Geochemical evidence for changes in provenance and paleoclimate during the Holocene obtained from a fluvial–eolian sequence in the southern Mu Us Desert, north-central China

Abstract

Identifying the sources and weathering degree of sediments in the northern deserts of China is crucial to understand the evolution of the Asian monsoon system and corresponding induced changes in surface processes. Great progress has been made in the reconstruction of paleoclimate and paleoenvironment from the Early Holocene onwards based on chronostratigraphy and proxies of aeolian sedimentary sequences in the Mu Us Desert (MUD). However, previous studies have given rise to some controversial over the timing of the Holocene optimum period, and there is a scarcity of studies on the sediment provenance evolution of the desert during the Holocene and its internal relationship with the East Asian summer (EASM) and winter (EAWM) monsoons. Here, we provide new elemental geochemical data from a fluvial–aeolian sedimentary sequence in the southern MUD; we then combine these data with other available elemental data and hydroclimatology information to investigate the characteristics of sediment source evolution and their possible connection with the EASM and EAWM on the southern margin of the MUD-desert/loess transition zone since the latter part of the Early-Holocene. The elemental composition and geochemical characteristics of the studied sedimentary sequence differed markedly in the periods before and after 6.4 ka BP, suggesting a change in the source of the sediments across this boundary. From the late Early-Holocene (10.6 cal. ka BP), sediments were dominated by the chemical weathering products of local source material, with a gradual increase in chemical weathering over time, indicating a gradual strengthening of the EASM; the EASM reached its peak Holocene strength at 8.4–6.4 ka BP, accompanied by a moderate level of chemical weathering. After 6.4 ka BP, with the weakening of monsoon precipitation, the EAWM began to strengthen, and long-distance transport of source material from the west and northwest increased; however, chemical weathering of local source material driven by the EASM remained dominant until 4.0 ka BP. After 4.0 ka BP, the EAWM became dominant, and sediments showed strong sedimentary cycles, mainly transported from long-distance sources by aeolian activity, and the MUD became an important sedimentary sink for the northern-central deserts. After 0.7 ka BP, the sediments in the MUD became a mixture of different-age sediments due to river scouring and the transport of pre-existing sediments along the shoreline. Collectively, the elemental geochemical and other proxy data from Holocene sediments on the southern margin of the MUD record an inverse relationship between the EASM and EAWM; these two monsoonal circulations have alternated in their control of the sedimentary source characteristics and geomorphological environmental evolution processes across the southern margin of the MUD during the Holocene.