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.     

Preliminary Results from Paleomagnetic Records on Lake Sediments from South America

The preliminary results of paleomagnetic and radiocarbon dating of late pleistocene-holocene sediments from two lakes of south-western Argentina (41°S, 71.5°W) are presented. The magnetic susceptibility, intensity and direction of the natural remanent magnetisation were measured. The stability of the natural remanent magnetisation was investigated by alternating field demagnetisation. The magnetic parameters allowed the cores within each lake to be correlated. ¹³ C analysis, total organic content measurements and C ¹⁴ dating were carried out. A model of sedimentation is suggested. Using this model and the correlation, curves of variations of magnetic inclination and declination in time are shown.     

Inversion of titanomaghemite in oceanic basalt during heating

We studied the change of magnetic behaviour upon laboratory heating of altered mid-ocean ridge basalt (MORB) samples in the age range of 16-35 Ma to determine the influence of titanomaghemite inversion on the thermal demagnetisation of natural remanent magnetisation (NRM) of these basalts. MORB samples were heated to successively higher temperatures and at the same time the temperature dependence of either saturation magnetisation or NRM was monitored continuously. After each heating step, hysteresis loops and remanent magnetisation curves between 10 K and room temperature were measured. With this procedure, it is shown that the dominant magnetic remanence carrier in our MORB samples is cation deficient titanomaghemite. Moreover, it is demonstrated that the titanomaghemite is gradually changing to a Ti-poor titanomagnetite as the final inversion product. During inversion, both the Curie temperature as well as the maximum unblocking temperature of the NRM are gradually increasing. We show that the paradox of unblocking temperatures above the Curie temperatures often observed for altered MORBs is an artefact of this gradual, heating induced inversion process.     

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.     

Post-depositional remanent magnetization in a re-deposited deep-sea sediment

Re-deposition of some deep-sea clay material in the earth's magnetic field is described. At the time of an induced reversal of the ambient field declination, calcium carbonate layers were introduced for identification purposes. NRM and bulk susceptibility measurements along the still moist sediment cores showed the presence of a time lag of around 10 days between a change in the ambient field direction and a corresponding change in the stable magnetization. AF-demagnetization of discs sampled from a dried core suggested that this time lag was caused by a post-depositional remanent magnetization mechanism. Magnetomineralogical investigations show the presence of the thermally unstable magnetic mineral phase, probably titano-maghemite. A positive correlation between NRM-intensities and water temperature during deposition is discussed. It is suggested that consolidation-rate is a major factor influencing the NRM intensity in deep-sea sediments.     

Utilization of magnetic susceptibility and intensity of remanent magnetisation for chronostratigraphic correlation of lake sediments

Summary Magnetic susceptibility (k) and intensity of remanent magnetisation (J) logs can be used for chronostratigraphic correlation of the late Holocene stratigraphic sequence of the southern basin of Mara Lake. Q-ratio (J/k) logs also show a good correlation from core to core within the lake basin.Chronostratigraphic correlations of cores from Mara, Shawnigan and Christina Lakes were carried out on the basis of paleodeclination and paleoinclination log information and substantiated by the relative position of the Mt. Mazama tephra in the Mara and Shawnigan Lake stratigraphic sequences. This information was used as dating control to construct and compare magnetic susceptibility, intensity of remanent magnetisation and the Q ratio record for each of the three lakes. None of these three parameters would appear to be useful for correlation from lake basin to lake basin. Therefore, on the basis of this study magnetic susceptibility, intensity of remanent magnetisation and Q ratio logs can be used for intra-lake basin correlation but not for lake to lake correlation.Presented at 2nd conference on New Trends in Geomagnetism, Castle of Bechyn, Czechoslovakia, September 24–29, 1990.     

Paleomagnetic and sedimentological studies at Lake Tahoe,California-Nevada

Three closely spaced 6-m piston cores were taken in the central part of Lake Tahoe. Cores were split into two complete replicates for paleomagnetic study and the remaining sections were used for stratigraphic and mineralogical analysis.Stratigraphic correlation of the cores is based on two distinctive horizons (volcanic ash and diatomite) and upon three different sedimentological regimes dominated by (1) poorly bedded silts and muds, (2) well bedded graded units, and (3) finely laminated silts. These correlations served as the standards for the evaluation of the paleomagnetic data. Extrapolation of¹⁴C dates obtained in the upper sections of the Lake Tahoe sediments suggests that the lower sections of the cores may reach ages of 25,000–30,000 years B.P.X-ray, optical, Curie point, and hysteresis measurements show that magnetite is the only important magnetic mineral in the sediments and occurs in the size range of 10 μm. Hematite is essentially absent. Based on large changes in the declination and inclination of the natural remanent magnetism (NRM) within single graded layers the paleomagnetic signature is a post-depositional remanent magnetism (PDRM). This PDRM is believed to be caused by magnetic orientation during compaction.Paleomagnetic measurements show three regimes that are correlated with the stratigraphic regimes. NRM declination and inclination data show good correlation between the three cores and agree well with the correlations based on sediment character. NRM intensity variations are due largely to the variations in magnetite content and its occurrence as either single detrital grains or as inclusions within the larger silicates. Thus the variation in paleo intensity was not determined.Comparisons of Lake Tahoe data with that from Mono Lake show fair correlations of declination and inclination. The occurrence of a short-wavelength, high-amplitude event in the lower section of the Lake Tahoe cores may provide confirmation of the Mono Lake geomagnetic excursion.     

The application of high resolution IRM acquisition to the discrimination of remanence carriers in Chinese loess

Summary High resolution isothermal remanent magnetisation (IRM) acquisition was performed on forty-one Chinese loess samples with the aim of investigating the effects of weathering on the remanence carrying mineralogy. Magnetic susceptibility was taken as a measure of the degree of weathering. This is a valid assumption based on previous studies which have demonstrated a close relationship between climate and magnetic susceptibility in the Chinese loess. Detailed analysis, based on fitting accumulative log-gaussian curves to IRM acquisition data, reveals up to four distinct coercivity components. The remanences of the two harder coercivity components are found to be almost independent of magnetic susceptibility. On the other hand, the remanences of the two softer coercivity components are found to be positively correlated with magnetic susceptibility. Most probably the two harder components are associated with detrital iron-oxides in the primary dustfall, whereas the softer components were created during pedogenesis.     

First archaeomagnetic results and dating of Neolithic structures in northern Greece

Archaeomagnetism in Greece has continuously developed during the last decades. Numerous studies have provided high quality data and accurate secular variation curves for the direction and intensity of the geomagnetic field have been constructed. The Greek Secular Variation Curves (SVCs) cover the last 8 millennia for intensity and 6 millennia for direction. The coverage of the archaeological periods remains uneven, with several gaps, mostly in the directional dataset, with only two results for periods older than 2500 B.C. In the present contribution, the first archaeomagnetic results from Neolithic settlements in northern Greece are presented. For the present study, samples were collected from three different archaeological sites: burnt structures in Avgi (Kastoria) and Vasili (Farsala) and one oven from Sosandra (Aridaia). The natural remanent magnetization (NRM) grouping of all specimens indicated that the majority of the samples were burnt in situ, providing thus a reliable direction of the ancient field. Magnetic cleaning (both alternating-field and thermal) revealed the presence of one stable component of magnetisation. Rock magnetic experiments (acquisition of isothermal remanent magnetization (IRM), thermal demagnetisation of the IRM, thermomagnetic curves) have been performed on pilot samples indicating that low coercivity magnetic minerals such as magnetite or Timagnetite are prevailing. The mean directions (declination D, inclination I and parameters of the Fisherian statistics), which arose from the three sites are as follows: Sosandra: D = 343°, I = 55.6°, ??₉₅ = 4.8°; Avgi: D = 10.1°, I = 53.4°, ??₉₅ = 4.2° and Vasili: D = 357.5°, I = 43.1°, ??₉₅ = 4.1°. The obtained data are in a very good agreement with results from Neolithic Bulgaria. This study represents the beginning of an effort to fill the gaps of the Greek secular variation curves and their extension to the Neolithic period.     

Particulate pollution monitoring in the Elefsis Gulf: The role of mineral magnetic studies

Measurements of ‘quadrature’ susceptibility (X_q) at low and high frequency on seabottom sediments of the Gulf of Elefsis supplement the results derived from the study of magnetic susceptibility (X) and saturation isothermal remanent magnetization (SIRM) (Scoullos et al., 1979) and make possible the distinction between the background soil erosional fraction and that of anthropogenic combustion origin. New devices speed up the on-site appraisal of the impact of discharges on marine sediment quality.     

Magnetic properties of olivine basalt: Application to Mars

The natural remanent magnetisation (NRM) of basalt lava containing (oxidised) magnetic phases is usually assumed to be proportional to the weight percent of magnetic phases. It is shown here that olivine basalt has a different behaviour. The NRM intensity increases at an increasing rate with the amount of magnetic phases. This is attributed to the oxidation of olivine during cooling of the basalt that leads to the exsolution of magnetite in a single domain state. In this way, olivine basalt is found to become an order of magnitude more magnetic than basalt that does not contain olivine. A simple explanation for the magnetic anomalies on Mars is offered, based upon these findings and Mössbauer spectroscopy data from Gusev crater on Mars.     

Analysis of geomagnetic secular variations 10 000 to 30 000 years bp,Lac du Bouchet,France

Cores of the bottom sediments from Lac du Bouchet, in the Haute Loire, France (44.9° N, 3.8° E) have been taken using Mackereth type pneumatic piston corers. The sediments have been dated by palynological control back to 15 000 years bp (years before present), and by radiocarbon age determinations using both conventional and accelerator methods. The depth/time transform provisionally used here dates the sequence from 10 000 to 30 000 years bp. The stacked records of demagnetised remanent magnetisation from eight cores have been examined. Fourier and maximum entropy method spectral analyses show good agreement. Inclination and declination spectra are different in that the dominant inclination periods tend to fall in the negative part of the complex spectrum, while the dominant declination periods tend to fall in the positive part. The VGP path traced out by the remanent magnetisation vector shows predominantly clockwise looping, consistent with essentially westward drifting geomagnetic sources. The detailed form of the VGP path is affected by the degree of smoothing, but clockwise looping always predominates averaging ∼ 67% over the whole time interval investigated. A band of frequencies between ∼ 0.9 and ∼ 0.3 cycles per thousand years is associated with the strongest bias towards clockwise rotation (∼ 80% to 90%) while a band between ∼ 0.3 and 0.1 cycles per thousand years is associated with only ∼ 50% clockwise rotation. Individual core records show some negative inclinations at horizons where inclination minima occur suggesting that ‘excursions’ should be regarded as extreme values of secular variation rather than aborted polarity reversals of the main field.     

The Blake magnetic polarity episode in cores from the Mediterranean Sea

Palaeomagnetic and oxygen isotope studies on samples taken from five cores in the Mediterranean area are described. Directions of the stable remanent magnetisation show the presence of an anomalous (reversed) zone during the transition between the oxygen isotope substages 5e and 5d. This position corresponds to the age of about 117 kyr B.P. and thus to the Blake polarity episode. The duration of this episode is estimated to be about 6000 years. The study of the anisotropy of the magnetic susceptibility shows no indication of sedimentological disturbances in recorded sections. The Blake episode therefore represents a stratigraphic marker which may be used to link the continental and marine palaeoclimatic and sedimentary records.     

New Palaeomagnetic Data from the Betic Cordillera: Constraints on the Timing and the Geographical Distribution of Tectonic Rotations in Southern Spain

—A palaeomagnetic investigation has been carried out at 14 sites on Jurassic red nodular limestones from the central and eastern part of the External Zones of the Betic Cordillera (Subbetic and Prebetic Zones). Progressive thermal demagnetisation of samples from the Subbetic Zone reveals the presence of two stable magnetic components of the natural remanent magnetisation: 1) a secondary Neogene syn-folding component and 2) the original Jurassic magnetisation. As similar characteristics have been reported in Jurassic limestones from the western Subbetic Zone, a widespread remagnetisation event took place within <10⁶ years in the entire Subbetic region during Neogene times. In contrast, in the Prebetic region, no evidence for a secondary overprint has been detected. Palaeomagnetic Jurassic declinations indicate variable and locally very large clockwise rotations (35°–140°), but the two sites in the north-westernmost part of the investigated region are not rotated. The use of both components of magnetisation and the incremental fold-test results allowed the timing of block rotations in the Subbetic Zone to be constrained. Rotations in the western Subbetic occurred after the acquisition of the secondary overprint, whereas in the central part of the Subbetic Zone they were completed by the time of the remagnetisation event.     

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.     

Magnetic monitoring of top soils of Merida (Southern Mexico)

This paper focuses on the study of the correlation between magnetic parameters with the level of contamination by heavy metals in urban soils. We report a magnetic investigation of urban soil samples from Merida, state of Yucatan, Southern Mexico. It appears that most of our samples contain ferrimagnetic minerals as the magnetic carriers, probably coming from the titanomagnetites/titanomaghemites series. This is inferred by the acquisition of isothermal remanent magnetization, which shows that most of samples are almost completely saturated at about 200 mT. The S₋₂₀₀ value (factor characterizing stability of remanent magnetization) is between 0.8 and 1.0, characteristic of ferrimagnetic minerals. The susceptibility vs. temperature measurements also point to some titanomaghemites and titanomagnetites as probable responsible for magnetization. On the other hand, measurements of magnetic susceptibility at high and low frequencies helped us to determine the high content of superparamagnetic grains in the majority of the samples, although not all of these showed high values of magnetic susceptibility. We found that the most contaminated samples by Pb and Zn showed the higher saturation isothermal remanent magnetization values, whereas the higher values in magnetic susceptibility correspond to samples contaminated by Cr. Finally, we noted that a high level in Sr decreases the magnetic susceptibility.     

Paleomagnetism and ore mineralogy of some basalts of the Geirud Formation of Late Devonian - Early Carboniferous age from the southern Alborz,Iran

Basaltic lavas from the southern Alborz, an area about 40 km northeast of Tehran, Iran, have been paleomagnetically investigated. The lavas are of Late Devonian-Early Carboniferous age, and belong to the basal member of the Geirud Formation. At 11 sites a total of 80 cores was drilled.Detailed analyses by means of progressive demagnetization of the natural remanent magnetization (NRM) were made both by the application of alternating magnetic fields and by heating. Also, on a number of specimens a study was done both with thin sections and with polished sections. There proved to be general agreement between the properties of the characteristic NRM and the kind of Fe-Ti oxides in the lavas. In the case of specimens containing magnetite only the characteristic NRM was entirely removed at temperatures just below 600°C, or in alternating fields up to 1500/2000 Oe peak value; on the other hand, in specimens containing both magnetite and a substantial part of hematite (martite) the final part of the characteristic remanence was removed at temperatures above 600°C, and this remanence resisted alternating fields above 2000 Oe peak value. From the characteristic site-mean directions of 5 sites an average paleomagnetic direction is computed withD = 210.8°,I = 66.9°, and α₉₅ = 3.9°.This result might be taken as an indication that at the Devono-Carboniferous transition the southern part of the Alborz was located in the present Indian Ocean off the Arabian coast.     

Magnetic anomalies and rock magnetism of basalts from Reykjanes (SW-Iceland)

This study presents rock magnetic properties along with magnetic field measurements of different stratigraphic and lithologic basalt units from Reykjanes, the southwestern promontory of the Reykjanes peninsula, where the submarine Reykjanes Ridge passes over into the rift zone of southwestern Iceland. The basaltic fissure eruptions and shield lava of tholeiitic composition (less than 11500 a old) show a high natural remanent magnetization (NRM, J_r) up to 33.6 A/m and high Koenigsberger ratio (Q) up to 52.2 indicating a clear dominance of the NRM compared to the induced part of the magnetization. Pillow basalts and picritic shield lava show distinctly lower J_r values below 10 A/m. Magnetic susceptibility (κ) ranges for all lithologies from 2.5 to 26×10⁻³ SI.     

Assessment of soils pollution extent in surroundings of ironworks based on magnetic analysis

This paper reports on magnetic and magnetomineralogical studies of soils influenced by ironworks activity. Researches were conducted in five areas, of which Ostrowiec Świętokrzyski ironworks area is described in detail. A map of magnetic susceptibility was created based on field measurements and samples taken from soil profile were analyzed in laboratory. Measurements of magnetic susceptibility in two frequency ranges, anhysteretic remanent magnetization, isothermal remanent magnetization thermomagnetic and geochemical analysis were carried out. SEM was used to identify ferromagnetic fractions. As a result the horizontal and vertical extent of heavy metals pollution was established.