Late Permian To Early Triassic Paleosol Sedimentary Sequences And Paleoclimate Reconstruction In North China

The Permian–Triassic transition witnessed the greatest mass extinction of the Phanerozoic.During this period,the climate and environment of both marine and land had significantly changed.The research on the marine environment is relatively deep-going,and a variety of hypotheses of environmental events have been proposed.Compared with the marine facies,the environment on land has more climatic factors,besides temperature,the humidity and aridity change,the concentration of atmospheric CO₂ are also critical to the extinction and recovery of terrestrial biotas.Therefore,this study selects a kind of typical terrestrial sediments,paleosols,as the research materials.Systematical petrologic studies on the paleosol sequences have been performed,in order to construct the developing model of paleosols,reveal the climate and geographic location of various paleosols,and reflect the qualitative climatic characteristics of its formation period.Then,some suitable paleosol sequences and samples for quantitative paleoclimate reconstruction are selected through the analysis of the paleosol diagenetic imprint and environment.Depths of calcium carbonate accumulation horizons and carbon isotopes of pedogenic carbonate are used for the reconstruction of the mean annual precipitation(MAP)and atmospheric CO₂ concentration(p CO₂)from Late Permian to Early Triassic.Correlated with the changes of terrestrial biodiversity from the Late Permian to Early Triassic in North China,the influence of climate on the biotic extinction and recovery on land is discussed.In order to reconstruct terrestrial climate change from the Late Permian to Early Triassic,three main terrestrial sections,i.e.Shichuanhe,Liulin and Yiyang sections,and two auxillary sections,Buliang and Xuecun sections,which contain well-developed paleosols in North China are herein selected for a systematic study on the paleosols.Three main orders of paleosols(Inceptisols,Aridisols and Vertisols)and six suborders(calcic Inceptisols,gleyed Inceptisols,vertic Aridisols,gleyed Aridisols,calcic Vertisols,and gleyed Vertisols)have been recognized in these studied sections.The Aridisol is the main type of paleosols in the lower and upper parts of the Sunjiagou Formation,correlated with micromorphological pedofeatures of Fe/Mn quasicoating and calcite hypocoating,representing a climatic feature of aridity;the Vertisol takes charge in the middle part of Sunjiagou Formation,indicative of sub-humid to semi-arid climate with relatively long humid periods.The moderate to high maturity of Aridisols and Vertisols requires a long duration of pedogenesis,indicating that the depositional environment of the Sunjiagou Formation was on a tectonically stable floodplain.The Inceptisol is the major type of paleosols in the Liujiagou Formation,especially the gleyed Inceptisol with sub-humid derived pedofeatures,and the multiple stages of low maturity gleyed Inceptisols showing that the pedogenesis started and stoped for several times controlled by flooding events.The main types of paleosols in the Heshanggou Formation change upwards from Aridisol-Inceptisol-Vertisol-Aridisol.Multi-periods of paleosols with switch of types may explain the repeatability of climatic conditions in the Heshanggou Formation,but the paleosol sequences of Inceptisols-Vertisols is preodominated in the Formation.Though the arid climate is indicated at the bottom and top of the Heshanggou Formation,the overall climate was sub-humid to semi-arid during the depositon of the main part of the formation and the sedimentary facies is floodplain with playa lake.In summary,according to the petrological analysis of paleosols,several developing models of paleosols are established,which can qualitatively reflect paleoclimate changes based on the main paleosol types from the Late Permian to Early Triassic in North China.The Late Permian was predominated by aridity;the Permian-Triassic transition(the middle Sunjiagou Formation)became humid;the Early Triassic dominated by semi-arid and semi-humid,with two climate cycles from arid to humid.Based on the systematic petrographical study of paleosols,the diagenetic imprints in paleosols,such as the content of microspar and sparry calcite,are used to assess the degree of diagenesis in paleosols;and comparing with the cathodoluminescence patterns,the main diagenetic conditions of paleosols are identified.The results show that the influence of diagenesis on all paleosols is less than 40%,and the diagenetic environment is predominated by meteoric vadose zone.Some samples which formed in the well-drained environment and with low diagenetic influence are selected for the study of carbon and oxygen isotopes,to conduct the MAP and p CO₂ calculations quantitatively.According to the calculation results,related with the paleoclimate reflected by paleosols qualitatively,the MAP change from the Late Permian to Early Triassic can be divided into four stages.Stage one is the Late Permian(Changhsingian),the climate was mainly semi-arid to arid,with an average precipitation of 370±147 mm/yr;Stage two is the Permian-Triassic transition and the climate was sub-humid to semi-arid,with the precipitation of 470±147 mm/yr;Stage three is the early Early Triassic(Induan)and the climate predominated by aridity;and Stage four is the late Early Triassic(Olenekian)and the climate mainly sub-humid to semi-arid,with an average MAP of 560±147 mm/yr during wet period while 260±147 mm/yr during dry period.The average value of p CO₂in the Late Permian was 2055 ppm V,wheras the average in the late Early Triassic 1031ppm V.Although the relationship between the late Permian arid climate and mass extinction is not clear due to the lack of fossil evidence in the study area,the declined aridity and increased precipitation during the late Early Triassic(Olenekian),accompanied with a decrease of atmospheric CO₂ concentration,suggests that the climate had returned to the normal situation from the early Early Triassic extreme conditions in North China.The improvement of the climate may be closely related to the recovery of various terrestrial biotas.