A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studie...A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studied loess-paleosol sediments retain primary sedimentary fabrics. Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from both loess and paleosol specimens. A low-temperature component, isolated between 100°C and 200°C, is consistent with the present geomagnetic field direction, and a high-temperature component, isolated between 200-300°C and 620-680°C, includes clearly normal and reversed polarities. Isothermal remanent magnetization and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite. The Day plot, together with the stratigraphic variations of rock-magnetic parameters, shows that the uniformity of magnetic mineralogy and grain size fulfills the criteria for relative paleointensity （RPI） studies. RPI records have been constructed using natural remanent magnetization （NRM） intensity after thermal demagnetization at 300°C normalized by low-frequency magnetic susceptibility （NRM 300 /χ）. The results show that the RPI record from the Baoji section, where pedogenesis is quite weak, is compatible with the stacked PISO-1500 paleointensity record, suggesting that it might reflect the paleointensity variation of the geomagnetic field. The RPI record from the Xifeng section, where pedogenesis is rather strong, indicates a clear dissimilarity with the stacked PISO-1500 paleointensity record, implying that it does not reflect the paleointensity variation of the geomagnetic field. Our new results show that the NRM 300 /χ from the strongly pedogenetic paleosols does not completely eliminate the pedogenetic （climatic） influence, so it might be unsuitable for a reliable paleointensity study.展开更多
基金supported by the National Natural Science Foundation of China (no. 41172038)the Programfor New Century Excellent Talents in University (no.NCET-11-0720)+2 种基金the Fundamental Research Funds for the Central Universities (no. 2010ZD05, 2011YXL010)the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education MinistryApart of this study was performed under the cooperative research program of CMCR (no. 07A017, 08A001)
文摘A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studied loess-paleosol sediments retain primary sedimentary fabrics. Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from both loess and paleosol specimens. A low-temperature component, isolated between 100°C and 200°C, is consistent with the present geomagnetic field direction, and a high-temperature component, isolated between 200-300°C and 620-680°C, includes clearly normal and reversed polarities. Isothermal remanent magnetization and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite. The Day plot, together with the stratigraphic variations of rock-magnetic parameters, shows that the uniformity of magnetic mineralogy and grain size fulfills the criteria for relative paleointensity （RPI） studies. RPI records have been constructed using natural remanent magnetization （NRM） intensity after thermal demagnetization at 300°C normalized by low-frequency magnetic susceptibility （NRM 300 /χ）. The results show that the RPI record from the Baoji section, where pedogenesis is quite weak, is compatible with the stacked PISO-1500 paleointensity record, suggesting that it might reflect the paleointensity variation of the geomagnetic field. The RPI record from the Xifeng section, where pedogenesis is rather strong, indicates a clear dissimilarity with the stacked PISO-1500 paleointensity record, implying that it does not reflect the paleointensity variation of the geomagnetic field. Our new results show that the NRM 300 /χ from the strongly pedogenetic paleosols does not completely eliminate the pedogenetic （climatic） influence, so it might be unsuitable for a reliable paleointensity study.
