Typical Mineralogical Characteristics And Their Responses To The Hyperthermals In The Early Eocene Sediments From The Qaidam Basin,China

The Early Eocene Climatic Optimum（EECO）which lasted a 2 Ma time interval began at～53 Ma,when the Earth attained their warmest Cenozoic surface temperature and elevated atmospheric CO₂ levels.Superimposed on the long-term global warming during the early Eocene were several transient（<200 kyr）global warming events,named hyperthermals.These hyperthermals provide geological analogues for human beings to understand the cause and consequences of global warming caused by the increase of CO₂ and other greenhouse gases（CH₄,N₂O,etc.）.Hyperthermals have been paid increasing attention on marine sedimentary records and topics such as the sources of carbon,concentration of CO₂ in the atmosphere,time scale of the hyperthermals,and temperatureand precipitation changes in a global scale were documented.However,limited studies have documented the continental paleoclimate system response to the hyperthermals.A comprehensive understanding of EECO,spatially and temporally,is still limited,despite increasingly emerged studies have indicated that varied paleoclimate and paleoenvironment occurred during the EECO in different regions.Paleosols can provide excellent archive to reconstruct characteristics of paleoclimate because they developed with the evolution of climate and environment changes in the continental interior directly.The Lulehe Formation,with a total thickness of～460 m,was the initial Cenozoic strata of the Dahonggou Section,Qaidam Basin.The fluvial clastic sediments at the middle of the Lulehe Formation strata（50～185 m）suffered intense pedogenesis and developed multiple paleosol layers.In this study,we incorporated multiple methods of isotopic geochemistry,geochemistry,mineralogy,and spectroscopy to analyze a high-resolution terrestrial sedimentary sequence,and to reconstruct the paleoclimate and paleoenvironment in Qaidam basin recorded by the Lulehe paleosols during the EECO:1)The Lulehe Formation is a set of sedimentary sequence with braided river facies,which is dominated by conglomerate,pebbled siltstone and mudstone.The Lulehe paleosols developed mainly from pebbled siltstone and mudstone.The finer deposits such as mudstones are overbank phase representing slowly deposited environment,while the coarser deposits such as pebbled siltstones are avulsion phase representing rapidly accumulated environment.Paleosols are brown reddish,in which some carbonate nodules,slickensides,burrows,and gray mottles are developed.Secondary minerals contain iron oxides such as goethite,hematite,and clay minerals such as smectite,kaolinite,illite and mixed-layer smectite-illite.2)Twelve paleosol samples were analyzed by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,X-ray fluorescence（XRF）,scanning electron microscopy（SEM）,and differential scanning calorimetry（DSC）to characterize mineralogy and origins of smectite.Samples from DHG section contained predominant dioctahedral smectites（60～65%）in the<2μm clay fractions.Minor dioctahedral illites,dioctahedral interstratified illite-smectite（I-Sm）,kaolinite,and traces of dioctahedral chlorites and palygorskites were present.Dioctahedral smectites comprised the dominance of montmorillonite in the fine-clay fractions（<0.2μm）with minor contributions of beidellite,as determined by the Greene-Kelly test,geochemical and FTIR analyses.SEM observations revealed two morphological types of dioctahedral smectites,the less abundant one with typical‘honeycomb’structure,interpreted as neoformed,and the other occurring as thin,well-defined plates and irregular masses considered to be detrital.DSC revealed two dehydroxylation temperatures（500～550℃ and 600～700℃）,indicating the presence of trans-vacant（tv）and cis-vacant（cv）layers,respectively.The cv layers were attributed to montmorillonite originating as catchment-delivered detritus,as the study Lulehe Formation exhibited no evidence of volcanic ash layers or diagenesis that would have promoted formation of montmorillonite with high dehydroxylation temperatures.The tv layers represented pedogenic beidellite formed via solution crystallization in a floodplain pedogenic environment.The accumulation of abundant inherited montmorillonite and neoformed beidellite in paleosols of the Lulehe Formation was probably promoted by a relatively warm climate with alternating wet-dry seasons during the Early Eocene.3)UV-Vis diffuse reflectance spectral（DRS）,visible and near-infrared reflectance spectroscopy（VNIR）and portable-X ray fluorescence（p-XRF）were combined to analyze bulk paleosol samples and several indices related to weathering intensity or soil moisture were measured.Here we named the indexes are i)iron oxides related indexes,including ratios of Geothite/Hematite（Gt/Hm）,VNIR parameters P₉₀₀ and D₉₀₀;ii)clay minerals related indices,including VNIR parameters D₁₄₀₀/D₁₉₀₀ and D₂₂₀₀/D₁₉₀₀;iii)weathering indices,including SiO₂/Al₂O₃ and SiO₂/（Al₂O₃+Fe₂O₃+TiO₂）.All the indices showed great correlations and showed good agreement with the carbon and oxygen isotopic data,indicating high sensitivity to changes of paleoclimate.4)Carbon and oxygen isotopic compositions(δ¹³C andδ¹⁸O)of pedogenic carbonate nodules were in good consistent with the contemporary benthic foraminifera carbon and oxygen isotopic record.Moreover,theδ¹³C andδ¹⁸O values fell in the same range of other terrestrial records.The estimated atmospheric p CO₂ based onδ¹³C peleo-barometers was 737±380 ppm V,which equals to about 2 to 3x pre-industrial atmospheric p CO₂.Manual mean temperature（MAT）based on theδ¹⁸O paleothermometer was 10°C±1.67°C,while the temperatures in warm months constrained by limited carbonate clump isotopes(Δ₄₇)was 33℃.The obvious difference between MAT and temperatures in warm months indicated a temperature dependent seasonality.Moreover,the MAT and temperature in warm months estimated from the Lulehe Formation paleosols showed similarity to other continental surface temperatures from similar latitude.Manual mean precipitation（MAP）calculated with CALMAG climo-funcation was 980±154 mm/yr,with obvious fluctuations.In summary,the warm climate characterized by hot summer showing a seasonal change recorded by the Lulehe Formation paleosols corresponds to the elevated p CO₂ and increased temperature during the EECO.5)Two rapid negative carbonate isotope excursions（CIEs）,with CIE of+2‰（150～159 m,P10 layer paleosols）and 1.5‰（175～185 m,P13 layer paleosols）,respectively,were recorded in the Lulehe Formation paleosols.Compared with the contemporary marine records,the CIE occurred at P10 paleosols and P13 paleosols might correspond to P（C23n.1n H）and S（C22r H3）hyperthermal events,respectively.Increased MAT and MAP were obsevered during the hyperthermal events.Iron oxides related indices Gt/Hm,P₉₀₀ and D₉₀₀ showed increased goethite in relative to hematite,suggested a more humid climate which favors to form goethite within the hyperthermal events.Clay minerals related indices D₁₄₀₀/D₁₉₀₀ and D₂₂₀₀/D₁₉₀₀ increased,indicating increased smectite and kaolinite.Weathering indices SiO₂/Al₂O₃ and SiO₂/（Al₂O₃+Fe₂O₃+TiO₂）indicated increased chemical weathering during the CIEs.All the results suggest a warm and humid climate during the hyperthermal events.6)An abrupt increase ofδ¹³C andδ¹⁸O,with an amplitude of>2‰and>1.5‰respectively,occurred at 110～120 m（P6 layer paleosols）.The positive shift was comparable with the marine records at～51 Ma.Estimated p CO₂ and MAT reached their maximums throughout the whole paleosol profile,with average values of～1300ppm V and～12°C,respectively,while the MAP decreased to its lowest with the average value less than 500 mm/yr.Iron oxides related indices indicated that hematite increased significantly during the period,suggesting a seasonally drier soil environment.Clay minerals related indices decreased to their minimums,indicating significantly decreased smectite and kaolinite.Weathering indices suggested obviously weakened chemical weathering.By characterizing the origins of smectites,we suggested that the decrease of detrital montmorillonite carried by the river should be the main cause resulting in the obvious decrease of smectite.Meanwhile,formations of beidellite and palygorskite were observed during the period favoring for an arid to semi-arid environment.We suggested two reasons to explain the hot and dry climate occurred at～51 Ma:i)the obviously elevated p CO₂ coupled with increased temperature caused stronger soil evaporation;ii)the regressive of Tethys Sea which served as the main source of precipitation in the Qaidam Basin resulted in decreased precipitation.The unusual hot and dry climate occurred at Qaidam Basin showed great difference to the contemporary warm and humid climate at Green River Basin,North America,suggesting possible regional differences during EECO.7)In summary,two stages of climate before and after the peak of～51 Ma were suggested by all the indices.StageⅠ（<51.7～51 Ma）:Each paleosols developed in a shorter length and interbedded frequently with the coarser conglomerate layers.Theδ¹³C was relatively positive.MAT increased from bottom to middle,with fluctuations.The relative low content of goethite and hematite,D₁₄₀₀/D₁₉₀₀,D₂₂₀₀/D₁₉₀₀ and weathering intensity（SiO₂/（Al₂O₃+Fe₂O₃+TiO₂）,SiO₂/Al₂O₃）indicated by the weathering indices were low with frequently fluctuations.All the results suggested that the climate was unstable before～51 Ma.The paleoclimate during this period was hot and dry,with wet fluctuations.StageⅡ（51 Ma～>50 Ma）:theδ¹³C of pedogenic carbonate nodules was relatively more negative.Compared to the MAT during the peak,the MAT during this stage decreased slightly but increased obviously during the two hyperthermals.MAP was relatively high.The relative content of goethite and hematite,D₁₄₀₀/D₁₉₀₀,D₂₂₀₀/D₁₉₀₀ and weathering intensity（SiO₂/（Al₂O₃+Fe₂O₃+TiO₂）,SiO₂/Al₂O₃）indicated by the weathering indices are higher than StageⅠ,indicating a warm and humid climate.