Seafloor sulphide deposits

Although less energetically efficient than the oxidation of hydrogen sulphide (Fig. 8.4), the oxidation of metal sulphides could potentially support chemo-synthesis at seafloor massive sulphide deposits long after hydrothermal activity had ceased, even in well-buffered seawater (Eberhard etal., 1995 Juniper Tebo, 1995). Newly formed sulphide deposits are rapidly subjected to oxidation upon contact with ambient seawater, and some show micro-scale weathering features (e.g. etch pits on mineral surfaces Verati etal., 1999),... 

Where undiluted hydrothermal fluids mix with cold seawater at and near the seafloor, minerals precipitate out of solution to form chimney structures and other deposits. These deposits are mineralogically complex, containing sulphides (e.g. pyrite, marcasite, chalcopyrite, sphalerite, wurtzite, galena), sulphates (e.g. anhydrite, barite), silica and oxyhydroxides (e.g. Hannington etal., 1995 ... 

These studies demonstrate that bacteria are likely exploiting the vast energy available in hydrothermal vent sulphides, and these deposits could potentially serve as long-term energy sources for the chemolithoautotrophic production of microbial biomass on the seafloor. However, the quantitative significance of this energy source and the environmental importance of this form of metabolism remains to be determined. 

Modem hydro- High-temperature hydrothermal vents currently active at mid-ocean ridges offer a thermal mineral- unique opportunity to study a hydrothermal mineral deposit in the process of ization at formation. The current working model assumes that cold seawater sulphate is mid-ocean ridges drawn down into sea-floor basalts, where it is heated in the vicinity of a magma chamber. Some sulphate is precipitated as anhydrite whilst the remainder is reduced to sulphide by reaction with the basalt. The fluid is vented back onto the seafloor at about 350 C laden with sulphides. On mixing with seawater these are precipitated onto the sea floor as a fine sulphide sediment whilst at the vent site itself the sulphides are built into a chimney a metre or so in height. 

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