AMS- (super 14) C chronology of a lacustrine sequence from Lake Langano (Main Ethiopian Rift); correction and validation steps in relation with volcanism, lake water and carbon balances.

Abstract

Located in the Ziway-Shala Basin of the Main Ethiopian Rift, Lake Langano is part of an asymmetric half-graben, defined by a series of north-northeast-trending faults in the tectonically active zone of the rift. A 15-m deep succession of organic homogeneous muds, silts, bioclastic sands, and pyroclastic layers was cored in 1994. The definition of a certified radiocarbon chronology on these deposits required the indispensable establishment of modern hydrological and geochemical balances. The isotopic contents of the total dissolved inorganic carbon (TDIC) of surface water clearly show the influence of a deep CO (sub 2) rising along the main fault crossing the lake basin. The 5.8 pMC disequilibrium existing in 1994 with the atmosphere likely produces the aging of authigenic materials developing at the lake surface. However, with a mean residence time of approximately 15 years, this apparent (super 14) C aging of Lake Langano water still integrates the (super 14) C produced by the nuclear tests in the 1960s. Reconstructing the natural (super 14) C activity of the lake TDIC allows for the quantification of the deep CO (sub 2) influence, and for the correction of AMS- (super 14) C datings performed along the core. The correction of the AMS- (super 14) C chronology defined on Lake Langano allows for a better understanding of paleohydrological changes at a regional scale for at least the last 12,700 cal BP.