Magneticenhancement mechanism of loess-soil sequence at Qinan during Middle Miocene and its paleoclimate significance

Magnetic susceptibility of loess-soil sequence increased significantly during Mid-Miocene, which has been used as a regional marker for stratigraphic correlation on the Loess Plateau. However, the mechanism of magnetic enhancement is still unclear. In order to reveal the magnetic enhancement mechanism of Miocene's Red clay during the Mid-Miocene and its paleoclimatic significance, 347 samples from 14 to 17.1 Ma are collected from QA-I loess-soil sequence in the Qin'an Area. In this paper, environmental parameters and rock magnetic measurements (thermomagnetic properties, hysteresis properties, ?ARM/SIRM, etc.) and color parameters of all samples are systematically analyzed. The results show that magnetic minerals of the loess-soil sequence are pseudo-single domain (PSD) magnetite, superparamagnetic/single domain (SP/SD) maghemite and hematite. More SP/SD maghemite presents in soil than in upper and lower loess layers. Magnetic characters of loess-soil samples before and after CBD (Citrate-Bicarbonate-Dithionite) treatment show that concentrations of detrital PSD magnetite of the loess-soil sequence are almost stable. The increase of pedogenic fine particle (SP/SD) maghemite concentrations is the main reason for the magnetic susceptibility enhancement of the loess-soil sequence during about 14.5 similar to 16.0 Ma. However, the redness of the loess-soil sequence correlated with a pedogenetic degree did not increase significantly during this period. This inconsistency indicates that these SP/SD maghemite minerals removed by CBD treatment may not be formed by local pedogenetic weathering. Low-temperature magnetic properties of samples before and after CBD treatment during about 14.5 similar to 16.0 Ma show that SP/SD maghemite may be the weathering crust of detrital magnetite particles, and their variations are consistent with the median grainsize of samples during about 14.5 similar to 16.0 Ma. These phenomena reveal that these magnetic particles with SP/SD maghemite possibly are formed in the source area, and their contents in loess-paleosol are mainly controlled by the strength of the wind. Although magnetic susceptibility of Miocene paleosol samples can reflect the pedogenic degree compared with the upper and lower loess layers, the variations of SP/SD maghemite contents possible are controlled by the local pedogenic degree and the input of the source area, resulting in a complex relationship between magnetic susceptibility and the local pedogenic degree. Therefore, the long-scale variation in magnetic susceptibility of Miocene loess-paleosol sequence cannot reflect the evolution of the East Asian Summer Monsoon.