Glacier variations, mudflow activity and landscape development in the Aksay Valley (Tian Shan) during the late Holocene

The object of this paper is to reconstruct the history of glaciation and the dynamics of mudflow activity as part of the development of landscapes in the Aksay Valley, Kirgizsky Ridge, over the last 2000 years. Glacial, mudflow and slope deposits were dated by lichenometry and the radiocarbon method. The results of 14C dating of buried soils shows that at about 1700 BP a period of activization of slope processes took place in the valley. The zone of alpine meadow was situated at that time at an elevation of about 2750 m and glacier termini were situated at about 3100-3200 m. The oldest mudflow surface, which was investigated by lichenometry, dates from the same time. New equations for the rate of growth of the lichens Aspicilia tianshanica and Rhizocarpon geographicum use the oldest mudflow surface as a control point. Two periods of reduced mudflow activity are identified by lichenometry, i.e., 350-450 and 130-240 BP, and the latter is confirmed by 14C dating of peat-soil interlayers in a mudflow stratum with the specific activity of samples exceeding NBS standard. Thus, a new modification of the 14C-dating method is demonstrated. Lichenometry on moraines of the Aksay Glacier shows that advances of the glacier took place in the fourteenth and late- nineteenth centuries as well as at the beginning of this century. The depression of equilibrium-line altitude (ELA) for the fourteenth and nineteenth centuries was 150-170 m and 100-120 m, respectively.