Stability of forsterite + CO2 and its bearing on the role of CO2 in the mantle

An experimental determination of the reaction MgCO₃ + MgSiO₃ = Mg₂SiO₄ + CO₂ between 20 and 40 kbars and in the range 1000–1500°C yields an average pressure effect on the equilibrium of 44 bars/°C. This result shows that the assemblage forsterite and carbon dioxide is not stable under most pressure and temperature conditions expected in the upper mantle. Hypotheses requiring the presence of free CO₂ in the low-velocity zone, CO₂ as a drive mechanism for kimberlite emplacement, or action of a free CO₂ phase in ultramafic rocks may need considerable revision.     

Form und Schichtungsgefüge periodischer Sandkörper im Strömungsfeld des Außenelbewatts

Zusammenfassung Periodisch gescharte Sandkörper auf den Sandwattflächen (große Rippeln und- hier besonders - Sandwellen) geben durch ihre im Luftbild erkennbare Asymmetrie Indizien für den Sandtransport. Diese Sandwellen sind, im Gegensatz zu Groß- und Riesenrippeln, aus einer Kreuzschichtung aufgebaut, die durch wandernde Kleinrippeln entstanden ist. Dadurch entsteht auch die Asymmetrie der Großformen. Die aus der Luftbildanalyse resultierende Richtungsverteilung stimmt mit einer Stromvektorenkarte (Göhren), die von normalen Tideverhältnissen abgeleitet ist, gut überein.Die Gesamtheit der auftretenden Strömungen spiegelt sich in der Schichtung nicht wider, sondern es dominiert diejenige Strömung, die am häufigsten mit einer relativ großen Geschwindigkeit eintritt. Extreme Bedingungen haben so geringere Chancen, im Sediment dokumentiert zu werden.

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The asymmetric character of periodic sedimentary sand bodies found on tidal flats (largescale ripples and here particularly sand waves) can be mapped on aerial photographs to give an indication of sand transport directions. These sand waves, in contrast to largescale and giant ripples, exhibit cross-lamination which formed from migrating smallscale ripples. The asymmetry of the large form is established through the migration of the smallscale ripples. The vector distributions obtained from aerial photograph analyses show good agreement with a map of current vectors (Göhren) based on normal tidal conditions.Not every current is reflected in the sediment; dominant are the frequently occurring currents having a moderately high velocity. Extreme conditions (which are infrequent) have a smaller chance to be recorded in the sediment.

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Résumé Dans la zone intertidale, on trouve des mégarides et d'autres crêtes sableuses, groupées periodiquement, qui sont appelées ici «Sandwellen» (vagues de sable). L'asymmetrie de ces formes, bien reconnaissable sur les photographies aériennes, donne des indications sur le mouvement du sable. Les vagues de sable montrent, au contraire des mégarides ordinaires, une stratification croisée de rides à l'intérieur. La migration de ces rides cause l'asymmetrie. La distribution des directions concorde avec une carte des vecteurs de courants (Göhren) dérivée des conditions ordinaires de marée.La totalité des courants présents dans la zone intertidale n'est pas réflétée dans la stratification, mais ce sont les courants, qui ont lieu le plus fréquemment avec des vélocités relativement fortes qui dominent. Les conditions extrêmes n'ont pas une grande chance d'être fossilisées.

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Enthalpy of formation of Fe3Al2Si3O12 (almandine) by high temperature alkali borate solution calorimetry

A high-temperature solution calorimetric method suitable for thermochemical studies of anhydrous minerals containing Fe²⁺ ions has been developed. The method is based on an oxide melt solvent with 52 wt% LiBO₂ and 48 wt% NaBO₂ maintained at a temperature of 750°C. In a first application of this method the enthalpies of solution of synthetic almandine, fayalite, a mixture of fayalite plus quartz on FeSiO₃ composition, and natural quartz were measured. For the reaction:

the enthalpy change at 1023 K is ?3.82 ± 0.87 kcal, based on fayalite, quartz, corundum and almandine, and ?5.96 ± 0.90 kcal based on the fayalite plus quartz mixture, corundum, and almandine. These values lead to standard molar enthalpies of formation of almandine from the oxides at 1023 K of ?14.10 ± 1.22 kcal and ?16.24 ± 1.74 kcal, respectively. The measured enthalpy of formation of almandine is less negative by several kilocalories than values derived from analysis of the phase equilibrium work of Hsu (1968), but in closer agreement with the phase equilibrium study of O'Neill and Wood (1979) and similar to the phase equilibrium deduction of Froese (1973).The agreement of the present almandine enthalpy of formation with O'Neill and Wood (1979) and Froese (1973) suggests that almandine entropies at 298 K to be obtained from their studies, in the range 79–81 cal/K, are more nearly correct than the several estimates based on oxide sum and volume-entropy systematics, most of which are much lower.     

The reversed alumina contents of orthopyroxene in equilibrium with spinel and forsterite in the system MgO-Al2O3-SiO2

Equilibrium alumina contents of orthopyroxene coexisting with spinel and forsterite in the system MgO-Al₂O₃-SiO₂ have been reversed at 15 different P-T conditions, in the range 1,030–1,600° C and 10–28 kbar. The present data and three reversals of Danckwerth and Newton (1978) have been modeled assuming an ideal pyroxene solid solution with components Mg₂Si₂O₆ (En) and MgAl₂SiO₆ (MgTs), to yield the following equilibrium condition (J, bar, K): $$\begin{gathered} RT{\text{ln(}}X_{{\text{MgTs}}} {\text{/}}X_{{\text{En}}} {\text{) + 29,190}} - {\text{13}}{\text{.42 }}T + 0.18{\text{ }}T + 0.18{\text{ }}T^{1.5} \hfill \\ + \int\limits_1^P {\Delta V_{T,P}^{\text{0}} dP = 0,} \hfill \\ \end{gathered} $$ where $$\begin{gathered} + \int\limits_1^P {\Delta V_{T,P}^{\text{0}} dP} \hfill \\ = [0.013 + 3.34 \times 10^{ - 5} (T - 298) - 6.6 \times 10^{ - 7} P]P. \hfill \\ \end{gathered} $$ The data of Perkins et al. (1981) for the equilibrium of orthopyroxene with pyrope have been similarly fitted with the result: $$\begin{gathered} - RT{\text{ln(}}X_{{\text{MgTs}}} \cdot X_{{\text{En}}} {\text{) + 5,510}} - 88.91{\text{ }}T + 19{\text{ }}T^{1.2} \hfill \\ + \int\limits_1^P {\Delta V_{T,P}^{\text{0}} dP = 0,} \hfill \\ \end{gathered} $$ where $$\begin{gathered} + \int\limits_1^P {\Delta V_{T,P}^{\text{0}} dP} \hfill \\ = [ - 0.832 - 8.78{\text{ }} \times {\text{ 10}}^{ - {\text{5}}} (T - 298) + 16.6{\text{ }} \times {\text{ 10}}^{ - 7} P]{\text{ }}P. \hfill \\ \end{gathered} $$ The new parameters are in excellent agreement with measured thermochemical data and give the following properties of the Mg-Tschermak endmember: $$H_{f,970}^0 = - 4.77{\text{ kJ/mol, }}S_{298}^0 = 129.44{\text{ J/mol}} \cdot {\text{K,}}$$ and $$V_{298,1}^0 = 58.88{\text{ cm}}^{\text{3}} .$$ The assemblage orthopyroxene+spinel+olivine can be used as a geothermometer for spinel lherzolites, subject to a choice of thermodynamic mixing models for multicomponent orthopyroxene and spinel. An ideal two-site mixing model for pyroxene and Sack's (1982) expressions for spinel activities provide, with the present experimental calibration, a geothermometer which yields temperatures of 800° C to 1,350° C for various alpine peridotites and 850° C to 1,130° C for various volcanic inclusions of upper mantle origin.     

Photogrammetric Reverse Engineering of the "Angel of the North"

The Department of Geomatics and the Regional Centre for Innovation in Engineering Design in the University of Newcastle upon Tyne collaborated in a project to produce precise measurements for use in the fabrication of the body of the "Angel of the North" sculpture. This 20 m high edifice, by the sculptor Antony Gormley, is located adjacent to the A1(M) in Gateshead, Tyne and Wear.
This paper reviews the photogrammetric reverse engineering work carried out on a 1:10 scale plaster maquette of the "Angel of the North", which is a life-size model of the sculptor's body without any arms. The three dimensional measurements of the maquette, made by analytical photogrammetric techniques, were then processed to produce a fully functional three dimensional CAD model from which the dimensions needed by the fabricating company, Hartlepool Steel Fabrications Ltd., could be extracted. Additionally, the VirtuoZo digital photogrammetric software package was used to investigate the potential of automated measurement for this type of work and the results were compared with those obtained analytically.     

Sediment accumulation in embayments controlled by bathymetric slope and wave energy: Implications for beach formation and persistence

High energy, rocky coastlines often feature sandy beaches within headland‐bound embayments. Not all such embayments have beaches however, and beaches in embayments can be removed by storms and may subsequently reform. What dictates the presence or absence of an embayed beach and its resilience to storms? In this paper, we explore the effect of offshore slope and wind conditions on nearshore sediment transport within idealised embayments to give insight into nearshore sediment supplies. We use numerical simulations to show that sand can accumulate near shore if the offshore slope is >0.025 m/m, but only under persistent calm conditions. Our modelling also suggests that if sediment in an embayment with an offshore gradient steeper than 0.025 m/m is removed during a period of persistent stormy conditions, it will be unlikely to return in sub‐decadal timescales. In contrast, sediment located in embayments with shallower gradients can reform swiftly in both calm and stormy conditions. Our findings have wide implications for contemporary coastal engineering in the face of future global climate change, but also for Quaternary environmental reconstruction. Our simple method to predict beach stability based on slope can be used to interpret differing responses of embayments to periods of changing coastal storminess such as the medieval climate anomaly‐little ice age (MCA‐LIA) transition. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Modelling climate change impacts on hydropower lake inflows and braided rivers in a mountain basin

This study models climate change impacts on the natural flow regime of braided rivers and inflows to hydropower lakes in a New Zealand mountain basin. Flow metrics include the magnitude, frequency, timing and duration of unaltered flows. The TopNet hydrological model was used to simulate impacts in the Upper Waitaki Basin of the South Island for the 1990s, 2040s and 2090s. An average emissions scenario and results from 12 global circulation models were used as input. Indicators of hydrological alteration and Kruskal-Wallis tests were used to evaluate flow differences. Modelled total inflows increase over time for all lakes, with most increases in winter/early spring and small decreases in summer/autumn. High flows generally increase, while low flows decrease. Although these changes may benefit hydropower and floodplain ecology, they may increase flood risk in winter and spring and drought risk in summer and autumn, causing additional challenges managing hydropower operations.

EDITOR M.C. Acreman

ASSOCIATE EDITOR S. Kanae     

Modelling grazed grassland systems: wether sheep grazing perennial ryegrass

A simple model or a lowland sheep flock grazing a ryegrass sward is described, which incorporates herbage growth and deterioration; the mechanics of grazing, herbage intake and animal production. The model takes into account the effects of climate, the availability of water and nitrogen and the frequency and intensity of defoliation on herbage production; diet selection by the grazing animal; the effects of herbage digestibility and animal body weight on intake; and the fate of the organic matter ingested by the ruminant. Preliminary results comparing continuous and rotational grazing systems and different levels of nitrogen fertilizer application are discussed. Some possible areas for further applied research are also outlined.