Lithology and structure of the Grenville-aged (∼1.1 Ga) basement of Heimefrontfjella (East Antarctica)

The Heimefrontfjella mountains, Western Dronning Maud Land (East Antarctica), are dominantly composed of Grenville-aged (≈ 1.1 Ga) rocks, which were reworked during the Pan -African orogeny at ≈500 Ma. Three discontinuity-bounded Grenville-aged terranes have been recognized namely (from north to south) the Kottas, Sivorg and Vardeklettane terranes. The terranes contain their own characteristic lithological assemblages, although each is made up of an early supracrustal sequence of metavolcanic and/or metasedimentary gneisses, intruded by various (predominantly granitoid) suites. No older basement upon which the protoliths of these older gneisses were deposited has been recognized. In each terrane the older layered gneisses were intruded by various plutonic suites ranging in age from ≈ 1150 to ≈1000 Ma. The Vardeklettane terrane is characterized by abundant charnockites and two-pyroxene granulite facies parageneses in metabasites, whereas the Sivorg and Kottas terranes were metamorphosed to amphibolite facies grade. P-T estimates show that peak metamorphic conditions changed from ≈600°C at 8 kbar in the south, to ≈700 °C at 4 kbar in the northern Sivorg terrane. Regional greenschist retrogression of high-grade assemblages may be of Pan-African age. The Heimefrontfjella terranes were juxtaposed and pervasively deformed during a complex and protracted period of E-W collision orogenesis in a transpressive regime at ≈ 1.1 Ga. This is manifest as early, gently dipping thrust-related shear fabrics (D₁), succeeded by the initiation of an important (D₂) steep dextral shear zone (Heimefront shear zone, HSZ), during which the early fabrics and structures were steepened and rotated in an anticlockwise sense. The HSZ is a curvilinear structure which changes from a dextral oblique strike-slip lateral ramp in the north to a steep dip-slip frontal ramp in the south, where it forms the boundary between the Sivorg and Vardeklettane terranes. The Pan-African event is manifested as discrete, low- to medium-temperature ductile to brittle shears (D₃) and numerous K/Ar cooling ages.     

The conductance of a maize crop and the underlying soil to ozone under various environmental conditions

Flux measurements of ozone and water vapour employing the eddy correlation technique were used to determine the surface conductance and canopy conductance to ozone. In the surface conductance to ozone, all surfaces at which ozone is destroyed and the transport process to these surfaces are included. The canopy conductance to ozone represents the ozone uptake of transpiring plant parts. The surface conductance to ozone of the maize crop and the underlying soil was generally larger than the canopy conductance to ozone. This means that beside the uptake by stomata, there was another important ozone sink. Under wet soil surface conditions, the surface conductance and the canopy conductance to ozone coincided. This indicates that the resistance of wet soil and the remaining plant parts (cuticle) to ozone was much larger than the stomatal or soil resistance. On the other hand, under dry soil conditions the conductances differ, largely caused by a variation in the transport process to the soil. The transport of ozone to soil increased with increasing friction velocity (u _*) and decreased with increasing atmospheric stability, leaf area index (LAI) or crop height (h). These effects for midday (unstable) conditions were parameterized with an in-crop aerodynamic resistance,r _inc in a very straightforward way;r _inc=13.9 LAIh/u _*+67 (cc.=0.77). If the ozone flux in air pollution models is described with a simple resistance model (Big Leaf model), the extra destruction at the soil should be modelled using an in-crop aerodynamic resistance. For these measurements the ozone flux to the soil was 0–65% of the total ozone flux measured above the crop. Under wet soil conditions, this was less than 20%; under dry soil conditions, this was 30–65%.     

Oblique collision at about 1.1 Ga along the southern margin of the Kaapvaal continent,south-east Africa

The 1.1 Ga Natal Metamorphic Province (NMP) lies at the heart of a world-wide system of Grenville age mobile belts which welded early continental fragments into the Mesoproterozoic supercontinent of Rodinia. Structural analysis of the three tectonostratigraphic terranes in Natal reveals a kinematic history characterized by prolonged NE-SW plate convergence, manifested as early thrust tectonics and later pervasive sinistral transcurrent shearing. Consequently, superimposed on the Natal tectonostratigraphic terranes is a kinematic subdivision into tectonic domains which are characterized by shallow, south-west dipping foliations, south-west plunging stretching lineations and north-east verging recumbent folds, and by younger domains with subvertical shear fabrics, subhorizontal to oblique lineations and folding about near-vertical axes. Microtextural and petrographic analyses suggest that the later shearing took place under high temperature conditions of at least 500°C. The recorded kinematic indicators suggest that early subhorizontal compressional tectonics gave rise to tectonic thickening of the crust, progressively followed by oblique transcurrent shearing within a transpressional regime. The shearing event in the southern arc-related terranes was associated with the widespread emplacement of late kinematic rapakivi granite -charnockite plutons, with A-type granite geochemical characteristics. This orogenic event took place around 1100 Ma during prolonged NE-SW collisional convergence along the southern margin of the stable Archean foreland, which lay to the north.     

New evidence for a relation between wind stress and wave age from measurements during ASGAMAGE

Data from the 1996 ASGAMAGE experiment, performed in the southern North Sea at research platform Meetpost Noordwijk (MPN), are analysed for the parameters affecting the momentum flux. The stress turns out to be quadratically related to the 10-m wind speed and linearly to the wind speed at a wavelength related level. The Charnock parameter (dimensionless roughness length) shows a pronounced correlation with wave age. This implies, due to a coupling between wave age and the steepness of the waves, a connection between the stress and the steepness. We find that our North Sea results are consistent withopen ocean observations. For a given wind speed the mean stress at MPN turns out to be higher because the wave age there is in general lower. We define and give an expression for a drag coefficient at a wavelength related level that can be calculated straightforwardly from the wave age and then reduced to a standard level.     

On the temperature dependence of the ratio of O VIII Ly-α and Ly-β radiation from the solar corona

The incorporation of two-photon transitions between the levels 2s and 1s into the calculation of level populations of the hydrogenic O⁷⁺ ion reduces the coronal temperature, deduced from the Ly- - Ly- ratio, from appr. 4 × 10⁶ K to about 1.5 × 10⁶ K.     

A comparative study of Pleistocene phosphorites from the continental slope off western India

Two types of phosphorite recovered from the continental slope off western India are described. The first type, phosphorite 1, comprises a hard, grey nodule composed of carbonate fluorapatite (CFA) and calcite as major minerals. The phosphorite consists of light‐brown microcrystalline apatite containing a few skeletal fragments and planktonic foraminifera. Scanning electron microscope (SEM) studies show evidence of dissolution of skeletal calcite and filling of the resulting cavities by phosphate composed of ovoid to rod‐shaped apatite microparticles. Apatite also occurs as coatings on these particles. The P₂O₅ content of the phosphorite is 29%, and the CO₂ content of the CFA is about 4·5%. The rare‐earth element (REE) abundance (ΣREE=2·02 μg g^–1) is lower than in other modern phosphorites. The ⁸⁷Sr/⁸⁶Sr ratio and ?Nd value of this sample are 0·70921 and –9·9 respectively. The ¹⁴C age found through accelerator mass spectrometry (AMS) dating (18 720 ± 120 years BP) is much younger than that determined by the U‐series method (100 ka). The second type, phosphorite 2, comprises a friable, light‐brown nodule consisting of CFA as the only major mineral, with a CO₂ content of the CFA of 4·5%. In thin section, the phosphate is light brown and homogeneous, and a few bone fragments are present. The P₂O₅ content is 33%, and REE contents (ΣREE = 0·18 μg g^–1) are lower than in phosphorite 1. The age of phosphorite 2 is >300 ka. Phosphorite 1 appears to have formed during the late Pleistocene through replacement of carbonate by phosphate; phosphorite 2 is also of Pleistocene age but is much older than phosphorite 1. The initial substrate for phosphorite 2 was a fish coprolite, which was subsequently phosphatized during slow sedimentation under low‐energy conditions. Microbial mediation is evident in both phosphorites. The colour, density and P₂O₅ content of the phosphorites are found to be dependent on the nature of the initial substrates and physico‐chemical conditions during phosphatization. The CO₂ content of the CFA is not related to the precursor carbonate phase. The nature of sediments, rates of sedimentation and the time spent undergoing phosphogenesis at the sediment–water interface may control REE concentrations in phosphorites.