DISCUSSION OF THE ORIGIN OF GAS HYDRATES

Gas hydrates, distributed in the sediments of the deep seas and lakes and the permafrost of the polar region, are potential sources of energy in the future. Gas hydrates mainly occur in the outer continental and insular margins, which are in agreement with the distribution of recent volcanoes. Carbon isotopic compositions indicate that methane of gas hydrates in marine sediments is generated by autotrophic methanogens, which reduce CO2 to CH4. There is not enough organic carbon ( 0.5%～1.0%) in the typical continental margins to produce high content of methane in the gas hydrate zone. The age of the sediments hosting gas hydrates primarily ranges from late Miocene to late Pliocene. The coexistences of gas hydrates with volcanic ash or sands to some extent demonstrate that their formation is associated with volcanohydrothermal system. The spatial consistency between volcanoes and gas hydrates indicates that the methane in hydrates may be derived from the reduction of large amounts of CO2 provided by submarine volcanohydrothermal system. From the fact that the hydrogens of microbial methane originate in water, it may be deduced that with the reduction of HCO3 a transfer of H atom occurs from the O atom to the C atom within HCO3 anions formed by the disassociation of dissolved CO2 in water, and furthermore, the formyl group (HCO) forms. Thus one of H atoms in CH4 generated by autotrophic methanogens by the reduction of HCO3 anions arises from water, but how the other three come from water remains to be explored next. According to some literatures, authigenic carbonates can present within the gas hydrate zone, which demonstrates that the precipitation of authigenic carbonates is associated with methane production and gas hydrate formation. Their precipitation results from the dissolution of CO2 supplied by submarine volcanohydrothermal system, not from the oxidation of methane in gas hydrate zones. Methane production consumes bicarbonate (HCO3), which restrains the further dissociation of HCO3 to CO23 an H+,and this may explain the decrease of carbonate mineral content within gas hydrate zones. The submarine volcanohydrothermal system is favorable to methane production by thermophilic methanogens, meanwhile, gigantic pressure produced by seawater column prevents methane escaping, and gas hydrates were formed and preserved with the drop of temperature. Ocean undercurrent may result in the migration of CO2 and methane. A preliminary model of origin for gas hydrates has been proposed from our discussions. On the other hand, autotrophic methanogens present us the prospects that methane may be artificially produced from CO2 and H2O. 