Reconstructing fluvial, lacustrine and aeolian process dynamics in Western Mongolia

Abstract

At the eastern rim of the Mongol Els, the largest dune field of Mongolia, the rivers Shurgyn Gol and Zavkhan Gol converge and form a flood plain at the dune front. Interfingering fluvio-lacustrine and aeolian sediments are found in different spatial contexts in the area. By investigating the chronostratigraphy of these sediments, valuable information can be derived for the reconstruction of geomorphological processes and forcing-response relations in the past. The flood plain is locally covered by lacustrine sediments representing an Early to Mid-Holocene lake only a few meters deep. This can be deduced from several 14C and OSL dates and tachymetric measurements, and is in accordance with high stands of other lakes in Western Mongolia. Relics of a lake sediment on top of an old dune 17 m above the floodplain have been OSL-dated to 114 10, 119 10, and 127 10 ka, indicative of an Eemian age. At that time a large and deep lake must have covered the plain. There is evidence of two causes of lake formation: increased discharge of the rivers due to increased precipitation in the mountainous catchment areas and, at the same time, ponding of the rivers by strong dune advance due to persistingly arid conditions in the foreland. The transition phase to more humid con ditions, together with a relatively high humidity gradient, appears to have been morphodynamically very efficient. No evidence could be found of the assumed modern climate change. Recent reduction of the sparse grass cover of the region reflects overgrazing, and the reduction of river discharge is due to water-consuming irrigation.