Evolution And Driving Mechanisms Of Wet/dry Palaeoclimate And Palaeo-vegetation Since Late Miocene In Eastern China

The Asian monsoon is an important part of the global climate system.Deciphering its temporal and spatial evolution is of great significance for people to fully understand global climate change in different time scales.In particular,the formation and evolution of the East Asian monsoon since the late Cenozoic have an important impact on both regional and global climatic changes.In the past decades,a large number of studies were focused on the evolution of the East Asian Monsoon on the orbital scale,with fewer studies on the tectonic scale being that most of them were mainly based on the records from the Chinese Loess Plateau,Tibetan Plateau and South China Sea.Due to the lack of long-term sedimentary profile in eastern China,the paleoclimate evolution since Neogene has always been a weak link.Also,although the evolution of the East Asian Monsoon on the tectonic time scale since the late Cenozoic has been generally suggested to be related to the uplift of the Tibetan Plateau,there are still many controversies about the potential mechanisms of the monsoon evolution.In particular,it is not clear whether the spatial pattern of the dry/wet palaeoclimate associated with the monsoon rainfall on tectonic scales in eastern China is as different from that of the modern dry/wet climatic changes?e.g.,the rainfall dipole or tripole pattern over eastern China?.If so,what about the driving mechanism?These questions need further discussions.Similarly,with the climate changes since the Neogene,the terrestrial ecosystem also underwent a major transformation.Especially,the emergence of C₄ grassland in the late Cenozoic led to the large-scale replacement of C₃ plants by C₄ plants,which greatly expanded the grassland habitat on a global scale.Many researches have been focused on the first global expansion of C₄ plants in the late Miocene,and the basic framework for this expansion from low to mid-high latitudes has been established.However,it is unclear whether there is a second expansion of C₄ plants in East Asia since the late Miocene.If so,what is the specific mechanism for the second C₄ expnasion?How is it different from the first extension event in the late Miocene?These questions also need further discussions.In recent years,based on the development of regional geological survey work,the continuous long-term sedimentary profile in eastern China?ca.8 Ma to present?could be acquired,which allows us to better understand the wet/dry paleoclimatic changes of eastern China since the late Miocene,and fully reveal the spatial variations of wet/dry paleoclimate in the East Asian Monsoon region together with their potential driving mechanism.Besides,the continuous breakthrough of biomarker indices in the past decade has created the conditions of reconstructing the wet/dry paleoclimatic changes.In particular,Glycerol Dialkyl Glycerol Tetraethers?GDGTs?,the membrane lipids derived from archaea and bacteria,have been widely used in the studies of palaeo-environmental and palaeo-climatic reconstructions due to their wide distributions in various environments and sensitivity to environmental changes.Therefore,it is of great significance to reconstruct the East Asian Monsoon evolution by using GDGTs-derived indicies.Long term borehole deposits in eastern China are mostly fluvio-lacustrine sedimentary.Among the GDGTs-derived indices,the indices that can be used to reconstruct long term wet/dry paleoclimate should be verified in modern river-lacustrine sedimentary environment.Thus,in this study,we started with the GDGTs distributions as well as the relationship between GDGTs-derived indices and environmental factors in modern river-lacustrine environment to choose reliable indicators for wet/dry palaeoclimatic changes.We selected the Qinghai Lake as the modern research area because this region is very sensitive to wet/dry climate changes.After choosing the relaibale GDGTs indices for lake and river environments,we apply the indices to two long term drillcores in the North China Plain and the Northern Jiangsu Plain spanning an interval from ca.8 Ma to present,trying to reconstruct the wet/dry paleoclimatic changes.Meanwhile,we present organic carbon isotopic records from these two drillcores to understand the vegetation evolution in eastern China since the late Miocene.The main innovative understandings obtained in this paper are as follows?some of the research results have been published in international journals?:1.Based on the investigations of GDGTs-derived indices in modern river-lacustrine environments,the reliable indices for wet/dry paleoclimate are proposed.By analyzing the isoGDGTs and br GDGTs distributions in lake,river and surrounding soils in the Qinghai Lake area,we discuss the relationship between GDGTs-derived indices and environmental factors in different environments.The variation of isoGDGTs in lake and river environments are more regular than that of br GDGTs,and the isoGDGTs-derived index GDGT-0/Cren is significantly correlated to the lake water depth,which can be used as a reliable indicator for wet/dry paleoclimate reconstruction.However,variations of br GDGTs-derived indices are more complicated and are controlled by multiple factors in lake and river environments.For example,the CBT index,which can reflect p H,has been found to be related to salinity in river sediments,and the MBT'index,which can be used to reconstruct temperature,is found to be related to water depth in the lake.The complicated relationship between br GDGTs-derived indices and environmental factors would limit the applications of these indices to the long term sedimentary drillcore,while the GDGT-0/Cren index has advantages in paleoclimate reconstruction because of its clear and simple controlled factors.Meanwhile,the GDGT data from global lake sediments?including some lake sediments in eastern China?and peatlands are analyzed to further support that GDGT-0/Cren index can be used as a useful dry/wet palaeoclimatic indicator for terrestrial water environmental changes.2.The tectonic-scared wet/dry paleoclimatic changes in eastern China show a‘tripole'pattern since the late Miocenen,and the changes of the equatorial Pacific SSTs gradients is suggested to be the driving mechanism.By analyzing the GDGTs compounds in the fluvial-lacustrine sedimentary profiles from drillcore G3 in Tianjin,North China Plain??8 Ma to present?and drillcore ZKA4in Yancheng,Northern Jiangsu Plain?ca.7.6 Ma to present?,we apply the new proxy GDGT-0/Cren and previously established index R_i/b to reconstruct the wet/dry paleoclimatic changes since the late Miocene in the eastern China.The biomarker indices from the North China Plain and the Northern Jiangsu Plain show that the climate was dry with less rainfall during the late Miocene to the early Pliocene,while the climate has become wet since the early Pliocene?ca.4.2-4.5 Ma?with an abrupt increase in East Asian Summer Monsoon?EASM?rainfall.This switch of wet/dry paleoclimate occurring in the early Pliocene is consistent with the records from the Chinese Loess Plateau and the South China Sea,but contrary to the records from the middle and lower reaches of the Yangtze River.These data demonstrate a meridional“tripole-modern-like”shift in monsoon rainfall across the region,where the observed dry conditions in northern China and central-southern South China Sea?-?,and wet conditions in central eastern China and northern South China Sea?+?during the late Miocene,completely reverses at around 4.2-4.5 Ma,that is,from a‘-,+,-'to a‘+,-,+'pattern.According to the modern interannual to interdecadal variability of summer monsoon rainfall anomalies over East Asia,together with the global paleoclimate records during the early Pliocene,we suggest that the switch of the‘tripole'pattern of rainfall in eastern China is mainly caused by the increasing zonal and meridional SSTs gradients in the Pacific since the early Pliocene,leading a strengthened convective activities over the Philippines.In addition,the enhanced Hadley and Walker circulation since the early Pliocene also contribute to the increase in poleward heat transport from equatorial western Pacific,which would have likely acted as a positive feedback on the East Asian Summer Monsoon.3.According to the organic carbon isotopic analysis,a second global C₄ expansion since the late Cenozoic occurred in the early Pliocene was found.The decreasing of atmospheric p CO₂ is likely to be the main driving factor for the early Pliocene global C₄expansion.By analyzing the bulk organic carbon isotope of the sediments from the G3 and ZKA4 drillcores,we reconstruct the evolution of C₃/C₄ vegetation in the eastern China since the late Miocene.The potential mechanism for this global C₄ expansion is discussed by comparing our data with carbon isotope records from other regions in East Asia and other continents.The organic carbon isotopes of the G3 show a significant positive shift at ca.4.1Ma,and those of ZKA4 similarly show a positive shift at?4.5 Ma.Consistent data from two drillcores indicate a significant C₄ expansion in eastern China during the early Pliocene.Our data mirrors carbon isotope shifts in loess-red clay deposits of the Chinese Loess Plateau,confirming the regional nature of this event.Furthermore,coeval carbon isotope records from Africa,North America,and South America provide evidence that the early Pliocene C₄-grassland expansion was global in scale and temporally distinct from the Late Miocene event.Whereas the late Miocene event may have been related to aridification and altered fire regimes,this mechanism cannot account for the Early Pliocene expansion owing to contemporaneous increases in humidity at many Northern Hemisphere sites.We hypothesize that the C₄-grassland expansion in the Early Pliocene was triggered by falling atmospheric CO₂ levels,as evidenced by multiple p CO₂ proxy records.This inference is supported by quantum photosynthetic efficiency modeling,which shows that,as p CO₂ and temperatures decreased during the early Pliocene,climate conditions in many regions crossed a threshold favoring C₄ floras,especially in mid-latitude regions such as the North China Plain and Norther Jiangsu Plain.