中国中部全新世古地磁长期变和相对古强度研究

A HIGH-RESOLUTION HOLOCENE PALEOMAGNETIC SECULAR VARIATION AND RELATIVE PALEOMAGNETIC INTENSITY RECORD FROM GONGHAI LAKE, CENTRAL OF CHINA

高分辨率的古地磁场长期变（Paleomagnetic Secular Variation，简称PSV）和相对古强度（Relative Paleomagnetic Intensity，简称RPI）研究是解决全新世湖泊研究定年难题的有效途径之一，也能为认识区域古地磁场的变化提供不可或缺的基础数据。然而，目前我国中部地区仍缺少年代序列可靠、时间分辨率在百年尺度的全新世PSV和RPI记录。先前的研究显示山西宁武公海全新世岩芯具有稳定的粉砂质粘土湖泊沉积，并通过25个（本文在全新世时段用到18个）陆生植物残体AMS14C测年建立了可靠的高分辨率年代序列，为重建高质量的全新世PSV和RPI记录提供了良好条件。本文通过特征剩磁磁倾角的变化建立了PSV序列，利用磁化率（χ）、非磁滞剩磁（ARM）、饱和等温剩磁（SIRM）等环境磁学参数对特征剩磁强度（NRM25mT）进行归一化，估算了RPI变化序列。结果表明，公海全新世湖泊沉积物的PSV和RPI记录与北半球其他湖泊和海洋记录具有较好的一致性，同时存在着显著的区域差异，可以作为我国中部地区湖泊沉积的对比曲线。

The Paleomagnetic Secular Variation （PSV） and Relative Paleomagnetic Intensity （RPI） are the behavior of dipole and non-dipole field of the Earth＇s interior. The PSV and RPI stacks can be applied for chronological study and regional stratigraphical comparation as the similar feature of dipole field of the earth. However, few of highresolution Holocene PSV and RPI records from central of China were reported, which hinders the understanding of the geomagnetic field variation and relationship between lacustrine sediments and climate change during the Holocene. In this study, a high-resolution paleomagnetic investigation was undertake in core GH14B from Gonghai Lake （38°54＇30.50＂N, 112°14＇05.80＂E; 1858m a. s. l. and water depth 9.3m）, Shanxi Province, central of China, for the purpose of acquiring direction and intensity of the geomagnetic field. This study is an effective method to solve dating problem of Holocene lacustrine sediments in central of China, which can also provide basic data for regional paleomagnetic field variation. The previous studies show that the dominant magnetic carrier is magnetite in the pseudo-single-domain （PSD） grain size range and with a relatively small change in concentration, thus is suitable for recording PSV and RPI. We obtained a sediment core （GH14B） from the central part of Gonghai Lake using a UWITEC Piston Corer and focus here on the upper 7.24m which belongs to the Holocene （11.5ka B.P.）. Gonghai Lake lacustrine sediments contains stable, continuous and homogeneous finegrained silty clay. The detailed description of the lacustrine sediments as follows：0-2.4m, brownish grey silty clay; 2.4-4.5m, light brownish grey mud; 4.5-7.1m, dark brownish organic mud; 7.10-7.24m, brownish yellow silt. There are 18reliable accelerator mass spectrometry （AMS） 14C data which are based on terrestrial plant macrofossils used for absolute age control. Total 159samples were selected using the plastic box in 8cm 3every 4cm interval which were used for the measurements of magnetic susceptibility （χ） and alternating demagnetization, and total 77samples were used for the measurements of anhysteretic remanent magnetization （ARM） and saturation isothermal remanent magnetization （SIRM） every 8cm interval. The relative paleomagnetic intensity proxy （RPI） was established by normalizing the natural remanent magnetization （NRM25mT） with the magnetic susceptibility （χ） after power spectrum tests of RPI and normalization parameters （χ, ARM and SIRM）. Here we present the vector paleomagnetic field reconstruction since Holocene. The new record is compared with other lacustrine and marine records in the Northern Hemisphere, revealing the generally consistent variabilities to certain extent, however, the obvious spatial differences still exist among the different regions. In addition, our results suggest that it does not exist geomagnetic excursion at least during the Holocene in central of China. Therefore, the PSV and RPI curves from Gonghai Lake are potential tools for stratigraphic correlation and further chronological framework of lake sediment sequence in central of China. The discrepancy in amplitude and pattern among the different regional records may be ascribed to the existence of non-dipole field which comes from magnetic flux lobes at the core-mantle boundary.