Environmental magnetic responses of urbanization processes: evidence from lake sediments in East Lake, Wuhan, China

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1 Geophys. J. Int. (2009) 179, doi: /j X x Environmental magnetic responses of urbanization processes: evidence from lake sediments in East Lake, Wuhan, China Tao Yang, 1 Qingsheng Liu, 2 Qingli Zeng 2 and Lungsang Chan 3 1 Institute of Geophysics, China Earthquake Administration, Beijing , China. yangtaosx@163.com 2 Department of Geophysics, China University of Geosciences, Wuhan , China 3 Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China Accepted 2009 June 25. Received 2009 June 24; in original form 2009 March 10 SUMMARY We present a study on the environmental magnetic response to urbanization processes from nine sediment cores from East Lake in Wuhan, China. The concentration of magnetic particles, heavy metals and organic matter in the upper 2 18 cm of the sediment cores have been significantly elevated due to the input of coarse magnetite grains from industrial (e.g. power generation and steelmaking) and other anthropogenic activities (e.g. vehicle emissions). Concentration-related magnetic parameters, such as magnetic susceptibility (χ), saturation isothermal remanent magnetization and anhysteretic remanent magnetization, are significantly correlated with concentration of heavy metals and organic matter, for example, Pearson s correlation coefficients are for χ-cu, for χ-pb and for χ-loss-on-ignition, respectively. The magnetic properties of the lake sediments document the pollution history caused by human impact on the lake catchment during urbanization. The environmental quality of the lake was fairly good before the 1960s. Magnetic, heavy metal and organic matter contents of the sediments were low and relatively constant, which indicates relatively stable natural inputs from the lake catchment. Pollution trends since the 1960s are reflected in downcore magnetic property variations of the lake sediment. The concentration of magnetic particles in the lake sediments started to increase since 1957, when the Wuhan Iron and Steel Company and the Qingshan Thermal Power Plant, which are located upwind of the lake, were built and put into production. Lake pollution was further aggravated since the acceleration of industrialization and urbanization in East Lake area in the 1980s, which is verified by the elevated concentration of heavy metals and organic matter in the sediments. The magnetic mineral concentration in the lake sediment increased continuously until it peaked in the 1990s and has remained a high level since then. These results suggest that magnetic properties respond sensitively to environmental status, and that the magnetic properties of sediments can provide an excellent record of the industrial and anthropogenic history in an urban lake catchment. Key words: Environmental magnetism; Rock and mineral magnetism. GJI Geomagnetism, rock magnetism and palaeomagnetism 1 INTRODUCTION Lake sediments provide valuable records of natural environmental and climatic changes (Bloemendal et al. 1979; Lotter et al. 1992; Lanci et al. 1999; Maher et al. 1999; Frank 2007), geomagnetic variations (Turner & Thompson 1979; Creer 1981), land-use within the watershed (Thompson & Oldfield 1986; Gaillard et al. 1991; Foster et al. 1998), as well as pollution caused by human impacts within the lake catchment (Locke & Bertine 1986; Petrovský & Elwood 1999; Mecray et al. 2001; Hu et al. 2003; Jordanova et al. 2003, 2004; Rose et al. 2004; Knab et al. 2005; Yang et al. 2007). In particular, lake sediment is a good collector of organic and inorganic contaminants (e.g. nutrients, toxic pollutants, heavy metals and hydrocarbons), which are liable to pass through different food chains in aquatic ecosystems and cause health risks for humans prior to deposition (Jordanova et al. 2003). Therefore, good knowledge of the distribution of contaminants, and of the processes that led to their accumulation or dispersal, are significant for predicting the environmental response to anthropogenic impacts, and for the design and implementation of cost-effective environmental monitoring strategies. Both are important for environmental protection, remediation and management. Magnetic investigations of sediments can provide abundant information on anthropogenic and industrial activities in a lake catchment. Locke & Bertine (1986) described the impact of coal burning over time by studying recent lake, estuarine and marine sediments that were contaminated by magnetite-bearing pollutants. Morris et al. (1994) and Versteeg et al. (1995) found that the C 2009 The Authors 873

2 874 T. Yang et al. magnetic properties of sediments in Hamilton Harbour in western Lake Ontario were useful for mapping the thickness of contaminated sediment across the harbour, and that the concentrations of hydrocarbons and certain heavy metals (e.g. Pb, Zn and Fe) correlate closely to magnetic iron-oxide contents. Numerous studies have shown good relationships between certain magnetic properties (e.g. magnetic susceptibility) and heavy metals (Charlesworth & Lees 1997; Georgeaud et al. 1997; Petrovský et al. 1998; Petrovský & Ellwood 1999; Desenfant et al. 2004; Yang et al. 2007; Chaparro et al. 2008, and references therein). These significant correlations make it possible to use magnetic parameters as proxies for pollution. A number of studies have demonstrated the anthropogenic origins of the high magnetic mineral concentrations in recent sediments due to accumulations of coarse-grained iron oxides from industrial and anthropogenic sources (Oldfield & Scoullos 1984; Scholger 1998; Chan et al. 2001; Rose et al. 2004; Knab et al. 2005; Yang et al. 2007; Chaparro et al. 2008), however, this research has mostly focused on contemporary/modern sediment and current environmental pollution, and rarely on the dynamic magnetic response of the environment to historical pollution. One exception involves a study of near-shore marine sediments offshore of southwestern Taiwan where variations in the history of industrial pollution have been clearly documented (Horng et al. 2009). Magnetic measurements make it possible to obtain historical records of lake pollution caused by human impacts if industrial and anthropogenic magnetic particles are preserved in the sediment column and a well-resolved sedimentary age model can be developed. These records can be verified by comparison with the input of pollutants (e.g. heavy metals and organic matter), and with historical data for anthropogenic and industrial activities and known pollution sources in the lake catchment. In this study, lake sediments were sampled spatially and stratigraphically from East Lake in Wuhan, China. We aim to examine the magnetic properties of the lake sediments, and to establish a historical link between the enhanced magnetization of the sediments and pollutants (heavy metals, e.g. Cu, Pb, Zn, etc., and organic matter) with well-determined industrial and anthropogenic activity in the lake catchment. Emphasis is placed on the magnetic response of the lake sediment to urbanization in the past 50 yr in the East Lake area. 2 STUDY AREA East Lake, the largest urban lake in China, is situated in the northeast of Wuhan, which is the largest city in central China (Fig. 1). The water area and the average depth of the lake are, respectively, 27.9 km 2 and 3 4 m, with a maximum depth of 4.75 m. A number of roadway dikes have been constructed across the lake, dividing it into several water bodies; Guozheng Lake and Tanglin Lake are the two major basins. Interbasin water exchange occurs only through bridge underpasses and culverts beneath the roadways. The prevailing wind direction is from the northeast throughout the year. The lake was an open lake that was connected with the Yangtze River before 1957 when the Wuhan Iron and Steel Company (WISC) and the Qingshan Thermal Power Plant (QTPP) were constructed. Since then, the lake has been isolated from the Yangtze River and is an artificially controlled inland water body. At present, the QTPP has a total capacity of kw and the WISC is the third largest iron and steel consortium in China. In addition, further industrial plants, such as cement plants, foundries, coal gasification (tar) and coking plants, etc., have been built in the Qingshan industrial area. 3 MATERIALS AND METHODS 3.1 Sample collections Nine sediment cores, with lengths of cm, were recovered from East Lake (Fig. 1) using a gravity corer in 2006 May. The cores were cut into 1-cm slices, each slice was air-dried at room Figure 1. Locations of the studied sediment cores ( ) in East Lake, Wuhan, China. The asterisk ( ) marks the location of the core with sedimentation rate of 0.74 cm a 1 determined by Yang et al. (2004).

3 Magnetic responses of urbanization processes 875 temperature, carefully disaggregated using a wooden pestle and mortar, and passed through a 1-mm sieve to remove refuse and small stones. 3.2 Magnetic measurements The low-field magnetic susceptibility (χ) was measured using an AGICO KLY-3S Kappabridge magnetic susceptibility metre. Magnetic susceptibilities were measured at dual frequencies (0.47 and 4.7 khz) using a Bartington Instruments MS2B sensor. The values are expressed as mass-specific susceptibility χ lf and χ hf,respectively. Frequency-dependent susceptibility was then calculated and expressed as a percentage χ fd per cent = (χ lf χ hf )/χ lf 100 per cent. An anhysteretic remanent magnetization (ARM) was imparted using a peak alternating field (AF) of 100 mt with a direct current (DC) bias field of 0.05 mt parallel to the AF, and was measured using a Molspin magnetometer. Isothermal remanent magnetization (IRM) measurements were carried out using an ASC Scientific model IM-10 impulse magnetizer and Molspin magnetometer. The IRM acquired in a peak field of 1 T was regarded as a saturation IRM (SIRM), whereas the IRM acquired with a back-field of 300 mt is named the IRM 300mT.TheIRM 300mT /SIRM (S-ratio) was used to define approximately the relative importance of antiferromagnetic (e.g. haematite) versus low-coercivity ferrimagnetic minerals (Thompson & Oldfield 1986). In addition, a number of representative samples were magnetized in stepwise increasing DC fields to obtain IRM acquisition curves; 12 steps from 7 to 1100 mt were used. Temperature dependence of the magnetic susceptibility was determined using a Bartington Instruments MS2 system with a high temperature attachment. Measurements were performed from room temperature up to 700 C, with a measurement interval of 2 Cand a heating and cooling rate of 5 and 10 Cmin 1, respectively. 3.3 Heavy metal and organic matter content analyses Heavy metal contents were determined for representative samples. A small portion of the sample (approximately 1 g) was digested in 12 ml of 68 per cent nitric acid. The digested solutions were diluted and filtered with 50 ml of deionized water. Concentrations of heavy metals, such as Cr, Cu, Mn, Ni, Pb and Zn, etc., were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS Intrepid II XSP, Thermo Electron Corporation, USA). A blank sample was prepared in the same way and was used as the standard sample. 10 per cent of samples were measured repeatedly for quality control. The analytical precision, which was measured by the relative standard deviation of parallel samples, was less than 5 per cent. The organic matter content was determined by means of the loss-on-ignition (LOI) method. Weighed subsamples (ca. 5g) were oven-dried overnight at 105 C, reweighed and then ignited at 650 C for 4 hr in a muffle furnace. The LOI value is then expressed as the percentage weight lost. 3.4 Radionuclides Subsamples of sediment from core TL3 were analyzed for 210 Pb, 226 Ra and 137 Cs by direct gamma assay using Ortec HPGe GWL series well-type coaxial low background intrinsic germanium detectors at the Nanjing Institute of Geography and Limnology, Chinese Figure 2. Downcore radioisotope activity for (a) 210 Pb and (b) 137 Cs. The presence of 137 Cs at a depth of 5 cm corresponds to the Chernobyl nuclear power plant disaster in Russia in Academy of Sciences. The absolute efficiencies of the detectors were determined using calibrated sources and sediment samples of known activity. Downcore radionuclide profiles for core TL3 are shown in Fig. 2. The presence of 137 Csaboveadepthof5cm (Fig. 2b) records the Chernobyl nuclear power plant disaster in Russia in 1986, and was used as a reference point to further constrain the 210 Pb age model. Using a model of constant rate of supply (CRS), 210 Pb dating revealed a mean sedimentation rate of 0.23 cm a 1 for core TL3. The sedimentation rate for the core marked by an asterisk ( ) in Fig. 1, which is close to the studied core GZ3, was determined as 0.74 cm a 1 by Yang et al. (2004) using the same method. 3.5 Microscopic observation of magnetic extracts Magnetic extracts were separated from selected sediments using a hand magnet sealed in a polyethylene bag. Microscopic observation of magnetic extracts was completed using an environmental scanning electron microscope (SEM) Quanta 200 equipped with an energy dispersive spectrometer (EDS), to determine the morphology and composition of particles. 4 RESULTS AND DISCUSSION 4.1 Magnetism of the lake sediments The magnetic properties of the sediment cores are summarized in Tables 1 and 2, and downcore variations of the magnetic properties are depicted in Fig. 3. All magnetic concentration-related parameters, such as χ, ARM and SIRM, have similar downcore characteristics, with an interval of high values restricted to the first 2 18 cm of the cores, but with stable and relatively low values for the lower parts of the cores (Fig. 3). For example, the mean values of χ, ARM and SIRM in the first 1 2 cm of core TL1 are approximately seven times those of the 3 16 cm interval (Table 2). The strong magnetization suggests that there was a much greater magnetic input to the upper sediment layers. However, χ fd per cent, which is sensitive to the superparamagnetic (SP) particle component, is much lower ( 1 per cent) in the upper layers (Tables 1 and 2), which suggests that these sediments are dominated by coarse-grained magnetic particles (probably pseudo-single domain/multidomain (PSD/MD) grains) and that the proportion of SP particles is low (Dearing et al. 1996). High values of χ fd per cent in the deeper layers (e.g. the

4 876 T. Yang et al. Table 1. Magnetic parameters for sediments from Guozheng Lake. Core χ χ fd ARM SIRM S-ratio χ χ fd ARM SIRM S-ratio (10 8 m 3 kg 1 ) per cent (10 5 Am 2 kg 1 ) per cent (10 3 Am 2 kg 1 ) 1 10 cm cm Min GZ1 Max Mean SD cm 5 37 cm Min GZ2 Max Mean SD cm cm Min GZ3 Max Mean SD cm 7 19 cm Min GZ4 Max Mean SD cm 3 21 cm Min GZ5 Max Mean SD Table 2. Magnetic parameters for sediments from Tanglin Lake. Core χ χ fd ARM SIRM S-ratio χ χ fd ARM SIRM S-ratio (10 8 m 3 kg 1 ) per cent (10 5 Am 2 kg 1 ) per cent (10 3 Am 2 kg 1 ) 1 2 cm 3 16 cm Min TL1 Max Mean SD cm 3 16 cm Min TL2 Max Mean SD cm 5 24 cm Min TL3 Max Mean SD cm 5 36 cm Min TL4 Max Mean SD mean value of χ fd per cent is 6 per cent within the 3 16 cm interval in core TL2, Table 2) indicate that the background magnetic assemblage contains a significant proportion of SP particles. IRM acquisition curves for representative samples from the upper 2 18 cm of cores (e.g. GZ1 1, GZ1 10, GZ3 1, GZ3 15, TL1 2 and TL4 1) are close to saturation below 300 mt (Fig. 4), and the S-ratio ranges from 0.86 to 0.96 (Tables 1 and 2), which indicates that low-coercivity ferrimagnetic minerals dominate the magnetic properties of the upper part of the cores. However, samples from deeper layers (e.g. GZ1 23, GZ3 40, TL1 12 and TL4 35) usually are still not magnetically saturated at above 500 mt, or even at 1000 mt (Fig. 4). S-ratios are correspondingly much lower (Tables 1 and 2), which suggests that the sediments in the deeper part of the cores are dominated by higher coercivity minerals.

5 Magnetic responses of urbanization processes 877 Figure 3. Downcore variations of magnetic properties for sediment cores from East Lake, Wuhan, China. Temperature-dependent magnetic susceptibility (κ T) curves (Fig. 5) are normalized with the room temperature value (κ RT ). The lack of reproducibility between heating and cooling curves suggests that the observed susceptibility peaks might not be true Hopkinson peaks, but rather that they are indicative of the thermal breakdown of magnetic minerals during heating. The upper sediments from Tanglin Lake and cores GZ3 and GZ4 in Guozheng Lake have similar behaviour (Figs 5a and b). The gradual susceptibility increase between room temperature and approximately 300 C, which is succeeded by a gradual susceptibility decrease between 360 and 400 C,

6 878 T. Yang et al. Figure 3. (Continued).

7 Magnetic responses of urbanization processes 879 Figure 4. Isothermal remanent magnetization (IRM) acquisition curves for representative samples from sediment cores from East Lake. The depth of the sample within each core is indicated by the number after the hyphen (in cm). Figure 5. Curves of temperature-dependent magnetic susceptibility for representative samples from sediment cores from East Lake. Each curve was normalized to its corresponding magnetic susceptibility at room temperature (κ RT ). The thick and thin lines denote heating and cooling runs, respectively. might be attributed to the transformation of maghemite to hematite, and accounts for per cent of the susceptibility decay after cooling. The marked T c of approximately C (Figs 5a c), indicates that magnetite dominates the magnetic properties of the uppermost sediments (Dunlop & Özdemir 1997). For the uppermost sediments from cores GZ1 and GZ2 (Fig. 5d), the peak at low temperature ( C) in the heating curve is suggestive of the thermal breakdown of greigite (with major decrease in magnetization between 270 and 350 C; Roberts 1995) or smythite (Fe 9 S 11 ;Hoffmannet al. 1993). Above 500 C, there is a neoformation of a large amount of secondary magnetite, which could be attributed to decomposition of Fe-rich silicates/carbonates (e.g. clay minerals) (Hoffmann et al. 1999) and/or to breakdown of pyrite to magnetite (Passier et al. 2001). Magnetite produced by

8 880 T. Yang et al. Figure 6. Representative SEM micrographs of magnetic extracts. Particles a d were extracted from sample TL3 1, while particles e and f were extracted from samples TL3 5 and TL3 10, respectively. Symbols (+) indicate the spot locations for energy-dispersive X-ray analyses. these alterations could be responsible for the significant increase in magnetic susceptibility after cooling, which is about two times higher than κ RT (Fig. 5d). For samples from deeper parts of cores (Figs 5c, e and f), the curves are noisy; however, several magnetic phases are clearly present: (1) the continuous κ increase between room temperature and approximately 200 C with subsequent stability to 320 C suggests the presence of maghemite (Fig. 5c); (2) the presence of magnetite is revealed by the T c of C (Fig. 5c) and (3) a T c of 680 C (Figs 5c and e) confirms the presence of hematite. Attempts to obtain κ T curves for samples from much deeper layers were hampered by a low signal-to-noise ratio due to the much lower initial magnetic susceptibility. SEM observations reveal that a large number of magnetic spherules with diameters of μm are present in the uppermost sediments. These particles have various morphologies: orange-peel (Fig. 6a), brain-like surface structure textures (Fig. 6b), a hightemperature oxidized pyrite framboid (Fig. 6c) and a slick surface (Fig. 6d). In sediments from the lower parts of cores, however, magnetic particles are much less abundant and have irregular shapes (Figs 6e and f). EDS results indicate that both pure Fe-oxide particles and a complex mixture of alumino-silicates and Fe-bearing compounds are present (Table 3). Table 3. Composition of magnetic particles shown in Fig. 6 (unit: wt per cent). Particles O Fe Al Si Mg K Cr a b c d e f In summary, the magnetic carriers in the uppermost 2 18 cm of the studied sediments are mainly coarser-grained (PSD/MD) magnetite particles, with variable amounts of maghemite and possiblegreigite.incontrast,sedimentfrombelow3to19cmappears to be dominated by high coercivity minerals (e.g. hematite). The similarities between the downcore magnetic variations of the multiple cores indicates that sedimentation rates were relatively constant in the lake, which allows selection of a single core to represent the lake in further analyses. In this study, cores GZ1, GZ3 and TL3 were selected for detailed chemical and organic matter analysis.

9 Table 4. Heavy metal concentrations, LOI and magnetic parameters for sediments from different depth intervals. Magnetic responses of urbanization processes 881 Core Depth interval Cu Fe Pb Zn LOI (mg kg 1 ) (mg g 1 ) (mg kg 1 ) (mg kg 1 ) (per cent) GZ1 GZ3 TL cm cm 1 18 cm cm 1 4 cm 5 12 cm cm 4.2 Heavy metal, organic matter and their association with magnetic properties Heavy metal contents and LOI for cores GZ1, GZ3 and TL3 are summarized in Table 4, and their downcore variations are shown in Fig. 7. Higher heavy metal contents and LOI are observed in the uppermost sediment layers, while values are generally relatively low and stable in the deeper layers, which represent the natural background signature. Pearson s correlation coefficient between heavy metal contents, LOI and magnetic parameters are listed in Table 5. The correlation matrix indicates that magnetic concentration-related magnetic parameters, such as χ, ARM, SIRMandS-ratio, correlate significantly with the content of heavy metals (Cr, Cu, Fe, Pb and Zn) and LOI in Guozheng Lake, but that they only correlate with Cu, Pb and LOI in Tanglin Lake and correlate with Cu, Pb and LOI for all samples. For example, the correlation coefficient between χ and Zn is (at 0.01 significant levels) in Guozheng Lake; however, it is in Tanglin Lake and for all samples, respectively. These differences result from the selection of the samples used for the statistical analyses, possibly due to different concentrations of heavy metals and their association with different magnetic minerals, and to various physical and chemical changes that occurred during their transportation to the lake and after deposition. Beckwith et al. (1986) found that the correlation between heavy metals and χ is lost if sediment samples with low χ values are involved in statistical analyses. Schmidt et al. (2005) argued that a strong correlation between χ and heavy metals only exists for polluted samples with enhanced χ, and proposed that the best correlation of results is obtained using samples with χ values that lie above a site-specific threshold. The exact relationship between magnetic minerals and heavy metals in lake sediments is an open problem that needs further study. Range Mean ± SD ± ± ± ± ± 0.89 Range Mean ± SD ± ± ± ± ± 2.17 Range Mean ± SD ± ± ± ± ± 1.17 Range Mean ± SD ± ± ± ± ± 0.04 Range Mean ± SD ± ± ± ± ± 2.11 Range Mean ± SD ± ± ± ± ± 1.11 Range Mean ± SD ± ± ± ± ± Sources of the magnetic particles and pollutants Magnetic minerals in the studied sediments can be categorized as natural or non-natural. The former come mainly from exposed soils that contain detrital (lithogenic) and neoformed (pedogenic) magnetic minerals, and from erosion of the rocks that entered the lake by surface runoff and/or by eolian transportation. These natural magnetic minerals should be the dominant magnetic carriers in the lower parts of the cores, where the magnetization is relatively weak and stable (Fig. 3). In the uppermost several centimetres of the cores, however, high values of χ, SIRM, S-ratio, low χ fd per cent and the presence of coarse-grained (PSD/MD) magnetite and spherules with morphologies (Figs 6a d) that are typical, as is the case in many other settings (Fialova et al. 2006; Magiera et al. 2006; Jordanova et al. 2006; Blaha et al. 2008), of non-natural origins. Based on the environmental setting of the lake, the nonnatural magnetic minerals are inferred mainly to represent: (1) fly ashes from WISC, QTPP and other industrial plants (e.g. cement works); (2) run-off from paving materials from roads, abrasion of tires and vehicle emissions; (3) anthropogenic atmospheric dust (e.g. construction materials, dusts from metallurgical and other plants, and household coal burning, etc.) and (4) industrial and domestic wastewater from sewage outlets. Among these sources, the first should be dominant, as indicated by the presence of high concentrations of coarse-grained Fe-rich spherules with industrial origin in the uppermost sediments (Figs 6a d). Natural sources of heavy metals include topsoil and weathered rocks exposed in the catchment. The natural organic matter in the lake sediments consists of fragments and leaves of plants that grew in the catchment or that entered the lake via runoff or eolian transportation, as well as communities of aquatic algae and bacteria (Kruge et al. 1998). For non-natural heavy metals and organic matter, the lakeshore has several major point sources in addition to

10 882 T. Yang et al. Figure 7. The concentration of heavy metals and LOI against depth for sediment cores (a) GZ1, (b) GZ3 and (c) TL3. the coal-fired power plant and steelworks, cement plants, and metal processing plants to the northeast. Fisheries significantly contribute heavy metals and organic matter to the lake sediments. In addition, discarded garbage, spilled fuel and lubricating oil from vessels are expected to be sources of organic matter in the lake sediments. Other non-point sources of contaminants could include combustion by-products from residential heating and from motor vehicles. 4.4 History of industrial and anthropogenic pollution Based on the 0.74 cm a 1 sedimentation rate of the core that is close to core GZ3 (Yang et al. 2004), and the sedimentation rate of 0.23 cm a 1 for core TL3, temporal constraints (Fig. 8) can be provided by correlation of magnetic properties for the studied cores. The magnetic concentration-related parameters vary synchronously with steel output from WISC, power generation from QTPP and coal consumption in Wuhan city (Fig. 8). All of the magnetic parameters are almost constant below 28 cm for core GZ3 (Fig. 8a) and below 13 cm for core TL3 (Fig. 8b), respectively; these lower parts of the cores can be regarded as recording the background magnetization of the sediment, and indicate a stable sedimentary environment before the 1960s. The weak magnetization and the high χ fd per cent values are attributed to natural magnetic sources, which are mainly detrital hematite/maghemite derived from soil and weathered rock. The lower contents of heavy metals and organic matter in the lower part of the cores further reflect a fairly clean natural environment with little pollution, which is also supported by historical evidences (Zhang & Wei 1998). Since 1957, the WISC and QTPP were put into production. Coal was the major fuel for power generation and steelmaking, while the QTPP was the major source of power (>95 per cent) for the city until Coal consumption has therefore risen, for example, up to

11 Table 5. Correlation coefficient between heavy metal concentration, LOI and magnetic parameters of lake sediments. Magnetic responses of urbanization processes 883 Cr Cu Fe Mn Ni Pb Zn LOI χ a a a a a a Guozheng χ fd per cent b b a Lake ARM a a a b a a a (n = 17) SIRM a a a a a a S-ratio b a b a a a χ a b a Tanglin χ fd per cent a a Lake ARM a b a (n = 10) SIRM a b a S-ratio a a χ a b a a All χ fd per cent a a a samples ARM a a a a (n = 27) SIRM a b a a S-ratio a b a a a a Correlation is significant at the 0.01 level (two-tailed). b Correlation is significant at the 0.05 level (two-tailed). Figure 8. Comparison of the downcore magnetic properties for cores (a) GZ3 and (b) TL3 with the outputs from QTPP and WISC over the last 50 yr. The age models were constructed based on sedimentation rates of 0.74 and 0.23 cm a 1, respectively, for the two cores. Power, steel and coal represent power generation from QTPP, the steel output from WISC and the coal consumption in Wuhan, respectively.

12 884 T. Yang et al. Figure 9. Schematic magnetic record of the history of anthropogenic inputs into East Lake, Wuhan, China million tons in 1957, which is four times higher than in With increasing coal consumption, fly ash and emissions produced by coal combustion and steel production were transported and deposited into the lake, causing the increase of magnetization of the lake sediment since 1957 (Fig. 8). However, industrial activity and urbanization developed much more rapidly since political reform of China in Coal consumption in Wuhan significantly increased since then (Fig. 8), for example, amounting to 12.3 million tons in 1998, which is 94 and two times that in 1946 and 1978, respectively. More industrial plants were built around the lakeshore; production capacity of the WISC and QTPP has enlarged since the 1980s, which is reflected in their outputs (Fig. 8). For example, the steel

13 output for WISC and power generation for QTPP were 13 million tons and 4.8 billion kwh in 2005, which were 3.3 and 1.9 times those in 1985, respectively. Consequently, increasing amounts of fly ash produced by coal combustion has been continuously emitted into the atmosphere, and has accumulated in the lake. These fly ashes contain abundant magnetic particles (mainly magnetite) with toxic metals (e.g. Pb, Zn, Cu, Cr and Cd) (Hansen et al. 1981; Hunt et al. 1984; Spiteri et al. 2005), which has contributed to the significant increases in the magnetic (Fig. 8) and heavy metal contents (Fig. 7) of the lake sediments. Industrial development and urbanization bought larger numbers of vehicles to the roads around the lake. For example, the motor vehicles in Wuhan in 2005 is 9.5 times higher than in Vehicle emissions and erosion/abrasion of vehicle bodywork, when washed into the lake, will contribute magnetic particles, heavy metals and organic matter to the lake sediment (Hoffmann et al. 1999; Kapička et al. 2003; McIntosh et al. 2007). These inputs will increase the total organic matter and heavy metal contents in the recent lake sediments (Fig. 7). In summary, we can establish a magnetic record of industrial and anthropogenic pollution in the East Lake area (Fig. 9). The environmental quality of the lake was fairly good before the 1960s. The magnetization of the sediments was much weaker (Fig. 9c); the magnetic carriers were dominated by the relatively stable inputs of natural and detrital material from the lake catchment and possible eolian inputs that could have originated from outside of the lake catchment. However, since 1957, after construction of the QTPP and WISC in the upwind area of the lake, and after isolation of the lake from the Yangtze River, the magnetization of the sediments started to increase (Fig. 9b). Since the 1980s, industrialization and urbanization around East Lake have accelerated and lake pollution increased, as reflected by the elevated contents of anthropogenic heavy metals, organic matter (Fig. 7), and magnetic minerals in the lake sediments. These parameters have been increasing continuously since then and peaked in the 1990s, and have remained at a high level since then (Fig. 9a). 5 CONCLUSIONS Detailed magnetic measurements, heavy metal and organic matter analyses were performed on sediment cores from East Lake in Wuhan, China, to examine the magnetic response of the lake sediments to the environmental history of industrialization and urbanization in the area. The following major conclusions can be drawn from this study. (1) Concentrations of magnetic particles, heavy metals and organic matter are significantly elevated in the uppermost sediments, but all have lower and more stable values in the deeper layers. The magnetization of recent sediments (upper 2 18 cm) is dominated by coarser-grained (mostly PSD/MD) magnetite-like particles resulting from anthropogenic and industrial activities. In contrast, sediments from the deeper parts of the cores are dominated by natural high coercivity minerals (e.g. hematite). (2) The elevated magnetization and concentration of heavy metals and organic matter in the uppermost sediments are attributed to anthropogenic and industrial activities, such as emissions from the power plant and steelworks upwind of the lakeshore, vehicle emissions on the roads encircling the lake, and industrial and domestic wastewater from these plants and residential areas around the lakeshore. This contrasts with the relatively weak magnetization, and stable and low heavy metal and organic matter content in the lower parts of the studied cores that are dominated by natural Magnetic responses of urbanization processes 885 inputs, such as from topsoil and weathered rocks from the nearby hills. (3) The magnetic properties of the sediment cores from East Lake document the environmental history that is linked to industrial and anthropogenic activities in the lake catchment. The environmental quality of the lake was fairly good before the QTPP and WISC were constructed and put into production in Since then, the magnetization of the sediment has increased continuously, especially after industrialization and urbanization around East Lake accelerated since the 1980s, peaked in the 1990s and has been maintained at a high level since then. (4) Variations in the magnetic properties of sediment cores can reveal valuable information about anthropogenic environmental impacts. We suggest that magnetic properties can respond sensitively to environmental status, and that magnetic measurements of lake sediments could be an increasingly powerful and efficient tool to reconstruct the anthropogenic and industrial history of the surrounding catchment. ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (No ). The authors thank Mr Weilan Xia at Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, for his generous help in the radionuclide analysis. We also thank Erwin Appel, Andrew Roberts and another anonymous reviewer for their thorough reviews and helpful suggestions that greatly improved the paper. REFERENCES Beckwith, P.R., Ellis, J.B., Revitt, D.M. & Oldfield, F., Heavy metals and magnetic relationships for urban source sediments, Phys. Earth planet. 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