upward sulfide flux via AOM. Furthermore, due to the introduction of oxygenated bottom water into the sediment via bioirrigation, the depth of the sulfide sink gradually deepens towards outer habitats. We therefore suggest that - in addition to the oxygen levels in the water column, which determine whether macrofaunal communities can develop or not - it is the depth of the SMT and thus of sulfide production that determines which chemosynthetic communities are able to exploit the sulfide at depth. We hypothesize that large vesicomyid clams, by efficiently expanding the sulfate zone down into the sediment, could cut off smaller or less mobile organisms, as e.g. small clams and sulfur bacteria, from the sulfide source.

Provisional guidance for quantitative risk assessment of polycyclic aromatic hydrocarbons.

Mode of access: Internet.

Control techniques for hydrocarbon and organic solvent emissions from stationary sources.

Includes bibliographies.

Monitoring equipment for hydrocarbon ozone precursors and for hazardous volatile organic compounds : cooperative research and development agreement with Perkin-Elmer Corporation.

Mode of access: Internet.

Ambient water quality criteria for polynuclear aromatic hydrocarbons /

Mode of access: Internet.

Experimental investigation of the effects of aromatic hydrocarbons on a sediment food web /

"July 1994."

ASNE-API code for installation, proving and operation of positive displacement meters in liquid hydrocarbon service; or, Petroleum P. D. meter code (short title)

At head of title: ... (Tentative)

Thermal reactions of aromatic hydrocarbons,

Mode of access: Internet.

The determination of aromatic, unsaturated and naphthene hydrocarbons in light oils and motor spirits,

Mode of access: Internet.

Economic impact of the proposed change in the hydrocarbon emission limitation for petrochemical manufacturing processes (R77-3) /

Project no. 10.065.

The Acute toxicity of chlorinated hydrocarbon and organic phosphorous insecticides to livestock /

Cover title.

Bioassay of soil containing residues of chlorinated hydrocarbon insecticides : with special reference to control of Japanese beetle grubs /

Literature cited: p. 38-44.

Hydrocarbon vapor standards for performance evaluation of combustible gas detectors /

"Issued October 1981."

Utah combined hydrocarbon leasing regional final EIS /

One map on folded leaf in pocket of Vol. 1.

A tissue enzyme assay for chlorinated hydrocarbon insecticides.

"Project 16030 ELZ; Program element 1BA027."