Lipid Biomarkers And The Sedimentation Of Peat And Lake Sediments From Langmusi, Southern Gansu

Present use of biomarkers to reconstruct paleoclimate and ancient human activities has become an important part of molecular stratigraphy.Biomarkers have the characteristics of stable compounds,long-time preservation and wide distribution, providing great information on climatic and environmental changes.They contain framework structures of parent materials,and they can record evolution of palaeoclimate,palaeovegetation and palaeoenvironment.Many types of biomarkers have been reported,such as alkanes,aromatics,alkanoic acids,alkanols,alkenones and esters.Most researches mainly focused on the category,concentration,relative abundance of biomarker compounds,and compositions of stable isotopes such as monomer carbon,hydrogen,oxygen,and nitrogen.The research areas have reached aerosols,marine sediments,lacustrine and peat sediments,loess and so on.Because of its special location and sensitive response to the global climate transformation,Tibetan Plateau has been regarded by scholars of the geoscience domain.The research area is located on the edge of the northeast Tibetan Plateau.The sampling site is in Langmusi County,with an altitude of 3559m(34°09′N,102°34′E).This sedimentary sequence was selectively investigated by lipid molecular stratigraphy by gas chromatography-mass spectrometer(GC-MS).A variety of molecular fossils are found in this profile,including n-alkanes,n-alkan-2-ones,pristane,phytane and so on.N-alkane and n-alkan-2-ones are concerned in our research.The n-alkanes are ranged from C₁₆～C₃₃.The n-alkan-2-ones are range from C₁₉to C₃₃,having a maximum at C₂₃or C₂₉.Higher plants are the predominant source of the n-alkane compositions.Combining the distribution of lipids and types of vegetation in and near this area,we can conclude that the source is mainly herbages.N-alkan-2-ones are from epicuticular waxes of higher plants.Based on the establishment of a reliable AMS dating,we have been able to establish climate change series of the profile using the distribution patterns of the TOC and the indexes concluding from lipids during the Holocene.The profile was formed from 11.3 to 4 cal ka BP.From the variation of those indexes,we can conclude as follow.From 11.3 to 7.3 cal ka BP,the Langmusi area was cold and arid.From 9.6 to 8.6 cal ka BP,there has been a fluctuation.Since 7.3 cal ka BP,this area became warmer,and the climate came into the optimum.Though this period,peat had been deposited,and we can devide it into three phases:from 7.3 to 6.5 cal ka BP,the climate began to become warmer;from 6.5 to 5.5 cal ka BP,the climate of this atea was warmest and most humid;since 5.5 cal ka BP,the climate became colder again,and peat cease to develop at about 4 cal ka BE During the Holocene,there were several cold events in Langmusi area.They happened in about 8.8 cal ka BP and 5 cal ka BP.It's very special at about 5 cal ka BP.The ACL and Pαq values of n-alkane indicate that there may be many submerged and floating macrophytes in this area,but other indexes such as TOC and L/H of alkane show the cold and dry climate.We conclude that it may be attribute to a particular species of plant in this area.