High-frequency polarity swings during the Gauss-Matuyama reversal from Baoji loess sediment

Paleomagnetic records of the Gauss-Matuyama reversal were obtained from two loess sections at Baoji on the Chinese Loes Plateau. Stepwise thermal demagnetization shows two obvious magnetization components. A low-temperature component iso lated between 100 and 200–250℃ is close to the present geomagnetic field direction, and a high-temperature component iso lated above 200–250℃ reveals clearly normal, reversed, and transitional polarities. Magnetostratigraphic results of both sec tions indicated that the Gauss-Matuyama reversal consists of a high-frequency polarity fluctuation zone, but the characteristic remanent magnetization directions during the reversal are clearly inconsistent. Rock magnetic experiments demonstrated tha for all the specimens with normal, reversed, and transitional polarities magnetite and hematite are the main magnetic carriers Anisotropy of magnetic susceptibility indicates that the studied loess sediments have a primary sedimentary fabric. Based on virtual geomagnetic pole latitudes, the Gauss-Matuyama reversal records in the two sections are accompanied by 14 short-lived geomagnetic episodes(15 rapid polarity swings) and 12 short-lived geomagnetic episodes(13 rapid polarity swings), respec tively. Our new records, together with previous ones from lacustrine, marine, and aeolian deposits, suggest that high-frequency polarity swings coexist with the Gauss-Matuyama reversal, and that the Gauss-Matuyama reversal may have taken more than11 kyr to complete. However, we need more detailed analyses of sections across polarity swings during reversals as well a more high-resolution reversal records to understand geomagnetic behavior and inconsistent characteristic remanent magnetiza tion directions during polarity reversals.

Contrasting magnetic properties from two loess/paleosol sections in Gansu Province, west China

We made comparison of magnetic property between two loess/paleosol sediments from theBeiyuan section, Linxia and the Shajinping section, Lanzhou. The two sections apart only 100 km inNE-SW direction. At the Beiyuan section, enhancement of the low-field magnetic susceptibility, ARMsusceptibility and SIRM are observed in paleosols. The positive correlations among these threeparameters suggest the presence of two magnetic components, i.e. the background component and theenhanced component. The unblocking curves of low-temperature IRM give strong evidence for the ideaof the two components. On the contrary, the magnetic parameters are almost constant throughout theShajinping section. The enhanced component is not found in the Shajinping section. The magneticproperties of the background component are indistinguishable between the two sections. Contrastingmagnetic properties imply the difference of soil formation process between the two sections.