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1.
Paleomagnetic and/or micropaleontological studies have been carried out on approximately 28 sedimentary cores of latest Pleistocene age from the Gulf of Mexico. Sedimentation rates range from 9 cm to 20 cm/1000 yr. A distinct excursion in the earth's magnetic field occurs in the upper parts of 8 of 15 cores for which paleomagnetic studies were conducted and is independently correlated with planktonic foraminiferal zones. An inclination change to zero or negative inclination is often associated with a declination change. The age of the excursion was determined by extrapolation of sedimentation rates from the Z-Y paleontological boundary which is dated at 11,000 B.P. The magnetic excursion occurred between 12,500 and 17,000 yr ago within the upper part of zone Y. This falls within the age range of the Laschamp Event as originally defined.  相似文献   

2.
A paleomagnetic study of sediments at the Baranova Gora and Podol III/1 archaeological sites, located near Lake Volgo on the northwestern Central Russian Upland (56.9°N, 33.2°E), was performed. The paleomagnetic studies at both sites for the first time revealed the development of the Gothenburg geomagnetic excursion (dated 13000-12350 BP) in this region. This made it possible to specify the time interval when the Alleroed climatic phase started developing on the Central Russian Upland.  相似文献   

3.
An analysis of the paleomagnetic characteristics of the bottom sediments taken in 2000 in the northern Barents Sea for the first time revealed the Gothenburg geomagnetic field excursion (13 000–12 000 years ago) at the time boundary of the transition from the glacial period to the recent warm epoch (the Holocene). The obtained data confirm the excursion complex structure: the presence of two successive time intervals of variations in the geomagnetic field inclination. An increase in the magnetic susceptibility and natural remanent magnetization of the samples at the above boundary and about 15 000 years ago indicates that the magnetic parameters of the sediments respond to climate changes in the environment in this time interval.  相似文献   

4.
This article commences by surveying the basic dynamics of Earth's core and their impact on various mechanisms of core-mantle coupling. The physics governing core convection and magnetic field production in the Earth is briefly reviewed. Convection is taken to be a small perturbation from a hydrostatic, “adiabatic reference state” of uniform composition and specific entropy, in which thermodynamic variables depend only on the gravitational potential. The four principal processes coupling the rotation of the mantle to the rotations of the inner and outer cores are analyzed: viscosity, topography, gravity and magnetic field. The gravitational potential of density anomalies in the mantle and inner core creates density differences in the fluid core that greatly exceed those associated with convection. The implications of the resulting “adiabatic torques” on topographic and gravitational coupling are considered. A new approach to the gravitational interaction between the inner core and the mantle, and the associated gravitational oscillations, is presented. Magnetic coupling through torsional waves is studied. A fresh analysis of torsional waves identifies new terms previously overlooked. The magnetic boundary layer on the core-mantle boundary is studied and shown to attenuate the waves significantly. It also hosts relatively high speed flows that influence the angular momentum budget. The magnetic coupling of the solid core to fluid in the tangent cylinder is investigated. Four technical appendices derive, and present solutions of, the torsional wave equation, analyze the associated magnetic boundary layers at the top and bottom of the fluid core, and consider gravitational and magnetic coupling from a more general standpoint. A fifth presents a simple model of the adiabatic reference state.  相似文献   

5.
Palaeomagnetic measurements have been carried out on 474 specimens taken at about 2.5-cm intervals along an 11 m long piston core.The magnetic inclination exhibits variations with a period of approximately 2800 yr deduced from five radiocarbon age determinations along the core. The assumption that this period was regular allows “magnetic” ages to be assigned to the core and when these are plotted against depth, two straight lines are obtained indicative of steady rates of deposition of 0.4 mm yr?1 since about 15,000 yr B.P. and 0.8 mm yr?1 before then. The radiocarbon ages, except that at 16,800 yr, fit these lines suggesting that the assumption that the magnetic inclination fluctuated with regular period is justified. These swings correlate well, but on a two to one basis, with swings in inclination described for core V10-58 from the Aegean Sea (N.D. Opdyke, D. Ninkovich, W. Lowrie and J.D. Hays, 1972). The extra detail carried by the Black Sea core is possibly explained by the faster rate of deposition.The close correspondence of the periodity of these inclination swings with that observed in declination at Lake Windermere (F.H. Mackereth, 1971) is noted and the possibility that they are both caused by oscillations in intensity of the same geomagnetically active region in the core is discussed.The ratio of NRM intensity to susceptibility varies between about 0.4 and 25 within the banded lutites. This range of values may reflect different magnetic mineralogy in the two types of band, of which there are typically 3–7 per specimen, as well as variations in geomagnetic field intensity.  相似文献   

6.
Paleomagnetic and rock-magnetic studies on a hydraulic piston core (Ver98-1, St.6) from Academician Ridge, Lake Baikal showed the occurrence of a reversal excursion at 670-696 cm depth, which is at the base of marine oxygen isotope stage 6. A correlation of X-ray CT values, as a proxy of relative density, to the marine oxygen isotope record provides an age of 177-183 ka for this reversal excursion. It can be correlated with other excursion records from Lake Baikal, found in Core 287-K2 from Academician Ridge [King et al., Russ. Geol. Geophys. 34 (1993) 148-162] and in core BDP93-1 drilled on the Buguldeika saddle [BDP-93, Quat. Int. 37 (1997) 3-17]. We correlate the Lake Baikal reversal excursion with a well documented excursion in the Brunhes Chron, the Iceland Basin event (186-189 ka) from ODP Sites 983 and 984 in the North Atlantic [Channell, J. Geophys. Res. 104 (1999) 22937-22951]. Also the relative paleointensity record agrees well with that from ODP Site 983 [Channell, J. Geophys. Res. 104 (1999) 22937-22951]. The Lake Baikal excursion and the Iceland Basin event correspond to the minimum of relative intensity at 188 ka in Sint-800 [Guyodo and Valet, Nature 399 (1999) 249-252]. We argue that it is distinct from the Jamaica/Pringle Falls excursion, estimated at 205-215 ka [Langereis et al., Geophys. J. Int. 129 (1997) 75-94]. This is supported by the recalibration of the age of another excursion found in Core St.16 in Lake Baikal [Sakai et al., Bull. Nagoya Univ. Furukawa Mus. 13 (1997) 11-22] with an age of ∼223 ka, which is close to the age of the Jamaica/Pringle Falls excursion, as suggested earlier [King et al., Russ. Geol. Geophys. 34 (1993) 148-162]. The VGP path of the reversal excursion (177-183 ka) consists of a southward swing through the North Atlantic, followed by a loop through Africa and the Indian Ocean. The path morphology is similar to that of the Iceland Basin event from the North Atlantic [Channell, J. Geophys. Res. 104 (1999) 22937-22951].  相似文献   

7.
对白令海北部陆坡B5-4孔进行了古地磁和岩石磁学研究,尝试获得该岩芯的地磁场相对强度和方向变化信息.结果表明:(1)除0~0.44 m沉积物的磁性矿物粒度比其余沉积物细以外,岩芯的磁学性质总体均一,其记录的地磁场相对强度可以与北大西洋ODP983孔相应记录进行高度对比.(2)根据B5-4孔与ODP983孔地磁场相对强度记录对比结果,并结合该孔4.54~4.56 m处有孔虫AMS14C测年结果,可以确定3个深度-年龄对比点,并据此初步建立了B5-4孔的年龄模型.(3)B5-4孔磁偏角和磁倾角记录与贝加尔湖、北美、欧洲全新世以来的记录和当地地磁场球谐模型结果一致,其对比点丰富了强度对比点年龄模型,揭示了14 cal ka B.P.以来近线性的沉积模式.(4)根据与中国东部陆架两个钻孔的磁倾角对比,我们推测B5-4孔9~14 ka之间两段浅化的磁倾角可能是哥德堡极性事件的记录,但是受到早期成岩或者沉积物平滑效应的影响.以上结果足以证明,地磁场相对强度和方向变化可以从适宜的白令海沉积物中获得,它可以为确定该海区沉积物年龄提供相关辅助信息,有助于解决北极、亚北极古环境和古海洋研究中由于有孔虫等钙质生物壳体缺乏导致的年龄信息匮乏问题.  相似文献   

8.
The Umbrian Apennines were the site of pelagic sedimentation throughout most of the Jurassic. Magnetic stratigraphy from four sections spanning many of the Jurassic stages indicates that the geomagnetic field at this time was characterized by two intervals of mixed polarity, separated by an interval of predominantly normal polarity corresponding to the Jurassic “quiet-zone” in the oceanic magnetic anomaly record. Unfortunately, ammonites are poorly preserved or absent throughout most of these sections; the duration of this “quiet-interval” cannot be well defined, although it is probably restricted to the Callovian and Oxfordian stages.  相似文献   

9.
Magnetic properties as well as the magnetomineralogy were investigated in rocks underlying a 7 km long aeromagnetic anomaly situated in the Moldanubian crystalline unit of the Bohemian Massif. The anomaly is caused by highly magnetic cordierite gneiss forming a stripe of NE — SW direction east of the town of Humpolec, eastern Bohemia. Magnetic susceptibility and its anisotropy (AMS), natural remanent magnetization, field and temperature variations of susceptibility were measured. Optical study of thin sections, electron microprobe and Mössbauer studies were also used to reveal the carrier of the high susceptibility and the high NRM. There appear to be two major generations of Fe-Ti oxides: older hematite with ilmenite exsolutions (Ti-hematite) which is the dominant remanence phase, and younger magnetite, the dominant susceptibility phase, usually associated with rutile. This indicates a reaction Hematite + Ilmenite → Magnetite + + Rutile; the trace elements in magnetite, as well as texture and morphology of the oxide grains support this assertion. An additional minor portion of maghemite is revealed by Mössbauer and thermomagnetic results. The Ti-hematite belongs to the oldest mineral assemblage in the rock, despite its anhedral morphology. Inclusions in Ti-hematite, among which corundum and abundant paragonite occur, record a strongly peraluminous and probably disequilibrium association during the crystallization of the Ti-hematite.  相似文献   

10.
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   

11.
The data that describe the long-term reversing behavior of the geodynamo show strong and sudden changes in magnetic reversal frequency. This concerns both the onset and the end of superchrons and most probably the occurrence of episodes characterized by extreme geomagnetic reversal frequency (>10–15 rev./Myr). To account for the complexity observed in geomagnetic reversal frequency evolution, we propose a simple scenario in which the geodynamo operates in three distinct reversing modes: i—a “normal” reversing mode generating geomagnetic polarity reversals according to a stationary random process, with on average a reversal rate of ～3 rev./Myr; ii—a non-reversing “superchron” mode characterizing long time intervals without reversal; iii—a hyper-active reversing mode characterized by an extreme geomagnetic reversal frequency. The transitions between the different reversing modes would be sudden, i.e., on the Myr time scale. Following previous studies, we suggest that in the past, the occurrence of these transitions has been modulated by thermal conditions at the core-mantle boundary governed by mantle dynamics. It might also be possible that they were more frequent during the Precambrian, before the nucleation of the inner core, because of a stronger influence on geodynamo activity of the thermal conditions at the core-mantle boundary.  相似文献   

12.
Our study concerned magnetic properties of soil profiles taken from polluted regions of Eastern Ukraine around the industrial centres Krivyj Rig, Mariupol and Komsomolsk. Soils represent chernozem and podzoluvisol. The low-field magnetic susceptibility shows enhancement in the topsoil caused by contamination by coarse-grain magnetite connected with industrial pollution. Magnetic mineralogy was determined by means of thermal demagnetisation of SIRM, monitoring of susceptibility changes during warming from ?196°C to room temperature and heating to 700°C, and Mössbauer analysis. Granulometry of magnetic particles was investigated by determination of hysteresis parameters, susceptibility, M s , SIRM and ARM ratios and frequency dependence of susceptibility. The chemical parameters, namely pH, organic carbon and iron content in different pedogenic and lithogenic minerals, measured for particular horizons determined pedogenic characteristic of soil profiles. Our study showed that differences in magnetic parameters of non-polluted and polluted soil profiles are not limited to the topsoil, but reach deep layers of the parent material. Industrial pollution promotes formation of the so-called “pedogenic” SP and SD particles in these layers.  相似文献   

13.
Paleomagnetic samples from the Nolichucky Formation (Late Cambrian), sampled at two sites in the Valley and Ridge Province of east Tennessee, yield a possibly penecontemporaneous characteristic magnetization that appears to reside in detrital magnetite. The paleomagnetic pole positions are “Paleozoic”, but differ: site I, lat. 41°N, long. 109°E,dp = 1°, dm = 2°; site II, lat. 39°N, long. 131°E,dp = 4°, dm = 7°. The difference in poles reflects a significant difference in declination between the site-mean directions, and this declination difference probably reflects relative tectonic rotation as the sites are in different thrust sheets. The paleontologic age of both sections is exceptionally well-constrained as they are sampled across an abrupt “biomere boundary” between contrasting trilobite faunas. Comparison of these results with paleomagnetic data from coeval strata elsewhere in North America reveals gross discrepancies, so that at least some of the published data must reflect remagnetization and/or tectonic rotation.  相似文献   

14.
The paper presents the history of measurements of the geomagnetic field parameters over the territory of Russia in the 18th century derived from archival and literature sources. Topographical mapping of the European territory and neighboring seas of the Russian Empire from the late 17th to the mid 18th century during which magnetic measurements were made was of great importance for determining magnetic declination. The magnetic declination in Siberia and its neighboring seas was measured for the first time during the first expedition of V. Bering in 1728 and then during his second expedition (the Great Northern Expedition) in 1733–1742. Magnetic measurements were carried out along the northern coast of Siberia and in the Bering Sea. The academic group of the expedition performed magnetic declination measurements over southern regions of Siberia (Krasnoyarsk, Irkutsk, Nerchinsk, Yakutsk, etc.) in 1735 and 1736. During the second half of the 18th century, Russian expeditions determined geographical coordinates of the cities of European Russia and carried out magnetic declination measurements for them. During these expeditions Inokhodtsev paid attention to the anomalous magnetic declination in the Kursk region. In his opinion, the anomaly could have been caused by the presence of iron ore.  相似文献   

15.
现代风成沉积物磁化率各向异性与风向关系的研究   总被引：19，自引：3，他引：16       下载免费PDF全文
通过对风洞及室内模拟定向风条件下的风成沉积物磁化率各向异性与风向之间关系的研究,发现风成沉积物磁化率各向异性的长轴方向的偏角不仅与风向有关,而且与粉尘沉积量有关。风成沉积物磁化率各向异性的长轴方向的偏角在α95（α95≤20°）的区间范围内与粉尘沉积量大的风向一致．在干旱、半干旱区,现代风成沉积物（沙丘、黄土的表层土）的磁化率各向异性的长轴方向的偏角在此范围区间内与常年盛行风向一致,尤其与沉积物沉降时的盛行风向一致．  相似文献   

16.
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.
Paleomagnetic measurements have been made on a continuously sampled, 5-m section of a core from Clear Lake, California. The sediments studied span an 8000-year interval centered at 25,000 years B.P., the approximate date of the large-scale, counterclockwise loop of the magnetic vector recorded in sediments from Mono Lake, California. The data from Mono Lake have been interpreted as a geomagnetic excursion with a duration of 600–1000 years. Because Clear Lake is only 320 km from Mono Lake and since each sample from Clear Lake represents 26 years of sedimentation, the magnetic signature of the Mono Lake geomagnetic excursion should be recorded in detail in the Clear Lake samples. Aside from a generally uniform shallowing of inclinations due to compaction, the paleomagnetic record from Clear Lake contains no anomalous features which would correspond to the Mono Lake excursion. Thus it has yet to be shown that the Mono Lake excursion was recorded anywhere besides Mono Lake. Even if the existence of the excursion is ultimately confirmed, its usefulness as a magnetostratigraphic horizon is limited.  相似文献   

18.
Unpreparedness is often the main cause of the economic and social damages caused by floods. To mitigate these impacts, short-term forecasting has been the focus of several studies during the past decades; however, less effort has been paid to flood predictions at longer lead times. Here, we use forecasts by six models from the North American Multi-Model Ensemble project with a lead time from 0.5 to 9.5 months to predict the seasonal duration of floods above four National Weather Service flood categories (“action,” “flood,” “moderate” and “major”). We focus on 202 U.S. Geological Survey gage stations across the U.S. Midwest and use a statistical framework which considers precipitation, temperature, and antecedent wetness conditions as predictors. We find that the prediction skill of the duration of floods for the “action” and “flood” categories is overall low, largely because of the low accuracy of the climate forecasts rather than of the errors introduced by the statistical models. The prediction skill slightly improves when considering the shortest lead times (i.e., from 0.5 to 2.5 months) during spring in the Northern Great Plains, where antecedent wetness conditions play an important role in influencing the generation of floods. It is very difficult to draw strong conclusions with respect to the “moderate” and “major” flood categories because of the limited number of available events.  相似文献   

19.
PKKP signals from Novaya Zemlya recorded at LASA at distances around 60° show consistent anomalies in both slowness and azimuth. The observed anomaly suggests that the signal is a BC branch arrival, although the arrival time corresponds to the DF branch. The BC branch, however, does not extend back to this distance. The azimuth of approach is in the range 229–245°, instead of the expected 186°. These anomalies are associated only with PKKP; analysis of the core phases PKiKP and P′P′ (BC) from the same events show that they arrived at LASA with the appropriate slownesses and azimuths.The PKKP signals can be interpreted as “scattered” PKKP; the scattering occurs on underside reflection at the core-mantle boundary and is probably caused by topographic irregularities on the boundary itself. The calculated scattering region has a surface projection at about 60°S, 134°E, which is outside the diametral plane through source and receiver, and about 21° from the expected PKKP reflection point at 76°S, 95°E.Both the “direct” and “scattered” arms of the PKKP signal have a PK path close to that of the “C” end of the BC branch. The unexpectedly large amplitude of the arrival suggests that there may be a focusing of energy at C, which would indicate a change in velocity gradient just above the inner core boundary. The observations nevertheless require, on the scattering interpretation, lateral variations in the topography of the core-mantle boundary and a region of relatively large topography responsible for the anomalous PKKP observations.  相似文献   

20.
We investigated a late Quaternary terrestrial sedimentary sequence (Uwa Formation) in core IC2, from a site adjacent to that of the reported core IC on NW Shikoku Island, SW Japan, and developed its tephra and pollen stratigraphy to refine the age model of the formation. First, we identified 19 horizons with high glass shard concentrations in the IC2 core sediments as possible tephras or cryptotephras, and correlated them with reported tephras on the basis of the major- and trace-element compositions of their glass shards. All correlated widespread tephras and cryptotephras were products of volcanoes in the Kyushu volcanic zone (Aso, Kakuto, Aira, Ata, and Kikai calderas). Second, we confirmed the presence in core IC2 of two pollen zones dominated by Quercus subgen. Cyclobalanopsis, which is an indicator of very warm interglacial vegetation. In the Japanese Islands, these two vegetation zones have usually been considered to characterize marine isotopic stages (MISs) 1 and 11. A previous study of the Uwa Formation correlated the upper pollen zone to MIS 1, but the lower zone was not correlated to MIS 11; rather, it was inferred to be older than MIS 12 because it was stratigraphically below the “Oda” tephra (equivalent to a distal Kasamori 5 [Ks5] tephra [MIS 12]). In this study, however, noting that the Naruohama-IV tephra (Nh-IV; MIS 10d) and Ks5 cannot be distinguished by their shard chemistries, we inferred that the suggested “Oda” tephra actually correlates to Nh-IV, rather than to the Ks5 tephra. By re-assigning the “Oda” tephra to Nh-IV, we could correlate the underlying Quercus subgen. Cyclobalanopsis-abundant zone to MIS 11 and, consequently, a pair of pollen zones indicating cool and warm conditions below the MIS 11 pollen zone to MISs 12 and 13, respectively. The resulting age model whereby tephra and pollen constraints are integrated showed a roughly constant sedimentation rate from MIS 13, without any long-term gaps; further, our MIS 13 horizon in core IC2 corresponds to the reported 1 Ma tephra horizon in core IC. Therefore, these findings represent a dramatic change in the Uwa Formation age model and validate the Uwa Formation as one of the most useful terrestrial archives of Quaternary tephrostratigraphy and paleoclimatic fluctuation in SW Japan.  相似文献   

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