Multiple rapid polarity swings during the Matuyama-Brunhes transition from two high-resolution loess-paleosol records

Two high-resolution magnetic records of the Matuyama-Brunhes (M-B) transition have been obtained from Xifeng and Baoji loess-paleosol sequences in the Chinese Loess Plateau. The results of stepwise thermal and alternating field (AF) demagnetizations confirm that both treatments are effective in isolating characteristic remanent magnetization (ChRM) from specimens of loess-paleosol sediments which suffer from weak pedogenesis, and that AF demagnetization clearly fails to isolate ChRM from specimens of paleosol sediments which have undergone strong pedogenesis. On the basis of virtual geomagnetic pole latitudes crossing the virtual geomagnetic equator, the M-B transition involves at least 15 rapid polarity swings. Rock magnetic experiments and anisotropy of low-field magnetic susceptibility show that high-frequency polarity swings during the M-B transition cannot be attributed to rock-magnetic and/or sedimentary causes. Extrapolating a constant accumulation rate between the Jaramillo-Matuyama and M-B boundary ages, the M-B transition interval of both the Xifeng and Baoji sections is estimated to have a duration of more than 10 kyr. Durations of each short polarity interval are estimated to range from about 0.3 to 5.6 kyr. Our new results, combined with previous paleomagnetic observations from lavas and marine sediments, suggest that multiple rapid polarity swings occurred during the M-B transition.