Seismic velocity model of the crust and uppermost mantle around the Mirnyi kimberlite field in Siberia     

V.D. Suvorov  E.A. Melnik  H. Thybo  E. Perchu&#x;  B.S. Parasotka 《Tectonophysics》2006,420(1-2):49

We present the first detailed seismic velocity models of the crust and uppermost mantle around the Mirnyi kimberlite field in Yakutia, Siberia. We have digitized vintage seismograms that were acquired in 1981 and 1983 by use of Taiga analogue seismographs along two perpendicular seismic profiles. The 370-km long, northwest striking profile I across the kimberlite pipe was covered by 41 seismographs, which recorded seismic signals from 21 chemical shots along the line, including one off-end shot. The perpendicular, 340-km long profile II across profile I ca. 30 km to the south of the Mirnyi kimberlite field was covered by 45 seismographs, which recorded seismic signals from 22 chemical shots, including four off-end shots. Each shot involved detonation of between 1.5 and 6.0 tons of TNT, distributed in individual charges of 100–200 kg in shallow water (< 2 m deep). The data is of high quality with high signal/noise ratio to the farthest offsets. We present the results from two-dimensional ray tracing, forward modelling.Both velocity models show normal cratonic structure of the ca. 45-km-thick crust with only slight undulation of the Moho. However, relatively small seismic velocity is detected to 25-km depth in a ca. 60-km wide zone around the kimberlite pipe, surrounded by elevated velocity (> 6.3 km/s) in the upper crust. The lower crust has a relatively constant velocity of 6.8–6.9 km/s. It appears relatively unaffected by the presence of the kimberlite field. Extremely large P-wave velocity (> 8.7 km/s) of the sub-Moho mantle is interpreted along profile I, except for a 70-km wide zone with a “normal” Pn velocity of 8.1 km/s below the kimberlite. Profile II mainly shows Pn velocities of 8.0–8.2 km/s, with unusually large velocity (> 8.5 km/s) in two, ca. 100-km wide zones, at its southwestern end, one zone being close to the kimberlite field. The nature of these exceptionally large, sub-Moho mantle velocities is not yet understood. The difference in velocity in the two profile directions indicates anisotropy, but the effect of unusual rock composition, e.g. from a high concentration of garnet, cannot be excluded.  相似文献   

An Early Cretaceous age for the Rajmahal traps, Panagarh area, West Bengal: palynological evidence     

Vijaya  T. K. Bhattacharji 《Cretaceous Research》2002,23(6):789

The discovery of traps in the subsurface Mesozoic succession of the Panagarh area, West Bengal, has drawn attention to their correlation with the traps in the Rajmahal Basin, Bihar. The objective of this paper is to assess the palynological dating of the intertrappean beds with the help of sequential first appearances of the stratigraphically important spore species Cicatricosisporites australiensis, Aquitriradites spinulosus, and Foraminisporis wonthaggiensis. C. australiensis, along with the first appearance of A. spinulosus, denotes the earliest Berriasian. This association is significant for recognition of the Jurassic/Cretaceous boundary. The lowest occurrence of F. wonthaggiensis indicates deposits of Berriasian–Valanginian age. The data on which these observations are based have been obtained from the intertrappean beds of the Rajmahal Formation in six boreholes: PGD-1A, 2, 4, 6, 8 and 9A. Further palynological studies on boreholes PGD-4, 8 and 9A have indicated the approximate time-span for the duration of volcanic activity in the Panagarh area. This seems to have been initiated in the early Berriasian and continued into the Hauterivian, which is earlier than the volcanism associated with the Rajmahal traps in the Rajmahal Basin.  相似文献   

东非大陆边缘地质特征及油气勘探前景   总被引：1，自引：0，他引：1  

马君  刘剑平  潘校华  王训练  刘权 《世界地质》2008,27(4)

非洲大陆油气资源潜力已越来越引起人们的广泛关注.与盛产油气的西非大陆边缘相比,东非大陆边缘至今还没有显著的原油发现.通过对东非大陆边缘构造演化特征、边缘构造性质及边缘形态等地质条件的分析,以及从油气成藏(烃源岩、储集层以及成熟度等方面)有利与不利因素的角度对其油气勘探前景进行的分析,认为东非大陆边缘具备油气成藏的基本地质条件.其中烃源岩分布,古近系构造抬升、剥蚀破坏作用,构造圈闭,特别是沿岸边缘海上区,以及三角洲地区的勘探工作应是今后研究工作的重点.  相似文献   

俄罗斯西伯利亚与远东地区城镇化发展水平测度及空间分异研究   总被引：4，自引：0，他引：4  

初楠臣  张平宇  李鹤  李欣 《地理科学》2018,38(7):1069-1078

构建城镇化评价指标体系,研究俄罗斯西伯利亚与远东地区2005~2015年人口、经济、社会城镇化发展水平及空间分异。研究表明：俄罗斯西伯利亚与远东地区城镇化综合发展水平呈小幅上升,西伯利亚城镇化发展水平高于远东,两联邦区人口城镇化水平下降、经济与社会城镇化发展缓慢,多数联邦主体的城镇化发展低于平均水平;空间上,城镇化发展水平高值区集中在西伯利亚联邦区西部（鄂木斯克-新西伯利亚-阿尔泰边疆区-克麦罗沃-克拉斯诺亚尔斯克-伊尔库茨克联邦主体）,低值区分布在远东联邦区东部（马加丹-楚科奇-勘察加联邦主体）,西高东低格局特征显著;最后探讨了促进西伯利亚与远东人口、经济、社会城镇化发展的具体对策。  相似文献   

Effects of Time-dependent Moisture Content of Surface Sediments on Aeolian Transport Rates Across a Beach,Wildwood, New Jersey,U.S.A.     

Nancy L. Jackson  Karl F. Nordstrom 《地球表面变化过程与地形》1997,22(7):611-621

A one-day field investigation on an unvegetated backbeach documents the importance of surface sediment drying to aeolian transport. Surface sediments were well sorted fine sand. Moisture content of samples taken in the moist areas on the backbeach varied from 2·9 to 9·2 per cent. Lack of dry sediment inhibited transport prior to 08:50. By 09:10 conspicuous streamers of dry sand moved across the moist surface. Barchan-shaped bedforms, 30 to 40 mm high and composed of dry sand (moisture content <0·10 per cent), formed where sand streamers converged. The surface composed of dry sand increased from 5 per cent of the area of the backbeach at 09:50 to 90 per cent by 12:50 Mean wind speeds were beetween 5·6 and 8·6 m s⁻¹ at 6 m above the backbeach. Corresponding shear velocities were always above the entrainment threshold for dry sand and below the threshold for the moist sand on the backbeach. Measured rates of sand trapped (by vertical cylindrical traps) increased during the day relative to calculated rates. The measured rate of sand trapped on the moist foreshore was higher than the rate trapped on the backbeach during the same interval, indicating that the moist foreshore (moisture content 18 per cent) was an efficient transport surface for sediment delivered from the dry portion of the beach upwind. Measured rates of sand trapped show no clear relationship to shear velocities unless time-dependent surface moisture content is considered. Results document conditions that describe transport across moist surfaces in terms of four stages including: (1) entrainment of moist sediment from a moist surface; (2) in situ drying of surface grains from a moist surface followed by transport across the surface; (3) entrainment and transport of dry sediment from bedforms that have accumulated on the moist surface; and (4) entrainment of sand from a dry upwind source and transport across a moist downwind surface. © 1997 John Wiley & Sons, Ltd.  相似文献   

The geological characteristics of the large-and medium-sized gas fields in the South China Sea     

Gongcheng Zhang  Dongdong Wang  Lei Lan  Shixiang Liu  Long Su  Long Wang  Wu Tang  Jia Guo  Rui Sun 《海洋学报(英文版)》2021,40(2):1-2

By the end of 2019, more than 220 gas fields had been discovered in the South China Sea. In order to accurately determine the geological characteristics of the large-and medium-sized gas fields in the South China Sea, this study conducted a comprehensive examination of the gas fields. Based on the abundant available geologic and geochemical data, the distribution and key controlling factors of the hydrocarbon accumulation in the South China Sea were analyzed. The geological and geochemical features of the gas fields were as follows:(1) the gas fields were distributed similar to beads in the shape of a "C" along the northern, western, and southern continental margins;(2) the natural gas in the region was determined to be composed of higher amounts of alkane gas and less CO_2;(3) the majority of the alkane gas was observed to be coal-type gas;(4) the gas reservoir types included structural reservoirs, lithologic reservoirs, and stratigraphic reservoirs, respectively;(5) the reservoir ages were mainly Oligocene, Miocene, and Pliocene, while the lithology was mainly organic reef, with some sandstone deposits; and(6) the main hydrocarbon accumulation period for the region was determined to be the late Pliocene-Quaternary Period. In addition, the main controlling factors of the gas reservoirs were confirmed to have been the development of coal measures, sufficient thermal evolution, and favorable migration and accumulation conditions.  相似文献   

Evaluating the efficiency of three methods for monitoring of native crayfish in Germany     

《Limnologica》2020

We compared the capture efficiency of three traditional sampling methods, hand captures at day, hand captures at night and captures by commercial baited traps (“Pirat type”) with a coarse mesh size of 10 × 45 mm, for the native crayfish species Austropotamobius torrentium (stone crayfish) and Astacus astacus (noble crayfish). These capture types are applied regularly in monitoring programs. Additionally, we tested modified traps with fine mesh size to capture stone crayfish. We hypothesized that the capture efficiency of the methods is different between the two species, different sex and size classes and seasons and that modified traps with small mesh size were more effective in capturing juvenile stone crayfish than traps with coarse mesh size. Hand captures at night were found to be the most effective sampling method for both species. It generates the highest relative abundances and is the least biased method in relation to size and sex of crayfish specimen. Hand captures at day result in lower capture rates than hand captures at night. However, there is a tendency to gather more small individuals and females in comparison to the night captures. Baited traps turned out to be completely inappropriate for capturing stone crayfish; even traps modified with small mesh size were not a suitable capture method. For noble crayfish all three methods seem to be suitable, depending on the sampling purposes. The relative differences between the methods did not change significantly during the seasons. However, when capture rates of all three methods were combined, September was found to be the month where the highest capture rates were achieved as both sexes of both species were on the peak of their activity.  相似文献   

末次间冰期以来门捷列夫洋脊南部冰盖对细颗粒沉积的控制作用   总被引：1，自引：1，他引：0  

叶黎明  于晓果  章伟艳  王嵘 《海洋学报(英文版)》2020,39(9):86-95

门捷列夫洋脊南部的粘土矿物沉积具有明确的物源,为追踪该区沉积环境的演变提供了良好的条件。末次间冰期以来,ARC7-E23孔中的粘土矿物记录表现出了非常显著的变化。结合沉积物粒度的端元组份和冰阀碎屑沉积,粘土矿物的变化模式表明,东西伯利亚冰盖(ESIS)的规模可能是控制细颗粒沉积的主要因素。在氧同位素2期(MIS2)和4期(MIS4),门捷列夫洋脊南部可能被ESIS所覆盖,几乎阻挡了所有来自加拿大和拉夫贴夫海陆架的沉积物,但允许大量来自东西伯利亚海陆架的细粒沉积物输入。只有当ESIS消融后,波弗特环流和越极流的相对强度以及搬运作用才成为了控制远源沉积物输入的主要因素。MIS3期的气候条件似乎最适合远源沉积物的输入,不仅提高了表层环流的流通性,也提供了足够多的搬运介质。  相似文献   

楚科奇和西伯利亚陆架颗粒有机物的来源：碳、氮同位素的线索   总被引：2，自引：1，他引：1  

贾仁明  牟新悦  陈敏  朱晶  王博  李晓鹏  Astakhov A S  郑敏芳  邱雨生 《海洋学报(英文版)》2020,39(9):96-108

The stable isotopic composition(δ~(13)C and δ~(15)N) and carbon/nitrogen ratio(C/N) of particulate organic matter(POM) in the Chukchi and East Siberian shelves from July to September, 2016 were measured to evaluate the spatial variability and origin of POM. The δ~(13)C_(POC) values were in the range of -29.5‰ to-17.5‰ with an average of -25.9‰±2.0‰, and the δ~(15)N_(PN) values ranged from 3.9‰ to 13.1‰ with an average of 8.0‰±1.6‰. The C/N ratios in the East Siberian shelf were generally higher than those in the Chukchi shelf, while the δ~(13)C and δ~(15)N values were just the opposite. Abnormally low C/N ratios(4), low δ~(13)C_(POC)(almost-28‰) and high δ~(15)N_(PN)(10‰) values were observed in the Wrangel Island polynya, which was attributed to the early bloom of small phytoplankton. The contributions of terrestrial POM, bloom-produced POM and non-bloom marine POM were estimated using a three end-member mixing model. The spatial distribution of terrestrial POM showed a high fraction in the East Siberian shelf and decreased eastward, indicating the influence of Russian rivers. The distribution of non-bloom marine POM showed a high fraction in the Chukchi shelf with the highest fraction occurring in the Bering Strait and decreased westward, suggesting the stimulation of biological production by the Pacific inflow in the Chukchi shelf. The fractions of bloom-produced POM were highest in the winter polynya and gradually decreased toward the periphery. A negative relationship between the bloom-produced POM and the sea ice meltwater inventory was observed, indicating that the net sea ice loss promotes early bloom in the polynya.Given the high fraction of bloom-produced POM, the early bloom of phytoplankton in the polynyas may play an important role on marine production and POM export in the Arctic shelves.  相似文献   

Organic biomarkers in the twilight zone—Time series and settling velocity sediment traps during MedFlux     

Stuart G. Wakeham  Cindy Lee  Michael L. Peterson  Zhanfei Liu  Jennifer Szlosek  Isabel F. Putnam  Jianhong Xue   《Deep Sea Research Part II: Topical Studies in Oceanography》2009,56(18):1437-1453

The transfer of material through the twilight zone of the ocean is controlled by sinking particles that contain organic matter (OM) and mineral ballast. During the MedFlux field program in the northwestern Mediterranean Sea in 2003, sinking particulate matter was collected in time series (TS) and settling velocity (SV) traps and analyzed for amino acids, lipids, and pigments (along with ballast minerals) [Lee, C., Armstrong, R.A., Wakeham, S.G., Peterson, M.L., Miquel, J.C., Cochran, J.K., Fowler, S.W., Hirschberg, D., Beck, A. Xue, J., 2009b. Particulate matter fluxes in time series and settling velocity sediment traps in the northwestern Mediterranean Sea. Deep-Sea Research II, this volume [doi:10.1016/j.dsr2.2008.12.003]]. The goal was to identify how organic chemical compositions of sinking particles varied as a function of their in-situ settling velocity. The TS record was used to define the biogeochemical character and temporal pattern in flux during the period of SV trap deployment. Temporal variations in organic and mineral compositions are consistent with particle biogeochemistry being driven by the seasonal succession of phytoplankton. Spring diatom bloom conditions led to a high flux of rapidly sinking aggregates and zooplankton fecal matter; summer oligotrophy followed and was characterized by a higher proportion of slowly sinking phytoplankton cells. Bacterial degradation is particularly important during the low-flux summer period. Settling velocity traps show that a large proportion of particulate organic matter sinks at 200–500 m d⁻¹. Organic compositions of this fast-sinking material mirrors that of fecal pellets and aggregated material that sinks as the spring bloom terminates. More-slowly sinking OM bears a stronger signature of bacterial degradation than do the faster-sinking particles. The observation that compositions of SV-sorted fractions are different implies that the particle field is compositionally heterogeneous over a range of settling velocities. Thus physical and biological exchange between fast-sinking and slow-sinking particles as they pass down the water column must be incomplete.  相似文献