Travertine formation in Plitvice National Park, Yugoslavia: chemical versus biological control

Hydrochemical studies of the Plitvice Lakes and their tributaries (Croatia/Yugoslavia) were coupled with micromorphological investigations on carbonate lake sediments and recent travertines. Karst springs discharge water from aquifers in Triassic and Jurassic dolomites and limestones and collect in lakes, which are ponded behind accreting travertine dams. Waters at springs have a high CO₂ partial-pressure (greater than 7000 ppm) and are slightly undersaturated with respect to calcite (saturation index less than —0·03). CO₂ partial pressure is quickly reduced in swift running streams, leading to very high supersaturation with carbonate minerals (saturation indices between 0·74 and 0·53). Calcite deposition, however, is restricted to the lake bottoms (formation of lake marl) and to the tufa dams. The annual carbonate precipitating capacity of the system based on water balance and downstream loss of dissolved ions is estimated to be on the order of 10 000 t CaCO₃ as cascade deposits (tufa dams) or as micrite in lakes behind the travertine dams. The initial stages of travertine formation as a result of morphological, biological, and chemical factors are (i) moss settling on small ridges in the creek courses, (ii) epiphytes (diatoms and cyanobacteria) settling on the moss surface, (iii) micrite particles resuspending from lake bottoms and being trapped on mucous excretions from bacteria and diatoms, and (iv) inorganic calcite precipitating as sparite at nucleation sites provided by these crystal seeds. Geochemical studies of the lake marl and tufa dams show that amino acids are dominated by aspartic acid. Carbohydrates come from structural polysaccharides of diatoms. The sticky excretions, rich in aspartic acid, are necessary for the initiation of calcite precipitation. They may be a response of algal and bacterial metabolism to environmental stress by either nutrient depletion or high calcium concentrations in ambient waters. The formation of tufa and micrite (lake marl) appears to be initiated by localized biological factors and is not governed by mere calcite supersaturation of the water. Oligotrophy may be an essential precondition for the formation of fresh water carbonate deposits.     

Petrology and Geochemistry of Post-Collisional Early Miocene Volcanism in the Karacada? Area (Central Anatolia, Turkey)

Early Miocene (ca.?21–18 Ma) volcanism in the Karacada? area comprises three groups of volcanic rocks: (1) calcalkaline suite (andesitic to rhyolitic lavas and their pyroclastics), (2) mildly-alkaline suite (alkali basalt, hawaiite, mugearite, benmoreite and trachydacite), and (3) a single trachyandesitic flow unit. Field observations, 40Ar/39Ar ages and geochemical data show that there was a progressive temporal transition from group 1 to 3 in a post-collisional tectonic setting. The calcalkaline suite rocks with medium-K in composition resemble those of subduction-related lavas, whereas the mildly-alkaline suite rocks having a sodic tendency (Na2O/K2O=1.5–3.2) resemble those of within-plate lavas. Incompatible element and Sr-Nd isotopic characteristics of the suites suggest that the lithospheric mantle beneath the Karacada? area was heterogeneously enriched by two processes before collision: (1) enrichment by subduction-related processes, which is important in the genesis of the calcalkaline volcanism, (2) enrichment by small degree melts from the astenosphere, which dominates the mildly alkaline volcanism. Perturbation of the enriched lithosphere by either delamination following collision and uplift or removal of the subducted slab following subduction and collision (i.e., slab breakoff) is the likely mechanism for the initiation of the post-collision volcanism.     

3rd International Conference on Unsaturated Soils (UNSAT 2002) Recife, Brazil, 10–12 March 2002

Conference Report

3rd International Conference on Unsaturated Soils (UNSAT 2002) Recife, Brazil, 10–12 March 2002     

THE SOLAR Mg II h AND k LINES – Observations and Radiative Transfer Modeling

This paper presents observations of the MgII h and k lines obtained with the UVSP instrument that flew aboard the SMM satellite. Both spatially averaged and resolved observations are compared with calculated profiles of the lines from standard, plane-parallel solar models. The radiative transfer calculations presented take full account of partial frequency redistribution and wavelength overlap of the h and k lines. A comparison between theoretical and observed wing profiles indicates that current one-dimensional models underestimate the temperature in the middle photosphere. The cores of spatially resolved dark intra-network profiles are well reproduced by the model calculations, while the spatially averaged profiles have in general broader emission peaks, indicating that the additional broadening is due to a contribution of magnetic network profiles.     

Constraint on Parameters of Inverse Compton Scattering Model for PSR B2319+60

Using the multifrequency radio profiles of pulsar PSR B2319+60, two parameters of inverse Compton scattering model, the initial Lorentz factor and the factor of energy loss of relativistic particles are constrained.     

Uv Line Profiles of Biv from a 10-Ps Krf-Laser-Produced Plasma

We use a KrF laser that generates pulses up to 8 × 10¹⁴ watt/cm² when focused onto a boron nitride target. We measured the line profile of the near UV, n = 0, n = 2 transitions of helium-like boron and we studied the triplet 1s2p³P-1s2s³S at 2825.85 (J = 1), 2821.68 (J = 2) and 2824.57 (J = 0) Å, at various positions from the target surface, in order to correlate details of this profile with the presence of self-generated magnetic fields and/or dynamic turbulence among other processes. We fitted the measured profiles to a 0-D model, that includes, Stark, Doppler, Zeeman and instrumental broadening. The effect of the magnetic field was included using an intermediate field calculations. Preliminary results of these measurements are presented and discussed.     

Arm and Interarm Star Formation in Spiral Galaxies

We present an outline of our study of the effects of star formation on the different components of the interstellar medium in the discs of spiral galaxies, both globally and as a function of arm and interarm environment. We are in the process of obtaining images of 57 spiral galaxies at low inclinations, and analysing them to study the distribution of recent massive star formation, old stars, young stars, gas and dust. We will dissect the images into arm and interarm regions and compare and contrast the morphology and scale lengths within these regions inH_α, HI, the near infrared, optical and (where available) CO. Modelling will show how the scale lengths are affected by star formation, how this differs between arms and interarms, and whether the Schmidt Law varies from the global values in the arm and interarm regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Two distinct morphological types of magnetic shear development and their relation to flares

From a morphological study of the evolution of six active regions, we found two types of processes for the development of magnetic shear configurations between sunspots: (A) collision of two sunspots of opposite magnetic polarities, and (B) successive emergence of twisted magnetic flux ropes. We conclude that the process (B) might be essential for the production of major flares.