Coupling Relationship Between Climate And Ecological Environment In The Agro-pastoral Ecotone In Northern China Since Holocene

The uncertainty of the climate evolution since the last glaciation will have a great impact on the survival and development of mankind,especially the abnormal climate since the Holocene.The climate instability of the Holocene warns us that the future living environment of mankind is very uncertain.With the increasing impact of human activities on the natural environment and the increase of“greenhouse gas”emissions year by year,the impact of global temperature rise on the environment in the future is one of the hotspots of general concern.The argo-pastoral ecotone is the transition zone between agriculture to grassland animal husbandry ecosystem in China,and it serves as the East Asian monsoon transition belt.The change of precipitation patterns caused by the East Asian summer monsoon intensity was the primary factor limiting the scope of the agro-pastoral range.In addition,human factors affect the distribution pattern of the modern agro-pastoral ecotone,resulting in frequent swings of the agro-pastoral boundary.Two sections in the central part of the northern agro-pastoral ecotone were selected for the research.The first component is the fluvial-lacustrine-eolian deposit from Dishaogowan section in the Salawusu River valley,located in the desert-loess ecotone.The second section is the lacustrine-eolian deposit from Tengger Nuur lake,in the sandy-steppe transition area.Based on the analysis of palaeoclimate proxies such as end-member of grain size,?¹³C_org,n-alkanes,TOC,C/N and Fe²⁺contents of sediments,the paleoenvironment of the northern agro-pastoral ecotone since Holocene was reconstructed.The evolution process and characteristics of the East Asian monsoon system and ecological environment were recognized,as was the interaction between wind-blown sand activities and climate,as well as the coupling relationship between climate and environment evolution since the Holocene was discussed.The main understanding of this paper is as follows:The end-member(EM)analysis of grain-size distribution in Dishaogouwan section yielded an optimal model with 3 EMs,EM1(30.2?m and 363.1?m)represents the aeolian sand,EM3(19.18?m and 195.1?m)represents aeolian sand from promixal or local-source and EM2(19.95?m and 120.2?m)belongs to the shore-shallow lacustrine sand component in lacustrine deposits.The sediment from Tengger Nuur lake is mainly composed of 4 EMs and gravel component as well.EM1(5.012?m)belongs to long-term suspended fine silt,reflecting the change of lake water level;EM2(17.38?m)belongs to offshore suspended particles,representing long-distant transportation of aeolian sand;EM3(104.7?m)belongs to nearshore suspended component of shore-shallow lacustrine facies,reflecting the change of runoff;EM4(363.1?m)belongs to jumping particles of Nearshore zone,representing promixal or local-source sand,reflecting the strength of winter monsoon.The gravel represents the rolling particles of the fluvial-lacustrine facies,indicating the extreme contraction of the lake surface of Tengger Nuur during this period.In the early Holocene,EASM began to strengthen during the period of10400?9270 cal a BP,when precipitation increased;the climate changed,and the temperature became warmer and more humid.That was suitable for the progressive development of higher vegetation,forest,and grassland.With the enhancement of EASM,temperature and precipitation continued to increase from 9270 to 5700 cal a BP.Salawusu paleo-lake reached its maximum during 8000 to 5700 cal a BP.Between5700 and 4590 cal a BP,the East Asian monsoon weakened,the westerly wind strengthened,temperatures and precipitation decreased,but the climate remained moderately humid.The Salawusu Palaeo Lake became shallow into swamps,and sparse forests and grasslands appeared.From 4590 to 2880 cal a BP,the winter monsoon intensified,the climate turned cold and dry with a decrease in the temperature and precipitation;the wind-blown sand activity became active again,the sparse forest-steppe degenerated into the desert steppe,and the environment of Dishaogouwan was gradually desertified.The lake of Tengger Nuur Lake was very shallow and small during the periods of10920?10450 cal a BP,10050?8750 cal a BP and 5450?5100 cal a BP,while the lake was deep and relatively stable between 8750 to 5450 cal a BP and 5100 cal a BP.The lake had the deepest water level in 5100?3700 cal a BP.After approximately 2000 cal a BP,the lake level fluctuated and declined but remained in a shallow lake state until1985,when it dried up.Between 3830 to 2700 cal a BP,wind and sand activity was heavy.Since about 2000cal a BP,human activities in the Tengge Nuur area have increasingly intensified,with evident consequences on Lake hydrological status.In the agro-pastoral ecotone of northern China,under the influence of the East Asian monsoon since the early Holocene,the temperature and humidity began to increase,the lakes expanded gradually.And a relatively stable lake environment was formed in the Middle Holocene.However,the period and genetic factors governing the large lake area and the deep waterbody vary by region.In the desert-loess transition areas with low vegetation cover and rainfall supply,deeper lakes were formed during the East Asian summer monsoon enhancement between 8000 to 6000cal a BP.While in the areas with higher vegetation cover and less rainfall brought by the summer monsoon in the sandy land-grassland,during the period from 5000 to4000 cal a BP,due to the weakening of the East Asian monsoon and the strengthening of the westerly,the decrease of temperature led to the weakening of evaporation and the increase of effective humidity.After that,when the East Asian monsoon weakens,the lakes become increasingly shallow and eventually dry up completely.Since about2000 cal a BP,human activities have gradually increased in the northern agro-pastoral ecotone.