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The Source And Paleoclimate/environmental Significance Of Carbon / Hydrogen Isotopic Composition Of Individual Hydrocarbon Compounds And Their Profile Variation In The Lower Cretaceous Yi-xian Formation Sediments Of Western Liaoning Province, China

Posted on:2017-01-26Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y LiFull Text:PDF
GTID:1220330485494159Subject:Institute of Geochemistry
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During the period of the Early Cretaceous Yi-xian Formation(K1y3) sedimentation in western Liaoning, organisms are thrived with frequent and intensive volcanic activity, and therefore provide a rare opportunity for studying the Cretaceous continental paleoclimate and terrigenous biological evolution. Many significant and well preserved fossils have been discovered within this Fromation and that constitute the famous Jehol Biota fossils. For a long time, the paleoclimate and environmental changes during the deposition of the Yi-xian Formation have been one of important research focuses by many geodisciplines including Sedimentology, Volcanology, Paleontology and Geochemistry etc. Combined with previous researches, this thesis has employed molecular organic geochemistry methods and has conducted systematical organic geochemical analysis, including GC, GC-MS, GC-IR-MS, to study the sedimentary organic matter in the Early Cretaceous Yi-xian Formation sediments from western Liaoning, particularly the biomarker composition and distribution, and the carbon and hydrogen isotopic composition variation of individual hydrocarbons. We aimed to investigate the sedimentary organic matter responses to volcanic activities and the regional paleoclimate/environmental variation. The major conclusions are as follows:1. The TOC content of samples from Yi-xian Formation ranges from 0.05% to 5.65%, varying significantly. The lower part(< 161.5 cm) which contains abundant fossils has a higher organic carbon content range than the upper part. The characteristics of the group components of EOM and thermal maturity-related biomarker indicators suggest that the thermal maturity of Yi-xian Formation are immature, indicating the influence of geological diagenetic evolution on organic matter of Yi-xian Formation is small. The detection of gammacerane and the low Pr/Ph values(< 0.8) suggest a salinity stratified water column associated with anoxic bottom waters and the sedimentary organic matter preserved well.2. The distribution and carbon isotopic composition of n-alkanes shows regularly change through the profile, with long-chain n-alkanes(n C27、n C29、n C31) dominate in the lower and upper parts of the profile, while the medium-chain n-alkanes(n C23)dominant in the middle section, indicating a major source input of terrigenous plants in the the lower and upper parts while a primary source input of aquatic plants in the middle section. The variation trends of the carbon isotopic composition of different carbon n-alkanes through the profile are quite consistent, with the δ13C of the n-alkanes being very negative in the lower part of the profile, and then a general upward positive excursion which reached the most positive value(-26‰) at 352.5 cm, and return to negative excursion after the positive shift. The carbon isotopic composition of atmospheric CO2 during the Early Cretaceous was reconstructed base on the carbon isotopic composition of n-alkanes derived from terrestrial plant wax, the result is in the range of-10‰ to-3‰, implying an impact of volcanic activity on the concentration of atmospheric CO2 and its carbon isotopic composition and cause the corresponding change of paleoclimate and paleoenvironment conditions.3. The correlation results of hydrogen isotope composition between dominated n-alkanes suggest that n C23 are primarily derived from aquatic plants, while n C29 and n C31 are mainly originate from terrestrial higher plants but n C27 may derived from multi-source. The calculated ΔδDmid-long composition changes from-28.8‰ to 85.0‰, suggesting that the climate fluctuated periodicity during Jehol Biota booming, but gradually change from being warm and dry to being warm and humid then return to being warm and dry, which consistent with the climate change indicated by dominated n-alkanes and their carbon isotopic composition, particularly the consistency with sedimentology and paleobotany evidences, confirmed that ΔδDmid-long is effective indicator for paleoclimate and paleoenvironment information.4. The steranes of Yi-xian Formation are dominated by regular steranes, diasteranes and C30 4α-methyl-24-ethyl steranes were only detected in low abundance in some samples. The relative abundance of the C27, C28 and C29 ααα steranes display an asymmetric ‘V’ or an inverse ‘L’ pattern, perhaps indicating mixed inputs of aquatic algae and land plants. The existence of diasteranes in some samples of middle part may be attributed to the catalysis of specific clay minerals in the sediments. The detection of C30 4α-methyl-24-ethyl steranes in some samples from upper part may suggests that the depositional and climate conditions during the formation of upper part were favourable for the growth of fresh water algae or bacteria.5. The C27- C31 αβ hopanes, C30 8,14- secohopane and C24 17,21- secohopane are detected in Yi-xian Formation. The δ13C composition of C30 αβ hopane varies significantly from-53.3‰ to-37.6‰, likely indicating varying types of bacteria. The more 13C-depleted C30 αβ hopane(-53.3 to-50.3‰) in the lower part suggest methanogenic bacteria inputs, whereas the 13C-enriched C30 αβ hopane(-43.7 to-37.6‰) in the upper part more likely originated from a combination of methanogenic and other autotrophic microorganisms. C30 8,14- secohopane were detected in all samples whereas C24 17,21- secohopane were only detected in lower part and some samples of middle part, implying that they had a different origins. Combined with carbon isotopic composition, C30 8,14- secohopane may have originated from bacteria that contained hopanoids while the origin of C24 17,21- secohopane are more complicated, possibly connected with the volcanic ash deposits and massive vertebrate remains, or derived from some 13C-enriched special bacteria.
Keywords/Search Tags:biomarker, carbon and hydrogen isotopic of individual hydrocarbons, source of organic matter, paleoclimate and paleoenvironment, Lower Cretaceous Yi-xian Formation sediments, western Liaoning
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