Holocene Environmental Evolution Recorded By Aeolian Deposits In The Northern Qinghai Lake Area

The Qinghai Lake area is situated in the northeast margin of Tibetan Plateau,and plays a vital role in responding to the evolution of regional climate and environment due to its unique geographic position and complex atmospheric circulation patterns.Aeolian deposits,widespreadly distributed in the Qinghai Lake area,are of vital implications to reveal the history of regional aeolian activities and the evolution of climate environment.Previous studies on the aeolian deposits have been implemented,but most of them focus on the east,west and south of the Qinghai Lake area,while the studies of the northern part are rarely reported.Additionally,there is also a lack of research regarding reconstruction of palaeoclimatic evolution by synthesizing representative sections of aeolian deposits around the whole Qinghai Lake area.Herein,we parsed the OSL ages results of aeolian deposits and palaeoclimatic implications of multi-proxies,including the grain size,magnetic susceptibility,color parameters and elemental geochemical proxies,from two sections of northern part in the Qinghai Lake,i.e.Gangcha(GC)and Reshui(RS)sections.We also assembled a comprehensive collection of published stratigraphy and chronology dates as well as compared with a range of proxy indicators from previous studies to reconstruct palaeoclimatic evolution in the study area since 11 ka and preliminarily explore its underlying forcing mechanisms.In this thesis,we obtain the following conclusions:1.The OSL ages of GC and RS sections are consistent with the stratigraphic sequence,and there may be a sedimentation discontinuity in the sediments at the top of RS section.Similar phenomenon also exists in other aeolian deposits in Qinghai Lake area.The bottom ages of GC and RS sections are 9.5 ± 0.8 ka and 11.0 ± 0.9 ka,respectively.2.The GC and RS sections are dominated by silt composition,and the grain size of the RS section is coarser than that of the GC section.The end-member modeling analysis(EMMA)of grain size distributions indicated that fine component(EM1)may represent a weak aeolian activity and link to a dominated component in the period of warm and humid climate,while the coarse component(EM2 and EM3)may reflect a relatively strong eolian activity with dry climate.The high magnetic susceptibility and low lightness(L*)values of palaeosols in the two sections indicate that the palaeosols had strong pedogenesis,which may be related to the warm and humid climate environment.3.The CIA and A-CN-K ternary diagram of sediments in GC and RS sections suggest that most sediments were in the weak and moderate chemical weathering intensity,and they were still at the incipient stage of Na and Ca removal due to plagioclase weathering.The study area had experienced the great alternation between dry and wet climate,reflected by the change of CIA values.On the whole,the geochemical indexes can correspond to the changes of stratigraphic sequence.4.The composite records of the stratigraphic units of the representative aeolian deposits sections in Qinghai Lake area show that aeolian sand deposits and loess deposits generally developed in the early Holocene,palaeosol deposits were pervasively developed in the middle Holocene,and then were replaced by modern soil or aeolian sand deposits in the late Holocene.5.Our comprehensive results indicate that the early Holocene(from 11.0 to 7.5ka)was characterized by the relatively dry climate and aeolian activity was strong in the Qinghai Lake area,consistent with widespread aeolian sand deposits and loess deposits in the representative aeolian sections.The humid climate conditions and weak aeolian activity occurred from 7.5 to 3.0 ka,supported by commonly developed palaeosol deposits in the Qinghai Lake area.After 3.0 ka,due to drier climate conditions the aeolian activity was again strengthened.The variation of Northern Hemisphere summer insolation and Asian monsoon may cause the increase and decrease of effective humidity,thereby lead to the dry and wet climate change in Qinghai Lake area during the Holocene.