共查询到20条相似文献，搜索用时 15 毫秒
1.
Kenneth L. Verosub Jonathan O. Davis S. Valastro 《Earth and Planetary Science Letters》1980,49(1):141-148
A well-dated, 3.3-m section of deep-lake clays from pluvial Lake Lahontan, now exposed along the shore of Pyramid Lake, Nevada, has provided a paleomagnetic record of secular variation during the time interval 25,000–36,000 years B.P. The measured ranges of inclination and declination are 40° and 75°, respectively, and represent values which are comparable to observed secular variation at northern mid-latitudes. These results and those from a previously reported study from Clear Lake, California, together provide strong evidence for the conclusion that northern California and western Nevada were not affected by any geomagnetic excursion during the time interval 21,000–36,000 years B.P. Such a conclusion puts severe constraints on the nature and timing of the proposed Mono Lake, Laschamp and Lake Mungo geomagnetic excursions. 相似文献
2.
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 from14C analyses on the Lomond sediment and comparison with other14C-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. 相似文献
3.
Donald F. Palmer Thomas L. Henyey Richard E. Dodson 《Earth and Planetary Science Letters》1979,46(1):125-137
Three closely spaced 6-m piston cores were taken in the central part of Lake Tahoe. Cores were split into two complete replicates for paleomagnetic study and the remaining sections were used for stratigraphic and mineralogical analysis.Stratigraphic correlation of the cores is based on two distinctive horizons (volcanic ash and diatomite) and upon three different sedimentological regimes dominated by (1) poorly bedded silts and muds, (2) well bedded graded units, and (3) finely laminated silts. These correlations served as the standards for the evaluation of the paleomagnetic data. Extrapolation of14C dates obtained in the upper sections of the Lake Tahoe sediments suggests that the lower sections of the cores may reach ages of 25,000–30,000 years B.P.X-ray, optical, Curie point, and hysteresis measurements show that magnetite is the only important magnetic mineral in the sediments and occurs in the size range of 10 μm. Hematite is essentially absent. Based on large changes in the declination and inclination of the natural remanent magnetism (NRM) within single graded layers the paleomagnetic signature is a post-depositional remanent magnetism (PDRM). This PDRM is believed to be caused by magnetic orientation during compaction.Paleomagnetic measurements show three regimes that are correlated with the stratigraphic regimes. NRM declination and inclination data show good correlation between the three cores and agree well with the correlations based on sediment character. NRM intensity variations are due largely to the variations in magnetite content and its occurrence as either single detrital grains or as inclusions within the larger silicates. Thus the variation in paleo intensity was not determined.Comparisons of Lake Tahoe data with that from Mono Lake show fair correlations of declination and inclination. The occurrence of a short-wavelength, high-amplitude event in the lower section of the Lake Tahoe cores may provide confirmation of the Mono Lake geomagnetic excursion. 相似文献
4.
《Physics of the Earth and Planetary Interiors》1986,44(1):1-14
Cores of the bottom sediments from Lac du Bouchet, in the Haute Loire, France (44.9° N, 3.8° E) have been taken using Mackereth type pneumatic piston corers. The sediments have been dated by palynological control back to 15 000 years bp (years before present), and by radiocarbon age determinations using both conventional and accelerator methods. The depth/time transform provisionally used here dates the sequence from 10 000 to 30 000 years bp. The stacked records of demagnetised remanent magnetisation from eight cores have been examined. Fourier and maximum entropy method spectral analyses show good agreement. Inclination and declination spectra are different in that the dominant inclination periods tend to fall in the negative part of the complex spectrum, while the dominant declination periods tend to fall in the positive part. The VGP path traced out by the remanent magnetisation vector shows predominantly clockwise looping, consistent with essentially westward drifting geomagnetic sources. The detailed form of the VGP path is affected by the degree of smoothing, but clockwise looping always predominates averaging ∼ 67% over the whole time interval investigated. A band of frequencies between ∼ 0.9 and ∼ 0.3 cycles per thousand years is associated with the strongest bias towards clockwise rotation (∼ 80% to 90%) while a band between ∼ 0.3 and 0.1 cycles per thousand years is associated with only ∼ 50% clockwise rotation. Individual core records show some negative inclinations at horizons where inclination minima occur suggesting that ‘excursions’ should be regarded as extreme values of secular variation rather than aborted polarity reversals of the main field. 相似文献
5.
M. A. Irurzun C. S. G. Gogorza A. M. Sinito M. A. E. Chaparro H. Nuñez J. M. Lirio 《Studia Geophysica et Geodaetica》2008,52(2):157-172
We present results of paleomagnetic and sedimentological studies carried out on three cores Lmor1, Lmo98-1, Lmor98-2 from
bottom sediments of Lake Moreno (south-western Argentina), and integrate them with data from our previous studies. Measurements
of directions (declination D and inclination I) and mass specific intensity of natural remanent magnetization (NRM intensity),
magnetic susceptibility (specific, χ and volumetric, κ), isothermal remanent magnetization (IRM), saturation of isothermal
remanent magnetization (SIRM), and back field remanent coercivity (B0CR) were performed. The stability of the NRM was investigated using alternating-field demagnetization. The results show that
these sediments meet the criteria required to construct a reliable paleomagnetic record. The cores were correlated very well
based on magnetic parameters, such as χ and NRM intensity, as well as with lithological features. Tephra layers were identified
from the lithological profiles and magnetic susceptibility logs. We obtained the D and I logs of the characteristic remanent
magnetization for the cores as a function of shortened depth. The data from the three cores were combined to form a composite
record using the Fisher method. A comparison between stacked inclination and declination records of Lake Moreno and those
obtained in previous works on Lake Escondido and Lake El Trébol shows good agreement. This agreement made it possible to transform
the stacked curves into time series spanning the interval 12–20 kyr. The results obtained improved our knowledge of SV and
the behaviour of the geomagnetic field and also allowed us to determine the range of past inclination variations from −70°
to −45° for the southern hemisphere, where data are scarce. 相似文献
6.
Continuous measurements of the natural remanent magnetization of six cores from central Lake Michigan have been made, using a cryogenic magnetometer designed to permit the passage of long cores through its sensing region. The output of each of the three-component sensors is deconvolved with the appropriate system response function and combined to yield continuous records of inclination, declination and intensity with a resolution of approximately 6 cm. The paleomagnetic record extends to about 13,500 years B.P. and includes two features which may be excursions of the earth's field. Other features may be correlated between different cores, and the magnetic sequence is, in general, consistent with the stratigraphic sequence. There is an indication of a repeated sequence of field changes, which is somewhat reminiscent of the solar field cycle. 相似文献
7.
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 104 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 104 yr. 相似文献
8.
Results from joint work between the Geophysical Institute (Sofia, Bulgaria), and the Geomagnetic Institute (Grocka, Yugoslavia) on the geomagnetic field variation in the prehistoric past are presented. Preliminary curves of variation of the three geomagnetic elements: declination, inclination and intensity are presented. The movement of the virtual pole position for the 6500 years time period B.C. is derived. The curves and the virtual pole positions provide a dating tool for archaeological purposes and determination of the periodicities in the geomagnetic secular variations in southeastern Europe. 相似文献
9.
Seven deep-sea sediment cores recovered in the central equatorial Pacific collectively span a magneto- and biostratigraphically determined age interval ranging from about 0.1 to 21 m.y. B.P. Measured values of paleomagnetic inclination and their systematic variation with depth in these cores denote relative motion between the central Pacific lithosphere and the magnetic field of the earth. Assuming that the position of the earth's dipole field remained essentially parallel to the present spin axis during the interval, the data provide evidence of a marked decrease in the northward rate of plate motion from about 11 cm/yr to about 6 cm/yr at approximately 12 m.y. B.P. This date of change of motion as well as the northward direction and overall average rate of about 8 cm/yr throughout the last 21 m.y., agree reasonably well with results of other studies of the tectonic history of the Pacific plate and ridge system. More significantly, however, these preliminary results demonstrate the usefulness of the paleomagnetic record in deep-sea sediment cores spanning sufficiently long intervals of time as an aid in reconstructing plate motions. 相似文献
10.
R.J. Suttill 《Earth and Planetary Science Letters》1980,49(1):132-140
Palaeomagnetic measurements of 1-m cores and a 9-m Delft core of Recent tidal-flat sediments from the Wash, England have shown that these sediments possess a record of the variation of the Earth's magnetic field. The record compares well with the historic-archaeomagnetic record for the period 0–1000 years B.P. but is offset down the core due to the remanence being of post-depositional origin. A period of at least 100 years is suggested for the alignment of particles during acquisition of the post-depositional remanence. Magnetite has been identified as the major carrier of this remanence. 相似文献
11.
Palaeomagnetic measurements on three cores from the Tyrrhenian Sea abyssal plain appear to provide evidence of short-period geomagnetic secular variation during historical times. Using these patterns of variation a method is proposed to determine the relative sedimentation rates between cores.Tentative correlation with dated secular variation curves from Lake Windermere, England (R. Thompson, 1973) gives absolute sedimentation rates for the Tyrrhenian cores in the range 65–130 cm/1000 years. 相似文献
12.
Compilations of historical observations, archaeomagnetic data from ancient fireplaces and palaemagetic results from short cores of sediment from lakes in southeastern Australia, particularly Lake Keilambete, provide a detailed record of the geomagnetic secular variation during the last 3000 years. The independent sets of data are in good agreement if the radiocarbon time scale for the lacustrine record is about 450 years too old. The error is attributed to systematic incorporation of ancient carbon into the lake floor sediments, mainly through erosion of sediment on the crater walls at times of low water level. A significant lag between deposition and the acquisition of stable magnetic remanence is ruled out. Inclination has been abnormally steep during the last 500 years but remained fairly close to the axial dipole field value prior to that. During the last 1000 years the predominant sense of looping of the magnetic vector corresponds to westward drift of the nondipole field. Secular variations on a time scale of ～ 100 years can be resolved by the lacustrine record. 相似文献
13.
Niels Abrahamsen Karen Luise Knudsen 《Physics of the Earth and Planetary Interiors》1979,18(3):238-246
Palaeomagnetic measurements have been made on specimens from Late Pleistocene sediments of a piston boring at Rubjerg in Vendsyssel, northern Jutland. The stratigraphy of the deposits is based on content of foraminifera. A total of 70 relatively oriented specimens were investigated palaeomagnetically. Normal steep inclinations close to that of the axial dipole field were found in the Upper Saxicava Sand and in the Younger Yoldia Clay (radiocarbon dated at 14,650 ± 190?12,650 ± 180 B.P.), and a secular variation with an amplitude of 10–12° in the inclincation and a “period” roughly estimated at about 350–400 years was found in the Younger Yoldia Clay.Seventeen relatively oriented specimens from undisturbed older marine deposits revealed a stable low inclination of 11° with α95 = 3°. The age of this apparent geomagnetic excursion falls somewhere between 23,000 and 40,000 B.P. (Older Yoldia Clay). Among other known geomagnetic excursions and events within this interval are Laschamp in France, Mono Lake in California and Lake Mungo in Australia. Until more definite ages have been obtained, the excursion is provisionally named the “Rubjerg Excursion”. 相似文献
14.
Q.Y. Wei T.C. Li G.Y. Chao W.S. Chang S.P. Wang 《Physics of the Earth and Planetary Interiors》1981,25(1):107-112
Curves of the variation of the inclination and declination of the geomagnetic field for the Loyang region during the last 2400 years are presented. An I-D diagram of secular variation, like that for London, has been drawn for Loyang. 相似文献
15.
根据19360年426个地磁测点和28个IGRF计算的地磁数据,计算地磁场和地磁异常场各个分量的曲面Spline模型,并绘制相应的地磁图和地磁异常图.依据我国部分地区的1909～1915,1915～1920,1920～1930,1930～1936年间地磁偏角长期变化图,1908～1917,1917～1922,1922～1936年间水平强度长期变化图和1908～1922,1922～1936年间垂直强度长期变化图,使用曲面Spline方法,分别计算上述9个时间段的磁偏角（D）、水平强度（H）和垂直强度（Z）长期变化的曲面Spline模型,并绘制相应的长期变化图.根据这些长期变化模型,将19360年426个点的三分量绝对值数据归算至1940,1930,1920,1910年和1900年,从而为计算这5个年代的地磁场模型奠定了坚实的基础. 相似文献
16.
The late Pleistocene geomagnetic field as recorded by sediments from Fargher Lake,Washington, U.S.A.
Measurement of the remanent magnetization of a 6.88-m oriented core of soft sediments and tephras from Fargher Lake near Mount St. Helens in southwestern Washington State shows that no significant geomagnetic reversals were recorded in the sediments of the lake. Radiocarbon and palynological dating of the tephra layers from the lake bed indicates deposition during the interval 17, 000–34, 000 years B.P. although geochemical correlation of a prominent tephra layer in the core with tephra set C of Mount St. Helens could mean that the maximum age of the sediments may be at least 36, 000 years B.P. The core was divided into specimens 0.02 m long, each representing approximately 55 years of deposition assuming a constant rate of sedimentation. Pilot alternating field demagnetization studies of every tenth specimen indicated a strong, stable remanence with median destructive field of 15 mT, and the remaining specimens were subsequently demagnetized in fields of this strength. The mean inclination for all specimens exclusive of the unstably magnetized muck and peat from near the surface is 56.1° which is 8° shallower than the present axial dipole field at this site, perhaps because of inclination error in the detrital remanent magnetization of the sediments, although because of the variability in the data, this departure from the axial dipole field may not be significant. The ranges of inclination and declination are comparable to those of normal secular variation at northern latitudes. Although three isolated specimens have remanence with negative inclination, these anomalous directions are due to sampling and depositional effects. Measurement of a second core of 6.86 m length also revealed only normal magnetic polarity, but this result is of little stratigraphic value as this core failed to penetrate the distinctive tephra found near the base of the former core.Studies of a concentrate of the magnetic minerals in the sediments by optical microscopy and X-ray diffraction indicate that the primary magnetic constituent is an essentially pure magnetite of detrital origin. The magnetite occurs in a wide range of grain sizes with much of it of sub-multidomain size (< 15 μm).As a whole, this study provides substantial evidence against the existence of large-scale worldwide geomagnetic reversals during the time interval of Fargher Lake sedimentation, a segment of geological time for which many excursions and reversals have been reported elsewhere. 相似文献
17.
第11代国际地磁参考场(IGRF-11)是国际地磁学与高空物理学联合会(IAGA)于2009年12月提出的最新与比较准确的IGRF.根据IGRF-11模型,计算了2005-2010年中国地区地磁长期变化(SVC).IGRF-11所描述的2005-2010年中国地区地磁长期变化与实际观测的地磁长期变化(SVO)是基本一致的,但亦有明显差异.分析比较了在中国地区34个台站上的SVO与SVC之间的差异,并得到了它们之间的差值及其均方误差σ,对于地磁偏角和倾角,σ分别为0.35′/a与0.53′/a;对于地磁总强度、水平分量、北向分量、东向分量与垂直分量,σ分别为5.12nT/a,8.91nT/a,8.89nT/a,3.27nT/a与3.59nT/a.引起IGRF-11所描述的2005-2010年中国地区地磁长期变化的误差原因是:中国地区的区域性与局部性的磁异常、IGRF忽略了外源场与IGRF模型的截断阶数、全球台站与测点的分布不均匀、地磁观测误差等因素.由于中国地磁模型(CGM)优于IGRF模型,并能比较准确地描述中国地区地磁场及其长期变化,故在实际应用中应选用CGM. 相似文献
18.
Superposition of paleomagnetic polarity logs of seven chronologically overlapping piston cores from the central equatorial Pacific, using the established tropical radiolarian zonation as a stratigraphic reference, produced a nearly continuous correlation of magnetic and radiolarian events ranging from late Pleistocene to earliest Miocene. Twenty magnetic polarity epochs, and possibly as many as 30 polarity events, occur during this time span. Epoch 16 (reversed polarity) appears to be the longest interval (～ 14.8–17.6m.y. B.P.) among these Neogene magnetostratigraphic units. The middle/late Miocene boundary is shown to fall within latest Epoch 11 (normal) and its approximate age is between 10.5 and 11 m.y. B.P. The early/middle Miocene boundary occurs within the top of Epoch 16 at a suggested age of about 15 m.y. B.P. 相似文献
19.
20.
Birger Schmitz 《Earth and Planetary Science Letters》1984,69(2):422-426
A study of the remanent magnetism in Swedish varved clay shows that, even in varves that macroscopically appear to be undeformed, the paleomagnetic record may be seriously altered due to stress-induced deformation of the micro-fabric in the sediment.In the studied area disturbed clays were rather the rule than the exception, but undisturbed sediments were found at two sites, Järna II and III. This clay formed 10,200 years B.P. and shows a declination of about 30° east, a value that could be obtained thanks to excellent lateral control of the magnetism. 相似文献