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1.
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
2.
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
3.
Abstract A single layer of widespread tephra deposits possibly can provide an instantaneous record of the past geomagnetic field and potentially can indicate even a small-scale tectonic rotation compared to a range of geomagnetic secular variations. We report paleomagnetic data of the Ebisutoge–Fukuda tephra, which is dated at approximately 1.8 Ma and is distributed in central Japan between the Osaka–Kyoto area and the Boso Peninsula. The Fukuda volcanic ash layer and its correlative ash deposits in the Osaka–Kyoto area, near Lake Biwa and in the Mie and Niigata areas yield identical site mean declinations of approximately −170° after tilt correction, whereas moderate inclination shallowing is observed in the upper unit at several localities. Anisotropy measurements both of low-field magnetic susceptibility and of anhysteretic remanent magnetization suggest that the inclination shallowing results from the biased alignment of magnetic grains, which were deposited in the fluvial environment. The source volcanic unit, Ebisutoge pyroclastic deposits in the Takayama area, yields a mean declination of approximately −155°, showing clockwise deflection from the magnetic directions of the correlative tephra deposits. These results suggest that no significant rotation occurred between the Osaka–Kyoto, Mie and Niigata areas, but that the Takayama area suffered a clockwise rotation in respect to the other areas during the Quaternary. This rotation might have been caused under an east–west stress field associated with the collision of the Okhotsk Plate with the Eurasia Plate. 相似文献
4.
A 5 mm thick tephra layer has been identified in the lacustrine sediments of Moon Lake in the Arxan-Chaihe volcanic field (ACVF) in Greater Khingan Mountains (NE China). The visible tephra layer is clearly revealed as a distinct peak in magnetic susceptibility measurements. The tephra layer consists mainly of brown vesicular glass shards and minor amounts of plagioclase, olivine and clinopyroxene. Major and minor element analysis has been carried out on the glass shards and plagioclase minerals. Glass shards show low concentrations of K2O, similar to the eruptive products derived from post-Miocene volcanoes of the ACVF. The plagioclase phenocrysts in both lava and tephra from ACVF, and in the tephra recorded in Moon Lake are labradorites. During the Late Pleistocene to Holocene, there were also extensive explosive eruptions in the nearby Nuominhe volcanic field (NVF). Volcanic rocks from the ACVF are easily distinguished from those derived from the NVF, having distinctly different K2O concentrations. This compositional variation is likely the result of different magmatic processes operating in the ACVF and NVF. Radiocarbon dating on organic materials from the lacustrine sediments dates the tephra layer to ca. 14,200 cal yrs BP, which implies that it was generated by a previously unknown Late Pleistocene explosive eruption in the ACVF. These results, for the first time, give a direct tephra record in this area, and suggest that identification of further tephra and/or cryptotephra in local sedimentary basins such as crater lakes of scoria cones and maars will be significant for dating the Late Pleistocene to Holocene volcanic eruptions and will help to establish a detailed record of the volcanic activity in the ACVF. The newly discovered tephra layer also provides a dated tephrochronological marker layer, which will in future studies provide a means to synchronise local sedimentary records of the climatically variable Late Glacial. 相似文献
5.
Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region. 相似文献
6.
Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region. 相似文献
7.
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
8.
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
9.
晚更新世以来以一系列地磁漂移事件是区域地层对比和确定沉积物年代框架的重要工具.但是在海相沉积物中,识别这些地磁漂移事件多依赖相对磁场强度(RPI)的变化,在地磁方向异常方面鲜有报到,从而影响对这些事件可靠性的讨论.本文选择南海南部巽他陆架附近重力活塞钻孔BKAS2PC的沉积物,进行系统的岩石磁学和古地磁研究,获得该钻孔沉积记录的RPI和磁倾角变化信息.岩石磁学及扫描电镜结果表明沉积物的原生主要载磁矿物为单畴(SD)和假单畴(PSD)钛磁铁矿.在一些层位,沉积后发生的还原作用形成胶黄铁矿等铁硫化物.磁性矿物的浓度和粒度等参数呈现显著的两阶段变化特征,上部(约220 cm以上)含量低而粒度细,下部含量高但粒度相对较粗,整体上均在一个数量级范围内波动.样品的交变退磁特征显示当交变场超过60 mT时,受胶黄铁矿影响产生旋转磁化,剩磁强度反而上升.因此,在20～60 mT之间确定原生特征剩磁,并把RPI定义为NRM_((20-40)mT)/ARM_((20-40)mT).通过~(14)C限定,并与其他有良好年代控制的相对磁场强度曲线进行对比,建立了钻孔的年代框架.结果显示,钻孔沉积物记录了几次显著的地磁漂移事件,这为联合应用RPI与磁场方向异常构建海相沉积物年代学框架提供了新的依据. 相似文献
10.
E. G. Guskova O. M. Raspopov A. L. Piskarev V. A. Dergachev 《Geomagnetism and Aeronomy》2007,47(6):781-786
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
11.
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
12.
Multiple-centres electron spin resonance (MC-ESR) dating of quartz grains has been commonly applied to fluvial and lacustrine deposits and can provide a precise chronological framework for depositional histories. However, the reliability of this method for quartz grains obtained from sediments of boreholes, which are usually deposited continuously and record information regarding basin evolution and climate change, has not yet been assessed. In this study, we have initially applied the MC-ESR dating method to borehole sediments from the Zhoulao core (ZLC), located in the depocenter of the Jianghan Basin, middle Yangtze River, China. Dating of quartz grains from the ZLC using MC-ESR yields estimated ages that are generally consistent with the established paleomagnetic chronological framework. For Middle Pleistocene samples (i.e., 0.7–0.3 Ma), the Ti–Li centre provides more accurate ages than those of Al centre, which are overestimated. For Early Pleistocene samples (i.e., 2.3–0.8 Ma), both the Al centre and Ti–Li centre give highly consistent estimate ages, indicating that this is a favorable dating range for MC-ESR. Overall, the Al centre shows promise for dating Pliocene samples, whereas the Ti–Li centre is more suitable for Middle-Early Pleistocene (2.3–0.3 Ma) sediments. In addition, the deposition rate of depth ＜170 m in the ZLC is greater than those of depth ＞170 m sediments; however, the specific tectonic, climatic, or geomorphic mechanism for this change in sedimentation rate is still unclear. 相似文献
13.
Kenneth L. Verosub 《Earth and Planetary Science Letters》1977,36(1):219-230
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
14.
A. V. Khokhlov 《Izvestiya Physics of the Solid Earth》2014,50(4):562-567
Understanding the character of variations in the magnetic field of the Earth in the geological past requires a mathematically substantiated method for testing the statistical hypotheses against the real paleomagnetic data. As known, the paleomagnetic data from lava flows are sort of momentary snapshots of the state of the ancient magnetic field. Being quite fragmentary in time and space, these data compose what is referred to as a sample in statistics: on the close discrimination of the lava flows, the internal correlations in the data are absent. It is well known that the distributions of the paleomagnetic directions from the sedimentary data differ from the distributions in lavas, which is mainly due to the effect of averaging of the magnetization over the time interval corresponding to the accumulation of sedimentary layers represented in the rock specimen. Assuming the rate of sedimentation to be known for each specimen, one can suggest the method for the quantitative testing the statistical consistency of the paleomagnetic data in the sediments with the model variations of the magnetic field of the Earth in terms of the Giant Gaussian Process (GGP). It turned out that the averaging effect can well be allowed for by the coefficients of GGP, and the scheme of the further testing is in this case identical to the scheme of testing the paleomagnetic data obtained from lavas. 相似文献
15.
本文报道青藏高原羌北地区中-晚侏罗世雁石坪群古地磁新结果.对采自青海省格尔木市唐古拉山乡雁石坪剖面(33.6°N, 92.1°E)11个灰岩采点(118块)和10个碎屑岩采点(99块)定向样品系统古地磁学研究表明,大部分样品的退磁曲线具有双分量特征.低温分量方向在地理坐标系下较为集中,应该为地层褶皱之后的黏滞剩磁.高温特征剩磁分量方向可分为两类:(1)索瓦组(J3s)和布曲组(J2b)灰岩,以磁铁矿为主要载磁矿物,高温特征剩磁分量(Ds=355.7°,Is=42.1°,k=58.2,α95=6°)可通过99%置信度的褶皱检验.(2)雪山组(J2x)和雀莫错组(J2q)碎屑岩,以赤铁矿、磁铁矿为主要载磁矿物,高温特征剩磁分量(Ds=3.3°,Is=28.9°,k=30.7,α95=8.9°)可通过95%置信度的倒转检验和99%置信度的褶皱检验.两组分量都应该是岩石形成时的原生剩磁信息.碎屑岩组的磁倾角比灰岩组偏低13°左右,其剩磁方向很可能存在着与压实作用相关的剩磁倾角变浅的状况.本文取灰岩组平均磁化方向作为雁石坪群的原生剩磁分量,获得羌北地区雁石坪群古磁极位置:80.0°N,295.2°E(dp/dm=7.4/4.5).古地磁结果表明,羌北-昌都地区晚石炭-晚二叠世期间位于南纬中低纬度地区,早三叠世以后开始大规模北向漂移,至中-晚侏罗世已到达24.3°N.其快速北向运动主要发生在早三叠至早侏罗世期间(3500 km左右),与现今位置相比中晚侏罗世之后的北向迁移总量为900 km左右. 相似文献
16.
V. G. Bakhmutov V. Ya. Yevzerov V. V. Kolka 《Izvestiya Physics of the Solid Earth》2009,45(7):567-582
The genesis, varve, and paleomagnetic informativeness of glaciolacustrine sediments are considered based on paleomagnetic and lithological studies. The magnetic properties of varved clays from periglacial basins have been considered using the sedimentological model of formation of varved clays as products of glacial turbidity currents, which explains the differences in the magnetic characteristics and degree of paleomagnetic informativeness of these clays. The distal zone sediments (formed due to precipitation from glacial turbidity currents, suspension, and sometimes along the periphery of the proximal zone) are most successfully used to study secular variations. The possible errors of the secular variation records in varved clays and such errors in estimating annual bedding of these clays have been analyzed. It has been indicated that information about the composition, concentration, and magnetic fraction size should be completed with detailed lithological studies, which make it possible to reconstruct the character of clastic material income into a basin and to decipher a paleomagnetic record. An analysis of the magnetic susceptibility anisotropy parameters and the magnetization formation in a sediment makes it possible to take into account disturbances caused by the sedimentation process and, thereby, to detect a pure geomagnetic signal 相似文献
17.
New paleomagnetic data from shallow-marine sediments of the Ichishi Group suggest a clockwise tectonic rotation of Southwest Japan in the Middle Miocene. Samples have been collected from mud or tuff layers at 17 sites. Stability of remanent magnetization has been examined by using alternating field and thermal demagnetization. The polarity sequence, composed of four normal and seven reversed polarity sites, is correlated to Polarity Epoch 16 (15.2–17.6 Ma), based on micropaleontological assignment of the upper Ichishi Group to Blow's Zone N8. The mean paleomagnetic direction of the 11 sites shows an anomalous declination toward the northeast. This result suggests that Southwest Japan was subjected to a clockwise rotation through 45° since 16 Ma. The clockwise rotation can be explained by the drift of Southwest Japan associated with the spreading of the Japan Sea during the Middle Miocene. 相似文献
18.
Volcanic glass compositions and tephra layer age are critical for anchoring their sources and correlating among different sites; however, such work may be imprecise when the tephra has varied compositions. The ash from Changbaishan Millennium eruption (940s AD), a widely distributed tephra layer, has been detected in the far-east areas of Russia, the Korean Peninsula, Japan, and in Greenland ice cores. There are some debates on the presence of this tephra from sedimentary archives to the west of Changbaishan volcano, such as lake and peat sediments in the Longgang volcanic field. In this paper, major element compositions for clinopyroxene and Fe-Ti oxides were performed on proximal tephra from Changbaishan and the Millennium eruption ash record in Lake Sihailongwan. Clinopyroxene and Fe-Ti oxides microlites from Sihailongwan show augite- ferroaugite and titanmagnetite compositions, similar to those from dark pumice in Changbaishan proximal tephra, but different from the light grey pumice, which has ferrohedenbergite and ilmenite microlite compositions. This result implies that the tephra recorded in Sihailongwan was mainly from the trachytic eruptive phase of the Millennium eruption, and the rhyolitic eruptive phase made a relatively small contribution to this area. Analyzing clinopyroxene and Fe-Ti oxides microlites is a new method for correlating tephra layers from Changbaishan Millennium eruption. 相似文献
19.
Fifty-three major explosive eruptions on Iceland and Jan Mayen island were identified in 0–6-Ma-old sediments of the North Atlantic and Arctic oceans by the age and the chemical composition of silicic tephra. The depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 Ma and paleoclimatic proxies (δ18O, ice-rafted debris) for the last 1 Ma. Major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off-rift volcanic zones in Iceland, and to the Jan Mayen volcanic system. The tholeiitic central volcanoes along the Iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4.9 Ma. They were the source of at least 26 eruptions of dominant rhyolitic magma composition, including the late Pleistocene explosive eruption of Krafla volcano of the Eastern Rift Zone at about 201 ka. The central volcanoes along the off-rift volcanic zones in Iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition, with three distinct episodes recorded at 4.6–5.3, 3.5–3.6, and 0–1.8 Ma. The longest and last episode recorded 11 Pleistocene major events including the two explosive eruptions of Tindfjallajökull volcano (Thórsmörk, ca. 54.5 ka) and Katla volcano (Sólheimar, ca. 11.9 ka) of the Southeastern Transgressive Zone. Eight major explosive eruptions from the Jan Mayen volcanic system are recorded in terms of the distinctive grain-size, mineralogy and chemistry of the tephra. The tephra contain K-rich glasses (K2O/SiO2>0.06) ranging from trachytic to alkali rhyolitic composition. Their normative trends (Ab–Q–Or) and their depleted concentrations of Ba, Eu and heavy-REE reflect fractional crystallisation of K-feldspar, biotite and hornblende. In contrast, their enrichment in highly incompatible and water-mobile trace elements such as Rb, Th, Nb and Ta most likely reflect crustal contamination. One late Pleistocene tephra from Jan Mayen was recorded in the marine sequence. Its age, estimated between 617 and 620 ka, and its composition support a common source with the Borga pumice formation at Sør Jan in the south of the island. 相似文献
20.
V. A. Bol’shakov 《Izvestiya Physics of the Solid Earth》2017,53(2):293-310
The results of numerous rock magnetic and paleomagnetic studies of Pleistocene deposits in the Loess Plateau in China which were obtained over a period of a few decades are analyzed. It is shown that two important problems remain unsolved. These are (1) developing the particular mechanism of “magnetic enhancement” in the soils, probably with a more accurate assessment of the level of effect of various natural factors causing qualitative changes in the magnetic fraction of the soil. Here, both the chemical composition of the newly crystallized magnetic mineral causing this enhancement and the parameters of the corresponding secondary (chemical) magnetization process should be determined. (2) Fixing the exact climate-stratigraphic position of the main paleomagnetic benchmarks of the Pleistocene, primarily the Matuyama–Brunhes reversal. In contrast to many conclusions, it is inferred that the Pleistocene paleoclimatic loess-soil record in China generally disagrees with the oxygen isotope (OI) record in the deep-sea sediments. This inconsistency is particularly significant for the Matuyama chron deposits. 相似文献