Evolution Of Hydrothermal Patterns In Eastern China Since Late Cenozoic:Evidence From Geological Lipids

The global climate has changed significantly since the late Cenozoic,and the climate of China has also changed from a tropical planetary climate system to a monsoon-arid system.Asian monsoon is an important part of the global climate system,and its evolution of hydrothermal patterns in eastern China since the late Cenozoic has great significance for people to fully understand the global climate change.However,the current research on the East Asian monsoon is mostly focused on the orbital time scale,and more on the China Loess Plateau,the Tibetan Plateau and the South China Sea,the research on the tectonic time scale in East China is less.Moreover,due to the lack of well-preserved and continuous sediment carrier,the records of the paleoclimate of the terrestrial environment in the East China are missing.Modern seasonal rainfall variability across East Asia is tightly coupled with regional temperatures,with summer heating driving the transportation of monsoon rainfall across the land.However,uncertainty remains as to the connection between temperature and hydroclimate on geological time scales,primarily due to the difficulty in separating the two different climate factors in sedimentary records.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 recent years,with the continuous breakthrough of biomarker indices in the past decade,more and more indicators are applied to reconstruct the paleoclimatic and paleoenvironmental in the geological history period.Especially the Glycerol Dialkyl Glycerol Tetraethers（GDGTs）,the membrane lipids derived from archaea and bacteria,have been widely used in the studies of paleoclimatic and paleoenvironmental reconstructions due to their wide distributions in various environments and sensitivity to environmental changes.In addition,based on the development of regional geological survey work,the continuous long-term sedimentary profile in eastern China,including the XA3 drill core in the North China Plain,ca.5.6 Myr,and the GYN1 drill core,ca.3.0Myr and ZKW4 drill core,ca.2.6 Myr in the middle and lower reaches of the Yangtze River in the mid-East China since the Quaternary,could be acquired.These allows us to better understand the paleoclimatic changes of the Eastern China since the late Cenozoic.The main conclusions of the paper is summarized as follows:1.According to the monitoring experiment of GDGTs and seasonal environmental factors in the modern lake system,this study proposed indicators for reconstructing the sedimentary environment in geological history.we collected samples from the top 1-2 cm of sediments and water column suspended particulate matter（SPM）at a monthly interval from June 2015 to May 2017 in Lake Donghu,a closed and shallow lake in central China.An updated liquid chromatography method was used to separate the 5-and 6-methyl br GDGTs.We analyzed the correlation between the fluxes and distributions of archaea iso GDGTs and bacteria br GDGTs and seasonal environmental factors,and discussed the applicability of climatic indicators based on GDGTs in the reconstruction of paleoclimate and paleoenvironment.A remarkable difference in the distribution and GDGT-based proxies between the lake sediments,SPM,and catchment soils suggested that microorganisms inhabiting the surface sediments contributed a significant proportion of iso GDGTs and br GDGTs to the GDGT pool in the lake sediments.And according to the differences of GDGTs distribution in lake sediments,SPM,and catchment soils,suggesting the water system is vulnerable to the impact of catchment soils.We found a clear seasonality in the concentration and distribution of fossil iso GDGTs and br GDGTs in the lake surface sediments,in contrast to the absence of seasonality in br GDGTs in mid-latitude soils reported by previous studies.The GDGT-0/Cren ratio for the lake sediments exhibited a negative correlation with the dissolved oxygen in the water column and for the SPM also exhibited negative correlation with precipitation（TPM）.This found indicating that the GDGT-0/Cren can be a useful index for reconstructing paleohydrological conditions in geological history.In addition,seasonal temperature variation clearly impacted on the methylation degree of C6-methylated br GDGTs(MBT′_6ME)but no influence on C5-methylated br GDGTs(MBT′_5ME),as opposed to the dependence of MBT’_5ME on temperature in global soils,peatlands and African lakes.Furthermore,the cyclisation degree of C5-methylated br GDGTs(CBT_5ME)was strongly related to water p H.The intact polar lipid/core lipid ratios for both iso GDGTs and br GDGTs in the lake surface sediments were significantly higher than those in SPM and catchment soils and were higher in the warm seasons than in winter.This collectively suggested that the turnover of br GDGTs in the lake sediments was faster than that in soils.We suggest that the seasonal variation of fossil GDGTs in lake sediments should be considered when developing a calibration using fossil GDGTs in shallow-water lake surface sediments.2.According to the GDGTs proxies in the drill corein North China,we reconstructed the first terrestrial paleotemperature variation in North China since5.6 Ma.By comparing the paleoclimate with the corresponding dry/wet paleoclimate records,this study found that the terrestrial temperature and hydroclimate is decoupled and the driving mechanisms are different.we deconvolved air temperature and hydrology in a record extending back to 5.6million years（Ma）on the basis of microbial lipid biomarkers preserved in a terrestrial sediment sequence from North China.The molecular records clearly show the decoupling of terrestrial temperature and hydroclimate changes since the Late Pliocene.The bacterial GDGT（glycerol dialkyl glycerol tetraethers）parameters MBT’5ME show that the temperature was relatively stable during 3.0-5.6 Ma period,and began to drop significantly at～3 Ma in lock-step with the onset of Northern Hemisphere glaciation（NHG）.Moreover,the variation of terrestrial paleotemperature is consistent with that of marine paleotemperature,indicating that the land and marine are co-evolution.At the same time,the marine-land temperature gradient may also have an important impact on the precipitation of China.As to hydroclimate change,two microbial GDGT indices(GDGT-0/Cren and R_i/b)show a great shift in hydroclimate at～4.2 Ma,consistent with the strengthening of Pacific zonal and meridional sea surface temperature（SST）gradient and increased marine-land temperature gradients.Thus,it was likely changes in the temperature gradient,rather than the global cooling,that was the primary driver of the hydrological changes in North China since the late Miocene.3.According to the dry-wet proxies of GDGTs in the drill core in the middle and lower reaches of Yangtze River,three dry intervals were clearly identified by the lipid parameters during 2.5-2.7 Ma,1.7-1.9 Ma,and 0.9-1.1 Ma,which correspond to the three great lake periods reported before in the Africa,indicating that there is a close relationship between the East Asian hydroclimate and the African monsoon in the Quaternary.The East Asian Summer Monsoon（EASM）and the African Monsoon are the two most important monsoon systems in the world,but their interaction remains largely unknown in Earth history,in particular on the tectonic time scale.Here we present two terrestrial microbial lipid records indicative of the hydroclimate evolution in Quaternary in the middle and lower Yangtze River of the EASM region.Three extreme dry intervals were clearly identified by the elevated lipid parameters to occur during 2.5-2.7 Ma,1.7-1.9 Ma,and 0.9-1.1 Ma.These episodes exactly correspond to the three African great lake periods reported before,and are featured by the global climate changes（i.e.,ice expansion,increased the Pole-Equator thermal gradient,and compressed ITCZ）.This coincidence shows the strong connection of the two monsoon systems on the tectonic time scale.In addition,the intervals of 2.5 to 2.7 Ma,1.7-1.9 Ma and 0.9-1.1 Ma corresponds to,respectively,the intensification of the Northern Hemisphere Glaciation,an important intensification and shift in the east-west zonal atmospheric circulation shown by the Walker circulation,and the initiation of the Mid-Pleistocene Revolution featured by the shift of glacial/interglacial cycles from 41-kyr to～100-kyr.Furthermore,the three periods caused the great environmental changes and exerted an important impact on early human evolution in the world.Compiled data suggest the momentous strengthening of the Africa monsoon,resulting from compressed ITCZ due to ice expansion and increased the Pole-Equator thermal gradient,might have caused the enhancement and a westward shift of the Walker circulation,and a La Ni?a-like state which in turn results in a reduction in monsoon rainfall in mid-east China.Our data suggest that potential changes in the Africa monsoon due to anthropogenic warming may influence monsoonal precipitation in EASM regions in the future.