黄河宁蒙段河道粗细双组分泥沙分选沉积特征

黄河宁蒙段河道淤积严重,阐明近源粗泥沙和远源细泥沙分选沉积规律对于河道淤积治理有重要意义。本研究基于120根河道钻孔泥沙样品,通过粒径分布和元素矿物组成,分析了河道>0.05 mm、>0.08 mm和>0.1 mm粗泥沙沿河分选沉积变化特征,并定量评估了近源粗泥沙和远源细泥沙沉积对于河道泥沙淤积的贡献。结果表明：黄河宁蒙段河道淤积的主要是来源于近源沙漠及其下伏砒砂岩区>0.08 mm的粗泥沙,而来源于上游黄土区<0.05 mm的细泥沙主要沉积在河道滩地,对河道淤积的贡献小于30%。因此,风水复合侵蚀引起的沿黄沙漠和下伏砒砂岩粗泥沙入黄是黄河宁蒙沙漠河段河道淤积的根源,也是该河段“悬河”的治理重点。     

Frequency dependence of magnetic susceptibility for populations of single-domain grains

A recent paper (Dearing et al . 1996) on the use of frequency dependence of magnetic susceptibility (Xfd) in magnetic grain-size investigations of environmental materic has proposed a fundamentally new model for the magnetic susceptibility of single-domain (SD) grains measured at different frequencies, and is divergent from the no well-established theory of SD behaviour. This new model appears to be founded on a confusion about the behaviour of SD grains, as predicted by theory, and magnetic measurements of real materials, which naturally have a distribution of magnetic grain sizes. Here I try to clear up this confusion by showing how different log-normal distributions of magnetic grain sizes affect the frequency dependence of magnetic susceptibility measurement. This analysis highlights that the limiting maximum Xfd value of ˜ 15 per cent obtained for environmental materials can be explained simply as a constraint imposed by the magnetic grain-size distribution having a finite minimum width (˜1.0 on a natural log scale). There is no reason to suppose that if the grain-size dispersion was narrower higher Xfd values could not be observed. Furthermore, the Xfd value is non-unique and so cannot be used quantitatively to determine the amount or grain-size distribution of SD grains in a sample     

Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz

This paper presents an improved method for the optical dating of Holocene sediments from a variety of geomorphic settings. We have measured the equivalent dose (D_e) in individual grains of quartz, using green laser light for optical stimulation, and have simulated the D_e distributions for multiple-grain ‘synthetic’ aliquots using the single-grain data. For 12 samples of known (independent) age, we show that application of a ‘minimum age model’ to the single-grain and ‘small’ (10-grain) aliquot D_e data provides the most accurate estimate of the burial dose for nine of the samples examined (3 aeolian, 5 fluvial, and 1 marine). The weighted mean D_e (as obtained using the ‘central age model’) gives rise to burial age overestimates of up to a factor of 10 for these nine samples, whether single grains, small aliquots, or ‘large’ (100-grain) aliquots are used. For the other three samples (two aeolian and one fluvial), application of either the minimum age model or the central age model to the single-grain, small aliquot, and large aliquot D_e data yields burial ages in accord with the independent age control. We infer that these three samples were well bleached at the time of deposition. These results show that heterogeneous bleaching of the optical dating signal is commonplace in nature, and that aeolian transport offers no guarantee that the sample will be well bleached at the time of deposition. We also show that grains sensitive to infrared (IR) stimulation can give rise to low D_e values, which will result in significant underestimation of the burial dose and, hence, of the age of deposition. We demonstrate that use of a modified single-aliquot regenerative-dose protocol incorporating IR stimulation prior to green light stimulation deals effectively with contamination by IR-sensitive grains. We conclude that application of the modified protocol to single grains or small aliquots of quartz, using the lowest D_e population to estimate the burial dose, is the best means of obtaining reliable ages for Holocene sediments from a wide range of depositional environments.     

Identification of magnetic Fe–Ti oxides in marine sediments by electron backscatter diffraction in scanning electron microscopy

In paleomagnetic and environmental magnetic studies the magnetomineralogical identification is usually based on a set of rock magnetic parameters, complemented by crystallographic and chemical information retrieved from X-ray diffraction (XRD), (electron) microscopy or energy dispersive spectroscopy (EDS) of selected samples. While very useful, each of these supplementary techniques has its limitations when applied to natural sample material which are related to low particle concentrations (down to the ppm range in marine sediments) and very fine grain sizes (down to the nm scale). Therefore, meaningful application of such techniques depends on sample quality. Electron backscatter diffraction (EBSD) of individual grains in scanning electron microscopy (SEM) enables mineralogical identification of grains down to ∼0.2 micrometer and is particularly powerful when combined with EDS. In this study, we show the merits of EBSD for rock magnetic investigations by analyzing titanomagnetites and hemoilmenites of various compositions and submicron lamella of titanomagnetite–hemoilmenite intergrowths. Such particles often occur in natural marine sediments where EDS often has a semi-quantitative character and compositionally similar intergrowths may be difficult to distinguish. With the mineralogical information provided by EBSD unambiguous identification of spinel-type and trigonal oxides is obtained. Optimal EBSD patterns are gathered from smooth, polished surfaces, but here we show that interpretable EBSD patterns can be obtained directly from the surface of unconsolidated, so called 'non-embedded' particles from marine sediments. This information enhances the interpretative value of rock magnetic parameters.     

The magnetic properties of the recent sediments of Brothers Water, N W England

The magnetic properties of recent sediments from five short cores from Brothers Water in the English Lake District are compared with those from a set of catchment soil samples. From the comparison, it is concluded that bacterial magnetosomes have made a significant contribution to the magnetic properties of the sediments of the last five decades. This contribution is superimposed on a continuing erosive input of haematite-rich sediment from the parent materials of the catchment and may obscure any possible increase in surface soil input during recent times. The results highlight the difficulty of making sediment-source linkages where biogenic contributions to the magnetic properties of the sediment are important relative to the input of magnetic minerals from catchment erosion.     

The palaeomagnetism and magnetic fabric of sediments from Peak Cavern, Derbyshire

Summary. This paper describes the magnetic remanence and susceptibility anisotropy of 42 samples of laminated clay from Peak Cavern. The sediments are shown to have acquired a stable remanent magnetization due to the depositional alignment of magnetite grains. The magnetic fabric data confirm the existence of alteration within a reddened surface layer. A record of geomagnetic secular variation in the underlying clay is compared to a dated lake sediment magnetostratigraphy to suggest that deposition occurred around 7000 yr bp . A method for estimating water current directions based on the remanence and magnetic fabric data is critically assessed before deducing the sense of palaeoflow in the cave.     

Mineral magnetic study of Late Quaternary South Caspian Sea sediments: palaeoenvironmental implications

Magnetic properties of sediments from a core (10 m long) in the southern basin of the Caspian Sea have been investigated. Varying concentrations of greigite (Fe₃S₄) dominate the magnetic fraction in Late Pleistocene sediments. The synsedimentary formation of greigite indicates that the Late Pleistocene Caspian Sea was a brackish or fresh-water, poorly ventilated basin and suggests a water level higher than at the present. The variation in magnetic parameters, with the detrital magnetite-bearing fraction remaining constant, is interpreted in terms of greigite grain-size variation and related to the slight variation in water salinity. The Holocene sediments are characterized by detrital magnetite. This indicates better ventilation of the basin and suggests lower water levels than in the Late Pleistocene. The gradual change in magnetic properties of the sediments between 90 and ≈60 cm depth, with decreasing quantities of greigite, indicates stepwise establishment of oxic conditions in the Holocene.     

A method for the selective removal of inorganic ash particles from lake sediments

Inorganic ash particles are formed by the fusing of inorganic material present during the high temperature combustion of fossil fuels. As they accumulate in lake sediments, they record the history of atmospheric contamination produced from such sources. A technique has been developed for concentrating these particles from lake sediments involving the stepwise removal of unwanted components of the sediment, including organic material and biogenic silica.When applied to a sediment core taken from Loch Tinker, central Scotland, a particle concentration profile, very similar to that of the carbonaceous particle profile (the other component of fossil-fuel combustion ash) is produced. The concentration of the inorganic ash spheres in the sediment is approximately an order of magnitude higher than the carbonaceous particles and there appears to be a continuous pre-industrial background value. This seems to imply a similar source (i.e. coal combustion rather than oil) for both inorganic ash and carbonaceous particles. This method has also been used with success on peat cores.     

The relationship between the magnetic properties of terrestrial igneous rocks and the composition and internal structure of their component Fe-oxide grains

Summary. Hysteresis loops and the demagnetization of saturation remanence (SIRM), before and after heating, are used to determine the high field magnetic behaviour and the coercivity spectra for common igneous rock types containing discrete types of magnetic carriers. These incude homogeneous to highly exsolved titanomagnetite grains, oxidized titanomagnetite grains (martite), exsolved hemo-ilmenite grains, and oxidized ilmenite grains. Specific internal structures and changes in composition due to oxidation are found to have characteristic effects on both high and low field magnetic properties.     

Paleoclimate changes inferred from stable isotopes and magnetic properties of organic-rich lake sediments in Arctic Norway

Stable isotope measures in organic matter are frequently used as indicators of past climate change. Although such analyses can provide valuable information, there is considerable uncertainty associated with studies of organic-rich sediments, especially those from Arctic lakes and bogs. We studied stable isotopes of carbon and nitrogen, and magnetic properties in a sediment core from a small alkaline lake with a high sedimentation rate, Lake Nattmålsvatn, Norway. There is good correspondence among the different sediment variables during the late glacial, and they seemingly reflect major climate variations such as the Allerød Interstade and the Younger Dryas, as well as the transition into the current interglacial. During the early Holocene, however, these relationships are more complex and δ¹³C and δ¹⁵N values do not stabilize until ~7,500 cal year BP. A significant excursion in all variables occurs between 6,850 and 6,500 cal year BP and is interpreted to represent climate deterioration. Holocene δ¹³C values vary little and indicate that isotopically-depleted dissolved inorganic carbon (DIC) in the lake, possibly influenced by methanotrophy and high pCO₂, dominated the lake’s carbon cycle. Holocene δ¹⁵N is similarly muted, likely due to the availability of abundant dissolved nitrogen. Bulk organic matter is probably dominated by phytoplankton remains produced beneath the ice cover in late spring and during ice breakup when isotopically-depleted DIC, pCO₂ and ammonium availability were maximal. Thus, use of δ¹³C and δ¹⁵N as indicators of Holocene paleoclimate and paleoproductivity variation can be challenging in a lake such as Nattmålsvatn, where ice cover isolates the basin for large parts of the year, allowing dissolved respiratory gases to accumulate in the water column. In contrast, magnetic variables appear to better track climate variations. In particular, runoff-driven influx of minerogenic sediments shows high variability that can be attributed to regional changes in Holocene winter precipitation. The most striking shifts occur between 4,000 and 2,300 cal year BP.     

Environmental magnetic measurements of marine sediments from Antarctica: implications to paleoclimate changes during the past 15 ka

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The complex-image method for calculating the magnetic and electric fields produced at the surface of the Earth by the auroral electrojet

For studying the auroral electrojet and for examining the effects it can produce in power systems on the ground, it is useful to be able to calculate the magnetic and electric fields that the electrojet produces at the surface of the Earth. Including the effects of currents induced in the Earth leads to a set of integral expressions, the numerical computation of which is complicated and demanding of computer resources. An approximate solution can be achieved by representing the induced currents by an image current at a complex depth. We present a simple derivation of the complex-image expressions and use them to calculate the fields produced by the auroral electrojet at the surface of an earth represented by layered conductivity models. Comparison of these results with ones obtained using the exact integral solution show that the errors introduced are insignificant compared to the uncertainties in the parameters used. The complex-image method thus provides a simple, fast and accurate means of calculating the magnetic and electric fields.     

The palaeomagnetism and magnetic fabric of cave sediments from Grønligrotta and Jordbrugrotta, Norway

Summary. Twenty-four oriented specimens of laminated clay were obtained from vertical sections in two caves in northern Norway. Studies of the magnetic remanence and susceptibility anisotropy show that the magnetization is depositional in origin and due to magnetite. The palaeomagnetic record in one section is correlated with results from a Swiss lake sediment core to suggest an age of 9600–6800 yr bp for the cave clay. It is shown that the susceptibility lineation produced on gently sloping surfaces was predominantly controlled by gravity and cannot therefore be used directly to estimate the palaeoflow or palaeofield directions.     

Environmental magnetic record of Antarctic palaeoclimate from Eocene/Oligocene glaciomarine sediments, Victoria Land Basin

The onset of continent-wide glaciation in Antarctica is still poorly understood, despite being one of the most important palaeoclimatic events in the Cenozoic. The Eocene/Oligocene boundary interval has recently been recognized as a critical time for Antarctic climatic evolution, and it may mark the preglacial–glacial transition. Magnetic susceptibility, intensity of natural and artificial remanences, hysteresis parameters and magnetic anisotropy of the lower half (late Eocene/early Oligocene) of the CIROS-1 core (from McMurdo Sound, Antarctica) reveal alternating intervals of high and low magnetic mineral concentrations that do not correspond to lithostratigraphic units in the core. Pseudo-single-domain magnetite is the main magnetic mineral throughout the sequence, and sharp changes in magnetite concentration match changes in clay mineralogy beneath and at the Eocene/Oligocene boundary. The detrital magnetite originated from weathering of the Ferrar Group (which comprises basic extrusive and intrusive igneous rocks). Weathering processes and input of magnetite to the Victoria Land Basin were intense during periods when the Antarctic climate was warmer than today, but during intervals when the climate was relatively cool, chemical weathering of the Ferrar Group was suppressed and input of detrital magnetite to the Victoria Land Basin decreased. Our results also indicate that a cold and dry climate was not established in Antarctica until the Eocene/Oligocene boundary, with major ice sheet growth occurring at the early/late Oligocene boundary. Some earlier cold intervals are identified, which indicate that climate had begun to deteriorate by the middle/late Eocene boundary.