Palaeomagnetic Secular Variation Research Articles

Sensitivity of the geomagnetic axial dipole to thermal core-mantle interactions

Since the work of William Gilbert in 1600 (ref. 1), it has been widely believed that the Earth's magnetic field, when suitably time-averaged, is that of a magnetic dipole positioned at the Earth's centre and aligned with the rotational axis. This 'geocentric axial dipole' (GAD) hypothesis has been the central model for the study of the Earth's magnetic field--it underpins almost all interpretations of palaeomagnetic data, whether for studies of palaeomagnetic secular variation, for plate tectonic reconstructions, or for studies of palaeoclimate. Although the GAD hypothesis appears to provide a good description of the Earth's magnetic field over at least the past 100 Myr (ref. 2), it is difficult to test the hypothesis for earlier periods, and there is some evidence that a more complicated model is required for the period before 250 Myr ago. Kent and Smethurst suggested that this additional complexity might be because the inner core would have been smaller at that time. Here I use a numerical geodynamo model and find that reducing the size of the inner core does not significantly change the character of the magnetic field. I also consider an alternative process that could lead to the breakdown of the GAD hypothesis on this timescale, the evolution of heat-flux variations at the core-mantle boundary, induced by mantle convection. I find that a simple pattern of heat-flux variations at the core-mantle boundary, which is plausible for times before the Mesozoic era, results in a strong octupolar contribution to the field, consistent with previous findings.

Palaeomagnetic secular variation as a function of intensity

We seek to establish whether or not secular variation, the rate at which the magnetic field is changing in time, is a function of field strength. Towards that end we examine a database consisting o...

The effect of thermal core–mantle interactions on the palaeomagnetic secular variation

We compare a numerical dynamo model with a model of the palaeosecular variation and find that, although there is good agreement in the meridional distribution of the secular variation, the amplitude of secular variation in the dynamo model is too small. Increasing the Rayleigh number does little to increase the amplitude of the secular variation, while it increases the axial dipole component of the field to geophysically unreasonably large values. However, by introducing lateral variations in heat flux across the core–mantle boundary into the dynamo model, we are able to match the amplitude of the secular variation. In addition, we find that the dynamo model then exhibits persistently anomalous behaviour beneath the Pacific region.

Palaeomagnetic dating of Late Holocene sediments in Fennoscandia

Palaeomagnetic secular variation curves, that have been dated by varve analysis (counting annual laminations), are used to date four Holocene lacustrine sediment sequences from central Finland and northwest Russia (lat. 62–66°N). Several fine-scale geomagnetic declination and inclination fluctuations can be identified during the last 3200 yr, which is the most accurately dated interval in the present data. Relative palaeomagnetic dating is a powerful dating tool for sediments of large and deep lakes in northern Europe, which are difficult to date using the 14C method because of low organic content, contamination by older carbon and/or scarcity of terrestrial macrofossils suitable for AMS 14C dating. The dating error during the last 3200yr is estimated to be less than ±50 yr. Such accuracy is not readily attainable using other methods.

Holocene palaeomagnetic secular variation at Lake Biwa, central Japan

SUMMARY We have conducted a palaeomagnetic study on three sediment cores obtained from a single site in Lake Biwa, central Japan. A total of 1430 discrete samples from three cores show strong and stable remanence, which is carried by pseudo-single-domain magnetite. Palaeomagnetic records from individual cores show little scatter of directions, and there is excellent correlation among the records from the site. A composite palaeomagnetic secular variation (PSV) record for the Holocene was constructed after fine-scale adjustment and stacking of the three records. The age model is based on two wide-spread tephra layers: Kawagodaira (3150 yr BP, calendar years) and KikaiAkahoya (7250 yr BP). Our results show that the declinations and inclinations are strikingly similar to archaeomagnetic data from southwest Japan for the last 2000 years. The PSV record from Lake Biwa shows good correlation in directions with that of Beppu Bay, located about 500 km west of Lake Biwa, although the Beppu Bay record shows relatively smaller amplitudes of both inclination and declination, probably due to heavy smoothing of the initial results. When the Lake Biwa record is compared with the proposed type curve for southwest Japan, there is considerable disagreement throughout the Holocene. A re-analysis of the data sets and other information used to construct the type curve revealed that the discrepancy in the results was due to inadequate age determination for the cores used in previous studies. We conclude that the stacked PSV record from Lake Biwa meets the criteria required for a reliable curve. Moreover, our results resolve the complications found in the correlation of PSV records in southwest Japan and thereby contribute to a better understanding of Holocene PSV.

Late Weichselian pollen stratigraphy, clay-varve chronology, radiocarbon chronology, and palaeomagnetic secular variations at Farslycke, Blekinge, southern Sweden

Pollen analysis, AMS 14C measurements and varve counting, carried out on a sequence of Late Weichselian lacustrine sediments in Blekinge in southeastern Sweden, show that the area was deglaciated during the middle part of the Bølling, at c. 12,500 14C years BP + c. 200 varve years. This date fits well with the regional deglaciation chronology. The deglaciation was accompanied by a typical late glacial vegetation succession. The varve chronology for the site is correlated to the regional chronology for Blekinge, and the ice recession rate is calculated to c. 100 m/year. The palaeomagnetic secular variation analyses show a characteristic westerly declination swing at c. 11,500-10,300 14C years BP, which is in accordance with curves from other sites in southern Sweden.

Long-term features of drifting and standing non-dipole fields as determined from Holocene palaeomagnetic secular variations

Geomagnetic field motions of Holocene secular variations are investigated using a separation method. The palaeomagnetic secular variations from Britain, North America and Australia have been subjected to maximum-entropy method analyses. Based on the results of spectral analyses, the secular variations are separated by band-pass filters into low-frequency components, generally including the period band 1800-3600 yr, and high-frequency components, generally including the period band 1000-1200 yr. There is an interval, from 4200 to 1700 yr BP, which shows clockwise rotational motions in the low-frequency components of all three sites. Westward drifting of geomagnetic fields may be globally dominant. Swinging or elliptical looping motions constrained to a certain direction were observed in the low-frequency components of the British data. The time duration for the persistence of the swinging motion constrained to a certain direction was 3500 years or so, which could be the lifetime of an oscillating stationary field. The duration of the transitional motion was 1000-1300 years, which may indicate the recurrence time of a stationary field.

A palaeomagnetic secular variation record from late Pleistocene-Holocene lacustrine sediments from Chalco Lake, Basin of Mexico

Palaeomagnetic secular variation is reported for the past 25,000 years from the lacustrine sequence of Chalco Lake, Basin of Mexico. The rock magnetic and palaeodirectional record is obtained from a 11.27 m core drilled in the central sector of the lake (19.25 ° N, 99 ° W). Detailed stratigraphy, pollen and diatom studies provide a reconstruction of the environmental evolution. The depth-time scale transformation is derived from nine 14C dates, which give depositional rates of 0.28 mm/year for the first 3.5 m and 0.80 mm/year between 3.5 m and 11.27 m. Spectral analysis of intensity and directional records shows low frequency components with characteristic periods of 10,500, 3200–3400, 2900–3000, 1400–1500 and 800–900 years. In phase oscillations of inclination and intensity records point to drifting non dipole field anomalies. Mean inclination anomaly is −2.4 °, which is similar to results from volcanic rocks of the Basin of Mexico and Hawaii, and which support a zonal character of the non dipole field. The palaeomagnetic vectors describe characteristic loops, with dominant clockwise sense in the intervals 2000–5600, 12,000–15,000 and 18,600–24,000 years, and dominant counterclockwise sense in the intervals from 6500 to 11,000 and 16,600–18,500 years. Clockwise vector rotation may represent westward drift of non dipole field anomalies, though westward drift may also result in counterclockwise vector motion. Major declination swings are related to environmental and depositional disturbances, and without further confirmation they cannot be considered as geomagnetic variations.

Palaeomagnetic chronostratigraphy of Holocene sediments, Niagara basin, Lake Ontario, Canada

The North American Great Lakes contain 18% of the world's fresh water. Lake Ontario is the lowermost (downstream) of the Great Lakes and has existed for the past 13 000 yrs, yet details of the evolution of this lake during this period remain sketchy. Over the summers of 1992 and 1993, piston cores sampled the late Pleistocene and Holocene sediments within the Niagara and Mississauga basins of Lake Ontario. The history of late Pleistocene and Holocene environments and deposition rates are assessed by palaeomagnetic secular variations and stratigraphical descriptions of the sediment. Pollen samples were taken from the sediment to better constrain the palaeomagnetic ages and to establish the boundary between Pleistocene and Holocene deposition at 10 500 yr BP. Secular variation type curves were used to provide estimates of the age of the late Pleistocene and Holocene sediments. A description of the stratigraphical sequence is included. The sediments can be divided into six stratigraphical units for which ages have been derived. The lowermost unit is a diamict (upper contact 12 500 yr BP). Next in the sequence is a deformed unit (upper contact 12 300 yr BP), followed by a thinly laminated unit (upper contact 12 000 yr BP), a massive unit (upper contact 11 800–11 000 yr BP), and an irregularly laminated unit (upper contact 10 500 yr BP). The unit deposited in the deep water of the basin since 10 500 yr BP is an iron-banded mud. Sedimentation rates of the basin compiled from pollen and from magnetic data are compared and the post-glacial geological history of the Niagara basin of western Lake Ontario is discussed.

Palaeosecular variation recorded by lava flows over the past five million years

We present a new global palaeomagnetic database, comprising lava flows and thin intrusive bodies, suitable for studying palaeosecular variation and the time-averaged field. The database is presented in some detail in the appendix and is available on-line from the authors. We review palaeosecular variation models to date, emphasizing the assumptions required and the rather arbitrary construction of many of these models. Preliminary studies of the statistical properties of the new database suggest that existing palaeosecular variation models are inadequate to explain the long-term temporal variations in the field. It is increasingly apparent that data distribution and quality are pivotal in determining the characteristics of the secular variation. The work presented here demonstrates the need for revised models of the time-averaged field structure for both normal and reverse polarities before reliable models for palaeosecular variation can be made.

A palaeomagnetic secular variation record for New Zealand during the past 2500 years

A record of the secular variation of the geomagnetic field over New Zealand for the past 2500 years has been compiled by measuring and averaging logs of the primary magnetization of five cores of lacustrine sediment from Lake Pounui. A time-scale is provided by four radiocarbon age determinations. The record is constrained by historical observations for the last 350 years and is in good agreement with instantaneous palaeomagnetic directions from lava flows. This is the first continuous record of secular variation from New Zealand and one of only a few from the southern hemisphere. Together with existing Australian records, the results show significant evidence of westward drift of the geomagnetic field over the past 2500 years. It contains new information for modelling the geomagnetic secular variation in the southern hemisphere and will also be valuable for dating applications within New Zealand.

Climatic and geomagnetic influences on the Lac du Bouchet palaeomagnetic SV record through the last 110 000 years

Abstract Palaeomagnetic secular variation records stacked from five cores ∼ 20 m long extend back through the last glacial cycle to ∼ 100 000 years ago. The time-scale has been constructed from conventional and accelerator radiocarbon ages back to ∼ 45 Ka BP and beyond this by comprehensive pollen studies which identify forest and steppe cycles that have been related to marine oxygen isotopic stages 4 and 5. Spectral analyses of the intensity and directional records using both Fourier and maximum entropy methods reveal strong low-frequency components which fall in the range of the Milankovitch obliquity and precession periodicities (40-20 Ka). It is concluded that although part of this signal may have fed indirectly into the natural remanence via the influence of climatic change on sediment facies, part has been impressed directly by perturbation of the geomagnetic dynamo process at the core-boundary by variations in the Earth's obliquity and precession. These long periods envelop a well-defined pattern, registered also by the independent ‘Mackereth’ core stack back to 50 Ka BP, of irregular secular oscillations of the geodynamo with periods of a few thousands of years.

Late Weichselian pollen stratigraphy, palaeomagnetic secular variations and radiocarbon chronology at the Torreberga ancient lake, Skåne, Sweden

Abstract Lake sediments from Torreberga, Skåne, southern Sweden, covering the Allerod and Younger Dryas, previously investigated with respect to palaeomagnetic secular variations, have been pollen analyzed and radiocarbon dated. The palaeomagnetic records from the lower parts of this site as well as from other sites in southern Scandinavia indicate that the sediments are younger than previously thought. The sedimentation in the ancient lake did not start until more than 1000 radiocarbon years after the deglaciation. Birch and pine was sparsely present during the Allerod, and disappeared at the transition Allcrod-Younger Dryas but reappeared shortly afterwards to form an open forest. In the later part of the Younger Dryas, pollen influx values increased significantly as a result of the climatic warming. The palaeomagnetism is characterised by a distinct westerly declination swing at about 11,500 to 10,500 BP. The inclination shows four major cycles.

The occurrence of Greigite in sediments from Loch Lomond

AbstractGreigite has been identified in the sediments of Loch Lomond using X‐ray diffraction. Greigite is the dominant magnetic mineral in the oldest sediments recovered, while magnetite predominates in the younger sediments which had previously been used for palaeomagnetic secular variation studies. A layer of sediment in between these two magnetic mineral regimes has very low magnetic concentrations, probably as a consequence of magnetite dissolution associated with sulphide rich pore‐waters produced at the time of a marine incursion.The greigite largely oxidises once the sediment is exposed to air, but if freeze dried the greigite becomes surprisingly stable. Following freeze drying the greigite can be heated to 280°C in air before it alters and loses its strong ferrimagnetic properties.

Dynamo waves and palaeomagnetic secular variation

Summary. A dynamic model is proposed for large amplitude geomagnetic secular variation with periodicity in the range of 1000-3000yr. Thermal convection in the lower mantle imposes a spatially variable heat flux at the core-mantle boundary. If the total heat loss from the core exceeds approximately 3 x 10'2W, then both (i) a convective layer and (ii) large scale, quasigeostrophic shear flows are likely to exist beneath the core-mantle boundary. The combined effects of convection and shear at the top of the outer core can generate three-dimensionally propagating magnetic field disturbances called cuw-dynamo waves. An analysis is made of the surface magnetic field disturbances due to dynamo wave propagation in the outer core in the presence of turbulent convection and an east-west shear flow. Nonaxisymmetric disturbances exhibit the following characteristics: (i) dynamo waves propagate in each hemisphere with a poleward or equatorward component, depending on the sign of the a-effect; (ii) both eastward and westward propagation is allowed althcugh westward propagation is preferred. The apparent velocity at the earth's surface is the sum of propagation and drift velocities in the core; (iii) dynamo wave disturbances are transient, with exponential growth and decay phases; (iv) traces of inclination versus declination of individual Fourier components obey Runcorn's circularity rule. A comparison is made with lake sediment secular variation records from central North America measured by Lund & Banerjee. The band-passed time series and the major inclination/declination loops can be modelled from 2600 to 10 000 yr BP by a single dynamo wave, periodic in 2400 yr, propagating under the site along an azimuth of 345. Between 0 and 2600 yr BP, the record can be modelled by addition of a wave with 1200yr period, propagating along an azimuth of 30. This indicates that the palaeomagnetic secular variation record in North America to 10 000 yr may be dominated by poleward propagating wave-like disturbances.

A palaeomagnetic secular variation record from c. 120 000 yr-old New Zealand cave sediments

Summary. A record of palaeomagnetic secular variations has been obtained from a 5 m section of cave sediments in New Zealand. The uppermost horizon has been dated at 120 000 yr, and the section probably spans about 10 000 yr. The mean direction of remanence is close to the field direction expected at the site from an axial geocentric dipole. In the upper part of the record the palaeomagnetic vector describes three open clockwise loops, ending in a swing to anomalously low inclinations. This swing is tentatively associated with a weakening of the main dipole field prior to the Blake geomagnetic event.

A preliminary palaeomagnetic study of the Holocene and late Wurmian sediments of Lac Du Bouchet (Haute-Loire, France)

Summary. Lac du Bouchet is a small maar type lake located at ∼45°N, 4°E, in central France. the bottom sediments have been sampled down to a depth of 6 m with Mackereth type corers. A chronology has been established from pollen analytical and diatom zonations and by radiocarbon age determinations. Throughout the period covered, which runs from ∼22 000 years ago through to the present time, declination and inclination variations consist of oscillations of up to about 30° amplitude. No abnormally large amplitude ‘excursions’ in direction are recorded. the main contribution of this study is the extension of the European secular variation (SV) records to the interval from ∼13000 BP (years before present) to ∼22000 BP: the sediments deposited through this interval (Ancient Dryas intc Wurm) originated from the erosion of the internal walls of the volcano. Spectrum analyses of the declination and inclination curves from 13000 to 22000 BP reveal peaks at periods of 3000–4000, ∼1800, ∼1150 and ∼700 yr. Cross-spectrum analysis shows strong coherence of the two signals at the longest of these periods. the phase relationship of all these peaks indicates clockwise rotation of the palaeomagnetic vector, possibly suggesting westward drifting sources. Although the palaeomagnetic SV record through the Holocene (0-10300 BP) is compressed into ∼1.7m of highly organic sediments, the principal features exhibited by the ‘type’ curves established for UK can be identified and the resulting ‘magnetic’ ages are consistent with the palynology: the sense of rotation of the magnetic vector is biased equally in both senses. the Late Glacial (∼10300 BP to ∼13000 BP) is characterized by oscillating climatic conditions (Recent Dryas, Allerod and Bolling) which resulted in poorly resolved and probably discontinuous palaeomagnetic SV records which show a strong bias to anti-clockwise rotation. All ages are given in radiocarbon years.

Late Weichselian palaeomagnetic secular variation from the Torreberga basin, south Sweden

Three 6 m long sediment profiles from the Torreberga basin (55°37′N, 13°14′E) in south Sweden have been analysed palaeomagnetically. The sediment successions cover the period 12 800-10 000 years B.P. which almost corresponds to the entire Late Weichselian (13 000-10 000 years B.P. sensu Mangerud et al.). The sediments were recovered with a Russian peat corer, a method which allows the declination to be determined in absolute degrees. Around the mean declination two maxima with an intervening minima is recorded, with a peak to peak amplitude of 60°. In the inclination records more variation can be seen, however, the amplitudes are much smaller. The mean inclination is 5° lower than expected with respect to the site latitude. On a general trend of gradually steeper inclination upcore 5 maxima and 5 minima are found. The peak to peak amplitudes range between 5° and 14°. The amplitudes of both parameters are comparable in magnitude to those documented in other records of Late Quarternary age from, e.g., North America or of Holocene age from Europe. The records show that there is no evidence of any geomagnetic excursion between 12 800 and 10 000 years B.P., as previously discussed in the literature, but the field has been of normal polarity. The palaeomagnetic records from the Torreberga basin, which give consistent and repeatable values, must so far be considered to be the best and most detailed European palaeomagnetic record from this period.

Construction of type curves of geomagnetic secular variation for dating lake sediments from east central North America

Type curves of palaeomagnetic secular variations of declination and inclination through the last 12 000 radiocarbon years have been constructed by combining data from the Minnesota Lakes with parallel data from the Great Lakes. The former records were well dated by reliable radiocarbon age determinations at frequent depth increments but the curves are not so well formed as the corresponding Great Lakes curves, which, however, cannot be so accurately dated. The same characteristic features can be identified in the records from both areas, particularly in inclination. Thus the Great Lakes records have been redated by correlation of the inclination features with the Minnesota records. Inclination and declination records from both areas were then stacked and the resulting curves serve as type curves for east-central North America. Standard error bars are shown at each time increment of 40 years. Significant swings on declination and inclination curves were used for reinterpretation of Lake Erie and Lake Michigan records.

Evidence for recent palaeomagnetic secular variation in lake sediments from the New Guinea Highlands

Palaeomagnetic declination and inclination data are reported from Mackereth mini cores taken from three lake sites in Papua New Guinea. 137Cs, 210Pb and 14C dating provide a timescale for the palaeomagnetic oscillations. Tephrastratigraphy aids correlation between lakes. The palaeomagnetic data resemble the geomagnetic secular variation pattern deduced from historical observations. The new results greatly extend the latitudinal coverage of palaeomagnetic secular variation recorded in the last 10 4 yr. Their significance in assessing secular variation source models is discussed. The data suggest that the edge of the Pacific region of low non-dipole field activity has remained near New Guinea during the last 10 4 yr.