PAGES Ethiopia

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1- GLACIAL GEOMORPHOLOGY

The high rising Ethiopian Mountains in Simien, Arsi and Bale appear to have been glaciated at the LGM, but without clear evidence of earlier glaciation (Mohammed et al. 2003; Osmaston et al. In press). On Simien the highest (4543m) but also the driest of all the mountains, small moraines, cirques and glacial striations extending only a few hundred meters below the peaks were surveyed and mapped with great detail and precision by Hurni (1982, 1989). The total glaciated area is estimated to have been ~13km2 and the ELA about 4100m on the northern and 4400m on the southern side. Recently further moraines have been observed by Osmaston, 250m lower (Osmaston et al. In press).
On Mt. Badda (Arsi mountains) clear terminal moraines have been observed at 3200-3700m with the glaciated area estimated to be 85km2 (Potter 1975,1976; Street, 19791,2, Mohammed et al. 2003) The ELAs descended to 3700m (East) and 3900m (West).
The Bale mountains were supposed to have contained the most extensive glaciation (Messerli et al. 1977) covering an area of about 600km2. The ELA descended to 3700m. However, recent observations revealed that there was a total glaciated area of only 150km2 (Osmaston et al. In press).
The above information is supported by a 2.6m core taken from a swamp at ca.4000m near the eastern edge of the plateau just north of Badegesa Hill, outside the area of the BBM. It has been dated to 17,763 BP near the base and is currently being analysed (Zech, Universität Bayreuth, personal comm. 2003). This probably shows the limits of the LGM ice on the eastern side and should yield important information.

There is an attempt to directly date the age of deglaciation on the Ethiopian mountains using a cosmogenic method (Osmaston et al. In press). So far estimates have been made from the basal radio carbon age of sedimentary cores. One such core, 1.8m long, was taken from a swamp at 3000m at Tamsaa (the Bale mountains) and dated to ca. 14000 yr. BP. This date is considered to be the minimum age for deglaciation (Mohammed and Bonnefille; 1998).
A 16m core in a small cirque lake (Lake Garba Guracha) dammed by a glacial moraine at 4000m on the Bale Mountains was recently recovered. It was taken at a water depth of 6m and was dated at the base to ~14000 yr. BP (Mohammed et al. 2003). The results of pollen, organic matter, and sedimentological analysis are in press. This study also reveals that deglaciation occurred prior to 14000 yr BP.


2- LAKE SEDIMENT RECORDS OF CLIMATE CHANGE

The longest (50m) and most continuous sedimentary sequence so far obtained comes from Lake Abhé, the terminal lake of the Awash River in central Ethiopia. Reconstruction of lake-level and salinity fluctuations over the past 40,000 years was based on sedimentary and diatom studies of that core and of numerous outcrops, and palaeo-beaches (Gasse 1977, Gasse et al. 1980, 1998). Following shallow, saline-alkaline conditions around 31 14C kyr BP (ca. 36 cal. kyr), Lake Abhé became a large (6,000 km2), deep (170 m), freshwater lake between 27 and 23 14C kyr BP (ca 31.6-27 cal. kyr). A stepwise drop to the modern lake-level occurred between ca. 23 and 17.1 14C kyr BP (27-20.2 cal. kyr).



The Main Ethiopian Rift experienced a comparable climatic evolution (Fig. 2), as shown by geomorphic and sedimentological studies of outcropping sequences and shorelines in the Ziway-Shalla Basin (Street 1979a, Gasse and Street 1978, Gasse et al. 1980, Benvenutti et al. 2003, Le Turdu et al. 1999). Today, this internal drainage basin contains four lakes of decreasing altitude and increasing salinity: L. Ziway, L. Langano, L. Abiyata, and L. Shalla. A highstand, at least 83m above the modern Lake Shalla, occurred between ca. 26.5 and 22 14C kyr BP (31-26 cal. kyr) (Street 1979a) (Fig. 2). The lake level then fell dramatically to levels at or below present, and remained low until about 12.5 14C kyr BP (ca. 14.5 cal. kyr). Assuming temperatures 3 to 6°C lower than today, water balance calculations suggest a decrease in annual precipitation of 9 to 32% compared to modern during the LGM (Street 1979a, 1979b).

All lacustrine and geological evidence in the region converge to show dry conditions during the LGM.

Much wetter conditions than today prevailed during the early-mid Holocene asa aresult of an orbitally-induced increase in monsoon strength. It has long been apparent that the modern vegetation altitudinal belts began to establish themselves during the Holocene in response to both wetter and warmer climatic conditions (Hamilton 1982, Lézine and Bonnefille 1982, Mohammed and Bonnefille 1998). The lakes experienced early-mid Holocene high stands followed by generally low water levels during the past 5000 cal. yr (e.g., Butzer 1972, Fontes and Pouchan 1975, Street and Grove 1978, Street 1979a, Williams et al.1977, Gasse 1977, Gasse and Street 1978, Gasse and Descourtieux 1979, Owen et al. 1982). However, most records show that vegetation and hydrological changes did not only respond to the smooth sinusoidal waves of orbital forcing (Gasse, 2000). This was the case when abrupt dry intervals occurred during the Early Holocene wet phase in particular at ca. 8.5 cal. kyr BP as well as during a wet interval in the mid-late Holocene dry phase at ca. 4-3.5 cal. kyr BP (Telford and Lamb 1999; Lamb et al. 2000, Chalié and Gasse, 2003)

There are also some recent high-resolution climatic reconstruction over the past few millennia (e.g. Bonnefille and Mohammed 1994, Legesse et al. 2002, Darbyshire et al. 2003). Preliminary results indicate Medieval Warm Epoch dry and Little Ice Age wet conditions as well as indications of Human Impact.


3- TRAVERTINE AND BURIED SOILS

In Northern Ethiopia travertine aggradation started during the Early Holocene and ceased ca. 3500 yr. BP. This created dammed basins in which swamps developed (Berakhi et al. 1998; Nyssen et al. 2003; Dramis et al. 2003). The relationship between travertine aggradation and climate change (cold to warm transition) is generally accepted (Dramis et al. In press). A similar event took place in Somalia (8000-3500 yr. BP) (Mohammed, 1995). Moreover important phases of soil formation including peat development took place during this period (Brancacco et al., 1997; .Berakhi et al., 1998; Dramis et al., 2003; Nyssen et al. 2003). The situation also corresponded to the Early Holocene phases of high lake stand (Gasse, 2000; Mohammed et al. 2004; Dramis et al. 2003).