Sediment history mirrors Pleistocene aridification in the Gobi Desert (Ejina Basin, NW China)

Central Asia is a large-scale source of dust transport, but it also held a prominent changing hydrological system during the Quaternary. A 223 m long sediment core (GN200) was recovered from the Ejina Basin (synonymously Gaxun Nur Basin) in NW China to reconstruct the main modes of water availability in the area during the Quaternary. The core was drilled from the Heihe alluvial fan, one of the world's largest alluvial fans, which covers a part of the Gobi Desert. Grain-size distributions supported by endmember modelling analyses, geochemical–mineralogical compositions (based on XRF and XRD measurements), and bioindicator data (ostracods, gastropods, pollen and non-pollen palynomorphs, and n-alkanes with leaf-wax δD) are used to infer the main transport processes and related environmental changes during the Pleistocene. Magnetostratigraphy supported by radionuclide dating provides the age model. Grain-size endmembers indicate that lake, playa (sheetflood), fluvial, and aeolian dynamics are the major factors influencing sedimentation in the Ejina Basin. Core GN200 reached the pre-Quaternary quartz- and plagioclase-rich “Red Clay” formation and reworked material derived from it in the core bottom. This part is overlain by silt-dominated sediments between 217 and 110 m core depth, which represent a period of lacustrine and playa-lacustrine sedimentation that presumably formed within an endorheic basin. The upper core half between 110 and 0 m is composed of mainly silty to sandy sediments derived from the Heihe that have accumulated in a giant sediment fan until modern time. Apart from the transition from a siltier to a sandier environment with frequent switches between sediment types upcore, the clay mineral fraction is indicative of different environments. Mixed-layer clay minerals (chlorite/smectite) are increased in the basal Red Clay and reworked sediments, smectite is indicative of lacustrine-playa deposits, and increased chlorite content is characteristic of the Heihe river deposits. The sediment succession in core GN200 based on the detrital proxy interpretation demonstrates that lake-playa sedimentation in the Ejina Basin has been disrupted likely due to tectonic events in the southern part of the catchment around 1 Ma. At this time Heihe broke through from the Hexi Corridor through the Heli Shan ridge into the northern Ejina Basin. This initiated the alluvial fan progradation into the Ejina Basin. Presently the sediment bulge repels the diminishing lacustrine environment further north. In this sense, the uplift of the hinterland served as a tipping element that triggered landscape transformation in the northern Tibetan foreland (i.e. the Hexi Corridor) and further on in the adjacent northern intracontinental Ejina Basin. The onset of alluvial fan formation coincides with increased sedimentation rates on the Chinese Loess Plateau, suggesting that the Heihe alluvial fan may have served as a prominent upwind sediment source for it.

Geomorphological evolution and palaeoenvironmental change in the Western Alashan Plateau, China

Though neotectonic activity is believed to be the major factor in the evolution of the topography of the western Alashan Plateau, detailed documentation and analysis of landform evolution and their Quaternary environmental implications in this area are lacking. The present study is a site-specific documentation of landforms developed in the wide terrain of the Ejina Basin (western Alashan), with the aim to identify the tectonic and climatic achieves of landforms and their implications on palaeoenvironmenal changes. The climate and hydrology at present in Ejina is relatively monotone and stable. Besides tectodynamics, the most widely spread landforms in the basin are climatic landforms. Both geomorphological and sedimentological abnormities of aeolian landforms occur in the whole basin, suggesting that the role of retention effect by river and related unenen ground and weak deflation processes nearby river, may be significant factors for the formation of the Ejina dunes other than arid climate. We propose that a wide interaction of aeolian and fluvial processes is the major mechanism to explain regional differences in Ejina landforms. Based on the gemorphological and sediment properties of the regional comparability, it is reasonable to attribute the formation of relic landforms in the margin of the Ejina Basin to the local glacial maximum of the last glaciation. The landform transformtion of pediments and desert gorges into desert plains and the occurrence of wide shifting dunes, together with the existing of large palaeo-lakebed areas, all indicate an increasing aridity index in the Ejina Basin at present. The climate and hydrological conditions of the basin during the last glaciation and during the Early Holocene were much better than at present, possibly having an average annual precipitation ranged between 60~350 mm on the basin during ca. 39-23 ka BP but great fluctuations during Holocene. However, the coexistence of diverse landforms and the consequent geomorphodiversity in the basin should be a compound result of surficial processes other than glaciations. The periods of lower aridity during the late Pleistocene in the bain could be induced by an increase of the westerlies and a weakening of the Asian winter monsoon on the arid areas of the Central Asia.

Sediment history mirrors Pleistocene aridification in the Gobi Desert (Ejina Basin, NW China)

Central Asia is a large-scale source of dust transport, but also holds a prominent changing hydrological system during the Quaternary. A 223-m-long sediment core (GN200) was recovered from the Ejina Basin (synonymously Gaxun Nur Basin) in NW China to reconstructing the main transitional modes of water availability in the area during the Quaternary. The core has been drilled from the Heihe alluvial fan, one of the world's largest continental alluvial fan, which covers a part of the Gobi Desert. Grain-size distributions supported by endmember modelling analyses, geochemical-mineralogical compositions (based on XRF and XRD measurements), and bioindicator data (ostracods, gastropods, n-alkanes with leaf-wax δD) are used to infer the main transport processes and related environmental changes during the Pleistocene. Magnetostratigraphy supported by radionuclide dating provides the age model. Grain-size endmembers indicate that lake, playa (sheetflood), fluvial, and aeolian dynamics are the major factors influencing sedimentation in the Ejina Basin. Core GN200 reached the Pre-Quaternary quartz- and plagioclase-rich Red Clay Formation and reworked material derived from it in the core bottom. This part is overlain by silt-dominated sediments between 217 and 110 m core depth, which represent a period of lacustrine and playa-lacustrine sedimentation that presumably formed within an endorheic basin. The upper core half between 110 and 0 m is composed of mainly silty to sandy sediments derived from the Heihe River that have accumulated in a giant sediment fan until modern time. Apart from the transition from a siltier to a sandier environment with frequent switches between sediment types upcore, the clay mineral fraction is indicative for different environments. Mixed layer clay minerals (chlorite/smectite) are increased in the basal Red Clay Formation and reworked sediments, smectite is indicative for lacustrine-playa deposits, and an increased chlorite content is characteristic of the Heihe river deposits. The sediment succession in core GN200 based on the detrital proxy interpretation demonstrates that lake-playa sedimentation in the Ejina Basin has been disrupted likely due to tectonic events in the southern part of the catchment around 1 Ma BP. At this time Heihe river broke through from the Hexi Corridor through the Heli Shan ridge into the northern Ejina Basin. This initiated the alluvial fan progradation into the Ejina Basin. Presently the sediment bulge repels the diminishing lacustrine environment further north. In this sense, the uplift of the hinterland served as a tipping element that triggered landscape transformation in the Northern Tibetan foreland (i.e., the Hexi Corridor) and further on in the adjacent northern intracontinental Ejina Basin. The onset of alluvial fan formation coincides with increased sedimentation rates on the Chinese loess plateau, suggesting that the Heihe fluvial/alluvial fan may have served as a prominent upwind sediment source for it.

Climate-Driven Landformof the Ejina Basin (NW China) in Central Asia and Its Paleoenvironmental Implications

<p>Though neotectonic activity is believed to be the major factor in the evolution of the topography of the Ejina Basin in Central Asia, detailed documentation and analysis of climatic landscape features and their environmental implications are lacking. The present study is a site-specific documentation of landforms developed in the wide part of the Ejina Basin, with the aim to identify the climatic landforms based on the method of climatic geomorphology and to evaluate its landscape evolution and response to palaeoclimate changes. The morphodynamics of older landscapes are recognized by making comparison with the present climate and its corresponding landscapes. Clear evidences testifying the basin-scale shifting and transformation of different morphoclimatic zones in the basin are observed, which prove that the main geomorphic unit is changed from an alluvial-lacustrine plain to a desert plain. The coexistence of diverse landscapes and the consequent geomorphodiversity in the basin should be a compound result of surficial processes other than glaciations. The climate and hydrological conditions of the basin during the last glaciation and during the Early Holocene were much better than at present, possibly having an average annual precipitation ranged between 60~350 mm on <del cite="mailto:unknown" datetime="2015-11-27T09:00"> </del>the basin during ca. 39-23 ka BP but great fluctuations during Holocene. The periods of lower aridity during the late Pleistocene in the bain could be induced by an increase of the westerlies and a weakening of the Asian winter monsoon on the arid areas of the central Asia.</p>