Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from Akilia, Southwest Greenland

A granodiorite from Akilia, southwest Greenland, previouslysuggested to date putative life-bearing rocks to 3·84Ga, is re-investigated using whole-rock major and trace-elementgeochemistry, and detailed cathodoluminescence image-guidedsecondary ion mass spectrometer analyses of zircon U–Th–Pband rare earth elements. Complex zircon internal structure revealsthree episodes of zircon growth and/or recrystallization datedto c. 3·84 Ga, 3·62 Ga and 2·71 Ga. Rareearth element abundances imply a significant role for garnetin zircon generation at 3·62 Ga and 2·71 Ga. The3·62 Ga event is interpreted as partial melting of ac. 3·84 Ga grey gneiss precursor at granulite facieswith residual garnet. Migration of this 3·62 Ga magma(or melt–crystal mush) away from the melt source placesa maximum age limit on any intrusive relationship. These earlyArchaean relationships have been complicated further by isotopicreworking in the 2·71 Ga event, which could have includeda further episode of partial melting. This study highlightsa general problem associated with dating thin gneissic veinsin polyphase metamorphic terranes, where field relationshipsmay be ambiguous and zircon inheritance can be expected. KEY WORDS: Archaean; geochronology; Greenland; secondary ion mass spectrometry; zircon     

Emplacement of the Cleveland Dyke: Evidence from Geochemistry, Mineralogy, and Physical Modelling

The igneous rocks of the British Tertiary Volcanic Province(BTVP) comprise intrusive central complexes and associated lavafields in northwest Scotland and northern Ireland. These centresare associated with linear dyke swarms which are radial aroundthe major central complexes. The most extensive dyke swarm isrelated to the Mull intrusive complex and includes the Clevelanddyke, which appears to extend some 430 km from Mull throughthe Scottish Midland Valley (SMV) to the coast of northeastEngland. The dyke may have been emplaced by lateral magma migrationfrom Mull, by vertical magma migration, or by a combinationof these processes associated with the emplacement of the Mullcentre and the presence of a regional stress field in northernBritain. Petrographic, mineralogical, and geochemical data for samplescollected across and along the Cleveland dyke have been usedto evaluate its petrogenesis and emplacement mechanism. Thesegment of the dyke north of, and along, the Southern UplandsFault, the southern boundary of the SMV, is not comagmatic withthat to the south, which is now defined as the Cleveland dykesensu stricto. The Cleveland dyke is an olivine-free, plagioclase-and pyroxene-phyric basaltic andesite. Plagioclase mineralogyand bulk composition indicate that it experienced a complexmagmatic history involving polybaric fractional crystallizationand minor crustal contamination. Despite this complex evolution,the dyke magma is relatively homogeneous and shows chemicalcharacteristics closely similar to tholeiitic rocks from Mull.The data substantiate lateral emplacement from this BVTP centre,rather than by vertical emplacement through heterogeneous lithosphere. Numerical modelling of dyke dynamics is consistent with emplacementof the Cleveland dyke as a single pulse of magma from the Mullcentre, flowing in a manner transitional between laminar andturbulent conditions. According to this model, the dyke (volumec. 85 km³ was initiated in a large magma chamber below Mullsubject to a small excess magmatic pressure. Lateral migrationat relatively high velocity (1–5 ms^–1) caused emplacementof the dyke in 1–5 days. Following emplacement, minorvertical ascent of magma may have contributed to the local enechelon distribution of dyke segments.     

Post Fordism Just-In-Time

Industrial geographers working from a labour process and organisation of production perspective argue that the production process is being transformed by post Fordism and flexible specialisation. An alternative interpretation is that a shift from just-in-case to Just-in-time systems is taking place. A review of these ideas and case studies of two New Zealand-based manufacturing companies concludes that the shift to just-in-time is the more useful conceptualisation of changing workplace relations. Some directions for geographical research interested in the diffusion and impact of just-in-time methods are outlined.     

A propensity for n‐ω‐amino acids in thermally altered Antarctic meteorites

Abstract– Asteroids and their fragments have impacted the Earth for the last 4.5 Gyr. Carbonaceous meteorites are known to contain a wealth of indigenous organic molecules, including amino acids, which suggests that these meteorites could have been an important source of prebiotic organic material during the origins of life on Earth and possibly elsewhere. We report the detection of extraterrestrial amino acids in thermally altered type 3 CV and CO carbonaceous chondrites and ureilites recovered from Antarctica. The amino acid concentrations of the thirteen Antarctic meteorites ranged from 300 to 3200 parts‐per‐billion (ppb), generally much less abundant than in amino acid‐rich CI, CM, and CR carbonaceous chondrites that experienced much lower temperature aqueous alteration on their parent bodies. In contrast to low‐temperature aqueously altered meteorites that show complete structural diversity in amino acids formed predominantly by Strecker–cyanohydrin synthesis, the thermally altered meteorites studied here are dominated by small, straight‐chain, amine terminal (n‐ω‐amino) amino acids that are not consistent with Strecker formation. The carbon isotopic ratios of two extraterrestrial n‐ω‐amino acids measured in one of the CV chondrites (δ¹³C approximately ?25‰) are consistent with ¹³C‐depletions observed previously in hydrocarbons produced by Fischer‐Tropsch type reactions. The predominance of n‐ω‐amino acid isomers in thermally altered meteorites hints at cosmochemical mechanisms for the preferential formation and preservation of a small subset of the possible amino acids.     

Assessment and control of organic and other contaminants associated with the Stardust sample return from comet 81P/Wild 2

Abstract– Numerous potential sources of organic contaminants could have greatly complicated the interpretation of the organic portions of the samples returned from comet 81P/Wild 2 by the Stardust spacecraft. Measures were taken to control and assess potential organic (and other) contaminants during the design, construction, and flight of the spacecraft, and during and after recovery of the sample return capsule. Studies of controls and the returned samples suggest that many of these potential sources did not contribute any significant material to the collectors. In particular, contamination from soils at the recovery site and materials associated with the ablation of the heatshield do not appear to be significant problems. The largest source of concern is associated with the C present in the original aerogel. The relative abundance of this carbon can vary between aerogel tiles and even within individual tiles. This C was fortunately not distributed among a complex mixture of organics, but was instead largely present in a few simple forms (mostly as Si‐CH₃ groups). In most cases, the signature of returned cometary organics can be readily distinguished from contaminants through their different compositions, nonterrestrial isotopic ratios, and/or association with other cometary materials. However, some conversion of the carbon indigenous to the flight aerogel appears to have happened during particle impact, and some open issues remain regarding how this C may be processed into new forms during the hypervelocity impact collection of the comet dust.     

桑沟湾和胶州湾沉积物耗氧率研究

为评价浅海海域底层在碳元素生物地化循环中的作用,采用现场取样培养法测定了黄海桑沟湾和胶州湾的沉积物耗氧率。桑沟湾5月和8月的沉积物耗氧率分别为15和24 mmol.m-2.d-1;胶州湾8月的沉积物耗氧率为18 mmol.m-2.d-1。回归分析表明,2个湾的沉积物耗氧率分别与沉积物表层的有机碳和总氮含量正相关。取呼吸商为0.8,得到桑沟湾底层对有机碳的矿化率占水层初级生产力的102%(5月)和67%(8月),胶州湾底层对有机碳的矿化率占水层初级生产力的28%(8月)。取桑沟湾颗粒有机碳的生物沉积速率为236 mg.m-2.d-1,得到底层对有机碳的埋置效率为40%(5月)和3%(8月),埋置的有机碳占水层初级生产力的69%(5月)和2%(8月);胶州湾颗粒有机碳的自然沉积速率为251 mg.m-2.d-1,底层对有机碳的埋置效率为32%(8月),埋置的有机碳占水层初级生产力的13%(8月)。     

Bank erosion in an Andean páramo river system: Implications for hydro-development and carbon dynamics in the neotropical Andes

The páramo of the Northern Andes provide critically important ecosystem services to the Northern Andean region in the form of water provisioning and carbon sequestration, both of which are a result of the páramo?s organic-rich soils. Little is known, however, about the hydro-geomorphic characteristics of the rivers that drain these ecosystems. With impending plans for widespread hydro-development and increasing implementation of carbon-sequestering compensation for ecosystem services programs in the region it is imperative that we develop a thorough understanding of the hydrogeomorphic role that rivers play in this unique ecosystem. The objective of this study was to quantify bank erosion along an Amazonian headwater stream draining a small, relatively undisturbed páramo catchment to gain a better understanding of the natural erosion regime and the resulting sediment contributions from this unique ecosystem. This study implemented a combination of field, laboratory, and Geographic Information Systems techniques to quantify bank erosion rates and determine a bank erosion sediment yield from the Ningar River, a small páramo catchment(22.7 km~2) located in the eastern Andean cordillera of Ecuador. Results show that bank erosion rates range from 3.0 to ≥ 390.0 mm/yr, are highly episodic, and yield at least 487 tons of sediment annually to the Ningar River. These results imply that 1) páramo ecosystems substantially contribute to the sediment load of the Amazon River basin; 2) bank erosion is a potentially significant flux component of basin-scale carbon cycles in páramo ecosystems; and 3) hydrologic alteration campaigns(dam building) will likely critically alter these contributions and concomitantly disconnect a critical source of sediment and nutrients to downstream ecosystems.     

The visualization of socio-economic GIS data using virtual reality tools

This paper addresses two issues: the use of 'realistic' environments for the visualization of socio-economic data, and the use of virtual reality tools for the exploration of data from existing GIS databases. 'Realistic' is here used in a narrow sense to imply the reconstruction of real-world scenes in which major components of the physical environment are reconstructed to facilitate geographical visualization. A review of visualization techniques for geographically-referenced 3-D data is presented, and the translation of an existing urban GIS database into virtual reality modelling language (VRML) demonstrated for a study area in Cardiff, UK. This work shows that the export of conventional 2-D GIS data into virtual reality toolkits can be relatively straightforward, and offers exciting new avenues for visualization and exploration. We argue that some of the parameters of the true scene such as colour, building shape, and texture may be directly modified in order to represent otherwise unobservable socio-economic characteristics, in the spirit of exploratory data analysis. These issues are of particular importance in the context of current advances towards the dynamic linkage of GIS and virtual reality modelling.