Grain-size record of the Late Pleistocene loess deposits in the south of West Siberia
Sizikova A.O.
V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia
sizikova_a@ngs.ru
To be able to make more reliable predictions, better knowledge on the functioning of short-term climate changes in the past is essential. The grain-size studies of loess-soil sequences intend to contribute to a better understanding of short-term climate oscillations.
There are unique loess-soil sections which reflect the fullest changes of a climate in the Quaternary in the south part of the West Siberia. This record of climate shows a good correlation with marine oxygen isotope curve and other global climate records [1, 2] and consequently loess-soil sequence of West Siberia is detailed reference scale for the accurate loess study. General intensity of an atmospheric circulation is reflected in composition and structure of the loess strata. Soil formation occurred in periods of weak circulation, whereas loess deposition was associated with periods of an active circulation of the atmosphere saturated with dust. According to stratigraphy there are three Late Pleistocene loess horizons in West Siberia: Bagan, Eltsovka and Tulino [3]. Both Bagan loess and Eltsovka one are a part of Sartan horizon and equal to MIS-2. The first lies directly under the Holocene soil, and the second lies on deposits of the Karga interstadial. The Tulino loess lies on deposits of the Kazantsevo interglacial.
The one of the most significant object to study grain-size of the loose sediments is to derive much more reliable information about environmental conditions, in which these deposits were formed.
Relatively coarse sediments are associated with a dynamic environment typified by strong winds and cold conditions, whereas fine-grained sediments are related to a low energy deposition under warmer conditions. It has been shown on the Chinese Loess Plateau that a decrease in grain size also corresponds with a decrease in deposition rate, because both grain size and deposition rate depend on the wind strength [4].
The Belovo key section is located on the right bank of the Ob’ river in Cisaltai Plain. It was investigated by two locations. There are all three Late Pleistocene loess layers in both locations. Grain-size sampling was at 5-cm intervals. The grain-size distribution was measured between 0 and 1000 μm and performed on a FRITSCH A22 laser grain size analyzer. Apart from the usual grain-size parameters the U-ratio was used. It is the ratio between the medium-coarse and the fine-coarse fraction of the silt-sized range which evidences about a wind [5].
Fig. Grain-size distribution of the Late Pleistocene loess horizons, Belovo section location 2.
‘Location 2’. The Bagan loess is composed by brownish-grey loam. The percentage of fine-grained sand is gradually decreased to bottom of the layer from 35.7% to 11.6%. The coarse-silt content alternates from 51.1% to 72.4%. Clay fraction is in interval 4.2% - 6.7% with clear tendency to increase toward the horizon’s foot. Mean grain size varies from 36.2 μm to 24.5 μm. The Eltsovka loess is yellowish-gray loam. The content of fine-sand fraction is lesser than in the Bagan loess and amounts to 2.1%-20.8%; it decreases from top to the bottom of layer. There is 6.3% of middle-coarse sand in the middle part of this loess on the depth 3.6-3.85 m. Coarse-silt fraction dominates (57.7%-72.6%) and slightly increases to the bottom. Clay content varies within the interval 5% - 10.2% and rises downwards. Median grain size decreases from 35.52 μm to 19.55 μm toward layer’s bottom. The Tulino loess is dark-brownish-gray loam. The content of fine-sand fraction is substantially lower in the upper and in the lower parts of the layer, than in the middle, where it is 28.9%. Also there is middle-grain sand in content of 6% on the depth 5.9 m in the middle part of the horizon. The percentage of coarse silt is high (47.7 – 73.9%). It slumps to 40% to the foot of the layer. The clay content is in a maximum content (to 14.0%) in the uppermost part and then it gradually decreases to 6% on the depth 7.0 m and slumps to 2.5% towards bottom. Mean grain size varies from 16 μm to 33.5 μm.
Distribution of the main grain-size fractions along section and horizons on ‘location 3’ is close with it in ‘location 2’ described above. The difference is only in the higher contents of middle and fine-grained sand fractions in ‘location 3’. It can be due to its situation on the slope of a ridge.
There is a trend to increasing of fine-grained sand fraction percentage from the Tulino loess to the Bagan. The same pattern of fine sand distribution is observed from the bottom to top of each loess layer. The minimal percentages of clay and coarse-silt fractions are in the Bagan loess with further increasing toward the Tulino loess. The quantity of these fractions is reduced from bottom to top in each loess layer.
The mean grain size variations coincide with these of fine-grained sand percentages. The increase of sand fraction percentage and mean grain size in uneven-aged loess horizons indicates the strengthening in environment activity during the sedimentation. So, while the Bagan loess was forming, wind strength was higher, than during the Eltsovka and Tulino loess accumulation periods. There is an increase of wind flow intensity in the middle part of the Eltsovka and Tulino loess horizons in both locations. It is reflected in the presence of middle-sand fraction and in the higher fine-sand values in the location 2 and by the peak on the fine-sand fraction curve for the location 3. The wide spacing of mean grain size values also indicates wind strength fluctuations during Late Pleistocene.
The U-ratio, which is the ratio of the fractions 44-16 μm and 16-5.5 μm [5], reflects the wind strength during loess formation. U-ratio values vary from 1.2 to 3.5. However, for the Tulino loess accumulation period U-ratio values are in the interval 1.5 – 2.5, for the Eltsovka – 1.5 – 3.0 and for the Bagan – 2.1 – 3.3. So there is intensification of environmental dynamics from the Tulino loess accumulation period to the Bagan one.
The distribution of fine-grained sand and coarse-silt fractions and the mean grain size rises from the Tulino loess horizon to the Bagan one. It is the evidence of environment activity intensification from the Tulino loess accumulation period to the Bagan. The wind strength expressed by U-ratio also confirms the intensification of environmental dynamics to the Last Glacial. It goes with previously obtained data of higher loess accumulation rate during that time [6] and also with the data of sand quartz grain microscopy and micromorphology [7, 8].
The study was supported by RFBR Grant № 10-05-00673а and by interdisciplinary integration projects of SB RAS № 53, №147.
References:
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