Concretions, isotopes, and the diagenetic history of the Oxford Clay (Jurassic) of central England

In interpreting the results of a petrographic and isotopic study of concretions, a range of subjects is discussed including the original texture of the Oxford Clay sediment, Jurassic palaeotemperatures, the diagenetic history of pore-waters and the palaeo-hydrology of central England. The concretions are all composed predominantly of calcite. They include precompactional, pyrite-rich concretions that later suffered an eposide of brecciation, and others that only commenced to form after compaction had crushed ammonite shells included in the bituminous clay sediment. Petrographic, chemical, and especially carbon isotope data demonstrate a dominantly organic source for the carbon in the early formed concretions. Oxygen isotopes indicate formation at the same temperatures (13-16°C) at which benthic molluscs were living. Concretion growth in pelleted, anaerobic mud proceeded concurrently with bacterial sulphate reduction and pyrite precipitation. Cracking of the concretions started at this stage: in a few concretions, the cracks were also partially filled with brown calcite. During post-compactional growth, δ¹³C increased and pyrite content decreased, showing waning organic influence; δ¹⁸O decreased. The brecciated concretions were intruded by clay in which baryte crystals grew; finally, most remaining voids were filled with strongly-ferroan calcite of δ¹⁸O about—7 PDB and δ¹³C about O PDB. This must indicate strong depletion of the pore waters in ¹⁸O. Mechanisms that might lead to this are reviewed. It is concluded that the sequence of mineralogical and chemical changes is most readily explained if originally marine porewaters, first modified by bacterial activity, were flushed from the compacting clays by water of ultimately meteoric origin. This had its source in palaeo-aquifers beneath the Oxford Clay. Speculative attempts are made to relate this history to the geology of the region.     

Remarkable Species Diversity on Adjacent Salt Lakes in South Australia

正Lakes Acraman,Gairdner,Harris and Everard are a cluster of large episodic salt lakes on Eyre Peninsula,South Australia.These lakes have a characteristic terrestrial invertebrate fauna including various spiders,     

Radar observations of asteroid 1998 ML14

Abstract— Goldstone and Arecibo delay‐Doppler radar imaging of asteroid 1998 ML 14 shortly after its discovery reveals a 1 km diameter spheroid with prominent topography on one side and subdued topography on the other. The object's radar and optical properties are typical for S‐class near‐Earth asteroids. The gravitational slopes of a shape model derived from the images and assumed to have a uniform density are shallow, exceeding 30° over only 4% of the surface. If 1998 ML14's density distribution is uniform, then its orbital environment is similar to a planetary body with a spheroidal gravitational field and is relatively stable. Integration of a radar‐refined orbit reveals that the 1998 apparition was the asteroid's closest approach to Earth since at least 1100 and until 2283, when it approaches to within 2.4 lunar distances. Outside of that time interval, orbit uncertainties based on the present set of observations preclude reliable prediction.     

Pyrite in ammonite-bearing shales from the Jurassic of England and Germany

Pyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias ε, Germany) and in highly bituminous shales (Lias ε, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected-light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro-environments, while the sediment generally was not wholly anoxic. Most pyrite pre-dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ³⁴S values ranging from ?55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas-filled. In the bituminous Lias ε shales pyrite occurs abundantly as early framboids and micro-nodules. Larger nodules show a variety of forms, some of which post-date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ³⁴S values in shales are grouped about a mode near ?20. Pyrite formed over a long time-span, and throughout the sediment, not just in protected cavities. Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void-fillings and concretions, and which help define the diagenetic history of a shale.     

Announcements

Abstract

This is an updated and extended presentation of the results of the IAHS Workshop on Quality Assurance in Hydrologic Measurement, held during the XXI General Assembly of the International Union of Geodesy and Geophysics (IUGG), Boulder, Colorado, USA, in July 1995. The main focus at Boulder was on experience with hydrological applications of the international standard ISO 9002 and developments in rating curve theory. A case study of a quality assurance system is presented. Progress in hydrometric quality assurance since the workshop and areas for future development are discussed.     

Book review

Ocean Acoustic Tomography Walter Munk, Peter Worcester, Carl Wunsch, Cambridge Monographs on Mechanics, Cambridge University Press, Cambridge .     

ISOLATING THE FOOTPOINT CHARACTERISTICS OF A SOLAR FLARE LOOP

We analyse the physical characteristics of a C5.7 class flare which was observed on 27 September, 1993 using data from the soft X-ray telescope (SXT), the Bragg crystal spectrometer (BCS), and the hard X-ray telescope (HXT) on Yohkoh. The flare takes the form of a simple loop which is much brighter at one of its footpoints than anywhere else for a period of 2 min. During this time there is an increase in the soft X-ray fluxes, and a corresponding peak in hard X-rays. The parameters derived from the hard X-ray and soft X-ray spectra and images are assumed to be from the footpoint. This flare showed two peaks in the non-thermal velocity, the first one simultaneous with the footpoint brightening. The non-thermal velocity corresponding to these first few minutes is unusually large – by a factor of 80%, 68%, and 26% relative to the second peak in the Fexxv, Caxix, and Sxv channels respectively.     

THE SUN AS AN X-RAY STAR: OVERVIEW OF THE METHOD

We present a method to study the solar-stellar connection, i.e., the close similarity of the physical phenomena occurring on the Sun and on late-type active stars, by taking advantage of Yohkoh/SXT X-ray images. From such images, we first generate distribution functions of the whole disk differential emission measure, and then synthesize from these spectra analogous to those collected by X-ray telescope instruments aimed at stars other than the Sun. Here we illustrate the application of this method to the ROSAT/PSPC and ASCA/SIS, and discuss test cases as well as future applications.