Sedimentary Geochemical Records Of The Late Oligocene-early Pliocene Palaeovegetation Revolution In The Ying-Qiong Basin,South China Sea

The Cenozoic sediments of the Ying-Qiong Basin in the South China Sea have preserved important sedimentary records of the complicated interaction between terrigenous sediments and the East Asian monsoon climate.The late Oligocene-early Pliocene palaeovegetation and palaeoclimate in the Ying-Qiong Basin,South China Sea were comprehensively reconstructed using higher plant-derived biomarkers,palynological records,kerogen maceral compositions and trace elements of shales.During the period from about 24.9-18.3 Ma,there are trends of higher relative abundances of plant-derived biomarkers including oleanane,des-A-oleanane,rearranged oleanane,taraxastane,bicadianes,high molecular weight n-alkanes(C_n?27),C₁₉ tricyclic terpane,C₂₉???20R sterane in the younger sediments.Similarly,over this time period,there are increasing amounts of tropical/subtropical angiosperms and pteridophytes in the younger sediments,and lesser amounts of temperate Pinus and Alnus species.The bloom of the dominant tropical-subtropical higher plants and the large influx of terrigenous organic matter during this period were likely associated with extra precipitation under a warming climate.This warming and more humid climate is related with the East Asian monsoon climate in the South China Sea.The establishment of the East Asian summer monsoon in the South China Sea is proposed at about 24.9 Ma in the late Oligocene,which is likely to have been caused by uplift of the northern Himalaya-Tibetan Plateau at the same time.During the period of about 8.2-2.7 Ma(corresponding to the deposition of the upper Huangliu Formation to the lower Yinggehai Formation),there were remarkable decline of inferred higher plant inputs and vitrinite proportion in the kerogen.Temperate species Pinus and Alnus have higher proportion and ferns are in less abundance in the lower Yinggehai Formation than in the late Miocene sediments.Various higher plant-derived aromatic hydrocarbons including cadalene,retene,1,2,7-trimethylnaphthalene,1-methylphenanthrene,1,7-dimethylphenanthrene,6-isopropyl-1-isohexyl-2-methylnaphthalene(ip-i HMN)and some polycyclic aromatic hydrocarbons(PAHs)were identified.Higher plant parameters suggest that the abundance of the total terrestrial organic matter in the early Pliocene strata was relatively lower than in the angiosperms-dominated organic matter in the late Miocene Huangliu Formation.The gymnosperm/angiosperm indices including retene/(retene+cadalene),higher plant index(HPI),1-MP/9-MP illustrate that gymnosperms made larger contribution to the organic matter when the lower Yinggehai Formation was deposited during the early Pliocene.The combustion-derived PAHs including fluoranthene,Benzo[a]anthracene,benzofluoranthenes,coronene predominating over petrogenic sources indicate the more prevailing wildfires in the deposition of the lower Yinggehai Formation and the upper Meishan Formation,different from the mainly mixed input of combustion-derived and petrogenic sources in the Huangliu Formation.As also supported by increasing abundances of Pinus and Alnus in palynological records,the combustion-derived PAHs and gymnosperm/angiosperm parameters provide unequivocal evidence for the weakened East Asian summer monsoon in the cooling climate from the late Miocene to the early Pliocene in the South China Sea.