琼州海峡及周边海域沉积污染特征及历史过程
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摘要
海洋自古以来便承载了人类无数的梦想,近代以来所出现过的数十个世界性大国无一例外均为海洋大国,近代以来中国人始终肩负着民族复兴之梦,而当前提出的海洋强国战略,经略海洋的方针政策更是将海洋开发推向了前所未有的高度。随着社会的发展,科学技术的日新月异,人们已越来越重视人类与地球的和谐发展,保护地球家园,保护海洋环境已成为了人类的共识,因此要开发海洋,发展海洋经济就必须先认识海洋,了解海洋才能做到人与海洋和平共处,使得新一轮海洋开发不会变成又一轮污染行径,最终达到可持续发展的目的。我国海洋开发的热潮自上世纪90年代兴起以来,极大地推动了沿海地区的经济发展,海洋开发已成为沿海地区新的经济增长点和地区发展战略,海洋经济在沿海地区的经济地位越来越重要。然而,在我国海洋经济快速发展的同时,也给海洋环境带来了极大的压力,如部分区域赤潮的发生、重金属的超标等给渔业、旅游业等带来了沉重打击,给人民生产生活及身体健康带来一定的威胁。因此,在发展海洋经济的同时也必须加强海洋环境保护,做到人与自然协调发展。
近年随着食品安全事故频发,引发人们对食品安全的普遍担忧,“镉大米”事件等重金属污染的报道也屡见报端,公众对重金属污染的状况也越来越关注。海洋污染具有污染源广,污染物质种类多,影响范围大,危害深远,控制复杂,治理难度大的特点。在各种污染物中,又以重金属的危害面大、持续时间久而受到人们的格外关注,随着沿海经济的发展和城市化进程的加剧,大量含重金属等有毒物质的废水排放,使河口生态环境遭到严重破坏,其中重金属污染物不易降解,能迅速由水相转入固相(悬浮物和沉积物),最终进入沉积物中蓄积,使沉积物成为重金属等化学物质的主要存储库。由于重金属的积聚会影响到生态安全,并最终威胁到人类健康,因此对海底重金属的分布及迁移规律的研究便显得十分重要。
当前对重金属的研究主要采用化学测定的方式,该方法能够直接确定重金属的含量,然而测定周期长,费用高,样品处理过程中还存在一定的污染,而且测试工作必须在实验室完成。环境磁学是近几十年发展起来的一门介于地球科学、环境科学和磁学之间的边缘学科,可提供大量有关区域或全球环境变化及人类活动对环境影响等的重要信息。由于环境磁学测试具有快速、经济、方便、无污染及无损样品的特点,部分参数(如磁化率)甚至可实现现场测试,因此,一直以来受到国内外众多学者的推崇,已有学者将其运用到重金属的污染监测之中。
琼州海峡是中国的三大海峡之一,东西长度约80公里,南北平均宽度近30公里,不仅是沟通北部湾和南海中、东部的海上走廊,同时也是广东至海南岛和越南等地的海上交通捷径。随着经济发展,该海区的海洋活动也越来越频繁,主要表现为渔业生产、航运发展、海底管道铺设及今后即将建设的跨海大桥等,在今后的区域经济建设中,琼州海峡将发挥其越来越重要的角色。然而当前对琼州海峡海洋环境的认识却并不高,对琼州海峡成因历史、沉积环境演变、沉积物类型及分布规律、沉积物来源及运移方向、重金属污染现状、重金属来源及成因历史等问题并没有深刻认识,本论文即首次运用目前最详细的调查资料及多种学科研究方法来综合分析琼州海峡及周边海域海洋环境特征,系统的阐述了该区域沉积物来源及运移趋势,深入认识该区域全新世以来海洋沉积环境演变,详细分析了该区域重金属分布现状及重金属历史沉积变化,并对重金属来源进行了分析,还首次将环境磁学的研究方法引入到该区域,分析了磁性特征与重金属污染的关系,判断污染物在琼州海峡及周边海区的环境磁学响应机制,为该区域海洋环境监测新手段的开拓提供了新的方向。
本论文共选取了琼州海峡及周边海域共计500多个表层站位及近20个柱状样进行了分析,其中表层样间距为5km,柱状样深度大多在1到2米之间。通过多种方法的综合分析,得出了以下认识：
粒度分析的结果表明琼州海峡内沉积物类型以砂及砾砂为主,由海峡中部向口外逐渐过渡到更细粒的粉砂及粘土质粉砂。琼州海峡东西口水动力条件不一致,在海峡东部,砂及粉砂沉积物界限为半弧形,沉积物类型变化较快,迅速由粗粒转变为了细粒沉积物,塑造了海峡东口的弧状潮流三角洲,说明海峡东口外存在较强的水动力,抑制了海峡内潮流作用的向外扩张；在海峡西部,砂及粉砂界限为不规则鸡爪状,形成了海峡西部指状三角洲,沉积物过渡较为平缓,说明海峡西口外水动力较弱,潮流作用向西部延伸较远,粗粒物质也能被输送到较远的区域。琼州海峡内沉积物粒径多在-0.73φ—7.06φ-之间,东西口粒径分布存在较大差异,分选性东部优于西部,说明东部水动力较稳定,研究区域主要存在5种峰态类型,反映不同区域沉积物来源组成不一致。
由主量元素分析可知,琼州海峡沉积物主要由SiO2、Al2O3、Fe2O3、CaO、K2O、MgO及Na20组成,这7种氧化物占沉积物总量的90%以上,该区域沉积物元素分布与粒径具有一定的相关性,反映了该区域“元素的粒度控制律”。
选用0.125mm～0.063mm粒级的碎屑矿物进行了矿物鉴定,结果表明该区域含金属矿物平面分布具有其独自的特征,部分含金属矿物的平面分布特征也大致为海峡中部低,两端高的特点,但与粒度及常量元素等可比性并不高,与磁化率特征也没多大可比性,主要由于矿物鉴定选用的是特定粒级矿物含量,而非样品总体。
环境磁学测定结果表明,研究区域磁化率值变化较大,其变化范围为(2.475～679.8)×10-6SI之间,小于50×10-6SI的区域主要位于琼州海峡内及东部,海峡内主要位于中间,呈东西条带状分布,东部呈扇状分布,西部也存在一片低值区域,主要呈指状分布,高值区主要位于琼州海峡东口东部区域及海峡西口北部。该区域频率磁化率最高值为18.98%,最低值为0.59%,平均值为8.56%,共有9个站位频率磁化率值低于3%,其所对应磁化率值也较高,说明这几处沉积物可能已遭受到人为污染。k-T曲线中磁化率在580℃左右急剧降低,表明起主导作用的磁性矿物为磁铁矿。扫描电镜结果显示,该海区沉积物中磁性矿物主要为钒钛磁铁矿及磁铁矿等,矿物形态主要为类球状,与燃烧形成的典型磁性矿物有一定相似度。
体积磁化率κ与沉积物粒度参数相关性分析结果表明,体积磁化率与沉积物平均粒径Mz(φ)及中值粒径MD(φ)存在显著相关,相关系数达到0.8以上,说明磁化率值的大小受沉积物粒度控制明显,沉积物越细,磁化率值越大。体积磁化率κ与0.125mm～0.063mm粒径内锐钛矿、磁铁矿及黄铁矿正相关,但相关性不显著,与钛铁矿、赤褐铁矿负相关。磁化率与大多数常量元素存在相关性,与Fe2O3、Al2O3、MgO、TiO2、P2O5、K2O、Na2O具有显著相关性,与Si02显著负相关,与CaO、MnO相关性不显著,体积磁化率与Ti02具有较好的相关性,也说明了该区域磁性矿物来源应为陆源物质的输入。
采用GSTA模型对该海域沉积物运移路径分析后认为,沉积物在琼州海峡内主要由海峡内向外运移,在海峡东部,沉积物具有比较明显的由口内向口外输运的趋势,沉积物在东口呈较强放射状向外海输运。海峡西部,沉积物出海峡口后主要显示了向北输运的趋势,与海峡东口相比,沉积物到达口外后并未呈放射状向外输运,而是以向北输运为主,说明东西口水动力环境存在较大差异。
重金属分析结果表明,该海区重金属平均值由大到小依次为Zn(73.64μg/g)> Cr(62.73μg/g)> Pb(30.88μg/g)> Cu(18.88μg/g)> Cd(0.08μg/g),变异系数由大到小依次为Cu(55.00)>Zn(44.50)>Cr(44.24)>Cd(40.26)>Pb(33.97),变异系数显示该区域Cu的迁移能力最弱,Pb的迁移能力最强。各重金属平面分布特征存在一定的相似性,高值区主要分布在琼州海峡东口以外及雷州半岛西部,低值区主要位于琼州海峡东部。与海洋沉积物一类标准对比,Cd、Pb及Zn最高值均未超过一类标准,Cr及Cu有少量超出。相关性分析显示Cu、Pb、Zn、Cr、Cd五种重金属具有较好的相关性,重金属与平均粒径(φ值)均呈正相关,说明细粒沉积物更容易吸附重金属。潜在生态危害评价显示该区域重金属污染程度及生态危害指数均较低,属于低风险。
磁化率与重金属相关性分析结果表明,磁化率与5种重金属相关性均较高,其中与迁移能力弱的重金属相关性要高于迁移能力强的,说明磁性矿物更易与迁移能力较弱的重金属分布在一起。由于存在较高的相关性,因此,在琼州海峡及周边海域采用磁化率值来指示重金属含量的大小具有一定的可行性,但由磁化率与重金属的一般线性模拟可见,在不同的取值范围,磁化率值与各重金属的相关性也存在差异,因此在运用过程中应考虑到不同阶段磁化率值对重金属含量的反应。
通过粒度对重金属及磁性特征的控制影响分析后发现,在该海域,粒度对重金属及磁化率的控制作用十分明显,主要表现在细粒组份与重金属及磁化率均呈明显的正相关,一定程度上也可能揭示了磁化率与重金属的相关性主要是被粒度所控制。其它磁性特征中,频率磁化率与获取的各粒级均没有显示出显著的相关性,剩磁的各参数与(5.75-6.25)φ及(8.75-11)φ这两组粒级显示了显著的相关性,可能反应了容易获得剩磁的磁性矿物主要位于这两段粒级组份之中。
利用柱状样对琼州海峡内沉积历史分析后表明,自距今4000年之前到现代,琼州海峡及周边海域未发生重大沉积事件,处于稳定状态,沉积物在潮流改造下逐渐变细,分选性也逐渐变好。除了海峡西南部的柱样B492外,其它柱状样磁化率变化趋势基本一致,均为底部缓慢增加,到顶部时表现为快速增加,说明这些区域磁性矿物的输入具有一致性,磁性矿物的来源具有一定的同源性。
柱状样分析结果还显示,在琼州海峡东西口外,沉积物粒径由底部到顶部在逐渐变细,沉积物重金属含量自底部到顶部大多为升高的趋势,但大多表现为底部升高缓慢,快到顶部时存在一个快速升高的阶段,将这种升高趋势与粒径变化对比后发现,底部的升高主要是由于粒径变细所引起,而顶部的升高受粒径变化的影响不大,反映了物源区重金属含量的升高。结合年代学分析得出该海域磁性矿物及重金属在顶部的升高具有一定的对应性,大致在距今120年左右开始,而此时正是洋务运动之后工业生产开始进入我国,说明这一时期磁化率及重金属的升高主要受到人类活动的影响。
通过对重金属来源分析后认为,琼州海峡内重金属应主要来自于珠江径流的输入,重金属及磁性矿物均主要赋存于细粒物质中,珠江入海后在沿岸流的作用下,大量物质西向输运,并在沿途大量沉积。而胶体及溶解态物质由于粒径小,搬运距离最远,当到达琼州海峡东部时,由于海峡内往复潮流的冲刷也带出了大量细颗粒物质,两种物质相遇后发生中和作用,而产生了大量细粒沉积物。由于该海区起主导作用的水动力因素为海峡内的往复潮流,这些因中和而产生的大量细粒物质便在潮流的作用下被带到海峡东西口沉积,最终形成了琼州海峡现今重金属的分布状况。
Since ancient times the ocean have been hold many dream of mankind, and there have been dozens of world powers since modern times, without exception, are major maritime countries. In modern times Chinese people always dream of national revival, and at present the strategic maritime power, the Ocean 's principles and policies were ptoposed, which pushed the development of ocean into an unprecedented height. For the advance of science and technology, there has been increasing emphasis on the harmonious devel(?) of mankind and the earth. protect the planet, protect the marine environment have become the general concent of mankind. Therefore before exploiting marine and developing the marine economy, first we must understand oceans, learn how to live peacefully between man and the ocean, in case the new development of sea becomes another marine pollution act, and ultimately achi(?) the sustainable development.
China's maritime development has been boomed since the1990s, which greatly promoted the economic development of coastal areas, coastal marine aent has become a new economic growth point and the regional development strategy,.. economy in the coastal areas is becoming more important than before. However, the rapid development of China's marine economy, also brought great pressure to the marine environment, such as the occurrence of red tide in some areas, excessive heavy metals to fisheries, which brought heavy blows to the fishery production and people's life. Therefore, strengthen marine environmental protection must be carry out, then the coordinated development can be last between man and nature for long time.
In recent years, along with food safety accidents, sparked widespread concern about food safety,"cadmium rice" incident reports and other heavy metals are also often found in newspapers, the situation of heavy metal pollution are concerned increasingly by public. For sources of marine pollution are widely and variety, the impact range of pollution is widely, and far-reaching damage, so which are very difficult to be controlled, once polluted, it's very hard to be recovered. Among all of the contaminants, heavy metals were particularly attended by public for the harm of large area and long duration. With the development coastal economic and urbanization, a large number of heavy metals and other toxic substances were discharged follow the wastewater, and the estuary ecological environment has been severely damaged. For heavy metal pollutants can not be degrade easily and can quickly move from the aqueous phase into the solid phase (suspended solids and sediments), and ultimately accumulate into the sediment, so the sediment is the main repository for heavy metals and other chemical substances. For the accumulation of heavy metals will affect the ecological security, and ultimately threaten the human health, so the research for the distribution and migration of heavy metals on the seabed has become very important.
Current research on heavy metals mainly measured by chemical methods, this method can directly determine the content of heavy metals, but the test time is very long, and cost high, some pollution will be caused during sample processing, and the test work must be completed in the laboratory. Environmental magnetism was developed between earth science, environmental science and magnetism in recent decades, which can record a lot of regional and global environmental changes information, and also can provide the information of human activities impact on the environment. Due to the characteristics of fast, economical, convenient, no pollution and destructive to the samples, some parameters (such as magnetic susceptibility) even can be achieved on field, therefore, environmental magnetism has been praised by many scholars at home and abroad, and many scholars already applied it to monitor the heavy metal pollution.
Qiongzhou Strait is one of China's three major Straits, the length from east to west is about80km, the average width form north to south is nearly30km, not only link Beibu Gulf and South China Sea as maritime corridor, but also is the very important maritime traffic shortcuts from Guangdong to Hainan and Vietnam. With the development of economic, the ocean activities have become more frequent in this area, mainly are fishery production, shipping development, submarine pipeline and the upcoming construction of the bridge, etc., so Qiongzhou Strait will play increasingly important role in future regional economic development. However, the current marine environment awareness in Qiongzhou Strait is not clear, such as the genesis history of Qiongzhou Strait, the evolution of sedimentary environment, the type and distribution of sediment, the sources and migration directions of sediment, pollution status of heavy metals, the historical pollution and sources of heavy metals, etc., there are still no deep understanding for these problems. And in this thesis, it is the first time to use the most detailed survey data and multi-disciplinary research approach to comprehensive analysis marine environmental characteristics of Qiongzhou Strait and the surrounding seas, expound the sediment sources and transport trends in this region, deeply understand the regional marine sedimentary environment in the Holocene, analyze the history and current situation and regional distribution of heavy metals detailed, clarify the sources of heavy metals, moreover, it is also the first time to use environmental magnetic research method in this region, analyze the relationship between the magnetic characteristics and heavy metal pollution, judge the environmental magnetic response mechanism to determine contaminants in Qiongzhou Strait and the surrounding sea area, and develop a new tools to monitor the marine environment in this region.
More than500surface samples and nearly20cores in Qiongzhou Strait and the surrounding area were analyzed in this topic, of which the interval for surface sample is5km, most columnar core depth is between1-2m. Through a comprehensive analysis by a variety of methods, reached the following conclusions:
Particle size analysis showed that the sediment type in Qiongzhou Strait mainly are sand and gravel sand, and from the central Strait to the estuary the sediment type gradually transform to fine-grained silt and clayey silt. For the hydrodynamic force is inconsistent between the east and west estuary in Qiongzhou Strait, the boundary from sand to silt in east is a semi-circular, and sediment types changes rapidly in the eastern Strait, changing rapidly from coarse to fine-grained sediments, which shaping arcuate trend Delta in East of the mouth of Qiongzhou Strait, indicating that there is a strong hydrodynamics in East estuary, which inhibit the tide current expanded outward to the east. But the sand and silt sediment boundary in west is irregular and like chicken feet, forming a finger liked delta in the western straits, sediment types change more gently, indicating there is a weak hydrodynamics in the west, and tidal action extends farther to the west, coarse materials can also be transported to distant regions. The sediment grain size in Qiongzhou Strait is mainly between-0.73(pand7.06φ, the particle size distribution is quite different between the east and west, and the better sorting in the east indicated that the hydrodynamic is more stable than west, there are mainly five kinds of peaks polymorphic in this study area, which reflecting the composition and sediment sources in different regional are inconsistent.
By the major element analysis, the sediments mainly composed by SiO2, Al2O3, Fe2O3, CaO, K2O, MgO and Na2O in Qiongzhou Strait, which accounted more than90%of total deposits, and there is a certain relevance for the distribution of elements and particle in this region, reflecting the "element granularity control law."
Detrital minerals of grain size0.125mm～0.063mm were identified, the results showed that the regional distribution of metalliferous minerals plane had its own characteristics, and some metallic mineral distribution characteristics were generally lower in the central Strait, but higher in both ends, the comparability with grain size and major elements was not high, and also do not comparable with magnetic susceptibility, mainly due to the mineral selection was a specific grain size content, rather than the overall sample.
Environmental magnetism test showed that the value of susceptibility changes widely in the study area, the range was (2.475～679.8)×10-6SI, and low areas (less than50×10-6SI) were mainly located in the Qiongzhou Strait and eastern of Strait, inner the Strait mainly located in the middle with banded structure, fan-shaped distributed in the eastern, and there was also a low value region existed in the west with a finger-like distribution, high-value areas were mainly located in far eastern region and the north of west side of Qiongzhou Strait. The maximum value of frequency susceptibility was18.98%in this region, the lowest value was0.59%. and the average was8.56%, totally nine stations frequency magnetic susceptibility values was less than3%, which corresponds to a higher magnetic susceptibility values, indicating that the sediments in that area may had been subjected to man-made pollution. The k-T curve showed that the susceptibility drastically reduced at about58O℃, indicating that the dominant part of the magnetic mineral was magnetite. Scanning electron microscopy showed that the magnetic minerals mainly were vanadium-titanium magnetite and magnetite in sediments, mineral forms were mainly spherical, had some similarity with typical combustion magnetic minerals.
The correlation analysis between bulk magnetic susceptibility k and grain size parameter showed that the bulk magnetic susceptibility, the mean particle diameter Mz(φ) and the median size MD(φ) had a significant correlation, the correlation coefficient was up to0.8, indicated that the magnetic susceptibility was controlled by the sedimental grain size obviously, the finer grain size, the larger magnetic susceptibility value. The magnetic susceptibility values had a positive correlation with anatase, magnetite and pyrite which selected in0.125mm～0.063mm particle size, but not significant correlation, had a negative correlation with ilmenite, hematite-limonite. There was a correlationship between magnetic susceptibility and most of the major elements, related to Fe2O3, Al2O3, MgO, TiO2, P2O5, K2Oand Na2O were significant dependencies, and for SiO2was significant negative correlation,but no significant correlation with CaO and MnO. For bulk magnetic susceptibility and TiO2had a good correlation, illustrated the sources of magnetic minerals in this region should be the terrigenous material.
From the analysis for the migration path of sediments by GSTA model used in this area, we can conclude that sediments mainly moved from inward to outward in strait, in the east sediment transport obviously from inner estuary to the outside with a strong radial pattern. In the west,the sediment transport trend is to the north after the sediment moving out of the mouth of the Strait. Compared with the east, sediment arriving at the estuary does not radiate outward, but mainly transport to the north, indicating the hydrodynamic force between east and west of Qiongzhou Strait are great different.
Heavy metal analysis showed that the descending order of the mean metal value in this sea area was Zn (73.64μg/g)> Cr (62.73μg/g)> Pb (30.88μg/g)> Cu (18.88μg/g)> Cd (0.08μg/g), the coefficient of variation order was Cu (55.00)> Zn (44.50)> Cr (44.24)> Cd (40.26)> Pb (33.97), the coefficient of variation shows that the weakest migration was Cu in this area, while Pb was the strongest. There was a certain similarity for the plane distribution of heavy metal, high-value areas were mainly distributed in the outside of the east of Qiongzhou Strait and the west of Leizhou Peninsula, while low-value areas were mainly distributed in the east of Qiongzhou Strait. Compare with the first class standard of marine sediments, Cd, Pb, and Zn did not exceed the maximum value of the standard, Cr and Cu with a small amount exceeded. Correlation analysis showed that the five heavy metal (Cu, Pb, Zn, Cr and Cd) had a good correlation with the mean particle size, indicating that heavy metals is easier to be absorbed by fine-grained sediments. Potential Ecological Risk Assessment of heavy metals pollution in the region showed that the extent and ecological risk index were lower, which was low risk.
Correlation analysis between magnetic susceptibility and heavy metals showed that the magnetic susceptibility associated with five heavy metals highly, and the correlation was higher for weak migrated heavy metals than strong migrated kinds, indicating magnetic minerals distributed with weak migrated heavy metals much easier. For there was a higher correlation, therefore, using the magnetic susceptibility values to indicate the heavy metal content in Qiongzhou Strait and the surrounding areas is feasibility, but from the general linear analog between the magnetic susceptibility and heavy metals, which showed that in different ranges magnetic susceptibility values associated with the heavy metals were also different, so the application process should take into account the different stages respond of the magnetic susceptibility values for heavy metals.
Through the impact analysis of magnetic characteristics and heavy metals controlled by the particle size in this area, found that particle size ability on the control of heavy metals and magnetic susceptibility effect was very obvious, the fine-grained component with heavy metals and magnetic susceptibility mainly showed a significant positive correlation, may also revealed that correlation between magnetic susceptibility and heavy metals were mainly controlled by particle size. For other magnetic characteristics, the frequency magnetic susceptibility did not show a significant correlation with grain size, the parameters of remanence showed significant correlation with size fraction of (5.75-6.25)φand (8.75-11)φ, may reflected magnetic minerals which can easily obtain remanence were mainly located in these two fractions.
Sedimentary history analyzing in Qiongzhou Strait by the cores shows that from4000years ago to nowadays, there were no major sedimentary events happened in Qiongzhou Strait and surrounding areas, sediments gradually taper off finer and sorting was also changed for better by tide currents. In addition to core B492in the southwest of the Strait, the susceptibility of other cores changed consistently, were slowly increasing in the bottom while rapidly increasing in the top, indicating that the input of magnetic minerals in these areas were consistent, the source of magnetic minerals were homology.
The analysis results of core sample also showed that sediment grain size tapered from the bottom to the top, in the east and west of Qiongzhou Strait estuary, the heavy metals were mostly rising slowly in the bottom, while approaching the top there was a rapid increase, compared the increasing trend of this change with the particle size, which indicated the rise at the bottom caused by the increasing of finer particle size, but in the top the particle size variation showed little effect, reflected the increase of heavy metals from the source area. Combined with Geochronology analysis results, the increase of magnetic minerals and heavy metals at the top of this area had some correspondence, roughly around120years ago, this time it was the Westernization Movement started in China after industrial production, indicating that the rise of magnetic susceptibility and heavy metals in this period mainly caused by human activities.
Heavy metals in Qiongzhou Strait should be mainly input from the Pearl River through the sources analysis, heavy metals and magnetic minerals mainly consisted in the fine-grained material, after the Pearl River flowed into the sea, a large number of substances transported and deposited westward along the coast by the alongshore current. For the small particle size of colloidal and dissolved substance, which can transport far away, when they reached to the eastern of Qiongzhou Strait, met another large number of eroded fine particulate matter brought out by the reciprocating tide current from Qiongzhou Strait, and the two substances neutralized after they encountered, then a lot of fine-grained sediments generated. As the reciprocating tide current played a leading role within the hydrodynamic factors in Qiongzhou Strait, the neutralized fine-grained material was brought to the east and west mouth and deposited finally, formed today's distribution of heavy metals in Qiongzhou Strait.
近年随着食品安全事故频发,引发人们对食品安全的普遍担忧,“镉大米”事件等重金属污染的报道也屡见报端,公众对重金属污染的状况也越来越关注。海洋污染具有污染源广,污染物质种类多,影响范围大,危害深远,控制复杂,治理难度大的特点。在各种污染物中,又以重金属的危害面大、持续时间久而受到人们的格外关注,随着沿海经济的发展和城市化进程的加剧,大量含重金属等有毒物质的废水排放,使河口生态环境遭到严重破坏,其中重金属污染物不易降解,能迅速由水相转入固相(悬浮物和沉积物),最终进入沉积物中蓄积,使沉积物成为重金属等化学物质的主要存储库。由于重金属的积聚会影响到生态安全,并最终威胁到人类健康,因此对海底重金属的分布及迁移规律的研究便显得十分重要。
当前对重金属的研究主要采用化学测定的方式,该方法能够直接确定重金属的含量,然而测定周期长,费用高,样品处理过程中还存在一定的污染,而且测试工作必须在实验室完成。环境磁学是近几十年发展起来的一门介于地球科学、环境科学和磁学之间的边缘学科,可提供大量有关区域或全球环境变化及人类活动对环境影响等的重要信息。由于环境磁学测试具有快速、经济、方便、无污染及无损样品的特点,部分参数(如磁化率)甚至可实现现场测试,因此,一直以来受到国内外众多学者的推崇,已有学者将其运用到重金属的污染监测之中。
琼州海峡是中国的三大海峡之一,东西长度约80公里,南北平均宽度近30公里,不仅是沟通北部湾和南海中、东部的海上走廊,同时也是广东至海南岛和越南等地的海上交通捷径。随着经济发展,该海区的海洋活动也越来越频繁,主要表现为渔业生产、航运发展、海底管道铺设及今后即将建设的跨海大桥等,在今后的区域经济建设中,琼州海峡将发挥其越来越重要的角色。然而当前对琼州海峡海洋环境的认识却并不高,对琼州海峡成因历史、沉积环境演变、沉积物类型及分布规律、沉积物来源及运移方向、重金属污染现状、重金属来源及成因历史等问题并没有深刻认识,本论文即首次运用目前最详细的调查资料及多种学科研究方法来综合分析琼州海峡及周边海域海洋环境特征,系统的阐述了该区域沉积物来源及运移趋势,深入认识该区域全新世以来海洋沉积环境演变,详细分析了该区域重金属分布现状及重金属历史沉积变化,并对重金属来源进行了分析,还首次将环境磁学的研究方法引入到该区域,分析了磁性特征与重金属污染的关系,判断污染物在琼州海峡及周边海区的环境磁学响应机制,为该区域海洋环境监测新手段的开拓提供了新的方向。
本论文共选取了琼州海峡及周边海域共计500多个表层站位及近20个柱状样进行了分析,其中表层样间距为5km,柱状样深度大多在1到2米之间。通过多种方法的综合分析,得出了以下认识：
粒度分析的结果表明琼州海峡内沉积物类型以砂及砾砂为主,由海峡中部向口外逐渐过渡到更细粒的粉砂及粘土质粉砂。琼州海峡东西口水动力条件不一致,在海峡东部,砂及粉砂沉积物界限为半弧形,沉积物类型变化较快,迅速由粗粒转变为了细粒沉积物,塑造了海峡东口的弧状潮流三角洲,说明海峡东口外存在较强的水动力,抑制了海峡内潮流作用的向外扩张；在海峡西部,砂及粉砂界限为不规则鸡爪状,形成了海峡西部指状三角洲,沉积物过渡较为平缓,说明海峡西口外水动力较弱,潮流作用向西部延伸较远,粗粒物质也能被输送到较远的区域。琼州海峡内沉积物粒径多在-0.73φ—7.06φ-之间,东西口粒径分布存在较大差异,分选性东部优于西部,说明东部水动力较稳定,研究区域主要存在5种峰态类型,反映不同区域沉积物来源组成不一致。
由主量元素分析可知,琼州海峡沉积物主要由SiO2、Al2O3、Fe2O3、CaO、K2O、MgO及Na20组成,这7种氧化物占沉积物总量的90%以上,该区域沉积物元素分布与粒径具有一定的相关性,反映了该区域“元素的粒度控制律”。
选用0.125mm～0.063mm粒级的碎屑矿物进行了矿物鉴定,结果表明该区域含金属矿物平面分布具有其独自的特征,部分含金属矿物的平面分布特征也大致为海峡中部低,两端高的特点,但与粒度及常量元素等可比性并不高,与磁化率特征也没多大可比性,主要由于矿物鉴定选用的是特定粒级矿物含量,而非样品总体。
环境磁学测定结果表明,研究区域磁化率值变化较大,其变化范围为(2.475～679.8)×10-6SI之间,小于50×10-6SI的区域主要位于琼州海峡内及东部,海峡内主要位于中间,呈东西条带状分布,东部呈扇状分布,西部也存在一片低值区域,主要呈指状分布,高值区主要位于琼州海峡东口东部区域及海峡西口北部。该区域频率磁化率最高值为18.98%,最低值为0.59%,平均值为8.56%,共有9个站位频率磁化率值低于3%,其所对应磁化率值也较高,说明这几处沉积物可能已遭受到人为污染。k-T曲线中磁化率在580℃左右急剧降低,表明起主导作用的磁性矿物为磁铁矿。扫描电镜结果显示,该海区沉积物中磁性矿物主要为钒钛磁铁矿及磁铁矿等,矿物形态主要为类球状,与燃烧形成的典型磁性矿物有一定相似度。
体积磁化率κ与沉积物粒度参数相关性分析结果表明,体积磁化率与沉积物平均粒径Mz(φ)及中值粒径MD(φ)存在显著相关,相关系数达到0.8以上,说明磁化率值的大小受沉积物粒度控制明显,沉积物越细,磁化率值越大。体积磁化率κ与0.125mm～0.063mm粒径内锐钛矿、磁铁矿及黄铁矿正相关,但相关性不显著,与钛铁矿、赤褐铁矿负相关。磁化率与大多数常量元素存在相关性,与Fe2O3、Al2O3、MgO、TiO2、P2O5、K2O、Na2O具有显著相关性,与Si02显著负相关,与CaO、MnO相关性不显著,体积磁化率与Ti02具有较好的相关性,也说明了该区域磁性矿物来源应为陆源物质的输入。
采用GSTA模型对该海域沉积物运移路径分析后认为,沉积物在琼州海峡内主要由海峡内向外运移,在海峡东部,沉积物具有比较明显的由口内向口外输运的趋势,沉积物在东口呈较强放射状向外海输运。海峡西部,沉积物出海峡口后主要显示了向北输运的趋势,与海峡东口相比,沉积物到达口外后并未呈放射状向外输运,而是以向北输运为主,说明东西口水动力环境存在较大差异。
重金属分析结果表明,该海区重金属平均值由大到小依次为Zn(73.64μg/g)> Cr(62.73μg/g)> Pb(30.88μg/g)> Cu(18.88μg/g)> Cd(0.08μg/g),变异系数由大到小依次为Cu(55.00)>Zn(44.50)>Cr(44.24)>Cd(40.26)>Pb(33.97),变异系数显示该区域Cu的迁移能力最弱,Pb的迁移能力最强。各重金属平面分布特征存在一定的相似性,高值区主要分布在琼州海峡东口以外及雷州半岛西部,低值区主要位于琼州海峡东部。与海洋沉积物一类标准对比,Cd、Pb及Zn最高值均未超过一类标准,Cr及Cu有少量超出。相关性分析显示Cu、Pb、Zn、Cr、Cd五种重金属具有较好的相关性,重金属与平均粒径(φ值)均呈正相关,说明细粒沉积物更容易吸附重金属。潜在生态危害评价显示该区域重金属污染程度及生态危害指数均较低,属于低风险。
磁化率与重金属相关性分析结果表明,磁化率与5种重金属相关性均较高,其中与迁移能力弱的重金属相关性要高于迁移能力强的,说明磁性矿物更易与迁移能力较弱的重金属分布在一起。由于存在较高的相关性,因此,在琼州海峡及周边海域采用磁化率值来指示重金属含量的大小具有一定的可行性,但由磁化率与重金属的一般线性模拟可见,在不同的取值范围,磁化率值与各重金属的相关性也存在差异,因此在运用过程中应考虑到不同阶段磁化率值对重金属含量的反应。
通过粒度对重金属及磁性特征的控制影响分析后发现,在该海域,粒度对重金属及磁化率的控制作用十分明显,主要表现在细粒组份与重金属及磁化率均呈明显的正相关,一定程度上也可能揭示了磁化率与重金属的相关性主要是被粒度所控制。其它磁性特征中,频率磁化率与获取的各粒级均没有显示出显著的相关性,剩磁的各参数与(5.75-6.25)φ及(8.75-11)φ这两组粒级显示了显著的相关性,可能反应了容易获得剩磁的磁性矿物主要位于这两段粒级组份之中。
利用柱状样对琼州海峡内沉积历史分析后表明,自距今4000年之前到现代,琼州海峡及周边海域未发生重大沉积事件,处于稳定状态,沉积物在潮流改造下逐渐变细,分选性也逐渐变好。除了海峡西南部的柱样B492外,其它柱状样磁化率变化趋势基本一致,均为底部缓慢增加,到顶部时表现为快速增加,说明这些区域磁性矿物的输入具有一致性,磁性矿物的来源具有一定的同源性。
柱状样分析结果还显示,在琼州海峡东西口外,沉积物粒径由底部到顶部在逐渐变细,沉积物重金属含量自底部到顶部大多为升高的趋势,但大多表现为底部升高缓慢,快到顶部时存在一个快速升高的阶段,将这种升高趋势与粒径变化对比后发现,底部的升高主要是由于粒径变细所引起,而顶部的升高受粒径变化的影响不大,反映了物源区重金属含量的升高。结合年代学分析得出该海域磁性矿物及重金属在顶部的升高具有一定的对应性,大致在距今120年左右开始,而此时正是洋务运动之后工业生产开始进入我国,说明这一时期磁化率及重金属的升高主要受到人类活动的影响。
通过对重金属来源分析后认为,琼州海峡内重金属应主要来自于珠江径流的输入,重金属及磁性矿物均主要赋存于细粒物质中,珠江入海后在沿岸流的作用下,大量物质西向输运,并在沿途大量沉积。而胶体及溶解态物质由于粒径小,搬运距离最远,当到达琼州海峡东部时,由于海峡内往复潮流的冲刷也带出了大量细颗粒物质,两种物质相遇后发生中和作用,而产生了大量细粒沉积物。由于该海区起主导作用的水动力因素为海峡内的往复潮流,这些因中和而产生的大量细粒物质便在潮流的作用下被带到海峡东西口沉积,最终形成了琼州海峡现今重金属的分布状况。
Since ancient times the ocean have been hold many dream of mankind, and there have been dozens of world powers since modern times, without exception, are major maritime countries. In modern times Chinese people always dream of national revival, and at present the strategic maritime power, the Ocean 's principles and policies were ptoposed, which pushed the development of ocean into an unprecedented height. For the advance of science and technology, there has been increasing emphasis on the harmonious devel(?) of mankind and the earth. protect the planet, protect the marine environment have become the general concent of mankind. Therefore before exploiting marine and developing the marine economy, first we must understand oceans, learn how to live peacefully between man and the ocean, in case the new development of sea becomes another marine pollution act, and ultimately achi(?) the sustainable development.
China's maritime development has been boomed since the1990s, which greatly promoted the economic development of coastal areas, coastal marine aent has become a new economic growth point and the regional development strategy,.. economy in the coastal areas is becoming more important than before. However, the rapid development of China's marine economy, also brought great pressure to the marine environment, such as the occurrence of red tide in some areas, excessive heavy metals to fisheries, which brought heavy blows to the fishery production and people's life. Therefore, strengthen marine environmental protection must be carry out, then the coordinated development can be last between man and nature for long time.
In recent years, along with food safety accidents, sparked widespread concern about food safety,"cadmium rice" incident reports and other heavy metals are also often found in newspapers, the situation of heavy metal pollution are concerned increasingly by public. For sources of marine pollution are widely and variety, the impact range of pollution is widely, and far-reaching damage, so which are very difficult to be controlled, once polluted, it's very hard to be recovered. Among all of the contaminants, heavy metals were particularly attended by public for the harm of large area and long duration. With the development coastal economic and urbanization, a large number of heavy metals and other toxic substances were discharged follow the wastewater, and the estuary ecological environment has been severely damaged. For heavy metal pollutants can not be degrade easily and can quickly move from the aqueous phase into the solid phase (suspended solids and sediments), and ultimately accumulate into the sediment, so the sediment is the main repository for heavy metals and other chemical substances. For the accumulation of heavy metals will affect the ecological security, and ultimately threaten the human health, so the research for the distribution and migration of heavy metals on the seabed has become very important.
Current research on heavy metals mainly measured by chemical methods, this method can directly determine the content of heavy metals, but the test time is very long, and cost high, some pollution will be caused during sample processing, and the test work must be completed in the laboratory. Environmental magnetism was developed between earth science, environmental science and magnetism in recent decades, which can record a lot of regional and global environmental changes information, and also can provide the information of human activities impact on the environment. Due to the characteristics of fast, economical, convenient, no pollution and destructive to the samples, some parameters (such as magnetic susceptibility) even can be achieved on field, therefore, environmental magnetism has been praised by many scholars at home and abroad, and many scholars already applied it to monitor the heavy metal pollution.
Qiongzhou Strait is one of China's three major Straits, the length from east to west is about80km, the average width form north to south is nearly30km, not only link Beibu Gulf and South China Sea as maritime corridor, but also is the very important maritime traffic shortcuts from Guangdong to Hainan and Vietnam. With the development of economic, the ocean activities have become more frequent in this area, mainly are fishery production, shipping development, submarine pipeline and the upcoming construction of the bridge, etc., so Qiongzhou Strait will play increasingly important role in future regional economic development. However, the current marine environment awareness in Qiongzhou Strait is not clear, such as the genesis history of Qiongzhou Strait, the evolution of sedimentary environment, the type and distribution of sediment, the sources and migration directions of sediment, pollution status of heavy metals, the historical pollution and sources of heavy metals, etc., there are still no deep understanding for these problems. And in this thesis, it is the first time to use the most detailed survey data and multi-disciplinary research approach to comprehensive analysis marine environmental characteristics of Qiongzhou Strait and the surrounding seas, expound the sediment sources and transport trends in this region, deeply understand the regional marine sedimentary environment in the Holocene, analyze the history and current situation and regional distribution of heavy metals detailed, clarify the sources of heavy metals, moreover, it is also the first time to use environmental magnetic research method in this region, analyze the relationship between the magnetic characteristics and heavy metal pollution, judge the environmental magnetic response mechanism to determine contaminants in Qiongzhou Strait and the surrounding sea area, and develop a new tools to monitor the marine environment in this region.
More than500surface samples and nearly20cores in Qiongzhou Strait and the surrounding area were analyzed in this topic, of which the interval for surface sample is5km, most columnar core depth is between1-2m. Through a comprehensive analysis by a variety of methods, reached the following conclusions:
Particle size analysis showed that the sediment type in Qiongzhou Strait mainly are sand and gravel sand, and from the central Strait to the estuary the sediment type gradually transform to fine-grained silt and clayey silt. For the hydrodynamic force is inconsistent between the east and west estuary in Qiongzhou Strait, the boundary from sand to silt in east is a semi-circular, and sediment types changes rapidly in the eastern Strait, changing rapidly from coarse to fine-grained sediments, which shaping arcuate trend Delta in East of the mouth of Qiongzhou Strait, indicating that there is a strong hydrodynamics in East estuary, which inhibit the tide current expanded outward to the east. But the sand and silt sediment boundary in west is irregular and like chicken feet, forming a finger liked delta in the western straits, sediment types change more gently, indicating there is a weak hydrodynamics in the west, and tidal action extends farther to the west, coarse materials can also be transported to distant regions. The sediment grain size in Qiongzhou Strait is mainly between-0.73(pand7.06φ, the particle size distribution is quite different between the east and west, and the better sorting in the east indicated that the hydrodynamic is more stable than west, there are mainly five kinds of peaks polymorphic in this study area, which reflecting the composition and sediment sources in different regional are inconsistent.
By the major element analysis, the sediments mainly composed by SiO2, Al2O3, Fe2O3, CaO, K2O, MgO and Na2O in Qiongzhou Strait, which accounted more than90%of total deposits, and there is a certain relevance for the distribution of elements and particle in this region, reflecting the "element granularity control law."
Detrital minerals of grain size0.125mm～0.063mm were identified, the results showed that the regional distribution of metalliferous minerals plane had its own characteristics, and some metallic mineral distribution characteristics were generally lower in the central Strait, but higher in both ends, the comparability with grain size and major elements was not high, and also do not comparable with magnetic susceptibility, mainly due to the mineral selection was a specific grain size content, rather than the overall sample.
Environmental magnetism test showed that the value of susceptibility changes widely in the study area, the range was (2.475～679.8)×10-6SI, and low areas (less than50×10-6SI) were mainly located in the Qiongzhou Strait and eastern of Strait, inner the Strait mainly located in the middle with banded structure, fan-shaped distributed in the eastern, and there was also a low value region existed in the west with a finger-like distribution, high-value areas were mainly located in far eastern region and the north of west side of Qiongzhou Strait. The maximum value of frequency susceptibility was18.98%in this region, the lowest value was0.59%. and the average was8.56%, totally nine stations frequency magnetic susceptibility values was less than3%, which corresponds to a higher magnetic susceptibility values, indicating that the sediments in that area may had been subjected to man-made pollution. The k-T curve showed that the susceptibility drastically reduced at about58O℃, indicating that the dominant part of the magnetic mineral was magnetite. Scanning electron microscopy showed that the magnetic minerals mainly were vanadium-titanium magnetite and magnetite in sediments, mineral forms were mainly spherical, had some similarity with typical combustion magnetic minerals.
The correlation analysis between bulk magnetic susceptibility k and grain size parameter showed that the bulk magnetic susceptibility, the mean particle diameter Mz(φ) and the median size MD(φ) had a significant correlation, the correlation coefficient was up to0.8, indicated that the magnetic susceptibility was controlled by the sedimental grain size obviously, the finer grain size, the larger magnetic susceptibility value. The magnetic susceptibility values had a positive correlation with anatase, magnetite and pyrite which selected in0.125mm～0.063mm particle size, but not significant correlation, had a negative correlation with ilmenite, hematite-limonite. There was a correlationship between magnetic susceptibility and most of the major elements, related to Fe2O3, Al2O3, MgO, TiO2, P2O5, K2Oand Na2O were significant dependencies, and for SiO2was significant negative correlation,but no significant correlation with CaO and MnO. For bulk magnetic susceptibility and TiO2had a good correlation, illustrated the sources of magnetic minerals in this region should be the terrigenous material.
From the analysis for the migration path of sediments by GSTA model used in this area, we can conclude that sediments mainly moved from inward to outward in strait, in the east sediment transport obviously from inner estuary to the outside with a strong radial pattern. In the west,the sediment transport trend is to the north after the sediment moving out of the mouth of the Strait. Compared with the east, sediment arriving at the estuary does not radiate outward, but mainly transport to the north, indicating the hydrodynamic force between east and west of Qiongzhou Strait are great different.
Heavy metal analysis showed that the descending order of the mean metal value in this sea area was Zn (73.64μg/g)> Cr (62.73μg/g)> Pb (30.88μg/g)> Cu (18.88μg/g)> Cd (0.08μg/g), the coefficient of variation order was Cu (55.00)> Zn (44.50)> Cr (44.24)> Cd (40.26)> Pb (33.97), the coefficient of variation shows that the weakest migration was Cu in this area, while Pb was the strongest. There was a certain similarity for the plane distribution of heavy metal, high-value areas were mainly distributed in the outside of the east of Qiongzhou Strait and the west of Leizhou Peninsula, while low-value areas were mainly distributed in the east of Qiongzhou Strait. Compare with the first class standard of marine sediments, Cd, Pb, and Zn did not exceed the maximum value of the standard, Cr and Cu with a small amount exceeded. Correlation analysis showed that the five heavy metal (Cu, Pb, Zn, Cr and Cd) had a good correlation with the mean particle size, indicating that heavy metals is easier to be absorbed by fine-grained sediments. Potential Ecological Risk Assessment of heavy metals pollution in the region showed that the extent and ecological risk index were lower, which was low risk.
Correlation analysis between magnetic susceptibility and heavy metals showed that the magnetic susceptibility associated with five heavy metals highly, and the correlation was higher for weak migrated heavy metals than strong migrated kinds, indicating magnetic minerals distributed with weak migrated heavy metals much easier. For there was a higher correlation, therefore, using the magnetic susceptibility values to indicate the heavy metal content in Qiongzhou Strait and the surrounding areas is feasibility, but from the general linear analog between the magnetic susceptibility and heavy metals, which showed that in different ranges magnetic susceptibility values associated with the heavy metals were also different, so the application process should take into account the different stages respond of the magnetic susceptibility values for heavy metals.
Through the impact analysis of magnetic characteristics and heavy metals controlled by the particle size in this area, found that particle size ability on the control of heavy metals and magnetic susceptibility effect was very obvious, the fine-grained component with heavy metals and magnetic susceptibility mainly showed a significant positive correlation, may also revealed that correlation between magnetic susceptibility and heavy metals were mainly controlled by particle size. For other magnetic characteristics, the frequency magnetic susceptibility did not show a significant correlation with grain size, the parameters of remanence showed significant correlation with size fraction of (5.75-6.25)φand (8.75-11)φ, may reflected magnetic minerals which can easily obtain remanence were mainly located in these two fractions.
Sedimentary history analyzing in Qiongzhou Strait by the cores shows that from4000years ago to nowadays, there were no major sedimentary events happened in Qiongzhou Strait and surrounding areas, sediments gradually taper off finer and sorting was also changed for better by tide currents. In addition to core B492in the southwest of the Strait, the susceptibility of other cores changed consistently, were slowly increasing in the bottom while rapidly increasing in the top, indicating that the input of magnetic minerals in these areas were consistent, the source of magnetic minerals were homology.
The analysis results of core sample also showed that sediment grain size tapered from the bottom to the top, in the east and west of Qiongzhou Strait estuary, the heavy metals were mostly rising slowly in the bottom, while approaching the top there was a rapid increase, compared the increasing trend of this change with the particle size, which indicated the rise at the bottom caused by the increasing of finer particle size, but in the top the particle size variation showed little effect, reflected the increase of heavy metals from the source area. Combined with Geochronology analysis results, the increase of magnetic minerals and heavy metals at the top of this area had some correspondence, roughly around120years ago, this time it was the Westernization Movement started in China after industrial production, indicating that the rise of magnetic susceptibility and heavy metals in this period mainly caused by human activities.
Heavy metals in Qiongzhou Strait should be mainly input from the Pearl River through the sources analysis, heavy metals and magnetic minerals mainly consisted in the fine-grained material, after the Pearl River flowed into the sea, a large number of substances transported and deposited westward along the coast by the alongshore current. For the small particle size of colloidal and dissolved substance, which can transport far away, when they reached to the eastern of Qiongzhou Strait, met another large number of eroded fine particulate matter brought out by the reciprocating tide current from Qiongzhou Strait, and the two substances neutralized after they encountered, then a lot of fine-grained sediments generated. As the reciprocating tide current played a leading role within the hydrodynamic factors in Qiongzhou Strait, the neutralized fine-grained material was brought to the east and west mouth and deposited finally, formed today's distribution of heavy metals in Qiongzhou Strait.
引文
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