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

Abstract

This work reports the results of an integrated investigation on Holocene faulted deposits exposed on the northeastern Lake Langano in the central sector of the Main Ethiopian Rift (MER). The Lake Langano is part of a closed basin and thus it is highly sensitive to climate fluctuations. The present study explored the foot of the Haroresa escarpment, where coarse-grained slope deposits interbedded with thin lake shore shell-rich coarse sands are well exposed. Stratigraphical analysis of these deposits, integrated with radiocarbon dating carried out on lacustrine gastropod Melanoides tubercolata shells, points to significant lake level fluctuations forced mainly by climate oscillations. These have been interplaying during the early and middle Holocene with a structurally-controlled threshold separating an embayment of the Lake Langano to the south, from the small perched Lake Haro Bu-a basin to the north. Significant differences in the 13C/12C isotopic ratio have been identified in the M. tubercolata shells collected on the opposite sides of such a threshold. The time-space variations of the isotopic signature of the shells are referred to the mutual relationships between the two main different CO2 sources (i.e., microbial activity and deep mantle degassing) dissolved in the lake water.