Palaeoenvironmental changes in the central part of theTaklamakan Desert, northwestern China sincethe late Pleistocene

Abstract

The Taklamakan Desert, with an area of 337600 km2 and located in the Tarim Basin of southern Xinjiang, is the largest sand sea in the mid-latitudes on Earth. Previous understanding about palaeoclimate of this vast arid landscape was mostly deciphered from loess sequences found in the surrounding mountains, while only a few in situ investigations in the sand sea were carried out. In light of the advances in the optically stimulated luminescence (OSL) dating techniques during the last decades, reconstructing palaeoenvironmental changes via studying aeolian sands in the interior of sand seas can now be achieved by careful observations in the field and by application of improved protocols of OSL dating methods. We used the new multiple-aliquot regenerative-dose (MAR) protocol with carefully selected preheat conditions of 230°C for 10 s to date all samples which are distinctly younger than the upper age limit of this dating procedure, confirming that our chronology is scientifically robust. This paper reveals facies changes and the OSL chronologies of nine sedimentary sections showing interactions between fluvial, lacustrine (including slack-water) and aeolian processes in the central part of the Taklamakan. At present all sites of these sections are undergoing aeolian processes. The occurrences of fluvial and lacustrine (including slack-water) sediments in these sections indicate that many parts of the sand sea were shaped by rivers and inundated by water for long durations in the past. The initiations of occurrence of rivers and lakes (including slack-water) show some cyclicity according to the OSL ages. Our palaeoenvironmental interpretations of the different sedimentary facies in the field are supported by the data of palaeoclimatic proxies including color, grain sizes and magnetic susceptibility. The nine sections collectively suggest that three moist periods occurred in the Taklamakan Sand Sea since the late Pleistocene, i.e., ~70-50 ka (1 ka = 1000 years before 2010 CE), the late glacial of the last glaciation (~17-11 ka) and the late Holocene (~5-2 ka). During these moist periods, wetlands like fluvial channels, lakes (including slack-waters) occurred in the interior part of the Taklamakan, in the lower reaches of the Keriya and Andier Rivers and in the southern side of the Tarim River. The timing of the moist conditions in the Taklamakan Desert concurred with the intervals when global temperatures shifted from cold to warm stages. Thus we deduce that the wetter conditions in the Taklamakan were likely associated with increased runoffs of the rivers flowing into the desert with headwaters in the surrounding mountains. The amount of runoffs was dependent on regional precipitation and more importantly on the melting of snow and glaciers in the headwater regions. Palaeoclimatic simulations indicate that moistures of the Taklamakan Desert were transported mainly by mid-latitude westerlies both during the last Glacial Maximum and the Mid-Holocene. It is likely that the southward shift of westerlies due to the intensifying of Arctic Oscillation resulted in more orographic precipitations in the surrounding mountains of the Taklamakan Desert. We conclude that the increase of runoffs due to the increased melting of snow and glaciers during the transitional periods from cold to warm intervals allowed for the development of wetlands and provided water resources for the prosperity of early civilizations in the largest desert of China. Thus the features and driving factors of late Quaternary palaeoenvironmental changes in the Taklamakan are different from those in the middle and eastern portions of the desert belt in northern China where the palaeoclimate fluctuations were largely triggered by the monsoon system.