基于硅酸盐中磁性矿物包裹体的长江和黄河沉积物区分

环境磁学指标已被用来区分中国近海沉积物中长江和黄河物质来源，但是该方法会受到潜在的成岩作用影响。硅酸盐中的磁性矿物包裹体不受成岩作用影响，具有可靠区分物源的潜力。本研究将盐酸（10 mol/L）处理后样品的磁性特征视作硅酸盐中磁性矿物包裹体的贡献，以提供区分长江和黄河沉积物的新指标。结果表明，盐酸处理后样品中与磁性矿物含量有关的参数（χ、SIRM、χ_ARM、HIRM）不到处理前样品参数值的10%；硅酸盐中的磁性矿物包裹体以低矫顽力的亚铁磁性矿物为主，相比原样中的亚铁磁性颗粒更细；盐酸处理后长江沉积物相比黄河，具有更高的SIRM、χ_ARM、HIRM值，表明前者包裹体中亚铁磁性矿物和不完整反铁磁性矿物含量更高。分粒级的磁学参数显示，两个河流沉积物磁性矿物包裹体SIRM与HIRM值随着粒级的增大而增大。由于长江支流众多，岩石类型复杂，包裹体磁性参数变化较大，而黄河物质主要来自于黄土高原的黄土，黄土由源区至高原的风力搬运过程中的分选和混合，使得包裹体磁性矿物含量较低，但磁性参数相对均一。本研究表明，利用硅酸盐中磁性矿物包裹体SIRM和S_-100散点图、磁性参数主成分分析和热磁曲线，可有效区分长江和黄河沉积物。

Distinguishing sediments from the Yangtze River and Yellow River using magnetic mineral inclusion within silicates

Environmental magnetic parameters have been widely used in China to differentiate sediment sources in coastal and marine deposits, specifically those from the Yangtze River and the Yellow River. It is important to consider the influence of early diagenesis on magnetic minerals, as they are sensitive to redox conditions. Magnetic mineral inclusions within silicates are not affected by diagenesis and can provide reliable source discrimination. In this study, we collected 19 samples from the Yangtze River(11 samples from the floodplain at Datong, Anhui Province) and the Yellow River(5 samples from the floodplain of the modern Yellow River Delta; 3 samples from the tidal flat of the Old Yellow River Delta). We extracted magnetic mineral inclusions from bulk and particle-sized sediment samples(i.e., < 16 μm, 16~32 μm, 32~63 μm, >63 μm) using published methods. The samples were treated with excess 10 mol/L HCl, heated in a water bath at 80℃ for 5 hours, and then allowed to react at room temperature for 19 hours. After treatment, the samples were centrifuged, acid-washed, and dried. The magnetic characteristics of the treated samples were attributed to magnetic mineral inclusions within silicates. The aim of this study was to develop a new approach for distinguishing sediments from the Yangtze and Yellow Rivers. The results showed that concentration-related magnetic parameters(χ, SIRM, χARM, HIRM) were reduced to less than 10% of the untreated bulk sediments after HCl treatment. The HCl-treated samples were nearly saturated in a 300 mT magnetic field, indicating a dominance of low coercivity ferrimagnetic minerals in the silicates. The parameters related to the size of magnetic mineral particles(χARM/SIRM and χARM/χ) increased significantly, indicating finer particles in the inclusions compared to the untreated samples. The treated Yangtze River sediments had higher SIRM, χARM, and HIRM values compared to the Yellow River sediments, indicating higher contents of ferrimagnetic and imperfect antiferromagnetic minerals in the inclusions. The variations of magnetic parameters with grain size showed that SIRM and HIRM of the inclusions in both river sediments increased with increasing grain size, indicating an increase in ferrimagnetic and imperfect antiferromagnetic mineral contents with larger grain sizes. Due to the numerous tributaries and complex rock types in the Yangtze River catchment, magnetic parameters of the inclusions showed a larger range compared to those of the Yellow River. The Yellow River sediments were mainly derived from the Chinese Loess Plateau, which was well-sorted and mixed during aeolian transport from Asian deserts and had a lower content of magnetic inclusions but relatively uniform magnetic parameters. This study demonstrates that scatter plots of SIRM(saturation isothermal remanent magnetization) and S-100, as well as principal component analysis of magnetic parameters of silicate inclusions and thermomagnetic analysis, can effectively distinguish sediments from the Yangtze and Yellow Rivers. On the SIRM-S-100 scatter plot, the S-100 values of the Yangtze River samples decrease significantly with increasing SIRM, indicating a correlation between higher coercivity and higher magnetic mineral content. The principal component analysis of magnetic parameters shows that the first principal component has higher loadings on SIRM, χARM(anhysteretic remanent magnetization susceptibility), and HIRM(hard isothermal remanent magnetization), while the second principal component has higher loadings on S-100 and S-300. The first and second components reflect the content and type of magnetic minerals, respectively. The Yangtze River samples have higher positive scores on both the first and second components compared to the Yellow River samples. The temperature-dependent magnetic susceptibility analysis reveals a clear Hopkinson effect in the < 16 μm fraction of the Yangtze River samples. These findings present a new, cost-effective, and efficient approach for source analysis of sediments in China's coastal areas.