Populating a cluster of galaxies – I. Results at

We simulate the assembly of a massive rich cluster and the formation of its constituent galaxies in a flat, low-density universe. Our most accurate model follows the collapse, the star formation history and the orbital motion of all galaxies more luminous than the Fornax dwarf spheroidal, while dark halo structure is tracked consistently throughout the cluster for all galaxies more luminous than the SMC. Within its virial radius this model contains about     dark matter particles and almost 5000 distinct dynamically resolved galaxies. Simulations of this same cluster at a variety of resolutions allow us to check explicitly for numerical convergence both of the dark matter structures produced by our new parallel N -body and substructure identification codes, and of the galaxy populations produced by the phenomenological models we use to follow cooling, star formation, feedback and stellar aging. This baryonic modelling is tuned so that our simulations reproduce the observed properties of isolated spirals outside clusters. Without further parameter adjustment our simulations then produce a luminosity function, a mass-to-light ratio, luminosity, number and velocity dispersion profiles, and a morphology–radius relation which are similar to those observed in real clusters. In particular, since our simulations follow galaxy merging explicitly, we can demonstrate that it accounts quantitatively for the observed cluster population of bulges and elliptical galaxies.     

Computer modelling of Al/Si ordering in sillimanite

Computer simulation is used to investigate the effect of Al/Si disordering over the tetrahedral sites on the lattice energy and the lattice constants of the mineral sillimanite Al₂SiO₅. A methodology for an atomistic assessment of the energy of the reaction 2(Si-O-Al)→(Si-O-Si)+(Al-O-Al) and its various contributions is established. This ordering energy is 0.97 eV for nearest neighbour sites in the ab-plane and 0.56 eV for those separated in the c-direction. The large difference is due to a greater constraint on the atomic relaxation in the ab-plane and shows the structural dependence of the ordering energy. Its magnitude appears to be determined by a complicated balance between Coulomb and short-range repulsive energy involving strain over many bonds, both in the ordered and disordered structures. There is also a significant interaction between second neighbour sites whereas the contribution of more distant neighbours is negligible. The lattice energies of most of the 154 configurations studied show a linear behaviour as a function of short-range order, specified by the number of Al-Al pairs. The ordering temperature T_c, estimated on the basis of a statistical mechanical model of disordering, and the calculated ordering energies are in semi-quantitative agreement with experimental values.     

Synoptic hydrography of a highly stratified estuary

This study investigates the hydrodynamic characteristics of the lower, middle, and upper sectors of a highly stratified estuary, the Itajaí-Açu river estuary (south of Brazil ∼27° S/48.5° W). The study is based on a 25-h field campaign with three sampling stations positioned at 2, 17, and 38 km inward from the river mouth, during low river discharge condition and spring tide. The experimental data gathered was reduced and analyzed in terms of distribution of variables in time and space tide average vertical profiles and decomposition of the advective transport of salt and suspended particulate matter (SPM). Tidal range was nearly constant along the estuary, presenting time lag of about 2 h between lower and upper estuary. The ebb discharge peaks were about twice the discharge flood peaks and occurred simultaneously. The tide was the main determining agent in the lower estuary, where currents, salt stratification, and SPM distributions presented a repetitive behavior. In the middle estuary, the tide effects were also observed, but the presence of saline waters decreased along the time due to increasing river discharge during the campaign. The distribution of SPM in the mid- and upper estuary presented patched pattern not associated with tides and may be attributed to short-term flood contributions of tributaries. Currents presented ebb dominance in all three sectors; in the middle and upper estuary, they presented also a time asymmetry, with ebb currents longer than flood. The advective transport of salt in the lower estuary was upstream, with dominance of gravitational circulation term. In the mid-estuary, there was practically no transport, with balance between fluvial discharge (downstream) and tidal correlation (upstream). The advective transport of SPM was upstream in the lower estuary and downstream in the mid- and upper estuary, being dominated by gravitational circulation in the former and fluvial discharge in the others.     

Inversion-related features along the southeastern margin of the North German Basin (Elbe Fault System)

A seismostratigraphic approach constrained by well data is used for the interpretation of the deformation style along the central Elbe Fault System (EFS) within the sedimentary succession. Structural analysis allows to qualify, to quantify, and to date tectonic events. The stratigraphic interpretation is complicated by the mobilized Upper Permian Zechstein salt and by erosional events. A first-order quantification of the inversion-related uplift is estimated from vertical fault offsets that reach up to 4 km. The main uplift occurred during the Maastrichtian/Paleocene. Amounts of erosion inferred from comparing the strata thickness on top of the Flechtingen High with the surrounding basinal areas range from 3 to 4 km. The data indicate a changing deformation style: Thick-skinned deformation with southwest-dipping thrusts that vertically offset the pre-Permian basement is observed along the Flechtingen High in the central part of the EFS. Thin-skinned deformation occurs in the North German Basin where salt detaches the post-Permian cover from the barely faulted basement. It is concluded that during the Late Cretaceous/Early Tertiary inversion, the EFS responded to regional compression with uplift and formation of an internal high, the Flechtingen High. A stress-sensitive crustal weak zone beneath the EFS could be the reason for the repeated strain localization in the area.     

Fluid inclusions in high-pressure granulites of the Pan-African belt in Tanzania (Uluguru Mts): a record of prograde to retrograde fluid evolution

The high-pressure granulites of the Uluguru Mountains are part of the Pan-African belt of Tanzania, the metamorphic evolution of which is characterized by an anticlockwise P-T path. Mineral assemblages that represent distinct metamorphic stages are selected for fluid inclusion studies in order to deduce the fluid evolution in metapelites and pyroxene granulites from the prograde to the retrograde stage. Fluid inclusion data improve the petrologically derived P-T path and confirm the anticlockwise evolution. Fluid inclusions in quartz enclosed in garnet porphyroblasts in metapelites preserve prograde fluids of CO₂–N₂ composition and later-trapped pure CO₂. During isochoric heating at temperatures near the peak of metamorphism, deformation and recrystallization led to fluid homogenization yielding N₂-poor CO₂ composition in the metapelites. Near-peak CO₂–N₂ fluid inclusions in quartz of metapelites and CO₂ inclusions in garnet-pyroxene granulites are characterized by perfect negative crystal shape. Garnet formed in veins and as coronas around orthopyroxene represent the near-isochoric/isobaric cooling stage which is characterized by high-density CO₂-rich fluid inclusions. Up to 15 mol% N₂ in some primary CO₂ inclusions in corona garnet indicate small-scale fluid heterogeneity during the static garnet growth. The fact that high-density fluid inclusions are preserved, suggests a shallow dP/dT slope of the uplift path. Nevertheless, some fluid inclusions decrepitated or re-equilibrated and low-density CO₂ inclusions were trapped in the garnet-pyroxene granulite while N₂–CH₄ inclusions formed in the metapelites. Different fluid compositions in metapelite and metabasite argue for an internal control of the fluid composition by phase equilibria. In shear zones where the pyroxene granulite was transformed into scapolite-biotite schist, CO₂–N₂ and low-density N₂–CH₄ fluid inclusions indicate several stages of tectonic activity and suggest fluid influx from the nearby metapelites. High- and low-salinity aqueous inclusions observed beside CO₂ inclusions in garnet-pyroxene granulites, in vein quartz and shear zones could be of high-grade origin but are mainly re-equilibrated or re-trapped along healed microfractures during lower-grade stages. Received: 21 May 1997 / Accepted: 6 October 1997     

Renaturalization of streams and rivers — the special importance of integrated ecological methods in measurement of success. An example from Saxony-Anhalt (Germany)

Since hydromorphology in about 80% of German streams and rivers is degraded to a high degree, increased efforts in hydromorphological renaturalization are necessary. A measurement of the success of the first realized projects shows that improvement in stream morphology has a remarkably positive influence on aquatic ecology. An example of a restored stretch of a lowland stream in Saxony-Anhalt is used to describe the possibilities of success measurement programs for improvement of poor renaturalization. Therefore, a combined morphological and hydrobiological approach was developed. An integrated ecological assessment is possible by using the multimetric index EQI_M (Ecological Quality Index using benthic Macroinvertebrates) and the GFI (German Fauna Index). The latter represents a tolerance measure to evaluate the hydromorphological status of a site by using certain taxa that indicate either positive or negative physical attributes. To consider the special characteristics of the stream in its landscape unit, specific reference conditions (‘Leitbild’) were defined for macroinvertebrate communities by sampling comparable but undisturbed streams in the same landscape unit. Only the combination of biological indices, hydromorphological mapping and comparison to the reference status allows for an expressive evaluation of renaturalization measures and precise conclusions for their improvement.