Holocene lacustrine fluctuations and deep CO2 degassing in the northeastern Lake Langano Basin (Main Ethiopian Rift)

► Langano and Haro Bu-a are hydrologically closed lakes sensitive to Holocene climate fluctuations. ► A structurally-controlled threshold acted between the two lakes. ► Melanoides tubercolata shells on the threshold sides show different 13C/12C isotopic ratios. ► Concurrent dissolved microbial- and deep mantle-derived CO2 account for these differences. ► The geochemical variability is due to connection or isolation between Lakes Langano and Haro Bu-a.