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1.
Until now, palaeoclimatic reconstructions for the major stages in the development of the Quaternary loess–palaeosol sequence on the Russian Plain have been based on pedological, palynological and faunal (vertebrates and molluscs) analyses. In order to demonstrate the palaeoclimatic influence on the magnetic properties of this sequence, the magnetic susceptibility signature of the Strelitsa type section in the Upper Don basin is compared with a detailed landscape – climate reconstruction of loess and soil from palynological data. Large amplitude fluctuations of palaeoclimate and palaeoenvironment are reflected clearly in the lithology and in the rock magnetic properties, which usually are enhanced in wet and warm interglacial periods, but stay at low levels during cold glacial epochs. Palynological climate zonation, however, is sometimes in conflict with the pedologic–magnetic record. Strong climate fluctuations, as indicated by changing pollen assemblages, are not always paralleled by corresponding changes in lithology and/or rock magnetic properties. Alternatively, light coloured illuvial horizons with low magnetic signal sometimes appear to have formed during early stages of interglacials, and the top parts of some palaeosols apparently formed during glacial stages. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

2.
The sediments deposited off south-western Baja California have recorded millennial-scale fluctuations in biogenic compounds and trace metals accumulation during the last glacial period [Ortiz, J.D., O’Connell, S.B., DelViscio, J., Dean, W.E., Carriquiry, J.D., Marchitto, T., Zheng, Y., van Geen, A., 2004. Enhanced marine productivity off western North America during warm climate intervals of the past 52 ky. Geology 32, 521–524; Dean, W.E., Zheng, Y., Ortiz, J.D., van Geen, A., 2006. Sediment Cd and Mo accumulation in the oxygen-minimum zone off western Baja California linked to global climate over the past 52 ka. Paleoceanography 21, PA4209]. Since the variations in trace metals concentration appear to result from dilution with nonbiogenic matter, the variability of the terrigenous sedimentation needs to be addressed. Therefore, we performed rock magnetic and geochemical analysis on a 38 m-long sediment core collected from the slope off Baja California. The temporal framework provided by 14C dating and identification of palaeoclimatic transitions allows assigning the sequence to the last glacial–interglacial cycle (last 120 ka). The comparison of magnetic and geochemical properties led to retain the hypothesis of a primary modulation of iron oxides quantity and quality by terrigenous input variations, with a secondary diagenetic amplification. Two magnetic mineral input regimes are distinguished: (i) magnetic susceptibility variations reveal changes in titano-magnetite concentration related to fluvial transport of the terrigenous fraction; (ii) coercivity changes allow detecting variations of hematite or goethite concentrations, minerals generally issued from aeolian deflation of weathered rock surfaces. These two regimes are paced by two distinct climatic forcing: the millennial-scale changes in titano-magnetite input are related to the northern hemisphere climatic variability whilst the record of wind-blown magnetic mineral input contains its major power in the precessional frequency band, with higher input during low insolation periods (and conversely). This record highlights the great sensibility of this region to high and low latitudes climatic regimes.  相似文献   

3.
We conducted rock magnetic and paleomagnetic research on two deep-sea sediment cores from the west Philippine Sea, located to the east of Benham Rise with the length of 4 m and water depth of over 5000 m. At the bottom of core 146 occurs a reversal of inclination and deflection of relative declination, which is recognized as Brunhes-Matuyama Polarity Boundary (MBPB). No reversal occurs in core 89, which implies a younger bottom age than that of core 146. Rock magnetic results reveal magnetic uniformities in mineralogy, concentration and grain size along the two cores, thus relative paleointensity variations are acquired. The three normalizers-anhysteresis remanent magnetization (ARM), magnetic susceptibility (k) and saturation isothermal remanent magnetization (SIRM) are used for normalization to obtain relative paleointensities. The three normalization results are averaged to indicate the paleoitensity of the cores and are further stacked together to get a synthetic curve for west Philippine Sea (named asWPS800 in this paper). Based on the magnetic correlation between cores and paleointensity to Sint800, we transfer the changes of rock magnetic parameters from depth to time. Furthermore, the astronomically tuned oxygen isotope from ODP site 1143 in the south China Sea is used for the glacial and interglacial indicator. Three concentration proxies (ARM, k and SIRM) and grain size indicators (k ARM/SIRM, k ARM/k) are examined according to the paleointensity-assisted chronology. The grain size changes in the two cores display a consistent pattern with the climatic changes embodied by oxygen isotope. The magnetic sizes are usually coarser in glacial periods and finer in interglacial times, which may reflect the influence of chemical erosion rather than fining from sea level rising on the source sediment. Furthermore, the sub-peaks and sub-troughs in interglaciations almost correspond with that of oxygen isotope records, which means sedimentation can reflect the subtle changes in interglaciations. This kind of revelation of climatic fluctuation by magnetic size is also found in the South China Sea, which shows a common pattern of magnetic signals to climate at least within East Asia. The concentration of ARM (representing more about fine grain) also shows similar response to glacial and interglacial cycles, that is, high in interglacial cycle and low in glacial cycle; but k and SIRM (reflecting more about coarse grain) lack the response to the climatic cycles. At the same time, S-ratio lacks the correlation with aeolian dust record and rhythmic changes, indicating the dominant source of main magnetic carrier (low coercivity magnetite) is the suspended matter instead of dust. The decreasing trend of sedimentation rate from west to east also reveals that the sediments are mainly from west Luzon and adjacent land. Grain sizes first became coarse and then stable around 400 ka B.P., and at the same time all the magnetic contents lowered and amplitude of magnetic mineral changes increased. The magnetic transition around 400 ka B.P. is simultaneous with the decreases of carbonate content, reflecting a global carbonate dissolution event, i.e. mid Brunhes event. The synchronization of magnetic content and grain size with climatic cycles of glacials and interglacials imply the validity of paleointesnityassisted chronology. Also, the response of rock magnetic signals to stable oxygen isotope changes and carbonate variation reveals that rock magnetismmethod can be an effective tool for paleoclimatic and paleoceanographic research. __________ Translated from Quaternary Sciences, 2007, 27(6): 1040–1052 [译自 : 第四纪研究]  相似文献   

4.
桂林水南洞洞穴沉积物的古地磁记录及其古环境意义   总被引：1，自引：0，他引：1  
通过对桂林水南洞2.5m厚的洞穴沉积物剖面的古地磁初步研究,其结果及地质证据表明该沉积剖面形成期可能与贾拉米洛正向期对应。根据磁化率、磁性矿物的特征和沉积学特征,对该地区这一地质时期的古环境变化提出以下认识:1.第三暖湿期,以钙华沉积为特征;2.第二干冷期,以含有少量钙质的黄褐色粘土为特征,并以含有高矫顽磁性矿物和低磁化率为特征;3.第二暖湿期,以互层的红褐色粘土、含高含量钙质的粘土或钙华为特征,并以含有低矫顽磁性矿物和高磁化率为特征;4.第一干冷期,以褐灰色粘土为特征,并以含有高矫顽磁性矿物和低磁化率为特征;5.第一暖湿期,以地下河沉积相(砂卵石为主)为特征。   相似文献   

5.
A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studied loess–paleosol sediments retain primary sedimentary fabrics. Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from both loess and paleosol specimens. A low-temperature component, isolated between 100°C and 200°C, is consistent with the present geomagnetic field direction, and a high-temperature component, isolated between 200–300°C and 620–680°C, includes clearly normal and reversed polarities. Isothermal remanent magnetization and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite. The Day plot, together with the stratigraphic variations of rock-magnetic parameters, shows that the uniformity of magnetic mineralogy and grain size fulfills the criteria for relative paleointensity (RPI) studies. RPI records have been constructed using natural remanent magnetization (NRM) intensity after thermal demagnetization at 300°C normalized by low-frequency magnetic susceptibility (NRM300/χ). The results show that the RPI record from the Baoji section, where pedogenesis is quite weak, is compatible with the stacked PISO-1500 paleointensity record, suggesting that it might reflect the paleointensity variation of the geomagnetic field. The RPI record from the Xifeng section, where pedogenesis is rather strong, indicates a clear dissimilarity with the stacked PISO-1500 paleointensity record, implying that it does not reflect the paleointensity variation of the geomagnetic field. Our new results show that the NRM300/χ from the strongly pedogenetic paleosols does not completely eliminate the pedogenetic (climatic) influence, so it might be unsuitable for a reliable paleointensity study.  相似文献   

6.
A magnetic investigation of a loess/paleosol sequences record in Ili area   总被引：1，自引：0，他引：1  
The pedogenic ferrimagnetic minerals have been considered to be the cause of magnetic susceptibility enhancement in loess deposits distributing in the Chinese Loess Plateau and Central Europe, while “wind intensity” mechanism is proposed to be responsible for the magnetic susceptibility enhancement of loess in Alaska and Siberia. However, the magnetic enhancement mechanism is still open for loess in Ili valley, Xinjiang Uygur Autonomous Region, China. To understand this, we conducted a rock magnetic investigation on Axike section that is located in Ili valley. Results show that transitional stage from magnetic (χlf) trough to magnetic (χlf) peak corresponds to soil units. The PSD and MD-grained magnetite dominate the magnetic properties of AXK sequences, and the main factor affecting magnetic properties is the concentration of ferrimagnetic fraction. For the “pedogenesis enhancement” and “wind intensity” model, it seems hard to explain the magnetic susceptibility enhancement in this area. For the concentration of fine-grained magnetite in magnetic mineral shows positive relationship with the intensity of pedogenesis, the magnetic parameters of loess deposit in Ili valley can be used to recover paleoclimatic variations.  相似文献   

7.
A Holocene lake sediment record is presented from Lake N14 situated on Angissoq Island 15 km off the main coast of southern Greenland. The palaeoclimatic development has been interpreted on the basis of flux and percentage content of biogenic silica, clastic material, organic material and sulphur as well as sedimentation rate, moss content and magnetic susceptibility. A total of 43 radiocarbon dates has ensured a reliable chronology. It is argued that varying sediment composition mainly reflects changing precipitation. By analogy with the present meteorological conditions in southern Greenland, Holocene climate development is inferred. Between 11 550 and 9300 cal. yr BP temperature and precipitation increase markedly, but this period is climatically unstable. From 9300 yr BP conditions become more stable and a Holocene climatic optimum, characterised by warm and humid conditions, is observed from 8000 to 5000 cal. yr BP. From 4700 cal. yr BP the first signs of a climatic deterioration are observed, and from 3700 cal. yr BP the climate has become more dry and cold. Superimposed on the climatic long‐term trend is climate variability on a centennial time‐scale that increases in amplitude after 3700 cal. yr BP. A climatic scenario related to the strength and position of the Greenland high‐pressure cell and the Iceland low‐pressure cell is proposed to explain the Holocene centennial climate variability. A comparison of the Lake N14 record with a terrestrial as well as a marine record from the eastern North Atlantic Ocean suggests that the centennial climate variability was uniform over large areas at certain times. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

8.
Carbonate rocks can be classified in terms of those properties relating to the pore system of lithified sediments, so‐called ‘petrophysical rock types’, or ‘depositional rock types’ which are categorized based on characteristics directly reflecting their original depositional environment. Whereas petrophysical rock types are typically used to identify and distribute rock bodies within a reservoir with similar flow characteristics, depositional rock types ignore pore types and capture sedimentary structures, lithology and fossils. Both classification systems are extensively used to describe reservoir rocks, but the degree of plurality between them remains poorly understood and is the motivation for this study. To examine the degree of congruency between the two classification schemes, a field assessment was conducted for a 175 km2 area situated offshore Al Ruwais, northern Qatar, encompassing depositional environments spanning supratidal, intertidal, shallow subtidal and open marine conditions. A total of 350 surficial sediment samples were collected along 24 shore‐normal transects. Each sample was assigned a ‘petrophysical rock type’ class based on analysis of sedimentary texture (grain size and sorting). ‘Depositional rock type’ classes, by contrast, were defined with reference to faunal content and, in turn, classes of mineralogy were delimited by weighting this content against the mineralogy of each faunal category. Of course, the samples studied correspond to unconsolidated sediments and not to indurated rocks. However, considering only primary porosity and permeability preservation, it is reasonable to assume that the classified sediments would become petrophysical rock types and depositional rock types when consolidated, following their primary grain size, sorting and grain type distribution. Therefore, the term ‘rock type’ is retained here for ease of terminology but, for clarity, these are sediment samples. The discrete samples were interpolated into continuous surfaces describing the distribution of depositional rock types, petrophysical rock types and mineralogy, and spatial correspondence between those surfaces was statistically evaluated. In order to link these parameters with environment of deposition, their correlation with water depth (as audited from airborne light detection and ranging) and ecological habitat (mapped from DigitalGlobe satellite imagery) was also assessed. The data reveal that spatial distributions of sedimentary faunal, petrographic and mineralogical properties do not show exactly congruent patterns. Other meaningful trends do exist, however. For example, the occurrence of certain depositional rock types is indicative of particular petrophysical rock types, and vice versa. Further, connections between petrophysical rock types and mineralogy are emphasized and offer insight as to how the evolution of matrix porosity might be predicted via diagenetic models tuned to specific sediment textures. Useful relationships are also identified between the occurrence of petrophysical rock types and depositional rock types, and both ecological habitat and water depth. The potential of such dualities is two‐fold. Firstly, they can be applied to more realistically distribute petrophysical rock types and depositional rock types by environment of deposition in reservoir models and, secondly, the use of modern carbonate systems as subsurface analogues might be enhanced.  相似文献   

9.
The piedmont aeolian sediment in Central Asia is has been widely used for Quaternary palaeoclimatic reconstructions. Here we present detailed sedimentological and rock magnetic investigations on 188 samples from the ~9.4‐m‐thick Urumqi River section (URS) in the southern Chaiwopu Basin of the northern Chinese Tian Shan foreland. Optically stimulated luminescence (OSL) dating constrains the time‐span of the URS loess sedimentation from c. 28 to c. 6.8 ka. This chronology implies that the onset of the URS loess sedimentation is approximately synchronous with the Shangwangfeng Glaciation (MIS 2) in the headwaters of the Urumqi River. Analyses of sediment grain size and rock magnetic parameters indicate that sediment grain‐size composition controls magnetic properties of the URS loess, i.e. the wind intensity model, distinct from the pedogenic model interpreting the variations in magnetic properties of the loess in the Chinese Loess Plateau (CLP). Overall, the variations in sediment grain size and magnetic properties throughout the URS indicate that the present study area was dominated by a climate of strengthened wind associated with gradually intensified aridity during the latest Pleistocene. In contrast, the intensity of aridity and coldness during the early Holocene gradually became relatively weakened.  相似文献   

10.
Measurements of palaeomagnetic horizontal remanent intensity, saturation isothermal remanence, coercivity of remanence, high field remanence and magnetic susceptibility, have been made on lateglacial and postglacial sediments from two small, inter-drumlin hollow lakes from central Northern Ireland. These magnetic measurements have been compared with pollen and chemical analyses from the same profiles and with complementary mineral magnetic data from local soils. Representative sediment samples have also been divided into a range of particle size fractions and each fraction has been subjected to magnetic investigation. Five distinctive stratigraphic horizons have been identified on the basis of the magnetic mineralogy. Two horizons relate to phases of pronounced erosion. One of these is connected with solifluction processes in the lateglacial and the other to an intensive period of farming activity which started in Medieval times. Two horizons are associated with periods of very high iron and manganese deposition and are possibly related to the precipitation of magnetic minerals within the lake. Hydrological changes during periods of local anthropogenic activity appear to produce magnetic mineral assemblages indicative of stream bed/bank substrate sources. The particle size data also support the suggestion that downcore changes in the mineral magnetic record are mainly caused by changes in the sediment source rather than particle size or sedimentological effects.  相似文献   

11.
中国黄土磁性矿物特征及其古气候意义   总被引：71，自引：11，他引：71       下载免费PDF全文
最直观、最直接的古气候记录莫过于陆相黄土地层,它由许多黄土和古土壤层叠覆而成,详细地记录了第四纪以来的古气候变化。黄土高原中部黄土和古土壤的风化和发育程度可以用磁学参数,如磁化率等来描述。本文作者对甘肃西峰剖面约13万年以来的黄土地层做了详细采样,测量其磁滞回线各参数,频率磁化率、低场和高场磁化率、温度磁化率、磁组构等参数。研究表明,黄土与古土壤中的磁性矿物组分相似而且稳定。磁铁矿、赤铁矿和磁赤铁矿(主要是磁铁矿)的含量及其颗粒大小决定了它们的磁学特征。黄土和古土壤的磁学性质差别主要受成土过程中形成细小磁铁矿的影响。  相似文献   

12.
Previous work has demonstrated that magnetic properties and sediment particle size of Chinese loess deposits provide information on past behaviour of Chinese summer and winter monsoons, respectively (Heller and Evans, 1995; Derbyshire et al, 1997). It has been suggested that the East Asian winter monsoon system in particular is teleconnected on earth orbital and sub-orbital timescales to climatic states and events in high northern latitudes, especially northern hemisphere ice volume and Heinrich events, (Porter and An, 1995; Chen et al., 1997). However, the majority of this research has been performed around the central part of the Chinese Loess Plateau where the rate of dust accumulation is relatively low, thus limiting the potential resolution of palaeoclimatic records. Here we present a high resolution (5mm / ~20 yr interval) magnetic susceptibility record from an thick loess deposit located at Caoxian in the north-western part of the Loess Plateau. The record spans the “Lateglacial” (last glacial / interglacial transition) and when placed on a palaeomagnetic chronology shows a relationship with the GISP2 proxy air temperature record from Greenland (Grootes et al., 1993; Meese et al., 1994; Stuiver et al 1995). The results demonstrate that several Lateglacial climatic fluctuations previously reported in the North Atlantic region and in Europe are also recorded in China. In addition, the apparent absence of a signal corresponding to the Bølling Interstadial in China during the early deglaciation suggests that, at certain times, the apparent North Atlantic — Asian monsoon teleconnection may have collapsed. The demonstrated ability of the loess deposits to resolve sub-millennial scale climate variations points to their potential as a previously unexplored archive for very high-resolution studies of terrestrial climate.  相似文献   

13.
对东秦岭地区洛南盆地的上白川、刘湾和丹江上游的二龙山黄土剖面进行了岩石磁学研究。结果表明,大部分黄土和古土壤样品的磁性矿物以磁铁矿和磁赤铁矿为主,古土壤中亚铁磁性矿物的含量比黄土的多,极少数黄土样品以反铁磁性矿物为主。亚铁磁性矿物和反铁磁性矿物的含量随成土作用增强而增加,成土作用形成的细粒亚铁磁性矿物包括超顺磁性和单畴（似单畴）颗粒,但以单畴和（或）似单畴为主。古土壤磁化率增强与这些土壤成因的细粒亚铁磁性矿物含量有关,显示出受气候变化控制的特点。东秦岭地区黄土岩石磁学性质与黄土高原地区的相似,但也存在一定差异,而且三个剖面之间磁化率值整体差别较大。温湿的气候和复杂的山区地形可能是导致这种差异的原因。  相似文献   

14.
1200a来洪湖演变的环境磁学记录   总被引：8，自引：2，他引：6  
对取自洪湖的 140cm沉积物样芯进行了环境磁学测量,分析了所获得的环境磁学参数随深度变化的曲线,结果表明,沉积物磁性特征由稳定单畴(SSD)和细粘滞性 (FV)亚铁磁性矿物所主导,磁性矿物颗粒由下至上粒径逐渐减小。结合AMS测年和地球化学分析,可以推断出洪湖在 1200a来经历了沼泽沉积和湖相沉积。并且磁性特征的变化与气候的变化大致对应:磁性参数较大值对应着冷干的气候条件;水热条件增强时,磁性参数值下降。较好记录了湖泊随气候的扩张和缩小过程,同时也反映了人类活动的影响。  相似文献   

15.
Hematite is a very abundant mineral in natural rock samples. Despite being one of the most important carriers of remanent magnetization, its magnetic anisotropy is not well understood partially due to its high coercivity and complex behavior. In particular, the field intensity beyond which the Rayleigh relation no longer holds varies from one crystal to another. This field threshold is usually less than the field used in most commercial instruments. The nonlinear behavior of low-field susceptibility may thus hinder the magnetic fabric analysis. We have carried out an intensive study of the low-field bulk susceptibility and anisotropy of magnetic susceptibility (AMS) at increasing low fields in the range of 2–450 A/m (effective value) in a collection of hematite natural crystals. Standard rock magnetic properties, X-ray diffraction, and mass spectrometry have also been determined in order to discover the parameters influencing the low-field susceptibility variations with field. The AMS principal directions, the shape of the AMS ellipsoid, and the degree of anisotropy are the parameters that can vary with different applied fields. It has been found that there is no correlation between magnetic properties like coercivity or saturation magnetization and the range in which the Rayleigh approximation is valid. However, there seems to be a correlation with the peak width determined from X-ray diffraction, suggesting that the Rayleigh region in hematite crystals is related to the spatial orientation of the physical domains within the basal plane.  相似文献   

16.
中国黄土和阿拉斯加黄土磁化率气候记录的两种模式探讨   总被引：45，自引：16，他引：29  
在中国和中欧黄土-古土壤研究中发现的磁化率与成壤作用(或古气候温湿程度)的正相关性已被第四纪科学家广泛认识,并应用于古气候研究中.成壤过程形成的亚铁磁性矿物被认为是古土壤磁化率增加的主要原因;然而,这一模式并不一定适用于其他黄土沉积地区,如阿拉斯加和西伯利亚黄土沉积显示了一个完全相反的磁化率行为,即在黄土层获高磁化率值而在古土壤层获低磁化率.这种相反的关系过去被解释为磁化率反映的是与风动力吹来的亚铁磁性矿物的含量,即与风速或风力大小有关.本研究发现阿拉斯加黄土与古土壤的磁性矿物性质有明显差异,不仅仅是粒径的大小,还有磁性矿物的种类即矿物相的差别.这一证据很难单纯以风力强度的大小来解释,意味着阿拉斯加古土壤的低磁化率至少部分是在成壤过程中亚铁磁性矿物发生改变(如溶解)而造成,表明阿拉斯加黄土和中国黄土的磁化率与古气候记录可能存在两种模式,即氧化和还原条件下的成土模式.黄土磁化率在不同的气候(温度湿度)条件下有着不同的对应关系:在低降水量、高蒸发量的干旱氧化成壤条件下,利于亚铁磁性矿物的生成,其磁化率与古气候的关系呈正相关,如中国和中亚黄土;在高纬高湿的还原成壤条件下,亚铁磁性矿物会被破坏被分解,其磁化率与古气候呈负相关关系,如阿拉斯加黄土.如果成壤条件在氧化和还原之间来回变换,那么就很难找到两者之间的联系.因此,将磁化率应用于古气候的重建时要加倍的小心.  相似文献   

17.
In this study, environmental magnetic, heavy metal and statistical analyses were conducted on 21 surface sediments collected from Chennai coast, India, to examine the feasibility of heavy metal pollution using magnetic susceptibility. The Chennai coastal sediment samples are dominated by ferrimagnetic minerals corresponding to magnetite-like minerals. The percentage of frequency dependent magnetic susceptibility reflects the presence of super-paramagnetic/single domain magnetic minerals in Chennai harbour, Cooum and Adayar rivers sediments. High pollution load index in sample E1, E2, CH7, C11, C12 and A16 is mainly due to anthropogenic activities such as, harbour activities, Cooum and Adayar rivers input and industrial effluent. Factor analysis shows that the magnetic concentration dependent parameters (χ, χ ARM and SIRM) covary with the heavy metal concentrations, suggesting that the input of magnetic minerals and heavy metals in Chennai coastal sediments are derived from the same anthropogenic sources. Strong correlation obtained between pollution load index (PLI) and concentration dependent parameters (χ, χ ARM and SIRM) for the polluted samples with magnetic susceptibility excess of 50×10 − 8 m3kg − 1. Significant correlations between heavy metals and magnetic susceptibility point out the potential of magnetic screening/monitoring for simple and rapid proxy indicator of heavy metal pollution in marine sediments.  相似文献   

18.
We combine environmental magnetism, geochemical measurements and colour reflectance to study two late Quaternary sediment cores: GeoB 4905‐4 at 2° 30′ N off Cameroon and GeoB 4906‐3 at 0° 44′ N off Gabon. This area is suitable for investigating precipitation changes over Central and West Africa because of its potential to record input of aeolian and fluvial sediments. Three magnetozones representing low and high degree of alteration of the primary rock magnetic signals were identified. The magnetic signature is dominated by fine‐grained magnetite, while residual haematite prevails in the reduced intervals, showing increase in concentration and fine grain size at wet intervals. Our records also show millennial‐scale changes in climate during the last glacial and interglacial cycles. At the northern location, the past 5.5 ka are marked by high‐frequency oscillations of Ti and colour reflectance, which suggests aeolian input and hence aridity. The southern location remains under the influence of the Intertropical Convergence Zone and thus did not register aeolian signals. The millennial‐scale climatic signals indicate that drier and/or colder conditions persisted during the late Holocene and are synchronous with the 900 a climatic cycles observed in Northern Hemisphere ice core records. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

19.
Diamond drilling typically constitutes a major part of costs in advanced mineral exploration programs. This generates thousands of meters of rock cores during major exploration campaigns, but the cores are not currently utilized to their full potential. They could supply three-dimensional information on physical properties, geochemistry and mineralogy; such data could be used to model the geology or physical properties in 3D, characterize hydrothermal alteration, or provide chemo-stratigraphic constraints, for example. But measuring all the parameters one by one at high spatial resolution by traditional methods would be impractical due to cost or time considerations and, for some parameters, it would destroy the core (e.g. geochemistry).In this paper we describe a multi-sensor core logger and its use on rock cores from exploration diamond drill holes. This semi-automated system can measure near-simultaneously, non-destructively and at high spatial resolution, the following parameters: (1) volumetric magnetic susceptibility; (2) density using gamma-ray attenuation; (3) several chemical elements through energy-dispersive X-ray fluorescence spectrometry; and (4) visible/near infrared spectrometry, which allows numerous minerals to be detected and characterized. The logger also acquires a continuous image of the core using a line-scan camera, which allows the user to compare other properties with the visual aspect of the core and creates a complete virtual archive. The aim of this mostly methodological paper is to describe the logger as a whole and then each instrument or sensor separately, outlining the numerous tests that have been performed to assess and improve data quality. We also present preliminary results from the Matagami mining camp of Canada, a base metal district.  相似文献   

20.
A deep-sea core over 16 m long from the crestal area of the Mediterranean Ridge has been investigated with different techniques, including quantitative micropaleontology, stable isotopes (measured on the epipelagic species Globigerinoides ruber and on the mesopelagic species Globorotalia inflata), and clay mineralogy. The resulting record of climatic fluctuations can be cross correlated to other Mediterranean cores by means of isochronous lithologies (tephra layers and sapropels). The climatic record of the Mediterranean is similar in character, phase, and chronology to the records investigated in the equatorial Pacific and in the Caribbean. Isotope stages 1 to 17 have been recognized. Cyclically repeated stagnant cycles resulting in sapropel deposition complicate both the isotopic and the faunal signal. The isotopic investigations reveal that the temperature change in the surface layers of the eastern Mediterranean was no greater than 8°C in the late “glacial” Pleistocene. The chronostratigraphic and biostratigraphic interpretation of Core KS09 indicate that the mean sedimentation rate was 2.4 cm/1000 years, a value very close to the 2.5 cm/1000 years calculated for the entire Quaternary section at DSDP Site 125, also located in the crestal area of the Mediterranean Ridge in the Ionian Basin. The base of KS09 is likely to be very close to the Brunhes/Matuyama boundary dated at 0.7 my.  相似文献   

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