Co-generation of hydrogen sulfide and methane in marine carbonate sediments

Sulfate reduction and methanogenesis are considered to be mutually exclusiv e microbial reactions in marine sediments. Typically, methane does not appe ar in significant concentrations in sediment pore waters until almost all d issolved sulfate has been reduced to sulfide, An exception to this commonly accepted pattern occurs in an approximately 500-meter thick sequence of Qu aternary carbonates on the continental margin of the Great Australian Bight . An unusual combination of geochemical and sedimentological conditions lea ds to extensive simultaneous sulfate reduction and methane production throu ghout the 500-m interval. A probable explanation for the co-production of t hese reduced gases in this deeper biosphere is the presence of noncompetiti ve substrates for the two types of microbiota.