Study Of Forming-chronology And Its Climatic Background Of Alpine Meadow Soil In The Qinghai Lake Basin

As the third pole of the world,Qinghai-Tibetan Plateau?QTP?,is sensitivity to global warming and human activities,and its ecological environment is extremely fragile.Meadow soil,also known as grass felt soil,alpine meadow soil and so on,is the main soil type on the QTP,widely distributed on the mountains?3000-4800 m above sea level?,with an area of approximately 1.2×10⁸hm²,accounting for 35%of the total plateau area.It plays a core role in the QTP ecosystem.The notable characteristics of meadow soil included high soil organic matter content?SOMC?,low bulk density,thin thickness?generally less than 1 m?.Because of low temperatures,meadow soil is usually frozen more than six months of a year.Grassroots are becoming intertwined and finally produced a grass felty layer?Afe?on the top soil,which consists of mineral particles,humified organic matter,large amounts of dead-living roots as well as rhizomes.The Afe,humus layer?Ah?,parent material layer?CAhb?and gravel layer?C?are found at the alpine meadow soil profile.However,there is few study of the forming-chronology on the meadow soil,which further limits the understanding of pedogenesis process under the changeable climatic background.Therefore,five typical alpine meadow soil profiles?QHL southwestern:XPSD1,XPSD2;QHL southeastern:RYS1,RYS2,LGR?and three transitional types of soil profiles?QHL eastern,near Haiyan county:ZHK;QHL northern:HEGHD;QHL northwestern:LHLQ?in the QHL basin were selected in this study.Fifty OSL samples in total from these sections were collected to obtain the age of different soil horizons and establish the chronological framework of pedogenic process in the alpine meadow soils using the method of optically stimulated luminescence?OSL?dating.The grain size?GS?,magnetic susceptibility?MS?and SOMC of soils in different horizons were measured to analyze its'physical and chemical properties and to estimate the dust accumulation rate based on pedogenesis.Combined with the paleoenvironmental evolution records in QHL,the alpine meadow soil formation patterns and the climatic background of pedogenesis process were discussed.It has an important theoretical significance for understanding the different climatic backgrounds of alpine meadow soil formation process,also has practical significance for soil protection and ecological security on the QTP.The results show that:1.Systemic OSL dating and self-test verified that OSL dating method is suitable and reliable for accurately getting the ages of different soil layers in the alpine meadow soil.Each OSL sample has sufficient exposure before burial,strong OSL signals,appropriate preheating temperature conditions,small standard deviation of D_e value and recuperation,recycling ratio is stable from0.9 to 1.1.Besides,the ages of the quartz and k-feldspar are in a good agreement on RYS1 and RYS2 alpine meadow soil sections.The calibrated ¹⁴C ages are always younger than OSL ages on XPSD1 and XPSD2 alpine meadow soil sections.2.From the field observation records,OSL dating results,GS and its distribution curves,aeolian dust accumulation rate,the alpine meadow soil profiles in this study could be divided into two types.By the source of alpine meadow soil forming parent materials,one of them is the parent materials were rich in aeolian dust,the other one is comprised of alluvial sediments.The aeolian dust in the alpine meadow soil mainly has an origin of low altitude area or long-distance dust.3.The alpine meadow soil of aeolian dust parent material was formed and developed in the Late Holocene?⁴ ka?,and the aeolian dust rapidly accumulated in this period.The parent materials in the alpine meadow soil have an aeolian origin was mainly deposited in the last deglaciation and Early Holocene.Alluvial sediments parent material was formed during the middle Holocene?⁷-5 ka?in the alpine meadow soil.4.Combined with the paleoenvironmental evolution records of sediments and lake-core in the QHL basin,the LGR profile of high-density OSL sampling independently recorded the pedogenesis process of alpine meadow soil with aeolian dust parent material in this region.It found that during the last deglaciation and Early Holocene,aeolian activity was strongest,the soil development was stagnated.Middle Holocene's climate conditions were much warmer and wetter,so the accumulation rate of aeolian dust was lower,the paleosol was dominant.In the Late Holocene,climate was relatively dry and cold and the aeolian activity enhanced,the aeolian dust parent material which deposited during the last deglaciation and Early Holocene developed into the alpine meadow soil.Meanwhile,the Kobresia pygmea on the mountain can capture a large amount of aeolian dust.That is,alpine meadow soil is weakly developed while accumulating aeolian dust.The formation process is dominated by aeolian dust accretionary model.The aeolian dust accretion is a source of parent materials and nutrients for alpine meadow soils,which enriched SOMC with continuous input of aeolian dust.