LIGHT-HYDROCARBON GASES IN LAKE TANGANYIKA HYDROTHERMAL FLUIDS (EAST-CENTRAL AFRICA)

Active hydrothermalism occurs in the shallow-water environment of northern Lake Tanganyika. Carbon dioxide of magmatic origin (deltaC-13 = - 9.1 to - 3.8 parts per thousand) is the major gaseous component. In addition to CO2 variable amounts of light hydrocarbons (C1 to C6) were detected. The isotopic composition of methane (deltaC-13 = - 58.0 to - 50.40 parts per thousand; deltaD= - 199 to - 149 parts per thousand) indicates both a bacterial and a thermogenic source for the hydrothermal gas. The mechanism for bacterial methane formation remains unknown. However, according to the deltaD-values it is likely that most of the bacterial methane formed by CO2 reduction. The isotopic compositions of ethane (deltaC-13 = - 36.6 to - 26.7 parts per thousand) and propane (deltaC-13 = - 31.1 to - 25.6 parts per thousand) reflect the cogenetic origin of these thermogenic higher hydrocarbons.