Remagnetization of Quaternary eolian deposits: A case study from SE Chinese Loess Plateau

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Abstract

[1] The loess-paleosol succession in the southeastern margin of the Chinese Loess Plateau retains high-resolution archives of sedimentary and environmental change. In this study, we report a detailed paleomagnetic and rock magnetic investigation of loess-paleosol sequences in the Sanmenxia area spanning the last 1.1 Myr. The results demonstrate that the Matuyama/Brunhes Boundary occurs at the top of soil S8 and the upper and lower boundaries of the Jaramillo Normal Subchron are encountered at the top of soil S10 and loess L13, respectively, in agreement with the classic Luochuan section. Loess L9, also referred to as “the upper sand layer,” is the coarsest loess unit over the last 1.1 Myr and carries an “imperfect” normal polarity magnetization. Although it is fully separated by a short reversed polarity interval, this thick normal polarity zone is too extensive to represent excursions like Kamikatsura and/or Santa Rosa and is concluded to represent an artifact rather than a record of geomagnetic field directions. Precise multiparameter environmental magnetic determination reveals that the short reversed polarity interval within the middle of this thick normal polarity zone exactly corresponds to a thin pedogenic horizon. Detailed rock magnetic analyses show that there are no intrinsic differences in magnetomineralogical properties between this pedogenic interval and other parts of L9, although the composition of magnetically soft components (magnetite/maghemite) in the short pedogenic interval is relatively higher than that of other parts of L9. Thus it is proposed that extensive remagnetization of parts of L9 most likely reflects a lithology-dependent process, and the dramatic coarsening of particle size across L9 is the fundamental reason for pervasive remagnetization. Our magnetostratigraphic results strongly imply that we need to reconsider the remanence-recording fidelity of coarse loess (L9, and probably L15) and care should be taken to interpret excursions in future paleomagnetic investigations of the Chinese loess.

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