Reliability Of The Matuyama-brunhes Geomagnetic Polarity Reversal In The Jingchuan Section

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. Chinese loess-paleosol sequences are excellent archives of terrestrial paleoclimate in different timescales, which can be correlated well with records of marine sediments. But there still remain inconsistencies in different Matuyama-Brunhes reversal(MB reversal) records in terms of their stratigraphic position at different localities, their duration, and the inferred morphology of the transitional field.In this study, L8 and S8 of Jingchuan loess section have been chosen to further clear some problems including magnetic mineral, process of MB reversal, and location of its boundary. We mainly focus on the rock magnetism and paleomagnetism of the loess stratigraphy, and then discuss the reliability of the MB reversal records. The main conclusions are as follows:1) Experiments of rock magnetism, M-T curves, IRM acquisition and demagnetization curves, hysteresis loops, Day plot were carried out. Rock-magnetic investigations carried on L8 and S8 show that the main magnetic minerals are multidomain(MD) magnetite and a small amount of hematite, maghemite;2) Based on the research of anisotropy of magnetic susceptibility(AMS), the sampled sediments have primary sediment fabric;3) Using progressive thermal demagnetization of 5 sets of parallel samples, the MB reversal is finally accurately determined with a thickness of 32.5 cm located in the transitional zone of L8 and S8;4) The characteristic remanent magnetization(ChRM) results from 5 sets of parallel samples within the MB transition are scattered, while parallel samples outside of the transition have consistent paleomagnetic directions. This difference is attributed to low efficiency alignment of detrital magnetite within a weak paleomagnetic field during the polarity transition. The morphology of five parallel specimens within the MB reversal are inconsistent, which indicates that Jingchuan section can hardly record the high-frequency polarity swings within the MB transitional zone, and that limitations associated with recording fidelity should always be considered when interpreting high-frequency paleomagnetic variations from the Chinese loess. Besides the low relative paleointensity during the MB reversal, the low sediment rate of loess dust which also can reduce the reliability of the paleomagnetic records, especially when the true paleomagnetic directions change rapidly.