Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identi- fied, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and a...Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identi- fied, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the pa- leoenvironment and paleovegetation history during S4 was reconstructed. The CPI (Carbon Predomi- nance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data dem- onstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes was dominated by C31 homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period.展开更多
To understand climate changes recorded in the Luochuan loess-palaeosols, Shaanxi province, northwes- tern China, clay mineralogy was studied using X-ray diffraction （XRD）, high-resolution transmission electron micro...To understand climate changes recorded in the Luochuan loess-palaeosols, Shaanxi province, northwes- tern China, clay mineralogy was studied using X-ray diffraction （XRD）, high-resolution transmission electron microscopy （HRTEM）, and scanning electron microscopy （SEM） methods. XRD results show that clay mineral compositions in the Luochuan loess-palaeosols are dom- inantly illite, with minor chlorite, kaolinite, smectite, and illite-smectite mixed-layer clays （US）. Illite is the most abundant species in the sediments, with a content of 61%- 83%. The content of chlorite ranges from 5%-22%, and the content of kaolinite ranges from 5%-19%. Smectite （or US） occurs discontinuously along the loess profile, with a content of 0-8%. The Kfibler index of illite （IC） ranges from 0.255°-0.491°, and the illite chemical index （ICI） ranges from 0.294-0.394. The CIA values of the loess- palaeosols are 61.9-69.02, and the R3＋/（R3＋ ＋ R2＋ ＋ M＋） values are 0.508-0.589. HRTEM observations show that transformation of illite to illite-smectite has occurred in both the loess and palaeosol, suggesting that the Luochuan loess-palaeosols have experienced a certain degree of chemical weathering. The Luochuan loess-palaeosols have the same clay mineral assemblage along the profile. However, the relative contents of clay mineral species, CIA, ICI, and IC values fluctuate frequently along the profile, and all these parameters display a similar trend. Moreover, climate changes suggested by the clay index are consistent with variations in the deep-sea 6180 records and the magnetic susceptibility value, and thus, climate changes in the Luochuan region have been controlled by global climate change.展开更多
Extraterrestrial particles have higher 3He/4He ratios than those of terrestrial sediments ( > 100 Ra versus < 0.03 Ra, where Ra is the 3He/4He ratio normalized to the atmospheric value of 1.4×10?6). The int...Extraterrestrial particles have higher 3He/4He ratios than those of terrestrial sediments ( > 100 Ra versus < 0.03 Ra, where Ra is the 3He/4He ratio normalized to the atmospheric value of 1.4×10?6). The inter- planetary dust particles (IDPs), which are very rich in 3He, continuously bombard the Earth and can accumulate in deposits. IDPs sedimentation rate changes can significantly influence 3He concentration in the Earth surface sediments. Since IDPs are not easy to detect in terrestrial deposits, measuring helium isotopes is a helpful approach to examine changes in IDPs. Helium concentration and helium isotopic composition magnetic substances and the quartz particles were examined for helium concen- tration and helium isotopic ratio. Results show that the 3He/4He ratio and the 3He concentrations of the magnetic substances are clearly higher than those of the bulk samples and the quartz particles, and, the 3He/4He ratio of the extracted magnetic substances is also higher than that of the average level of the Earth’s crust. The higher helium content in the magnetic fractions can be explained by an influx of IDPs.展开更多
基金the Hundred Talent Project of Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant No. 40371117)
文摘Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identi- fied, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the pa- leoenvironment and paleovegetation history during S4 was reconstructed. The CPI (Carbon Predomi- nance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data dem- onstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes was dominated by C31 homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period.
基金This work was supported by the National Natural Science Foundation of China （Grant Nos. 41272053 and 41472041）. C.W. acknowledges a postdoctoral science foundation of China （2015M582301）, Fundamental Research Funds for the Central Universities, China University of Geosciences （Wuhan）, and National Natural Science Youth Foundation of China （Grant No. 41602037）. The authors wish to thank Dr. Yu J. S. for XRD analysis, Dr. Liu X. W, for HRTEM analysis and Dr. Yang H. and Dr. Yang Q. for SEM analysis.
文摘To understand climate changes recorded in the Luochuan loess-palaeosols, Shaanxi province, northwes- tern China, clay mineralogy was studied using X-ray diffraction （XRD）, high-resolution transmission electron microscopy （HRTEM）, and scanning electron microscopy （SEM） methods. XRD results show that clay mineral compositions in the Luochuan loess-palaeosols are dom- inantly illite, with minor chlorite, kaolinite, smectite, and illite-smectite mixed-layer clays （US）. Illite is the most abundant species in the sediments, with a content of 61%- 83%. The content of chlorite ranges from 5%-22%, and the content of kaolinite ranges from 5%-19%. Smectite （or US） occurs discontinuously along the loess profile, with a content of 0-8%. The Kfibler index of illite （IC） ranges from 0.255°-0.491°, and the illite chemical index （ICI） ranges from 0.294-0.394. The CIA values of the loess- palaeosols are 61.9-69.02, and the R3＋/（R3＋ ＋ R2＋ ＋ M＋） values are 0.508-0.589. HRTEM observations show that transformation of illite to illite-smectite has occurred in both the loess and palaeosol, suggesting that the Luochuan loess-palaeosols have experienced a certain degree of chemical weathering. The Luochuan loess-palaeosols have the same clay mineral assemblage along the profile. However, the relative contents of clay mineral species, CIA, ICI, and IC values fluctuate frequently along the profile, and all these parameters display a similar trend. Moreover, climate changes suggested by the clay index are consistent with variations in the deep-sea 6180 records and the magnetic susceptibility value, and thus, climate changes in the Luochuan region have been controlled by global climate change.
基金Supported by the National Natural Science Foundation of China (Grant Nos. 40173006 and 40325007)Key Research Project of Ministry of Education, China (Grant No. 03106)National Basic Research Program of China (Grant No. 2004CB720506)
文摘Extraterrestrial particles have higher 3He/4He ratios than those of terrestrial sediments ( > 100 Ra versus < 0.03 Ra, where Ra is the 3He/4He ratio normalized to the atmospheric value of 1.4×10?6). The inter- planetary dust particles (IDPs), which are very rich in 3He, continuously bombard the Earth and can accumulate in deposits. IDPs sedimentation rate changes can significantly influence 3He concentration in the Earth surface sediments. Since IDPs are not easy to detect in terrestrial deposits, measuring helium isotopes is a helpful approach to examine changes in IDPs. Helium concentration and helium isotopic composition magnetic substances and the quartz particles were examined for helium concen- tration and helium isotopic ratio. Results show that the 3He/4He ratio and the 3He concentrations of the magnetic substances are clearly higher than those of the bulk samples and the quartz particles, and, the 3He/4He ratio of the extracted magnetic substances is also higher than that of the average level of the Earth’s crust. The higher helium content in the magnetic fractions can be explained by an influx of IDPs.
