Late Matuyama Geomagnetic Excursions From High-resolution Loess-paleosol Records In Xifeng

The Earth’s magnetic field is one of the most important component of the physical fields. The geomagnetic excursion, showing anomalous geomagnetic field directions that deviate considerably from the expected geocentric axial dipole field, is a distinct characteristic of the Earth’s magnetic field and can be documented in the Chinese Loess-paleosol sequences. In order to identify the potential geomagnetic excursions in the Late Matuyama C hron and obtain the detailed magnetic variability information, a detailed paleomagnetic and rock magnetic investigation has been carried out in high resolution from S7 to L12 at the Xifeng section, Gansu province. Stepwise thermal demagnetization displays two clear magnetization components, a low-temperature components isolated between 100-200 ℃ and consists with the modern geomagnetic field direction, a high- temperature components which decayed towards the origin isolated between 300-500℃ and shows obvious normal, reversed, transitional polarities. We found two abnormal direction of the characteristic remanent magnetization from 18.7-19.8 meter and 6.7-16.8 meter and tentatively named them XF-1 and XF-2. Rock magnetic investigation indicated that the two distinct anomaly have a same feature with the adjacent interval which include both normal, reversed, and transitional, and magnetic and maghemite are the main magnetic carriers. Anisotropy of magnetic susceptibility appears all the specimens have the same performance that maintain a primary sedimentary fabric. So, some conclusions can be drawn as follows:1. The abnormal direction of the characteristic remanent magnetization of XF-1 and XF-2 not because of magnetic minerals and(or) sedimentary disturbance but rather records of the true variability of the Earth’s magnetic field.2. XF-1 lay on the transitional zone of S9-1 and L9-2, and showed an absolute normal polarity characteristics. Assuming constant sedimentation rate between the Jaramillo-Matuyama and Matuyama-Brunhes magnetic polarity boundaries, the midpoint ages for XF-1 is estimated at 0.94 Ma. The excursion is probably correlated with the Santa Rosa excursion.3. XF-2 located at L9-1, thermal demagnetization results of the XF-2 showed complex feature. we divided XF-2 into two parts and named them anomaly 1 and anomaly 2, anomaly 1 appears almost 6 meter normal polarity interval and anomaly 2 displays random directions of the C haracteristic remanent magnetization. If we use the age model like XF-1, then the XF-2’age is 0.87 Ma, which consists with the Kamikatsura excursion, so it may present the Kamikatsura excursion.