Motion of the Earth’s magnetic poles in the last decade

Based on vector magnetic data from the CHAMP German satellite, average daily spherical harmonic models of the main geomagnetic field to n = m = 10 have been constructed for the period from May 2001 to the end of 2009 at an interval of 4 days. The obtained 16 models, which were averaged over half a year, have been used to calculate the coordinates of the north and south magnetic poles (the points where magnetic field lines are vertical). The changes in these coordinates during these eight and a half years have been traced. Both poles continue moving northward and westward. The north magnetic pole has traveled 400 km during this period. The velocity of its motion has increased up to the year 2003, reaching 62.5 km yr⁻¹, and then started decreasing and reached 45 km yr⁻¹ by the end of 2009. In addition, the direction of motion changed from north-northwestward to northwestward; i.e., the pole started turning slightly towards Canada. The south magnetic pole moved slower by an order of magnitude and has traveled 42 km during this period. The coordinates of the geomagnetic (dipole) poles and the eccentric dipole parameters have also been calculated. The dynamics of these poles has been traced.     

Paleomagnetism and Cyclostratigraphy of the Middle Ordovician Krivolutsky Suite,Krivaya Luka Section,Southern Siberian Platform: Record of Non-Synchronous NRM-Components or a Non-Axial Geomagnetic Field?

The Middle Ordovician Volginsky and Kirensky fossil zones were sampled in the Krivaya Luka section (Krivolutsky suite) that outcrops along the Lena river in Siberia. The Volginsky and Kirensky zones are coeval to the Llandeilo in the global geologic time scale. The Krivaya Luka section consists of siltstones, clays, sandstones, and limestones, and displays a remarkably distinct sedimentary cyclicity, especially in its reddish middle part.Stepwise thermal demagnetization yields three NRM components. Component A, isolated in the 100—250°C interval can be either normal or reversed. The normal A-component has a direction close to recent local magnetic field. The reversed A-component directions are scattered around a direction close to that of the lower Triassic Siberian traps. Component B has unblocking temperatures that range from 400 to 500°C and is represented mainly by normal polarity directions. The B-component, isolated from rocks of the middle part of the section is of a normal polarity with D = 176.5°, I = 30.0° and a North pole position at 16.2°S, 111.3°E. The other parts of the section are characterized by intermediate B-directions, which resulted possibly by partially overlapping A- and C-components. The highest temperature dual-polarity component C was isolated in the 550—670°C interval, resulting in the detection of two complete polarity zones and three magnetic reversals. The C-component is characterized by the following mean directions: for the reversed component D = 335.7°, I = 6.9°, and for the normal component D = 188.6°, I = 28.0°, which is very close to the normal polarity directions of the B-component. The corresponding paleomagnetic North pole for reversed polarity rocks is 32.6°S, 137°E, which is typical of Middle Ordovician rocks from Siberia – the mean pole for Llanvirn-Llandeilo is 30°S, 136°E (cf. Smethurst et al., 1998) – whereas for normal polarity rocks the pole position 17.2°S, 99.1°E is markedly different. Nevertheless, we assume that the C-component records the ancient geomagnetic field of Ordovician times, even though it does not pass the reversals test. This could be explained by overlapping NRM unblocking temperature spectra for the B and C components. In this case, the paleomagnetic pole positions should be interpreted with some caution.In addition, the section was logged and sampled in detail for cyclostratigraphic purposes. Spectral analysis in the depth domain using the high-field susceptibility as input parameter showed that the observed cyclicity is most likely orbitally forced. Detected spectral peaks (significant at the 95% confidence level) were close to the expected positions of the periodicities of precession, obliquity and eccentricity for the Ordovician. Consequently, the average sediment accumulation rate is estimated at 3.5 cm/kyr. Extrapolating this sedimentation rate yields a total duration of at least 1 Myr for the Volginsky fossil zone and 1.2 Myr for the entire Krivaya Luka section.     

An analytic signal approach to the interpretation of total field magnetic anomalies1

The analytic signal (AS) is defined as the square root of the sum of the squares of the vertical and the two horizontal derivatives of the total magnetic field ΔT. This paper verifies theoretically that peaks of the AS correlate directly with their magnetic causative bodies and are positioned symmetrically over them, i.e. the main feature of the AS is that it is independent of the inclination of the magnetic field. This avoids the difficulties that are often faced in the conventional process of reduction to pole for ΔT, when the direction of magnetization of the causative bodies is not known. In addition, the AS has characteristics similar to the derivative features of the magnetic field, so that it is very sensitive to edge effects of the causative magnetic bodies. The theoretical derivations are tested by comparison with calculations on models, and, in a field example from Hunan Province, China, the AS is applied successfully to the interpretation of ΔT, whereas the conventional process of reduction to pole fails, due to the reverse magnetization of the causative body.     

Chinese tombs oriented by a compass: Evidence from paleomagnetic changes versus the age of tombs

Extant written records indicate that knowledge of an ancient type of compass in China is very old — dating back to before the Han dynasty (206 BC-220 AD) to at least the 4th century BC. Geomancy (feng shui) was practised for a long time (for millenia) and had a profound influence on the face of China’s landscape and city plans. The tombs (pyramids) near the former Chinese capital cities of Xi’an and Luoyang (together with their suburban fields and roads) show strong spatial orientations, sometimes along a basic South-North axis (relative to the geographic pole), but usually with deviations of several degrees to the East or West. The use of the compass means that the needle was directed towards the actual magnetic pole at the time of construction, or last reconstruction, of the respective tomb. However the magnetic pole, relative to the nearly ‘fixed’ geographic pole, shifts significantly over time. By matching paleomagnetic observations with modeled paleomagnetic history we have identified the date of pyramid construction in central China with the orientation relative to the magnetic pole positions at the respective time of construction. As in Mesoamerica, where according to the Fuson hypothesis the Olmécs and Maya oriented their ceremonial buildings and pyramids using a compass even before the Chinese, here in central China the same technique may have been used. We found a good agreement of trends between the paleodeclinations observed from tomb aligments and the available global geomagnetic field model CALS7K.2.     

磁赤道处化极方法

化向地磁极(化极)是最基本的磁测资料处理方法之一,化极能消除或减少斜磁化影响,提高对磁测资料的认识程度和解释水平,对研究地壳产生的磁异常具有重要意义.但低纬度地区特别是磁赤道处,化极处理很不稳定甚至奇异,一直是位场研究的难点.针对地磁纬度较低特别是磁赤道地区磁异常化极的困难,利用从磁北极处垂直磁化向低纬度地区水平磁化方向转换稳定的特点,提出"狭义化赤"概念,并将其与低纬度磁异常"倒相"解释方法结合,提出专门用于磁赤道处化极的方法.该方法扩展了现有的化极理论,实现了磁赤道处的稳定化极.区别于目前任何方法,专门用于(近)水平磁化条件下的化极计算,具有原理简单,实现方便,收敛速度快等特点.对理论模型和实际资料计算表明这种针对磁赤道地区磁异常的化极处理方法是稳定、可靠的.     

SOME NEW MAGNETIC TRANSFORMATIONS*

All magnetic transformations are governed by one simple differential relation between the observed and the transformed quantities. A magnetic map for any component, at any location, and for any given direction of magnetization can be converted into one for which any one, two, or all three parameters differ. Three new magnetic transformations are introduced: (i) reduction to equator, (ii) orthogonal reduction, and (iii) elimination of remanence. The first eliminates (or minimizes) the asymmetry and the lateral shift of the measured total field anomalies, exactly as in Baranov's reduction to pole. The second produces perfect asymmetry so that a symmetrical target lies vertically below the zero anomaly point, midway between the maximum and minimum. When remanence is a contributing factor, the direction of resultant magnetization must be known a priori in all cases, except for transformation of one component into another in the same area. Explicit working formulae are presented for reduction to equator and pole, and orthogonal reduction.     

Paleomagnetic study of some Cretaceous and tertiary rocks on Hispaniola

Summary A reconnaissance paleomagnetic study of Hispaniola shows that three igneous units in the Dominican Republic possess meaningful directions of magnetism. A Late Cretaceous tonalite, an Eocene pyroxene diorite and a Miocene andesite porphyry have been investigated. The rock material studied is fresh, and has not been affected by secondary oxidation except in the case of the andesite which is occasionally weathered and reveals some hydrothermal alteration. Alternating field and thermal demagnetization result in removal of viscous remanence in some samples, while others reveal a good stability of NRM and little change in direction. The results disclose directions of magnetization substantially different from that of the present earth's field in Hispaniola and from those obtained from contemporaneous rocks of North America. They yield paleomagnetic poles at 23.1° N, 144.9° W for the Cretaceous tonalite and at 17.4° N, 138.0° W for the Eocene diorite, the positions of which are not significantly different from each other, suggesting no change of geomagnetic field direction during the two epochs. These poles have generally similar positions to those obtained from Late Cretaceous rocks on Jamaica and Puerto Rico. The Miocene data fall into two groups, one having a direction corresponding to a pole closely coinciding with the Miocene North American pole and the other giving a paleomagnetic pole at 68.3° N, 151.9° W coinciding with the Miocene pole for Jamaica. Paleotectonic interpretation of the results suggests that like other Greater Antilles, Hispaniola has been subject to large anticlockwise rotation since Late Cretaceous.     

First estimate for the age of a mesoproterozoic palaeomagnetic pole from the Volodarsk-Volynsky Massif,the Ukrainian Shield

Palaeomagnetic data are presented from the southern Volodarsk-Volynsky Massif (VVM) of the Korosten Pluton, the Ukrainian Shield. Laboratory experiments (AF and thermal demagnetization, IRM acquisition, thermal separation), field tests (consistency and secular variation methods) and optical observations indicate that single domain and nearly single domain magnetite is the dominant carrier of a primary TRM in the anorthosites. Palaeomagnetic poles from the three sampling sites (Golovino and Turchinka quarries) are indistinguishable at the 95% confidence level and have been combined to yield a mean pole at P_lat = 30 °N, P_lon = 178 °E, a₉₅ = 3.4 °.In the large slow cooling Korosten Pluton the U-Pb zircon/baddeleyite (U_zb) technique gives an age for the anorthosites, which are not equivalent to the time of magnetic blocking. Based on integrated analysis of geochronologic information and blocking-temperature data for magnetic minerals proposed by Briden et al. (1993), a first attempt has been undertaken to estimate the palaeomagnetic pole age from the Mesoproterozoic anorthosites. The Korosten Pluton has cooled from 850 °C (the closure temperature of U-Pb systematics in zircon/baddeleyite) to 350 °C (the closure temperature of K-Ar systematics in biotite) during 150 Ma after the emplacement of the anorthosites. Assuming a uniform cooling of the intrusion yields a rate of 3.3 °C/Ma. The cooling rate for the granites is 3.1 °C/Ma. The mafic and acid rocks have an average rate of 3.2 °C/Ma. Using the cooling gradient for the VVM (3.2 °C/Ma) and the mean natural blocking temperature of magnetite (520 °C) can be determined a remanence age. The estimate for TRM acquisition is 1656 ± 10.0 Ma.The magnetic pole for the VVM is in good agreement with the mean pole from the Baltic quartz porphyry dykes with an age of 1630 – 1648 Ma. The VVM pole is best dated and requires a revision of the latest paleogeographic reconstructions for the Fennoscandian and Ukrainian Shields at 1770 and 1650 Ma. (Pesonen et al., 2003).     

Inner core anisotropy, anomalies in the time-averaged paleomagnetic field, and polarity transition paths

The diffusion of the dynamo-generated magnetic field into the electrically conducting inner core of the Earth may provide an explanation for several problematic aspects of long-term geomagnetic field behavior. We present a simple model which illustrates how an induced magnetization in the inner core which changes on diffusive timescales can provide a biasing field which could produce the observed anomalies in the time-averaged field and polarity reversals. The Earth's inner core exhibits an anisotropy in seismic velocities which can be explained by a preferred orientation of a polycrystalline aggregate of hexagonal close-packed (hcp) iron, an elastically anisotropic phase. Room temperature analogs of hcp iron also exhibit a strong anisotropy of magnetic susceptibility, ranging from 15 to 40% anisotropy. At inner core conditions the magnetic susceptibility of hcp iron is estimated to be between 10⁻⁴ and 10⁻³ SI. We speculate here that the anisotropy in magnetic susceptibility in the inner core could produce the observed anomalies in the time-averaged paleomagnetic field, polarity asymmetry, and recurring transitional virtual geomagnetic pole (VGP) positions.     

Sulitjelma Gabbro,northern Norway: A palaeomagnetic result

The Sulitjelma Gabbro situated at 67.2°N, 15.4°E was intruded close to the Late Ordovician climax of regional metamorphism in the northern Scandinavian Caledonides. Magnetic properties have been examined from samples collected at seven localities in the south western part of this body. Total NRM directions show a tendency to be aligned near the present earth's magnetic field direction in this region. Stability to a.f. demagnetisation is low and commensurate with low Koenigsberger ratios (0.001–0.16) and the presence of unoxidised magnetite as the principal remanence carrier. After cleaning the site mean directions no longer show an alignment near the present earth's field and of six statistically significant sites three are approximately reversed with respect to remainder. The combined mean direction after cleaning isD = 195°,I = 15° (precision parameterk = 6) and the derived virtual geomagnetic pole is at 0°E, 14°S (α₉₅ = 23°). This pole is close to poles of comparable age from the British Isles and suggests that Britain and Norway were part of the same crustal plate in Ordovician times. Discrepancies between Siluro-Devonian results from the two regions may be due to inadequate age coverage of present results.     

Secular variation in numerical geodynamo models with lateral variations of boundary heat flow

We study magnetic field variations in numerical models of the geodynamo, with convection driven by nonuniform heat flow imposed at the outer boundary. We concentrate on cases with a boundary heat flow pattern derived from seismic anomalies in the lower mantle. At a Rayleigh number of about 100 times critical with respect to the onset of convection, the magnetic field is dominated by the axial dipole component and has a similar spectral distribution as Earth’s historical magnetic field on the core-mantle boundary (CMB). The time scales of variation of the low-order Gauss coefficients in the model agree within a factor of two with observed values. We have determined the averaging time interval needed to delineate deviations from the axial dipole field caused by the boundary heterogeneity. An average over 2000 years (the archeomagnetic time scale) is barely sufficient to reveal the long-term nondipole field. The model shows reduced scatter in virtual geomagnetic pole positions (VGPs) in the central Pacific, consistent with the weak secular variation observed in the historical field. Longitudinal drift of magnetic field structures is episodic and differs between regions. Westward magnetic drift is most pronounced beneath the Atlantic in our model. Although frozen flux advection by the large-scale flow is generally insufficient to explain the magnetic drift rates, there are some exceptions. In particular, equatorial flux spot pairs produced by expulsion of toroidal magnetic field are rapidly advected westward in localized equatorial jets which we interpret as thermal winds.     

Paleosecular variation and absolute geomagnetic paleointensity records retrieved from the Early Cretaceous Posadas Formation (Misiones,Argentina)

The Early Cretaceous may be considered a key period for understanding the evolution of the Earth’s magnetic field. Some still unsolved problems are related to the mode of paleosecular variation (PSV) of the Earth’s magnetic field before and during the Cretaceous Normal Superchron. We report here a detailed rock-magnetic, paleomagnetic and paleointensity investigation from 28 lava flows (331 standard paleomagnetic cores) collected in the Argentinean part of the Parana Flood Basalts (Formation Posadas) in order to contribute to the study of PSV during the early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South America. The average paleofield direction is precisely determined from 26 sites, which show small within-site dispersion and high directional stability. Five sites show evidences for the self-reversal of thermoremanent magnetization. 23 sites yielded normal polarity magnetization and only 3 are reversely magnetized. Moving windows averages were used to analyze the sequential variation of virtual geomagnetic pole’s (VGP) axial positions. Interestingly, the axial average VGP path traces an almost complete cycle around the geographical pole and passes near the location of all previously published Paraná Magmatic Province poles. Both paleomagnetic poles and average VGP paths are significantly different from the pole position suggested by fixed hotspot reconstructions, which may be due to true polar wander or the hotspot motion itself. Only 15 samples from 5 individual basaltic lava flows, yielded acceptable paleointensity estimates. The site mean paleointensities range from 25.2 ± 2.2 to 44.0 ± 2.2 μT. The virtual dipole moments (VDMs) range from 4.8 to 9.9 × 10²² Am². This correspond to a mean value of 7.7 ± 2.1 × 10²² Am² which is 96% of the present day geomagnetic field strength. These intensities agree with the relatively high values already reported for Early Cretaceous, which are consistent with some inferences from computer simulations previously published.     

Palaeomagnetism of silurian lavas of Somerset and Gloucestershire,England

Detailed alternating field demagnetisation of Upper Llandovery volcanics of the Mendip Hills and Gloucestershire has isolated remanence directions interpreted as primary from each of five sites. Well-defined high-coercivity secondary magnetisation is present in six samples of one site and low-coercivity secondary remanence is present in all samples from another site; the former component was apparently acquired in Permo-Triassic times. Primary directions of magnetisation show marked improvement in precision after correction for penecontemporaneous folding, and show a late Llandovery reversal in the sense R → N.The group mean directions of magnetisation isD = 243.5°,I = 47.5° (precision parameterk = 29). Petrographic examination confirms observations from magnetic properties that relict titanomagnetite (oxidation classes 3 to 5) is the remanence carrier in most samples. Hematite, probably mostly late magmatic in origin, is widely developed in all samples, but only the principal remanence carrier where it has thoroughly replaced the titanomagnetite. Low-coercivity remanence is apparently caused by weathering effects but there is no clear visible cause for secondary high-coercivity remanence carried by some samples.The mean virtual geomagnetic pole position is close to Upper Silurian/Lower Devonian pole positions from other parts of Britain and defines a minimum apparent polar shift of 60° between late Ordovician and Upper Llandovery times. Reference to absolute age dates suggests that this shift took place between ca. 447 and 434 m.y. followed by slight polar movement between ca. 434 and 394 m.y.     

Magnetic field transforms with low sensitivity to the direction of source magnetization and high centricity

Magnetic data interpretation faces difficulties due to the various shapes of magnetic anomalies and the positions of their extrema with respect to the causative bodies for different directions of the source magnetization. The well‐known transforms — reduction to the pole, pseudogravity field, and analytic signal (total gradient) — help in reducing the problem. Another way to achieve the required effect is the transformation of magnetic data, ΔT or Z, into values of the anomalous magnetic intensity T. In this respect, we have found some transforms based on differential operators such as the gradient of T and its modulus R = |?T|, the Laplacian L = ?²T, the product T ?²T and its square root Q, and the Laplacian ?²(T²) and its square root E, to be useful. They are slightly sensitive to the magnetization orientation and their extrema occur above the sources. For a 2D anomaly of a homogeneous causative body, the proposed transforms do not depend on the inclination of magnetization. In the 3D case, such independence does not exist even for the elementary field of a point dipole. The influence of the magnetization direction is estimated by an integral coefficient of sensitivity. This coefficient takes values of up to 2.0 for ΔT or Z anomalies, while their transforms T, R, E, Q and L have values of less than 0.28, 0.29, 0.24, 0.16 and 0.07, respectively, i.e. on average, 10 times less. The estimation of the centricity is carried out using the relative deviation of the principal extremum of the anomaly or its transforms from the epicentre of the model body at a depth equal to 100 units. For a ΔT anomaly this deviation is up to 67%; for the L transform it is less than 8%; for Q, E, R and T it is less than 10%, 15%, 20% and 25%, respectively. The proposed transforms take only non‐negative values. With respect to their shape, the peripheral magnetic extrema are removed, the anomalous configuration is simplified and the resolution of complicated interference patterns is improved. Their calculation does not require additional data for the direction of magnetization, which is an essential advantage over the reduction‐to‐the‐pole and pseudogravity‐field transforms. A joint analysis of the measured field and its transforms T, E and L offers possibilities for more confident separation of the anomalous effects and direct correlation to their sources. The model tests performed and the 3D field applications to real magnetic data confirm the useful properties of the transforms suggested here.     

The average electric current system for the sudden commencements of magnetic storms

Summary A statistical investigation of the world-wide electric current-systems corresponding to s.c's has been carried out. The storm-time variationDst and the disturbance diurnal variationDs of s.c's are derived separately from the results of the average local-time variations in the magnetic field at different latitudes.A conspicuousDs current-system is found in the inner polar regions, and a concentrated current-flow is also confirmed along the geomagnetic equator resembling the current-system of an enhancedSq-field. The inner polar region currents flow towards the meridian at about 9h local-time and are supposed to complete their path along the outer auroral zone in opposite directions in the forenoon and afternoon hemispheres. It is of interest that the pattern of this atmospheric electric current-system is very similar to that caused by an electric doublet centred on the highiy conducting region near the pole.Some observed facts concerning the statistical nature of s.c's can be explained by these current-systems.     

New Early Cretaceous paleomagnetic results from Qilian orogenic belt and its tectonic implications

Lower Cretaceous red sedimentary rocks from the depositional basin of East Qilian fold belt have been collected for a paleomagnetic study. Stepwise thermal demagnetization reveals two or three components of magnetization from dark red sandstones. Low-temperature magnetic component is consistent with the present Earth Field direction in geographic coordinates. High-temperature magnetic components are mainly carried by hematite. The mean pole of 19 sites for high-temperature magnetic components after tilt-correction is λ=62.2°N, φ=193.4°E, A95=3.2°, and it passes fold tests at 99% confidence level and reversal tests at 95% confidence level. The paleopole is insignificantly different from that of Halim et al. (1998) from the same sampling area at the 95% confidence level. Compared with paleomagnetic results for North China, South China, and Eurasia, our results suggest that no significant relative latitudinal displacement has taken place between Lanzhou region and these blocks since Cretaceous time. Remarkably, the pole of Lanzhou shows a 20° clockwise rotation with respect to those of North China, South China, and Eurasia. Geological information indicates that the crustal shortening in the western part of Qilian is greater than that in eastern part. In this case, the clockwise rotation of sampling area was related to India/Eurasia collision, and this collision resulted in a left-lateral strike-slip motion of the Altun fault in north Tibetan Plateau after the Cretaceous.     

The new Permian–Triassic paleomagnetic pole for the East European Platform corrected for inclination shallowing

The results of detailed paleomagnetic studies in seven Upper Permian and Lower Triassic reference sections of East Europe (Middle Volga and Orenburg region) and Central Germany are presented. For each section, the coefficient of inclination shallowing f (King, 1955) is estimated by the Elongation–Inclination (E–I) method (Tauxe and Kent, 2004) and is found to vary from 0.4 to 0.9. The paleomagnetic directions, corrected for the inclination shallowing, are used to calculate the new Late Permian–Early Triassic paleomagnetic pole for the East European Platform (N = 7, PLat = 52.1°, PLong = 155.8°, A95 = 6.6°). Based on this pole, the geocentric axial dipole hypothesis close to the Paleozoic/Mesozoic boundary is tested by the single plate method. The absence of the statistically significant distinction between the obtained pole and the average Permian–Triassic (P–Tr) paleomagnetic pole of the Siberian Platform and the coeval pole of the North American Platform corrected for the opening of the Atlantic (Shatsillo et al., 2006) is interpreted by us as evidence that ~250 Ma the configuration of the magnetic field of the Earth was predominantly dipolar; i.e., the contribution of nondipole components was at most 10% of the main magnetic field. In our opinion, the hypothesis of the nondipolity of the geomagnetic field at the P–Tr boundary, which has been repeatedly discussed in recent decades (Van der Voo and Torsvik, 2001; Bazhenov and Shatsillo, 2010; Veselovskiy and Pavlov, 2006), resulted from disregarding the effect of inclination shallowing in the paleomagnetic determinations from sedimentary rocks of “stable” Europe (the East European platform and West European plate).     

A Late Eocene paleomagnetic pole for the Pacific plate

Seamount magnetic anomaly inversions as well as DSDP paleomagnetic and equatorial sediment facies data constrain a paleomagnetic pole for the Pacific plate of Late Eocene age. The location of the pole at 77.5°N, 21.2°E implies 12.5 ± 1.6° of apparent polar wander for the Pacific plate during the last 41 ± 5 m.y. The Late Eocene pole is significantly different from the Pacific Maastrichtian pole at the 95% confidence level and indicates 7.2° of apparent polar motion of the Pacific between 69 and 41 m.y. B.P. The data source locations for the Late Eocene pole are scattered over a large area of the North Pacific and thus the consistency of the data supports the hypothesis that the north central Pacific plate has been rigid since the Eocene. The agreement of the Late Eocene pole with the motion predicted for the Pacific from hotspot models suggests that relative motion between the spin axis and hotspots has been small since that time. Additionally, this finding dictates that the significant amounts of hotspot versus spin axis motion inferred by other authors to have occurred since the Cretaceous must have instead occurred at a faster rate and concluded before the Eocene.     

The S pole distribution on magnetic grains in pyroxenite determined by magnetotactic bacteria

North-seeking bacteria (NSB) with 1 μm diameters migrate to the S pole only. They were applied to identify the S pole determination on a polished surface of magnetite-rich pyroxenite whose natural remanent magnetization (NRM) intensity was 5.64 × 10⁻³ Am² kg⁻¹. The microscopic observations were performed under dark-field illumination in a controlled magnetic field to 10 μT. The NSB formed clusters on limited areas of magnetite grains and scattered over the whole magnetite grains.

The NRM decreased to 1.02 × 10⁻⁵ Am² kg⁻¹ by alternating field (AF) demagnetization to 60 mT but no clusters appeared, while small populations of the NSB scattered on each grain. These scattered bacteria may gather toward the S pole resulting from magnetic domain walls.

When the sample acquired saturation isothermal remanent magnetization (SIRM) to 1 T, the NSB formed dense clusters at the opposite side to the applied field direction on the many grains as expected. This evidence indicated that the NSB can be useful micro-organisms for the determination of fine magnetic structures. Some grains also had NSB clusters at the edge of the grains toward the field direction or did not exhibit any clusters. The complicated distribution of the clusters (the S poles) may be explained by shape anisotropy of the magnetic grains.     

安徽休宁地区下震旦統休宁羣砂岩的交变場退磁研究

本文使用交变場退磁方法对安徽休宁地区下震旦统休宁羣砂岩定向标本进行了剩磁稳定性检验,结果表明该岩层的天然剩余磁性基本上是稳定的,并得出它们的平均剩余磁化强度方向D_r=311°,I_r=76°,相应时代的古地极位置λ=90°E,φ=45°N,标本产地在当时所处的古纬度是90°-p=63°。最后,文章肯定了交变場退磁方法对古地磁学研究的重要意义;根据实验结果,对研究地区在早震旦世时的古气候提出了看法。