Characterizing the superparamagnetic grain distribution of Chinese red-clay sequences by thermal fluctuation tomography

Although magnetic susceptibility (χ) is a widely accepted East Asian summer monsoon (EASM) intensity proxy for Quaternary loess, debates exist about whether χ can be used to indicate EASM intensity for the red-clay sequence. In order to use χ of the red-clay sediments to indicate EASM intensity, χ should have the same sort of enhancement mechanisms as in Quaternary loess. However, this similarity has not been rigorously demonstrated. Previous estimates of magnetic grain size distribution of Chinese loess and red-clay are based on out-of-phase magnetic susceptibility inversion technique. This technique assumes that the coercivity of pedogenic magnetic grains is independent of temperature, and at any temperature, only a single grain size contributes to frequency-dependent or quadrature susceptibility, rather than a distribution of sizes. Thermal fluctuation tomography is a recently developed more rigorous inversion technique for characterizing the grain-size distribution of superparamagnetic (SP) and single domain grains with fewer assumptions. Here we apply this technique to the Chinese red-clay sequence of the Chaona section, central Chinese Loess Plateau, and the overlying loess–paleosol sequence to better characterize the size distribution of SP grains of the Chinese red-clay sequence. We found that SP grain-size distributions of both paleosol and red-clay from the Chaona section are similar. The results reinforce a previous conclusion supporting the idea that magnetic susceptibility (χ) can be used to indicate the intensity of the EASM in the red-clay sequence.