RESEARCH ARTICLES CURRENT SCIENCE, VOL. 120, NO. 6, 25 MARCH 2021 1066 *For correspondence. (e-mail: jinqiang@upc.edu.cn) Origin and source characterization of methane in the shallow-water environment of Southern Lake Tanganyika Rift Basin, Tanzania Januarius Matata Bishanga and Jin Qiang* School of Geoscience, China University of Petroleum, Qingdao 266000, China Lake Tanganyika, located in the western part of Tanzania between 3°S and 9°S lat, harbours hydro- carbon and non-hydrocarbon gases in its northern and southern shallow-water environment. In this study, a geochemical analysis of stable carbon and hy- drogen isotopes and an interpretation of individual gas molecular composition were done in order to determine the origin and composition of the naturally occurring hydrocarbon gases in Tanganyika Basin. Nitrogen, a non-hydrocarbon gas is a major compo- nent (76.69%–78.31%) by volume percentage followed by methane (11.68%–12.94%) and other higher hydro- carbons (0.16%–1.63%). The isotopic composition of carbon δ 13 C 1 and hydrogen δ DC 1 ranges from –65.32‰ to –65.81‰ and –272.5‰ to –275.9‰ respec- tively. The isotopic compositions of ethane (δ DC 2 = –36.7‰ to –35.2‰) and propane (δ DC 3 = –31.3‰ to –27.5‰) reflect the thermogenic origin of these higher hydrocarbons. According to molecular characterization of carbon and hydrogen isotope ratios and δ D-values, methane gas falls in the biogenic origin category and is formed by carbon dioxide reduction. The isotopic composition of CO 2 varies between –8.6‰ and –3.4‰. CO 2 reduction is also regarded as a mechanism of bio- genic methane formation based on carbon isotope fractionation factors (greater than 0.16). Keywords: Biogenic origin, isotopic composition, methane, molecular characterization, shallow-water envi- ronment. METHANE (CH 4 ) gas venting in Lake Tanganyika, Tanza- nia has drawn the attention of several researchers, debat- ing on its origin and mechanism of occurrence. The origin, molecular characterization and maturity evalua- tion of hydrocarbon gases in lacustrine settings have long been propounded using hydrocarbon molecular content (methane/(ethane + propane); C 1 /C 2+ or C 1 /(C 2 + C 3 ); commonly termed the Bernard parameter) and isotope carbon composition (δ 13 C 1 ) 1–5 . These gases are composed mainly of methane (C 1 ) with minor amounts of ethane (C 2 ), higher hydrocarbons (C 3+ ), nitrogen (N 2 ) and carbon dioxide (CO 2 ). The gases can be produced through bio- genic (microbially-derived) or thermogenic (petroleum- derived) processes 6 . The significant range in carbon and hydrogen molecular composition and stable isotope com- position ratios of hydrocarbon gases render interpretative details regarding the gases and their concentration in lacustrine basins depending on environmental settings. Globally, hydrothermally active regions are characte- rized by traces of gas anomalies in the freshwater/ seawater (plumes) or in the sediments 7–10 . Tropical lakes accumulate approximately one-quarter of the Earth’s freshwater 11 and bear a substantial amount of hydrocar- bons like methane and non-hydrocarbons like CO 2 (greenhouse gases). Most East African rift lakes are hy- drothermally active and contribute an enormous amount of carbon in the lakes’ water column. Till date, the con- tribution of these rift lakes of CH 4 gas to the atmosphere is unknown 12 . The Ocean Drilling Programme in Middle Valley, Juan De Fuca Ridge, Leg 1391, East African rift lakes showed the presence of thermogenic gases in the area. The evidence for thermogenic hydrocarbons inclu- ded elevated contents of higher hydrocarbons (δ 13 C 1 = –30.0‰ to –45.0‰) and based on the geologic setting, it is highly probable that the thermogenic gases were formed by hydrothermal processes. However, there is possible evidence of a bacterial gas component admixed with the thermogenic gases. The biogenic hydrocarbons in the free gas are concentrated below the thermocline where bacterial sulphate reduction has removed most or all of the dissolved sulphate 3 . Schoell et al. 13 discussed the origin of significantly- high methane gas concentration in Lake Kivu, north of Lake Tanganyika along the rift system, stating that methane gas (C 1 ) was formed by bacterial activity through acetate fermentation and CO 2 reduction. Further they mentioned that along Lake Kivu and the northern trough of Lake Tanganyika, thermal springs exist and gases vent through the water column, which is associated with both biogenic and hydrothermal origin 13–18 . Climate variability, dry and hot conditions lead to decreased lake level by evaporation and changes in aqueous CO 2 concen- tration. This phenomenon coupled with increased pH and salinity led to reduced dissolved CO 2 concentration 19 .