Magnetic Properties of Recent Sediments at Tianxing Centralshoal of Wuhan: Implication for Sedimentary Environment Change in the Middle Reaches of the Yangtze River
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摘要: 利用多磁性参数(包括质量磁化率、频率磁化率、饱和等温剩磁、退磁系数以及热磁曲线)及磁组构分析，以长江中游武汉天兴洲近代河流沉积物为研究对象, 探讨了近100年以来沉积物磁性参数变化特征及其对长江中上游水文变化及气候环境的指示.结果表明，上部组合带(0.60~1.50 m)和下部组合带(2.40~3.30 m)沉积物的磁性载体以亚铁磁性矿物磁铁矿为主，同时存在少量较稳定的不完整反铁磁性物质，中部(1.50~2.40 m)亚铁磁性矿物相对较少，且剖面上部沉积物的超顺磁(SP)颗粒对沉积物χ贡献较大.整个沉积剖面自下而上磁性矿物含量呈“C”字型变化，反映了“软”-“硬”-“软”的磁性特性.沉积物磁组构组合带研究显示，历史时期沉积环境水动力强度、颗粒排列有序化程度以及长江流速发生了明显的变化，沉积环境水动力强度经历了“不稳定”－“稳定”－“不稳定”的变化过程，反映了长江古水文状况的变化.近代沉积物磁性特征变化格局与1900年以来以来长江中上游“暖湿”-“温凉”-“暖湿”气候环境变化和“强降水”-“弱降水”-“强降水”以及长江汉口站流量变化过程相吻合.这一研究成果为深刻认识历史时期长江流域气候环境变迁其及对长江古水文和沉积环境的影响提供了重要的参考资料.Abstract: Multi-proxy of environmental magnetism (such as mass susceptibility, frequency dependent susceptibility, saturation isothermal, demagnetizing factor and thermalmagnetic curve) and magnetic fabric parameters derived from the recent sediments at Tianxing Centralshoal of Wuhan are employed to reconstruct the history of paleo-hydrology and climate changes in the mid-upper reaches of the Yangtze River. The assemblage zones of multi-proxy of environmental magnetism indicate that both upper (0.60-1.50 m) and lower (2.40-3.30 m) zones are dominated by the ferromagnetic magnetite. The upper zone is marked by the superparamagnetic grains, which make greater contributions to the mass susceptibility. The percentage of antiferromagnetic minerals in the middle zone (1.50-2.40 m) increases in inverse proportion to the percentage of the ferromagnetic. Therefore, the depth variation of the magnetic mineral contents is like letter C reflecting the magnetic characteristics of "SOFT", "hard" and "SOFT" throughout the section. The analysis on the magnetic fabric parameters demonstrates that hydrodynamic intensity of sedimentary environment, ordering degrees of particle arrangement, and flow velocity of the Yangtze River have undergone significant changes in the past 100 years, which is related to the paleo-hydrological change of the Yangtze River. Variation patterns of magnetic properties in the recent sediments are consistent with the climate change and discharge change at Hankou Station. Especially, hydrodynamic intensity of sedimentary environment has changed from unstable to stable, then unstable, corresponding to the temperature and precipitation changes from hot-wet to cool-dry and hot-wet. Our research has provided significant evidence for further understanding the influence of climate change on the paleo-hydrology and sedimentary environment of the Yangtze River in the historical period.
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Key words:
- sediments /
- environmental magnetism /
- magnetic fabric parameters /
- geophysics
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图 1 长江中游武汉天兴洲地理位置及采样地点分布
Fig. 1. Map showing the sampling site at the Tianxing Centralshoal of Wuhan, the middle reaches of Yangtze River
图 2 长江中游天兴洲沉积物A剖面磁性参数随深度变化特征
Fig. 2. Depth variation of magnetic parameters of sediments at the Tianxing Centralshoal of Wuhan, the middle reaches of Yangtze River
图 3 代表性样品的χ-T曲线
粗(细)线分别代表加热(冷却)曲线
Fig. 3. χ-T variation curve of certain representative samples
图 4 代表性样品的SIRM充磁曲线(a)和AF退磁曲线(b)
Fig. 4. (a) SIRM charging magnetization curve of certain representative samples and (b) AF demagnetization curve of certain representative samples
图 5 长江中游天兴洲沉积物B剖面磁组构参数随深度变化特征
Fig. 5. Depth variation of magnetic fabric parameters in the sediments at the Tianxing Centralshoal of Wuhan, the middle reaches of Yangtze River
图 6 长江中游天兴洲沉积物磁化率张量主方向的赤平投影
方块、三角、圆点分别表示κ1、κ2、κ3的主方向
Fig. 6. Equatorial plane projection map of major direction of magnetic susceptibility tensor in the sediments at the Tianxing Centralshoal of Wuhan, the middle reaches of Yangtze River
图 7 Fs、κ1轴方位角随深度的变化与汉口站流量(Chen et al., 2001)和近100年长江上游降水(图 7d, A)与温度(图 7d, B)曲线(陈家其等，2006)对比(图 7a中虚线为现代流速)
Fig. 7. Azimuth angle variation of Fs and κ1 axle with depth and its correlation with discharge volumes at Hankou station (from Chen et al., 2001), and precipitation (Fig. 7d, A) and temperature (Fig. 7d, B) curves (from Chen et al., 2006), in the upper reaches of the Yangtze River during the past 100 years (Dashed line in Fig. 7a represents current flow velocity)
表 1 长江中游武汉天兴洲采样剖面A岩性
Table 1. Lithology description of sampling section A at the Tianxing Centralshoal of Wuhan, the middle reaches of Yangtze River
分层 岩性 采样点数/样品数 深度(m) 层厚(m) ① 耕土 1/3 0.20~0.60 0.40 ② 粉砂质粘土 6/18 0.60~0.95 0.35 ③ 粘土质细砂 6/17 0.95~1.25 0.30 ④ 粉砂质粘土 8/19 1.25~1.65 0.40 ⑤ 粉细砂 3/9 1.65~1.85 0.20 ⑥ 粘土质粉砂 2/6 1.85~1.95 0.10 ⑦ 粉细砂 14/40 1.95~2.65 0.70 ⑧ 粘土质粉砂 14/39 2.65~3.30 0.65 
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