The yellow hypergiants     

Cornelis de Jager 《Astronomy and Astrophysics Review》1998,8(3):145-180

We list the main stellar data of known hypergiants and similarly luminous stars, and then concentrate on a review of the yellow hypergiants. These stars are post-red supergiants evolving along blueward loops in the Hertzsprung-Russell diagram. Their properties, their location in the Hertzsprung-Russell diagram and their occasional mass ejections are related to a region of atmospheric instability in the H-R diagram, the Yellow Void. The ‘bouncing against the border of the Void’ of three objects: Cas, HR8752 and IRC+10420, is described. The apparent atmospheric instability of yellow hypergiants is related to the atmospheric pulsations. There are indications that the approach to the Void is associated with an increased amplitude of the pulsation and with enhanced mass loss. The observed small-scale motion field is only apparently strongly supersonic; the observed large stochastic velocities are the quasi-stochastically varying thermal motions in the many hot sheets that occur in the wakes of many small shocks, while the real hydrodynamic velocity component is small and subsonic. This shock-wave field is also responsible for the observed rate of mass loss and for emission in the wings of H. Most yellow hypergiants have envelopes containing gas and dust, but a thick extended envelope, presumably dissipating and showing bipolar outflow, is only known around IRC+10420. At the interface of the bipolar wind and the interstellar medium one or more stationary shocks may develop as is observed in the case of IRC+10420 and suspected with Cas. Received 1 July 1997  相似文献   

Crabs,leeches and trypanosomes: an unholy trinity?     

Hemmingsen W  Jansen PA  Mackenzie K 《Marine pollution bulletin》2005,50(3):336-339

The red king crab Paralithodes camtschaticus was deliberately introduced to the Barents Sea in the 1960s and 1970s from its native area in the North Pacific. The carapace of these crabs is a favoured substrate for the leech Johanssonia arctica to deposit its eggs, and the leech is a vector for a trypanosome blood parasite of marine fish, including cod. We examined cod for trypanosome infections during annual cruises along the coast of Finnmark in North Norway over three successive years from stations along a gradient of over 1000 km. In every year the level of trypanosome infection in cod was significantly highest in the area with the greatest density of king crabs. We propose the hypothesis that the burgeoning population of red king crabs in this area is indirectly responsible for increased transmission of trypanosomes to cod by promoting an increase in the population of the leech vector.  相似文献   

Some geochemical controls on lead and barium concentrations in ferromanganese deposits     

Cornelis H. Van Der Weijden  E.C. Kruissink 《Marine Chemistry》1977,5(2):93-112

The Pb content of natural deep-sea ferromanganese deposits decreases with depth. This is commonly explained by assuming oxidation of Pb²⁺ and uptake as (Mn, Pb)O₂ in normally oxygenated shallow water, a process that is less likely to occur in deep water which is somewhat depleted in oxygen. This concept is discussed and rejected, both on theoretical grounds and based on experimental results of Pb²⁺ sorption by Mn oxides. Comparative sorption experiments on Mn oxides were carried out with Pb and Ba. It is shown that Pb is preferentially adsorbed by birnessite at $\text{pH}\text{? 4}$. The uptake ratio is, however, much lower than the relative enrichment factor in natural nodule material. Suggested explanations for this discrepancy are: influence of hydrostatic pressure and extent of hydrolysis of the cations at the pH of ocean water.  相似文献   

Trace elements in suspended matter from the anoxic hypersaline Tyro and Bannock Basins (eastern Mediterranean)     

van der Sloot Hans A  Dirk Hoede  Gert Hamburg  Joost R.W Woittiez  van der Weijden Cornelis H 《Marine Chemistry》1990,31(1-3)

In the anoxic hypersaline Tyro and Bannock Basins of the eastern Mediterranean, extremely high concentrations of Co (0.015%), Cu (1.35%) and Zn (0.28%) were found in suspended matter collected at the sharp interface between seawater and the anoxic brine. The high particulate Co, Cu and Zn concentrations can be explained by the sharp increase in dissolved sulphide at these interfaces, and the resultant precipitation of metal sulphides. The particulate As, Sb and Mo concentrations also showed a sharp maximum at or close to the interface. However, the contributions of As, Sb and Mo contents in suspended matter to the total concentrations in the water column are small. Scanning electron microscopy-energy-dispersive X-ray analysis (SEM-EDAX) of suspended particulate matter from the Tyro Basin revealed spherical particles strongly enriched in Fe, Cu and Zn at the seawater-brine interface.  相似文献   

Experiments on the uptake of zinc and cadmium by manganese oxides     

Cornelis H. van der Weijden 《Marine Chemistry》1976,4(4):377-387

Experiments on the uptake of Zn and Cd by synthetic hydrous Mn oxides were carried out in an ionic medium at pH 3.5 and at pH 4. A slight preference for uptake of Cd²⁺ over Zn²⁺ was observed with both birnessite and nsutite, the Cd/Zn ratio being different for each mineral. Subsequently, the desorption of Zn and Cd from the obtained products in artificial seawater was studied. In this medium Cd is desorbed from the Mn oxides to a much higher extent than is Zn. The latter observations can be satisfactorily explained by the large difference of complex formation for the two metals in seawater, slightly counteracted by the preferential uptake of Cd²⁺ over Zn²⁺. The order of magnitude of the Zn/Cd ratio in natural manganese nodules is compatible with the ratio calculated on the basis of experimental results, taking fair estimates of the actual inorganic Zn/Cd ratio in seawater and of the pH of deep ocean water.  相似文献   

Fast reconnaissance of carbonate dissolution based on the size distribution of calcareous ooze on Walvis Ridge, SE Atlantic Ocean     

Jan-Berend W. Stuut  Maarten A. Prins  J. H. Fred Jansen 《Marine Geology》2002,190(3-4):581-589

We present a new index of carbonate fragmentation based on the size distribution of bulk sediments in core MD962094 from Walvis Ridge (SE Atlantic Ocean). The carbonate fragmentation index is constructed by taking a ratio of the two coarsest fractions in the grain size distributions of the bulk calcareous ooze. The coarsest two fractions (25–90 μm and >90 μm) of the bulk sediments consist primarily of complete shells and fragments of adult foraminifera shells, and juvenile foraminifera shells and fragments, respectively. The ratio of the proportions of the two fractions is interpreted as a measure of fragmentation of the foraminifera shells caused by carbonate dissolution. Downcore changes in our carbonate fragmentation index compare very well with those in the coarse-carbonate fragmentation index in sediments from a nearby core on Walvis Ridge. The latter commonly used fragmentation index is defined as a ratio of foraminifera fragments over whole foraminifera in the >150-μm fraction as seen with a light microscope. Fragmentation is relatively high during glacial stages and relatively low during interglacial stages during the last 300 kyr, caused by the combined effect of wind-driven upwelling of corrosive water and increased production of organic matter, decreasing the preservation potential of carbonates both during and after deposition. The carbonate fragmentation index we present here provides a precise and fast method to establish a downcore fragmentation record. It can be applied to bulk sediments that are carbonate-rich (CaCO₃>68%) and to all other deep-marine sediments of which the grain size distribution of the carbonate-free fraction is available.  相似文献   

The use of SfM-photogrammetry to quantify and understand gully degradation at the temporal scale of rainfall events: an example from the Ethiopian drylands     

Wouter Lannoeye  Cornelis Stal  Etefa Guyassa  Amanuel Zenebe  Jan Nyssen  Amaury Frankl 《自然地理学》2016,37(6):430-451

Abstract

With the recent technological advances offered by SfM-photogrammetry, we now have the possibility to study gully erosion at very high spatial and temporal scales from multi-temporal DEMs, and thus to enhance our understanding of both gully erosion processes and controls. Here, we examine gully degradation and aggradation at a gully headcut and at four re-incisions along a gully reach in Northern Ethiopia. Environmental controls recorded are topography rainfall, runoff, land use and cover, land management, and soil characteristics. The overall vulnerability of the catchment to erosion is low as calculated from the RUSLE (average 11.83 t ha^?1 y^?1). This reflects the successful land management of the past years. The runoff coefficient was on average 7.3% (maximum 18.2%). Runoff events caused most geomorphic change in the gully, but slumping of the gully bank also occurred on dry days. Most geomorphic change was caused by one major rainfall event of 54.8 mm d^?1, and smaller runoff events caused both degradation and aggradation, often asynchronous between studied sites. Although most research focuses on gully heads alone, re-incisions at lower locations can still cause important gully degradation, which ultimately will reach the gully head and cause instability.  相似文献   

Determination of efficiency of Vaseline slide and Wilson and Cooke sediment traps by wind tunnel experiments     

Feras Youssef  Gunay Erpul  Pieter Bogman  Wim M. Cornelis  Donald Gabriels 《Environmental Geology》2008,55(4):741-750

The trap efficiency of a catcher in wind erosion measurements plays a significant role, and in many cases suspension trap efficiencies at high wind velocities are still unknown. The sediment trap efficiency generally changes with particles size and with wind speed. In this study, the efficiency of Vaseline Slide (VS) and Modified Wilson and Cooke (MWAC) catchers were determined with different sand particle sizes (<50, <75, 50–75, 200–400, and 400–500 μm) at a fixed wind speed (13.3 ms⁻¹) and with different soil textures at different wind velocities (10.3, 12.3, and 14.3 ms⁻¹) in the wind tunnel of the International Center for Eremology (ICE), Ghent University, Belgium. The traps were placed at different heights (4, 6.5, 13, 20, 120, and 192 cm for VS and 1.5, 3, 5, 8, 11, and 30 cm for MWAC) to catch saltating and suspended sediments in a 12-m long, 1.2-m wide and 3.2-m high working section of the wind tunnel. In the sand particle experiments, the efficiency of the VS catcher was 92% for particles smaller than 50 μm and decreased with increasing particles size, falling to 2.2% for 400–500 μm particle size at 13.4 ms⁻¹. However, the MWAC’s efficiency was 0% for particles smaller than 50 μm and increased with increasing particle size to 69.5% at 400–500 μm. In the experiments with different soil textures, the efficiency of each catcher significantly changed with soil and with wind speed. It also considerably varied with the catchers: for instance, for sand (S), the MWAC efficiency was very high (67.4, 113.4, and 90.5% at 10.3, 12.3, and 14.4 ms⁻¹, respectively) while the efficiency of VS was relatively very low (5.2, 4.4, and 1.9% at 10.3, 12.3, and 14.4 ms⁻¹, respectively). Results indicated that the efficiency depends critically on the particle size, type of catcher, and wind speed, and these could be helpful to increase the robustness of wind erosion measurements.  相似文献   

Natural and anthropogenic forcing of Holocene lake ecosystem development at Lake Uddelermeer (The Netherlands)     

Stefan Engels  Rogier van Oostrom  Chiara Cherli  Jennifer A. J. Dungait  Boris Jansen  J. M. van Aken  Bas van Geel  Petra M. Visser 《Journal of Paleolimnology》2018,59(3):329-347

Lake Uddelermeer (The Netherlands) is characterized by turbid conditions and annual blooms of toxic cyanobacteria, which are supposed to be the result of increased agricultural activity in the twentieth century AD. We applied a combination of classic palaeoecological proxies and novel geochemical proxies to the Holocene sediment record of Lake Uddelermeer (The Netherlands) in order to reconstruct the natural variability of the lake ecosystem and to identify the drivers of the change to the turbid conditions that currently characterize this lake. We show that the lake ecosystem was characterized by a mix of aquatic macrophytes and abundant phytoplankton between 11,500 and 6000 cal year BP. A transition to a lake ecosystem with clear-water conditions and relatively high abundances of ‘isoetids’ coincides with the first signs of human impact on the landscape around Lake Uddelermeer during the Early Neolithic (ca. 6000 cal year BP). An abrupt and dramatic ecosystem shift can be seen at ca. 1030 cal year BP when increases in the abundance of algal microfossils and concentrations of sedimentary pigments indicate a transition to a turbid phytoplankton-dominated state. Finally, a strong increase in concentrations of plant and faecal biomarkers is observed around 1950 AD. Canonical Correspondence Analysis suggests that reconstructed lake ecosystem changes are best explained by environmental drivers that show long-term gradual changes (sediment age, water depth). These combined results document the long-term anthropogenic impact on the ecosystem of Lake Uddelermeer and provide evidence for pre-Industrial Era signs of eutrophication.  相似文献   

Spatial development of X-ray emission during the impulsive phase of a solar flare     

Cornelis De Jager  André Boelee  David M. Rust 《Solar physics》1984,92(1-2):245-258

The flare of 11 November, 1980, 1725 UT occurred in a magnetically complex region. It was preceded by some ten minutes by a gradual flare originating over the magnetic inversion line, close to a small sunspot. This seems to have triggered the main flare (at 70 000 km distance) which originated between a large sunspot and the inversion line. The main flare started at 172320 UT with a slight enhancement of hard X-rays (E > 30 keV) accompanied by the formation of a dark loop between two H bright ribbons. In 3–8 keV X-rays a southward expansion started at the same time, with - 500 km s ^–1. At the same time a surge-like expansion started. It was observable slightly later in H, with southward velocities of 200 km s^–1. The dark H loop dissolved at 1724 UT at which time several impulsive phenomena started such as a complex of hard X-ray bursts localized in a small area. At the end of the impulsive phase at 172540 UT, a coronal explosion occurred directed southward with an initial expansion velocity of 1800 km s^–1, decreasing in 40 s to 500 km s^–1.Now at Fokker Aircraft Industries, Schiphol, The Netherlands.  相似文献