Enrichment of sediments in iron hydroxides at the Mesozoic-Cenozoic boundary: A synthesis of petromagnetic data

Results of petromagnetic studies of epicontinental sediments at the Mesozoic-Cenozoic (K/T) boundary are generalized. Their analysis shows that an abrupt rise in the concentration of iron hydroxides is confined to this boundary. Their concentration is highest at the base of the boundary layer and drops by more than two times at it top. As distinct from iron hydroxides, other magnetic minerals were variously accumulated, depending on their origin and local deposition conditions of terrigenous material (for example, accumulation of particles of metallic iron and nickel, aerial transport of titanomagnetite grains of volcanic origin, and deposition of terrigenous particles of magnetite and ilmenite).     

Petromagnetic and paleomagnetic characterization deposits at Mesozoic/Cenozoic boundary: The Tetritskaro section (Georgia)

Petromagnetic and magnetostratigraphic characteristics are obtained for the Tetritskaro section. The boundary layer at the Mesozoic/Cenozoic (K/T) boundary is fixed primarily by an abrupt rise in the paramagnetic magnetization (total Fe concentration) and, to a lesser degree, by an increase in the concentration of such magnetic minerals as goethite, hemoilmenite, and magnetite. The along-section distribution of titanomagnetite of volcanic origin and metallic iron of cosmic origin does not correlate with the K/T boundary and lithologic properties of the sediments.The boundary of the Mesozoic and Cenozoic geological eras lies within the reversed polarity chron C29r and is marked by an abrupt rise in the geomagnetic field paleointensity and an instability of paleomagnetic directions, rather than by a polarity change. The accumulation time of the boundary clay layer is about 1.5–2 kyr, while abrupt changes in the paleointensity and direction of the geomagnetic field encompass 30–40 kyr. Such long occurrence intervals of the events in question cannot be related to a short-term impact phenomenon.     

Magnetic properties of the boundary layer at the Cretaceous/Tertiary boundary in the Gams section,Eastern Alps,Austria

The paper is concerned with the detailed sectional petromagnetic study of boundary clay in four 1A, 1B, 2A, and 2B Gams sections (in Austria). The composition of basic magnetic minerals in the boundary clay of all sections is similar. They are composed of iron hydroxides, hemoilmenite, titanomagnetite, magnetite, hematite, and iron. The difference is the presence in the Gams-1 section of metallic nickel, which is absent in the Gams-2 section, and the presence in the latter of iron sulfides of the pyrite type. Grains of titanomagnetite and ilmenite, connected with volcanic activity, are nonuniformly distributed in the boundary layer, which indicates their irregular precipitation in time. The ensemble of magnetic grains is characterized by high coercitivity. The boundary layer is characterized by an increased content of iron hydroxides. This effect is a global phenomenon and is irrelevant to the local physicogeographical conditions. Such a characteristic of impact events as the particles of metallic iron is almost absent in the boundary layer.     

Magnetic minerals in sediments at the Cretaceous/Paleogene boundary (the Gams Section,Eastern Alps)

The paper presents results of detailed magnetomineralogical and microprobe studies of sediments at the Cretaceous/Paleogene (K/T) boundary in two epicontinental sections in the Eastern Alps (Austria), where deposits, including the K/T boundary, outcrop along the Gams River and its tributaries. K/T boundary layers in these sections are similar in the set of such magnetic minerals as iron hydroxides, ferrospinels, hemoilmenite, titanomagnetite, magnetite, hematite, and metallic iron. However, the boundary layer in the Gams-1 section is distinguished by the presence of metallic nickel and its alloy with iron and by the absence of iron sulfides, whereas nickel has not been discovered in the Gams-2 section, which, however, contains iron sulfides of the pyrite type. Therefore, these minerals occur locally. It is suggested that enrichment in iron hydroxides of a common origin can be regarded as a global phenomenon inherent in the K/T boundary and unrelated to an impact event.     

First finding of natural rhenium in the transitional clay layer at the Cretaceous-Paleogene boundary in the Gams section (Eastern Alps,Austria)

As a result of detailed studies by the method of microprobe analysis, a particle of natural rhenium was found for the first time in the transitional clay layer at the Cretaceous-Paleogene boundary in the Gams section (Eastern Alps). It is concluded that the formation of natural rhenium was related to a volcanic aerosol that arose due to the magmatic activity of a mantle plume 65 Myr ago.     

Magnetolithologic and magnetomineralogical characteristics of sediments at the Mesozoic/Cenozoic boundary: The Koshak section (Mangyshlak peninsula)

Results of a detailed petromagnetic study of sediments of the Koshak section, including the Mesozoic/Cenozoic (K/T) boundary, are presented. The rocks are shown to have a very low magnetization. A relatively high magnetization is characteristic of two thin clayey beds, one at the K/T boundary and the other 0.6 m above it: x up to 2.5 × 10^?9 m³/kg, M _s up to 0.6 × 10^?3 A m²/kg, and M _rs up to 0.4 × 10^?3 A m²/kg. This is related to relatively high concentrations of hemoilmenite (up to 0.2%), magnetite (up to 0.01%), and goethite (up to 0.24%) in these beds. It is evident that the distribution of these magnetic minerals is lithologically controlled (the predominant occurrence in clayey beds), which is expressed, in particular, in the relation between the paramagnetic (clayey) and diamagnetic (carbonate) contributions to the magnetization of the sediments. Thus, clayey interbeds are sharply distinguished by the value of the paramagnetic magnetization (M _p = (83–86) × 10^?5 A m²/kg) as compared with purely diamagnetic chalk (M _d = ?(26–35) × 10^?5 A m²/kg). Minor concentrations of metallic iron (up to ～0.002%) discovered in the sediments have a lithologically uncontrolled distribution (metallic iron is more often observed near the K/T boundary rather than in clayey beds). Most probably, magnetite, hemoilmenite, and goethite were accumulated mostly with clay and other terrigenous material, while fine particles of iron are likely to have been dispersed by air. The whole set of the data of this work suggests that the K/T boundary is not distinguished by characteristic magnetomineralogical and magnetolithologic features.     

The first data on helium isotopy in a transitional clay layer at the cretaceous-paleogene boundary (Gams,Eastern Alps)

The paper presents preliminary results derived from the study of He isotopy in a transitional clay bed at the critical 65-Ma boundary in the Gams area (Eastern Alps). The ratio ³He/⁴He is shown to decrease from bottom to top of the transitional layer, and the difference between its values in the lower-and uppermost sublayers of the layer exceeds by more than ten times the measurement uncertainty (5%). The ratio ³He/⁴He is smallest in the upper part of the section, where all features of an impact event (spherules of pure nickel, diamonds, and awaruite) are established. The general inference of our study is that ³He enrichment in the lower part of the Gams section is related not to cosmic matter but to a change in the conditions in the source area, most likely, to volcanic activity. The effect of the latter has been established, independent of He isotopic studies, from anomalous concentrations of As, Pb, Cu, and Zn and from the presence of particles of gold, copper, and titanomagnetite similar in composition to its basaltic varieties.     

Size sorting of fine-grained sediments during erosion: Results from the western Gulf of Lions

Sediment cores from the western Gulf of Lions France were subject to known bottom shear stresses with the goal of understanding size-specific sediment erodibility. On cruises in October 2004, February and April 2005, cores with an undisturbed sediment–water interface were collected along a transect extending seaward from the Tet river mouth. The cores were exposed to increasing shear stresses (0.01–0.4 Pa) onboard the vessel shortly after collection by using a Gust erosion chamber. Samples of the suspensate were collected during the erosion experiments and analyzed for disaggregated inorganic grain size (DIGS) using a Coulter Multisizer IIe. Size-specific mobility plots were generated by dividing the proportion of each grain size in suspension at each shear stress by its proportion in the sediment before erosion. If all grain sizes that make up the bottom sediment are eroded equally from the bed, then mobility equals one for all grain sizes. Values >1 indicate that the suspended sediment is enriched in the size class and values <1 indicate that the size class is enriched in the bed. Results show that in non-cohesive, sandy silts, fine grains (clays and fine silts) are eroded preferentially from the bed at low shear stresses. With increasing bottom stress progressively larger grains are eroded from the bed. In cohesive silts, preferential erosion of the finer sizes no longer occurs, with all sizes up to medium silts eroding at approximately the same rate. Effectively, a sandy silt can be winnowed of its fine grain fraction during erosion while cohesive silts cannot. This difference in the sortability of cohesive and non-cohesive sediment during erosion may control the position and maintenance of the sand–mud transition and the sequestration of surface-adsorbed contaminants.     

Evolution of a section of the Africa-Europe plate boundary: Paleomagnetic and volcanological evidence from Sicily

New paleomagnetic data relative to Upper Cretaceous, Neogene and Quaternary volcanic rocks from eastern Sicily definitively indicate that Sicily is a part of the African plate, which collided with the European continental plate in Middle Miocene times. These data and the tectonic evolution of Sicily as inferred from the nature, age and distribution of volcanic products, are broadly consistent with the motions of Africa relative to Europe since the Upper Trias. During the Mesozoic, eastern Sicily was affected by extensional tectonics with associated alkali basaltic volcanism, and oceanic crust was produced in the meantime between the diverging African and European plates. Near the end of Mesozoic times the two plates started to converge with consequent consumption of oceanic crust. Different times of oceanic plate consumption along the Sicily-Calabria section of the plate boundary are suggested by the occurence of andesitic volcanism of different ages. The tectonic significance of late Tertiary to present basaltic activity in eastern Sicily is also discussed.     

Mercury in water,organisms and sediments from a section of the Turkish Mediterranean coast

Samples of water, marine organisms and sediment collected along the Turkish Mediterranean coast in the vicinity of Mersin were analysed for total mercury. The levels of mercury found in the samples from this area are generally low in comparison with the levels found in other regions of the Mediterranean.     

Noble gas isotopic abundances and noble metal concentrations in sediments from the Cretaceous-Tertiary boundary

Noble gases in four sediment samples from the Cretaceous-Tertiary boundary collected near Stevns in Denmark were investigated to test the possibility of the presence of noble gases indicative for meteorites. All samples were also analyzed for the noble metals Os and Ir and twelve other elements. The observed enrichments of³He,⁴He and of Ar, Kr, and Xe relative to atmospheric abundances can be explained without invoking the addition of extraterrestrial material. The ²⁰Ne/²²Ne ratio and the Kr and Xe isotopic compositions are identical with the isotopic ratios in the terrestrial atmosphere. In contrast, the high noble metal concentrations indicate the presence of material with elemental abundances similar to chondritic matter. Compared to noble metals the noble gases are less sensitive tracers of an admixture of extraterrestrial matter in sediments.     

Magnetostratigraphy of the Permo-Triassic boundary section at Shangsi (Guangyuan, Sichuan Province, China)

Continous marine sedimentation characterizes many Late Permian to Early Triassic sections on the Yangtze terrane in South China. The Permo-Triassic (P/Tr) boundary section at Shangsi (Sichuan Province) consists of limestones intercalated with clays and mudstones which belong to the Wuchiapingian and Changxingian (Upper Permian) and the Griesbachian and Dienerian (Lower Triassic) stages. The P/Tr boundary is formed by a clay horizon with an unusually high iridium concentration. The intensity of natural remanent magnetization is very low with a mean of 0.15 mA m⁻¹. About 40% of the samples contain secondary or unstable magnetization components only, whereas the remaining samples carry a characteristic remanent magnetization thought to reflect the polarity of the geomagnetic field during deposition with sufficient accuracy. Normal and reversed polarity of the characteristic magnetization constitute a pattern of at least six polarity zones, the P/Tr boundary being situated very close to the transition from a reversed to a normal polarity zone. The Shangsi polarity sequence represents part of the Illawarra interval of mixed polarity, the exact beginning of which has still to be determined.     

Paleomagnetism of paleozoic sediments from the Kozhim River section: On the problem of palinspastic reconstructions of the Subpolar Urals and Pai-Khoi

The collections of the Silurian, Devonian, Carboniferous, and Early Permian rocks from the Kozhim River section in the Subpolar Urals are studied. It is established that the pre-Permian Paleozoic deposits from the Kozhim River were remagnetized during the Kiama hyperchron (C₂-P₂) probably due to magnetoviscous processes that were blocked as these sediments were leaving the zone of increased temperature during the collision between the East European Platform and the Urals. The thrusts that took place at the last stage of this collision rendered the paleomagnetic directions of the studied structures different from those extrapolated from the East European Platform. The estimates of local rotations and displacements are obtained and applied in the combined analysis with the paleomagnetic determinations for the Pai-Khoi Ridge and Subpolar Urals. The amplitudes of the thrusts average 260 km and 180 km for the structures of Pai-Khoi and Subpolar Urals, respectively. The reconstruction of the prethrusting layout of the studied structures is presented.     

泥河湾盆地大长梁剖面河湖相沉积物的岩石磁学性质

以热磁分析为主,对中国北方泥河湾盆地更新世河湖相地层中灰绿色粉砂和灰黄色粉砂/细砂两种典型沉积物进行了详细的岩石磁学研究,有效确定了这两类沉积物中磁性矿物的种类、粒度特征以及加热过程中磁性矿物的变化过程和产物,并对其包含的古环境意义进行了初步探讨.灰绿色粉砂样品主要含有磁铁矿和赤铁矿两种磁性矿物,磁性相对较弱,颗粒相对较细;灰黄色粉砂/细砂主要含有磁铁矿和磁赤铁矿,磁性相对较强,颗粒相对较粗.在氩气环境中经700℃加热处理后,这两种沉积物中的绿泥石都分解,并生成超细粒（处于超顺磁和单畴颗粒区间）的磁铁矿,导致磁化率大幅升高.因此泥河湾盆地沉积物的热磁特征可以用来检测样品中绿泥石的相对含量,进而反映该地区化学风化作用强度的变化.此外,灰绿色粉砂样品中绿泥石含量比灰黄色粉砂/细砂样品的含量高,在氩气中加热后,磁化率升高幅度也较高,可能反映了化学风化相对较弱的沉积环境.     

Petromagnetic and paleomagnetic study of Jurassic sedimentary rocks of the southeast of Russia

In the present paper the new petro- and paleomagnetic data on the Jurassic terrigenous complexes of the Mesozoic sedimentary basins of the Amur River region, Trans Baikal region, and Yakutia are presented. The magnetic properties of the sedimentary rocks of coastal-marine (paleo-shelf) and lake genesis are investigated in the contemporary intracontinental riftogenic Mesozoic superimposed troughs of the Siberian and Amur plates: the Chulmansk, the Unda-Dainsk, the Sredne-Amur, the Amuro-Zeisk, and the Verkhne-Bureinsk troughs. The statistically significant differences in the magnetic (anisotropic) characteristics of continental and marine deposits were inferred. The correlation of the scalar and tensor characteristics of magnetic susceptibility anisotropy and the parameters of its linearity, which depend indirectly on the intensity of the folding, is established. The preferential directions of regional stress during the fold formation are determined based on the analysis of the distribution of the axes of the tensor ellipsoid of the magnetic susceptibility anisotropy. The Jurassic positions of the paleomagnetic pole, which are close to the Mesozoic section of the trajectory of its apparent motion for the North Chinese plate and which differ from the Jurassic poles of the Siberian plate, are defined more accurately. The intraplate rotations of geoblocks within the limits of the amalgamated to the end of the Jurassic-to the beginning of the Cretaceous terrains as a part of the Amur tectonic plate are inferred. The calculated coordinates of the paleomagnetic pole indicate the larger than the present-day difference of the latitudinal positions of the southern part of the Siberian plate, and also of the Amur and North Chinese plates in the Early-Middle Jurassic time. This can be indicative of the fact that the total width of the shallow residual basins of the Paleo-Asian Ocean, which separated the geoblocks indicated in the Early-Middle Jurassic, attained the first thousands of kilometers, and/or such a difference in the paleolatitudes reflects the total value of the shortening (crowding) of the crust during the accretion and the fold formation. The time of the completion of the accretion of the terrains of the Amur and the North-Chinese plates and their attachment to the Siberian plate is not earlier than the end of the Late Jurassic-the beginning of the Cretaceous.     

The diffusive boundary layer of sediments: oxygen microgradients over a microbial mat

Oxygen microelectrodes were used to analyze the distribution of the diffusive boundary layer (DBL) at the sediment-water interface in relation to surface topography and flow velocity. The sediment, collected from saline ponds, was covered by a microbial mat that had high oxygen consumption rate and well-defined surface structure. Diffusion through the DBL constituted an important rate limitation to the oxygen uptake of the sediment. The mean effective DBL thickness decreased from 0.59 to 0.16 mm as the flow velocity of the overlying water was increased from 0.3 to 7.7 cm s-1 (measured 1 cm above the mat). The oxygen uptake rate concurrently increased from 3.9 to 9.4 nmol cm-2 min-1. The effects of surface roughness and topography on the thickness and distribution of the DBL were studied by three-dimensional mapping of the sediment-water interface and the upper DBL boundary at 0.1-mm spatial resolution. The DBL boundary followed mat structures that had characteristic dimensions > 1/2 DBL thickness but the DBL had a dampened relief relative to the mat. The effective surface area of the sediment-water interface and of the upper DBL boundary were 31 and 14% larger, respectively, than a flat plane. Surface topography thereby increased the oxygen flux across the sediment-water interface by 49% relative to a one-dimensional diffusion flux calculated from the vertical oxygen microgradients.     

Planetary features of aurorae: Results of the IGY (a review)

In the period of the International Geophysical Year (IGY), almost the entire planet was covered for the first time by ground-based geophysical observations. Their analysis led to two fundamental results: the existence of the auroral oval and auroral (magnetospheric) substorm. At the final stage of the IGY, satellite explorations of the near-Earth space began. The auroral luminosity appeared to be related to the plasma structure of the magnetosphere. That opened new possibilities for parameters diagnostics of the Earth’s magnetosphere on the basis of ground-based aurora observations. The concepts of auroral oval and magnetospheric substorm became paradigms of the new science of solar-terrestrial physics.     

太白山三清池湖泊沉积记录的全新世气候变化及周期讨论

通过对太白山三清池湖泊沉积柱芯的低频磁化率(χlf)、频率磁化率(χfd)、总有机碳(TOC)、粒度参数指标的时序序列进行小波分析,探讨太白山全新世气候变化过程及周期波动性.结果显示:中、晚全新世太白山气候变化具有明显的阶段性特征;千年尺度上,χlf、χfd、TOC和平均粒径参数序列反映的主周期分别为1427、1427、1427和1452 a,晚全新世5410 cal a B.P.距今期间存在8次显著的冷暖交替震荡;在百年尺度上,χlf、χfd、TOC和平均粒径参数序列还揭示出分别以492、492、467和467 a为周期的次一级变化,且整体上经历近似10次的短期冷暖波动.此外,以上结果与全球范围内全新世气候周期有着较好的一致性,表明我国东部高海拔地区在千年和百年尺度上,对全新世气候振荡同样具有相似的响应.     

松辽盆地西北边界部分地球物理特征

松辽盆地是我国东北地区重要的地质构造单元.为探测松辽盆地西北部边界基底分布、深部结构与构造关系,沿扎兰屯经齐齐哈尔至林甸实施了约240 km的综合地球物理的探测研究,利用重力、电法等地球物理方法对其进行了综合地球物理解释.研究结果揭示了研究区的速度结构分布、重力分布、基底起伏形态及电性结构等地球物理特征:(1)满-绥断面速度资料显示,嫩江岩石圈断裂有明显的速度分界,其东南侧的松辽盆地与大杨树坳陷具有连续统一的速度结构;(2)重力反演基底显示,松辽盆地与大杨树坳陷基底连续,具有统一的基底特征;(3)二维电性反演显示,大杨树坳陷和松辽盆地两构造电性层连在一起,低阻分布形态也是一样的.以上结果结合满-绥断面速度结构分布表明大杨树坳陷是松辽盆地西部的延续,松辽盆地西部边界应扩展到大杨树坳陷西部边界,即嫩江岩石圈断裂.     

Downdrift coarsening of beach foreshore sediments at tidal inlets: An example from the coast of New Jersey

Analysis of grain size statistics of upper foreshore sediments on sand beaches at two tidal inlets in New Jersey, U.S.A. reveals that sediments are coarser at beaches flanking the inlets than updrift, although sediments become finer downdrift at the broad, regional scale. The local reversal of the regional trend in size grading is attributed to: (1) the offshore diversion of the finer sands along the surf zone on the ebb tidal delta, and (2) the removal of the finer sands from the inlet flank beach caused by low wave energy conditions at low stages of the tide and by deflation. Sediments thus become coarser at inlet beaches as a result of alterations in the interaction of waves with the beach and as a result of aeolian processes, not solely as a result of increased tidal current velocities as previously reported. The distance along the New Jersey barrier islands over which inlet processes are likely to affect changes in sediment size updrift averages less than 1100 m, but the impacts of inlets on the sedimentary record can be extended greater distances as a result of inlet migration.