The Distribution Of Sedimentary Lipids In Wetland And Their Paleoenvironment Implications

Wetland is an important,unique and multifunctional ecosystem on the earth.It plays an extremely important role in the global ecological balance and also plays an important role in the global terrestrial carbon cycle.The carbon cycling process and different evolution stages of wetland are mainly controlled by the changes of environmental conditions?such as pH,water level?and microbial activities.Therefore,the biogeochemical process of wetland in which microorganisms participate can respond and record the climate change and environmental evolution process of wetland sensitively.In the East Asian monsoon region,wetlands are the source of water conservation for many important rivers,which plays an important role in ensuring the safety of water quality and biodiversity.At the same time,there are obvious seasonal to interannual fluctuations of precipitation in the East Asian monsoon region,and frequent occurrence of drought and flood.These changeable climate disasters have brought serious consequences to human beings.In the context of global warming and acidification of lakes and marshes,it is a scientific issue to study the response of wetland microbial processes in the East Asian monsoon region to the unique climatic and environmental conditions.Lipids,especially those hopanoid derived from bacteria,are usually stored in wetland sediments.Although hopanoid and its compound-specific carbon isotope composition have been applied to some extent in the study of paleoenvironment and paleoecology,it is still misty about the diagenetic transformation process and the main controlling factors of the early diagenetic transformation of hopanoid.The potential of paleo-ecological application of hopanoid is still lack of systematic understanding.The relationship between the distribution,mutual transformation and environmental factors of hopanoid is in urgent need of breakthrough,which can not only better understand the response mechanism of microbes to environmental change at the molecular level.It may also provide new theoretical support for paleoenvironment and paleoecology research based on hopanoid in geological history.In view of the above problems,this paper discusses the distribution characteristics and influencing factors of hopanoid in wetland from several typical wetlands in China.On the basis of this,the carbon isotopes of hopanoid in the eastern monsoon region of China and the difference between the carbon isotopes of hopane and n-alkanes were studied in the Dajiuhu wetland.The responses of the carbon isotopes to paleohydrological changes since 18ka were discussed.Then,the paleohydrological conditions in the middle and lower reaches of the Yangtze River are restored by using the depositional lipids and the grain size in Longgan wetland.The environmental factors of methane oxidation reflected by the lipids in the Yangtze lake wetland were discussed.The main conclusions and findings are as follows:1)There are a wide variety of hopanoids in the Hani,Zoigê,shiwangutian and Dajiuhu wetlands,with C₃₁??hopane,C₃₂??hopanol,C₃₂??hopanoic acid and Bacteriohopanetetrol?BHT?at the highest levels.In addition,analysis of different environmental factors revealed that C₃₁??hopane is rich in pH less than 6.This study supports the previous work that pH plays an important role in the early diagenetic transformation of BHPs into geohopanoids.2)Based on the distribution and relationship between BHPs and geohopanoid in four peat wetlands,we propose that in wetlands,C₃₂??hopanoic acid is produced mainly by the oxidation of BHT,then the C₃₁??hopane and C₃₁??hopane form by the decarboxylation of C₃₂??hopanoic acid.pH value promotes the isomerization of C₃₁??hopane.A linear relationship between C₃₁??hopane and C₃₁??hopane with pH is established as follow,which provides a new idea for the reconstruction of pH deposits in other wetlands.pH=3.4169*C₃₁??/???+???+4.0045?n=63,r²=0.66,p<0.01,RMSE=0.81?3)A comparison of the hydrogen isotope of n-alkanes since 18 ka with other carriers identified several drought events in the mid-Holocene in Dajiuhu wetland.During these drought events,the carbon isotopes of the hopanoids especially C₂₉??hopane were negative,drought enhanced methane oxidation in wetlands,and a large amount of CH₄was used to synthesize BHPs.4)In the ZK-5 core of Dajiuhu wetland,the difference of carbon isotope between n-alkanes and hopane can respond to the change of environmental conditions.The difference of carbon isotope between n-alkanes and hopane is smaller in the middle Holocene drought event.During the drought period,aquatic plants contributed less n-alkanes,which resulted in positive carbon isotope values of n-alkanes,while wetland microorganism hopane was mainly made by aerobic heterotrophic bacteria.The drought increased the activity of these microorganism and resulted in positive carbon isotope of hopane,which indicated that the reaction of?¹³C reflected more detailed information about wetland ecosystem changes to environment.5)In Longganhu wetland,we analyse the diplotene and squalene which are associated with the methane oxidation activities.The concentration and the carbon isotope values of these compounds have responded differently to the change of the lake water level.Based on comprehensive analysis of Longganhu wetland and Dajiuhu wetland,the dry and wet condition is an important environmental factor to influence the methane oxidation activities in wetlands.