Observations of limb flares with a soft X-ray telescope

The structure and evolution of 26 limb flares have been observed with a soft X-ray telescope flown on Skylab. The results are:

1. One or more well defined loops were the only structures of flare intensity observed during the rise phase and near flare maximum, except for knots which were close to the resolution of the telescope in size (≈2 arc seconds) and whose structure can therefore not be determined.
2. The flare core features were always sharply defined during the rise phase.
3. For the twenty events which contain loops, the geometry of the structure near maximum was that of a loop in ten cases, a loop with a spike at the top in four cases, a cusp or triangle in four cases, and a cusp combined with a spike in another two cases.
4. Of the fifteen cases in which sufficient data were available to allow us to follow a flare's evolution, five showed no significant geometrical deviation from a loop structure, one displayed little change except for a small scale short-lived perturbation on one side of the loop 10 seconds before a type III radio burst was observed, eight underwent a large scale deformation of the loop or loops on a time scale comparable to that of the flare itself and one double loop event changed in a complex and undetermined manner, with reconnection being one possibility.

Based on observation of the original film, it is suggested that the eight flares which underwent large scale deformations had become unstable to MHD kinks. This implies that these flares occurred in magnetic flux tubes through which significant currents were flowing. It is suggested that the high energy electrons responsible for type III bursts accompanying these flares could have been accelerated by the V x B electric field induced by a small scale short-lived perturbation of parts of a flaring flux tube, similar to the one perturbation which was observed having these characteristics.     

Late- to post-orogenic exhumation of the Central Pyrenees revealed through combined thermochronological data and modelling

Apatite (U–Th)/He and fission track thermochronometry have been combined with 3D thermal modelling to constrain the late- to post-orogenic exhumation history of the Central Pyrenees, Spain. Data from four massifs immediately north and south of the present drainage divide of the mountain belt reveal a diachroneity in the transition from syn- to post-orogenic forcing of exhumation. Immediately south of the drainage divide, rapid exhumation of ∼1.5 mm year⁻¹ decelerated after ∼30 Ma to ∼0.03 mm year⁻¹. A similar transition occurred immediately north of the drainage divide at the same time. Further south, in the core of the Axial Zone antiformal stack of the Pyrenees, rapid (∼1 mm year⁻¹), syn-orogenic exhumation continued to ∼20 Ma, but slowed to ∼0.1–0.2 mm year⁻¹ soon after that time. This order of magnitude decrease in exhumation rates across the orogen records the diachronous transition into a post-orogenic state for the mountain belt. These data do not record rejuvenation of exhumation in Late Miocene or Pliocene times driven either by large-scale base-level change or an evolution to more erosive climatic conditions.     

Enhancement of In Situ Bioremediation of BTEX-Contaminated Ground Water by Oxygen Diffusion from Silicone Tubing

A field study of oxygen-enhanced biodegradation was carried out in a sandy iron-rich ground water system contaminated with gasoline hydrocarbons. Prior to the oxygen study, intrinsic microbial biodegradation in the contaminant plume had depleted dissolved oxygen and created anaerobic conditions. An oxygen diffusion system made of silicone polymer tubing was installed in an injection well within an oxygen delivery zone containing coarse highly permeable sand. During the study, this system delivered high dissolved oxygen (DO) levels (39 mg/L) to the ground water within a part of the plume. The ground water was sampled at a series of monitors in the test zone downgradient of the delivery well to determine the effect of oxygen on dissolved BTEX, ground water geochemistry, and microbially mediated biodegradation processes. The DO levels and Eh increased markedly at distances up to 2.3 m (7.5 feet) downgradient. Potential biofouling and iron precipitation effects did not clog the well screens or porous medium. The increased dissolved oxygen enhanced the population of aerobes while the activity of anaerobic sulfate-reducing bacteria and methanogens decreased. Based on concentration changes, the estimated total rate of BTEX biodegradation rose from 872 mg/day before enhancement to 2530 mg/day after 60 days of oxygen delivery. Increased oxygen flux to the test area could account for aerobic biodegradation of 1835 mg/day of the BTEX. The estimated rates of anaerobic biodegradation processes decreased based on the flux of sulfate, iron (II), and methane. Two contaminants in the plume, benzene and ethylbenzene, are not biodegraded as readily as toluene or xylenes under anaerobic conditions. Following oxygen enhancement, however, the benzene and ethylbenzene concentrations decreased about 98%, as did toluene and total xylenes.     

Classes of Anisotropic Media: A Tutorial

The purpose of the present article is to give a precise definition and analysis from first principals of anisotropy, as the term applies to elastic media, taking care to avoid unnecessary assumptions. Two fundamental concepts, material invariance and symmetry group of a material, are defined purely in terms of the stress-strain relation. The implications of material symmetry, or in other words, of anisotropy, for the structure of the stiffness tensor are then investigated. Using the reduced notation of Voigt, these results are presented as the well-known simplifications in the form taken by the six-by-six stiffness matrix that represents the material's stiffness tensor. A new, simple proof is given for the remarkable fact that an elastic medium cannot have rotational symmetry by an angle of less than 90° without being transversely isotropic. In addition, the mutual relation that the notions of elastic symmetry and crystal symmetry have with respect to the so-called orthogonal group is sketched. Despite the historical association between anisotropic elastic materials and the study of crystals, the given presentation shows that conceptually the notion of anisotropy in elastic media is entirely independent of that of crystal symmetry.     

Ankerite carbonatite from Swartbooisdrif, Namibia: the first evidence for magmatic ferrocarbonatite

Although general accounts of carbonatites usually envisage Ca-Mg carbonate melts evolving by fractional crystallisation to Fe-rich residua, there is longstanding concern that ferrocarbonatites may actually be products of hydrothermal rather than magmatic processes. All previously published examples of ankerite- and/or siderite-carbonatites fail to show one or more of the isotopic criteria (all determined on the same sample) thought to be diagnostic of crystallised magmatic carbonate liquids. Ferrocarbonatite dykes cut Archaean-Proterozoic basement at Swartbooisdrif, adjacent to the NW Namibia-Angola border. Their age is uncertain but probably ~1,100 Ma and their associated fenites are rich in sodalite. Where unaffected by subsequent recrystallisation, their petrographic textures resemble those of silicate layered intrusions; ankerite, magnetite and occasionally calcite are cumulus phases, joined by trace amounts of intercumulus pyrochlore. Ankerite is zoned, from Ca(Mg, Fe²⁺)(CO₃)₂ cores towards ferroan dolomite rims. Calcite contains ~1.7% SrO, plus abundant, tiny exsolved strontianite grains. Magnetite is close to pure Fe₃O₄. Pyrochlore has fine-scale euhedral oscillatory zoning and light-REE-enriched rims. ICP-MS analysis of magnetite and pyrochlore from the carbonatite allows calculation of their modal amounts from mass-balance considerations. Sodalite from the fenite is REE poor. Geothermometry, using either the calcite-dolomite solvus or oxygen isotope fractionation between calcite and magnetite, gives temperatures in the range 420-460 °C. Initial Sr, Nd and Pb isotopic ratios of the ferrocarbonatites (⁸⁷Sr/⁸⁶Sr=0.7033; )Nd=0.2-1.0; ²⁰⁶Pb/²⁰⁴Pb=16.37; ²⁰⁷Pb/²⁰⁴Pb=15.42; ²⁰⁸Pb/²⁰⁴Pb=36.01) are appropriate for an ~1,100-Ma magmatic carbonatite. Likewise, carbonate '¹⁸O=8.0 and '¹³C=-7.36 indicate little or no subsequent shift from magmatic values. It appears that dense ankerite and magnetite dominated crystal accumulation from a melt saturated in these phases, plus calcite and pyrochlore, with prior fractionation of a silicate mineral and apatite. The resulting ferrocarbonatite lacks a silicate mineral (excluding fenite xenocrysts) and apatite. It has unusually low (basalt-like) REE abundances and (La/Lu)_n, and low concentrations of Ba, Rb, U, Th, Nb, Ta, Zr and Hf. Very high Nb/Ta and low Zr/Hf imply that the evolution of the parental magma involved immiscible separation of a carbonate from a silicate melt. The sodalite-dominated Swartbooisdrif fenites suggest that the parental melt also had a substantial Na content, in contrast with the ferrocarbonatite rock.     

Automated Digital Photogrammetry: A Valuable Tool for Small‐scale Geomorphological Research for the Non‐photogrammetrist?

Digital photogrammetry provides a tool with which to automatically generate digital elevation models (DEMs). The necessary equipment is now both readily available and affordable: thus there is considerable potential for this technique to be widely adopted in geomorphological studies. But is it possible for geomorphologists without a background in photogrammety to use it successfully? As part of a larger study into rill initiation by overland flow, a non‐metric digital camera and ERDAS IMAGINE OrthoMAX software were used to generate small‐scale DEMs of soil surface microtopography. This paper reports on the procedure used, highlights potential pitfalls, and comments on the quality of the resultant DEMs. Whilst acquisition of high‐quality images using a digital camera is relatively straightforward, problems were subsequently encountered due to the small size of the internal imager and the need for camera calibration. Potential stumbling blocks in the use of the software lay in the setting‐up of ground control points and the use of tie‐points and check‐points, as well as several software glitches not identified in the current manual. Nonetheless, once these problems were overcome the technique proved to be a simple, effective and fast tool for generating high quality microtopographical DEMs. This methodology shows great promise for future geomorphological studies that require these kinds of surface data.     

Global Change and Health — The Good, The Bad and The Evidence

Debates over the merits and demerits of globalisation for health are increasingly polarised. Conclusions range from globalisation being essentially positive for health, albeit with a need to smooth out some rough edges, to one of utter condemnation, with adverse effects on the majority of the world's population. Anyone wading into this debate is immediately confronted by seemingly irreconcilable differences in ideology, opinion and interests. Both camps agree that global changes are occurring, and with them many of the determinants of population health status. While some skepticism persists about whether “globalisation” has value beyond being a fashionable buzzword, most agree that we need better understanding of these changes. Two difficult questions arise: (i) What are the health impacts of these changes; and (ii) how can we respond more effectively to them? To move beyond the stand-offs that have already formed within the health community, this paper reviews the main empirical evidence that currently exists, summarises key points of debate that remain, and suggests some ways forward for the research and policy communities. In particular, there is need for an informed and inclusive debate about the positive and negative health consequences of globalisation.     

The significance of the Vredefort Dome for the thermal and structural evolution of the Witwatersrand Basin, South Africa

Summary The Vredefort Dome represents an area of significant ( 10 km) structural uplift within the central parts of the economically important Witwatersrand Basin. Its rocks experienced higher grades of metamorphism than the equivalent stratigraphic horizons exposed around the periphery of the basin. Recent studies of this medium- to high-grade metamorphism, as well as new evidence concerning the origin of the dome, have contributed to a metamorphic model for the Witwatersrand Basin as a whole. This evidence shows that the gold-bearing strata experienced at least two metamorphic events at ca. 2 Ga. The unusually high strain rate and shock deformation features exposed in the rocks of the dome rule out an endogenous origin by tectonic or diapiric processes. Recent work on these features has shown that the dome is best explained as the central uplift of a large, 250–300 km diameter, 2023±44 Ma old meteorite impact structure, the extent of which closely correlates with the present-day limits of the Witwatersrand Basin. Impact-related deformation features in the Vredefort rocks facilitate the separation of metamorphic textures developed during a pre-impact event associated with the 2.05–2.06 Ga Bushveld magmatism, and textures developed during a slightly lower-grade, post-impact, static overprint. The post-impact overprint decreases in intensity outwards from the dome. It is attributed to the massive disturbance of the thermal structure of the crust by impact-induced exhumation, and to shock heating of the rocks as a consequence of the impact event.

---

Die Bedeutung des Vredefort-Domes für die thermische und strukturelle Entwicklung des Witwatersrand-Beckens, Südafrika

Zusammenfassung Der Vredefort Dom ist ein Gebiet von signifikantem (ca. 10 km), strukturellem Uplift im Zentralbereich des wirtschaftlich bedeutungsvollen Witwatersrand-Beckens. Die Ges-teine des Doms haben höhere Metamorphosebedingungen erfahren als die stratigraphisch äquivalenten Lagen, die im Randbereich des Beckens aufgeschlossen sind. Kürzlich durchgeführte Untersuchungen dieser mittel-bis hochgradigen Metamorphose und neueste Ergebnisse zur Entstehung des Domes haben einen Beitrag zu einem Metamorphose-Modell für das gesamte Witwatersrand-Becken geleistet. Diese neuen Befunde zeigen, daß die Gold-hältigen Gesteinsschichten zumindest zwei metamorphe Ereignisse vor ca. 2 Ga erfahren haben. Die ungewöhnlich hohen Beanspruchungsraten und die Stoßwellendeformationsstrukturen, die in den Gesteinen des Doms belegt sind, sprechen gegen einen endogenen Ursprung durch tektonische oder diapirische Prozesse. Neuere Arbeiten an diesen Phänomenen haben gezeigt, daß der Dom am besten als die zentrale Struktureinheit (Zentralberg) einer sehr grossen, 250–300 km weiten und 2023±4 Ma alten Meteoriteneinschlagsstruktur verstanden werden kann, deren Ausmaß eng mit den jetzigen Grenzen des Witwatersrand-Beckens übereinstimmt. Die Gegenwart von Impakt-bezogenen Deformationsstrukturen in Vredefort-Gesteinen erlaubt es, die metamorphen Texturen, die während eines hochgradigen, mit dem 2.05–2.06 Ga Bushveld Magmatismus korrelierten, metamorphen Stadiums vor dem Impaktereignis entstanden sind, von den Texturen zu trennen, die ein statisch metamorphes Ereignis von etwas geringerer Stärke, das nach dem Impakt stattfand, produzierte. Die Spuren des post-Impakt Ereignisses nehmen in ihrer Stärke zum Rand des Domes ab. Dieser Effekt wird durch eine massive Störung der thermischen Krusten-Struktur erklärt, die als Resultat einer Kombination von Impakt-induzierter Exhumierung, von Schock-Aufheizung der Krustengesteine, und von Erwärmung durch einen gewaltigen, jetzt erodierten Impaktschmelzgesteinskörper gesehen wird.

     

Elemental mobility during zeolite facies metamorphism of the Tertiary basalts of eastern Iceland

Major and trace element analyses are reported for 70 lavas from the Tertiary succession of eastern Iceland. The low grade regional metamorphism responsible for the zeolite zones appears to have caused significant mobilization of some elements, particularly Si, Mg, K, Rb, Sr and light rare earth elements (LREE). In contrast, values for Ti, P, Zr, Y, Nb, Ta, Hf and some of the rare earth elements show a high degree of correlation and this is taken to imply that these elements have been relatively unaffected by metasomatic transport. The demonstrated mobility of Sr and LREE suggest that Sr isotopic and rare earth data obtained from the eastern Iceland lavas must be interpreted with caution.