extended abstracts - Geomorphic Processes and Geoarchaeology

Geomorphic processes and geoarchaeology
Lacustrine silts in the interval 5.80-5.20 m evidence the impulse of lake sedimentation
dated to 11.0-10.5 ka BP (cal). It was followed by another phase of alluvial sedimentation
(1.9-m thick sandy loams). The final establishment of lacustrine regime is marked by
change to carbonate lacustrine silts at depth 3.30 m dated to 9.5 ka BP (cal).
To estimate the age of the island the interruption of lacustrine sedimentation (the
top of lake sediments at the land surface) should be dated. Radiocarbon dating of mollusk
shells from a bank exposure gave dates 2-4 ka older than expected from stratigraphy and
extrapolation of sedimentation rates. It is explained by contamination of lake carbonates
by old carbon from Paleozoic marbles that compose a big portion of the lake catchment.
Sediment composition excluded also luminescent dating techniques. Therefore to date the
topmost lacustrine silts in the island geological correlation to a well dated lake core was
undertaken. The lake core was divided into 7 sedimentation units based on composition
of biological remains (mainly algae) and bulk chemical composition. The lower unit
corresponds to underlying alluvial sediments. Sequence of lacustrine sedimentation units
reflects succession of phases in lake development with different hydrological regime
(more or less active renovation of water because of changing proportion between outflow
and evaporation in water balance). Borders between units were radiocarbon-dated.
Distinguishable changes of biological and chemical composition of units were used as a
correlation tool. The top of lacustrine sediments in Por-Bajin Island were correlated to the
second half of Unit V, which give the age estimation between 4-5 ka BP (cal).
Given the above data, history of the Tere-Khol Lake and Por-Bajin Island has been
reconstructed as follows. The lake appearance was pre-conditioned in the Late Glacial by
active development of alluvial fans of big and small rivers that drained surrounding
mountains. The fans protruded into the bottom of the Terekhol Basin and isolated its
south-western corner. Shortly after the start of the Holocene ca 11,000 years BP the
initial lake reservoir appeared probably during an interval of relatively high river runoff.
During this initial phase the lake occupied a vast area that included also the location of
the modern Por-Bajin Island that did not exist at that time. About 10,500 years BP the
lake shrank considerably and the future Por-Bajin Island area changed to lowland terrain
subject to overbank alluvial sedimentation. Probable reason for the lake shrinkage was
thermokarstic (permafrost melting) subsidence. Lowering of the lake level favored
incision of tributary small rivers that in turn resulted in cessation of overbank alluvial
sedimentation in surrounding plain.
The next phase of lake development was its final transgression and spread about
its modern dimensions that is dated to ca 9,500 years BP. Since that moment the area of
the Por-Bajin Island had been a part of lake bottom for about 5,000 years. Between 4,000-
5,000 years BP the lake bottom rose locally by 5-7 m to make the island. Mechanism of
this rise was frost heaving favored by the lake winter freeze through during one of dry
phases when water depth was even shallower than usually. In the emerged part of the lake
bottom permafrost continued to accrete in subsequent years and led to formation of
palsa – frost heave hill. The mechanism was as follows. Freezing of water-saturated
sediments and attraction of additional water to the frost caused formation of ice lenses
that considerably increased the total volume of sediments and finally had led to formation
of a flat hill with the top 4-5 m above the lake and gentle sides sloping down to the lake
bottom. This palsa island had existed in the lake for several thousand years before in the
mid-VIII century it was chosen as a location for the fortress of Por-Bajin.
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