Spectral analysis of Quaternary palaeomagnetic data from British Columbia and its bearing on geomagnetic secular variation

Palaeomagnetic results obtained from a 7-m sedimentary sequence in southern British Columbia spanning approximately 9000 yr (˜ 22 000 to ˜31 000 yr BP based on radiocarbon dates) are reported and analysed. Remanence directions from 37 horizons spread throughout the section have been determined, and no evidence of the geomagnetic excursion observed at Lake Mungo, Australia (˜ 31 000–28 000 yr BP), or that observed at Mono Lake, California (25 000–24 000 yr BP) is present. However, regular oscillations in the remanence vectors are observed, and maximum entropy spectral analysis, after mapping the remanence directions on to the complex plane, reveals peaks at periods of approximately 2000 and 5000 yr. The 2000-yr peak is associated with clockwise looping of the geomagnetic vector, and therefore most likely represents the time associated with one full cycle of the westward drift of the non-dipole field. The 5000-yr peak is associated with elliptical counterclockwise looping of the local geomagnetic vector and may be indicative of counterclockwise motion of the geomagnetic dipole axis.     

Two geomagnetic excursions during the Brunhes chron recorded in Chinese loess-palaeosol sediments

Detailed palaeomagnetic integrated with rock magnetic studies have been carried out on a loess-palaeosol sequence in Baoji, Shaanxi province, southern Chinese Loess Plateau. For most samples stepwise thermal demagnetization revealed two well-defined magnetization components. A low-temperature component (LTC), which was isolated between 100 and 200 °C, is consistent with the present geomagnetic field direction. A high-temperature component (HTC), which was isolated between 250 and 620–680 °C, shows normal, reversed or transitional polarities. Our new magnetostatigraphy revealed two distinct geomagnetic excursions recorded in loess unit of L5 and palaeosol unit of S7, respectively, and the Matuyama-Brunhes (M-B) polarity boundary in loess unit of L8. Rock magnetic experiments demonstrated that the specimens from the excursion zones have the same magnetic properties as those from the Brunhes normal or Matuyama reversed polarity zones. Measurements of anisotropy of magnetic susceptibility (AMS) showed that the sediments have primary sedimentary fabrics. Based on the palaeoclimatological and magnetostratigraphic age models, the middle Brunhes excursion in loess L5 is dated at 413–433 ka, and the early Brunhes excursion is estimated to occur 23–33 ka after the M-B reversal. Comparing with previously reported geomagnetic excursions in the Brunhes chron, the middle Brunhes excursion (L5) is likely global. For the early Brunhes excursion (S7), we need further studies to examine its global occurrence.     

Geomagnetic excursion in the late Cretaceous

Summary. Two sedimentary cores from the western Pacific display a palaeo-magnetic record of the late Cretaceous long normal interval and the boundary reversed interval corresponding to seafloor spreading anomalies 33–34. Near the young end of this reversed interval, a systematic excursion of inclinations is observed in both cores. Samples are very stable to both alternating field and thermal demagnetization. Blocking temperatures and Curie points suggest that the remanence is carried primarily by magnetite, but with an additional contribution from hematite. Approximate sedimentation rates derived from biostratigraphy suggest that the excursion had a duration of between 46 000 and 54 000yr and occurred about 236000–303000 yr before the succeeding polarity reversal. The excursion, thus, may represent an aborted geomagnetic field reversal.     

Fidelity and time resolution of the magnetostratigraphic record in Siwalik sediments: high-resolution study of a complete polarity transition and evidence for cryptochrons in a Miocene fluviatile section

The Surai Khola section in southwest Nepal, a 5000 m continuously exposed record of fluvial sedimentation since Middle Miocene, was revisited for high-resolution magnetostratigraphy in sequences with expected cryptochrons and reversals of the geomagnetic field. Polarity intervals with durations of a few tens of thousands of years are recorded as zones of stable palaeomagnetic directions. Polarity transitions are recorded as zones with complex demagnetization behaviour of specimens in the sedimentary column. Almost antiparallel palaeoremanence directions, residing in different haematite phases in the same specimens, could generally not be separated properly by thermal demagnetization. Differing demagnetization paths for neighbouring specimens during a reversal suggest that measured transitional directions are not true geomagnetic field directions, but rather are generated by the superposition of variable amounts of at least two almost antiparallel components of magnetization. Accompanying studies of recent river sand deposits demonstrate that these sediments acquire a true depositional remanent magnetization (DRM) with considerable inclination errors and scattered directions for individual specimens.     

The British Tertiary igneous province: palaeomagnetism of the dyke swarm along the Sleat coast of Skye

Summary. A palaeomagnetic investigation has been made of a swarm of more than 400 dykes along the south coast of Skye, Scotland, by the Sound of Sleat.
Seven red lamprophyre dykes have palaeomagnetic directions inconsistent with Tertiary age, and not inconsistent with their previously held Caledonian age. The remaining 409 dykes have palaeomagnetic directions that are consistent with a Lower Tertiary age. We present evidence suggesting that the Tertiary dykes might have been emplaced during a short time, over which the geomagnetic polarity occupied as few as three polarity intervals (NRN or RNR).
Certain 'intermediate' directions of magnetization have also been found, and are presented here.     

东南极普里兹湾NP93－2柱样古地磁结果

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Preliminary palaeomagnetic results of an Archaean dolerite dyke of west Greenland: geomagnetic field intensity at 2.8 Ga

The geomagnetic field intensity during Archaean times is evaluated from a palaeomagnetic and chronological study of a dolerite dyke intruded into the 3000 Ma Nuuk Gneisses at Nuuk (64.2°N, 51.7°W), west Greenland. Plagioclase from the dolerite dyke yields a mean K-Ar age of 2752 Ma. Palaeomagnetic directions after thermal demagnetization of the dyke and the gneiss reveal a positive baked-contact test, indicating that the high-temperature-component magnetization of the dyke is primary. Thellier experiments on 12 dyke specimens yield a palaeointensity value of 13.5±4.4 μT. The virtual dipole moment at ca. 2.8 Ga is 1.9±0.6 × 10²² Am², which is about one-quarter of the present value. The present study and other available data imply that the Earth's magnetic field at 2.7 ∼ 2.8 Ga was characterized by a weak dipole moment and that a fairly strong geomagnetic field similar to the present intensity followed the weak field after ca. 2.6 Ga.     

Magnetostratigraphy of Palaeocene basalts from the Vaigat Formation of West Greenland

A palaeomagnetic study comprising the directional results from 289 individual lava flows, sampled along eight sections in the Palaeocene basalts of West Greenland, is reported. The eight individual sections are correlated using lithostratigraphical marker horizons to form a single composite profile. Generally, the lithological correlation is in good agreement with the record of geomagnetic secular variation.
  The total composite palaeomagnetic profile represents a stratigraphic thickness of 1.6  km through the Vaigat Formation, which is the lowermost of the two volcanic formations formed during the main stage of plateau volcanism. Only two polarity zones are found in the composite profile, suggesting a very short duration for the West Greenland main plateau-building volcanism. ⁴⁰Ar/³⁹Ar dates support a high extrusion rate and also indicate that the lower normal polarity zone is Chron C27n and that the upper reverse polarity zone is Chron C26r.
  The C27n–C26r transition is fully recorded along one of the sections (Nuusap Qaqqarsua), with intermediate directions covering a 200  m thick succession of lavas. A combined palaeomagnetic, field and geochemical study along this profile showed good agreement; that is, geochemically and geologically derived single magmatic events show groupings of the palaeomagnetic directions. Supposing a duration for the geomagnetic transition of 5000 years, the eruption frequency during this period was as high as one flow every 80 years.     

Palaeomagnetism of A Core Sediment From the Inland Sea, Japan (Seto Naikai)

Palaeomagnetic measurements on a giant core sample 20 cm in diameter and 7.38 m long collected from Mizushima-Nada, the Inland Sea. Japan (Seto Naikai) provide evidence of post-depositional magnetization. the geomagnetic secular variation from about 4000 to 8000 yr BP is characterized by a long period of westerly declination before 6600 yr BP. the maximum deflection is beyond 50°W during this period. the palaeomagnetic record further demonstrates that there is a hiatus over 3000 yr in sedimentation due 10 the sea-level change around 6.85 m from sea bottom, and that the lower limit of the sea-level around 8000 yr BP is 18.5 m beneath the present sea-level.     

Record of the younger part of the Pringle Falls excursion at Long Valley, California

The younger of two closely spaced palaeomagnetic excursions at Pringle Falls, Oregon, is recorded in lacustrine silts that crop out in Long Valley, California. Assigned an age of about 220 000 years, the virtual geomagnetic poles of the younger excursion form a clockwise loop that reached 35 °S latitude east of South America before returning to the northern hemisphere in the Pacific Ocean west of Central America. The poles then form a narrow band across North America while moving to high northern latitudes. This record matches extremely well feature B of the original excursion record from Pringle Falls reported by Herrero-Bervera et al. (1994) and is similar to this excursion at Summer Lake, Oregon ( Negrini et al. 1994 ), in that the pole path is confined primarily to the east–central Pacific Ocean. On the basis of an assumed sedimentation rate of 30  cm per thousand years, the younger excursion (feature B at Pringle Falls) spans an estimated 1200 years and followed by about 1000 years a larger excursion (feature A at Pringle Falls) previously discovered at the same Long Valley site. At a second Long Valley site 30  m away, the younger excursion (feature B) is only partially recorded because of a presumed small hiatus in the sedimentary section.     

Geomagnetic variations recorded in Older (≫ 23 000 B.P.) and Younger Yoldia Clay (∼ 14 000 B.P.) at Nørre Lyngby, Denmark

Summary. Detailed palaeomagnetic results from a rapidly deposited 12.5-m Younger Yoldia Clay sequence of age around 14 000 B.P. at the coast cliff at Nørre Lyngby (northern Jutland, Denmark) and a 2-m Older Yoldia Clay sequence of age somewhere between 23 000 and 40 000 B.P. are presented and discussed. The Younger Yoldia Clay spans some 1000–1500 yr and shows swings in inclination and declination of about that period, and also more rapid oscillations which are particularly marked in inclination, showing that rapid secular variations as have occurred during historic times were indeed also present back in time. There exist easterly declinations of 80° to 90° in the upper half of the Younger Yoldia Clay which cause the virtual geomagnetic pole to migrate clockwise to around 50° away from the rotation pole. This we name the 'Nørre Lyngby declination excursion'.
In the Older Yoldia Clay, as well as secular variations in both declination and inclination, significant low inclination values are found, confirming the existence of the recently named 'Rubjerg low inclination excursion', with the virtual geomagnetic pole moving first in a clockwise then in an anticlockwise sense at 40° to 60° away from the rotation pole.
It is therefore inferred that models for the Earth's geomagnetic field should involve at least local rapid eastward as well as westward 'drift' of the non-dipole field components at various times in the past.     

A physical model for palaeosecular variation

Summary. A new model to describe the latitude dependence of the angular dispersion of the palaeomagnetic field (palaeosecular variation) is developed following previous models, but with crucial differences. It is shown that if the probability distribution of virtual geomagnetic poles (VGPs) is circularly symmétric about the rotation axis then the geométry of the distribution of field directions is latitude dependent. This has a significant effect on the latitude dependence of dispersion and is accounted for in the model. The dipole and non-dipole parts of the field are not artificially separated but are intimately linked through an observationally determined relation that the time averaged intensity of the non-dipole field is dependent upon the intensity of the dipole field. It is shown that a consequence of this relation is that no knowledge of the probability distribution of the geomagnetic dipole moment is required. This is a fundamental improvement over previous models.
The model provides excellent fits to the palaeodata and, unlike previous models, is not inconsistent with the latitude variation of the non-dipole field dispersion determined from the present field. For the past 5 Ma the point estimate of the VGP dispersion due to dipole wobble is 7.2° and of the VGP dispersion at the equator due to variation in the non-dipole field is 10.6°. This estimate of the dispersion due to variation in the non-dipole field is in excellent agreement with the value predicted from an analysis of the variation in field intensities over the same period. Fits of the model to data from earlier periods indicate that dispersion due to variation in the non-dipole field is essentially independent of the geomagnetic reversal rate while dipole wobble is positively correlated with reversal rate.     

Gyroremanent magnetization and the magnetic properties of greigite-bearing clays in southern Sweden

The acquisition of a gyroremanent magnetization (GRM) by single-domain (SD) greigite particles during alternating-field (AF) demagnetization is demonstrated. Previous palaeomagnetic studies failed to identify the presence of authigenic greigite in the glacio-marine clays studied. These clays formed the subject of an earlier debate about the validity of a Late Weichselian geomagnetic excursion (the Gothenburg Flip) in southern Sweden. The greigite carries a stable chemical remanent magnetization (CRM), which coexists with a detrital remanent magnetization (DRM) carried by magnetite. AF demagnetization could not isolate the primary remanence in the sediments where magnetite and greigite coexist, due to the overlapping coercivity spectra of the two minerals and the inability to determine the time lag between sediment deposition and CRM formation. Thermal demagnetization removed the CRM at temperatures below 400 C, but this method was hindered by the unconsolidated nature of the sediments and the formation of secondary magnetic minerals at higher temperatures. The results suggest that the low-coercivity DRM carried by magnetite was mistaken for a 'viscous' component in the earlier studies. Hence the former debate about the record of the Gothenburg Flip may have been based on erroneous palaeomagnetic interpretations or non-reproducible results. AF demagnetization procedures applied to samples suspected of bearing SD magnetic particles (such as greigite) should be carefully selected to recognize and account for GRM acquisition.     

Spectral analysis of Pennocarboniferous geomagnetic variation data from glacial rhythmites

Summary. Palaeomagnetic results for a sequence of Permocarboniferous rhythmites presented in the previous paper have been submitted to maximum entropy spectral analysis to test whether these palaeomagnetic data could supply information on geomagnetic variations. There is a good correlation between the thickness of the rhythmites and sunspot spectra, suggesting that these sediments are really seasonal. The palaeomagnetic spectra are compared with those of observatory records. Periods of approximately 24.4, 12.4, 8.6, 6.7 and 5.5 found for palaeomagnetic data have corresponding values in the geomagnetic spectrum. Most of these periods, however, are the same as those found in the thickness data, implying that magnetization can be influenced by the sedimentation process as suggested by other investigators. On the other hand, both geomagnetic and climatic (thickness) variations seem to be related to solar activity. Therefore, at least indirectly, palaeomagnetic data may reflect geomagnetic variations.     

The magnetic polarity stratigraphy of a Hauterivian/Barremian carbonate sequence from southern Italy

Remanence directions, measured at 2  cm intervals along a composite 88  m bore-core, enable mean palaeomagnetic poles to be defined at 13.6°S, 25.2°W and 13.6°N, 154.8°E. The directions of remanence vary very smoothly away from each palaeomagnetic pole, extending more than 90° from them. This raises doubts about the physical meaning of polarity definitions based on the distance between virtual and mean palaeomagnetic poles. For practical purposes, intermediate polarity is defined as directions whose virtual poles lie more than 25° from the mean pole, enabling at least five normal subchrons to be specified within the upper predominately reversed quarter of the core and 11 reversed subchrons within the lower predominantly normal three-quarters of the core. The stratigraphic thickness between these subchrons shows a very high linear correlation ( r >0.99) with the stratigraphic thickness of other terrestrial sequences and the distances between marine polarity sequences of comparable age. The analysed sequence contains wavelength spectra which, when transformed to the temporal realm, match periodicities determined for three marine magnetic anomaly profiles of similar age. These also match planetary orbital periodicities for the Cretaceous. These observations suggest that secular variations and polarity transitions are driven by common core processes whose surface expression is influenced by changes in the planetary orbits. Such detailed geomagnetic features enable far greater reliability in establishing magnetostratigraphic correlations and also enable them to be dated astronomically.     

Palaeomagnetism of some Late Cretaceous and Miocene igneous rocks on Jamaica

Summary. In order to contribute to the resolution of the problem of the plate tectonic character of the Caribbean, a palaeomagnetic study has been carried out on some Jamaican igneous rocks. Sixteen Late Cretaceous intrusives and lavas and one Late Miocene lava sampled in five sites have been investigated. Because of widespread maghaemitization of the predominantly large-grained deuteric class 1 titanomagnetites, some difficulty was experienced in identifying stable directions of magnetization in the Cretaceous rock units. Using thermal demagnetization technique, two distinct directions of magnetization were obtained, significantly different from those observed in contemporaneous North American rocks. Nine units yield a palaeomagnetic pole at 143.8°W, 44.1°N, referred to as 'normal', while seven units yield'equatorial'poles situated both east and west of Jamaica. It is not possible to decide which of the two directions of magnetization reflects the Late Cretaceous geomagnetic field in Jamaica, but from other evidence they appear to merit palaeotectonic interpretation. They are consistent with the plate tectonic behaviour of the Caribbean since Late Cretaceous and its motion from the southwest relative to the present day frame of reference, with a simultaneous large anticlockwise horizontal rotation of Jamaica. Late Miocene lavas, containing high-coercivity magnetic material, yield closely grouped directions giving a palaeopole at 152.4°W, 73.3°N, supporting the deductions made from the Cretaceous data.     

Geomagnetic secular variation in historical lavas from the Canary Islands

Summary. The age of historical lava flows on the Canary Islands can be traced back at least to 1585 ad . During the last 400 years eruptions occurred at fairly regular intervals. The palaeomagnetic directions of these lavas record the ambient field direction with high precision so that an historical secular variation curve (SVC) can be constructed for the Canaries which closely resembles the curve registered at Paris. The SVC can be used as a dating tool for historical eruptions of unknown age and can probably be extended into the last 4000–5000 yr of recent volcanic activity on the Canaries.     

The palaeomagnetism and magnetic fabric of cave sediments from Grønligrotta and Jordbrugrotta, Norway

Summary. Twenty-four oriented specimens of laminated clay were obtained from vertical sections in two caves in northern Norway. Studies of the magnetic remanence and susceptibility anisotropy show that the magnetization is depositional in origin and due to magnetite. The palaeomagnetic record in one section is correlated with results from a Swiss lake sediment core to suggest an age of 9600–6800 yr bp for the cave clay. It is shown that the susceptibility lineation produced on gently sloping surfaces was predominantly controlled by gravity and cannot therefore be used directly to estimate the palaeoflow or palaeofield directions.     

Remanent magnetization of Precambrian and Cretaceous kimberlites in South Africa

Summary. Middle Precambrian and Cretaceous kimberlites were collected from three sites (Premier, Montrose and National) and two sites (Wesselton and Koffyfontein) in South Africa respectively. The natural remanent magnetization of these rocks remains stable to both alternating field and thermal demagnetization. The virtual geomagnetic pole-positions derived from the directions of stable remanence of the Precambrian rocks can be correlated with palaeomagnetic poles obtained from other Middle-Late Precambrian rocks in Africa. The Cretaceous poles for the Wesselton and the Koffyfontein rocks coincide with other Cretaceous poles.