Jets of moderate optical thickness from thick acceretion disks

The intense, super-Eddington, well-collimated radiation field generated by non-Keplerian accretion disks orbiting black holes can accelerate matter to high velocities. By using an energy balance argument which is both simple and physically appealing, we show that ordinary electron-proton plasmas can only reach relativistic gamma-factors up to 1.5. A light electron-positron plasma could be accelerated up to 5. The approach is valid if the optical depth along the accelerating funnel is significant, and the optical depth across the funnel is negligible. It is, therefore, appropriate for long, narrow funnels. Our method could easily be generalized to disk geometries other than the examples we give.     

Reversible Markov grain sequences in granite

A key feature of an ideal granite is the occurrence of grain sequences which are reversible Markov chains. This property was tested using a ² test on a 2 × 6 contingency table consisting of reversible grain pairs for microcline, plagioclase, quartz, and biotite, and on a 2 × 12 contingency table consisting of reversible grain triads. All 28 samples examined from the Pacolet Mills pluton, South Carolina, passed the ² test for grain pairs, and all but three of these passed the ² test for grain triads. The coefficients of the reversibility matrix were examined for statistical significance after normalization, using a logarithmic transformation. For all three phases of the Pacolet Mills pluton, the average coefficients were in the range 1.08–0.89. Elevated and depressed values of these coefficients suggested possible differences among the three phases of the granite in their crystallization paths.     

Effects of the duration of exposure to mercuric chloride on the embryogenesis of the estuarine teleost, Fundulus heteroclitus

Embryos of Fundulus heteroclitus in the 4–8 cell stage were exposed nine Hg⁺⁺ (as mercuric chloride) concentrations from 0 to 100 μg/litre (ppb) for 32 days, 5 days, 2 days and 1 day. All but the chronically (32-day) exposed embryos were allowed to continue development in mercury-free water (20% S) after the exposure period to determine the effect of duration of exposure on three parameters of development. Ninety-six hour survival, total hatching success and per cent incidence of lateral spinal curvature were measured. Only embryos exposed to mercury for a single day showed a significant increase in survival in comparison with embryos exposed for a longer period of time after four days of development. Under all conditions of exposure, survival was reduced at concentrations greater than 40 ppb Hg⁺⁺. Hatching success of chronically exposed embryos was significantly reduced at concentrations greater than 10 ppb Hg⁺⁺. Significant increases in total hatchability were effected by reducing the duration of exposure to five days and one day. Of eleutheroembryos emerging following chronic exposure to mercury, increases in the incidence of lateral spinal curvature were noted at all concentrations above 20 ppb Hg⁺⁺. Significant reductions of this disorder were observed among eleutheroembryos exposed to mercury for 5 days, 2 days or 1 day. Of the three parameters measured, the incidence of lateral spinal curvature and the resulting VH₅₀ value gave a more sensitive evaluation of the embryotoxic effects of Hg⁺⁺ as mercuric chloride on the estuarine teleost, Fundulus heteroclitus.     

Similarity method for modelling hydroelastic offshore platforms

Similarity methods provide a very powerful technique for modelling offshore platforms. Such methods are preferable for situations where the wave-structure interaction process is complex, and its modelling cannot be expressed by rigorous mathematical formulations.In modelling the interaction between waves and an offshore platform, the dynamic properties of the wave and structure must be considered together to determine the similitude parameters. The paper gives details of such an investigation using a frequency response function approach. Three different models, which are termed general, distorted and nondistorted Froudian models, are developed. From the general model, it was found that the velocity scales of the wave and structure could be treated differently. Internal and external force scales could also be different. It is observed then that a restrictive modelling criterion is used when the velocity scale of a structure is obtained from Froudian scale modelling. Furthermore, detailed scales for practical use are developed. The model parameters for an acrylic plastic tripod tower platform with length scales of 1/50 and 1/70 are given, and the accuracy involved among the three models determined for these scales. The sensitivity of the results to slight variations in thickness and Young's modulus are also discussed.     

Final dissolved organic carbon broad community intercalibration and preliminary use of DOC reference materials

A broad community intercalibration exercise for accurate measurement of dissolved organic carbon (DOC) in seawater has been carried out over a period of 5 years. A set of 10 natural samples with DOC content from 40 to 200 μM C were accompanied by two glucose standards and a “zero C” blank; all sealed in glass ampoules. Samples were sent to all interested analysts for “blind” analysis; 62 laboratories in 17 countries participated. A total of 59 separate analyses were determined to be acceptable by screening criteria based on standards and blank; another nine sets of analyses did not pass the screening. The majority of the analyses, both those passing and those that did not, were performed with high temperature combustion (HTC) methods, six sets of analyses were done using wet chemical oxidation methods.From the 53 sets of acceptable HTC analyses, the coefficient of variation (%CV) for analytical comparability of the samples was 10% (“community precision”). It is estimated that the individual replicate injection precision for most instruments was approximately 2% and that no additional variability was caused by differences within the ampoules of individual samples. The additional variability over 2% was likely a result of both random and systematic differences in analytical capabilities from instrument to instrument and from day to day for individual instruments. With an arbitrary selection after the fact, smaller subsets of analysts can show comparability better than 10% and duplicate or triplicate runs on different days of the full sets of samples in several laboratories showed comparability in the 2–6.5% range. Experienced oceanic analysts, with internal or shared reference materials, can now show reproducibility and comparability at a level closer to 2%.Preliminary use of DOC reference materials by 14 participants showed day-to-day reproducibilities for their laboratories in the 2–6% range in most cases; several with poorer reproducibility do not normally perform DOC analyses on samples with concentrations as low as the deep ocean reference used here. Use of these reference materials can also give a demonstration of comparability between laboratories. For credibility of DOC analyses, it is necessary for analysts to use community reference materials and report results of their analytical performance with these references.This paper does not identify individual data nor should it be considered an evaluation of individual laboratories or analysts. The purpose is to show the summary picture of the international community of DOC analysts as it existed in the mid- to late 1990s.     

Seismic response of earth dam with varying depth of liquefiable foundation layer

Earthquake induced liquefaction continues to be a major threat to many engineered structures around the world. Analysis of liquefaction becomes particularly difficult for two-dimensional (and 3D) problems such as dam/foundation systems. Predominantly, analyses for such systems are performed utilizing some type of finite element or finite difference procedure. Verification or validation of the analyses relies on very limited field performance data with reduced knowledge of the full scope of system conditions or loading conditions.Research reported in this paper represents a portion of ongoing work to obtain a database of information useful for numerical model calibration and to gain a better understanding of the complex dynamics of liquefying foundations under earth dams. Specifically, a highly instrumented model of an earth dam with clay core founded on a liquefiable foundation subjected to earthquake loading is being studied. Properties of the liquefiable foundation are varied to determine the related effects on the overlying earth dam. In this paper, results from three centrifuge physical models will be presented. The models are identical, with the exception of the location (depth) of a liquefiable layer in the foundation, and are subjected to the same dynamic excitation. Results and discussion related to the significance of the liquefiable layer location within the foundation and damage to the earth dam are presented.     

Book Review: Using Geochemical Data: Evaluation,Presentation, Interpretation. By Hugh R. Rollinson. Longman,Edinburgh Gate, 1993, 352 p., £39.99 (U.K.), paperback,ISBN 0-582-06701-4

Mathematical Geosciences -     

Changing women's roles, changing environmental knowledges: evidence from Upper Egypt

The aim of this paper is to investigate the ways in which changing gender roles in a Bedouin community in Upper Egypt, brought about by settlement over the last 20 years on the shores of Lake Nasser, have impacted on the accumulation and development of indigenous environmental knowledges by Bedouin women. The research was carried out among four groups of Ababda Bedouin in the Eastern Desert of Egypt and involved in-depth monthly conversations carried out over a period of 12 months. The main conclusions of the study are that the women of the study area have had to develop new knowledges which, in some cases, are now different from those held by men because of the different economic circumstances in which many find themselves; that these knowledges are fluid, dynamic and ever-changing with their own internal dynamism; and that socially constructed notions of gender are vital in the development process, notions that are sensitive to both men's and women's interests and their interrelationships.     

A plate tectonic origin for diamond-bearing kimberlites

It is proposed that diamond-bearing kimberlites are the result of activity at the deep end of a subducted oceanic plate. The source material consists of oceanic plate along with entrained hydrous and carbonaceous material that upon metamorphism or melting form diamond-bearing kimberlite. The emplacement should occur deep under continents and parallel to fold mountain ranges. The South African fields are cited as a specific example of the suggested process.     

A method for determining the reversibility of a Markov sequence

This paper describes, given a tally matrix with strictly positive entries, a method to determine whether the associated Markov process is reversible, and (for reversible Markov processes) methods to compute the reversibility matrix from the tally matrix. If the tally matrixN is symmetric, then it is shown that the Markov process must be reversible and the reversibility matrixC equalss (R ^–1NR^–1), whereR is the diagonal matrix whosei ^th diagonal entry is the sum of the entries of thei ^th row ofN (for everyi) ands denotes the sum of all the entries ofN. Because a symmetric tally matrix is of special importance in applications, a ² test is proposed for determining, in the presence of experimental errors, whether such a matrix is symmetric.