共查询到20条相似文献，搜索用时 31 毫秒
1.
A number of ‘weathering’ horizons have been identified within glacigenic sediment sequences in North Wales and have been instrumental in determining various proposed Late Quaternary event chronologies. This paper applies the techniques of mineral magnetic analysis to such a sediment sequence at Glanllynnau. The presence (or otherwise) of evidence for weathering processes on the upper surface of the Criccieth Till, which is overlain by further glacial materials, is examined. Despite a marked colour variation between the ‘weathered’ and non-weathered components of the Criccieth Till, their mineral magnetic properties suggest little or no variation in iron oxide assemblages. The consequences of this for the environmental interpretation of the sediment sequence are discussed. 相似文献
2.
Rock-magnetic characterization of early, redoxomorphic diagenesis in turbiditic sediments from the Madeira Abyssal Plain 总被引：6，自引：0，他引：6
Rock‐magnetic measurements of two sediment cores from the Madeira Abyssal Plain (MAP), north Atlantic, are used to investigate post‐depositional changes in the concentration, grain size and composition of magnetic minerals in the sediments that have occurred within organic‐rich turbidite horizons. The changes are associated with an initial stage of suboxic (reductive) diagenesis, following depletion of porewater O2, and a later stage of oxidative diagenesis associated with the slow descent of an oxidation front through the sediment, as a result of diffusion of O2 from the overlying sea water. The turbidites are of late Quaternary age (δ18O stages 1–3) and derive both from different sites on the NW African continental margin, and from the flanks of the Canary Islands. Thus, the turbidites are variable compositionally, especially in terms of carbonate, detrital magnetic mineral and organic carbon content. Diagenetic changes in these sediments have been identified using solid‐phase geochemical data (U, Mn, Corg and CaCO3) reported previously in more than one study. Rock‐magnetic parameters of the sediments, when expressed on a carbonate‐free basis, reveal that significant depletion of detrital ferrimagnetic iron (Fe2+/Fe3+) oxide grains has occurred within organic‐rich turbidites during redoxomorphic diagenesis. Normalized quotients of magnetic parameters also show that reductive diagenesis is a ferrimagnetic grain size‐selective process, but it has a minimal effect on the canted‐antiferromagnetic Fe3+ oxides in the sediment. Such components, if present, therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are dissolved progressively, and bulk magnetic concentration is thus depleted. There is clear evidence in both cores for the existence of ultrafine ferrimagnetic grains at depth within the suboxic zone of the organic‐rich turbidites, beneath both active and fossil oxidation fronts. These grains are most probably associated with populations of live magnetotactic bacteria, which commonly inhabit such organic‐rich horizons and play a part in the chain of bacterially mediated reactions normally associated with suboxic diagenesis. These results show that simple and rapid rock‐magnetic techniques can be used to characterize early diagenetic processes involving iron phases in deep‐sea sediments, at least as effectively as more laborious, time‐consuming and sample‐destructive geochemical measurements. 相似文献
3.
杭州湾北岸芦潮港潮滩沉积物磁性特征的年际变化及其粒度控制 总被引：1，自引：1，他引：0
对杭州湾北岸芦潮港潮滩2007-2010年固定点沉积物进行了逐月的磁性测量和粒度分析,探讨了磁性特征的月际、年际变化及其与粒度的关系。结果显示,2007-2009年,芦潮港潮滩沉积物的磁性特征呈现出相似的季节性变化特征,磁性参数χ、χARM、SIRM以及比值χARM/SIRM总体上在12月次年7至月表现为高值,8-11月为低值,S-300变化趋势与之相反。粒度分析显示,每年12月至次年7月沉积物粒度总体较细,8-11月较粗,这说明潮滩较细沉积物中含有较多的亚铁磁性矿物,与已报道的2004-2005年研究结果相似。相比前三年,2010年1-7月及12月沉积物磁性矿物含量相对下降,8-11月份磁性矿物含量相对上升,导致2010年磁性参数月际变化幅度减小。同时,2010年沉积物粒度月际变化特征也发生了改变,即1-7月及12月沉积物粒度变粗,8-11月沉积物变细。相关分析表明,磁性参数与粒度之间的关系在2007-2010年中没有发生明显的变化,说明2010年沉积物粒度月际变化模式的改变是磁性特征月际变化改变的主导因素。尽管如此,长江流域来沙下降对河口潮滩沉积物物源的可能影响,仍需予以长期关注。 相似文献
4.
Ian Snowball 《地学学报》1993,5(3):258-270
A mineral magnetic study of lacustrine sediments and soils from the Kårsa valley in Lappland, Sweden was undertaken to estimate the potential of mineral magnetic techniques in elucidating the variability of climate, recorded by glacier fluctuations and soil development. Sediment cores from two lake basins receiving sediment input from a glacial outflow stream, and their respective catchments, show a mineral magnetic record that can be interpreted in terms of glacial activity and soil development. Dissolution of magnetite is indicated during periods of climatic amelioration and causes a high S-ratio. During periods of glacial advance (and colder, wetter climate) the minerogenic sediment supply is increased and magnetite is preserved in the sediment, resulting in a low S-ratio. Variations in the S-ratio can then be used as a proxy-climate indicator. 相似文献
5.
对长江三角洲地区7个晚新生代钻孔沉积物的岩石地层和磁化率特征进行详细对比,并进一步选择浦东机场孔（PD孔）进行了详细的岩石磁学测试,讨论了该地区的晚新生代沉积物物源的演变。结果显示沉积物磁性的4次显著变化： 1）各钻孔上新世磁性强弱差异明显; 2）早更新世早期本区普遍表现出弱磁性; 3）从早更新世中晚期至全新世,磁性强度一般随粒度粗细变化波动,即砂质沉积磁性强,泥质沉积磁性弱; 4）晚更新世晚期以来,细颗粒沉积物的磁性较明显增强。磁性参数分析显示上新世和早更新世早期磁性矿物晶体颗粒较细,早更新世中晚期-晚更新世以粗晶粒磁性矿物为主,全新世细晶粒磁铁矿再次占主导。我们认为上述磁性变化反映构造沉降控制下现代长江三角洲地区物源区的不断扩大。 相似文献
6.
FRANK OLDFIELD QINGZHEN HAO JAN BLOEMENDAL ZOË GIBBS‐EGGAR SHIVA PATIL ZHENGTANG GUO 《Sedimentology》2009,56(7):2091-2106
Using a combination of particle size analysis, magnetic measurements, scanning electron microscopy and transmission electron microscopy imaging, this study shows that in a wide range of depositional environments, there is a strong link between particle size classes and magnetic response, especially below the upper limit of stable single domain magnetic behaviour. Ferrimagnetic grain assemblages dominated by stable single domain magnetosomes regularly have peak susceptibility and remanence values in coarser grades than do those containing finer‐grained, viscous and superparamagnetic secondary magnetic minerals formed during pedogenesis. This effect is despite the fact that there is a one to two orders of magnitude size difference between the particle size boundaries (at 1 or 2 μm) and key domain state transitions (mostly below 0·05 μm). The implications of these results are explored using samples spanning 22 Myr of loess accumulation on the Chinese Loess Plateau. The results from the loess sections, complemented by data from low‐temperature magnetic experiments, show that there are subtle distinctions in mean ferrimagnetic grain‐size between the Pleistocene and Miocene parts of the record, thus allowing more refined rock magnetic interpretations of the fine‐grained ferrimagnetic mineral assemblages arising from the effects of weathering, pedogenesis and possibly diagenesis in the sections studied. 相似文献
7.
Geert‐Jan Vis Sjoerd J. P. Bohncke Heike Schneider Cornelis Kasse Sander Coenraads‐Nederveen Koen Zuurbier Jouke Rozema 《第四纪科学杂志》2010,25(8):1222-1238
The present paper aims to reconstruct the Lower Tagus Valley flooding history for the last ca. 6500 a, to explore the suitability of pollen‐based local vegetation development in supporting the reconstruction of flooding history, and to explain fluvial activity changes in terms of allogenic (climate, human impact) and autogenic (system intrinsic) processes. The flooding history has been determined by cored sedimentary records located ～18 km apart in distal, low‐energy backswamps on both sides of the Tagus channel. In these low‐energy backswamps, fine‐grained sediment layers deposited from suspended load of overbank flood water reflect periods with multiple overbank floods. By means of a multi‐proxy approach (sedimentology, magnetic susceptibility, grain size, loss‐on‐ignition, carbonate content and pollen), sedimentary and environmental changes were identified. At both sites, synchronous lithological intervals accumulated, suggesting a common origin for the changes in fluvial activity since ca. 6500 cal. a BP. Based on lithological changes, three phases of high fluvial activity (6500–5500, 4900–3500 and 1000–0 cal. a BP) and two phases of low fluvial activity (5500–4900 and 3500–1000 cal. a BP) were identified. Two periods with dominant allogenic controls on fluvial activity in the Lower Tagus Valley were identified: relative sea level (6500–5500 cal. a BP) and human impact (1000–0 cal. a BP). During the intermediate period, changes in fluvial activity may have been caused by climate (5500–1000 cal. BP), but unambiguous correlations are difficult to make. This is due to the way allogenic controls are translated through the fluvial system, the geomorphological differences between upstream and downstream studies and autogenic processes. The comparison of local vegetation development and flooding phases as reconstructed using sedimentology shows a limited added value of using local palynology as a proxy for fluvial activity. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
8.
Ian F. Snowball 《第四纪科学杂志》1993,8(4):339-346
Geochemical and mineral magnetic studies of lake sediments from the Kårsa valley in north Sweden demonstrate that down-core variations in magnetic properties have been determined by post-depositional magnetite dissolution rather than by sediment source changes. A low sediment accumulation rate during times of climatic amelioration combined with a higher organic carbon content produced intense reducing conditions with almost complete magnetite loss. An increase in the sedimentation rate and a reduction in the organic carbon content, primarily due to glacier reformation, allowed magnetite preservation. Thus the mineral magnetic profiles record lake sediment palaeoredox conditions brought about by climate change. This new interpretation of mineral magnetic transformations in freshwater sediments may have far reaching implications for other mineral magnetic studies concerned with the alternation between glacial (stadial) and interglacial (interstadial) episodes during the Quaternary period. 相似文献
9.
Pleistocene sedimentary sequences in the East Anglian region of Britain record both major and minor climatic oscillations, and the impact of isostatic and eustatic variations. Intensively studied in terms of their lithology and biostratigraphy, the sequences have been difficult to place in an absolute timeframe. Dating and correlation by magnetostratigraphy has been attempted over a number of years. However, these sediments are difficult to date by palaeomagnetic means because they are poor in detrital magnetite, are subject to post‐depositional deformation and diagenesis, and have unknown rates of sedimentation. Determining whether their natural remanence magnetisation (NRM) directions are reliable thus requires information on the mode and timing of remanence acquisition. Here, we apply palaeomagnetic, rock magnetic and mineralogical analyses to identify the NRM carriers in these sediments and hence their palaeomagnetic reliability. Within oxidised fluvial sediments (the Kesgrave Formation), the magnetic carriers appear to be relict magnetic minerals (ferrian ilmenites, chromites, haematite and goethite), which sometimes carry a reliable primary depositional remanence (DRM) but often an overprinting viscous (time‐varying) remanence (VRM). Within some reduced marine and intertidal sediments (within the Crag basin), the iron sulphide, greigite, has been found to carry a reliable, ‘syn’‐depositional chemical remanence (CRM). In all the sediments, magnetic inclusions within silicates are abundant, are significant for the mineral magnetic signal but contribute little to any recoverable palaeomagnetic information. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
10.
潮滩沉积的物源一直是沉积学领域重要的研究内容。近30年长江入海泥沙量持续减少,致使长江入海泥沙对浙闽潮滩沉积的物质贡献降低。在此背景下,探究浙闽沿岸潮滩沉积的物源具有重要的现实意义。对杭州湾至福州湾沿岸的泥质潮滩表层沉积开展了详细地磁学研究,结果显示,研究区沉积物的磁性主要受磁铁矿主导,但磁性矿物含量、磁畴和矫顽力等磁性特征存在一定差异,这可能意味着沉积物受到多物源影响。皮尔森相关性分析进一步表明,沉积物磁性特征受粒度效应影响有限,主要与物源差异有关。物源判别结果指示,浙闽泥质潮滩沉积的物源包括长江、海岸基岩和浙闽河流。杭州湾至福宁湾,沉积物普遍受到长江和浙江诸河混合物质的影响,且浙江诸河的物质贡献较大;隘顽湾和象山港沉积物还受到其附近海岸基岩的影响;福宁湾沉积物可能还受到闽江源物质和其附近海岸基岩的影响;罗源湾和福州湾沉积物的潜在物源是闽江和其附近海岸基岩,长江和浙江诸河的物质贡献相当有限。 相似文献
11.
扫描电镜定量分析沉积物矿物组合的方法,可以快速准确得到大量详细的矿物组合、矿物颗粒大小和岩屑组成等信息。尝试利用这一方法,研究渭河流域现代河流样品的矿物组合和沉积物碎屑组分,探究其在沉积物源示踪上的应用。研究表明,渭河流域现代沉积重矿物组合主要以角闪石、帘石类矿物和石榴石为主,不同区域沉积物矿物组合通过主成分分析图表（Principal Component Analysis,PCA）可以较好地区分开。轻矿物以石英和长石为主。碎屑组分石英—长石—岩屑（QFL）三角图解显示渭河干流和流经鄂尔多斯高原的两大主要支流北洛河和泾河的沉积物碎屑组分,主要位于再旋回造山带区域;而北秦岭山前河流沉积碎屑组分则主要位于岩浆岛弧区域。这一结果符合区域地质构造背景,表明扫描电镜定量分析手段可很好地应用于沉积物碎屑组分QFL三角图解、重矿物组合物源示踪研究。 相似文献
12.
13.
为了研究白垩纪中期大洋缺氧事件及其后古海洋环境的变化,对藏南贡扎剖面白垩纪赛诺曼阶/土伦阶和三冬阶/坎 潘阶界线附近的浅海相沉积地层开展了详细的岩石磁学对比研究。 结果显示,这两个时间段的沉积物中磁性矿物含量和粒 度无显著区别,但赛诺曼阶/土伦阶地层中含有高矫顽力磁性矿物,如赤铁矿和针铁矿,而三冬阶/坎潘阶地层中则主要为 低矫顽力磁性矿物,如磁铁矿。 由于海平面位置在这两个时间段相近,海面变化对沉积物的磁学特征的影响很小。 磁性矿 物种类的变化可能主要是由于海洋沉积环境的变化所引起的。 高矫顽力磁性矿物在赛诺曼阶/土伦阶的出现及其在三冬阶/ 坎潘阶的缺失,表明赛诺曼阶/土伦阶氧化程度可能比三冬阶/坎潘阶更高。 这与深海沉积所记录的赛诺曼阶/土伦阶为缺氧 以及三冬阶/坎潘阶为富氧的特征明显不同。 这说明以江孜地区为代表的深海-半深海环境和以岗巴定日地区为代表的浅海 环境对白垩纪中期气候变化有着不同的响应。 深水和浅水环境的演化在这两个时间段的显著差异表明相应时期的大洋环流 也可能比以往所认识的更复杂。 相似文献
14.
ACHIM BRAUER THOMAS LITT JÖRG F. W. NEGENDANK BERND ZOLITSCHKA 《Boreas: An International Journal of Quaternary Research》2001,30(1):83-88
In this paper we briefly compare previously published data on four lateglacial sediment profiles from lakes Holzmaar and Meerfelder Maar in the Eifel region. Two of these profiles (one from each lake) are varvedated, whereas the other two pollen profiles are on depth scales. In general, the palynological signal in all profiles compares well, proving the regional validity of the individual data sets. There are some discrepancies between Holzmaar and Meerfelder Maar, however, mainly concerning varve dating of major lateglacial features, and these are explained after detailed correlation using a combination of varve dating and palynological signals. The result is a consistent varve-dated biostratigraphy for the Lateglacial in the Eifel region. Moreover, it is demonstrated that a combination of varve chronology and high resolution palynostratigraphy in several lake profiles enables even gaps of a few decades up to a few centuries to be detected, thus providing a valuable tool for precise regional palaeoenvironmental study. This multi-core study on two lakes from the same region demonstrates the likelihood of undetected errors in single-core studies on non-varved sediments. 相似文献
15.
David S. Leigh 《Geoarchaeology》1998,13(3):309-330
Artifacts are commonly buried by approximately 50 cm of sediment at prehistoric archeological sites (early Archaic through Mississippian) on uplands of the Sandhills of the upper Coastal Plain of the southeastern United States. Bioturbation, eolian sedimentation, and colluviation are the primary processes that can explain artifact burial because of the upland position of the sites in an erosional landscape setting. Colluvial sedimentation is discounted at most of the sites because they occur on interstream divides and upper hillslope positions. Thus, the focus is on eolian sedimentation versus bioturbation as burial agents. Six sites in the midst of the Sandhills region along the corridor of South Carolina Highway 151 in Chesterfield County provide the data. The Sandhills consist primarily of Cretaceous and Tertiary marine, fluvial, and eolian sediments that are highly dissected and overlie crystalline rocks in the deep subsurface. Two of the sites are on high fluvial terrace remnants that predate 12 ka and serve as controls where bioturbation is the only reasonable burial process. Hillslope positions of the sites are on erosional elements of the landscape (crests, shoulder slopes, and upper backslopes) where sediment transfer operates (colluvial and overland flow), but where deposition is minimal. The sites occur on very sandy soils having a texture of loamy sand to sand. In some instances, a fine textured cover sand, which is about 1.5 m thick, overlies a clayey subsoil or Bt horizon. This cover sand has been interpreted by some as an eolian sand sheet that buries a second parent material and paleosol, but standard particle size and heavy mineral data indicate that it is simply a thick E horizon over a Bt horizon. Standard particle size fractionation at whole phi intervals, and particle size analysis of the heavy mineral fraction, indicate that eolian sedimentation is unlikely at five of the six sites. Heavy minerals were analyzed with respect to the sedimentological principle of hydraulic equivalence, which provides clear separation of eolian versus water-laid sediment. Results of particle size analysis suggest that the cover sands are water-laid (probably fluvial) at five of the six sites, which favors the bioturbation process of artifact burial. Heavy mineral analysis corroborates the standard particle size data, indicating that only one site, 38CT16, possibly is composed of eolian sediment. Soil profile development suggests that the age of the sediment at site 38CT16 probably is older than 12 ka and was in place prior to human occupation. Therefore, possible eolian sedimentation at that site is not relevant to artifact burial, which also suggests bioturbation is the primary process of artifact burial. Additional evidence favoring bioturbation as a vigorous artifact burial process in the Sandhills comes from the two sites on high elevation sandy fluvial terraces (38CT34, 38CT17) where artifacts are also buried. At these terraced sites bioturbation is the only possible burial process. Overall results suggest that bioturbation best explains the occurrence of buried artifacts and that eolian sedimentation processes are not readily apparent, and are not required, in explaining artifact burial. © 1998 John Wiley & Sons, Inc. 相似文献
16.
潮滩表层沉积物磁性特征的季节性变化及其对沉积动力作用的响应:以杭州湾北岸为例 总被引：2，自引：1，他引：1
根据2004年9月—2005年8月杭州湾北岸中潮滩固定观测点重复采集的表层沉积物的磁学参数和粒度分析,探讨了研究区域潮滩沉积物磁学性质的季节性变化及其对沉积动力的响应。 结果表明, 该观测点潮滩沉积物磁学参数存在显著的月际变化,2004年12月底—2005年5月上旬的半年中,SIRM、χARM、χfd%、χARM/χ、χARM/SIRM等参数出现高值,指示了沉积物中含有较多的细晶粒亚铁磁性矿物,对应于潮滩淤积时段和较细的沉积物粒级组成,其余各月上述磁参数值较低,对应于潮滩沉积物较粗的侵蚀期。上述结果表明,沉积物的磁性特征可以很好地反映潮滩冲淤过程中沉积动力的变化。 相似文献
17.
18.
近期长江口-杭州湾邻近海域沉积物粒径的时空变化及其影响因素 总被引：3，自引：1，他引：2
为了研究长江口-杭州湾外近海沉积物粒径的现状和近期变化,于2008年4月在该区域用箱式取样器取得了33个表层样,用震动活塞采样器取得7个柱状样;在室内用激光粒度仪对沉积物样品进行了粒度分析并与历史时期的研究成果进行对比。结果表明,1）表层沉积物总体上呈东粗（砂）西细（泥）的特点,粒径总体上的空间格局与历史时期相似;但砂-泥区界线在研究区北部（长江口和杭州湾外）有向西迁移（蚀退）迹象（西移10~30 km）,而在南部（舟山群岛以南）则出现明显向东迁移（淤进）现象（最大超过50 km）。2）当前的表层沉积物形成一条南北连续的泥质带（粒径向南逐渐变细）,反映过去存在于舟山群岛以东的最细组分（“黏土”）带不连续现象在表层现已趋于消失。3）表层沉积物粒径趋势分析揭示长江入海泥沙的运移方向主要是向南-东南。4）泥质区柱状沉积物具有粉砂和黏土为主但在垂向上（反映在时间上）有粗细多变的特点（侵蚀区表层沉积物有粗化迹象）。研究认为,长江口-杭州湾邻近海域现代沉积物对流域和沿海重大人类活动有较敏感的响应。 相似文献
19.
ERIC GALLAWAY ALAN S. TRENHAILE MARIA T. CIOPPA ROBERT G. HATFIELD 《Sedimentology》2012,59(6):1718-1734
A combined field and laboratory study in northern Lake Erie has provided new insights into the origin and dynamics of heavy mineral placer deposits on beaches consisting primarily of non‐magnetic sediment. Work was conducted on the cross‐shore and longshore transport of heavy magnetic minerals using magnetic susceptibility and fluorescent paints to trace the movement, in the field, of samples of magnetic (magnetite) and non‐magnetic (quartz and calcite) grains, respectively. Laboratory experiments examined how the burial of small, dense magnetic minerals is affected by the grain size of the non‐magnetic host material, and how grain burial affects magnetic susceptibility measurements at the surface. The field experiments demonstrated that the magnetic mineral tracers were buried rapidly beneath coarser, non‐magnetic grains under low to moderate wave conditions, and subsequently were unable to move in the longshore or cross‐shore directions. The laboratory experiments showed that the magnetic susceptibility rapidly decreased with the rate and depth of burial of the magnetic minerals, and that magnetic grain burial was most effective beneath coarser rather than finer non‐magnetic sand and, for the latter sediments, under less rather than more energetic conditions. The results imply that magnetic mineral concentrations develop in this area through magnetic grain burial under fairly mild conditions, and subsequent settling, exposure and concentration in the upper swash zone during more energetic periods, when the non‐magnetic grains are eroded. It is probably during these erosional periods, when the magnetic minerals are exposed in fairly homogeneous deposits, that longshore and cross‐shore transport takes place. 相似文献
20.
磁学可能是南海深海平原古环境研究取得突破的关键方面之一。众多磁学参数中,磁化率是最基本、最综合反映沉积物磁性矿物总体状况的参数,非磁滞剩磁对单畴颗粒含量敏感,而且单畴颗粒是剩磁的主要携带者,结合粒度、碎屑矿物丰度等数据探索南海深海平原柱状样QD189磁化率和非磁滞剩磁地质含义。在磁化率大于45×10-5层位和481 cm以浅的磁化率小于45×10-5的层段,磁化率、2~5粒级含量、ARM和ARM@20m T变化一致性较好,2~5粒级软磁性多畴颗粒含量变化可能是这些层位磁化率变化的主要原因。在808~488 cm层段内,小于45×10-5的磁化率与2~5粒级含量、ARM和ARM@20m T变化总体上一致性较差,磁化率变化机制有待进一步研究。2~5粒级是QD189重要的较粗粒级组分,大体上以深度350 cm为界具有下层段含量较高且粒度变化大、上层段含量较低且粒度变化小的特点。ARM@20m T比ARM更加真实地表示QD189单畴颗粒含量,除了单畴颗粒外,9~12粒级假单畴、多畴颗粒矿物对665~48 cm层段的ARM可能有一定贡献,其中,层段548~310 cm的ARM中可能还有6~9粒级多畴颗粒的贡献。通过碎屑矿物丰度、火山玻璃丰度和(或)磁化率异常峰值的对应关系识别出6次火山活动,火山玻璃丰度的分布特点还揭示出QD189中普遍发生生物扰动作用。 相似文献