Middle Matuyama Geomagnetic Excursions From High-resolution Loess-paleosol Recorded In Xifeng

The geomagnetic field is the basic physical field, and it has been reversed many times during the past geological history. The geomagnetic excursion is an important systematic and distinct component of the Earth’s magnetic field variability between geomagnetic reversals. Recent paleomagnetic studies have recognized numerous geomagnetic excursions that occurred in the Brunhes Chron, in contrast, the number of papers reporting on the Matuyama Chron is much smaller. To clarify the reason of fewer occurrences in the Matuyama Chron, a detailed paleomagnetic and rock-magnetic investigation on the middle Matuyama loess-paleosol sediments has been carried out at the Xifeng section, Gansu province. Stepwise thermal demagnetization shows two obvious magnetization components. A low-temperature component isolated between 100 and 200℃ is close to the present geomagnetic field direction, and a high-temperature component isolated between 300 and 600℃ reveals clearly normal, reversed, and transitional polarities. Our new magnetostatigraphy reveals six normal or mixed polarity intervals within the Matuyama reverse Chronozone. The uppermost and lowest normal polarity zones are consistent with Jaramillo and Olduvai subchronozones, respectively. Other four mixed polarity zones tentatively named as E1, E2, E3 and E4 geomagnetic excursions, have been recognized in the L13, L14, L15, and S22, respectively. Results of the anisotropy of low-field magnetic susceptibility show that the loess-paleosol sediments retain the primary sedimentary fabric, implying that the intervals with mixed polarity directions are not caused by rock magnetism and sedimentary structure disturbance. Rock magnetic experiments reveal that magnetite, and maghemite are the main magnetic carriers in the loess-palsosol sediments, and that the sediments from the mixed polarity intervals have the same magnetic characteristics as those from the surrounding normal and reversed polarity intervals. Assuming a constant accumulation rate between the lower Jaramillo and upper Olduvai polarity boundaries,the midpoint age of these four excursions are estimated to be at about 1.11 Ma(E1),1.18 Ma(E2), 1.22 Ma(E3), 1.56 Ma(E4), respectively. Comparing with previouslyreported geomagnetic excursions in the Matayuma chron, the E1, E2, E3 and E4 geomagnetic excursions can be correlated with the Punaruu, Cobb Mountain, Bjorn,and Gilsa geomagnetic excursions, respectively. Our high-resolution magnetostratigraphic result supports that the geomagnetic excursions is likely global occurrence, and that the short-lived geomagnetic excursions may also be a basic characteristic of the geomagnetic field during the middle Matuyama Chron.