Mineral magnetic studies of the vermiculated red soils in southeast China and their paleoclimatic significance

摘要

Vermiculated red soil, the main pedostratigraphical unit of the red soil sequences widespread in subtropical and tropical China, has experienced a higher degree of pedogenesis than loess/paleosols and the underlying red clays in the Loess Plateau of northern China. Previous studies suggested that magnetic properties of the vermiculated units of the red soils are significantly degraded compared to those of the non-vermiculated counterparts. The mechanism for this magnetic decrease, however, has not been well determined. In order to resolve this problem, this study has investigated two red soil sequences of different origin in southeast China by integrating environmental and mineral magnetic research. The results show that the magnetic depletion in the vermiculated units was probably caused by both dissolution of magnetic minerals in the white veins and transformation of highly magnetic maghemite to weakly magnetic hematite. It appears that stronger pedogenesis favors the transformation of more maghemite to hematite and magnetic properties associated with the relative concentration of maghemite to hematite can be used as climate indicator in the vermiculated red soils. The variation trends of the magnetic parameters of the vermiculated red soils prior to 0.85-0.9. Ma suggest a strengthening of the East Asian summer monsoon. The reverse trend of these parameters after 0.85-0.9. Ma may correlate with a significant strengthening of the East Asian winter monsoon since 0.85. Ma.