Had The Monsoon Climate Existed During The Early Eocene In The SE Tibetan Plateau? ——A Case Study From Lacustrine Sediments Of The Gonjo Basin

The monsoon system is one of largest climate systems and profoundly affects earth surface processes and life activities.In particular,the modern monsoon climate has exerted long-term influences on the well-being of humans and the development of civilizations.With the intersifying global warming in response to anthropogenic forcing,the intensity and scale of modern monsoons are constantly changing.To better ascertain monsoon changes under an intensifying anthropogenic warming and predict the future climate,we need to reveal how monsoon climate has behaved during the past warm periods.The early Eocene was one of the past warm periods during the Cenozoic,and characterized by global warming,high atmospheric carbon dioxide concentrations,increased precipitation and amplified seasonality.Some studies indicated that monsoon climate could already exist in the southeastern Tibetan Plateau during the extremely warm early Eocene.However,as those studies suffer from low sensitivity of climatic proxies and/or large uncertainties in chronology,whether monsoon climate has indeed established in the southeastern Tibetan Plateau at this period remains elusive.The Gonjo Basin is a typical Cenozoic sedimentary basin located in the southeastern Tibetan Plateau.The basin contains thick and well-dated Paleogene sedimentary rocks,which can be subdivided into the Late Cretaceous sandstones(>69 Ma),Gonjo Formation(?69-63.5Ma)and Ranmugou Formation(63.5-50 Ma)from bottom to top;and after 53.2 Ma,the Upper Ranmugou Formation(54-50 Ma)contains alternating layers of brown-reddish and grey-greenish mudstones,which are likely related to the periodic changes of monsoon climate,and thus,is the ideal material to study the early Eocene climate in the Tibetan Plateau.Here,this study has investigated sedimentary facies and chemical weathering indices of Gonjo Basin,to reconstruct regional climatic/environmental changes.To further evaluate whether a monsoon climate had established in the southeastern Tibetan Plateau,this study has also analyzed the sedimentology,diffuse reflectance spectroscopy,elementary geochemistry,and clay mineralogy of alternating layers.The major findings from this study are listed below:(1)The Late Cretaceous sandstones are characterized with large-scale planar and trough cross-bedding with compositional and structural maturities,and interpreted as dune deposits in a desert environment;the Lower Gonjo Formation is dominated by upward-fining sedimentary sequences which contains lag gravels at the scoured base,thick cross-bedded sandstones as main body and is topped by thin-bedded mudstones,and interpreted as braided river deposits;the Upper Gonjo Formation is characterized by interbedded purple-reddish mudstone and siltstone,with caliche nodules,paleosols and mud cracks,and interpreted as floodplain deposit;the Lower Ranmugou Formation is composed of massive and cross-bedded sandstones with thin mudstone and siltstone,which belongs to the sedimentary products of braided river environment;and the Upper Ranmugou Formation is predominated by laminated mudstones and siltstones,characterized by the alternating layers of brown-reddish mudstones,and interpreted as lacustrine depoist.(2)The average CIA value from the Late Cretaceous sandstones to Lower Rnmugou Formation is 62.98 and the average CIW' value is 84.62.Those values manifest weak chemical weathering and less precipitation,and thus reflect an arid to semi-arid climate in the Gonjo Basin from ?69 to 54 Ma.The average CIA and CIW' values of the Upper Rnmugou Formation are 70.05 and 90.77,respectively,which manifest moderate chemical weathering and more precipitation,and thus reflect a semi-humid climate from 54 to 50 Ma.(3)The changes in sedimentary facies and chemical weathering indices in the Gonjo Basin sediments reveal a critical transition from dry to wet climate in the southeastern Tibetan Plateau at ?54 Ma: the sedimentary environment changed from eolian-fluvial to humid lacustrine;the chemical weathering changed from weak to moderate;and precipitation increased.(4)The data from diffuse reflectance spectroscopy,elementary geochemistry,and clay mineralogy analyses on the uppermost Ranmugou Formation suggest that the brown-reddish mudstones were deposited in a shallow lake environment with stronger chemical weathering and more precipitation,whereas the grey-greenish mudstones were deposited in a deep lake environment.The alternating layers of brown-reddish and grey-greenish mudstones therefore represents a periodicity of relatively dry(but with increased precipitation)and wet(but with decreased precipitation)climate conditions,indicating the climate may have acquired the monsoonal features during the early Eocene in the southeastern Tibetan Plateau.(5)Combined with the above findings,this study concludes that the arid to semi-arid climate found in Gonjo Basin from ?69 to 54 Ma was likely resulted from the long-term control of the subtropical high-pressure belt;at ?54 Ma,the Eocene Thermal Maximum 2event had driven the Intertropical Convergence Zone deep into higher latitudes,which brought a large amount of precipitation and triggered the climatic transition in the southeastern Tibetan Plateau;and after 53.2 Ma,the Early Eocene Climatic Optimum event had driven the migrations of Intertropical Convergence Zone(global monsoon)more intense,which eventually gave rise to the monsoon climate in the southeastern Tibetan Plateau.