Indications of man-induced changes in a sequence and composition of floodplain deposits in Belarus
S. SAVCHIK1, N.MAKHNACH2
1The Institute of Geological Sciences, 220141 Minsk, st Kuprevich 7, Belarus
e-mail:,
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INTRODUCTION
Changes in a sequence and composition
of floodplain deposits may be caused by both natural and man-induced processes which occur in river valleys or within the entire river drainage areas. The immediate agents influencing
the character of deposition are divided into three main groups: (1)fluctuations of the river channel position (meandering, bifurcation); (2)changes in river discharge caused by climatic and other factors; and (3)landscape pattern change within the river drainage area. The factors corresponding to all the three groups may have as natural as anthropogenic origin. But considering recent time (the last several thousand years),
the landscape pattern change is caused predominantly by human activities in the river drainage basin. These activities include deforestation, agricultural development (ploughing and pasturing), and watershed relief change due to other actions such as urbanisation, melioration, mining and so far. Recent changes in the landscape pattern lead to an accumulation in river valleys of specific floodplain deposits called “muds” or “madas” (Starkel ed. 1996). The muds related to a human development of watersheds are referred to as poorly sorted clayey, silty or/and sandy floodplain deposits, covering the upper part of a section of river valley sediments (Kalicki 1996). The process of an accumulation of anthropogenic muds starts from
the deforestation and agricultural development
of river watershed. This facilitates sheet and linear erosion and provides an abundance
of sediment supply. During the spring flooding rivers become overloaded with sediments which results in a deposition of unsorted muds that differ significantly from the natural floodplain deposits.
Features of anthropogenic-related muds
Previously the muds accomplishing
the sequence of fluvial deposits in river valleys were referred as deposits associated mainly with human impact on landscapes within the river drainage areas (Mensching 1951). The later studies revealed that poorly sorted muds have been deposited in different environments of the Late Glacial and Holocene (Starkel 1960, Rutkowski 1987). But there are several character features of man-induced muds which differ them from the natural ones. The deposits composing anthropogenic muds are poorly sorted with
an obvious coarsening upward the profile. In other cases this feature hasn’t been observed and series of disorderly placed cycles of coarsening-fining upwards deposits have been recorded. Some researches completely exclude a possibility of presence of gradually coarsening upwards cycles in natural fluvial deposits (Allen 1970) so far as the meandering rivers form cycles of fining upwards sequences interrupted by thin layers
of abruptly coarsening sediments. The floodplain muds of the both natural and man-induced origin have been studied thoroughly in Poland.
The character features of these deposits, their classification and palaeogeographical reconstruction of sedimentation process are reported in several comprehensive papers by T. Kalicki (1987,1996), E. Niedzialkowska (1991). Nevertheless there is much controversy seen in an interpretation of the muds origin and in
a reconstruction of environmental conditions under which they have accumulated.
anthropogenic-related muds in Belarus
There are very few sections containing floodplain anthropogenic-related muds have been studied and reported in Belarus. In the Dnieper river valley an occurrence of man-induced muds marks starting point of wide-spread deforestation and ploughing at 940 years BP. The large-scale changes in landscape pattern of the Berezina river basin near Borisov have started to be recorded in floodplain sediments much later – 340 years BP (Kalicki, Sanko 1992; Kalicki 1999). Unfortunately, these two studied sites and several more don’t provide enough evidences for further reconstruction of human impacts on sedimentation process in Belarussian river valleys, which doesn’t make it possible to trace the history of landscape alteration by man in this area of Europe. Two more sections with muds were studied by authors in the Adrov river and Pochalitsa river valleys (tributaries of Dnieper and Adrov respectively). In both sites muds form from 1,5 to 1,0 meter thick covers, overlaying peat and floodplain deposits. The radiocarbon date obtained from the Pochalitsa site allows to assume that an accumulation of muds started there 520+80 BP (IGSB-627), that very likely to be conditioned by human activity in the drainage basin. The granulometric composition of muds from both sites show irregular coarsening and fining of deposits upwards which is very different from underlying floodplain sediments. Mud deposits are sorted much less then underlying “natural” muds and especially then channel alluvium. X-ray spectroscopy analysis showed a significant decrease in Al2O3 content in muds, which may indicate a lack of clay fraction that was removed by flood flow. All these features of muds from the Pochalitsa and Adrov sites allow to refer them as man-induced sediments, which mark the beginning of large-scale development of neighbouring areas.
CONCLUSIONS
Man-induced changes in a sequence and composition of floodplain deposits are used to determine the starting point, duration and intensity of large-scale human impact on
the landscapes of the river drainage basin. This indicator together with archaeological, geochemical and geomorphological data allows to trace a development of human impact since it appearance till the recent time. Nevertheless there are serious difficulties seen while interpreting data obtained from studies of “man-related” muds. It leads to controversial conclusions, which oftenly found in the papers on this subject (sometimes even written by the same author). Therefore the reconstruction
of anthropogenic alteration of landscape, based on study of floodplain deposits should be supplemented by studies of sequence and composition of lake and peat deposits. It also may be useful to pay attention on
the geoindicator of dune formation and movement, which may mark the starting point and an intensity of mass deforestation.
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