The geomagnetic field originates from the convection of iron-rich fluids in the outer core of the Earth, and is a window into the internal structure and dynamics of the Earth. The geomagnetic field can shield some charged energetic particles and cosmic rays, protecting life on Earth. It is of great significance to study the variation law of the geomagnetic field to understand the dynamic mechanism of the deep earth, the surface processes and the environment. Sediments with high sedimentation rates (e.g., loess profiles, marine and lake sediment drill cores, etc.) are ideal materials for high-resolution long-term geomagnetic field variation (PSV) studies. In the past 20 years, almost all of the published long-term geomagnetic field variation data in East Asia have been obtained through drilling cores from marine or lake sediments, and there is a lack of absolute change records of magnetic declination data, and the resolution of some studies is low. In terms of spatial distribution, the long-term variation of the geomagnetic field in East Asia is mainly studied from Japan and China, and the study area is mostly concentrated in the middle and low latitudes, and there are few regions above 35°N (Fig. 1). The above situation restricts the comprehensive and systematic understanding of the geomagnetic field variation law in East Asia.
Fig.1 Distribution of relevant PSV records and folk music profiles in East Asia
Fan Yunchang, a Ph.D. student from the Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, under the guidance of Prof. Yongxin Pan and Associate Prof. Shuhui Cai, cooperated with Prof. Chenglong Deng from the State Key Laboratory of Lithospheric Evolution and Francisco Javier from the Complutense University of Madrid, Spain Associate Professor Pavón-Carrasco and researcher Xiong Jianguo of the Institute of Geology of the China Earthquake Administration conducted a long-term study on the long-term variation of the geomagnetic field in the Holocene loess-paleosoil high sedimentation rate profile (Minle profile) in northwest Gansu Province, obtained the absolute change record of the direction (declination and dip) of the Holocene geomagnetic field in this area, and established a reference curve for the long-term variation of the East Asian geomagnetic field in the past 14 ka and a model of the change of the geomagnetic field direction in the East Asia region since 14 ka.
The magnetic results of the Minle profile showed that the magnetic mineral species of the section were dominated by low-coercivity single-domain pseudo-single-domain magnetite, and contained a small amount of maghematite. After the system alternating demagnetization and thermal demagnetization tests, most of the samples were separated from the stable characteristic remanence direction, and the consistency of the two demagnetization methods was good (Fig. 2), indicating that the results of the folk music profile had good reliability. Compared with other latitude records in East Asia and the prediction results of the global model, the results of the folk music profile reveal more detailed changes, indicating that the temporal resolution of the profile is higher, and there are obvious and comparable characteristic peaks and valleys in different recorded periods, as well as inconsistent change periods (Fig. 3), indicating that the geomagnetic field change in East Asia has been dominated by the dipole component since 14 kA, and the influence of the non-dipole component in some periods has been significantly enhanced.
Fig.2 The demagnetization results of the folk music profile samples and the related magnetic parameters vary with depth and age
Fig.3 Comparison between the PSV records of folk music profiles and the existing records and model predictions in East Asia
At the same time, they systematically sorted out and screened the published long-term variation data of sedimentary records in East Asia, combined with the new folk music profile data, calculated the long-term variation reference curve of East Asian sediment records in the past 14 ka (EASed14k) (Fig. 4), and established a long-term change model of geomagnetic field direction in East Asia since 14 ka (SCHA. EAS14k) (Figure 5). The Holocene high-precision long-term variable record of geomagnetic field obtained from the loess profile can be used as a representative profile for regional results comparison, and the established East Asian PSV reference curve and regional geomagnetic field model provide a basis and basis for regional and global geomagnetic field comparison.
Fig.4 The EASed14k reference curve and the East Asian PSV ensemble curve, Chinese archaeological magnetism PSV curve, SCHA. Comparison of prediction results of EAS14k region model
Fig.5 SCHA. The regional distribution of geomagnetic field direction of 4.35 ka, 9.15 ka, and 11.55 ka predicted by the EAS14k model. The white pentagram is the center of the model (30ºN, 108ºE) and the white circle is the position of the East Asian PSV record used to calculate the model
研究成果发表于国际权威学术期刊JGR: Solid Earth（樊耘畅，蔡书慧*，Francisco Javier Pavón-Carrasco，熊建国，邓成龙，潘永信. High-resolution paleomagnetic secular variation since ~13 ka from a loess section in Northwest China and a regional geomagnetic directional model for East Asia [J]. Journal of Geophysical Research: Solid Earth, 2024,129: e2023JB028094. DOI: 10.1029/2023JB028094.）。 研究受国家自然科学基金项目（42488201，42388101，41974077，41621004）、中国科学院地质与地球物理研究所重点部署项目（IGGCAS-201905）、中国科学院百人计划项目和国家第二次青藏高原科学考察项目（2019QZKK0704）联合资助。
Editor: Fu Shixu (East China Normal University)
Proofreading: Wan Peng