Material versus isobaric internal boundaries in the Earth and their influence on postglacial rebound

Most previous earth models used to calculate viscoelastic relaxation after the removal of the Late Pleistocene ice loads implicitly assume that there is no exchange of mass across the mantle density discontinuities on periods of tens of thousands of years (the material boundary formulation). In the present study, simple incompressible models are used to determine the Earth's behaviour in the case where the density discontinuity remains at a constant pressure rather than deforming with the material (the isobaric boundary formulation). The calculation of the movement of the boundary is more rigorous than in earlier studies and uses the local incremental pressure calculated at the depth of the boundary and allows for the vertical deformation caused by the change in volume as material changes phase. It is shown that the buoyancy modes associated with the density discontinuities decrease in strength and increase in relaxation time analogous to what results when the density contrast is reduced. Also, two viscoelastic modes arise from an isobaric boundary, which is also predicted when there is a contrast in rigidity or viscosity across a material boundary. The difference in predicted radial deformation between the isobaric boundary model and the material boundary model is largest for long-wavelength loads for which the material incremental pressure at depth is largest. If the isobaric boundary model is appropriate for the treatment of the mineral phase changes in the mantle on glacial rebound timescales, then previous inferences of the deep-mantle to shallow-mantle viscosity ratio based on large-scale deformation (spherical harmonic degree < 10) of the Earth and including data from the early part of the glacio-isostatic uplift are too small.     

Structural variation in Ca(TixSi1−x)O3 perovskites (1>x>0.65) and the ordered phase Ca2TiSiO6

High pressure perovskites in the system CaTiO₃?CaSiO₃ in the composition range from 0 to 50% CaSiO₃ have been re-examined using powder x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. In runs performed at 9 GPa and 1200 °C, a solid solution is obtained with compositions ranging from CaTiO₃ to near Ca(Ti_0.65Si_0.35)O₃, and with the orthorhombic distortion in the CaTiO₃ end-member diminishing to pseudocubic for Ca(Ti_0.65Si_0.35)O₃. Raman spectra show intensity changes and band broadening which are attributed to the reduction of orthorhombic character, but still indicate lower than cubic symmetry for the entire solid solution range. An ordered intermediate, Ca₂TiSiO₆, has been recovered from a run at 14 GPa at 1200 °C. The space group is Fm3m, with a=7.410(2)?Å, and the compound has the double perovskite structure. In this structure, Si-rich octahedral sites and Ti-rich octahedral sites alternate along the three principal axes. Transmission electron microscopy confirms the presence of the Fm3m ordered structure, and also documents the presence of lesser amounts of at least three other ordered perovskite structures which are not resolved in the x-ray data. These may account for extra features observed in the Raman spectrum which are inconsistent with mode assignments for the Fm3m phase. Non face-centered ordering schemes for these regions are suggested based on modeling of the HRTEM patterns.     

Structure and resilience of overwintering habitats of Calanus finmarchicus in the Eastern Norwegian Sea

Very high concentrations of overwintering Calanus finmarchicus were found in the eastern Lofoten Basin of the Norwegian Sea close to the shelf break in January 2001–2002. A coupled 3D hydrodynamic and ecological model was used to study the formation of this deep overwintering aggregation and its stability. The ecological model includes nutrients, phytoplankton and microzooplankton in addition to a stage-structured model of C. finmarchicus. Using a Eulerian approach, the model was initiated with an overwintering stock evenly distributed in the oceanic regions of the Norwegian Sea, i.e. where depths>600 m. Spawning and development of the new generation take place in response to vertical mixing and phytoplankton development. Animals are assumed to begin their descent to overwintering depths of 700–1000 m as late stage Vs. Model results show that, in late summer, high concentrations of animals were found at overwintering depths near the shelf break north of the North Sea, off the northeastern Vøring Plateau and in the eastern Lofoten Basin along the slope of the Barents Sea shelf. They remained there for months due to deep eddies and southward, deep currents along the Norwegian shelf. The simulation experiments indicate that the combined effect of deep anticyclonic circulation and vertical migration behavior of the animals may explain the high concentrations of overwintering C. finmarchicus found in field surveys in the Eastern Lofoten Basin, close to the shelf break.     

Opportunistic Feeding in Mediterranean Blennioid Fishes (Blenniidae and Tripterygion) as Revealed by Experimental Food Provisioning in the Field

Abstract. Consistent differences were found in trophic opportunism between species and between sexes by food provisioning experiments in an assemblage of 15 species of Mediterranean blennioid fishes. The degree of trophic opportunism in the different species was significantly and positively correlated with body size and two features indicative for intensity of male parental care. The degree of trophic opportunism correlated neither with trophic level, depth distribution, or relative abundance. Females of all species in which sex could be distinguished visited the feeding stations at higher frequencies than conspecific males.     

Factors that influence water exchange in spring-fed tidal canals

Environmental factors that influenced the rates of water exchange in a series of tidal, dead-end canals in Crystal River, Florida, were investigated by measuring the rates of dilution of the fluorescent dye Rhodamine WT. Water temperature regime was the most influential of these factors, with type of tidal cycle (spring or neap) and density of submersed vegetation (dominated byHydrilla verticillata) only affecting dye dilution rates in winter treatments. Dye half-lives were shortest in the summer (12–24 h), longest in the fall and spring (60–120 h), and most sensitive to water temperature patterns in the winter (26–60 h). These data provide useful information relating to the timing of hydrilla management using aquatic herbicides in spring-fed tidal systems. A mechanism is proposed that relates the canal water temperatures to tidal water movement and outputs from the adjacent Three Sisters Springs.     

The misrepresentation of the planktonic diatom assemblage in traps and sediments: southern Lake Malaŵi,Africa

Sediment trap collections near Cape Maclear, Lake Malai, were compared to phytoplankton and surface sediment diatoms to assess taphonomic variations. The sedimenting diatom community became progressively different from the diatom plankton with increasing depth: long Nitzschia species were strongly under-represented in the traps (annually, 53% among planktonic diatoms vs. 14% in the offshore 29 m trap; p0.005 by Kruskal-Wallis test), while Melosira was greatly over-represented in traps (32% vs. 57%; p<0.005). The abundances of the minor taxa (Rhopalodia, Fragilaria, Cymbella, and Surirella) were greatly enhanced in traps relative to the plankton, but they were still relatively uncommon (<3% of all diatoms each). Differences in grazing, dissolution, and sinking rates alone are insufficient to account for these distortions; a combination of these, plus perhaps unknown factors, strongly influence the deposited assemblage.These misrepresentations were also present at the sediment surface. The greatest discrepancy was noted for Melosira (32% of plankton vs. 53% of sediment surface diatoms; p<0.005) and for elongate Nitzschia species (53% of plankton vs. 0.8% in sediments; p<0.005). In Lake Malai, at least, paleolimnologists must not assume a straightforward correlation of modern and fossil assemblages.     

Routine analysis of sediment pore water of high ionic strength

Pore water of sediments plays an important role in aquatic systems as mediator and as the reactive zone between the sediment and surface water. Sediment pore waters with high ionic strength from acidified pit lakes were investigated to obtain information about the influence on the lake water quality. The analysis of soluble reactive phosphorus, nitrate, ammonium, silica, dissolved organic carbon, ferrous iron, sulfate, chloride, sodium, potassium, calcium, magnesium, manganese, and total dissolved iron was predetermined as the dataset required for evaluation of water quality. The data collection procedure was optimised by designing a methodology for stabilisation, dilution of pore water samples and adaptation of analytical methods. The developed methodology was evaluated with respect to the effort required in the laboratory under routine conditions. In the first round of analyses, 72% of 638 individual analyses from a random selection of pore water profiles were found to be within the calibration ranges. Remedial actions to handle the remaining 28% of invalid analytical results are exemplified. Differences between comparative analyses of some ions by continuous flow analysis, ion chromatography, and atomic emission spectroscopy were evaluated. The majority of results measured by ion chromatography differed on a highly significant level from results measured by atomic emission spectroscopy. Possible reasons, originating from the extreme sample matrix, are discussed. Finally, the designed methodology and the results of the method comparison are used to recommend the selection of analytical methods under specific conditions.     

Adsorptive Separation of 1, 2‐Dichloroethane from Model Wastewater by Natural Clinoptilolite

The adsorptive separation of 1, 2‐dichloroethane from model wastewater by different Slovak natural and octadecylammonium‐modified clinoptilolites has been studied. It was found that the 1, 2‐dichloroethane will be adsorbed by natural as well as by surfactant‐modified clinoptilolites; all clinoptilolites have a similar adsorption loading. There was also no remarkable difference between the natural and surfactant‐modified clinoptilolites in respect to the adsorption kinetics.