Filamentation instability of Alfvén waves

The filamentation instability of finite amplitude left-hand circularly polarized Alfvén waves has been investigated taking into account the second-order density and magnetic field perturbations that are created by the Alfvén wave pressure. The minimum scale length and time over which the filamentation occurs, are found. Our results are applied to Alfvén waves that should scatter cosmic rays in the interstellar medium (ISM).     

Mineral magnetic and physical properties of surficial sediments and onshore samples from the southern basin of Lake Baikal, Siberia

Results of lithostratigraphic and mineral magnetic analysis of two surficial sediment cores (21 cm and 45 cm in length) collected from the Southern basin of Lake Baikal at a water depth of 1390 m, are presented. The sediments have been measured for a wide range of mineral magnetic parameters in order to assess their value in the identification of turbidite layers. Particle size and geochemical data are also presented and these explain some of the down core variations in magnetic mineralogy. It is suggested that changes in the particle size frequency distributions down core may be related to fossil diatom shells. One of the cores has been dated using ²¹⁰Pb. The sediment cores were cross-correlated using low frequency magnetic susceptibility (f) and these cores can also be correlated with a nearby core collected earlier in 1992. Changes in the magnetic parameters of lf, IRMs and HIRM₂₁₀ suggest that there are significant changes in the concentration of ferrimagnetic minerals in the sediment cores, indicating changing sediment sources and/or increasing concentrations of spheroidal carbonaceous particles and the dissolution of minerals through reduction below the oxidised layer within the sediment core.     

Core waves recorded in central Europe

Summary The core waves recorded at four seismograph stations in Central Europe were investigated with the aim to find the most suitable set of travel-time curves and a fitting model of the core structure. Another travel-time curve was constructed by means of time differences between the core phases. The differences in the features of the travel-time curves were studied using records of shallow, intermediate and deep shocks from the same focal region (Tonga).     

Sediment oxygen profiles in a super-oxygenated antarctic lake

Perennially ice-covered lakes are found in the McMurdo Dry Valleys of southern Victoria Land, Antarctica. In contrast to temperate lakes that have diurnal photic periods, antarctic (and arctic) lakes have a yearly photic period. An unusual feature of the antarctic lakes is the occurrence of O2 at supersaturated levels in certain portions of the water column. Here we report the first sediment O2 profiles obtained using a microelectrode from a perennially ice-covered antarctic lake. Sediment cores collected in January and October 1987 from Lake Hoare in Taylor Valley show oxygenation down to 15, and in some cases, 25 cm. The oxygenation of sediments several centimeters below the sediment-water interface is atypical for lake sediments and may be characteristic of perennially ice-covered lakes. There is a significant difference between the observed January and October sediment O2 profiles. Several explanations may account for the difference, including seasonality. A time-dependent model is presented which tests the feasibility of a seasonal cycle resulting from the long photoperiod and benthic primary production in sediments overlain by a highly oxygenated water column.     

A technique to assess dispersion

A technique to arrive at conditions congenial to maintaining a stable suspension of fine particles has been described. The technique involves continuous monitoring of apparent loss of weight of a body suspended in settling slurries. Such a technique is useful in fixing the best conditions for gravity separation of fine particles involving Bartles-Mozley concentrators or hydrocyclones.     

A major lava tube system from Undara Volcano,North Queensland

The Undara Volcano erupted 0.19 m.y. ago and formed lava fields covering 1,500 km² with a volume of approximately 23 km³. One of the flows extended 160 km on a gradient that averaged only 0.3°. This great length was a result of very high effusion rates, favourable topography and lava tube efficiency. The Undara lavas are rather uniform hawaiites. Lava temperatures are estimated to have been somewhat less than 1175–1220°C and viscosities greater than 10 to 30 Pa s. Long, apparently single lava tubes are well preserved in many places. They are marked by depressions, caves and long level ridges. A system of lava tubes extends for perhaps more than 100 km. The features of the lava tubes are comparable with those described elsewhere. Aligned depressions associated with caves appear to have formed contemporaneously. Most are much wider than the caves and probably represent collapsed lava ponds. The lava tubes appear to have formed by roofing over of lava channels. Close to lava tubes, the rocks developed strongly oxidised characteristics, such as oxidised olivine phenocrysts, ferric clinopyroxene and extensively developed hematite. Differentiated lava forms drips in some caves and is also oxidised.     

Protein Digestibility by Khosti Fish Colisa fasciatus (Pisces,Anabantidae) under the Influence of Certain Factors

Protein digestibility was determined via two feeding trials in glass aquaria by the indicator (Chromium oxide) method using a mixture of shrimp powder with oil cake and wheat bran respectively (1: 1 W/W) as food. Analyses of feed were made to calculate the crude protein, carbohydrate, lipid and ash contents. Feces were analysed to determine their total nitrogen and chromium oxide. Protein digestibility was studied in relation to salinity of the medium size and weight of the fish body. The apparent digestibility of crude protein of a corresponding die has shown that 1st: Younger fish do possess a lower capability of protein digestion than fish having a medium weight and medium size; 2nd: Older fish with a high weight and large size exhibit no improvement in efficiency to digest the food protein, though their weight and size are significantly greater than those of middle weight and medium size fish; 3rd: The replacement of oil cake by wheat bran decreases the protein digestibility of shrimp powder mixture with them; 4th: A salinity (NaCl) of the aquatic medium at or above 3 g/l depletes the protein digestibility when fish of the same size were experimented with; but a salinity up to 2 g/l does not result in a significant decrease in the protein digestion rate.     

Erosion rates and mechanisms of knickzone retreat inferred from 10Be measured across strong climate gradients on the northern and central Andes Western Escarpment

A steep escarpment edge, deep gorges and distinct knickzones in river profiles characterize the landscape on the Western Escarpment of the Andes between ~5°S and ~18°S (northern Peru to northern Chile). Strong north–south and east–west precipitation gradients are exploited in order to determine how climate affects denudation rates in three river basins spanning an otherwise relatively uniform geologic and geomorphologic setting. Late Miocene tectonics uplifted the Meseta/Altiplano plateau (~3000 m a.s.l.), which is underlain by a series of Tertiary volcanic‐volcanoclastic rocks. Streams on this plateau remain graded to the Late Miocene base level. Below the rim of the Meseta, streams have responded to this ramp uplift by incising deeply into fractured Mesozoic rocks via a series of steep, headward retreating knickzones that grade to the present‐day base level defined by the Pacific Ocean. It is found that the Tertiary units on the plateau function as cap‐rocks, which aid in the parallel retreat of the sharp escarpment edge and upper knickzone tips. ¹⁰Be‐derived catchment denudation rates of the Rio Piura (5°S), Rio Pisco (13°S) and Rio Lluta (18°S) average ~10 mm ky^?1 on the Meseta/Altiplano, irrespective of precipitation rates; whereas, downstream of the escarpment edge, denudation rates range from 10 mm ky^?1 to 250 mm ky^?1 and correlate positively with precipitation rates, but show no strong correlation with hillslope angles or channel steepness. These relationships are explained by the presence of a cap‐rock and climate‐driven fluvial incision that steepens hillslopes to near‐threshold conditions. Since escarpment retreat and the precipitation pattern were established at least in the Miocene, it is speculated that the present‐day distribution of morphology and denudation rates has probably remained largely unchanged during the past several millions of years as the knickzones have propagated headward into the plateau. Copyright © 2011 John Wiley & Sons, Ltd.     

Dynamic simulation of littoral zone habitats in low Chesapeake Bay. II. Seagrass habitat primary production and water quality relationships

Seagrasses are indicators of ecosystem state because they are sensitive to variations in water composition and clarity resulting from watershed-level impacts. A simulation model designed to studyZostera marina (eelgrass) habitat dynamics in a variable littoral zone environment was used to address the potential ecological responses to eutrophication in lower Chesapeake Bay. The adjacent channel boundary environment is a source of dissolved and particulate materials to the littoral zone. In the simulations, concentrations of key water quality variables in the adjacent estuarine channel boundary were either halved or doubled relative to the base case to investigate light versus nitrogen effects. The role of the seagrass meadow in littoral zone carbon and nitrogen dynamics was evaluated when meadow size was changed in the model. Particulate and dissolved organic carbon accounted for 83% of the submarine light attenuation in the seagrass meadow. In all model runs, the water column concentrations of chlorophylla and dissolved inorganic nitrogen (DIN) were below the habitat criteria proposed as critical to seagrass survival. Eelgrass community production was carefully regulated by the interactive effects of light, nitrogen, and grazing on epiphyte growth. Increased eelgrass coverage in the littoral zone led to a simulated doubling of ecosystem primary production but reduced the fraction of production by planktonic and sediment microalgae. The simulation model presented here demonstrated the importance of material input from the channel in littoral zone biogeochemical dynamics. Submarine ligh regulated primary production more strongly than inorganic nitrogen concentrations in the model. External DIN concentrations influenced seagrass survival indirectly: enrichment stimulated growth of epiphytes and phytoplankton and promoted shading of the seagras leaf. The model was based upon a unimpacted ecosystem and deteriorated water quality negatively influenced primary production greater than the increases triggered by improved condition. Increased material loading to the littoral zone reduced submarine light availability, increased phytoplankton production, lowered ecosystem production, and reduced subtidal vegetated habitat. This simulation model of the estuarine littoral zone model combines hydrodynamics, biogeochemical sources and sinks, and living resources in order to better understand structure, function, and change in aquatic ecosystems.