Magnetic characteristics of dustfall in urban area of north China and its environmental significance

Urban particulate pollutants not only affect urban air quality but also directly threaten public health. Lanzhou has been one of the most seriously polluted cities in the world due to its special geophysical location and weather conditions, which makes it an ideal place for urban particulate pollution study. In this study a set of environmental magnetic parameters (χlf, χfd%, χARM, SIRM, HIRM, SOFT and back field IRM) were measured and analyzed for the dustfall samples collected monthly during 1997.4―2000.5 and 2004.11―2005.10 at Lanzhou University campus (KLD and CCS) and at a clear site of Gaolanxian (GLX). Results demonstrate that the main magnetic minerals in the dustfall are magnetite, with some maghaemite and haematite. Cross plots of Mrs/Ms vs. Bcr/Bc and χfd% vs. χARM/SIRM indicate that the main magnetic grain sizes are pseudo-single-domain (PSD). The high concentration of magnetic min-erals in the dustfall indicates that Lanzhou is seriously polluted. The main pollution sources are an-thropogenic activities and nature dust, with the former posing a great threat during the whole year, which is different from the general view that the natural dust is the main contributor in summer. With great efforts of the government and local authority in the past decades, the air quality in Lanzhou has been improved in winter, but it is not so obvious in summer. This study also reveals that as a simple, rapid and accurate technology, environmental magnetism could be a valuable tool for urban pollution study.     

Relative paleointensity of the geomagnetic field during the past 200 ka from the West Philippine Sea and its chronological significance

Our rock magnetic analysis of core Ph05 from the West Philippine Sea demonstrates that the core preserves a strong, stable remanent magnetization and meets the magnetic mineral criteria for relative paleointensity (RPI) analyses. The magnetic minerals in the sequence are dominated by pseudosingle-domain magnetite, and the concentration of magnetic minerals is at the same scale. Both the conventional normalizing method and the pseudo-Thellier method were used in conjunction with the examination of the rock magnetic properties and natural remanent magnetization. Susceptibility (χ), anhysteretic remnant magnetization (ARM) and saturation isothermal remnant magnetization (SIRM) were used as the natural remanent magnetization normalizer. However, coherence analysis indicated that only ARM is more suitable for paleointensity reconstruction. The age model of core is established based on oxygen isotope data and AMS¹⁴C data, which is consistent with the age model estimated from RPI records. The relative paleointensity data provide a continuous record of the intensity variation during the last 200 ka, which correlates well with the global references RPI stacks. Several prominent low paleointensity values are identified and are correlated to the main RPI minima in the SINT-200 record, suggesting that the sediments have recorded the real changes of geomagnetic field. Supported by National Natural Science Foundation of China (Grant No. 90411014) and Pilot Project of the Knowledge Innovation Program of Chinese Academy of Sciences (No. KZCX2-YW-211)     

Environmental mechanism of magnetic susceptibility changes of lacustrine sediments from Lake Hulun, China

The changes of magnetic susceptibility(κ) are correlated with those of corresponding sedimentological, geochemical, mineralogical and biological results, which verifies thatκ can be taken as one of the environmental proxies. However, usually the exact origin of magnetic signal is poorly understood, and is difficult to relate with the environmental evolution. Magnetic properties of material derived from the catchment and sedimentary environment may affect the accumulation, preservation, or authigenesis and diagenesis of magnetic minerals. In the Lake Hulun region in Inner Mongolia, it is found that muddy sediments, deposited during high water level period (corresponding to humid climate), have comparatively highκ values. In contrast, the sandy sediments, deposited during low water level period (corresponding to arid climate), have lowκ values. Detailed rock magnetic investigation confirms that detrital magnetite derived from volcanic rocks in the catchment exists in both muddy and sandy sediments. During high water level period, secondary ferrimagnetic iron sulphide was produced in muddy sediments under relatively reductive conditions. Ferrimagnetic iron sulphide, coexisting with detrital magnetite, predominates the magnetic properties of muddy sediments, resulting in increasingκ. This paper reveals the significance of authigenic ferrimagnetic iron sulphide produced after sediment deposition.     

Magnetic and mineralogical characteristics of hydrocarbon microseepage above oil/gas reservoirs of Tuoku region, northern Tarim Basin, China

The Tuoku region in northern Tarim Basin of China is a key area for studying oil/gas reservoir rocks. The magnetic and mineralogical parameters of well cuttings from two wells, well S₇, situated on oil/gas field, and well S₆, at an oil/water interface, were measured. The two wells are located in the same structure with similar strata and types of lithology, but well S₆ is a showing well of oil and gas 5 km northwest of well S₇. The purpose of this paper is to evaluate the possibility and distribution of secondary magnetic alteration that may have occurred due to hydrocarbon migration above an oil/gas accumulation. It is concluded that the magnetism of well cuttings from major strata in well S₇, including source rocks, oil reservoir rocks and cap rocks, and in Quaternary (Q) soil is higher than that from well S₆. The Cambrian oil-bearing strata and cap rocks have even higher magnetism in well S₇. The shape and parameters of magnetic hysteresis loops indicate that soft (H _c<20 mT,H _s<0.3 T) ferrimagnetic components dominate the magnetic carriers within the strongly magnetic strata of well S₇, whereas a mixed paramagnetic and ferrimagnetic distribution occurs in well S₆ (for example, low coecivityH _c and nonsaturating magnetized character). Analysis of heavy minerals shows that the contents of iron oxide (magnetite, maghemite and hematite) in well S₇ are often higher than those in well S₆. The magnetite content in samples of cuttings from Cambrian rocks can reach 9.7% in oil-bearing strata in well S₇, and in strata Ekm and N₁j are 1.215% and1.498%, respectively. Typical spherical magnetite grains are found within the main source rocks and the soils in well S₇. By analysis of surface microtexture and of trace element contents, we infer that the spherical magnetite is composed of aggregates of ultrafine particles that are probably authigenic magnetite formed in a hydrocarbon halo background. Project supported by the National Natural Science Foundation of China and the Geological Industry Foundation of China (Grant No. 49374216) and Foundation of the State Key Laboratory of Oil/Gas Reservoir Geology and Exploitation of China (Grant No. 9608).     

Carriers of magnetism and vertical inhomogeneities in ultramafic-mafic rocks of the Kondyor Massif (Aldan shield)

Dunite samples from a borehole drilled in the platiniferous concentrically-zoned Kondyor Massif are studied by electron spin resonance (ESR). The spectrum profiles, relative intensities I, and volume magnetic susceptibilities κ are analyzed. These values experience strong irregular variations, sometimes by an order of magnitude, in the upper and medium parts of the column, at depths from 100 to 400 m; and the variations decay at greater depths. The magnetic properties of the samples are determined by iron (II) ions in the olivine lattice and by iron (III) ions in the magnetite and pyrrhotite microphases and in the products of breakdown of the solid solution: chromiferous magnetite, chromoferrite, etc. The I and κ values are directly related: κ_max = 27.8 × 10⁻³ SI units, κ_min=2.63 × 10⁻³ SI units, and κ_mean = 12.7 × 10⁻³ SI units. The maximum κ values are found in the zones with elevated contents of magnetite and pyrrhotite particles, and the minimum ones, in zones with few medium and small clusters with Fe³⁺ ions. The uneven distributions of solid solutions and magnetic phases over depths are suggested to be related to the disturbances in the conditions of crystallization.     

Rock-magnetic properties of Neogene sediments, northern flank of Tianshan Mountains

Analyses of rock-magnetic properties of Neogene sediments of the Taxihe section, northern Tianshan Mountains, show that the section can be classified into three categories including lacustrine facies, fluvial facies and alluvial facies, which correspond to the lower, middle and upper of the Taxihe section respectively. The magnetic minerals of the lacustrine facies may be affected by the process of weath- ering, lithogenesis and biolithogenesis besides the source of the sediments. The natural remanence intensities are between 10-3 A/m and 10-2 A/m. The minerals are dominated by magnetite and the high coercive magnetic mineral may be goethite. The magnetic grains are the mixture of PSD SD or SD SP. The natural remanence intensities of the strata of fluvial facies are between 10-2 A/m and 10-1 A/m, about ten times that of the lacustrine facies. The magnetic minerals are mainly magnetite and hematite, and the magnetic grains are mainly PSD. The characteristic remanence (ChRM) carriers are magnetites. In the alluvial facies, the natural remanence intensities are mostly less than 1×10-2 A/m. The magnetic minerals of the series are dominated by magnetite and hematite, almost the same as the fluvial facies. But the difference is that most of the stepwise demagnetization can reveal two components and the ChRM carriers are hematites. The magnetic grains are PSD in terms of the hysteresis parameters.     

Magnetic signature of different vegetation species in polluted environment

Detailed magnetic study on vegetation samples from several strongly polluted and clean sites in Bulgaria is carried out in order to evaluate suitability of different species as passive dust collectors in magnetometry. From each location, available species among lichens, mosses, poplar leaves, dandelion, needles have been sampled. Magnetic susceptibility calculated on mass-specific basis shows wide variability between diamagnetic signal up to 846 × 10⁻⁸ m³/kg. Lichens and mosses are found to be the species, showing magnetic signals with the strongest contrast between clean and polluted environment. The main magnetic phase is magnetite-like according to the results from thermomagnetic analysis of susceptibility on magnetic extracts. Scanning electron microscopy (SEM) microphotographs reveal the presence of abundant particulate matter on vegetation surface both with anthropogenic (spherules) and lithogenic origin. Magnetic grain size deduced by the ratio of saturation remanent magnetization (SIRM) and mass-specific magnetic susceptibility (χ) and coercivities (B_c and B_cr) suggest that different species accumulate preferentially small SD-like grains from pollution emissions. Contrasting relationship of the ratio of anhysteretic remanent magnetization (ARM) and χ for polluted vs clean sites deduced by needles and lichens may be related to transformation of the accumulated dust particles within lichens’ tissue. This finding indicates that the exact species used as biological dust collector is of importance when studying spatial grain size distribution of magnetic dust particles. Pilot study on polycyclic aromatic hydrocarbons (PAH) content and its relation to magnetic parameters shows good correspondence between high levels of PAHs and high SIRM values for locations affected by non-ferrous industrial production.     

Transformation relationship among different magnetic minerals within loess-paleosol sediments of the Chinese Loess Plateau

The dominant magnetic minerals and carriers of magnetic signals within the Chinese Loess Plateau are magnetite, maghemite, hematite, and goethite. In this study, we investigated the provenance and evolution of magnetic minerals during loess pedogenesis, using X-ray diffraction (XRD) and optical and electron microscopy, including field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). Our results reveal that single- and multiphase mineral assemblages among magnetic minerals in the loess-paleosol sequence have been formed. Partial oxidation of coarse eolian magnetite has occurred in the desert source area and the oxidation degree is enhanced after deposition of the dust upon the Chinese Loess Plateau. This mode of origin resulted in a microtexture consisting of an inner magnetite core surrounded by a hematite rim, and strongly affected the magnetic characteristics of the loess. Goethite coexists with hematite in the loess and paleosol, and nanometer-scale hematite is formed upon goethite rims via dehydration. Our study provides direct mineralogical evidence of the magnetic record and paleoclimatic implications of the loess-paleosol sequence of the Chinese Loess Plateau. Supported by National Natural Science Foundation of China (Grant Nos. 40772032 and 40573054) and National Basic Research Program (Grant No. 2007CB815603)     

The role of dust source area and pedogenesis in three loess-palaeosol sections from North Bulgaria: A mineral magnetic study

Rock magnetic studies of three loess-palaeosol sections from Bulgaria (Harletz and Orsoja in the NW and Durankulak in the NE at the Black Sea coast) have been carried out. Thermomagnetic analyses of magnetic susceptibility point to magnetite as the major ferrimagnetic carrier in the loess and palaeosol units. Maghemite gives a significant contribution to the total magnetic signal in the recent soils (S₀), while in one of the sections-Orsoja-hematite may also be present. The effective magnetic grain sizes deduced from the hysteresis measurements and the ratios M_rs/M_s and B_cr/B_c fall in the pseudo single domain (PSD) range, but the data distributions in a Day diagram for the three different sections are shifted. This is suggested to be caused by different detrital input (e.g. different dust source areas) and varying degrees of pedogenic modification. The calculated background susceptibilities χ_bg differ significantly as well. The lowest value is obtained for the Durankulak section-χ_bg = 9.95 × 10⁻⁸ m³/kg, which is in accordance with the data from other studies of loess-palaeosol profiles from the Black Sea area. The corresponding background susceptibilities for the other two sections studied-Harletz and Orsoja-are significantly higher (33.1 and 17.75 × 10⁻⁸ m³/kg, respectively). Both parent material and pedogenesis are found to be responsible for the observed differences in the magnetic characteristics.     

Magnetic Susceptibility Screening of Anthropogenic Impact on the Danube River Sediments in Northwestern Bulgaria - Preliminary Results

Several events of anthropogenic impacts on the floodplain river sediments of the Danube along a section near the town of Oryakhovo (NW Bulgaria) have been detected using magnetic technique. In the field, magnetic susceptibility was measured using three susceptibility meters – MS2D, KT-5 and SM30. The differences in the surface susceptibility values measured with the different instruments are ascribed to the various penetration depths which depend on several parameters like sensor diameter, frequency and field strength. This is supported by the detailed laboratory study on penetration depth and sensitivity of the new SM30 susceptibility meter and the comparison with the already existing data for the other two sensors. Boundaries between different flooding events are clearly linked with significant variability of the measured susceptibility values corresponding to one and the same level. Sediment cores, taken at different distances from the water level, show the history of depositional events and corresponding degree of anthropogenic pollution. Identification of the magnetic phases responsible for the signal was carried out by means of thermomagnetic (T) curves. The main carrier is magnetite with Tc of 580°C or oxidized magnetite with Tc of 600°C. A second kink at about 300°C is better expressed for samples with lower susceptibilities. It may correspond to several minerals, e.g. maghemite, pyrrothite, titanomagnetite, as well as different mineral transformations of paramagnetic minerals (decomposition, dehydroxilation, etc. for example siderite, lepidocrocite). The samples showing strong magnetic enhancement are characterized by the predominance of magnetite. Optical microscopy on magnetic extracts shows the presence of small spherical particles, typical for the anthropogenic magnetic phases from high-temperature technological processes. Well expressed susceptibility variations along the cores suggest the presence of several stages of different degree of pollution, covering an estimated period of about 30 years.     

Rock-magnetic properties of topsoils and urban dust from Morelia (>800,000 inhabitants), Mexico: Implications for anthropogenic pollution monitoring in Mexico’s medium size cities

In this work, we investigate the correlation between some magnetic parameters and the level of contamination by heavy metals in urban soils from Morelia city, western Mexico. The magnetic study was carried out on 98 urban soils samples belonging to distinct land uses. Most of analyzed samples contain ferrimagnetic minerals as the responsible for magnetization, most probably corresponding to the titanomagnetites/ titanomaghemites solid solutions. This is inferred from the susceptibility vs. temperature measurements and the isothermal remanent magnetization (IRM) experiments. These measurements also indicate that most of samples are almost completely saturated before 300 mT. Additionally, the S_-200 values (S_-200 = IRM_-200/ SIRM, where IRM_-200= Back-field of 200 mT after magnetic saturation) are between 0.7 and 1.0, characteristic of low coercivity magnetic minerals. The averaged saturation isothermal remanent magnetization (SIRM) curves can be used as an indicator of pollution level, as these curves show different saturation values according to the level of contamination by heavy metals: Cu, Ni, Cr and Sr. These associations of (titano)magnetite with heavy metals were observed by Scanning Electron Microscope revealing some complex aggregates rather than commonly detected spherules.     

Paleosecular variations 12–20 Kyr as recorded by sediments from Lake Moreno (Southern Argentina)

We present results of paleomagnetic and sedimentological studies carried out on three cores Lmor1, Lmo98-1, Lmor98-2 from bottom sediments of Lake Moreno (south-western Argentina), and integrate them with data from our previous studies. Measurements of directions (declination D and inclination I) and mass specific intensity of natural remanent magnetization (NRM intensity), magnetic susceptibility (specific, χ and volumetric, κ), isothermal remanent magnetization (IRM), saturation of isothermal remanent magnetization (SIRM), and back field remanent coercivity (B_0CR) were performed. The stability of the NRM was investigated using alternating-field demagnetization. The results show that these sediments meet the criteria required to construct a reliable paleomagnetic record. The cores were correlated very well based on magnetic parameters, such as χ and NRM intensity, as well as with lithological features. Tephra layers were identified from the lithological profiles and magnetic susceptibility logs. We obtained the D and I logs of the characteristic remanent magnetization for the cores as a function of shortened depth. The data from the three cores were combined to form a composite record using the Fisher method. A comparison between stacked inclination and declination records of Lake Moreno and those obtained in previous works on Lake Escondido and Lake El Trébol shows good agreement. This agreement made it possible to transform the stacked curves into time series spanning the interval 12–20 kyr. The results obtained improved our knowledge of SV and the behaviour of the geomagnetic field and also allowed us to determine the range of past inclination variations from −70° to −45° for the southern hemisphere, where data are scarce.     

Response of the paleomagnetic record to environmental changes in the late Pleistocene

During revisiting the Upper Pleistocene Pekla loess-soil section located on the Sea of Azov coast of the Taman Peninsula, its lower 6 m were continuously sampled, which led to an increase in the age range from ∼50 to 400 ka. The detailed rock magnetic study of the structure, grain-size, and concentrations of magnetic mineral (natural remanent magnetization (NRM) carrier) in the collected rock samples revealed regular changes in rock magnetic characteristics along the section and their correlation with climatic fluctuations. Magnetite and hematite both deposited during the transport of sedimentary material and formed during pedogenesis, which involved the entire section to a varying extent, represent the main magnetic minerals in the examined rocks. Automorphic paleosoils that were formed during warm and humid periods corresponding to odd stages of the MIS scale are characterized by elevated concentrations of magnetic mineral (NRM, magnetic susceptibility (K _lf), saturation isothermal remanent magnetization (SIRM), and anhysteresis (ideal) remanent magnetization (ARM)) parameters and share of superparamagnetic particles (up to 80%, according to elevated values of the frequency-dependent magnetic susceptibility K _td) as well as by lowered rigidity parameter (B _cr) and grain size (ARM/K parameter). Such changes in the paleosoils may be explained by the occurrence of newly formed fine-grained magnetite particles close in size to its superparamagnetic and single-domain varieties due to the activation of bio/geochemical processes during warm stages. The growth of the above-mentioned rock magnetic parameters in automorphic soils may be considered as serving a quantitative criterion for defining the boundary between warm and cold periods even in poorly developed soils.     

Characteristics of low-frequency oscillation intensity of air-sea turbulent heat fluxes over the northwest Pacific

Based on the daily turbulent heat fluxes and related meteorological variables datasets (1985–2006) from Objectively Analyzed air-sea Fluxes (OAFlux) Project of Woods Hole Oceanographic Institution (WHOI), characteristics of low-frequency oscillation intensity of air-sea turbulent heat fluxes over the northwest Pacific are analyzed by linear perturbation method and correlation analysis. It can be concluded that: 1) the distribution of low-frequency oscillation intensity of latent heat flux (LHF) over the northwest Pacific is mainly affected by that of low-frequency oscillation intensity of anomalous air-sea humidity gradient (Δq′) as well as mean air-sea humidity gradient (), while the distribution of low-frequency oscillation intensity of sensible heat flux (SHF) is mainly affected by that of low-frequency oscillation intensity of anomalous air-sea temperature gradient (ΔT′). 2) The low-frequency oscillation of turbulent heat fluxes over the northwest Pacific is the strongest in winter and the weakest in summer. And the seasonal transition of low-frequency oscillation intensity of LHF is jointly influenced by those of low-frequency oscillation intensity of Δq′, low-frequency oscillation intensity of anomalous wind speed (U′), and mean wind speed (Ū), while the seasonal transition of low-frequency oscillation intensity of SHF is mainly influenced by those of low-frequency oscillation intensity of ΔT′ and Ū. 3) Over the tropical west Pacific and sea areas north of 20°N, the low-frequency oscillation of LHF (SHF) is mainly influenced by atmospheric variables q _a ′ (T _a ′) and U′, indicating an oceanic response to overlying atmospheric forcing. In contrast, over the tropical eastern and central Pacific south of 20°N, q _s ′ (T _s ′) also greatly influences the low-frequency oscillation of LHF (SHF). Supported by National Natural Science Foundation of China (Grant No. 40675028) and National Basic Research Program of China (Grant No. 2006CB403600)     

Inversion of stress field in Guangdong and its adjacent area

We determine the stress field of Guangdong and its adjacent area by using focal mechanism solutions of 137 earthquakes and obtain tectonic stress tensors in 12 zones. The result shows that the azimuth of maximum principal stress σ1 is approximately WNW in southwestern Fujian, southern Jiangxi, Guangdong’s Heyuan and the Pearl River Delta, NW in Guangdong’s Yangjiang, and nearly NNW in the two zones of eastern Guangxi and Beibuwan Gulf (the Northern Gulf), varying clockwise in WNW-NW-NNW from east to west. The azimuth of minimum principal stress σ3 varies from NNE to ENE. The relative magnitude of medium principal stress σ2 (R value), is the smallest in Beibuwan and largest in Longyan of Fujian. Strike-slip faulting is dominated in the study area.     

Study of changes in the rock formation medium of the Karadzha section (Azerbaijan) from the results of petromagnetic investigations

The petromagnetic characteristics of marine and subaqueous deposits on the upper marine terrace of the Karadzha section (Azerbaijan) are investigated. These deposits correspond to the great Khvalynsk transgression of the Paleocaspian. Thirteen meters of the section encompassing an age interval of ～45–20 ka are sampled continuously. Regular along-section changes are revealed, and their relation to environmental changes is established. Magnetite, maghemite, and hematite that were formed in the process of alteration of the maternal rocks in the Paleocaspian wash-down basin are the main ore minerals of the studied rocks. The ratios of these minerals in the deposits depend on the degree of alteration of the maternal rocks. Both the total amount of ore mineral (K and SIRM parameters) and the rigidity parameter B _cr regularly increase in the basal part of the section and in the sediments reflecting finer variations in the basin level. An increased magnetic rigidity and the sensitivity of petromagnetic parameters to weak variations in the sea level are characteristic features of sediments in this part of the Paleocaspian compared with normal marine sediments.     

ARM,TRM and magnetic interactions: Concentration dependence

ARM (anhysteretic remanence)/SIRM (saturation isothermal remanence) and TRM (thermoremanence)/SIRM were measured as a function of the concentration (volume fraction) of single-domain magnetite (3 × 10^?6 ? C ? 2 × 10^?2), ARM/SIRM increases with decreasing concentration, showing that there is magnetic interaction between fine particles. The role of magnetic interaction in TRM acquisition is also important at higher concentrations of magnetite (C ? 0.1%), where the value of TRM/SIRM increases with decreasing concentration. It is only for concentrations ofC ? 0.1% that the value of TRM/SIRM is fairly constant and interactions among magnetite grains seem to be ignored. The ratio TRM/ARM decreases from seven to almost unity as the concentration of magnetite decreases.     

Magnetic properties and heavy metal pollution of soils in the vicinity of a cement plant,Xuzhou (China)

This study characterized the magnetic property and levels of heavy metals of the topsoils near a cement plant. The concentrations of five selected heavy metals (Pb, Cu, Zn and Cd) were measured on 32 topsoil samples (0–20 cm) collected near a cement plant via inductively coupled plasma/mass spectroscopy (ICP-MS). The orders of enrichment factors (EF), on average, were Cd (7.3) > Cu (3) > Zn (2.9) > Pb (2.1), respectively. A self-organizing map (SOM) was applied to the concentrations of heavy metals for “correlation hunting”. Mineral magnetic concentration parameters, such as the specific magnetic susceptibility (χ), susceptibility of anhysteretic remanent magnetization (χ_ARM), saturation isothermal remanent magnetization (SIRM), together with interparametric ratios (such as IRM_− 100mT/SIRM, SIRM/χ, χ_ARM/SIRM) show that ferrimagnetic, superparamagnetic (SP) and multi-domain (MD) minerals dominated the soils. The results of correlation analysis indicate that copper showed a significant correlation with χ, χ_ARM and SIRM but such a relationship with χ, χ_ARM and SIRM was only weakly identified for Zn, Cd and Pb.     

Rock magnetic properties of a loess-paleosol couple along an N-S transect in the Chinese Loess Plateau

We have conducted detailed rock magnetic experiments on samples from loess unit 8 (L8) and paleosol unit 8 (S8) in Jingbian, Yichuan and Duanjiapo loess sections along an N-S transect in the Chinese Loess Plateau. Major rock magnetic results are as follows: (i) An increase of high field susceptibility (χ_h) in the same level of loess or paleosol from north to south is observed, suggesting an enhancement of pedogenesis. (ii) The low field susceptibility(χL) in loess unit L8 is almost the same in three sections. In contrast, the χ_L of paleosol unit S8 in Yichuan is highest, and the χ_L of Duanjiapo is lower than that in Yichuan section, suggesting that there is not correlation between the χ_L and the degree of pedogenesis in loess-paleosols. (iii) With the increasing of χ_L, both the contents of the superparamagnetic (SP) and the ferrimagnetic grains in loess-paleosol increase, however, the enhancement of magnetic susceptibility is probably dependent more on the increase of the ferrimagnetic concentration than on a change in the grain size. (iv) The content of the maghemite in loess unit 8 increases from the northern to the southern part of the Chinese Loess Plateau, and is positively correlated with the pedogenesis of the loess.     

福州盆地全新世沉积物的磁学性质及其对环境变化的响应

福州盆地位于海陆过渡地带,在海陆变迁过程中,沉积物记录了高分辨率的环境信息,是揭示沉积特征对环境变化响应过程及模式的理想区域.本文选择位于福州盆地的FZ5钻孔进行岩石磁学、环境磁学和古地磁学方面的研究,以期阐明该区域沉积物磁学性质对陆源碎屑输入、海平面变化和成岩作用的响应.岩石磁学结果表明钻孔沉积物以低矫顽力的亚铁磁性矿物为主体,但是在不同的环境变化阶段,磁性矿物的类型有较大变化.在9~3 cal. ka BP的海侵过程中,沉积物中以磁铁矿为主体,存在菱铁矿和铁硫化物等还原性矿物.硫化作用使细粒磁铁矿溶解形成胶黄铁矿和黄铁矿,其峰面随碎屑磁性矿物的浓度变化而迁移.但硫化作用没有完全消除磁铁矿携带的特征剩磁和陆源碎屑输入量以及海平面升降对该阶段沉积物磁性的控制.在~3 cal. ka BP以来随着海平面下降、沉积环境向陆相氧化环境转化,虽然早期还原作用仍然存在,但后期氧化作用使磁性矿物向高矫顽力的赤铁矿等矿物转变,氧化作用基本扰乱了磁铁矿携带的剩磁.沉积及其后期成岩作用过程中,发生在约~8.2、~7.7、~7.5、~2.7、~1.5、~0.5 cal. ka BP六次强烈的古氧化界面反映了福州盆地当时异常干旱或湿热的气候事件.