Record Of Geomagnetic Secular Variation Research Articles

A Holocene palaeomagnetic secular variation record from Lake Pupuke, New Zealand

ABSTRACT Palaeomagnetic records from three cores of lake sediment have been merged, producing a new Holocene geomagnetic secular variation record for northern New Zealand. The cores are from Lake Pupuke, a maar lake in Auckland Volcanic Field (AVF). They contain tephra from Rangitoto Volcano (530 ± 10 yr BP), Taupō (1718 ± 10 yr BP), Tūhua/Mayor Island (7637 ± 100 yr BP) and Rotomā tephra from Ōkataina Volcanic Centre (9423 ± 120 yr BP). These tephra dates are supplemented by selected radiocarbon age estimates. The cores were correlated using tephra and fine-scale variations in magnetic susceptibility. The natural remanent magnetisation is strong, stable and carried by fine-grained titanomagnetite from AVF’s basaltic volcanoes. The resulting palaeosecular variation (PSV) record spans from 10,000 to 1500 yr BP. It shows well-defined, millennial-scale swings in direction between 10,000 and 8000 yr BP and between 4000 and 1500 yr BP, while between 8000 and 4000 yr BP variations are of higher frequency and lower amplitude and centre on the geocentric axial dipole field direction. This is in excellent agreement with published PSV records from Mavora Lakes, 1000 km further south, indicating the regional nature of PSV – resulting from broad-scale changes in the circulation of conducting iron-rich fluid of Earth’s outer core.

A new Holocene record of geomagnetic secular variation from Windermere, UK

Paleomagnetic secular variation (PSV) records serve as valuable independent stratigraphic correlation and dating tools for marine and terrestrial sediment sequences, and enhance knowledge of geomagnetic field dynamics. We present a new radiocarbon-dated record (WINPSV-12K) of Holocene geomagnetic secular variation from Windermere, updating the existing 1981 UK master PSV curve. Our analyses used continuous U-channel samples taken from the center of four sediment cores retrieved from Windermere in 2012. The natural remanent magnetization (NRM) of each U-channel was measured before and after stepwise alternating field (AF) demagnetization on a superconducting rock magnetometer at intervals of 0.5-cm or 1-cm. The NRM data reveal a stable and well-defined primary magnetization.Component declinations and inclinations estimated using Principal Component Analysis (PCA) of NRM data from the four Windermere cores correlate well on their independent radiocarbon age models. The four records were stacked using a sliding window bootstrap method, resulting in a composite Holocene PSV record (WINPSV-12K).On millennial timescales WINPSV-12K correlates well with other records from Western Europe and the northern North Atlantic to a resolution of ∼1 kyr, given age uncertainties and spatial variability between records. WINPSV-12K also compares well to the CALS10k.2 and pfm9k.1a model predictions for Windermere. Key regionally-significant PSV inclination features of WINPSV-12K which correlate with other North Atlantic records include peaks at 5–6, 8.5, and 10 cal ka BP, and a trough at 7 cal ka BP. Key PSV declination features include the eastward swing from 5.5–2.3 cal ka BP followed by a major westward excursion at 2.3 cal ka BP, peaks at 1.1 and 7 cal ka BP, and troughs at 5.4 and 8.2 cal ka BP, with the caveat that an estimated magnetic lock-in delay of at least 100–200 yr is present. PSV variations on 1–3 kyr timescales are interpreted to represent strengthening and weakening of the North American versus the Siberian and European–Mediterranean high-latitude flux lobes, based on the close similarities between the North Atlantic regional records and the antiphase existing in the East Asian Stack record and the North East Pacific inclination stack. WINPSV-12K provides a regionally-important new PSV reference curve whose prominent features may serve as stratigraphic markers for North Atlantic paleo-records.

Potential role of humoral immunity in cardiac dysfunction of patients suffering from dilated cardiomyopathy

A record of geomagnetic secular variation for the last 7500 years has been obtained from three cores of lake sediment from Sorø Sø in Denmark. The time-scale has been derived from six radiocarbon age determinations on one of the cores. Variations in geomagnetic declination and inclination agree well with those found in a previous study using sediments from another Danish lake, Skanderborg Sø, some 120 km from Sorø Sø. A correlation with the record from the U.K. has been made, but with age differences of up to 1000 years, which are attributed to the dating errors inherent in the radiocarbon method when applied to lake sediments in both the U.K. and Danish sediments. The virtual geomagnetic pole path since 7500 BP appears to have had periods of both clockwise and anticlockwise motion of 1000-year duration, where the looping is open, interspaced with shorter periods of ∼ 500 years in which the geomagnetic behaviour is more irregular. It is suggested that this indicates a possible regularity in the pattern of field behaviour during the Post Glacial.

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.

A Late Holocene geomagnetic secular variation record from Erhai Lake, southwest China

A secular variation record of the geomagnetic field direction for the last 6.5 kyr has been obtained from the magnetization of sediment cores from Erhai Lake, southwest China. In order to make a comparison with this record, secular variation in east-central China was investigated by combining available magnetic field data from historical records and archaeomagnetic measurements since about 350 bc. The secular variation in Erhai Lake shows features consistent with the combined record, except for the oldest three observed declination swings in Sian from 720 to 900 ad. Many features of declination and inclination in China also occur in Japan. From 500 to 1000 ad, declination was westerly ranging from about −20° to −5° in Erhai Lake, east-central China, and Japan.

A Geomagnetic Secular Variation Record for the Last Glacial Period, Reconstructed from Coastal Bog Sediments on the Inland Sea of Japan.

A paleomagnetic secular variation record of geomagnetic field direction for the time span from 25 to 16ka was obtained from oriented bog sediment samples in a coastal area of the eastern Inland Sea, Japan. The plot of this secular variation record, combined with Holocene secular variation data, shows that a long-term westerly declination feature began after 16ka and ended at 7.5ka. This feature lasted more than 4, 000 years and less than 8, 500 years. The declinations at 25-16ka are dominated by easterly values, and no westerly declination was observed in the time span of 24-19.5ka. The inclinations at 25-16ka are characterized by slightly shallow values with small amplitude changes, compared to those in the Holocene. Prior to 18ka a large clockwise looping was dominant in the motion of the geomagnetic vector, while counter-clockwise rotations were dominant after it. Many features of secular variation in the last glacial period have the potential to yield good age constraints for use in paleomagnetic dating.

High-resolution record of geomagnetic secular variation from Late Pleistocene Lake Lisan sediments (paleo Dead Sea)

We measured geomagnetic secular variation in Lake Lisan sediments (paleo Dead Sea). More than 1500 oriented samples were collected from a 27.3-m section of alternating aragonite and detritus laminae in the Dead Sea basin ranging in age from 67 to 32 ka. The natural remanent magnetization (NRM) is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic. The NRM is very stable and was acquired several hundred years after deposition. The mean direction of 878 horizons is D = 005°, I = 45° ( α 95=1°; κ=22). We observed three modes of directional geomagnetic variation as a function of (and by inference, time): very rapid inter-sample changes, slow variation in mean direction, and inclination shallowing of about 1°/m. The overall rate of change in direction is 0.57±0.57°/year, not significantly different from zero. For about 83% of the record the rate of change is less than 1°/year and comparable to historical values. High rates of change are observed more frequently in the Lisan than in historical records, and peak rates are up to ten times faster. A smoothed curve resulting in a maximum rate of change of 0.66°/year and a mean 0.10±0.10°/year may be a more realistic representation of the field behavior. No reverse NRMs were observed, but geomagnetic field excursions may be present where the VGPs deviate by more than 40° from the geographic north at about 52 and 41 ka; the latter may represent the Laschamp event.

Geomagnetic secular variation through the last 3500 years as recorded by Lake Aslikul sediments from eastern Europe (Russia)

A paleomagnetic record of geomagnetic secular variation during the last 3500 yr has been obtained from three lake sediment cores from lake Aslikul in the western Pre‐Ural region (Russia). Pollen data provide a time‐scale and relative paleotemperatures which can be compared with radiocarbon dated paleotemperature data for this region. Magnetic susceptibility measurements have been used for correlation between cores and reconstruction of composite core data. The paleosecular variations which have been recorded in the lake Aslikul sediments are consistent with results from West‐European lakes. Westward shifting inclination features are observed between the more western European and the Pre‐Ural regions which have apparent age differences between 200 and 600 years resulting in drift rates which are of the order of 0.1 degrees/year.

Correlation of Late Pleistocene Glaciolacustrine and Marine Deposits by Means of Geomagnetic Secular Variation, with Examples from Northern New York and Southern Ontario

The geomagnetic secular variation record retained by glaciolacustrine and marine sediments at nine sites in northern New York and southern Ontario provides a means for stratigraphic correlation of glacial deposits for the time period between about 12,600 to 9900 14C yr B.P. Measurement of the depositional remanent magnetism of sediments deposited in Glacial Lake Iroquois and the Champlain Sea has produced a geomagnetic secular variation curve that represents the time period immediately following deglaciation about 12,600 14 C yr B.P. The curve varies from about 358° to 344° declination and 51° to 61° inclination over approximately 180 valve years. Marine sediments of the Champlain Sea have preserved a record approximately 1500 yr long that varies from about 2° to 29° declination and 47° to 60° inclination. These combined glacial-paleomagnetic records may also correlate with those from glacial sequences beyond our study area. The shape and amplitude of the secular variation record in glaciolacustrine and marine sediments from the western Adirondack borderland show agreement with other glacial varve secular variation records and suggest possible correlations with secular variation curves from lake cores.

A Late Quaternary geomagnetic secular variation record from Lake Waiau, Hawaii, and the question of the Pacific nondipole low

A continuous paleomagnetic record for the last 13,000 years was obtained from the sediments of Lake Waiau, located near the summit of Mauna Kea Volcano on the island of Hawaii. The secular variation (SV) of the geomagnetic field was studied using spectral analysis and precession analysis of the magnetic vector. The hypothesis that there has been anomalously low geomagnetic SV in the central Pacific was tested using angular dispersion analysis. The inclination record has a mean value of 27° that is much lower than the 35° predicted by the geocentric axial dipole hypothesis. The spectrum obtained from spectral analysis shows major inclination peaks at periodicities of approximately 3600 and 7900 years and major declination peaks at periodicities of approximately 1100, 2000, 3300, and 7800 years. The approximately 8000‐ and 3500‐year periodicities shown in both inclination and declination spectrums may represent the dipole and nondipole variation, respectively. The approximately 1100‐ and 2000‐year periodicities are probably derived from either the sources that affect declination only or the offsets between the core sections. In addition, Bauer plots of the magnetic vector are dominated by clockwise motion. The angular dispersion calculated from lake sediments is 5.9° with respect to the mean virtual geomagnetic pole (VGP) and 9.1° with respect to the geographic pole. Both values are much lower than the 13.5° expected from paleosecular variation model G which separates the angular dispersion into components from the dipole and the quadruple families. Our results support the hypothesis that an anomalously low secular variation is a characteristic feature of the central Pacific. Furthermore, this geomagnetic behavior persists on time scales of 104 years. Regional anomalies in temperature, topography or electrical conductivity at the core‐mantle boundary are inferred to cause the low SV.

Geomagnetic secular variation from Holocene lake sediments of Sorø Sø, Denmark

A record of geomagnetic secular variation for the last 7500 years has been obtained from three cores of lake sediment from Sorø Sø in Denmark. The time-scale has been derived from six radiocarbon age determinations on one of the cores. Variations in geomagnetic declination and inclination agree well with those found in a previous study using sediments from another Danish lake, Skanderborg Sø, some 120 km from Sorø Sø. A correlation with the record from the U.K. has been made, but with age differences of up to 1000 years, which are attributed to the dating errors inherent in the radiocarbon method when applied to lake sediments in both the U.K. and Danish sediments. The virtual geomagnetic pole path since 7500 BP appears to have had periods of both clockwise and anticlockwise motion of 1000-year duration, where the looping is open, interspaced with shorter periods of ∼ 500 years in which the geomagnetic behaviour is more irregular. It is suggested that this indicates a possible regularity in the pattern of field behaviour during the Post Glacial.

A Late Devensian Lateglacial palaeomagnetic record from Llyn Gwernan, North Wales: comparison with other NW European records

SUMMARY Palaeomagnetic analysis of 222 specimens from a high-resolution Late Devensian Lateglacial sedimentary sequence at Llyn Gwernan, North Wales has provided the first detailed record of geomagnetic secular variation for the Lateglacial period in Britain. This provides the opportunity to extend the British Holocene Geomagnetic Master Curve possibly as far back as 15 000 yr bp. The declination log records six well-defined oscillations with peak-to-peak amplitudes of 20–30°. The inclinations are shallower than expected for this latitude (52°44′N) which are probably due to the magnetic recording process although low geomagnetic inclinations are not ruled out. The inclination log defines a number of relatively smooth swings with peak-to-peak amplitudes of 10-20°. No evidence for any geomagnetic excursions (such as the Gothenburg ‘Flip’) was found. The Llyn Gwernan Lateglacial palaeomagnetic record is compared with the published palaeomagnetic records for a number of European sites. Correlations between records suggest that the ages of the Llyn Gwernan palaeomagnetic features, derived from a series of radiometric radiocarbon measurements, are in general compatible with those of the European sites. Discrepancies between the records are probably due to errors in radiocarbon dating of Lateglacial sediments and variability of the palaeomagnetic recording process and do not represent spatially-variable geomagnetic field behaviour.

A sixty thousand year paleomagnetic record from Gulf of California sediments: secular variation, late Quaternary excursions and geomagnetic implications

A unique inclination record of geomagnetic secular variation for the past 60, 000 years (60 ka) has been obtained from continuously deposited sediments in the Gulf of California, recovered during Deep Sea Drilling Project (DSDP) Leg 64 by hydraulic piston coring at Site 480. The chronology of Hole 480 was determined by δ 18O stratigraphy and varve counts, indicating sedimentation rates approaching 1 m/ka. The paleomagnetic results of the upper 49 m show: (1) The average inclination over this interval is identical to the geocentric axial dipole value at the sampling site. (2) Excursion directions occur between about 53 and 22 ka before present (BP). During this time, the geomagnetic field was generally “noisier” than in the overlying and underlying sections, with greater dispersion of the inclination. (3) The Laschamp excursion was apparently recorded at Hole 480 between about 51 and 49 ka BP and the Mono Lake excursion between about 29 and 26 ka BP. In addition, a narrow 0.3–0.4 m zone near 23 ka BP has a very similar paleomagnetic signature as the excursion observed at Summer Lake, Oregon [1], and we suggest that the Summer Lake is distinct from and younger than the Mono Lake excursion by 3–5 ka and of considerably shorter duration, lasting no more than a few hundred years. (4) Recurring inclination fluctuations were identified at Site 480, characterized by end points with steep inclinations and shallow intermediate value(s), as compared with the geocentric axial dipole. The inclination cycles are particularly apparent from 54 to 24 ka BP with a characteristic period of about 4.4 ka. (5) The “noisier” inclination record between 54 and 24 ka BP might be related to the generally reduced dipole moment between about 20 and 50 ka, and particularly low paleointensities for the Laschamp and Mono Lake excursions.

Magnetization of a stalagmite in Akiyoshi plateau as a record of the geomagnetic secular variation in West Japan.

The investigation of natural remanent magnetization (NRM) of a stalagmite has been carried out during these several years with the aim of producing a secular variation record of the geomagnetic field for the last several thousands of years. The NRM is relatively weak of the order of 10-5 to 10-7 Am2/Kg but has a stable component. The youngest specimens, at the upper surface of the stalagmite, show a magnetization in almost the same direction as that of the present geomagnetic field at the sampling site (declination=-6.4° and inclination=48.3°). Variation of the declination and inclination of the magnetization is relatively consistent with those obtained from archaeomagnetic studies and palaeomagnetic studies of sediments in Japan. The geomagnetic field intensity during the studied period has apparently decreased gradually to the present as deduced from a decrease of the intensity ratio of the NRM to the saturation isothermal remanent magnetization. The continuous geomagnetic secular variation record obtained from this study is considered to presumably cover the past 4000 years.

Detransformation of the British geomagnetic secular variation record for Holocene times

In this note we replot the British post-glacial secular variation curve in terms of actual values of declination and inclination, rather than the ‘transformed’ declination and inclination system used in our original paper (Turner & Thompson 1981). This should facilitate the comparison of our record with contemporaneous European archaeomagnetic data. Our composite record was obtained by averaging the dated records from 10 cores of lake sediment. The sequence of unit vectors describing the magnetic direction record of each core was rotated so as to place its mean direction (the mean over the section of core dated between 0 and 7000 BP, the time span of the shortest core) at zero transformed declination and inclination (dT = O”, IT = 0”). The transformed coordinate frame of each core is thus derived from its mean direction, assumed to represent the mean geomagnetic field direction over the 7000 yr period, rather than from the core tube and its orientation during coring. Hence the problem of not knowing the azimuthal orientation and inclination of the corer during core collection is avoided, and the 10 records can be averaged in this coordinate system. Transformed coordinates then, provide a convenient, generalised method of plotting averaged sediment records. However, in the archaeomagnetic method, sample orientation is straightforward, and the results can readily be plotted as absolute declination and inclination values. So for consistency when comparing the two, we have attempted here to ‘detransform’ our record. The following procedures, assumptions and approximations were taken to produce the detransformed magnetograms and Bauer-type plots of Figs 1 and 2. (1) Amplitude correction. Although the combination of 10 data sets produces a good estimate of the signature of the geomagnetic variations, the following factors contribute to an overall loss of amplitude in both declination and inclination. (a) The quality of magnetic recording varies widely between different sediments, depending on the porosity of the sediment, the size and shape distributions and the nature of the magnetic carrier (Verosub 1979). The Loch Lomond sediments carry a good, high amplitude pattern of remanent directions, of which the most recent closely follow the London observatory records. The much wetter and less compacted sediments of Llyn Geirionydd, on the

Time series analysis of the 10 000 yr geomagnetic secular variation record from SE Australia

Summary. A 10 000 yr continuous secular variation record from intensively dated lake sediments in SE Australia has been subjected to periodogram and maximum entropy method analysis. Tests on synthetic data reveal some of the limitations of the latter method, particularly when applied to complex number series. Anticlockwise precession of the magnetic vector at a period of 5000 ± 1000 yr is tentatively ascribed to dipole precession, and clockwise precession at a period of about 2800 yr is probably due to westward drift of features of the non-dipole field. The effect of calibrating the radiocarbon time-scale is important and results in periodicity shifts of up to 25 per cent. Even for well-dated lacustrine sequences power spectra are poorly constrained: it is thus possible that the geomagnetic secular variation on a time-scale of thousands of years is more uniform than often supposed. Mismatches between declination and inclination spectra can arise as a natural consequence of certain types of source mechanism and should not be simply attributed to figments of the analysis employed.

A 10 000 yr geomagnetic secular variation record from three Australian maars

Summary. Detailed within-lake magnetic stratigraphies enable composite declination and inclination versus depth curves to be derived for sets of 6m cores from each of three volcanic crater lakes in western Victoria. Comprehensive radiocarbon chronologies for each sequence enable reliable timescales to be assigned to the variations in direction of the remanence. The resultant time-scaled curves are in satisfactory agreement and are therefore thought to provide an estimate of the geomagnetic secular variation in SE Australia during the last 10 000 yr. Amplitude attenuation occurs as an increasing function of age, and probably results from long time-scale remagnetization processes. Mean directions coincide with the geocentric axial dipole field directions at the three sites. Inclination oscillations are stronger and better resolved than those in declination, whereas the opposite tends to be true for the European records. Reduction of records to a common ‘virtual’ site suggests that the difference is largely of geometrical origin. Comparisons of VGP paths do not reveal a common dipole wobble component. The sense of looping of the magnetic vector favours westward rather than eastward drift of features of the non-dipole field. Relative field strength estimates based on normalization of NRM with respect to measures of the magnetic mineral content do not agree with Australian and worldwide’ archaeointensity results.

Archaeomagnetic Dating in the American Southwest

Although archaeomagnetic dating seems straightforward in principle, there are practical limitations which are not generally understood. Unlike rate-dependent processes such as isotopic dating, archaeomagnetic dating requires the construction of a master record of geomagnetic secular variation. Error is inherent in such a master curve due to statistical uncertainties regarding both the magnetic directions and ages of the samples used to create the curve. The master curve itself is thus best represented as a ribbon rather than a line. Features being dated have their own error of measurement of magnetic direction, and deriving a date involves an interpretation based on the relation between the oval of confidence for the unknown and the ribbon representing the master curve. Thus a practical precision limit for archaeomagnetic dating is about ± 20 years under optimal circumstances, but the limit will generally be higher. Our pilot study revealed no major discrepancies between our work and the curves of DuBois (1975).

Behaviour of the earth's magnetic field as recorded in the sediment of Loch Lomond

A palaeomagnetic record of geomagnetic secular variation during the last 7000 years has been obtained from the sediments of Loch Lomond, Scotland. The magnetic direction fluctuations repeat well between cores and show greater detail, especially over the last 5000 years, than other European records. A time scale has been derived from 14C analyses on the Lomond sediment and comparison with other 14C-dated sediments. Investigation of relative palaeointensity determination methods has shown that the widely used normalization parameter of partial ARM is insensitive to even small sediment grain size fluctuations. The new high-fidelity direction record and improved time scale show that geomagnetic field changes have not followed a simple oscillatory pattern during the last 7000 years. The record enhances the application of palaeomagnetism to dating recent sediments, as the main declination swings are now characterized by fine detail, and paired inclination data are also available. The problem of mismatching swings when correlating with other paired directional records is thus reduced. The palaeomagnetic record agrees well with some archaeomagnetic results. It confirms the period of anticlockwise motion of the geomagnetic field vector, between 1000 and 600 years B.P., which was first documented by English archaeomagnetic investigations. Clockwise motion is shown to predominate during the remainder of the last 5500 years. The VGP path does not correlate with that of Japanese archaeomagnetic results nor North American sediment data from 2000 to 0 years B.P. This suggests that the secular changes are dominated by local non-dipole sources rather than wobbling of the main geomagnetic dipole.