Holocene Climate Change Recorded By Aeolian Deposits In The Southern Tibetan Plateau And Its Driving Mechanisms

The Tibetan Plateau（TP）is regarded as a“Third Pole of the Earth”and“Asian Water Tower”,and it has an important role in global climate change and human activities,as well as the safety of the living environment.The TP’s surface system and barrier function,as well as the natural landscapes like glaciers,lakes,and vegetation,have all undergone significant changes over the past 50 years under the influence of climate warming,which has created significant challenges for human living environment and regional sustainable development in the TP and its surrounding regions.The Yarlung Zangbo River（YZR）basin in the southern TP is not only an important component of the Asian Water Tower region,but it is also a major gathering area for the plateau population,with strong regional dust activity.Aeolian dust activities have a significant impact on regional human activities and the global climate and eco-environment.However,the research on climate change in the southern TP is still limited.Our understanding of the landscape changes of the TP’s surface system and their spatial analysis is constrained by the region’s geographical location and mountain-valley environment,which aggravate the complexity of regional climate response.In this research,we used extensive fieldwork,remote sensing analysis,and laboratory experiments to discuss the spatiotemporal distribution characteristics and physicochemical properties of aeolian sediments in the YZR basin.We also analyzed the environmental implications of the physicochemical proxies of the aeolian sediments.On this basis,four typical aeolian sedimentary sequences in the YZR basin from west to east have been used as research materials,and optically stimulated luminescence dating and accelerator mass spectrometry ¹⁴C dating were used to establish chronologies for these sequences and further reconstruct reliable age-depth models.We first used related climate proxies with clear implications to reconstruct the Holocene moisture history in the study area and then discussed spatial variations in moisture evolution and its driving mechanisms.Second,we reconstructed the Holocene aeolian dust activity history in the southern TP and revealed the similarities and differences in aeolian activities and their driving mechanisms in the study area and the eastern arid Central Asia based on the studied and reported aeolian sedimentary sequences.At last,we discussed the possibility of aeolian sequences in revealing the Holocene cold season temperature history based on the understanding of the temperature change and aeolian dust activity in the TP.The main conclusions are as follows:（1）We developed a new map/atlas of the distribution of aeolian sediments in the YZR basin in the southern TP,which is mainly composed of loess and sand dunes.The map/atlas includes a small-scale overview map and six large-scale regional maps with high-resolution satellite images and field photos attached.In the southern TP’s YZR basin,loess and sand dunes decreased from west to east.Aeolian sediments are mainly distributed in the wide valley of the river,particularly in the middle reaches’wide valleys of Shigatse and Shannan.The sediments earlier than the last glacial maximum are less preserved,and the aeolian deposits have predominantly originated and accumulated since the last glacial maximum and mainly during the Holocene.The sedimentary thickness increases from west to east in the basin,and the age is relatively old.The response of sediments in the YZR basin to climate is relatively complex,with obvious regional differences in the climatic significance of environmental proxies.The magnetic grains of sediments in the upper reaches of the YZR were significantly determined by detrital input and can indicate changes in near-surface wind intensity,whereas the magnetic properties of sediments in the middle reaches of the YZR were controlled by both detrital input and pedogenesis,and secondary magnetic minerals related parameters can reflect moisture/temperature changes.The response of proxies to climate is not sole,and the implications of various proxies need to be thoroughly evaluated when reconstructing paleoclimate change.（2）During the Holocene,there was a clear spatial difference in the evolution of moisture in the southern TP.The Holocene moisture evolution in the upper reaches of the YZR in the west part of the southern TP was mainly controlled by changes in the relative intensities of the Indian Summer Monsoon（ISM）and the winter Mid-latitude Westerlies（MLW）,which were fundamentally dependent on variations in summer and winter insolation at 30°N.In response to high summer insolation during the early Holocene,the region was primarily influenced by a heavy water vapor supply from evaporation from the surface of the Indian Ocean driven by the strong ISM.At～7.6 ka,the effectiveness of moisture delivery abruptly decreased,most likely due to the weakening of the ISM.During this period,the winter MLW had no effect on moisture supply.From the middle to late Holocene,precipitation delivered by the ISM gradually decreased as summer insolation decreased.As winter insolation increased,the intensified winter MLW penetrated into the southern TP,regulating the ISM-derived moisture supply.The Holocene moisture evolution in the middle reaches of the YZR in the middle part of the southern TP was similar to that in the upper reaches and was still controlled by a synergy of the ISM and winter MLW.However,the mountain-valley environment,may complicate this process,resulting in moisture changes since the late Holocene that did not completely correspond to changes in the winter MLW.The moisture evolution during the Holocene was controlled mainly by change of the ISM in lower reaches of the YZR in the east part of southern TP,and the influence of the winter MLW was significantly reduced.The moisture was relatively high during the early to middle Holocene and then gradually decreased,which was mainly controlled by southward shift of the ITCZ and related rainfall belt in response to changes in summer insolation at 30°N.（3）The aeolian dust activity in the southern TP gradually decreased from the early to middle Holocene and then gradually increased since the late Holocene,which was generally in response to the change in cold season insolation at 30°N,as well as the internal oscillation of the Earth’s climate system.The near-surface wind intensity decreased from the early to middle Holocene as cold season insolation increased,along with the rate during which dust accumulated and the size of the dust grains.Simultaneously,the decrease in warm season insolation caused a decrease in ISM precipitation,resulting in a decrease in vegetation cover and material supply,which may also affect the decrease in dust accumulation rate.The aeolian dust activity in the southern TP was significantly affected by cold events in the Earth,during this period of relatively cold conditions with high ice volume and low CO₂ concentration.With the decrease in cold season insolation since the late Holocene,the dust accumulation rate has increased and the grain-size of dust has become coarse in response to the intensified near-surface wind in the southern TP.However,due to the continuous decrease in warm season insolation,it had only a slight impact on dust activity.At this time,the feedback of dust activity in the southern TP on cold events on Earth was not obvious due to the relatively warm conditions with low ice volume and high CO₂ concentration.By comparing with the aeolian sedimentary records in the eastern arid central Asia,we propose that the driving mechanisms of aeolian dust activity in the two regions are similar.That is,changes in cold season insolation,ice volume,and CO₂ concentration co-controlled the regional temperature change and further affected the near-surface wind strength by affecting the TP High and the Siberia High respectively,thus eventually controlling the regional dust activity.In the context of future climate warming,the regional dust activity will continue to weaken.Finally,it is proposed that the aeolian sedimentary sequences in the TP may be a potential carrier reflecting the regional cold season temperature based on the understanding of the driving mechanisms behind the aeolian dust activity in the TP and the comparison with the winter temperature history recorded by ice core.