Planktoneustonic algae in the surface films of Lake Zürich: Occurrence and dependence on phytoplankton succession     

Astrid Nägeli  Ferdinand Schanz 《Aquatic Sciences - Research Across Boundaries》1990,52(3):269-286

Samples of surface films and of the underlying bulk water at 0.2 m depth were taken in Lake Zürich on 48 occasions in 1986, 1988 and 1989, with the aim of assessing the appropriateness of applying existing neuston nomenclature to the organisms found in these films, of determining which organisms accumulate there during which periods of time, and of assessing the importance of the phytoplankton community in the development of neustonic biocoenoses. Lake Zürich surface films were found to support a community of high population density, consisting of organisms which had migrated from the benthal or pelagial, or which had entered the film via the atmosphere or inflowing rivers. In all samples, species originating from within the phytoplankton community accounted for the greatest proportion of the total abundance. Several of these species were found to exhibit a special preference for the surface biotope, viz. those with a relative frequency (rF) exceeding 10%, a mean enrichment factor (Ef) exceeding 100, and, in the majority of cases, with a greater concentration of individuals in the surface film than in the underlying bulk water layer. We suggest that only those organisms occurring in the surface films of large water bodies which fulfil the above criteria should be considered as belonging to the planktoneuston, in order to distinguish them from other species, the occurrence of which in surface films is merely coincidental. The fact that planktoneustonic algae are dominant in the surface films of Lake Zürich means that changes in the species composition of the planktonic algal biocoenosis directly affect the species composition of the neustonic algal biocoenosis. Thus, just as in the underlying bulk water, a succession can be observed in the algal biocoenosis of the surface films. However, because of the higher abundance of planktoneustonic algae there, this succession is distinct from that occurring in the pelagial. Factors bringing about these differences which are discussed for the case of the dominant planktoneustonic algae are: alterations in specific gravity; positive phototaxis; and enhancement of growth rates in surface films as compared with the underlying bulk water.  相似文献   

Pelagic petroleum pollution off the south-west coast of Puerto Rico     

Jorge E. Corredor  Julio Morell  Astrid Méndez 《Marine pollution bulletin》1983,14(5):166-168

Petroleum pollution in the form of floating tar balls and dissolved or dispersed hydrocarbons has been monitored off the south-west coast of Puerto Rico on a weekly basis since November 1980. Tar ball abundance estimates agree well with previously published data for the area. Dissolved/dispersed hydrocarbon concentrations are close to, but below, those found in the Mediterranean Sea and considerably lower than those for more polluted areas. Tar ball abundance is closely related to the intensity of southerly winds. Dissolved/dispersed hydrocarbon concentrations are highest during the fall and coincident with the highest surface water temperatures.  相似文献   

The distribution of selected chlorinated microcontaminants in Baltic waters, 1992 to 1994     

Dirk Dannenberger  Astrid Lerz 《Ocean Dynamics》1995,47(4):301-312

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An Observational Case Study on the Influence of Atmospheric Boundary-Layer Dynamics on New Particle Formation     

Andreas Platis  Barbara Altstädter  Birgit Wehner  Norman Wildmann  Astrid Lampert  Markus Hermann  Wolfram Birmili  Jens Bange 《Boundary-Layer Meteorology》2016,158(1):67-92

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An assessment of the upstream migration and reproductive behaviour of allis shad (Alosa alosa L.) using acoustic tracking   总被引：3，自引：0，他引：3  

Acolas  M.L.; Begout Anras  M.L.; Veron  V.; Jourdan  H.; Sabatie  M.R.; Bagliniere  J.L. 《ICES Journal of Marine Science》2004,61(8):1291-1304

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Revision and recalibration of existing shock classifications for quartzose rocks using low‐shock pressure (2.5–20 GPa) recovery experiments and mesoscale numerical modeling          下载免费PDF全文

Astrid Kowitz  Nicole Güldemeister  Ralf Thomas Schmitt  Wolf‐Uwe Reimold  Kai Wünnemann  Andreas Holzwarth 《Meteoritics & planetary science》2016,51(10):1741-1761

A combination of shock recovery experiments and numerical modeling of shock deformation in the low‐shock pressure range from 2.5 to 20 GPa for two dry sandstone types of different porosity, a completely water‐saturated sandstone, and a well‐indurated quartzite provides new insights into strongly heterogeneous distribution of different shock features. (1) For nonporous quartzo‐feldspathic rocks, the traditional classification scheme (Stöffler 1984 ) is suitable with slight changes in pressure calibration. (2) For water‐saturated quartzose rocks, a cataclastic texture (microbreccia) seems to be typical for the shock pressure range up to 20 GPa. This microbreccia does not show formation of PDFs but diaplectic quartz glass/SiO₂ melt is formed at 20 GPa (~1 vol%). (3) For porous quartzose rocks, the following sequence of shock features is observed with progressive increase in shock pressure (1) crushing of pores, (2) intense fracturing of quartz grains, and (3) increasing formation of diaplectic quartz glass/SiO₂ melt replacing fracturing. The formation of diaplectic quartz glass/SiO₂ melt, together with SiO₂ high‐pressure phases, is a continuous process that strongly depends on porosity. This experimental observation is confirmed by our concomitant numerical modeling. Recalibration of the shock classification scheme results in a porosity versus shock pressure diagram illustrating distinct boundaries for the different shock stages.  相似文献   

Tenoumer impact crater,Mauritania: Impact melt genesis from a lithologically diverse target          下载免费PDF全文

Dina Simona Schultze  Fred Jourdan  Lutz Hecht  Wolf Uwe Reimold  Ralf‐Thomas Schmitt 《Meteoritics & planetary science》2016,51(2):323-350

Impact melt rocks from the 1.9 km diameter, simple bowl‐shaped Tenoumer impact crater in Mauritania have been analyzed chemically and petrologically. They are heterogeneous and can be subdivided into three types based on melt matrix color, occurrence of lithic clast components, amount of vesiculation (melt degassing), different proportions of carbonate melt mingled into silicate melt, and bulk rock chemical composition. These heterogeneities have two main causes (1) due to the small size of the impact crater, there was probably no coherent melt pool where a homogeneous mixture of melts, derived from different target lithologies, could be created; and (2) melt rock heterogeneity occurring at the thin section scale is due to fast cooling during and after the dynamic ejection and emplacement process. The overall period of crystal growth from these diverse melts was extremely short, which provides a further indication that complete chemical equilibration of the phases could not be achieved in such short time. Melt mixing processes involved in the generation of impact melts are, thus, recorded in nonequilibrium growth features. Variable mixing processes between chemically different melt phases and the formation of hybrid melts can be observed even at millimeter scales. Due to extreme cooling rates, different mixing and mingling stages are preserved in the varied parageneses of matrix minerals and in the mineral chemistry of microlites. ⁴⁰Ar³⁹Ar step‐heating chronology on specimens from three melt rock samples yielded five concordant inverse isochron ages. The inverse isochron plots show that minute amounts of inherited ⁴⁰Ar* are present in the system. We calculated a weighted mean age of 1.57 ± 0.14 Ma for these new results. This preferred age represents a refinement from the previous range of 21 ka to 2.5 Ma ages based on K/Ar and fission track dating.  相似文献   

A multi-proxy record of abrupt cooling events during the Windermere Interstadial at Crudale Meadow,Orkney, UK     

Christopher P. Francis  Stefan Engels  Ian P. Matthews  Adrian P. Palmer  Rhys G. O. Timms  Anne-Lise Jourdan  Ian Candy 《第四纪科学杂志》2021,36(3):325-338

Three clearly defined abrupt cooling events (ACEs) can be observed within Greenland Interstadial (GI)-1 in the Greenland ice-core records. However, the spatial variation in amplitude and timing of these ACEs is poorly understood due to the paucity of well-dated records with quantified temperature reconstructions. This study presents high-resolution chironomid-inferred July air temperature (T_Jul) and oxygen isotope (δ¹⁸O) records from Crudale Meadow (Orkney Isles, UK). Three centennial-scale ACEs punctuate the Windermere Interstadial at Crudale Meadow. The largest ACE shows an amplitude of 5.4 °C and a 1% isotopic decline and is centred on ~14.0 ka bp , consistent with the timing of the GI-1d event in the Greenland stratigraphy. The two other observed ACEs are of smaller magnitude and are centred on ~13.6 ka bp and ~13.2 ka bp , with these smaller magnitude events tentatively correlated with the GI-1cii and GI-1b events, respectively, but lack sufficient chronological constraint to fully assess their timing. When comparing the Crudale Meadow record with other locations in the British Isles a strong relationship can be observed between the magnitude of T_Jul cooling and latitude, with a reduced signal in more southerly locations, indicating that oceanic forcing may be a key driver of the ACEs.  相似文献   

Isotopic fractionation of zinc in tektites     

Frederic Moynier  Pierre Beck  Fred Jourdan  Qing-Zhu Yin  Uwe Reimold  Christian Koeberl 《Earth and Planetary Science Letters》2009,277(3-4):482-489

Tektites are terrestrial natural glasses produced during a hypervelocity impact of an extraterrestrial projectile onto the Earth's surface. The similarity between the chemical and isotopic compositions of tektites and terrestrial upper continental crust implies that the tektites formed by fusion of such target rock. Tektites are among the driest rocks on Earth. Although volatilization at high temperature may have caused this extreme dryness, the exact mechanism of the water loss and the behavior of other volatile species during tektite formation are still debated. Volatilization can fractionate isotopes, therefore, comparing the isotope composition of volatile elements in tektites with that of their source rocks may help to understand the physical conditions during tektite formation.For this study, we have measured the Zn isotopic composition of 20 tektites from four different strewn fields. Almost all samples are enriched in heavy isotopes of Zn compared to the upper continental crust. On average, the different groups of tektites are isotopically distinct (listed from the isotopically lightest to the heaviest): Muong-Nong type indochinites (δ^66/64Zn = 0.61 ± 0.30‰); North American bediasites (δ^66/64Zn = 1.61 ± 0.49‰); Ivory Coast tektites (δ^66/64Zn = 1.66 ± 0.18‰); the Australasian tektites (others than the Muong Nong-type indochinites) (δ^66/64Zn = 1.84 ± 0.42‰); and Central European moldavites (δ^66/64Zn = 2.04 ± 0.19‰). These results are contrasted with a narrow range of δ^66/64Zn = 0–0.7‰ for a diverse spectrum of upper continental crust materials.The elemental abundance of Zn is negatively correlated with δ^66/64Zn, which may reflect that isotopic fractionation occurred by evaporation during the heating event upon tektite formation. Simple Rayleigh distillation predicts isotopic fractionations much larger than what is actually observed, therefore, such a model cannot account for the observed Zn isotope fractionation in tektites. We have developed a more realistic model of evaporation of Zn from a molten sphere: during its hypervelocity trajectory, the molten surface of the tektite will be entrained by viscous coupling with air that will then induce a velocity field inside the molten sphere. This velocity field induces significant radial chemical mixing within the tektite that accelerates the evaporation process. Our model, albeit parameter dependent, shows that both the isotopic composition and the chemical abundances measured in tektites can be produced by evaporation in a diffusion-limited regime.  相似文献   

Changes in the timing of spawning of Baltic cod: possible causes and implications for recruitment   总被引：11，自引：5，他引：6  

Wieland  Kai; Jarre-Teichmann  Astrid; Horbowa  Katarzyna 《ICES Journal of Marine Science》2000,57(2):452-464

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