Beyond a certain depth in the sediment, all the pore-water sulphate ions are used up (Fig. 8.1). If there is still a supply of useable organic matter, the anaerobic bacteria here must feed by fermentation, much like yeast in the fermentation of beer or wine, producing methane and bicarbonate gas as waste products:
The energy yield here is even lower than for sulphate reduction. These methanogenic bacteria continue the process of organic decomposition so that very little organic matter may remain in the sediment. The hydrogen ions thus released can reduce available Fe3+ to Fe2+ ions, and the latter may combine with dissolved bicarbonate ions to precipitate out as FeCO3 (siderite) concretions. The methane produced in this way is also important as a 'greenhouse' gas.
The release of sulphide during this form of anaerobic respiration is highly toxic to most anaerobes, and can lead to the dissolution of carbonate (including fossils) in the sediment. Commonly, iron monosulphides and then pyrite are precipitated:
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