Diatom-inferred water balance variability in the Main Ethiopian Rift during the Holocene.
by
Françoise Chalié
CEREGE, UMR 6635 CNRS, BP 80, 13545 Aix-en-Provence Cedex 04, France
Coauthors: Françoise Gasse, Bruno Gemmiti

The Main Ethiopian Rift (MER) experiences a semi-arid climate with a long summer monsoon season (June- September) bringing heavy rains from the Indian and Atltantic Oceans, a dry season (October-February), and a 'small rain' season (March-May) bringing moisture from the Indian Ocean and the Red Sea.

Among the four lakes lying today in the Ziway-Shala basin (central part of the MER), Lake Abiyata is especially susceptible to climate changes due to its terminal position and its shallowness. The lake responds rapidly to changes in rainfall in the surrounding highlands by fluctuations in water depth and salinity. Diatom analyses of two cores (12.6 m and 1.16 m long, respectively) retrieved from the same site in Lake Abiyata allow quantitative reconstructions of ionic concentration (Gasse et al., 1995) on the centenial to the interannual time-scales..

The long, 14C-dated core spans the past 13.5 cal. kyr BP, and was analyzed at 100-200 years interval (Chalié and Gasse, submitted). The lower part of the core shows reconstructed salinity peaks around 13.5 and 12.5 cal. kyr BP separated by a brief freshwater stage centered around 13 cal. kyr BP. The Early-Mid Holocene registered a deep, freshwater lake from ca. 12 to 5.5 cal. kyr BP, with minimum salinity-values from 11 cal. kyr BP. Generally shallow, saline conditions then established abruptly. Salinity peaks around 5.4-5, 3, 2 , 1.2 and 0.8 cal. kyr BP, while fresher conditions re-established around 4-3.5 and 2.7 cal. kyr BP. The comparison with other lake profiles from Ethiopia, from the northern African and Asian tropics, and with climate model simulations, confirms that the variations recorded in Lake Abiyata reflect climate changes in the northern monsoon domains..

The short core, 210Pb-dated at 1800 AD at 80 cm (Dagnachew Legesse et al., submitted), was analyzed at 2-6 years interval. Detailed absolute dating control is still needed to confirm the chronology. However, environmental changes recorded in the dated section of the core are apparently consistent with historical events The fluctuations in diatom-inferred water depth and salinity in the upper 41 cm (1940-1998 AD) are roughly in phase with measured changes in water level. The interval 85 -41 cm (> 1800-1940 AD) records an overall water deficit. The 210Pb age of maximum salinity (68-66 cm; ca. 1890 AD) matches that of the extremely severe drought in Ethiopia during 1888-1892. The lower part of the core indicates an episode much wetter than over the last two centuries which ended before 1800 AD. Further work is needed to relate these changes with recent, rapid climate changes recorded elsewhere.

References: Chalié F. & Gasse F., A 13,500 year diatom-inferred water chemistry and lake level from the tropical East African Lake Abiyata (Ethiopia). Submitted to Palaeo-3 ; Gasse F. et al., 1995. Diatom-based transfer functions for inferring past hydrochemical characteristics of African lakes. Palaeo-3, 117, 31-54; Dagnachew Legesse et al., Environmental changes in a tropical lake (Lake Abiyata, Ethiopia) during recent centuries. Submitted to Palaeo-3.

Date received: May 15, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahr-03.