Moisture Changes And Hydrological Dynamics Recorded By Lake Sediments In Northern Xinjiang Over The Past 2400 Years

Northern Xinjiang is an important region of the Silk Road Economic Belt and the ancient Silk Road,as the fragile environment is highly sensitive to climate change.Therefore,the rise and decline of the ancient Silk Road and the regional human activities were strongly dependent on climate change.A full study of the climate change and hydrological dynamics in this region will help to provide a climate environment background for the rise and fall of the Silk Road over the historical period.Although many studies on regional climate change have been conducted in this region,there was still a lack of high-resolution humidity and hydrological records in low-altitude areas over the historical period.The studies on the correlation of past hydrological processes between high mountains and lower basin in northern Xinjiang was relatively scarce,and little knowledge is known about the impacts of historical human activities on regional environment.Here we present environment change records from two lakes in northern Xinjiang: Jili Lake(482.8 m),a low-altitude lake in Junggar Basin,Xinjiang,and Tiewaike Lake(1524 m),a lake in the south of Altai Mountain.Based on the analysis of pollen and charcoal data from core JL18-02-A,combined with historical literatures,we reconstructed the moisture changes in northern Xinjiang over the past 2400 years,and discussed the relationship between climate change and human activities in low altitude areas at centennial timescale.As hydrological dynamics of oasis lake in low altitude area is closely related to mountain precipitation,based on the analysis of grain size data and quartz data under microscope of core TWK19 A,we reconstructed the hydrological dynamics of south slope of Altai Mountain.The main conclusions are as follows:(1)The regional climate generally presents an increasing trend of humidity at millennial timescale,and the climate model shows a “warm-dry” and “cold-wet”pattern at centennial timescale.Pollen assemblages in Jili Lake over the past 2400 years showed that since 400 CE the vegetation variation in this area mainly changes from desert vegetation to desert steppe vegetation.The regional climate generally presents an increasing trend of humidity at millennial timescale,but there are still with three relatively dry phases at centennial timescale: 400 BCE-400 CE,850-1200 CE and 1880 CE to present.In contrast,400-850 CEand 1200-1880 CE were rather wet in climate.(2)Compared with the humidity changes recorded by the lakes in the monsoon margin area,it could be found that the moisture changes in Jili Lake were very different from those in the monsoon margin area.The moisture changes of Jili Lake were mainly affected by solar activities.The stronger westerly wind and the southward movement of the westerly jet corresponded to the lower total solar irradiance and colder conditions,which might benefit more water vapors to be transported to the arid central Asia and less evaporations.At the same time,lower temperature also inhibits evaporation and increases the effective humidity of the area.(3)Based on the pollen records,higher content of aquatic pollen during1600-1880 CE may indicate the lower lake level or strong hydrodynamic force.During the period of 1750-1880 CE,the hydrodynamic force enhancement is supported by the hydrological reconstruction records of Tiewaike Lake in mountainous areas.However,the lower lake level may be the result of the regional agricultural irrigation.Considering that the climate was generally humid during1600-1880 CE,the increase of cereal-type Poaceae and macro-charcoal concentration indicates the enhancement of human activities,and historical documents show that regional population development and agricultural irrigation activities have increased,so the lower lake level is more likely to be caused by intensified irrigation for agriculture in the catchment.(4)Study shows that the frequency of strong hydrological events in high-altitude areas was higher under the relatively warm and humid climate.The grain size and counted quartz particle data over the past 2400 years in Tiewaike Lake exposed that the content of coarse grain size was relatively higher in five phases: 30 BCE-300 CE,700-900 CE,1080-1300 CE,1400-1650 CE,and 1750-1880 CE.Considering the vegetation and drainage basin around Tiewaike Lake,it is speculated that strong hydrological events occurred in these five stages.The concrete process may be as follows: The rapid warming in spring causes the rapid melting of regional accumulated snow,resulting in spring floods;the coarse particles brought by the snow meltwater floods are more easily transported to the unmelted lake ice,which then enters the lake after the ice has melted.Therefore,it is considered that the coarse grain of the sediments may be related to the snowmelt events.