首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Study of several cosmic ray effects, such as VH track density, spallogenic26Al and53Mn activity,21Ne and22Ne/21Ne ratio, made in the same sample or in cores taken from different meteorites can identify parameters related to the exposure history of meteorites and cosmic ray flux variations. Meteorites with single or multiple exposure can be distinguished from a track production rate —22Ne/21Ne correlation diagram and cosmic ray flux variations over 106–107 years can be deduced from a three-isotope correlation diagram of26Al,53Mn and21Ne. Isotopic data based on chondrites with simple, one-stage exposure are consistent with the same average galactic cosmic ray intensity over the past 2 million years as that during the past 107 years.  相似文献   

2.
Long term characteristics of solar and galactic cosmic rays, as revealed by the study of their nuclear effects in lunar, meteoritic and terrestrial samples are summarised. The data so far available on radioisotopes, noble gases and tracks, though limited, are consistent with nearly constant fluxes and composition during different epochs over billions of years; one exception is14C activity in the earths atmosphere over the past few hundred years, suggesting a variation in the solar activity. Other small or brief variations, which cannot be ruled out as yet, require better estimation of depth and size dependence of nuclear effects in rocks before they can be attributed to cosmic rays.  相似文献   

3.
Variations in gravitational potential energy contribute to the intraplate stress field thereby providing the means by which lithospheric density structure is communicated at the plate scale. In this light, the near equivalence in the gravitational potential energy of typical continental lithosphere with the mid‐ocean ridges is particularly intriguing. Assuming this equivalence is not simply a chance outcome of continental growth, it then probably involves long‐term modulation of the density configuration of the continents via stress regimes that are able to induce significant strains over geological time. Following this notion, this work explores the possibility that the emergence of a chemically, thermally and mechanically structured continental lithosphere reflects a set of thermally sensitive feedback mechanisms in response to Wilson cycle oscillatory forcing about an ambient stress state set by the mid‐ocean ridge system. Such a hypothesis requires the continents are weak enough to sustain long‐term (108 years) strain rates of the order of ～10?17 s?1 as suggested by observations that continental lithosphere is almost everywhere critically stressed, by estimates of seismogenic strain rates in stable continental interiors such as Australia and by the low‐temperature thermochronological record of the continents that requires significant relief generation on the 108 year time‐scale. Furthermore, this notion provides a mechanism that helps explain interpretations of recently published heat flow data that imply the distribution of heat‐producing elements within the continents may be tuned to produce a characteristic thermal regime at Moho depths.  相似文献   

4.
A process‐based model that simulates fluvial erosion in the River Somme Valley over the last million years is presented here. The model takes into account lithology and climatic influences and allows the simulating of undercapacity and overcapacity sediment transport behaviour. The model has been calibrated to a family of terraces within the River Somme Valley. When matched to this field data, simulation trials suggest that bedrock incision occurred principally from 120 to 60–40 kyr during the last climatic cycle and before the last glaciation. The impact of a progressive tectonic uplift (c. 60 m over c. 1 million years) on the River Somme has also been studied here. Extended over a longer period of time, the simulations suggest that 1 million years ago the profile of the River Somme had a lower slope gradient than today, with little relief throughout the Paris Basin.  相似文献   

5.
The results derived from geological data show that the half-spreading rate between the African and South American plates has remained relatively constant at 2 cm/year over the past 80 million years (Silver et al. in Science 279:60–63, 1998). In this paper, we have reestimated a new relative angular velocity of Africa–South America plates using the selected space geodetic station data through a new method. Our angular velocity estimates the spreading rates of Africa–South America plate over several years that are similar in azimuth but significantly slower in rate than the NUVEL-1A predictions averaged over the past 3 million years. The implied rates of deceleration coincide with longer-term trends over the past 35 million years and may reflect the effects of plate interaction and coupling of Africa–South America plates. An erratum to this article can be found at  相似文献   

6.
We consider the evolution of galaxies in dense galactic clusters. Observations and theoretical estimates indicate that this evolution may be specified to a large extent by collisions between galaxies, as well as interactions between the gaseous components of disk galaxies and intergalactic gas. We analyze collisions between disk galaxies with gaseous components using a simple model based on a comparison of the duration of a collision and the characteristic cooling time for the gas heated by the collision, and also of the relative masses of stars and gas in the colliding disk galaxies. This model is used to analyze scenarios for collisions between disk galaxies with various masses as a function of their relative velocities. Our analysis indicates that galaxies can merge, lose one or both of their gaseous components, or totally disintegrate as a result of a collision; ultimately, a new galaxy may form from the gas lost by the colliding galaxies. Disk galaxies with mass M G and velocities exceeding ～300 (M G/1010 M ⊙)1/2 km/s in intergalactic gas in clusters with densities ～10?27 g/cm3 can lose their gas due to the pressure of inflowing intergalactic gas, thereby developing into E(SO) galaxies.  相似文献   

7.
Cenozoic climatic and environmental changes in the arid Asian interior, and their possible relations with global climatic changes and the Tibetan Plateau uplift, have been intensively investigated and debated over past decades. Here we present 40-Myr （million years）-long n-alkane records from a continuous Cenozoic sediment sequence in the Dahonggou Section, Qaidam Basin, northern Tibetan Plateau, to infer environmental changes in the northern basin. A set of n-alkane indexes, including ACL, CPI and Paq, vary substantially and consistently throughout the records, which are interpreted to reflect relative contributions from terrestrial vascular plants vs. aquatic macrophytes, and thus indicate depositional environments. ACL values vary between 21 and 30; CP1 values range from 1.0 to 8.0; and Paq values change from 〈0.1 to 0.8 over the past 40-Myr. We have roughly identified two periods, at 25.8-21.0 Ma （million years ago） and 13.0-17.5 Ma, with higher ACL and CPI and lower Paq values indicating predominant lacustrine environments. Lower ACL and CPI values, together with higher Paq values, occurred at 〉25.8 Ma, 17.5-21.0 Ma, and 〈13.0 Ma, corresponding to alluvial fan/river deltaic deposits and shallow lacustrine settings, consistent with the observed features in sedimentological facies. The inferred Cenozoic environmental changes in the northern Qaidam Basin appear to correspond to global climatic changes.  相似文献   

8.
Cosmogenic nuclides in extraterrestrial matter provide a wealth of information on the exposure and collision histories of small objects in space and on the history of the solar and galactic cosmic radiation. The interpretation of the observed abundances of cosmogenic nuclides requires detailed and accurate knowledge of their production rates. Accelerator experiments provide a quantitative basis and the ground truth for modeling cosmogenic nuclide production by measurements of the relevant cross sections and by realistic simulations of the interaction of galactic protons with meteoroids under completely controlled conditions, respectively. We review the establishment of physical model calculations of cosmogenic nuclide production in extraterrestrial matter on the basis of such accelerator experiments and exemplify this approach by presenting new experimental and theoretical results for the cosmogenic nuclide44Ti. The model calculations describe all aspects of cosmogenic nuclide production and allow the determination of long-term solar and galactic cosmic ray spectra and a consistent interpretation of cosmogenic nuclides in extraterrestrial matter.  相似文献   

9.
The flux of particulate extraterrestrial (ET) matter to the deep-sea has been calculated using a four-component mixing model based on osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the Pacific Ocean. Nineteen published bulk-leach osmium isotope data pairs that cover the last 80 million years have been used for the calculation. The calculated annual particulate flux, averaged over several 100 ka by slow accumulation and bioturbation, ranges from ∼18,000 to ∼67,000 tons, with a mean value of ∼37,000 ± 13,000 tons. The data indicate no significant variability in the flux with time, except at the K-T boundary and are thus compatible with results based on Ir accumulation in deep-sea sediments. The inferred constancy of the ET matter flux is in contrast to recent results based on the ET 3He flux that integrates only a small size fraction of the entire ET matter flux. 3He data indicate variability by a factor of 6 over the past 70 Ma and a fivefold increase in the ET flux between 3 Ma and 1 Ma that is not seen in the bulk ET matter flux based on osmium isotope data.The apparent constancy of the cosmic matter flux over the past 80 Ma stands in marked contrast to the dynamic nature of the marine osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record, except at the K-T boundary. A comparison between osmium isotope and Ir data from two pelagic sediment cores from the Pacific (DSDP Site 596 and LL44-GPC3) indicates that the seawater-soluble fraction of ET Os cannot exceed ∼36 kg/a and most likely is significantly smaller. The maximum value of 36 kg/a can account for approximately one half of the seawater-soluble ET matter flux necessary to balance the radiogenic continental runoff of Os without any additional unradiogenic source. Simple mass balance calculations indicate that an additional unradiogenic source of Os to the oceans, most likely alteration of oceanic crust, is required to balance the present-day seawater osmium-isotopic composition. This source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater.  相似文献   

10.
京津冀地区国土资源环境地质条件分析   总被引：5，自引：4，他引：1       下载免费PDF全文
统计分析了京津冀地区土地资源、地下水、湿地、矿产、地热和地质景观等资源条件分布,结果显示,平原区土壤质量总体良好,良好及以上等级土壤分布面积约占平原区面积的80.89%,适宜种植绿色农产品的面积为96363 km~2,富硒耕(园)地面积为1894 km~2;地下水可开采资源总量为188亿m~3/a,但呈现空间分布不均的特征;衡水湖等五大湿地分布面积约为614 km~2,近30年来减少了35.57%;金属矿产和非金属矿产资源丰富,例如铁矿资源储量98.4亿t,铜矿资源量111.50万t,石油地质储量249635.02万t;地热资源丰富,开发利用地热资源可替代3.43亿t标准煤;地质遗迹资源丰富,约有300余处可纳入环首都国家公园规划建设。同时,分析了活动断裂与地震、地面沉降、地裂缝、崩滑流和地面塌陷、地下水污染和湿地退化等主要环境地质问题现状;在此基础上,针对城镇发展和重要基础设施建设、湿地保护与修复、地下水资源开发利用、优质耕地资源保护和地质遗迹资开发利用等方面提出了地学建议,为区域规划建设提供地质安全保障和资源保障。  相似文献   

11.
The structure and evolution of the zero-acceleration surface around wide triple systems of galaxies are studied in detail. (The zero-acceleration surface is the boundary separating regions in which (i) the Newtonian gravitational attraction of the galactic matter and (ii) the Einsteinian universal repulsion of the cosmic vacuum dominate.) For a typical system, this surface is spherical in shape and several megaparsecs in size, and remains nearly unchanged throughout the lifetime of the system. The concept of a boundary surface can also be extended to systems on the largest possible scales, and its general properties are discussed in relation to clusters, superclusters, and voids.  相似文献   

12.
We present a new reconstruction of summer sea‐surface salinity (SSS) over the past 15 000 years based on a diatom record from piston core 17940, located on the northern slope of the South China Sea (SCS). The reconstructed diatom‐based summer SSS values for the modern period are in accord with instrumental observations of summer SSS in the area. Here, the modern summer SSS is primarily controlled by river runoff, in particular from the Pearl River. The reconstruction presented in this study shows that the summer SSS varied between 33.3 and 34.2 psu over the past 15 000 years. The long‐term summer SSS trend closely followed the trend of the orbitally controlled solar insolation at 20°N, suggesting that orbital forcing was the dominant driver of changes in summer SSS in this area. Comparisons to speleothem δ18O data and studies of surface hydrography in the region suggest that changes in solar insolation affected the summer SSS through changes in the East Asian Monsoon and sea‐level changes associated with the last deglaciation. Univariate spectral analyses indicate that centennial‐scale oscillatory variations in summer SSS were superimposed on the long‐term trend. During the deglacial period (c. 12 000–9000 cal. a BP), the dominant periodicity was centred around 230–250 years, whereas a ～350‐year oscillation dominated in the period 2200–4500 cal. a BP. The balance of evidence suggests that these centennial‐scale changes in summer SSS may have been driven by solar‐induced changes in the East Asian Monsoon, but further evidence is needed to firmly establish this relationship.  相似文献   

13.
《Sedimentology》2018,65(5):1667-1696
Multi‐proxy analysis of sediment cores from five key locations in hypersaline, alkaline Lake Bogoria (central Kenya Rift Valley) has allowed reconstruction of its history of depositional and hydrological change during the past 1300 years. Analyses including organic matter and carbonate content, granulometry, mineralogical composition, charcoal counting and high‐resolution scanning of magnetic susceptibility and elemental geochemistry resulted in a detailed sedimentological and compositional characterization of lacustrine deposits in the three lake basins and on the two sills separating them. These palaeolimnological data were supplemented with information on present‐day sedimentation conditions based on seasonal sampling of settling particles and on measurement of physicochemical profiles through the water column. A new age model based on 210Pb, 137Cs and 14C dating captures the sediment chronology of this hydrochemically complex and geothermally fed lake. An extensive set of chronological tie points between the equivalent high‐resolution proxy time series of the five sediment sequences allowed transfer of radiometric dates between the basins, enabling interbasin comparison of sedimentation dynamics through time. The resulting reconstruction demonstrates considerable moisture‐balance variability through time, reflecting regional hydroclimate dynamics over the past 1300 years. Between ca 690 and 950 AD , the central and southern basins of Lake Bogoria were reduced to shallow and separated brine pools. In the former, occasional near‐complete desiccation triggered massive trona precipitation. Between ca 950 and 1100 AD , slightly higher water levels allowed the build‐up of high pCO 2 leading to precipitation of nahcolite still under strongly evaporative conditions. Lake Bogoria experienced a pronounced highstand between ca 1100 and 1350 AD , only to recede again afterwards. For a substantial part of the time between ca 1350 and 1800 AD , the northern basin was probably disconnected from the united central and southern basins. Throughout the last two centuries, lake level has been relatively high compared to the rest of the past millennium. Evidence for increased terrestrial sediment supply in recent decades, due to anthropogenic soil erosion in the wider Bogoria catchment, is a reason for concern about possible adverse impacts on the unique ecosystem of Lake Bogoria.  相似文献   

14.
Geochronological data, combined with field and petrological evidence, constrain the timing and rate of near‐isothermal decompression at granulite facies temperatures in rocks from the Lützow‐Holm Complex of East Antarctica. Granulite facies gneisses from Rundvågshetta in Lützow‐Holm Bay experienced a peak metamorphic temperature of over 900 °C at c. 11 kbar, as evidenced by primary orthopyroxene–sillimanite‐bearing assemblages, and secondary cordierite–sapphirine‐bearing assemblages in metapelites. Peak metamorphic assemblages show strong preferred mineral orientation, interpreted to have developed synchronously with pervasive ductile deformation. Zircon from a syndeformational leucosome has a U–Pb age of 517±9 Ma, which is interpreted as a melt crystallization age. This age provides the best estimate of the time of peak metamorphic conditions. The post‐peak metamorphic history is characterized by near‐isothermal decompression, recorded by mineral textures in a variety of rock compositions. Field and textural relations indicate that decompression post‐dated pervasive ductile deformation. K/Ar and 40Ar/39Ar ages from hornblende and biotite represent closure ages during cooling subsequent to decompression, and indicate cooling to temperatures between c. 350 and 300 °C by c. 500 Ma, thus placing a lower time limit on the duration of the high‐temperature isothermal decompression episode. The combination of the zircon age from a syndeformational melt with K/Ar and 40Ar/39Ar closure ages indicates that near‐isothermal decompression from c. 11 to c. 4 kbar at granulite facies temperatures, followed by cooling to c. 300 °C, took place within a time interval of 20±10 Myr. Simple one‐dimensional models for exhumation‐controlled cooling indicate that these data require exhumation rates of the order of c. 3 km Myr?1 for several million years, then cessation of exhumation followed by relatively isobaric cooling during thermal re‐equilibration.  相似文献   

15.
The Halls Creek Orogen in northern Australia records the Palaeoproterozoic collision of the Kimberley Craton with the North Australian Craton. Integrated structural, metamorphic and geochronological studies of the Tickalara Metamorphics show that this involved a protracted episode of high‐temperature, low‐pressure metamorphism associated with intense and prolonged mafic and felsic intrusive activity in the interval ca 1850–1820 Ma. Tectonothermal development of the region commenced with an inferred mantle perturbation event, probably at ca 1880 Ma. This resulted in the generation of mafic magmas in the upper mantle or lower crust, while upper crustal extension preceded the rapid deposition of the Tickalara sedimentary protoliths. An older age limit for these rocks is provided by a psammopelitic gneiss from the Tickalara Metamorphics, which yield a 207Pb/206Pb SHRIMP age of 1867 ± 4 Ma for the youngest detrital zircon suite. Voluminous layered mafic intrusives were emplaced in the middle crust at ca 1860–1855 Ma, prior to the attainment of lower granulite facies peak metamorphic conditions in the middle crust. Locally preserved layer‐parallel D1 foliations that were developed during prograde metamorphism were pervasively overprinted by the dominant regional S2 gneissosity coincident with peak metamorphism. Overgrowths on zircons record a metamorphic 207Pb/206Pb age of 1845 ± 4 Ma. The S2 fabric is folded around tight folds and cut by ductile shear zones associated with D3 (ca 1830 Ma), and all pre‐existing structures are folded around large‐scale, open F4 folds (ca 1820 Ma). Construction of a temperature‐time path for the mid‐crustal section exposed in the central Halls Creek Orogen, based on detailed SHRIMP zircon data, key field relationships and petrological evidence, suggests the existence of one protracted thermal event (>400–500°C for 25–30 million years) encompassing two deformation phases. Protoliths to the Tickalara Metamorphics were relatively cold (～350°C) when intruded by the Fletcher Creek Granite at ca 1850 Ma, but were subsequently heated rapidly to 700–800°C during peak metamorphism at ca 1845 Ma. Repeated injection of mafic magmas caused multiple remelting of the metasedimentary wall rocks, with mappable increases in leucosome volume that show a strong spatial relationship to these intrusives. This mafic igneous activity prolonged the elevated geotherm and ensured that the rocks remained very hot (≥650°C) for at least 10 million years. The Mabel Downs Tonalite was emplaced during amphibolite facies metamorphism, with intrusion commencing at ca 1835 Ma. Its compositional heterogeneity, and the presence of mutual cross‐cutting relations between ductile shear zones and multiple injections of mingled magma suggest that it was emplaced syn‐D3. Broad‐scale folding attributable to F4 was accompanied by widespread intrusion of granitoids, and F4 fold limbs are truncated by large, mostly brittle retrograde S4 shear zones.  相似文献   

16.
Microbial life below the Earth's surface (the deep biosphere) has probably varied significantly since the Archaean. Reconstructing changes in deep biosphere activity over geological timescales is necessary to understand its role in biogeochemical cycling. Even for the last few million years, such changes are often not captured by studying the distribution of present activity. However, several studies using samples from scientific drilling have revealed mineralogical, geochemical, isotopic and fossil organic molecule imprints in the sedimentary record that document rather different past deep biosphere conditions. Changing deep biosphere conditions can also be simulated using geochemical models. While some processes occurring in the past can be understood by comparing them with the present deep biosphere, others lack any modern analogue – they are defined as non‐actualistic. A non‐actualistic consideration of the deep biosphere is therefore essential for a better understanding of how Earth and life co‐evolved through time.  相似文献   

17.

Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) analysis of zircons confirm a Late Devonian to Early Carboniferous age (ca 360–350 Ma) for silicic volcanic rocks of the Campwyn Volcanics and Yarrol terrane of the northern New England Fold Belt (Queensland). These rocks are coeval with silicic volcanism recorded elsewhere in the fold belt at this time (Connors Arch, Drummond Basin). The new U–Pb zircon ages, in combination with those from previous studies, show that silicic magmatism was both widespread across the northern New England Fold Belt (>250 000 km2 and ≥500 km inboard of plate margin) and protracted, occurring over a period of ～15 million years. Zircon inheritance is commonplace in the Late Devonian — Early Carboniferous volcanics, reflecting anatectic melting and considerable reworking of continental crust. Inherited zircon components range from ca 370 to ca 2050 Ma, with Middle Devonian (385–370 Ma) zircons being common to almost all dated units. Precambrian zircon components record either Precambrian crystalline crust or sedimentary accumulations that were present above or within the zone of magma formation. This contrasts with a lack of significant zircon inheritance in younger Permo‐Carboniferous igneous rocks intruded through, and emplaced on top of, the Devonian‐Carboniferous successions. The inheritance data and location of these volcanic rocks at the eastern margins of the northern New England Fold Belt, coupled with Sr–Nd, Pb isotopic data and depleted mantle model ages for Late Palaeozoic and Mesozoic magmatism, imply that Precambrian mafic and felsic crustal materials (potentially as old as 2050 Ma), or at the very least Lower Palaeozoic rocks derived from the reworking of Precambrian rocks, comprise basement to the eastern parts of the fold belt. This crustal basement architecture may be a relict from the Late Proterozoic breakup of the Rodinian supercontinent.  相似文献   

18.
The elastic moduli of magnesioferrite spinel, MgFe2O4, and their temperature dependence have been determined for the first time by ultrasonic measurements on a polycrystalline specimen. The measurements were carried out at 300 MPa and to 700°C in a gas-medium high-pressure apparatus. On heating, both the elastic bulk (K S) and shear (G) moduli decrease linearly to 350°C. By combining with extant thermal-expansion data, the values for the room-temperature K S and G, and their temperature derivatives are as follows: K 0 = 176.3(7) GPa, G 0 = 80.1(2) GPa, (∂K S/∂T) P = −0.032(3) GPa K−1 and (∂G/∂T) P = −0.012(1) GPa K−1. Between 350 and 400°C, there are abrupt increases of 1.4% in both of the elastic moduli; these closely coincide with the magnetic Curie transition that was observed by thermal analyses at about 360°C.  相似文献   

19.

The paper highlights the importance of using site-specific shear modulus reduction (G/Gmax versus shear strain, γ) curves and damping ratio (D versus shear strain, γ) curves for ground response analysis. In order to develop comprehensive G/Gmax–γ and D–γ curves (i.e. over a wide range of strain level), two types of apparatus, viz. resonant column and cyclic simple shear, have been used. The case study considered the geological deposits from the river beds of Yamuna River originating from the Himalayan seismic zone of North India. The tests results have been analysed to develop G/Gmax–γ and D–γ curves and compared with standard curves. It has been observed that upper and lower boundaries for the standard curves are remarkably different for the geological deposit under consideration. In order to assess the impact of using standard curves rather than site-specific curves, ground response analysis has been carried out at five sites along the Yamuna River using two types of curves (standard and site-specific developed in this study). The study showed that the amplification of shear waves at these sites based on the experimentally derived curves is much higher as compared to the standard curves. The proposed curves better represent dynamic behaviour of the soil deposits of the region and will provide a realistic response as far as practically possible, for the structures constructed in the states of Haryana and Delhi and nearby areas. It is anticipated that the data presented in this paper will have wide application and usage.

  相似文献   

20.
Gibraltar, a 6km2 peninsula jutting south from Spain at the western entrance to the Mediterranean Sea, is dominated by its 424 m‐high Rock: famous as a landmark to seafarers since ancient times. Twenty‐five years ago, an article in Geology Today (1991, v.7, pp.95–101) interpreted the Rock as a partly overturned mass of Early Jurassic dolomitic limestone, thrust into position during continent–continent collision about 15–20 million years before present and shaped finally by shoreline processes active during Quaternary tectonic uplift. Later articles featured the complex of tunnels and chambers excavated between 1782 and 1968 by British military engineers within its 2.6 km‐long main ridge (1992, v.8, pp.92–98), and the Neanderthals known to have used some of its caves (1997, v.13, pp.179–184). Significant aspects of the Rock's geological history have clarified since then.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号