Cosmic ray composition estimation below the knee of the spectrum from the time structure of erenkov light in EAS     

M. D. Roberts 《Astroparticle Physics》2000,12(4):471

Monte Carlo simulations show that the pulse profile of erenkov photons measured near the core of an extensive air shower is sensitive to the secondary muon/electron ratio of the cascade. erenkov pulses can easily be measured with a single large area mirror viewed by a photomultiplier tube subtending a small field of view (1°). Even for such a simple experiment, exposed to EAS from a range of core locations and arrival directions, strong statistical differences are shown to exist between the pulse parameter distributions of primary protons and those of heavier primary particles. A range of primary energies can be investigated by varying the zenith angle of observations. In this paper, results from simulations of primaries in the energy range 20 TeV to 400 TeV are presented, although in principle the technique could be extended to include the knee of the spectrum. At the lower end of this energy range results can be compared to direct measurements of the composition, while measurements at the upper end can augment results from existing ground based experiments.  相似文献   

Formation of iron sulfide nodules during anaerobic oxidation of methane   总被引：1，自引：0，他引：1  

Bart E. van Dongen  Andrew P. Roberts  Wei-Teh Jiang  Richard D. Pancost 《Geochimica et cosmochimica acta》2007,71(21):5155-5167

The biomarker compositions of iron sulfide nodules (ISNs; upper Pliocene Valle Ricca section near Rome, Italy) that contain the ferrimagnetic mineral greigite (Fe₃S₄) were examined. In addition to the presence of specific terrestrial and marine biomarkers, consistent with formation in coastal marine sediments, these ISNs contain compounds thought to originate from sulfate reducing bacteria (SRB). These compounds include a variety of low-molecular-weight and branched alkanols and several non-isoprenoidal dialkyl glycerol diethers (DGDs). In addition, archaeal biomarkers, including archaeol, macrocyclic isoprenoidal DGDs and isoprenoidal glycerol dialkyl glycerol tetraethers are also present. Both SRB and archaeal lipid δ¹³C values are depleted in ¹³C (δ¹³C values are typically less than −50‰), which suggests that the SRB and archaea consumed ¹³C depleted methane. These biomarker and isotopic signatures are similar to those found in cold seeps and marine sediments where anaerobic oxidation of methane (AOM) occurs with sulfate serving as the terminal electron acceptor. Association of AOM with formation of greigite-containing ISNs could provide an explanation for documented remagnetization of the Valle Ricca sediments. Upward migration of methane, subsequent AOM and associated authigenic greigite formation are widespread processes in the geological record that have considerable potential to compromise paleomagnetic records.  相似文献   

Geometry and chronology of growth and drowning of Middle Triassic carbonate platforms (Cernera and Bivera/Clapsavon) in the Southern Alps (northern Italy)     

Peter Brack  Hans Rieber  Roland Mundil  Wolfgang Blendinger  Florian Maurer 《Swiss Journal of Geoscience》2007,100(3):327-348

The depositional architecture and the geometric relationships between platform-slope deposits and basinal sediments along with paleontological evidence indicate the time interval of the younger Anisian Reitziites reitzi ammonoid zone to largely represent the main stage of platform aggradation at the Cernera and Bivera/Clapsavon carbonate platforms. Published and new U-Pb age data of zircons from volcaniclastic layers bracketing the stratigraphic interval of platform growth constrain the duration of platform evolution to a time span shorter than 1.8±0.7m.y., probably in the order of 0.5-1m.y., reflecting fast rates of vertical platform aggradation exceeding 500 m/m.y. In the range of growth potentials for shallow-water carbonate systems estimated in relation to the time span of observation, this high rate is in agreement with values for short intervals of 10⁵-10⁶yrs (e.g., Schlager 1999). After drowning, the platforms at Cernera and Bivera/Clapsavon were blanketed by thin pelagic carbonates. On the former platform flanks the draping sediments in places comprise red nodular pelagic limestones (Clapsavon Limestone) similar in facies to the Han Bulog Limestones occurring elsewhere in Middle Triassic successions of the Mediterranean Tethys. The drowning of vast areas of former carbonate platforms possibly triggered the onset of bottom-water circulation in adjacent basins as suggested by the abrupt transition from laminated to bioturbated pelagic nodular limestones in the Buchenstein Formation which occurred close to the time of initial platform submergence. During the Late Ladinian the topographic features of the drowned platforms were onlapped by rapidly deposited, predominantly clastic successions including coarse breccias and volcanic rocks sealing and preserving the peculiar stratigraphic setting.  相似文献   

Establishing the time of initial human occupation of Liang Bua, western Flores, Indonesia     

K.E. Westaway  M.J. Morwood  R.G. Roberts  J.-x. Zhao  T. Sutikna  E.W. Saptomo  W.J. Rink 《Quaternary Geochronology》2007,2(1-4):337-343

Liang Bua, a limestone cave in western Flores, has an archaeological and faunal sequence known to span the last 95 ka and is the type-site for a small-bodied hominid species, Homo floresiensis. This paper describes the geomorphic history of this significant site, and presents numerical ages for key events as determined by thermoluminescence dating of sediments (using a dual-aliquot regenerative-dose protocol) and uranium-series dating of flowstones. Our age estimates indicate that Liang Bua existed as a subterranean chamber by 400 ka ago. It was subsequently exposed by the incision of the Wae Racang River, which invaded the cave around 190 ka and deposited an upward-fining sequence of water-rolled boulders and cobbles. The latter conglomerate was partially reworked around 130 ka ago and was capped by flowstones at 100 ka. The conglomerate also contains stone artefacts, which implies that the occupation of the surrounding area is at least as old as the time of emplacement of the conglomerate. After 190 ka, a complex sequence of erosion and deposition led to the accumulation of at least 11 m of sediment in the front chamber of the cave, which proved suitable for hominid occupation. Such geochronological and geomorphological information is extremely valuable for interpreting the archaeological record at Liang Bua and its wider significance.  相似文献   

Variations in Melt Productivity and Melting Conditions along SWIR (70{degrees}E-49{degrees}E): Evidence from Olivine-hosted and Plagioclase-hosted Melt Inclusions   总被引：1，自引：0，他引：1  

Font  L.; Murton  B. J.; Roberts  S.; Tindle  A. G. 《Journal of Petrology》2007,48(8):1471-1494

Melt inclusion and host glass compositions from the easternend of the Southwest Indian Ridge show a progressive depletionin light rare earth elements (LREE), Na₈ and (La/Sm)_n, but anincrease in Fe₈, from the NE (64°E) towards the SW (49°E).These changes indicate an increase in the degree of mantle meltingtowards the SW and correlate with a shallowing of the ridgeaxial depth and increase in crustal thickness. In addition,LREE enrichment in both melt inclusions and host glasses fromthe NE end of the ridge are compatible with re-fertilizationof a depleted mantle source. The large compositional variations(e.g. P₂O₅ and K₂O) of the melt inclusions from the NE end ofthe ridge (64°E), coupled with low Fe₈ values, suggest thatmelts from the NE correspond to a variety of different batchesof melts generated at shallow levels in the mantle melting column.In contrast, the progressively more depleted compositions andhigher Fe₈ values of the olivine- and plagioclase-hosted meltinclusions at the SW end of the studied region (49°E), suggestthat these melt inclusions represent batches of melt generatedby higher degrees of melting at greater mean depths in the mantlemelting column. Systematic differences in Fe₈ values betweenthe plagioclase- and the olivine-hosted melt inclusions in theSW end (49°E) of the studied ridge area, suggest that theplagioclase-hosted melt inclusions represent final batches ofmelt generated at the top of the mantle melting column, whereasthe olivine-hosted melt inclusions correspond to melts generatedfrom less depleted, more fertile mantle at greater depths. KEY WORDS: basalt; melt inclusions; olivine; plagioclase; Southwest Indian Ridge  相似文献   

The energy balance experiment EBEX-2000. Part II: Intercomparison of eddy-covariance sensors and post-field data processing methods     

Matthias Mauder  Steven P. Oncley  Roland Vogt  Tamas Weidinger  Luis Ribeiro  Christian Bernhofer  Thomas Foken  Wim Kohsiek  Henk A. R. De Bruin  Heping Liu 《Boundary-Layer Meteorology》2007,123(1):29-54

The eddy-covariance method is the primary way of measuring turbulent fluxes directly. Many investigators have found that these flux measurements often do not satisfy a fundamental criterion—closure of the surface energy balance. This study investigates to what extent the eddy-covariance measurement technology can be made responsible for this deficiency, in particular the effects of instrumentation or of the post-field data processing. Therefore, current eddy-covariance sensors and several post-field data processing methods were compared. The differences in methodology resulted in deviations of 10% for the sensible heat flux and of 15% for the latent heat flux for an averaging time of 30 min. These disparities were mostly due to different sensor separation corrections and a linear detrending of the data. The impact of different instrumentation on the resulting heat flux estimates was significantly higher. Large deviations from the reference system of up to 50% were found for some sensor combinations. However, very good measurement quality was found for a CSAT3 sonic together with a KH20 krypton hygrometer and also for a UW sonic together with a KH20. If these systems are well calibrated and maintained, an accuracy of better than 5% can be achieved for 30-min values of sensible and latent heat flux measurements. The results from the sonic anemometers Gill Solent-HS, ATI-K, Metek USA-1, and R.M. Young 81000 showed more or less larger deviations from the reference system. The LI-COR LI-7500 open-path H₂O/CO₂ gas analyser in the test was one of the first serial numbers of this sensor type and had technical problems regarding direct solar radiation sensitivity and signal delay. These problems are known by the manufacturer and improvements of the sensor have since been made. The National Center for Atmospheric Research is supported by the National Science Foundation.  相似文献   

Strain parameters of discontinuities in rock for finite element calculation   总被引：1，自引：0，他引：1  

Roland Pusch  Lennart Brgesson 《Engineering Geology》1998,49(3-4):201-206

The bulk strain induced by excavation of rooms for storage of highly radioactive waste is usually unimportant, but the change in aperture and shear displacement of permeable fractures may be of practical importance. Hence, the normal and shear of such discontinuities have to be considered and are commonly predicted by using special deformation moduli termed “normal” and “shear” stiffnesses in numerical calculations. For the use of some finite element methods, these have to be converted to compression and shear moduli ,which, in turn, requires that the thickness of the discontinuity is known. The normal and shear strain can be expressed in terms of soil mechanical parameters, yielding the compression modulus M and the shear modulus G, with the required form for finite element calculations. By definition they are functions of the normal and shear stiffness K_n and K_s. It is concluded that calculation of the normal and shear strain of discontinuities with clastic fillings by the use of finite element methods and deformation moduli derived from stiffness numbers is very uncertain except when the geometry of the weaknesses can be accurately defined.  相似文献   

The magnetisation of Rosemary Bank Seamount,Rockall Trough,northeast Atlantic     

P.R. Miles  D.G. Roberts 《Earth and Planetary Science Letters》1981,54(3):442-448

Rosemary Bank is a non-uniformly magnetised seamount in the northern Rockall Trough. The reversely magnetised major component of the anomaly field was simulated by a numerical method and modelled using the Talwani three-dimensional magnetics program. The results suggest a higher Koenigsberger ratio than earlier reported for Rosemary Bank and a remanent magnetisation vector compatible with post-Jurassic formation and probably of a Late Cretaceous to Tertiary age. The limited depth to the base of the model implies that Rosemary Bank post-dates the underlying basement in agreement with a volcanic origin. The residual of the observed anomaly field is interpreted as being caused by normally magnetised bodies within and on top of the bank. This suggests subsequent volcanic activity during an interval of normal polarity.  相似文献   

Age and structure of the southern Rockall Trough new evidence     

D.G. Roberts  D.G. Masson  P.R. Miles 《Earth and Planetary Science Letters》1981,52(1):115-128

The Rockall Trough separates the Rockall Plateau microcontinent from the shelf and slope west of the British Isles. The structure and age of the trough has been the source of considerable discussion. Although widely considered to be of oceanic origin, postulated ages for the spreading range from Permian to Cretaceous. New seismic profiles linked to the IPOD sites in the Bay of Biscay and to oceanic anomalies of known age are used to present a new assessment of the age and structure of the southern Rockall Trough. It is concluded that about 120 km of ocean crust is present in the trough and that spreading took place in the Albian-Maastrichtian interval.  相似文献   

Palaeomagnetic evidence for large-scale dextral movement along the Trollfjord-Komagelv Fault,Finnmark, north Norway     

J. Kjøde  K.M. Storetvedt  D. Roberts  A. Gidskehaug 《Physics of the Earth and Planetary Interiors》1978,16(2):132-144

Palaeomagnetic data from Late Precambrian dykes from the northern part of Varanger peninsula, north Norway, suggest a two-axis magnetization structure. The dominant component is considered to be syn- to late-tectonic and probably acquired at around 640 m.y. B.P. Superposed on this magnetization is a minor component which is compatible with the relative Lower-Middle Palaeozoic field; i.e. it was most likely imposed during the climax of the Caledonian orogenic movements in north Norway. The estimated relative Late Precambrian palaeopole cannot easily be reconciled with the European Late Precambrian polar path. This disagreement can be accounted for by assuming a post-magnetization dextral megashear, of the order of 500–1000 km, along the Trollfjord-Komagelv fracture zone. This type of displacement is in line with geological evidence and the palaeomagnetic reconstruction supports the long-held view of there having been continuity between the depositional environments of the Varanger Peninsula Barents Sea Group, the Eleonore Bay Group of east Greenland and the Hecla Hoek Formation of east Spitsbergen. The character and age of the horizontal displacement, post-640 to pre-500 m.y. B.P., is seen in conjunction with the opening up of the lapetus Ocean and reactivation of ancient deep-seated fractures during both the spreading and the contraction phases of ocean development.  相似文献