The Paleoclimate And Paleoenvironment Reconstruction Based On Glycerol Dialkyl Glycerol Tetraethers During Late Cretaceous In The Songliao Basin And Its Vicinity

The Cretaceous was an interval of greenhouse climate,with high sea levels and intermittent oceanic anoxia.Most of our knowledge on the greenhouse climate and anoxic events in Earth's history comes from the marine records.In contrast,relatively little data has been collected from the Cretaceous continental depositions.The Songliao Basin?SLB?in northeastern China is one of the largest Cretaceous continental sedimentary basins in the world.The sedimentary deposit within the SLB includes the Upper Jurassic,the Lower Cretaceous and the Upper Cretaceous,offers a unique opportunity to understand late Cretaceous terrestrial paleoenvironment.In recent years,lots of achievements about late Cretaceous paleoenvironment,paleontology and terrestrial chronostratigraphy in the Songliao Basin were obtained,with the implementation of the“SK-1 Continental Scientific Drilling Project”.However,the research of paleotemperature in the SLB is scarce duing to the lack of effective proxy.In this dissertation,166 samples were collected from four typical outcrops which seated in southeast of SLB including Yaojia Station Section,Houjin'gou Section,Yuewangcheng Section and Lijiatuozi section,as well as a core?LD6–7?located in northwest of SLB.We used glycerol dialkyl glycerol tetraethers?GDGTs?proxy and other biomarker data(e.g.pristine/phytane ratio,gammerance index,aryl isoprenoids relative to phenanthrene ratio,methytrimethyltridecyl chromans index,as well as,C₃₀-cholestanes)to reconstruct paleoclimate and paleoenvironmental conditions in a lacustrine environment during the Late Santonian.Besides,we investigated the distribution of GDGTs in Jiayin Basin and reconstructed the paleotemperature across the Cretaceous–Paleogene boundary?K–Pg?with GDGTs proxy.This dissertation first carried out a comparative study of two GDGTs analysis methods.Liquid chromatography mass spectrometry?HPLC–MS?and fourier transform ion cyclotron resonance mass spectrometry?FTICR–MS?were used to detect one modern marine sediment sample in South China Sea and one source rock sample in the SLB,respectively.The results showed that six kinds of isopreniod GDGTs?isoGDGTs?are detected by HPLC–MS,including GDGT-0,GDGT-1,GDGT-2,GDGT-3,Crenarchaeol?Cren?and isomer Crenarchaeol?Cren'?in two samples.However,GDGTs were can't detected by FTICR–MS in mudstone sample in the SLB.The TEX₈₆^L,GDGT-0/Cren and MI values obtained by HPLC–MS and FTICR–MS show significant difference indicating that the response of each compound of GDGTs on the mass spectra between HPLC–MS and FTICR–MS are quite different.Therefore,the HPLC–MS method is considered more suitable for the analysis of GDGTs in SLB.The OM of Lijiatuozi section,Yaojia station section,Yuewangcheng section,Houjin'gou section,and the LD6–7 well are in the low-mature and immature stages.The OM maturity of Houjingou section and of LD6–7 well are relatively lower than the other sections.Samples contains GDGTs are mainly concentrated in the Yaojia station section and Houjingou section covers the Member 1 of Nenjiang Formation?K₂n?and LD6–7 core deposit the Member 2 of K₂n.The results show that the GDGTs are under detection limit when T_max>436?.The TEX₈₆ values are potential for the reconstruction of paleotemperature in the SLB.The concentriation of iso GDGTs is 0.02 to 1.32 ng/g in K₂n¹.The GDGT-0/Cren ratio range from 0.15 to 0.98.The lake surface temperatures?LST?reconstructed by TEX₈₆6 paleothermometer is 14.2 to 26.0? with an average of 18.2? in this interval.The concentriation of GDGTs vary from 0 to 6.39 ng/g in K₂n²,characteristiced by high proportion of isoGDGTs.The ratio of GDGT-0/Cren range from 0.38 to 1.03.The LST was 15.1 to 22.9? with an average value of 19.1? in this interval.All these temperature data show that the SLB was a subtropical climatic during the Late Santonian period,which is consistent with the semi-humid northern subtropical climate indicated by pollen records.Six kinds of isoGDGTs and 9 kinds of brGDGTs are detected in XHY-2008borehole sample across the K–Pg boundary in Jiaying Basin.The concentration of GDGTs vary from 0.1 to 78.4 ng/g.The concentration of brGDGTs is much higher than that of isoGDGTs with a mean BIT value is 0.94.The MAT_mr calibration show that the MAT range from 4.4 to 23.4? during the late Maastrichtian to early Danian.The MAT varied from 10.0 to 17.3? in uppermost Cretaceous and then abruptly warmed 6.7? at the K–Pg boundary.After this interval,the MAT abruptly cooled by 5.5? and then progressively returned to 14.5?.The K–Pg boundary warm interval may relate to the frequent volcanic activity during the late Cretaceous.Finally,we give a comprehensive analysis of the paleoenvironment and paleocliamte of SLB.During the lower part of the K₂n¹ and K₂n²,relative high concentration of 24-n-propyl and 24-iso-propyl cholestanes indicate that seawater incursion occoured.Along with seawater enters into Songliao paleo-lake,the abundant nutrients enter into the lake and promote the boom of primary productivity,leading to anoxia environments in the bottom water.Meanwhile,the lake water salinity increased and a brackrich environment occurred in bottom water.All these conditions benefit for the formation of OM-rich deposits.The evolution of the SLB in late Santonian is affected by regional tectonic activity and global climate change.