The spread of subducted lithospheric material along the mid-mantle boundary

Studies of phase transitions in silicate minerals at high temperatures and pressures suggest that the bulk density of subducted lithosphere at the mid-mantle boundary is intermediate between the densities of the upper and lower mantle. We argue that, if this is the case, then the lithospheric material will intrude along the mid-mantle boundary driven by buoyancy forces resulting from the compositional density differences between the intrusion and its surroundings. The rate of spread of the intrusion is given by a balance between these buoyancy forces and the viscous resistance of the mantle to motion. Using results from our recent studies of the fluid mechanics of such viscous gravity currents, we find that lithospheric material can propagate between one thousand and two thoudand kilometres in a hundred million years and can cover the entire boundary in one to six billion years. This spreading may be reflected in the global distribution of the isotopic characteristics of oceanic basalts.     

THE PREPARATION OF THREE BASE-METAL REFERENCE MATERIALS: LEAD CONCENTRATE, IGS-42, ZINC CONCENTRATE, IGS-43 AND COPPER MATTE, IGS-44

Three base-metal concentrates, each weighing 25 kg, were ground to -175 mesh (90 μm) and homogenised. They were then sub-sampled into foil-lined envelopes and sealed under argon before being issued to 27 participating laboratories for analysis. Over 4000 analytical results were returned and, after evaluation, a total of 37 elements within the three samples have been given recommended or more cautious probable values. The high standard of precision attained has enabled clear-cut distinctions to be made between methods for the determination of some elements. The most important of these is between titrimetry and electrogravimetry in the determination of copper.     

Structural history of high-pressure metamorphic rocks in the southern vanoise massif,french alps,and their relation to alpine tectonic events

A nappe of amphibolite-facies metamorphic rocks of pre-Permian age in the southern Vanoise massif (the Arpont schist) has been affected by an Alpine HP/LT metamorphism. The first mesoscopically recognizable deformation (D₁) post-dated the high-pressure peak (jadeitic pyroxene + quartz, glaucophane + ?lawsonite), and was associated with glaucophane + epidote. D₁ produced a flat-lying schistosity and a NW-trending glaucophane lineation, and was probably associated with nappe displacement involving NW-directed subhorizontal shear. D₂ formed small-scale folds and a foliation associated with chlorite + albite. The changing parageneses during the period pre-D₁ to D₁ to D₂ suggest decreasing pressure, so that the deformation appears to have been related to the uplift history, rather than to the process of tectonic burial. D₂ was followed by a static metamorphism (green biotite + chlorite + albite), possibly of Lepontine age. SE-directed backthrusting and folding (D₃), and later differential uplift along steep faults, took place under low-grade conditions.     

Late quaternary sediments in Lake Zürich,Switzerland

Lake Zürich occupies a glacially overdeepened perialpine trough in the northern Middlelands of Switzerland. A total of 154.4 m of Quaternary sediments and 47.3 m of Tertiary Molasse bedrock has been cored from the deepest part of the lake, some 10 km south of the city of Zürich. Some 16.8 m of gravels and sands directly overlying the bedrock include basal till and probably earliest subglacial fluvial and lacustrine deposits. These are overlain by 98.6 m of fine-grained, glacial-aged sediments comprising completely deformed proglacial and/or subglacial lacustrine muds, separated by four basal mud tills. The lack of interglacial sediments, fossils, and other datable material, and the presence of severe sediment deformation and unknown amounts of erosion prevent the establishment of an exact chronostratigraphy for sediments older than the upper mud till. Above it some 8.6 m of lacustrine muds were deposited, folded, faulted, and tilted during the final opening of the lake at about 17,500–17,000 years ago. Superimposed are 30.4 m of final Würm and post-glacial sediments comprising (from oldest): cyclic proglacial mud, thick-bedded and laminated mud, a complex transition zone, laminated carbonate, laminated marl, and diatom-calcite varves. These sediments reflect changing catchment and lacustrine conditions including: glacial proximity, catchment stability, lake inflow characteristics, thermal structure, chemistry, and bed stability. Average sedimentation rates ranged from 11 cm yr⁻¹ immediately after glacier withdrawal, to as low as 0.4 mm yr⁻¹ as the environment stabilized. The lack of coarse outwash deposits separating the fine-grained glaciolacustrine sediments from a corresponding underlying basal till suggests that deglaciation of the deep northern basin of Lake Zürich was by stagnation-zone retreat rather than by retreat of an active ice-front.     

Kaolinisation of granite in an urban environment

Under natural acid and wet conditions, one of the main weathering processes affecting granitic rocks is the kaolinisation of Na, Ca and K-feldspar minerals by H₂O + CO₂ attack. We here report the occurrence of authigenic kaolinite on the surface of an eighteenth century granitic monument covered with sulphate-rich weathering patinas. We suggest that, in humid mesothermic climates, anthropogenically derived gaseous SO₂ from air pollution is responsible for accelerated kaolinisation of feldspars in an urban environment; SO₂ from air pollution thus plays a dual role in the weathering of silicate rocks being responsible for the well known sulphation of Ca-bearing materials leading to the formation of sulphate salts such as gypsum as well as the weathering of feldspar minerals to kaolinite. The kaolinisation reaction weakens the granite microfabric leading to enhanced decay of the building stone.     

Geoscience of ancient Mediterranean harbours

Although much has been written on the subject of ancient Mediterranean harbours, the relatively new area of harbour geoarchaeology remains dispersed in the geoscience and archaeological literature. Over a decade of research has amassed rich and varied datasets of anthropogenically forced coastal evolution, with a remarkable number of between-site analogies. This new research field also shows the rich potential of geoscience to reconcile important archaeological questions. No single publication, however, has yet drawn on these geological patterns to yield a detailed overview suitable for geoscientists and environmental archaeologists. The aim of this review article is to (1) discuss how ancient harbours have come to be preserved in the geological record; (2) expound the basic principles and palaeoenvironmental tools underpinning ancient harbour geoarchaeology; (3) outline some of the most significant research advances made; and (4) discuss a new chrono-stratigraphic model applicable to harbour sequences.     

Erosion and natural revegetation associated with surface land drains in upland peatlands

Open cut drains are the most frequently used drainage technique in peatlands and are common throughout the world. Land drains increase the drainage density and promote enhanced coupling of hillslope sediment sources with streams. They may be major sources of fine sediment to peatland stream systems but data on drain sediment production in peatlands are rare. Many drains are now being dammed by humans, yet some revegetate naturally. This paper presents a survey of the erosion and natural revegetation of peat drains related to variables such as slope, drainage area and shading. The paper also provides measured comparison of sediment flux from open drains, drains that have been blocked by peat dams, undisturbed subcatchments, and the catchment outlet. Natural infilling of drains was often found to occur on gentle slopes <4°. Drains on slopes <2° were rarely eroded, while drains on slopes >4° were rarely infilled. Nick‐point retreat at the confluence of drains was often observed. Revegetation of drains was uncommon where drains have been cut or incised into the underlying mineral substrate. Where the drain floor was peat‐based, revegetation was much more common. Revegetation of drain floors was affected by overhanging vegetation, which provided shading. Drain floors with less than 60% shading tended to have a greater vegetation cover than drains with 60–90% shading. However, drains that were almost totally shaded (>90%) had the greatest floor vegetation. The intact drains were found to be major sources of suspended sediment within the survey catchments, with 18·3% of the sediment originating from the unblocked drains which drained 7·3% of the area. The winter quarter of the year was more important than other seasons for producing suspended sediment, even though precipitation totals were not greatest during this period. Drains which had been dammed at intervals along their length using peat blocks had very low sediment yields. Even poorly dammed drains, where water could still flow along the full course of the drain, had 54 times less suspended sediment production than unblocked drains. Copyright © 2007 John Wiley & Sons, Ltd.     

Pulling apart: new perspectives on the spatial dimensions of neighbourhood income disparities in Canadian cities

Journal of Geographical Systems - The spatial configurations of changes in the distribution of incomes within Canada’s eight largest metropolitan areas are examined using a new approach based...     

Upholding labour productivity under climate change: an assessment of adaptation options

Changes in labour productivity feed through directly to national income. An external shock, like climate change, which may substantially reduce the productivity of workers is therefore a macroeconomic concern. The biophysical impact of higher temperatures on human performance is well documented. Less well understood are the wider effects of higher temperatures on the aggregate productivity of modern, diversified economies, where economic output is produced in contexts ranging from outdoor agriculture to work in air-conditioned buildings. Working conditions are at least to some extent the result of societal choices, which means that the labour productivity effects of heat can be alleviated through careful adaptation. A range of technical, regulatory/infrastructural and behavioural options are available to individuals, businesses and governments. The importance of local contexts prevents a general ranking of the available measures, but many appear cost-effective. Promising options include the optimization of working hours and passive cooling mechanisms. Climate-smart urban planning and adjustments to building design are most suitable to respond to high base temperature, while air conditioning can respond flexibly to short temperature peaks if there is sufficient cheap, reliable and clean electricity.

Key policy insights

• The effect of heat stress on labour productivity is a key economic impact of climate change, which could affect national output and workers’ income.

• Effective adaptation options exist, such as shifting working hours and cool roofs, but they require policy intervention and forward planning.

• Strategic interventions, such as climate-smart municipal design, are as important as reactive or project-level adaptations.

• Adaptation solutions to heat stress are highly context specific and need to be assessed accordingly. For example, shifting working hours could be an effective way of reducing the effect of peak temperatures, but only if there is sufficient flexibility in working patterns.