Compound Specific Carbon And Hydrogen Iso-Topes Of Long Chain N-alkanes Extracted From Past15Ka Lacustrine Sediments Of Gonghai Lake, Shanxi Province, And Their Paleoclima-Tic Significance

Whether the East Asian Monsoon (EAM) was the strongest during the early or middle Holocene is a controversy in academia in China. Recently, the widely-concerned oxygen isotope records of speleothem indicate that they are the most negative during the early Holocene. Since such records are opined that implying the variation of the strength of EAM, especially in South China, it seems that the strong EAM and humid climate joined hand in hand over the early Holocene. However, their paleoclimatic significance is still debatable, which deserves more discussions from different fields.According to these scientific issues above, the compound specific carbon and hydrogen isotopes of long chain n-alkanes extracted from lacustrine sediments of Gonghai Lake, Shanxi Province over the past15ka have been investigated. We analyzed the data of carbon isotope of long-chain n-alkanes originating from terrestrial higher plants in the lake basin and compared it with the pollen results from existing parallel core, then carried on paleoclimatic reconstruction based on the successional process of paleo vegetation. After analyzing the data of hydrogen isotope of long-chain n-alkanes and comparing it with speleothem oxygen isotope, which are regarded as documenting the hydrogen and oxygen isotope of atmospheric precipitation respectively, we further discussed the relationship between precipitation isotopes and paleoclimatic history revealed by the aforementioned carbon isotope of n-alkanes.The acquired carbon isotope of long-chain n-alkanes presents two periods of negative anomaly during Younger Dryas (YD) and between8ka BP and5ka BP. In the light of the distribution intervals of carbon isotope of modern C3and C4plants, together with the research about their corresponding contemporary process, we judge that the terrestrial higher plants could be viewed as pure C3plants over these two stages in Gonghai Lake, whilst in others C3plants played a dominant role, mixing with C4plants. This is supported by pollen identification from the parallel core in Gonghai Lake. The percentage of tree and herb pollen shows that in YD, tree pollen was basically lower than25%whereas herb pollen was larger than75%, which illustrates that the vegetation type belonged to C? steppe within the lake basin at that time. From8ka BP to5ka BP, the proportion of tree and herb pollen accounted for more than75%and less than25%, separately, which undoubtedly tells us that C3forest was in growth in Gonghai area at this stage, since all the tree are C3plants. The reasons may be interpreted as their adapted ecosystem, especially for C4plants, which must grow under high enough temperature. Thus we estimated that the temperature in YD was too low for C4plants; climate in the latter age was warm and wet but C4plants were limited by over-humid conditions. C3-dominated plants and C3/C4-mixed vegetation appeared at other times, the relation of which and climate was rather complicated, which might be related to the negative effects of temperature, precipitation on the growth of C4plants. During the early Holocene, though the rising temperature was beneficial to C4plants, the increasing rainfall would inhibit them. In late Holocene, the reducing precipitation brought well but the decreasing warmth was unfavorable to their growth. Nevertheless, seeing from the results, the moistest period throughout out the Holocene should be the early-mid Holocene, when EAM was also the strongest.For a long-term trend, the hydrogen isotope in Gonghai Lake is consistent with the oxygen isotope of stalagmite in South China over the past15ka, i.e. they are all positively (negatively)-biased in YD (the early Holocene) and gradually positively-biased in the mid-late Holocene. There are differences in details between these two isotopes, probably because except that the hydrogen isotope from terrestrial higher plants mainly records the hydrogen isotope of atmospheric precipitation, they are affected by the variation of secondary photosynthesis (C3or C4) and vegetation ecotype (herb, shrub or tree). Anyway, the abovementioned consistence indeed takes the signal from hydrogen/oxygen isotope of atmospheric precipitation. Yet they are significantly different from climate process, since rainfall isotope was the most negative in the early Holocene but the climate in Gonghai Lake was not the most humid.Precipitation isotope is controlled by temperature effect in high latitude (e.g. the polar regions), hence hydrogen/oxygen isotope from ice cores is widely used to reconstruct the variation of temperature. In tropical areas, it is subject to rainfall effect, so the related records (hydrogen isotope from terrestrial higher plants) are employed to reconstruct the variation of precipitation. However, in mid-latitude, the relation between precipitation isotope and climate factors (majorly temperature and rainfall) is complex and it is hard for direct paleoclimate reconstruction. That is why in the long-term, the hydrogen isotope in Gonghai Lake accords with the oxygen isotope of stalagmite in South China while disaccords with the wettest climate in the early-mid Holocene.It is noticeable that the most humid early-mid Holocene reflected by carbon isotope from terrestrial higher plants and pollen among the lacustrine sediments of Gonghai Lake is in accordance with the newly-reported consequence from the Loess Plateau, which considered that it was wetter over8ka BP-3ka BP But on the whole they negatively correlate with the environmental magnetic results of stalagmite from Heshang Cave and the organic geochemistry study of peatland samples from Dajiu Lake in the middle reaches of the Yangzte River, where climate in the mid-Holocene was arid. It is possible that the summer monsoon might be the strongest then and longlastingly stayed in North China, which brought more moisture there, whilst it is relatively dry in South China. If so, the increased rainfall in the North can be utilized to infer the variation of monsoon strength. Due to the various paleoclimate processes in different regions under the same circulation system, up til now, it is still questionable to recover the paleoclimate variability or the monsoon strength in this area by precipitation isotope.Looking ahead, we suggest that on the one hand, it is imperative to discover more proxies with definite paleoclimatic significance in speleothem research; on the other hand, it is necessary to farther understand the rules and processes of paleoclimatic variation by relying on the records from South China.