Chapter VII — 109 — Activities in Deposits 



cient marine sediments, and the almost universal formation of methane in 

 recent marine sediments rich in organic matter, suggest the importance of 

 this gas as an end product of decomposing organic matter. 



The evolution of methane and hydrogen from Lake Glubokoje has 

 been described by Kusnetzow (19356) who discusses the importance of 

 these so-called "mud gases." He believes that the oxidation of methane 

 and hydrogen by autotrophic bacteria in the overlying oxygenated water 

 plays an important role in depleting the oxygen content of lakes. The 

 bacterial production of methane, hydrogen, CO2, and H2S has been ob- 

 served by BuTKEViCH (1938) in mud from the Caspian Sea. 



The anaerobic fermentation of organic matter usually results in the 

 formation of a certain amount of hydrogen (Buswell, 1936). Several 

 species of bacteria are endowed with this ability. Due to the chemical 

 reactivity of nascent hydrogen and the tendency of various bacteria to 

 activate molecular hydrogen, it is difficult to estimate how much hydrogen 

 may be produced in bottom deposits. However, the fact that free hydro- 

 gen is often detectable is evidence that it is formed, probably in very sig- 

 nificant quantities. Sugawara et al. (1937) noted the presence of hydro- 

 gen, along with methane and nitrogen, in lake deposits of high organic 

 content. 



Large quantities of hydrogen may be consumed in the production of 

 methane in mud. Sohngen (1910) postulated that methane is formed 

 by the bacterial reduction of CO2 in the presence of hydrogen: 



4 H2 -I- COo = CH4 + 2 H2O 



This mechanism of methane formation has been amply confirmed (Bar- 

 ker, 1936a), but it is still problematical whether free hydrogen is always 

 involved as an intermediate reactant in methane formation. 



Stephenson and Stickland (1931) observed that certain anaerobes 

 use molecular hydrogen to reduce sulfate to H2S in the proportions indi- 

 cated by the following equation : 



H2SO4 -I- 4 H2 = H2S -h 4 H2O 



This observation has been confirmed by the work of Starkey and Wight 

 (1943) and others. Hydrogen may play a role also in the hydrogenation 

 of organic matter with the formation of petroleum hydrocarbons. 



Nitrogen may be Hberated from nitrogenous organic matter undergo- 

 ing anaerobic decomposition, although the mechanism of the reaction is 

 not well understood. Ammonia ordinarily is hberated from decomposing 

 proteins but it will appear as a gas only when present in high concentra- 

 tions or in an alkaline environment. Nitrogen may be liberated from 

 nitrates or nitrites by the process of denitrification, but since neither 

 nitrates nor nitrites occur in bottom deposits having a low Eh, they are 

 not likely sources of nitrogen. 



Often appreciable quantities of H2S are formed during the decomposi- 

 tion of proteins and from the reduction of sulfate. The HoS may be either 

 fixed in the sediments as metallic sulfides or may escape into the overlying 

 water. Most black muds are rich in ferrous sulfide. If enough H2S is pro- 

 duced to escape into the water, it tends to deplete the dissolved oxygen 

 either abiogenically or through the activities of sulfur bacteria. H2S pro- 

 duced on the bottom of the Black Sea and in other stagnant bodies of 

 water is primarily responsible for the prevailing anaerobic conditions in 

 the overlying water. Copenhagen (1934) records that HoS generated in 



