Reconstructions of late Oligocene-early Pliocene palaeovegetation variation in the Ying-Qiong Basin, South China Sea, and palaeoecology during deposition of the middle Permian Lucaogou Formation, Junggar Basin, China
The sedimentary rocks of the Yinggehai-Qiongdongnan Basin in the northern South China Sea preserve important records of the complicated interactions between terrigenous sediments and the East Asian monsoon climate. It remains obscure how higher plant-derived biomarkers and the inferred floral changes respond to the East Asian monsoon climate in the South China Sea. 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 oleananes, taraxastane, bicadinanes, high molecular weight n-alkanes (Cn ≥ 27), C19 tricyclic terpane, and C29 ααα 20R sterane in the younger sediments. Similarly, over this period, there are records of 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 to the establishment of the East Asian monsoon in the South China Sea at about 24.9 Ma in the late Oligocene, which was likely caused by uplift of the northern Himalaya-Tibetan Plateau. During the period from about 8.2−2.7 Ma (corresponding to the deposition of the upper Huangliu Formation to the lower Yinggehai Formation), there was a remarkable decline of inferred higher plant inputs and vitrinite proportion in the kerogen. The temperate species Pinus and Alnus became more dominant, and ferns became less abundant in the lower Yinggehai Formation compared to the underlying late Miocene sediments. Various higher plantderived aromatic hydrocarbons including cadalene, retene, 1,2,7- trimethylnaphthalene, 1-methylphenanthrene (1-MP), 1,7-dimethylphenanthrene (1,7- DMP), 6-isopropyl-1-isohexyl-2-methylnaphthalene (ip-iHMN) and other polycyclic aromatic hydrocarbons (PAHs) were identified. Higher plant parameters suggest that the relative abundance of total terrigenous organic matter in the early Pliocene was lower than in the angiosperm-dominated organic matter in the late Miocene Huangliu Formation. Various gymnosperm/angiosperm indices including retene/(retene + cadalene), 1-MP/9-MP, and 1,7-DMP /(1,3- + 3,9- + 2,10 + 3,10-DMP), and the higher plant index, indicate that gymnosperms made a larger contribution to the organic matter when the lower Yinggehai Formation was deposited during the early Pliocene. Combustion-derived PAHs including fluoranthene, benzo[a]anthracene, benzofluoranthenes, and coronene predominate over petrogenic PAHs, indicating that there were more wildfires during deposition of the lower Yinggehai Formation and the upper Meishan Formation. In contrast, there is a mixed input of combustionderived and petrogenic PAHs in the Huangliu Formation. As also supported by increasing abundances of Pinus and Alnus in palynological records, the combustionderived PAHs, the gymnosperm/angiosperm parameters, and the trace element proxies provide unequivocal evidence for a weakened East Asian summer monsoon during a cooling climate from the late Miocene to the early Pliocene in the South China Sea.
The low maturity oil shales and associated tight oils in the middle Permian Lucaogou Formation were formed in a brackish-saline and alkaline lacustrine environment in the Jimusaer Sag, Junggar Basin. Biological diversity during deposition of the Lucaogou Formation was reconstructed using stable carbon isotope compositions of the organic matter, and biologically-informative molecules, including n-alkanes, steranes, and hopanes. C30 4α-methyl,24-ethyl sterane, C31 lanostane and abundant C30−C31 3β- methylhopanes were detected in the organic matter, suggesting the presence of type Ⅰ aerobic methanotrophs including the Methylococcaceae Methylosphaear hansonii. C30-C31 2α-methylhopanes and 7- and 8-methylheptadecanes are suggestive of the presence of cyanobacteria. Cyanobacteria and green algae are proposed to have made the dominant contribution to the organic matter. The presence of more 13C-enriched hopanoids in the lower unit compared to the upper unit of the Lucaogou Formation suggest a greater contribution of cyanobacteria to the organic matter in the lower unit. The high abundances of C28 and C29 steranes relative to C27 steranes is likely due to green algae from the Chlorophyta, including the class Chlorophyceae. Dinosteranes that are diagnostic for dinoflagellates were not detected. The very low concentrations of C40 isorenieratane, renieratane, β-isorenieratane, renierapurpurane, and β- renierapurpurane that are indicative of photosynthetic green and purple sulfur bacteria indicate a minor contribution from Chlorobiaceae and Chromatiaceae to the organic matter. It is inferred that low to moderate amounts of ciliates grew at or below the relatively unstable chemocline in the lake. The presence of C19-norisopimarane, 8β(H)-labdane, and 4β(H)-19-norisopimarane suggests the presence of coniferous plants growing around the lake, probably including Pinaceae, Araucariaceae, Cupressaceae.