Preflare state

Discussion on the preflare state held at the Ottawa Flares 22 Workshop focused on the interpretation of solar magnetograms and of H filament activity. Magnetograms from several observatories provided evidence of significant build up of electric currents in flaring regions. Images of X-ray emitting structures provided a clear example of magnetic relaxation in the course of a flare. Emerging and cancelling magnetic fields appear to be important for triggering flares and for the formation of filaments, which are associated with eruptive flares. Filaments may become unstable by the build up of electric current helicity. Examples of heliform eruptive filaments were presented at the Workshop. Theoretical models linking filaments and flares are briefly reviewed.Report of Team 1, Flares 22 Workshop, Ottawa, May 25–28, 1993     

Ecological classification of macrophytes and phytobenthos for rivers in Germany according to the water framework directive

A new assessment system for macrophytes and phytobenthos in German rivers meeting the requirements of the Water Framework Directive (WFD) of the European Community is described. Biocoenotic types based on biological, chemical and hydromorphological data from over 200 river sites covering the main ecoregions, hydromorphological stream types and degradation forms have been defined. For developing a classification system the quality element macrophytes and phytobenthos was divided into three components: macrophytes, benthic diatoms and remaining phytobenthos. For macrophytes seven types including one subtype, for benthic diatoms 14 types including three subtypes and for the remaining phytobenthos five river types were identified. The benthic vegetation at reference condition was described for most of the river types. Degradation is characterised as deviation in benthic vegetation species composition and abundance from the reference biocoenosis. For classification in five ecological status classes, several metrics were developed and used in combination with existing indices. For some of the described river types additional investigations are necessary before a classification system can be developed.     

DIffusion of Eu and Gd in basalt and obsidian

Tracer diffusion coefficients of ¹⁵³Gd and ¹⁵²Eu in olivine tholeiite have been determined at temperatures between 1150 and 1440°C. The results are identical for both tracers within experimental error. Between 1440 and 1320°C the diffusion coefficients are given by D(Eu, Gd) = 0.058 exp(?40,600/ RT). Between 1320 and 1210°C, the diffusion coefficients are constant at D = (1.4 ± 0.4) × 10^?7 cm²s^?1 and between 1210 and 1150°C, the D values drop irregularly to 4 × 10^?9 cm²s^?1. The liquidus temperature (1270°C) lies within the region of constant D. Such anomalous behavior has not been encountered in previous studies of Ca, Sr, Ba and Co diffusion in basalt. To explain the constant D value near the liquidus, we speculate that the structure of the melt changes as a function of temperature in such a way that the normal temperature dependence of the diffusivity is compensated. For example, the rare earth ions may be displaced from their (high temperature) octahedral coordination sites to other sites where they are more readily dissociated and therefore become progressively more mobile. The behavior below 1210°C may be the result of relatively stable complexes or molecules in the melt or of the formation of a REE bearing crystalline phase that has so far escaped detection. Preliminary results for Eu diffusion in obsidian are D (Eu, 800°C) = 5 × 10^?13 cm² s^?1 and D (Eu, 950°C) = 1.5 × 10^?11 cm² s^?1. These data are consistent with an activation energy of 59 Kcal mole^?1. These low diffusivities indicate that the partitioning of REE in crystallizing intermediate and acidic melts may be controlled by diffusion in the melt rather than equilibrium between the crystal surface and the bulk melt.The diffusion data are applied to partial melting in the mantle, in an attempt to explain how LREE enriched tholeiites may be derived from a LREE depleted mantle source. In this model LREE diffuse from garnet bearing regions that have small melt fractions into garnet free regions that have relatively large melt fractions. REE diffusion is so slow that this process is quantitatively significant only in small partially molten bodies (diameter ～1 km or less) or in larger, but strongly flattened bodies. Internal convective motion during diapiric rise would also increase the efficiency of the process.     

The geochemistry of Archaean shales derived from a Mafic volcanic sequence, Belingwe greenstone belt, Zimbabwe: provenance, source area unroofing and submarine versus subaerial weathering

Shales of the ∼2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2-2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1-100 m thick units intercalated with shallow-water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K₂O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (La_N/Yb_N = 2.84-4.45) and no significant Eu anomaly (Eu/Eu* = 0.93-0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (La_N/Yb_N = 0.69-2.19) and a small, negative Eu anomaly (average Eu/Eu* = 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering.     

The sediment record of the past 200 years in a Swiss high-alpine lake: Hagelseewli (2339 m a.s.l.)

Sediment cores spanning the last two centuries were taken in Hagelseewli, a high-elevation lake in the Swiss Alps. Contiguous 0.5 cm samples were analysed for biological remains, including diatoms, chironomids, cladocera, chrysophyte cysts, and fossil pigments. In addition, sedimentological and geochemical variables such as loss-on-ignition, total carbon, nitrogen, sulphur, grain-size and magnetic mineralogy were determined. The results of these analyses were compared to a long instrumental air temperature record that was adapted to the elevation of Hagelseewli by applying mean monthly lapse rates.During much of the time, the lake is in the shadow of a high cliff to the south, so that the lake is ice-covered during much of the year and thus decoupled from climatic forcing. Lake biology is therefore influenced more by the duration of ice-cover than by direct temperature effects during the short open-water season. Long periods of ice-cover result in anoxic water conditions and dissolution of authigenic calcites, leading to carbonate-free sediments.The diversity of chironomid and cladoceran assemblages is extremely low, whereas that of diatom and chrysophyte cyst assemblages is much higher. Weak correlations were observed between the diatom and chrysophyte cyst assemblages on the one hand and summer or autumn air temperatures on the other, but the proportion of variance explained is low, so that air temperature alone cannot account for the degree of variation observed in the paleolimnological record.Analyses of mineral magnetic parameters, spheroidal carbonaceous particles and lead suggest that atmospheric pollution has had a significant effect on the sediments of Hagelseewli, but little effect on the water quality as reflected in the lake biota.     

GREGOR ‐ optical design considerations

After many years of successful work the Gregory Coudé Telescope (GCT) is going to be replaced by GREGOR. This new 1.5 m telescope is the result of design considerations which to a wide extent are based on new technologies. Special aspects of the design are presented, including measures to compensate for optical aberrations caused by instrumental and atmospheric effects (Adaptive Optics). First light is expected for the end of 2004.     

Modelling inter-annual and spatial variability of ice cover in a temperate lake with complex morphology

The formation of ice cover on lakes alters heat and energy transfer with the water column. The fraction of surface area covered by ice and the timing of ice-on and ice-off therefore affects hydrodynamics and the seasonal development of stratification and related ecosystem processes. Multi-year model simulations of temperate lake ecosystems that freeze partially or completely therefore require simulation of the formation and duration of ice cover. Here we present a multi-year hydrodynamic simulation of an alpine lake with complex morphology (Lower Lake Constance, LLC) using the three-dimensional (3D) model Aquatic Ecosystem Model (AEM3D) over a period of 9 years. LLC is subdivided into three basins (Gnadensee, Zeller See and Rheinsee) which differ in depth, morphological features, hydrodynamic conditions and ice cover phenology and thickness. Model results were validated with field observations and additional information on ice cover derived from a citizen science approach using information from social media. The model reproduced the occurrence of thin ice as well as its inter-annual variability and differentiated the frequency and extent of ice cover between the three sub-basins. It captured that full ice cover occurs almost each winter in Gnadensee, but only rarely in Zeller See and Rheinsee. The results indicate that the 3D model AEM3D is suitable for simulating long-term dynamics of thin ice cover in lakes with complex morphology and inter-annual changes in spatially heterogeneous ice cover.     

Analysis and results of cooperative magnetographic measurements. I. Correction,comparison and discussion of measurements

On June 24, 1983 cooperative magnetographic measurements were made with the vectormagnetographs of the Sayan Observatory (Irkutsk) and the Potsdam Solar Observatory “Einsteinturm” and with the longitudinal magnetograph of the Ondrejov Observatory. Additionally, the maximum field strengths in the sunspot were measured with the photographic method. Using the photographically measured field strengths as reference values we provide a method to eliminate the influences of stray light in a time series of vectormagnetograms. A comparison of nearly simultaneous magnetograms shows a good correspondence in general. The deviations of the zero points as well as the scales are comparable with the results of other authors. Regarding the magnetic field distribution the magnetograms reflect a substantial nonsymmetric structure in the spot under study. The magnetic field lines tend to concentrate into several flux tube clusters. In one region of the penumbra we find a magnetic field with nearly longitudinal character in close neighbourhood of a strong, nearly horizontal flux tube bundle. This indicates a strong nonradial horizontal field gradient in the penumbra.     

Holocene glacier fluctuations and migration of Neolithic yak pastoralists into the high valleys of northwest Bhutan

Here we present geomorphologic, palaeoenvironmental and archaeo-botanical data which elucidate the Late Pleistocene and Holocene glacial history of the high, mountain-locked Himalayan valleys in northwest Bhutan and provide one of the earliest proofs of human activity yet known for the High Himalaya range. In this area, difficult to access, close linkage between climatic change, glacier fluctuations and human migration patterns has been discovered. Glacier systems in the studied area are characterized by avalanching and debris mantled glacier snouts, with the significant local influence of the Indian summer monsoon causing decoupling of glacier responses from temperature changes but supporting the idea of monsoonal forcing. Geomorphologic mapping, together with Optically Stimulated Luminescence (OSL) and radiocarbon dating of ice-proximal sediments, has been used to construct a local glacial chronology. Local ice-stream networks developed during the Early Holocene (ca 10,000–9000 a ago) and during the early part of the Mid Holocene (6710 ± 90–4680 ± 155 cal a BP) at which times there were ice advances of about 5 km from the modern glacier termini. At such times, the intensity of pro- and periglacial processes would have intensified and ice-dammed lakes were probably common as well, rendering human colonization of the high valleys in northwest Bhutan impossible. An abrupt shift to dry climatic conditions on the Tibetan Plateau between 5000 and 4500 a BP coincided with glacial decay and the onset of morphodynamically stable conditions on the broad valley floors of the high valleys in this part of the Himalaya. Palynological data suggest that the sudden disappearance of juniper and rhododendron pollen, the immediate onset of pollen input from cereals (confirmed by detailed SEM analysis) and a clear pattern of over-grazing, trampling and peat deterioration can be linked to human arrival in the valleys at ca 4280 ± 130 cal a BP. Extensive charcoal horizons dating to 4745 ± 250 and 4680 ± 155 cal a BP are interpreted as evidence for human use of fire and forest clearances and agree spatially and temporally with the pollen-based picture. Charcoal occurrences as old as 6710 ± 90 cal a BP might be linked to yet earlier exploration of these Himalayan valleys during phases of low glacial activity. We provide an account of the colonization of these high valleys in response to glacial and monsoonal change and argue that the most likely founder societies come from the Tibetan Plateau, where yak and barley based pastoralism and Neolithic settlements are known to have existed since the Mid Holocene.     

Niobium-tantalum Fractionation During Slab Subduction:Implications for the Formation of Continental Crust

Compared with the oceanic crust, knowledge about the formation of the continental crust (CC) is relatively poor. Although melting of subducted slabs in the early history of the Earth has been considered as the major way that shaped the chemical characteristics of the CC by most geologists, as the CC shares many characteristics with modern adakites, some geologists argued that Archean TTG was formed in the same way as modern arcs rather than slab melting, whereas others proposed that melting at the bottom of the thickened oceanic crust was more important. Recently,the debate is mainly focused on the unique subchondritic Nb/Ta value of the CC, and particularly, how Nb and Ta fractionated from each other and consequently how, in detail, the CC was built.