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

摘要

Central Asia is a large-scale source of dust transport,but it also held a prominent changing hydrological system during theQuaternary. A 223?m long sediment core (GN200) was recovered from the EjinaBasin (synonymously Gaxun Nur Basin) in NW China to reconstruct the mainmodes of water availability in the area during the Quaternary. The core was drilled from the Heihe alluvial fan, one of the world's largestalluvial 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-pollenpalynomorphs, and n -alkanes with leaf-wax δ D) are used to infer the maintransport processes and related environmental changes during thePleistocene. Magnetostratigraphy supported by radionuclide dating providesthe age model. Grain-size endmembers indicate that lake, playa (sheetflood),fluvial, and aeolian dynamics are the major factors influencingsedimentation in the Ejina Basin. Core GN200 reached the pre-Quaternaryquartz- and plagioclase-rich “Red Clay” formation and reworked materialderived from it in the core bottom. This part is overlain by silt-dominatedsediments between 217 and 110?m core depth, which represent a period oflacustrine and playa-lacustrine sedimentation that presumably formed withinan endorheic basin. The upper core half between 110 and 0?m is composed ofmainly silty to sandy sediments derived from the Heihe that haveaccumulated in a giant sediment fan until modern time. Apart from thetransition from a siltier to a sandier environment with frequent switchesbetween sediment types upcore, the clay mineral fraction is indicative ofdifferent environments. Mixed-layer clay minerals (chlorite/smectite) areincreased in the basal Red Clay and reworked sediments, smectite isindicative of lacustrine-playa deposits, and increased chlorite content ischaracteristic of the Heihe river deposits. The sediment succession in coreGN200 based on the detrital proxy interpretation demonstrates thatlake-playa sedimentation in the Ejina Basin has been disrupted likely due totectonic events in the southern part of the catchment around 1?Ma. Atthis time Heihe broke through from the Hexi Corridor through the HeliShan ridge into the northern Ejina Basin. This initiated the alluvial fanprogradation into the Ejina Basin. Presently the sediment bulge repels thediminishing lacustrine environment further north. In this sense, the upliftof the hinterland served as a tipping element that triggered landscapetransformation in the northern Tibetan foreland (i.e. the Hexi Corridor)and further on in the adjacent northern intracontinental Ejina Basin. Theonset of alluvial fan formation coincides with increased sedimentation rateson the Chinese Loess Plateau, suggesting that the Heihe alluvial fanmay have served as a prominent upwind sediment source for it.