Paleoenvironmental Changes And Paleoecological Response Recorded By Geolipids From The Late Neogene Lacustrine Sediments In The Zhaotong Basin,Yunnan Province

As the transitional period of Cenozoic global climate change,a series of climate-related geological events occurred during the period from the late Miocene to the Pliocene?late Neogene?,such as the strengthen of the Asian monsoon and the rapid expansion of Northern Hemisphere icesheets,the uplift of Tibetan Plateau and the closure of the Panama Seaway,the change of C₃/C₄ vegetation and the evolution of early hominoids.In the global changes,the Asian monsoon system is an important compent.As a part of the Asian monsoon system,the research on the evolution of the South Asian monsoon climate during the late Neogene is relatively less than that of the East Asian monsoon climate,especially in the terrestial environments.Meanwhile,the existing research conclusions on the evolution of South Asian monsoon are still controversial.In order to have a better understanding of the relationship between the evolution of South Asian monsoon and climate change,it is necessary to search for long time scale paleoclimate change in terrestrial environment.Yunnan Province,located at the southeastern margin of the Tibetan Plateau,develops a series of intermontane basins with continuous and widely distributed sediments,and the climate of this area is mainly controlled by the influence of the South Asian monsoon.However,due to the lack of precise chronological control,previous studies rarely involve the environmental and climatic change during the late Miocene to Plicene in this region.Furthermore,the southeastern margin of the Tibetan Plateau serves as a“refuge”for the late Miocene ape,which provides a chance for exploring the relationship between the ape and the environment.As a result,the paleoclimate changes in southwest China during the late Neogene are discussed to provide terrestrial record of climate change related to the South Asian monsoon.By analysing the lipids from the leaf wax and microbial activity,the ecological response to the climate change are reconstructed.In addition,the new hominoid fossil with the terminal Miocene age was recently discovered in the Shuitangba outcrop of the Zhaotong Basin,it is necessary to explore more detailed living environment for the latest Miocene Lufengpithecus lufengensis by using lipid proxies.With respect to the above issues,this study chooses two lacustrine sedimentary sequences in the Zhaotong Basin,which are named ZK1 borehole with 8.80-2.62 Ma sediments and Shuitangba outcrop with the late Miocene sediments,respectively.The Zhaotong Basin is located at the northeastern of Yunnan Province.Combined with depositional environment change and other bulk geochemical properties of organic matter,analyses of n-alkanes distributions and their compound-specific carbon isotope cmpositions,pentacyclic triterpenoids and glycerol dialkyl glycerol tetraethers?GDGTs?membrane lipids were conducted to decipher the paleoclimate,paleoenvironment and paleoecological implications of these lipids in these older lacustrine-swampy sediments.The main findings are as following:A series of lipids related to higher plants and microbial activities are found in the late Neogene lacustrine-swampy sediments in the Zhaotong Basin.Among the lipids of the plant leaf wax,des-A-triterpenoids,with oleanane,ursane and lupane carbon skeletons,are widely accepted as general biomarkers for angiosperms,the non-emergent?submerged/floating?macrophytes mainly produce the mid-chain length n-alkanes(C₂₃ and C₂₅).Based on these basic knowledge,a proxy ratio,des-A/C₂₃₊₂₅,has been established in this research to reflect the terrestial plants input to the lacustrine sediments relative to that from the non-emergent macrophytes.As one of the important lipids produced by the microorganisms,the hopanoids are dominated by the biological configuration???-stereoisomers?in the lacustrine-swampy sediments from the Zhaotong Basin.However,within the Quaternary peat deposit,the C₃₁hopane is present as the“thermally-mature”17?,21??H?stereoisomer.Even though lignite is tranformed by the peat,the hopanes show distinct distribution characteristics in thses two deposits,which may implicating that hopane isomerisation is controled by the environment change during the depositonal process.The long-term paleoclimate?hydrology?changes recorded by ZK1 borehole sediments in the Zhaotong Basin are reconstructed by using lipids.During the period of 8.8-6 Ma,the swampy facies is deposited when the climate is relatively dry,especially the driest period at about 7-6 Ma.During the period of 6-3 Ma,the climate changes to wet cndition and is in a relatively stable state.During the period of 3-2.6Ma,the climate experiences further wetting.The MBT'_5MEindex is used to reconstruct the long-term paleotemperature change trend,reflecting obvious decrease at about 4-3 Ma with frequent fluctuations.It's worth to note that the combination of IR_6MEand C₃₁??/???+???indicators consistently indicate the drier climate event occurs around 6.5 Ma,which is different from the drier climate event reflected by the proxies based on plant leaf wax lipids.In summary,the lipids characterize a more detailed paleoclimate change process in the Zhaotong Basin.The paleoecological response to climate change in different ecological components recorded by Shuitangba outcrop sediments in the Zhaotong Basin are reconstructed by using lipids.The discrepancy in the response of different vegetation types?aquatic plants and terrestrial plants?to the climate change in the ecosystem are revealed by the use of n-alkanes and des-A-triterpenoids.Terrestrial higher plants are the main contributors to the sediments of Zhaotong Basin.Except the in situ peat-forming plants,the input from the plants living on hillsides around the basin to the organic matter of the sediments increases when the climate gets wetter.The wetting climate also has an impact on the plants?such as the Betulaceae?living in the basin.Meanwhile,aquatic plants appear relatively prosperous phenomenon.The occurrence of des-A-arbor-9?11?-ene in lacustrine sediments proves that the contribution of aquatic plants increases when the sedimentary environment is dominated by lake.The change of paleo-vegetation is closely related to the climate which gradually gets wet.The hopanoids?including the 3-methylhopane?and their carbon isotope compositions,as well as the archaeol and iso GDGT-0 compound,combined with iso GDGTs/crenchaeol ratio indicate biogeochemical processes related to methane cycling in the late Neogene sediments in Zhaotong Basin.Combined with the climate change,a comprehensive analysis shows that the change of hydrological condition in the Zhaotong Basin is the main factor for microbial activities involved in the biogeochemical processes related to methane.Besides,temperature may also be one of the factors affecting the methanotrophy.According to the paleotemperature reconstruction of the Shuitangba outcrop,it was found that a mildly warm climate for the STB hominoid.Meanwhile,lithology and bulk geochemical indicators illustrate that the climate for the STB hominoid is relatively humid.The appropriate climate for the STB hominoid is consistent with the mildly warm and humid climate established by the clay mineralogy and carpological remains.By comparing the living environments for hominoid from other sites in Yunnan,it is found that the STB hominoid lives in a humid and densely vegetated forest environment.Meanwhile,there are shallow lake or marsh at the edge of the forest,but,the water level is in a relatively stable state.It is notable that the changes of the molecular and carbon isotope compositions of long-chain n-alkanes do not parallel with the abrupt increase of Poaceae pollen percentage during the STB hominoid living stage.The percentage of Poaceae pollen is sensitive to anthropogenic disturbance.Thus,it is interesting to infer whether the different source is the factor.Alternatively,the increase of Poaceae is caused by the disturbance from the STB hominoid and other vertebrate animals.Vertebrate fossils were abundant and shown high diversity in the hominoid-bearing peaty clay layer.