Drilling of Lakes, Northern Caucasus
The progress in the Holocene climate reconstructions largely relays on the analysis of the lake sediments, which provide a continuous uninterrupted chronicle of the natural processes and events of the past that can be decoded with fine time resolution. Lacustrine sediments are of a special value in glaciology because they enable uninterrupted reconstructions of the change of glacier size and equilibrium-line altitude compared to a discrete moraine chronology both for advance and retreat states of a glacier.
The pioneer works in lake sediment studies of the new generation belong to Scandinavian researchers (Karlén, 1976, Dahl, Nesje, 1994). Due to this approach, very detailed high-resolution reconstructions of climate and glacier variations in Scandinavia have been achieved (Nesje et al., 2000, Bakke et al., 2005, Nesje, 2005). The application of this approach in Russia is still very limited (Kalugin et al., 2007, 2009). In Caucasus, reconstructions of this kind do not exist so far.
In general the Holocene glacier and climate variations in Caucasus are very poorly studied. The existing reconstructions are primarily based on geomorphic descriptions of moraines and biostratigraphical data (Serebryanny et al., 1984, Knyazev et al., 1992). The available high-resolution reconstructions of the summer temperatures based on tree rings cover only 200 years (Dolgova, Solomina, 2010). Chronologies of glacier fluctuations of the last millennium are based on the results of lichenometric dating.
During the summer of 2010 the sediment coring of Lake Kakakyol (Western Caucasus, N 43° 26` 16``, E 41° 44` 38``, altitude 1335 m, depth 6-11 m) was performed by the Institute of Geography RAS. According to the radiocarbon dating, the age of basic layers of clay in this sediment is 9760 ± 80 yr BP. The initial palynological and geochemical analyses of the uppermost sediments unit (2235 ± 35 yr. BP) allowed assessing the climatic changes in this area with high resolution (50 years for palynology, 4-5 years for geochemistry). Both records show the evidence of very strong warming between the early IX and ХIV centuries as well as in the first half of XVIII century (Solomina et al., in press).
In the field season of 2012 the expedition hosted by Institute of Geography, Russian Academy of Sciences successfully cored the sediments of three other lakes: Bolshoe Dzitakskoe (N 43°45'15" E 40°23'12", altitude 1915 m, depth 6-10 m), Bolshoe Khmelevskoe (N 43°43,03’ E 40°11,98', altitude 1752 m, depth 2m) and Lake Donguz-Orun (N 43°13'26", E 42°29'35", altitude 2545 m, depth 4-14 m). All the three lakes have different origin and morphology with the latter being a classical periglacial lake dammed by the lateral moraine of Donguz-Orun glacier. Especially noteworthy is the thin varved clay of Donguz-Orun sediment core. The age of moraines of Donguzorun glaciers will be estimated basing on lichenometry and tree-ring analysis and they were sampled for the 10Be dating. Preliminary analysis of the sediment cores including grain-size composition and magnetic susceptibility reveals the highest priority of the study of the sediment of Caucasus lakes in reconstructions of the history of glaciation and climate of the region.
Elbrus region – Central Caucasus
Altitude: 2545 m
Lake Donguz-Orun is situated in the upper reaches of Donguz-Orun-Kyol, a tributary of Baksan river in the Elbrus region of Caucasus, a typical periglacial lake dammed by a lateral moraine of Donguz-Orun Glacier. It is a drainage lake with several inflowing glacial streams and effluent river Donguz-Orun. The surface area is around 105 000 m2, volume: 465 000m3. The average depth is around 4,5 m with maximum up to 14 m. The deepest part is found close to the moraine dam in the narrow northern part of the lake what is consistent with this type of glacial lake systems. An intensive gravitational drift of the moraine material towards the lake is observed. These nonrounded moraine rocks largely constitute the lakebed. Lakustrine sediment is present though.
The right moraine bank is 30-40° steep with sparse herbaceous vegetation, the left bank represents a gentle slope with more pronounced vegetation including bushes and young trees.
The upper part of the lake is largely affected by inflowing glacial streams that deposit significant amount of sand. This southern part of the lake is therefore shallow (0,2-0,5 m) with a tendency of overgrowing with sedge and similar swampy vegetation.
The expedition of Institute of Geography, Russian Academy of Sciences (august 2012) used a gravity corer mounted on the inflatable catamaran to obtain sediment of Lake Donguz-Orun. A sample 40-45 cm thick has been collected at the depth around 12m. Actual thickness of the sediment (reddish and dark-grey fine clay) might be larger.
Bolshoe Dzitakskoe Lake
Krasnaya Polyana region – Western Caucasus
Altitude: 1915 m
Bolshoe Dzitakskoe Lake lies in the head of low inclined trough valley of Dzitaku – left tributary of Urushten river. It is the biggest lake in the marsh-lake complex of Dzitaku with the area around 20 000 m2 and the depth reaching 10 m. It is situated around the axis of the Main Caucasus Ridge with separate summits exceeding the altitude of 3000 m.
According to the earlier radiocarbon dating the lake was generated around 6000 yr BP within the extent of former trough glacier.
The deepest part of the lake is situated in the north-east and represents a depression of an avalanche origin. Frequent avalanche activity and talus deposits of steep south slopes facing the lake prevent water reservoir from overgrowing. Surrounding land cover represents swamp grassland, predominant plant species is sedge.
In august of 2012 the expedition of Institute of Geography, Russian Academy of Sciences reached Bolshoe Dzitakskoe Lake in order to obtain first lacustrine sediments of the lake. A gravity corer mounted on the inflatable catamaran was used. The thickness of the sediment at the depth 9,5-10 m appears to be around 30-35 cm. The sediment represents fine clay with inclusions of fine detritus underlain by coarser rocks. A sample 30-35 cm thick had been obtained.
BOLSHOE KHMELEVSKOE LAKE
Krasnaya Polyana region – Western Caucasus
Altitude: 1752 m
Bolshoe Khmelevskoe Lake is situated northward of Krasnaya Polyana. It is a part of the protected area and recreation centre `Khmelevskie Lakes` consisting of four lakes (Bolshoe is the biggest one) and surrounding area. The centre includes a guesthouse building – about 100 m from the edge of Bolshoe Khmelevskoe Lake and several other recreation facilities.
The lakes are considered to have tectonic origin.
Bolshoe Khmelevskoe Lake lies in the internal-drainage basin surrounded by banks 25-30 m high and around 25° steep. The lake has an oval shape with dimensions approx. 200 by 80 m and total area 9 400 m2. About 65-70% of the lake’s surface is overgrown with sedge – that forms a distinctive vegetative floating ground, often capable of bearing human weight. The depth is constant throughout all the lake and constitutes 2 m.
The expedition of Institute of Geography, Russian Academy of Sciences (august 2012) used a gravity corer mounted on the inflatable catamaran and on the lake’s foating ground for coring at the depth of 2m. The sediment consists of fibric peat with abundant lest decomposed organic remnants. A sample 30 cm thick had been obtained.