Crustal structure of the Barents Sea from seismic data

In 1976, the Institute of Physics of the Earth and the Institute of Oceanology, the U.S.S.R. Academy of Sciences, carried out deep seismic soundings in the Barents Sea along a profile 700 km long northeast of Murmansk. A system of reversed and overlapping traveltime curves from 200 to 400 km long has been obtained. The wave correlation was effected by several independent approaches, which identified on the records the refracted and reflected waves from boundaries in the Earth's crust and the upper mantle. Different methods were applied for the solution of the inverse problem: the isochrone method, the intercept-time method, and the iteration method.The use of these different methods gives an indication of the general applicability of the interpretation and of the most reliable elements in the seismic model.All the interpretations and representations of the section positively establish an essentially horizontal inhomogeneity of the Earth's crust in the Barents Sea. On the whole the structure is similar to that of deep sedimentary basins of the East European platform. The thickness of the sedimentary layer varies from 8 to 17 km, the average crustal thickness is about 35–40 km; the velocities in the upper part of the consolidated crust are 5.8–6.4 km/s; in the lower crust they are 6.8–7.0 km/s and higher.     

Methane cycle in the Barents Sea

Biogeochemical cycle of methane in the Barents Sea was studied using isotope geochemistry to determine the rates of microbial methane oxidation. It was established that microbiological processes (glucose consumption, ¹⁴CO₂ assimilation, sulfate reduction, and slow methane oxidation) in oxidized surface and weakly reduced sediments are marked by only insignificant change in SO ₄ ^2? concentration and absence of notable growth of total alkalinity and N/NH₄ downward the sediment core. Microbial methane productivity was 0.111 × 10⁶ mol day^?1. Taking into account the volume of water column, microbial methane consumption therein can be as much as 1.8 × 10⁶ mol day^?1.     

Iron and Manganese Minerals in Suspended Matter from the Barents Sea

The mineralogy of suspended matter from surface and bottom waters is studied at two sites in the Barents Sea. Along with terrigenous minerals, the suspended matter samples contain authigenic mineral phases of iron and manganese oxyhydroxides. Mn-feroxyhite, Fe-vernadite, goethite, and proto-ferrihydrite were identified in samples from surface waters, whereas birnessite and nonferruginous vernadite were registered in samples from bottom waters. The formation of suspended manganese minerals in bottom waters is explained by an additional Mn supply from underlying reduced sediments during their early diagenesis and oxygen depletion in the near-bottom nepheloid layer. Bacteria are supposed to take part in the authigenic mineral formation.     

Recent climatic tendencies in the Barents Sea

Based on the results of multiannual oceanographic research at the Murmansk Marine Biological Institute in the “Kola Meridian” secular section, the recent tendencies of cyclic climate changes in the Barents Sea are discussed on the basis of the data on ice coverage and coastal meteorological conditions. It has been shown that a return from the extremely warm and warm state to the normal one was observed in the period of winter 2005/06–winter 2010/11. Further, this may be changed both to cooling and warming, which verifies the hypothesis about predominance of cyclic natural variability in the thermal and ice characteristics of the Arctic Region.     

从浅水陆架走向深水陆坡--南海深水扇系统的研究

南海深水扇系统是近年来在南海海域的重大发现,证实自30Ma以来深水扇系统密集地分布在我国的近海海域内,并以其宏大的规模、典型的结构及巨大的勘探前景引起国内外的关注。深入展开深水扇系统的研究将是我国油气界与科学界的重大任务。近20年来,深水扇系统已成为国际石油产量与储量增长的主体,已经有60多个国家进行深水扇研究,并在国际学术界获得高度重视。据认为:“深水浊流及其有关储层在今后至少25年内将成为油气勘探开发研究的前缘”。南海珠江口盆地深水扇系统发育区域的地质条件与国际上已有重大油气发现的深水扇系统发育区有许多重要的相似。古珠江大河充足的物源供应,长期沉降的深水凹陷与海平面周期性下降是纵向上呈良好叠置状态的深水扇系统发育的要素;而立足于精度较好,保真程度较高的高分辨地震剖面是识别层序界面,识别低水位体系域,辨识陆架坡折带的技术关键,这就使研究领域从浅水陆架进入到深水陆坡。应当给深水扇系统赋予科学的定义;然而国际学术界对深水扇系统的沉积物性质、类型及预测方法技术存在争议。但是,毫无疑义的是,深水扇系统具有可识别性,它对油气的运移、聚集、隐蔽圈闭的形成以及指导油气勘探有着重要的意义。     

Silica and volatile-element metasomatism of Archean mantle: a xenolith-scale example from the Kaapvaal Craton

Textural evidence in a composite garnet harzburgite mantle xenolith from Kimberley, South Africa, suggests metasomatism of a severely melt-depleted substrate by a siliceous, volatile-rich fluid. The fluid reacted with olivine-rich garnet harzburgite, converting olivine to orthopyroxene, forming additional garnet and introducing phlogopite, and small quantities of sulfide and probable carbonate. Extensive reaction (>50%) forming orthopyroxenite resulted from channelized flow in a vein, with orthopyroxene growth in the surrounding matrix from a pervasive grain-boundary fluid. The mineralogy of the reaction assemblage and the bulk composition of the added component dominated by Si and Al, with lesser quantities of K, Na, H, C and S, are consistent with experimental studies of hybridization of siliceous melts or fluids with peridotite. However, low Na, Fe and Ca compared with melts of eclogite suggest a fluid phase that previously evolved by reaction with peridotitic mantle. Garnet and phlogopite trace element compositions indicate a fluid rich in large-ion lithophile (LIL) elements, but poor in high field-strength elements (HFSE), qualitatively consistent with subduction zone melts and fluids. An Os isotope (T_RD) model age of 2.97 ± 0.04 Ga and lack of compositional zonation in the xenolith indicate an ancient origin, consistent with proposed 2.9 Ga subduction and continental collision in the Kimberley region. The veined sample reflects the silicic end of a spectrum of compositions generated in the Kimberley mantle lithosphere by the metasomatizing effects of fluids derived from oceanic lithosphere. These results provide petrographic and chemical evidence for fluid-mediated Si-, volatile- and trace-element metasomatism of Archean mantle, and support models advocating large-scale modification of regions of Archean subcontinental mantle by subduction processes that occurred in the Archean.     

Origin of diamictons on the Barents Sea shelf

Occurrence conditions and lithological-paleontological characteristics of two diamicton units developed in the southeastern segment of the Barents Sea shelf are considered. It is shown that they are lithologically analogous to the present-day (undoubtedly aqueous) diamicton mud. Correlation between bulk densities of both varieties and consolidation degree (consistency) is described by a single regression equation. We present pieces of evidence contradicting the widespread point of view about the diamicton as a glacial till moving along the seafloor. They are primarily represented by paleontological remains, mainly foraminiferal assemblages. It is shown that dislocations observed in cores and traditionally considered as glaciotectonic movements are confined to fault zones in the acoustic basement and they represent fragments of secondary neotectonic structures. Ridges and hills mapped previously as glacial accumulation morphostructures reflect a relict topography of the recent subaerial shelf development not masked by sedimentation so far.     

Bitumoids of Holocene Sediments in the Barents Sea

Doklady Earth Sciences - The results of studying the content, composition, and distribution of chloroform bitumoids (CBs) in the Holocene sediments of the Barents Sea (cruise 68 of the R/V Akademik...     

Spring melt patterns in the Kara/Barents Sea: 1984

Ice-atmosphere interactions in the Seasonal Sea Ice Zone undergo rapid changes during the spring melt period with the transition from winter to summer conditions. The nature of these interactions is strongly dependent on the characteristics of the surface, which also experiences large changes during this same time period. This paper describes a methodology, based on Extended Principal Components Analysis, which is used to categorize the spatial and temporal patterns of surface change that occur in the Seasonal Sea Ice Zone during the spring/early summer. The methodology is demonstrated for the Kara/Barents Sea in spring 1984 using data from the Nimbus-7 Scanning Multichannel Microwave Radiometer. The analysis shows conditions in the Barents Sea to be largely controlled by ice advection, while the variance in the Kara Sea is dominated by surface melt.     

Late Quaternary glaciation in the south-western Barents Sea

Moraine ridges have been morphologically and seismically identified in the south-western Barents Sea. Some of these ridges were deposited in front of ice lobes from the northern part of the Fennoscandian Ice Sheet, others in front of glaciers located on the southern Barents Sea shelf. The moraine ridges were probably deposited during the Weichselian, possibly the Late Weichselian.     

Cratonic reactivation and orogeny: An example from the northern margin of the North China Craton

Reactivation of cratonic basement involves a number of processes including extension, compression, and/or lithospheric delamination. The northern margin of the North China Craton (NCC), adjacent to the Inner Mongolian Orogenic Belt, was reactivated in the Late Paleozoic to Early Mesozoic. During this period, the northern margin of the NCC underwent magmatism, N–S compression, regional exhumation, and uplift, including the formation of E–W-trending thick-skinned and thin-skinned south-verging folds and south-verging ductile shear zones. zircon U–Pb SHRIMP ages for mylonite protoliths in shear zones which show ages of 310–290 Ma (mid Carboniferous–Early Permian), constraining the earliest possible age of deformation. Muscovite within carbonate and quartz–feldspar–muscovite mylonites from the Kangbao–Weichang and Fengning–Longhua shear zones defines a stretching lineation and gives ⁴⁰Ar/³⁹Ar ages of 270–250 Ma, 250–230 Ma, 230–210 Ma, and 210–190 Ma. Deformation developed progressively from north to south between the Late Paleozoic and Triassic. Exhumation of lower crustal gneisses, high-pressure granulites, and granites occurred at the cratonic margin during post-ductile shearing (~ 220–210 Ma). An undeformed Early Jurassic (190–180 Ma) conglomerate overlies the deformed rocks and provides an upper age limit for reactivation and orogenesis. Deformation was induced by convergence between the southern Mongolia and North China cratonic blocks, and the location of this convergent belt controlled later deformation in the Yanshan Tectonic Province. This province formed as older E–W-trending Archean–Proterozoic sequences were reactivated along the northern margin of the NCC. This reactivation has features typical of cratonic basement reactivation: compression, crustal thickening, remelting of the mid to lower crust, and subsequent orogenesis adjacent to the orogenic belt.     

Neodymium isotopes in seawater from the Barents Sea and Fram Strait Arctic-Atlantic gateways

The neodymium concentration, C_Nd, and isotopic composition, ε_Nd, in seawater have been determined in the water column at five sites in the Barents Sea-Fram Strait area where most of the water exchange between the Arctic Ocean and the North Atlantic takes place. In the main Arctic Ocean inflow branch across the Barents Sea the concentration and isotopic composition (C_Nd = 15.5 pmol/kg and ε_Nd = −10.8) are similar to those reported for the northeastern Nordic Seas, which is consistent with this region being a source area for the Arctic inflow. Due to the addition of Nd from Svalbard shelf sediments, the C_Nd in the surface waters above 150 m, in the Fram Strait inflow branch is higher by a factor of 2 and the ε_Nd is shifted to lower values (−11.8).In the stratified Nansen Basin, where cold low salinity water overlies warmer Atlantic water the C_Nd and ε_Nd do not vary with the vertical temperature-salinity structure but are essentially constant and similar to those of the Atlantic inflow throughout the entire water column, down to 3700 m depth, which indicates that the Nd is to a large extent of Atlantic origin.Compared to the Atlantic inflow water, the Nd in the major Arctic Ocean outflow, the Fram Strait, show higher C_Nd in the surface waters above 150 m, and a higher ε_Nd (−9.8) throughout the entire water column down to 1300 m depth. Sources for the more radiogenic Nd isotopic composition in deep water of the Fram Strait outflow most likely involve boundary exchange with sediments on the shelf and slope as the water passes along the Canadian archipelago. River water is a possible source in the surface water but it also seems likely that Pacific water Nd, modified by interactions on the shelf, is an important component in the Fram Strait surface outflow. Changes in the relative proportions of inflow of river water and flow of Pacific water through the Arctic Ocean could thus influence the isotopic composition of Nd in the North Atlantic.     

Origin of friable sediments of the Barents Sea shelf

The origin of friable sediments blanketing the Barents Sea shelf is considered. It is shown that their characteristic seismoacoustic record patterns reflect low degree of diagenetic transformations and indicates continuous sedimentation. According to traditional views, this single sedimentary complex also includes diamicton, and the section is interpreted as a three-unit structure: diamicton, which is considered a till; the overlying friable sediments accumulated under different conditions of deglaciation in glaciomarine settings; and the postglacial marine sediments. It is demonstrated that such views are inconsistent with geomorphologic features (datings by physical methods included) indicating a prolonged hiatus that separates epochs of the diamicton accumulation and formation of friable sediments. The analysis revealed that the composition, vertical succession, and lateral distribution of different lithological types of friable sediments are related to the regular spatiotemporal replacements of different facies settings in the transgressing Arctic sea rather than by the glacial process. This inference is confirmed by the composition of foraminiferal assemblages.     

Indications of neotectonic activity at the Barents Sea shelf

Geologic-geomorphic and structural indications of neotectonic, virtually present-day, activity at the Barents Sea shelf are considered. Wide belts of the secondary ruptures—linear zones of dynamic effects of faults with a strike-slip component in the acoustic basement—are mapped in the areas studied in detail. Some of these ruptures displace recent sediments. As a result, allochthonous blocks of Mesozoic or Paleozoic rocks occasionally barren of recent marine sediments arise under transpressional conditions. Other signs of the present-day secondary faulting include shallow-seated injection folds and a peculiar wavy topography of mud in deep areas of the bottom. The relationship of exotic submarine mounds and gas emissions in the eastern Pechora Sea with recent mud volcanoes controlled by the neotectonic activity of the Pai-Khoi-Novaya Zemlya Foldbelt under conditions of lateral compression is substantiated for the first time. A superimposed aggradational height is revealed in the most subsided portion of the Central Trench; the origin of this height is referred to the effect of seismic vibration of the seafloor that brings about partial fluidization of surficial marine mud and its ductile-viscous flow and local accumulation in a particularly favorable area of the bottom. The indications of neotectonic activity may be used as a tool for testing the tectonic concepts that are put forward.     

Crust structure of the northeastern Barents Sea basin area

Processing of data from regional geophysical surveys completed in the northern Barents Sea has provided updates to gravity and magnetic databases, structural maps of seismic interfaces, and positions of anomaly sources, which made a basis for 3D density and magnetic models of the crust. The new geological and geophysical results placed constraints on the boundaries between basement blocks formed in different settings and on the contours of deposition zones of different ages in the northeastern Barents Sea. The estimated thicknesses of sedimentary sequences that formed within certain time spans record the deposition history of the region. There is a 20-50 km wide deep suture between two basins of Mesozoic and Paleozoic ages in the eastern part of the region, where pre-Late Triassic reflectors have no clear correlation. The suture slopes eastward at a low angle and corresponds to a paleothrust according to seismic and modeling data. In the basement model, the suture is approximated by a zone of low magnetization and density, which is common to active fault systems. The discovery of the suture has important geological and exploration implications.     

Mapping lithospheric boundaries using Os isotopes of mantle xenoliths: An example from the North China Craton

The petrology, mineral compositions, whole rock major/trace element concentrations, including highly siderophile elements, and Re-Os isotopes of 99 peridotite xenoliths from the central North China Craton were determined in order to constrain the structure and evolution of the deep lithosphere. Samples from seven Early Cretaceous-Tertiary volcanic centers display distinct geochemical characteristics from north to south. Peridotites from the northern section are generally more fertile (e.g., Al₂O₃ = 0.9-4.0%) than those from the south (e.g., Al₂O₃ = 0.2-2.2%), and have maximum whole-rock Re-depletion Os model ages (T_RD) of ∼1.8 Ga suggesting their coeval formation in the latest Paleoproterozoic. By contrast, peridotites from the south have maximum T_RD model ages that span the Archean-Proterozoic boundary (2.1-2.5 Ga). Peridotites with model ages from both groups are found at Fansi, the southernmost locality in the northern group, which likely marks a lithospheric boundary. The Neoarchean age of the lithospheric mantle in the southern section matches that of the overlying crust and likely reflects the time of amalgamation of the North China Craton via collision between the Eastern and Western blocks. The Late Paleoproterozoic (∼1.8 Ga) lithospheric mantle beneath the northern section is significantly younger than the overlying Archean crust, indicating that the original lithospheric mantle was replaced in this region, either during a major north-south continent-continent collision that occurred during assembly of the Columbia supercontinent at ∼1.8-1.9 Ga, or from extrusion of ∼1.9 Ga lithosphere from the Khondalite Belt beneath the northern Trans-North China Orogen, during the ∼1.85 Ga continental collision between Eastern and Western blocks. Post-Cretaceous heating of the southern section is indicated by high temperatures (>1000 °C) recorded in peridotites from the 4 Ma Hebi suite, which are significantly higher than the temperatures recorded in peridotites from the nearby Early Cretaceous Fushan suite (<720 °C), and likely reflects significant lithospheric thinning after the Early Cretaceous. Combining previous Os isotope results on mantle xenoliths from the eastern North China Craton with our new data, it appears that lithospheric thinning and replacement may have evolved from east to west with time, commencing before the Triassic on the eastern edge of the craton, occurring during the Jurassic-Cretaceous within the interior, and post-dating 125 Ma on the westernmost boundary.     

The chemistry and mineralogy of the clay fraction of sediments from the southern Barents Sea

The separated clay fraction (material <μ) of surface and sub-surface sediment samples from the southwestern Barents Sea is described. The partitioning of the major and minor elements within the different grain-size fractions of the sediment and between detrital and non-detrital phases demonstrates that the clay chemistry of these oxic shelf sediments is terrigenous in origin.The clays are a variable mixture of micaceous debris and illite with chloritic material, minor expandable clay and occasional kaolinite. Carbonate debris and amphibole occur locally. The semi-quantitative analysis suggests the existence of a considerable variation in the relative content of the principal mineralogical components and this is confirmed by the investigation of the major and minor elements. The terrigenous chemistry provides a more sensitive index of clay variability and allows the recognition of three distinct petrographic provinces.