◎ 张卫国

　　环境磁学是20世纪70年代中期发展起来的一门介于地球科学、环境科学和磁学之间的新兴交叉学科。尽管其历史不长，它的发展却异常迅速，在样芯对比、物质来源鉴别、泥沙运移示踪、流域生态环境演替、古气候和古环境研究、土壤发生学、环境污染、油气勘探、古地磁学等领域得到了广泛应用。某些参数，如磁化率，已被视为全球变化研究的重要指标。
在环境磁学应用领域不断扩展的同时，环境磁学参数解释的多解性问题也引起了重视。阐明磁性矿物的变化机制，及其在磁性参数上的体现，是当前环境磁学研究难点和重点领域之一。本文的研究正是朝着这一方向的努力之一。选择长江口潮滩沉积物，开展环境磁学研究工作，主要基于下述理由：
首先，环境磁学研究对象大量集中以湖泊、海洋沉积物和黄土为对象的古气候、环境演变等领域, 潮滩是环境磁学研究较少涉足的环境系统。
其次，潮滩物源多样，动力环境复杂，沉积物粒度组成差异较大，早期成岩作用也较为显著，为研究不同环境因素对沉积物磁性特征的影响提供了较好的材料。
再次，潮滩是人类活动影响强烈的地区，往往受到不同程度的污染。环境磁学近年来在环境污染领域的应用得到了广泛的重视，并成为当前环境磁学研究的前沿领域之一。但已有的研究中，污染物的来源较为单一，如钢铁厂污水排放口附近的沉积物，或高位沼泽（污染物主要来自大气输入），在潮滩开放的环境系统中，磁性参数能否作为污染物指示指标？如果能，其内在机制是什么？
本文通过潮滩沉积物磁性测量、粒度分析和地球化学分析，拟回答下述问题：
长江口潮滩沉积物的磁性特征及其反映的磁性矿物类型、晶粒大小和物质来源;
沉积物磁性特征变化的因素是什么？粒度和早期成岩作用的影响是如何体现的？
磁性特征在重金属污染研究中的应用机制是什么？它可以发挥什么作用？
为较为完整地反映潮滩沉积物的磁性特征，在已有的研究基础上，着重选择了五个站点，分别是位于长江口南岸边滩紧邻排污口的石洞口、白龙港，位于郊县的东海农场和崇明东滩东旺沙，以及刚刚成陆的九段沙。在每一站点分别采集了柱状样及表层样（0~5 cm）。样品采集主要在高潮滩或相应部位进行，因为高潮滩物质通常最细，污染物质也最为富集。
所有样品进行了室温下的系列磁性测量，根据测量结果，选择代表性样品进行进一步的磁学、矿物学、地球化学、粒度和同位素分析。磁性测量包括低频（0.47 kHz）和高频
（4.7 kHz）磁化率（ ）、非滞后剩磁（ARM，交变磁场峰值100 mT, 直流磁场0.04 mT）、不同磁场强度下获得的等温剩磁（IRM）、低温磁化率、磁滞洄线和热磁分析。对部分样品进行了磁性矿物提取和电镜分析。化学分析包括重金属（Cu、Zn、Pb、Cr和Fe、Mn）"总量"（浓HNO3、HCl提取，非严格的总量）、形态（三步提取法，可还原态、可氧化态和残渣态）、氧化铁（连二亚硫酸钠－柠檬酸钠－重碳酸钠（DCB）提取和草酸铵（AOD）提取分别估计总氧化铁和无定形氧化铁）、总硫和有机质，并比较了DCB提取前后样品磁性特征的变化。粒度采用激光粒度分析仪进行，并进行了典型样品粒级分离，并用于磁性测量。210Pb分析应用 谱仪进行。
主要结论如下：
与磁性矿物含量有关（ 、饱和等温剩磁SIRM、软剩磁SOFT、硬剩磁HIRM、非滞后剩磁磁化率 ARM）参数空间变化较大，而频率磁化率 fd%、 ARM/ 、 ARM/SIRM、SIRM/ 、S-100 mT、F300mT、剩磁矫顽力Bcr等反映磁性矿物的晶粒特征和类型的参数变化较小，显示了潮滩沉积物磁性特征的差异性和共性。磁铁矿主导了长江口潮滩沉积物的磁性特征。磁铁矿晶粒以假单畴（PSD）－多畴（MD）为主。除了碎屑成因磁铁矿外，在磁性最强的石洞口沉积物中还出现了球状磁性颗粒，与化石燃料（如煤）高温燃烧过程中生成的球状磁铁矿相似，主要系人类活动的产物。
沉积物粒度组成是影响长江口潮滩沉积物磁性特征的重要因素。研究区沉积物主要属于粉砂。磁性参数与粒级组成的相关分析表明，磁性矿物的大小，以<32 m为主，其中细晶粒单畴（SD）、超顺磁（SP）磁铁矿在< 4 m粒级中富集，而较粗的磁铁矿（MD）在8～16 m粒级中富集。鉴于 ARM 、 fd%与粘土含量的高度相关性，因此可将它们视作沉积物粘土含量的代用指标。无论细晶粒磁铁矿的成因如何，其在粘土中富集是普遍现象。
长江口潮滩沉积物中无定形铁占游离氧化铁67 %以上。相关分析表明，与亚铁磁性矿物含量有关的参数（*、SIRM、*ARM、*fd%）和游离氧化铁、无定形铁之间存在着显著的相关性，尤其是与无定形铁的相关性更为显著。结合DCB测量，估算磁铁矿占游离氧化铁10 %左右，只是氧化铁中的一小部分。而顺磁性氧化铁约占氧化铁的68%。
样品DCB提取前后的比较发现，*、SIRM和SOFT绝对下降量与Fe-DCB呈显著正相关，因此，这些参数的变化量可作为游离氧化铁含量的一个指示指标。粒级分离基础上的结果表明，DCB提取能有效地溶解<1 m的细晶粒磁铁矿，不仅如此，还能溶解相当一部分粗晶粒磁铁矿。值得注意的是，<2 m粒级中，相对磁铁矿而言，DCB提取后不完整反铁磁性氧化铁含量相对上升，这可能与该粒级中氧化铁总量较高，DCB溶解不完全有关。尽管Fe-DCB被视作游离氧化铁的含量，实际上仍有一部分氧化铁未能溶解，这里主要因素有二，一是氧化铁的颗粒大小，二是氧化铁的含量。通过测量DCB提取前后磁性的变化，可以监测样品的提取效率。
长江口潮滩沉积物有机质含量为0.28~1.34 %，平均为0.93 %。Fe的含量为1.57~4.02 %，平均为2.45 %。总硫的含量为85～1109 mg/kg，平均为341 mg/kg。有机碳与总硫的重量比（C/S）为6.9~44.1，平均为17.8，这一比值远高于正常的海洋沉积物（2.8），显示了淡水沉积物的特征。总体而言，沉积物氧化程度较高，硫酸盐还原在有机质降解中的作用是微弱的。随深度的增加，氧化铁的还原在磁性特征的垂向变化得到了反映。所有柱样，S-100随着深度的增加而升高，Bcr、SIRM/ 则成反向变化趋势。这一特征反映了，随深度增加，沉积物中不完整反铁磁性氧化铁发生还原，导致含量的相对下降和磁铁矿含量相对上升。除石洞口站点外，磁铁矿还原在长江口潮滩沉积物中是微弱的，与该区沉积物偏碱性，沉积物还原程度较弱以及磁铁矿晶粒较粗有关。
除石洞口站点外，长江口潮滩沉积物重金属元素含量与前人报道的长江口沉积物及悬浮颗粒物中的重金属含量相近，与大陆页岩的元素含量相近。除石洞口站点外，重金属元素Cu、Zn、Pb、Cr以残渣态和铁锰氧化物态为主，铁锰氧化物很大程度上控制了Cu、Zn、Pb、Cr的地球化学循环。石洞口沉积物重金属残渣态比例要低于其他站点，有机-硫化物态要高于其他站点，因此，石洞口柱样上部（50 cm以上）沉积物重金属含量升高显然是污染所致，这与石洞口排污口及附近的发电厂、钢铁厂等大型工业企业有关。
除石洞口站点外，重金属含量的变化与 ARM、 fd%、 ARM/ 和 ARM/SIRM具有相似的垂向变化特征，而和 、SIRM的差异较大。 ARM等参数与重金属含量的密切相关性反映了沉积物粒度组成及细晶粒氧化铁、有机质对重金属含量的影响，即细晶粒（SD、SP）亚铁磁性矿物含量较多的沉积物中重金属含量较高，而与磁性强弱的变化并非同步。在选择重金属含量的替代指标时，应视具体情况而定。当沉积物中重金属以铁锰氧化物态为主时，磁参数 ARM可作为重金属含量粒度校正的指标。
磁性测量快速、简便，对样品不具破坏性，作为一种手段，与地球化学、矿物学手段相结合，可在沉积物地球化学和环境化学研究中发挥重要作用。

　

◎Wei-guo ZHANG

　　Environmental magnetism is a new subject which develops on the interplay of geoscience, environmental science and magnetism since mid-1970s. In spite of its short history, it develops rapidly and is widely used in core correlation, material source identification, sediment tracing, ecosystem evolution, paleocliamtic and paleoenvironmental study, pedology, environmental pollution, oil and gas exploration and paleomagnetism. Some magnetic parameters, such as susceptibility, have been regarded as important proxies in the study of global change.
During the expansion of applications of environmental magnetism, the problem of multi-choices in the explanation of magnetic parameters receives great attention. To clarify the mechanism of magnetic mineral changes and its characterization by magnetic parameters, is one of the key areas of the present environmental magnetic study. This thesis is an attempt to such goals. The reason for selecting tidal flat sediments of the Yangtze Estuary as the research subject is as follows:
Firstly, most of the environmental magnetic studies focus on lake sediments, ocean sediments and loess for the purpose of paleoenvironmental and environmental evolution study. Tidal flat is an environmental system less touched.
Secondly, tidal flat is characterized by multiple sediment sources and complicated hydrodynamics, where sediments show significant variations of particle size composition. Early diagenesis is also active here. It therefore provides ideal material for studying the impact of different factors on magnetic properties.
Thirdly, tidal flat is heavily influenced by human activities and often contaminated with various degrees. Applying environmental magnetism to the study of environmental pollution is highly appreciated and forms one of the research frontiers. However, in previous studies, the source of pollutants is relatively simple, e.g. sediments near the outfall of steel works, or ombrotrophic peat (pollutants derived mainly from atmosphere). In an open system like tidal flat, can magnetic parameters be used as indicators of pollutants? If so, what is the mechanism?
Combining magnetic measurements with particle size and geochemical analyses, this thesis aims to address the following questions:
1. Magnetic properties of tidal flat sediments of the Yangtze Estaury, and the type, grain size and source of magnetic minerals.
2. The factors influencing the magnetic properties of the sediments, and the role of particle size and early diagenesis.
3. The mechanism of applying magnetic properties to the study of heavy metal pollution, and what role can environmental magnetism play?
To characterize the magnetic properties of tidal flat sediments fully, five sites were selected based on previous studies, which were Shidongkou and Bailonggang at the southern bank of the estuary, Donghai and Dongwangsha in the rural area of Shanghai and emerging Jiuduansha shoal. At each site, core and surface samples （0~5 cm）were taken. Sampling was mainly carried out at higher tidal flat, where the sediments tend to be finest and therefore contain higher concentrations of pollutants .
All samples were subjected to magnetic measurements at room temperature. Based on the results of magnetic measurements, representative samples were selected for further magnetic, mineralogical, geochemical, granulometric and radioactive 210Pb analyses. Magnetic mesaurments at room temperature include magnetic susceptibility ( ) at low (0.47 kHz) and high (4.7 kHz) frequency. Anhysteric Remanent Magnetsization (ARM, peak value of alternating field is 100 mT and DC bias field is 0.04 mT) and Isothermal Remanent Magnetization (IRM) induced at various fields, low temperature susceptibility, magnetic hysteresis loop and thermomagnetic analysis. Some of the samples were subjected to magnetic mineral extraction and electronic microscope analysis. Geochemical analyses include 'total' (concentrated HNO3 -HCl digestion) and chemical forms (three step sequential digestion, reducible, oxidizable and residual forms) of heavy metals Cu, Zn, Pb, Cr,Fe and Mn, iron oxides (dithionite-citrate-bicarbonate (DCB) and ammonium in the dark (AOD) extraction as the estimation of total and amorphous iron oxides, respectively), total sulfur and organic matter. Variations of magnetic properties before and after DCB treatment were also monitored. Particle size analysis was carried out with laser granulometer, and tpical samples were separated into distictive size groups for magnetic measurements. 210Pb analysis was conducted with low background gamma counting.
The main conclusions are as follows:
Despite the inter- and intra-sites variability of concentration-dependent magnetic parameters, e.g. susceptibility ( ), saturation isothermal remanent magnetization (SIRM), soft remanece (SOFT), hard remanece (HIRM), susceptibility of anhysteretic remanent magnetization ( ARM), parameters fd%, ARM/ , ARM/SIRM, SIRM/ , S-100 mT, F300mT and Bcr indicate a similar magnetic mineral type and grain size. Magnetite, pseudo single domain (PSD)-multi domain (MD) in size, dominates the magnetic properties of the intertidal sediments in the study area. The ferrimagentic mineral is mainly detrital in origin, but in the magnetic enhanced sediments at site Shidongkou occur some spherical magnetic particles derived from anthropogenic sources.
Particle size strongly influences the magnetic properties of the sediments in the study area. The sediments belong mainly to silts. It is found that the size of the magnetic minerals are less than 32 m in general. Single domain (SD)/superparamagnetic (SP) magnetic grains reside primarily in the clay fraction while coarse MD grains enriched in the fraction of 8～16 m。Considering the strong positive relationships between ARM, fd% and the clay fraction, this two parameters can be used as proxies for clay content. It is believed that, whether fine magnetite being bacterial or detrital, its enrichment in the clay fraction is a common feature.
In the study area, amorphous iron oxides represent 67% of total iron oxides on average. Magnetic parameters *, SIRM, *ARM, *fd% show strong relationships with amorphous iron oxides, and to a less degree with total iron oxides. Combined with dithionite-citrate-bicarbonate (DCB) extraction, it is estimated that iron bounded with magnetite occupies only a small portion of total iron oxides (10 % on average). Paramagnetic iron oxides are the dominant ones, with a mean value of 68 % of total iron oxides.
Comparison between the pre- and post-DCB extraction reveals that the losses of *, SIRM and SOFT correlate positively with total iron oxides (Fe-DCB)，and therefore the changes in these parameters can be used as indicators of total iron oxide content. Results on sized samples show that DCB extraction can dissolve finer magnetite less than 1 m. Furthermore, it can dissolve a part of coarse MD magnetite as well. It is of note that the proportion of imperfect anti-ferromagnetic iron oxides in the fraction of <2 m increase relative to magnetite after DCB treatment. This may be related to the incomplete dissolution of iron oxides due to higher concentrations in this size. Although Fe-DCB is commonly regarded as the content of toal iron oxides, part of iron oxides remains untouched during DCB extraction. show aDCB diBoth magnetic grain size and concentration exert strong influence on DCB extraction efficiency. The main reasons are twofolds, one is the grain size of iron oxides, the other is the content of iron oxides. Comparison of magnetic properties before and after DCB treatment can monitor the efficency of DCB extraction.
Organic matter content in the intertidal sediments ranges between 0.28 % and 1.34 % with a mean value of 0.93 %. Fe concentration varies from 1.57 % to 4.02 % with a mean value of 2.45 %. Total S varies from 85 mg/kg to 1109 mg/kg with a mean value of 341 mg/kg. The ratio of organic carbon to total S is 6.9~44.1 in range, with a mean value of 17.8, which is significantly higher than the value (2.8) of normal marine sediments. Such a higher C/S ratio is typical of freshwater sediments. In general, the sediments are under a relatively oxidizing environment and the role of sulfate reduction in organic matter degradation is minor. With the increase in depth, iron oxide reduction takes place which is reflected in the profiles of magnetic properties. In all cores, S-100 increases downward while Bcr and SIRM/* show the opposite trends, suggesting a relative decrease of imperfect anti-ferromagnetic iron oxides with depth.. Except for site Shidongkou, magnetite diagenesis is not active, which may related to the alkaline (pH 7~8), moderate reducing environment as well as the relative coarse size of magnetite.
Except for site Shidongkou, the concentrations of heavy metals are relative low , which are comparable to the reported values of suspended matters and sediments of the Yangtze Eatuary as well as the average values of shales. Except for site Shidongkou, heavy metals Cu, Zn, Pb and Cr reside mainly in the residual and Fe/Mn oxides bounded forms, and Fe/Mn oxides largely control the cycle of elements Cu, Zn, Pb and Cr. In sediments from site Shidongkou, the residual phase of heavy metals is lower than that of other sites, while metals bounded with organic matters and sulfides are much higher. Therefore, the rise of heavy metal concentrations in the upper 50 cm of core sediment at Shidongkou is due to the anthropogenic input, which is linked to the sewage outfall and big industrial enterprises nearby (power plants, steel works etc.).
Except for site Shidongkou, the concentrations of heavy metals correlate well with ARM, fd%, ARM/ and ARM/SIRM, but not so good with , SIRM as found elsewhere. The close links between ARM and heavy metals are suggestive of the association of the latter with clay minerals, iron oxides as well as organic matters. It means that sediments with higher concentrations of fine-grained (SD/SP) ferrimagnetic minerals contain more heavy metals. The changes in the concentration of ferrimagnetic minerals are not necessary related to heavy metal contents. The selection of magnetic proxy of heavy metal concentrations should consider the environmental settings. When heavy metals reside mainly in the Fe/Mn oxides bounded phase, ARM can be used to normalize heavy metal concentrations for particle size effect.
Magnetic measurments is a simple, rapid and non-destructive technique, its combination with conventional geochemical and mineralogical methods can play important roles in the studies of sedimentary geochemistry and environmental chemistry.

　　作者简介：张卫国，1971年生，男，华东师范大学2001年博士研究生，师从俞立中教授，主要从事环境磁学和环境地球化学的研究。现在华东师范大学河口海岸国家重点实验室工作。

（责任编辑：王苏娜）