Causation and mechanism of magnetic susceptibility trend in Upper Miocene-Pliocene red clay deposits of the eastern Chinese Loess Plateau

摘要

The Upper Miocene-Pliocene red clay deposits of the Chinese Loess Plateau (CLP), like the overlying Quaternary loess, have recorded abundant paleoclimatic evolution information. In this study, we found that the variation in the magnetic susceptibility of seven sections across a north-south climate gradient in the eastern CLP exhibited an oscillatory upward trend. Prior to this research, much less was known about the causation and mechanism of this trend. Among the seven sections, the Jiaxian and Duanjiapo sections are located in the northernmost and southernmost of the climatic gradient, respectively. These representative sections were therefore selected to investigate the causation and mechanism of the aforementioned trend using magnetic measurements. The results were as follows. (1) The absolute and relative content of fine-grained ferrimagnetic minerals displayed an oscillatory upward increasing trend in both of the red clay sequences, which was the primary reason for the susceptibility increase in fluctuations. (2) The absolute content of antiferrimagnetic minerals was nearly equivalent in each red clay sequence. (3) Based on the combination of the results of present study and previous study findings, we found that the weathering and pedogenesis intensity of the red clay sequence showed an oscillatory upward trend during 7-2.6 Ma, which was closely linked with the strengthened East Asian summer monsoon in oscillations. These findings are probably the main cause for the oscillatory upward increasing finegrained ferrimagnetic minerals in Upper Miocene-Pliocene red clay sequence. Further, this is likely the mechanism for the oscillatory upward increase of chi in the red clay sequence.