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1.
The intensity of rotational remanent magnetization (RRM) acquired by single-domain greigite at a rotation frequency of 5 rps was combined with measurements of anhysteretic remanent magnetization (ARM) to calculate the effective biasing field ( Bg ) that produced the RRM. Samples of single-domain greigite had Bg values between -137 and -84 μT, and a MDFRRM of c. 80 mT. By contrast, a suite of natural and synthetic ferrimagnetic iron oxide samples, including single-domain magnetite and y Fe3O4 tape particles, acquired Bg values between -3 and -14 μT, and MDFRRM ranged between 43 and 68 mT (when RRM was acquired). Multidomain magnetite did not acquire a RRM. Bg values at 5 rps were calculated from previously published data for magnetite particles of different grain sizes, which revealed a minimum Bg value of -24 μT and a MDFRRM of 57 mT for the finest fraction (0.2-0.8 μm in diameter). In a geological example, measurements of Bg and MDFRRM were used to detect the presence of greigite in a 4 m long Late Weichselian sediment core. Variations in inclination, declination and the intensity of the natural remanent magnetization (NRM) correlate with changes in magnetic mineralogy.  相似文献   

2.
The Middle and Upper Jurassic Bathonian-Oxfordian shallow-water carbonate rocks from the Paris Basin, France, consist mainly of oolitic and bioclastic limestones that are hydrocarbon reservoirs in the subsurface. Despite a preliminary positive study, these deposits have been considered to be largely remagnetized (Rochette, private communication), and hence not amenable to palaeomagnetic dating. To establish their magnetic mineralogy and test this remagnetization hypothesis, we have used an integrated investigation combining petrographic, geochemical, rock-magnetic and palaeomagnetic measurements on samples extracted from five cores from the Paris Basin and from outcrops in Burgundy. Magnetic minerals in the Bathonian-Oxfordian carbonates include: (1) primary biogenic single-domain magnetite and detrital multi-domain Ti-magnetite and their oxidized form, maghemite; (2) authigenic spheres of magnetite probably related to hydrocarbons; and (3) goethite, either restricted to ferruginous ooid layers or resulting from surficial alteration, notably replacement of pyrite framboids. Rock-magnetic experiments carried out on 68 samples reveal H cr/ H c and M rs/ M s ratios ranging from 1.88 to 5.58 and 0.017 to 0.314, respectively. These values are clearly distinct from diagnostic values for a chemical remagnetization. Pyrrhotite was not identified within these sediments. Moreover, the average H cr/ H c ratio of 3.14 is significantly different from the value of 1.333 for natural pyrrhotite (Dekkers 1988). These results have a direct implication for the preservation of the primary magnetization; consequently, these deposits are selectively amenable for magnetostratigraphic dating and possible regional correlations.  相似文献   

3.
Rock magnetic parameters are often used to recognize variations in the original magnetic mineralogy and for normalizing purposes in palaeointensity studies. Incipient weathering, however, is shown to have a profound but partly reversible influence on the rock magnetic properties of the marls of the Early Pliocene Trubi formation in southern Sicily (Italy). The remanence in the marls resides in single-domain (SD) magnetite grains, but the remanent coercive force (Hcr) shows a strong variation and most values observed are anomalously high ( Hcr) range 36–188 mT).
The enhanced coercivities are attributed to stress in the magnetite grains induced by surface oxidation at low temperature. Upon heating to 150 °C a reduction of coercivities occurs that can be explained by a stress reduction as a result of a reduction of Fe2- gradient due to a higher diffusion rate at elevated temperature. After heating to 150 °C, coercivities are quite uniform throughout the outcrop and the values are characteristic of SD magnetite (Hcr range 30–38 mT). The bulk susceptibility increases by 4–24 per cent, and the isothermal remanent magnetization (IRM) decreases by 5–11 per cent. The increase in anhysteretic remanent magnetization (ARM) is large: 20–242 per cent. The magnitude of the changes is related to the degree of weathering.
Another effect of heating the marl samples to 150 °C is a substantial reduction of the coercivities of the secondary overprint in the natural remanent magnetization. After heating. separation of the secondary and primary components by alternating-field demagnetization is more efficient. The usual difficulties of thermal demagnetization above 300 °C may thus be avoided by a combination of moderate heating to 150 °C and subsequent alternating-field demagnetization.  相似文献   

4.
Summary. ARM and I sr stability with respect to alternating field and low-temperature treatment were measured for a set of artificial rock specimens. Each of the specimens contained a known amount of a natural magnetic mineral of a known grain size. The following minerals were used: a homogeneous natural magnetite, a natural titanomagnetite with submicroscopic Fe-rich inclusions, and a natural titanomagnetite and a natural titano-maghemite that both show exsolution phenomena. The grain size of the fractions varied from < 5 to 250 μm. It is found that the stability of ARM and I rs of multi-domain grains with respect to alternating fields and low-temperature treatment is similar, whereas for the smallest grains studied (< 5 μm) ARM is more stable than I sr. Multi-domain ARM data from this study are in agreement with current multi-domain ARM theory. Low-temperature behaviour of ARM and I sr can be explained either in terms of a model in which stable moments in the bulk of the grains are screened by the surrounding matrix or in terms of a model in which regeneration of the bulk remanence plays a role and in which stable moments are situated on the surfaces of the grains.  相似文献   

5.
A detailed record of mineral magnetic properties of a loess–palaeosol profile comprising seven loess horizons, six interbedded palaeosols and recent soil at the top in NE Bulgaria is analysed. A strong contrast between the soil and loess susceptibilities as well as other concentration-dependent hysteresis parameters is present, similar to the well-documented magnetic characteristics of the Chinese loess ( Hus & Han 1992 ; Maher & Thompson 1992 ; Heller & Evans 1995 ; Hunt et al. 1995 ). The magnetic enhancement of the palaeosol units is caused by very fine-grained pedogenic magnetite with superparamagnetic behaviour. Thermomagnetic analyses on bulk material suggest magnetite and maghemite as the main ferrimagnetic carriers in both soil and loess horizons. Their relative proportions are shown to reflect different palaeoclimatic conditions. Chernozem soils, which include recent soil S0 and first and second palaeosols S1 and S2 developed under steppe vegetation, show a high degree of low-temperature oxidation of the pedogenic magnetite to maghemite. This material is characterized by coercive force H c showing even higher values than those of the parent loess material. The older palaeosols (S4 to S6 ) were formed during more humid climatic conditions and therefore probably developed as forest types. Rock magnetic data suggest the existence here of only partly oxidized magnetite grains. The behaviour of the thermomagnetic curves, characterized by a kink at 200 °C, may be due to either a release of internal stress (built up as a result of partial low-temperature oxidation) or interactions between two phases.  相似文献   

6.
TRM deviations in anisotropic assemblages of multidomain magnetite   总被引：2，自引：0，他引：2  
Anisotropic assemblages of multidomain magnetite particles develop an anisotropy of magnetic susceptibility (AMS), which in turn induces deviations of thermo-remanent magnetization (TRM) from the field direction. From the theories of multidomain TRM acquisition, it is shown that the TRM anisotropy tensor has its eigenvalue ratios ( T i) related to the principal weak-field susceptibility ratios ( P i) by the order of magnitude T i≃ P 2i. This relation has been experimentally verified on two sets of highly anisotropic rock samples. The exponent has been determined to be 1.94 in the samples from a Peruvian gabbro, and 1.81 in those from the granite of Flamanville (NW France). Accounting for experimental difficulties in determining the TRM anisotropy tensors, these exponents are judged to agree well with the expected one. It is therefore stressed that AMS measurements provide a good means of evaluating the magnetic field direction from deviated TRM directions, providing magnetic carriers are mainly multidomain magnetites.  相似文献   

7.
A continental sequence of red beds and interbedded basaltic layers crops out in the Sierra Chica of Córdoba Province, Argentina (31.5°S, 64.4°W). This succession was deposited in a half-graben basin during the Early Cretaceous. We have carried out a palaeomagnetic survey on outcrops of this basin (147 sites in seven localities). From an analysis of IRM acquisition curves and detailed demagnetization behaviour, three different magnetic components are identified in the volcanic rocks: components A, B and X are carried by single- or pseudo-single-domain (titano) magnetite, haematite and multidomain magnetite, respectively. Component A is interpreted as a primary component of magnetization because it passes conglomerate, contact, tilt and reversal tests. The carrier of the primary magnetization, fine-grained (titano)magnetite, is present in basalts with a high degree of deuteric oxidation. This kind of oxidation is interpreted to have occurred during cooling. Components B and X are discarded because they are interpreted as recent magnetizations. In the sedimentary rocks, haematite and magnetite are identified as the carriers of remanence. Both minerals carry the same component, which passes a reversal test. The calculated palaeomagnetic pole, based on 55 sites, is Lat. 86.0°S, Long. 75.9°E ( A 95=3.3, K =35). This palaeomagnetic pole supersedes four with anomalous positions reported in previous papers.  相似文献   

8.
The low-temperature magnetic properties of magnetite are reviewed, and implications for rock magnetism considered. The behaviour of fundamental properties of magnetite at low temperatures near the Verwey transition ( T v ) are documented, and attention is given to various Verwey transition theories. The low-temperature behaviour of the magnetic energies that control domain structure is reviewed in detail. For the first time in rock magnetic literature, the low-temperature anomaly in spontaneous magnetization ( M s ) is documented and the differences between the saturation magnetization and M s near the Verwey transition are discussed. It is argued that the low-temperature behaviour of the magnetocrystalline anisotropy, and in particular the anomaly at T v , is most likely to affect multidomain remanence during low-temperature cycling. For multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through T v is likely to reduce the stability of closure domains.  相似文献   

9.
Loess-palaeosol deposits in the lower Danube area represent the southeastern edge of the loess cover in Europe. Detailed rock magnetic investigations of the loess/palaeosol sequence in Viatovo in NE Bulgaria reveal that magnetite and maghemite of very fine superparamagnetic grain size are responsible for the magnetic enhancement of palaeosol units. A detailed palaeoclimatic record is obtained through high-resolution measurements of magnetic susceptibility, frequency dependent magnetic susceptibility and CaCO3 content. Magnetic proxies indicate a more warm and humid climate during the development of the older palaeosol units (S4–S6).  相似文献   

10.
Rock magnetic properties of the maar lake sediments of Lac St Front (Massif Central, France) reflect environmental changes during the last climatic cycle. High magnetic concentrations are measured in the sediments deposited under glacial climatic conditions, while lower concentrations correspond with more temperate climatic periods. Low- and high-temperature measurements indicate that the remanence is carried by (titanium-poor) magnetite. However, some maghemite and haematite is present in sediments deposited under temperate conditions.
Normalized intensities and coercivities of the anhysteretic remanent magnetization (ARM) are clearly higher for the sediments deposited during the temperate climatic periods of the Eemian, St Germain I, II and Mid-glacial than for glacial sediments, but other magnetic parameters hardly differ between these groups. Due to slight differences in magnetic composition and possible effects of grain interactions, it is not straightforward to relate this different ARM behaviour to magnetic grain-size variations. For the Holocene sediments, rock magnetic parameters indicate a larger grain size. This trend is also suggested by granulometric experiments with an optical laser granulometer. Dissolution of smaller grains is the most likely explanation for this larger grain size.
Changes in magnetic composition and grain size are extremely limited for the glacial sediments, but magnetic concentration varies considerably. Magnetic concentration maxima in the glacial sediments of Lac St Front correlate with those of the nearby Lac du Bouchet (Thouveny et al. 1994). Correlating the susceptibility records of these sequences with the δ18O record of the GRIP ice cores (Thouveny et al. 1994) suggests that magnetic concentration maxima may correspond with short cold climatic episodes, associated with Heinrich events.  相似文献   

11.
EPR evidence for maghemitization of magnetite in a tropical soil   总被引：1，自引：0，他引：1  
Electron paramagnetic spectroscopy (EPR) was used in combination with standard rock magnetic methods to study magnetic minerals in a tropical soil. The susceptibility and hysteresis measurements showed magnetite grains with a Curie temperature near 850 K as the dominant magnetic remanence carriers in the soil. A minor Ti content in the magnetite was found by energy dispersive X-ray analysis. In order to get insight into the weathering status of the magnetite, different chemical treatments, including oxalate and citrate–bicarbonate–dithionite (CBD) extraction, were applied to the soil samples. The hysteretic properties exhibited no significant differences between the untreated and the CBD or oxalate treated samples. By contrast, the comparison of the EPR spectra revealed a significant broadening of the linewidth (δB) and a shift of the g -values ( g eff) to lower fields after the CBD treatment. Furthermore, the spectral parameters g eff and δB exhibited an angular dependence. At low temperature, the CBD treated samples showed a jump in δB between 120 and 100 K, the temperature range characteristic for the Verwey transition in magnetite. The changes in the spectral properties after the CBD treatment, which dissolves ferric oxides, were attributed to the removal of maghemite formed by the oxidation of magnetite, that is, during the maghemitization of the magnetite grains.  相似文献   

12.
Thermal generation of ferromagnetic minerals from iron-enriched smectites   总被引：1，自引：0，他引：1  
In recent years remagnetization of orogenic belts has been explained by fluid migration through rocks undergoing deformation. A laboratory study of remagnetization is presented in which varying amounts of iron (0-13.5 weight per cent Fe2O3) are adsorbed onto smectite surfaces. All smectite samples contain structural Fe (III) which is located in octahedral sites and is thermally stable up to 700 C. An increase in the amount of iron adsorbed onto the clay surface leads to the formation of ferric nanophases in which parts are magnetic. Mineralogical changes that occur during thermal treatment between room temperature and 700 C were monitored using electron spin resonance (ESR), bulk susceptibility, acquisition of isothermal remanent magnetization (IRM) and Curie temperature analysis. After heating the samples to 250 C, a new ferrimagnetic phase is created as indicated by ESR and IRM acquisition. ESR spectra, IRM acquisition and Curie analyses suggest that magnetite is the predominant phase that is being created. These grains continue to be created and grow with heating up to 500 C. Above this temperature a decrease in the intensity of the IRM at 1T suggests that the phase is being transformed into haematite. The thermal experiments on iron-loaded smectites show that surface-induced processes can lead to the formation of new magnetic minerals under conditions characteristic of low-grade metamorphism.  相似文献   

13.
Coercive force of single crystals of magnetite at low temperatures   总被引：1，自引：0，他引：1  
The temperature dependence of coercive force H c was studied on well-characterized and stoichiometric millimetre-sized single crystals of magnetite at a series of 16 temperatures from 300 to 10 K using a SQUID magnetometer. H c decreases gradually with cooling to the isotropic temperature, T i = 130 K, where the first magnetocrystalline anisotropy constant K 1 becomes zero. H c exhibits a sharp increase at the Verwey transition, T v = 120 K, where the structure changes from cubic to monoclinic. In crossing the Verwey transition, H c increases by more than two orders of magnitude, from 20 μT to 2.4 mT, and the shape of the hysteresis loops becomes wasp-waisted.
Observed coercivity between 300 K and 170 K varies with temperature as λ s / M s , where λ s is the magnetostriction constant and M s is the saturation magnetization, indicating that the coercivity in MD magnetite is controlled mainly by internal stress associated with dislocations or other crystal defects. It seems likely that the stable single-domain-like magnetic memory observed in large MD magnetite crystals is due to magnetoelastically pinned domain walls. The discontinuous change in H c at the Verwey transition is controlled by abrupt changes in magnetocrystalline and magnetostriction constants due to crystal deformation from cubic to monoclinic structure.  相似文献   

14.
Mineral and isothermal magnetic properties of Al-, Mn- and Ni-substituted haematites were characterized and their relationships evaluated in order to interpret better the results of magnetic analyses of soils and recent sediments. Aluminium, manganese and nickel haematites generally behaved as single-domain (SD) particles. The influence of incorporated Al on the magnetic behaviour of haematite was consistent with Al acting as a paramagnetic dilutent. Mass magnetic susceptibility ( χ ) and SIRM800 decreased as the level of Al substitution increased. Incorporation of Mn and Ni increased χ , which could be associated with enhancement of the spin canting effect of haematite. The stability of SIRM800 to demagnetization for Al-haematite appears to be related to a defect mechanism associated with the development of smaller crystallites arising from Al substitution. Magnetic domain rotation or flipping was probably inhibited, being blocked by structural defects during magnetization and demagnetization, and resulted in a low but stable partial SIRM (SIRM800 ). %IRM/SIRM800 demagnetization curves and estimated ( B o )CR values of ≤100  mT for Mn-haematite indicate pseudo-single-domain/multidomain-like behaviour despite Mn-haematite having particle and crystallite dimensions similar to Ni-haematite, which did not show this behaviour. Data indicate that parameters involving unsaturated, partial SIRM should be used with caution in magnetic studies of soils and sediments.  相似文献   

15.
Magnetic properties of sediments from a core (10 m long) in the southern basin of the Caspian Sea have been investigated. Varying concentrations of greigite (Fe3S4) dominate the magnetic fraction in Late Pleistocene sediments. The synsedimentary formation of greigite indicates that the Late Pleistocene Caspian Sea was a brackish or fresh-water, poorly ventilated basin and suggests a water level higher than at the present. The variation in magnetic parameters, with the detrital magnetite-bearing fraction remaining constant, is interpreted in terms of greigite grain-size variation and related to the slight variation in water salinity. The Holocene sediments are characterized by detrital magnetite. This indicates better ventilation of the basin and suggests lower water levels than in the Late Pleistocene. The gradual change in magnetic properties of the sediments between 90 and ≈60 cm depth, with decreasing quantities of greigite, indicates stepwise establishment of oxic conditions in the Holocene.  相似文献   

16.
Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter.  相似文献   

17.
Rayleigh hysteresis, as defined by the well-known Rayleigh relations, has been observed not only when magnetization of pyrrhotite-bearing KTB-samples is measured in parallel to a weak dc magnetic field, but also in experiments where field and measuring directions have been adjusted strictly perpendicularly to each other. Nine-tupels of independent Rayleigh hysteresis loops could thus be compiled. Their characteristic coefficients X ijk of initial susceptibility together with the Rayleigh loss coefficients αjk have been proved to determine completely the samples' weak-field magnetic anisotropy. Interpreting the coefficient matrices ( X ijk) and (αjk) as the tensor of initial susceptibility and the Rayleigh tensor, respectively, generalization of the isotropic Rayleigh relations in terms of corresponding tensor relationships has been suggested for the anisotropic case. Application to the KTB samples showed 3-D Rayleigh hysteresis measurements to be an excellent tool for rock magnetic analysis in terms of ore content and crystalline texture. In particular, a magnetocrystalline double texture of the basal planes of pyrrhotite precipitates and their [1120] directions of easy magnetization have been clearly detected. Surprisingly, the welt-known theorem α= const. X 2I, formulated by Néel (1942) for the isotropic case, has been found to hold true even in tensor generalization (αjk) = const ( X 2jk). To reach sufficient resolution for the measurements performed, a sensitive vibrating coil magnetometer (VCM) has been developed.  相似文献   

18.
Gao  Xing  Kang  Shichang  Liu  Qingsong  Chen  Pengfei  Duan  Zongqi 《地理学报(英文版)》2020,30(9):1481-1494
Heavy metals,one of the most toxic classes of pollutants,are resistant to degradation and harmful to the biological environment.The lakes that have developed on the Tibetan Plateau are ideal regions to investigate historic heavy metal pollution,particularly through the use of the reliable 210 Pb dating technique.Environmental magnetism has been successfully applied to estimate heavy metal pollution in different environmental systems due to its characteristics of simple processing steps,good sensitivity,and non-destructibility.However,it has not yet been applied to assess heavy metal pollution in lake sediments on the Tibetan Plateau.A series of environmental magnetic investigations of Qiangyong Co Lake sediments(southern Tibetan Plateau) was therefore conducted to explore the relationship between magnetic minerals and mercury(Hg) concentrations.The results showed that the magnetic mineral species in lake sediments remained stable,with similar levels of four different components from 1899 to 2011.However,the proportion of component 1(C1,hematite) increased continuously with the corresponding decrease in the proportion of C2(goethite),while the proportions of C3 and C4(magnetite) did not change significantly.As a result,the bulk magnetic signals(e.g.,SIRM and clf) were unsuitable for the evaluation of the Hg concentration;however,the proportion of hematite had a strong positive correlation with the Hg concentration.It is possible that the Qiangyong Glacier(the main water supply for Qiangyong Co Lake) has experienced faster melting with global and local warming,and the Hg trapped in cryoconite and ice was released.Hematite,with a large specific surface area,has a strong capacity for absorbing Hg,and both materials are ultimately transported to Qiangyong Co Lake.The proportion of hematite in a sample is therefore a suitable semi-quantitative proxy that can be used to evaluate the Hg concentration in Qiangyong Co Lake sediments.This study confirmed that the variation of magnetic minerals can provide a new method to estimate the variation of Hg concentrations and to study the process of Hg deposition in lakes in the southern Tibetan Plateau on the basis of a detailed environmental magnetic analysis.  相似文献   

19.
Summary. Palaeomagnetic investigations were carried out on nine lava flows around the Dhar region, which constitute the northern part of the Deccan traps. The stability of remanent magnetism of these specimens was analysed by an alternating magnetic field, thermal demagnetization and memory tests. Six flows exhibited characteristic components of magnetization, with a mean direction of D =143°, I =+46° ( K = 107.1, α95=5.5°). This gives a VGP located at 29° N, 67° W (δp=4.5°, δm= 7.0°). The lower site with normal magnetization and the upper five sites with reverse magnetization indicate a geomagnetic field reversal during the initial phases of Deccan volcanism in the Early Tertiary period. A rapid northward migration of about 18° in latitude and a simultaneous anticlockwise rotation of 37° is calculated for the subcontinent.  相似文献   

20.
This study presents the age control and environmental magnetism components of a new, late Pleistocene paleoclimate record for the Great Basin of western North America. Two new cores from the Summer Lake sub-basin of pluvial Lake Chewaucan, Oregon, USA are correlated to basin margin outcrops on the basis of tephrochronology, lithostratigraphy, sediment magnetism and paleomagnetic secular variation. Eleven tephra layers were found in the cores that correlate to tephra identified previously in the outcrop. The Olema ash was also found in one of the cores; its stratigraphic position, relative to 3 dated tephra layers, indicates that its age is 50-55 ka, somewhat younger than has been previously reported. The Summer Lake sediments are divided into deep and shallow lake lithosomes based on sedimentary features. The stratigraphic position of these lithosomes support the tephra-based correlations between the outcrop and the cores. These sediments contain a well resolved record of the Mono Lake Excursion (MLE) and an earlier paleomagnetic excursion as well as a high quality replication of the paleosecular variation immediately above the MLE.Relative sedimentation rates increased dramatically toward the depocenter during intervals of low-lake level. In contrast, during intervals of high-lake level, relative sedimentation rates were comparable along the basin axis from the basin margin to the depocenter. The magnetic mineralogy of the Summer Lake sediments is dominated by pseudo-single domain (titano)magnetite and intervals of high/low magnetite concentration coincide with lithosomes that indicate high/low lake levels. Magnetic grain size also varies in accord with bulk sediment grain size as indicated by the silt/clay ratio. To a first order, variations in magnetic parameters, especially those attributable to the concentration of magnetic minerals, correlate well with global glacial/interglacial oscillations as indicated by marine oxygen isotope stages. This relationship can be explained by increased dissolution of (titano)magnetite minerals as lake level dropped and the lake became more productive biologically. This inference is supported by a correspondence between lower concentrations of magnetite with higher levels of total organic carbon and vice-versa.  相似文献   

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