The Distributions Of Lipids And Its Response To Holocene Climate And Vegetation Changes In The Gushantun Peat Bog In Changbai Mountains

The Changbai Mountains in Northeast?NE?China lies in the northernmost part of the East Asian Summer monsoon?EASM?region and is influenced by the interplay of different atmospheric circulation,predominantly the Asian monsoon system and the northern-part of the Westerlies.The climate evolution in this region since the Holocene period has been reconstructed in lake sediments and peat deposits.However,many questions remain unclear,such as how did the combination of moisture and temperature evolved during the Holocene.On the one hand,modern process studies have shown that maybe n-Alkanes and their carbon isotopic compositions were affected by multiple external factors and their paleoclimatic significances are of multiple solutions.The application of n-Alkanes in the geological record needs further clarification.On the other hand,the applicability of Hopanoids compounds synthesized by bacteria still needs to be verified in paleoclimatic reconstruction.Therefore,in this study,a 735 cm peat column collected in Gushantun peat in NE China was analysed by using GC-MS and GC-IRMS to reveal the composition and distribution characteristics of n-Alkanes,hopanes and hopenes,analyzed its possible sources and influencing factors.We investigated such proxies response to Holocene climate and vegetation changes,as n-Alkane indexs,?¹³C_alk values,C₃₁hopane ratio and the relative abundance of diploptene.The main results are as follows:?1?In all peat samples,nC₁₅ⁿC₃₃ n-Alkanes were present,dominated by nC₂₇,nC₂₉and nC₃₁.Medium and long chains n-Alkanes showed a strong odd over even predominance.The?¹³C_alk values of the long chain n-Alkanes range from-29.7‰to-33.8‰revealed the development of C₃ plants.All these results indicated that n-Alkanes were mainly contributed by terrestrial and aquatic plants.?2?The P_aq and ACL values indicated a dry early Holocene and a wet middle-late Holocene climate.CPI was affected by both temperature and moisture changes,resulting in a small range of changes throughout the Holocene.However,preservation and vegetation input might have an impact on these proxies,indicating that the records of n-Alkanes on climate change are different,thus showed different chemical behaviors.When interpreting the paleoclimatic indicator significance of these indicators,it is better to combine with other indicators and consider it comprehensively according to the actual situation.?3?The?¹33 C_alk values and its difference value(??¹³C_31-27)may be controlled by the moisture and the change of carbon source.Therefore,their response to Holocene moisture and vegetation changes were not obvious.We need to be particularly careful when using it to interpret moisture changes in peatlands.?4?The changes of nC₂₃/(nC₂₃+nC₃₁),nC₃₁/(nC₃₁+nC₂₇)ratio showed that herbaceous plants were more developed during 11.5 to 8 ka BP approximately and the most recent 2 ka BP.Aquatic plants were more prosperous during 8 to 2 ka BP approximately,which was roughly similar to the pollen records.Therefore,it can be regarded as a good supplement for reconstructing past vegetation.The combination of different n-Alkanes ratio and pollen can effectively improve the resolution and accuracy of vegetation reconstruction.?5?Hopanes and hopenes including that C₂₉hopanes,C₃₀hopanes,C₃₁hopanes and diploptene,were detected in all samples in Gushantun peat.C₃₁??hopane and diploptene were the most abundant.They might be derived from different bacteria,C₃₁??hopane was mainly affected by moisture and pH value,while the relative abundance of diploptene might be mainly affected by temperature primarily.?6?The relative abundance of diploptene was higher and the ratio of hopane C₃₁??/??ratio was lower when the warm and dry early Holocene in northeast China respectively.The relative abundance of diploptene decreased significantly and the ratio of hopane C₃₁??/??ratio increased in the middle and late Holocene when the climate became cold and wet respectively.The overall changing trend of these two indexes can roughly respond to the change of temperature and moisture in this area.It is suggested that they can be regarded as two important proxies for paleoclimatic studies,which needs further intensified.?7?The pH was ranging from 3.38 to 7.03,and that was based on C₃₁??/???+???ratio,which was significantly lower than the measured values of modern surface waters?6.51±0.06?.This result indicated that it may not be ideal for estimating the specific pH value in peatlands,while it is basically consistent with the overall pH change trend of brGDGTs-CBT proxy reconstruction.It showed that this index has a certain potential in recovering the pH change trend of peatlands which should be further examined in the future.