Evaluating approximations for halo merging histories     

Rachel S. Somerville  Gerard Lemson  Tsafrir S. Kolatt  Avishai Dekel 《Monthly notices of the Royal Astronomical Society》2000,316(3):479-490

We study the merging history of dark matter haloes in N -body simulations and semi-analytical 'merger trees' based on the extended Press–Schechter (EPS) formalism. The main focus of our study is the joint distribution of progenitor number and mass as a function of redshift and parent halo mass. We begin by investigating the mean quantities predicted directly by the Press–Schechter (PS) and EPS formalism, such as the halo mass and conditional mass functions, and compare these predictions with the results of the simulations. The higher moments of this distribution are not predicted by the EPS formalism alone and must be obtained from the merger trees. We find that the Press–Schechter model deviates from the simulations at the level of 30–50 per cent on certain mass scales, and that the sense of the discrepancy changes as a function of redshift. We show that this discrepancy is reflected in the higher moments of the distribution of progenitor mass and number. We investigate some related statistics such as the accretion rate and the mass ratio of the largest two progenitors. For galaxy sized haloes ( M ∼10¹² M_⊙), we find that the merging history of haloes, as represented by these statistics, is well reproduced in the merger trees compared with the simulations. The agreement deteriorates for larger mass haloes. We conclude that merger trees based on the extended Press–Schechter formalism provide a reasonably reliable framework for semi-analytical models of galaxy formation.  相似文献   

Slip Sequences in Laboratory Experiments Resulting from Inhomogeneous Shear as Analogs of Earthquakes Associated with a Fault Edge     

Shmuel M. Rubinstein  Itay Barel  Ze’ev Reches  Oleg M. Braun  Michael Urbakh  Jay Fineberg 《Pure and Applied Geophysics》2011,168(12):2151-2166

Faults are intrinsically heterogeneous with common occurrences of jogs, edges and steps. We therefore explore experimentally and theoretically how fault edges may affect earthquake and slip dynamics. In the presented experiments and accompanying theoretical model, shear loads are applied to the edge of one of two flat blocks in frictional contact that form a fault analog. We show that slip occurs via a sequence of rapid rupture events that initiate from the loading edge and are arrested after propagating a finite distance. Each successive event extends the slip size, transfers the applied shear across the block, and causes progressively larger changes of the contact area along the contact surface. Resulting from this sequence of events, a hard asperity is dynamically formed near the loaded edge. The contact area beyond this asperity is largely reduced. These sequences of rapid events culminate in slow slip events that precede a major, unarrested slip event along the entire contact surface. We suggest that the 1998 M5.0 Sendai and 1995 off-Etorofu earthquake sequences may correspond to this scenario. Our work demonstrates, qualitatively, how the simplest deviation from uniform shear loading may significantly affect both earthquake nucleation processes and how fault complexity develops.  相似文献   

Controls on calcium isotope fractionation in cultured planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera     

B. K?sakürek  A. Eisenhauer  E.C. Hathorne  J. Erez 《Geochimica et cosmochimica acta》2011,75(2):427-5818

Specimens of two species of planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera, were grown under controlled laboratory conditions at a range of temperatures (18-31 °C), salinities (32-44 psu) and pH levels (7.9-8.4). The shells were examined for their calcium isotope compositions (δ^44/40Ca) and strontium to calcium ratios (Sr/Ca) using Thermal Ionization Mass Spectrometry and Inductively Coupled Plasma Mass Spectrometry. Although the total variation in δ^44/40Ca (∼0.3‰) in the studied species is on the same order as the external reproducibility, the data set reveals some apparent trends that are controlled by more than one environmental parameter. There is a well-defined inverse linear relationship between δ^44/40Ca and Sr/Ca in all experiments, suggesting similar controls on these proxies in foraminiferal calcite independent of species. Analogous to recent results from inorganically precipitated calcite, we suggest that Ca isotope fractionation and Sr partitioning in planktic foraminifera are mainly controlled by precipitation kinetics. This postulation provides us with a unique tool to calculate precipitation rates and draws support from the observation that Sr/Ca ratios are positively correlated with average growth rates. At 25 °C water temperature, precipitation rates in G. siphonifera and G. ruber are calculated to be on the order of 2000 and 3000 μmol/m²/h, respectively. The lower δ^44/40Ca observed at ?29 °C in both species is consistent with increased precipitation rates at high water temperatures. Salinity response of δ^44/40Ca (and Sr/Ca) in G. siphonifera implies that this species has the highest precipitation rates at the salinity of its natural habitat, whereas increasing salinities appear to trigger higher precipitation rates in G. ruber. Isotope effects that cannot be explained by precipitation rate in planktic foraminifera can be explained by a biological control, related to a vacuolar pathway for supply of ions during biomineralization and a pH regulation mechanism in these vacuoles. In case of an additional pathway via cross-membrane transport, supplying light Ca for calcification, the δ^44/40Ca of the reservoir is constrained as −0.2‰ relative to seawater. Using a Rayleigh distillation model, we calculate that calcification occurs in a semi-open system, where less than half of the Ca supplied by vacuolization is utilized for calcite precipitation. Our findings are relevant for interpreting paleo-proxy data on δ^44/40Ca and Sr/Ca in foraminifera as well as understanding their biomineralization processes.  相似文献   

Fabric development in a weathering profile at a basement–cover interface,the sub-Cambrian peneplain,Israel: Implications for decollement tectonics     

T. Angerer  R.O. Greiling  D. Avigad 《Journal of Structural Geology》2011,33(5):819-832

The present study examines a fossil saprock–saprolite–laterite-profile beneath the sub-Cambrian peneplain in the Pan-African Roded Granite, Israel, with regard to structure and magnetic fabrics (anisotropy of magnetic susceptibility, AMS), and image analysis of compaction. The deformed granite shows two pre-weathering foliations, S_1m (magmatic) and S_2g (gneissic). Pre-Early Cambrian weathering comprised weathering-brecciation in saprock and saprolite, and chemical weathering with clay-formation in saprolite and laterite. During subsequent Phanerozoic burial the laterite was vertically compacted to 73% of its original thickness. In the laterite, compaction produced an unconformity-parallel cleavage (S_3d) with increasing intensity towards the unconformity. Bulk susceptibility (κ_bulk) and anisotropy (P′) decrease from the unweathered granite into the saprolite, as a result of progressive magnetite breakdown, martitization and weathering-brecciation. In the laterite, an enrichment of haematite and relic Fe–Mg–mica lead to increased κ_bulk. Here, magnetic fabrics trace the compaction fabrics. The subhorizontal, compactional clay–/mica-fabric S_3d defines a structurally weak and impermeable layer. The mechanical weakness of a clay-enriched weathering horizon with an unconformity-parallel, planar shape-preferred orientation, combined with the potentially overpressured state due to the sealing character of such a zone provides a viable explanation for the abundant localization of decollement horizons at or beneath basement-cover interfaces.  相似文献   

Response of the Nile and its catchment to millennial-scale climatic change since the LGM from Sr isotopes and major elements of East Mediterranean sediments   总被引：1，自引：0，他引：1  

M.R. Box  M.D. Krom  R.A. Cliff  M. Bar-Matthews  A. Almogi-Labin  A. Ayalon  M. Paterne 《Quaternary Science Reviews》2011,30(3-4):431-442

Changes in ⁸⁷Sr/⁸⁶Sr and major element geochemistry, from two sediment cores (9509 and 9501) in the Eastern Mediterranean (EM), were used to resolve changes in sediment provenance and, hence, determine climate changes in the Nile catchment and Eastern Sahara desert over the past 25 ka. The sediment was described by a three end-member system comprising Blue Nile (BN; ⁸⁷Sr/⁸⁶Sr = 0.7506; Sr = 210 ppm), White Nile (WN; ⁸⁷Sr/⁸⁶Sr = 0.7094; Sr = 72.5 ppm) and Saharan dust (SD; ⁸⁷Sr/⁸⁶Sr = 0.7183; Sr = 99 ppm). The sedimentary record of these cores represents the suspended load carried down the Nile river and discharged into the S.E. Levantine basin and thus records palaeoclimatically controlled changes in erosion and transport in the catchment. During arid periods (0–5 ka BP) and prior to 11 ka BP, fluxes of BN sediment at 9509 (～6 g/cm²/yr & 10–12 g/cm²/yr, respectively) were greater than during the peak of the African Humid Period (AHP) from 5 to 11 ka BP (<2 g/cm²/yr); this latter period witnessed the deposition of the youngest organic-rich sediment, termed sapropel (S-1), in the EM basin. By contrast the flux of WN increased during the AHP from ～5 g/cm²/yr at ～13 ka BP to >15 g/cm²/yr. In the Ethiopian Highlands (BN catchment) increases in the amount and duration of the monsoon during the AHP caused more vegetation to grow resulting in less soil erosion. In the WN catchment increased rainfall caused more catchment erosion and higher sediment flux through the Sudd marshes. The sedimentation rate in core 9509 increased during the AHP because of the greater importance of the WN sediment flux relative to the BN sediment flux. Saharan dust flux also decreased during the AHP reaching a minimum at ～6 ka BP (core 9509) due to ‘greening’ of the Sahara desert. At the onset of S-1, the changes in Nile flow as determined by ⁸⁷Sr/⁸⁶Sr and climatic changes in the EM basin determined by δ¹⁸O of planktonic foraminifera were simultaneous, confirming that such isotopic tracers cannot be used directly to determine the cause of the circulation changes in the EM at this time. The increase in the proportion of BN sediment at 9509 with a somewhat higher grain size during the H-1 period (15–17 ka BP) was caused by erosion and redistribution of sediment from the Nile delta and/or the Israeli coast as sea-level rose.  相似文献   

Chemical and isotopic composition of diagenetic carbonate cements and its relation to hydrocarbon accumulation in the Heletz-Kokhav oil field (Israel)     

Ran Calvo  Avner Ayalon  Amos Bein  Eytan Sass 《Journal of Geochemical Exploration》2011

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Provenance of Neoproterozoic and early Paleozoic siliciclastic rocks of the Teplá-Barrandian unit (Bohemian Massif): Evidence from U–Pb detrital zircon ages     

Kerstin Drost  Axel Gerdes  Teresa Jeffries  Ulf Linnemann  Craig Storey 《Gondwana Research》2011,19(1):213-231

New LA-ICP-MS U–Pb detrital zircon ages from Ediacaran and Paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the Teplá-Barrandian unit (TBU) in the centre of the Bohemian Massif (Central Europe, Czech Republic). The samples taken span the period from ≤ 635 Ma to ～ 385 Ma and permit recognition of provenance changes that reflect changes in geotectonic regime. Detrital zircon age spectra of two Ediacaran, one Lower Cambrian and three Upper Ordovician samples resemble the ages known from the NW African proportion of Gondwana, particularly the Trans-Saharan belt, while three rocks from higher Lower Cambrian to Lowermost Ordovician strata contain detritus that may have been derived exclusively from local sources. The age spectrum of the Devonian rock is a combination of the NW Gondwanan and local features. These new findings in combination with a wide range of published data are in agreement with a Neoproterozoic subduction-related setting at the margin of Gondwana followed by a Cambrian/Early Ordovician rifting stage and an Ordovician passive margin setting. Furthermore the data are in favour of a position of the TBU at the Gondwanan margin throughout pre-Variscan times.  相似文献   

Observing the new Moon     

Roy E. Hoffman 《Monthly notices of the Royal Astronomical Society》2003,340(3):1039-1051

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On the streaming motions of haloes and galaxies     

Ravi K. Sheth  Antonaldo Diaferio  Lam Hui  Román Scoccimarro 《Monthly notices of the Royal Astronomical Society》2001,326(2):463-472

Little is known about the statistics of gravitationally lensed quasars at large (7–30 arcsec) image separations, which probe masses on the scale of galaxy clusters. We have carried out a survey for gravitationally lensed objects, among sources in the FIRST 20-cm radio survey that have unresolved optical counterparts in the digitizations of the Palomar Observatory Sky Survey. From the statistics of ongoing surveys that search for quasars among FIRST sources, we estimate that there are about 9100 quasars in this source sample, making this one of the largest lensing surveys to date. Using broad-band imaging, we have isolated all objects with double radio components separated by 5–30 arcsec that have unresolved optical counterparts with similar BVI colours. Our criteria for similar colours conservatively allow for observational error and for colour variations due to time delays between lensed images. Spectroscopy of these candidates shows that none of the pairs are lensed quasars. This sets an upper limit (95 per cent confidence) on the lensing fraction in this survey of 3.3×10⁻⁴, assuming 9100 quasars. Although the source redshift distribution is poorly known, a rough calculation of the expected lensing frequency and the detection efficiencies and biases suggests that simple theoretical expectations are of the same order of magnitude as our observational upper limit. Our procedure is novel in that our exhaustive search for lensed objects does not require prior identification of the quasars in the sample as such. Characterization of the FIRST-selected quasar population will enable use of our result to constrain quantitatively the mass properties of clusters.  相似文献   

Large-scale power spectrum and structures from the ENEAR galaxy peculiar velocity catalogue     

S. Zaroubi  M. Bernardi  L.N. da Costa  Y. Hoffman  M.V. Alonso  G. Wegner  C.N.A. Willmer  P.S. Pellegrini 《Monthly notices of the Royal Astronomical Society》2001,326(1):375-386

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