Book reviews

The accumulation of geographical scholarship since the late nineteenth century has encouraged a fusion of historical and geographical skills and interests. In the process, the study of the history of geographical thought should continue to strengthen its central position within the subject as a whole. Yet at the same time, research into this modern era insists upon a more generous incorporation of significant contributions to environmental appraisal and landscape authorship developed by government and non-government actors, and a consideration of vernacular or non-scientific modes of inquiry. The Commission's work draws attention to these important trends and lays the foundation for a broader discussion of their implications.     

Hydrothermal silica chimney fields in the Galapagos Spreading Center at 86°W

Silica chimneys were discovered in 1985 at 86°W in the rift valley of the Galapagos Spreading Center at 2600 m depth (“Cauliflower Garden”). The inactive chimneys lack any sulfides and consist almost entirely of amorphous silica (up to 96 wt.% SiO₂, opal-A); Fe and Mn oxides are minor constituents. Oxygen isotope data show that formation of the silica chimneys took place at temperatures between 32°C (+29.9‰ δ¹⁸O) and 42°C (+27.8‰ δ¹⁸O).Th/Udating reveals a maximum age of 1440 ± 300y. Amorphous silica solubility relations indicate that the silica chimneys were formed by conductive cooling of pure hydrothermal fluids or by conductive cooling of a fluid/seawater mixture. Assuming equilibrium with quartz at 500 bars, initial fluid temperatures of more than 175°C (i.e., a concentration of > 182 ppm SiO₂) were required to achieve sufficient supersaturation for the deposition of amorphous silica at 40°C and 260 bars. If the silica chimneys originate from the same or a similar fluid as higher-temperature ( < 300°C) sulfide-silica precipitates found nearby (i.e., 2.5 km away), then subsurface deposition of sulfides may have occurred.     

Die Bilanz des Längstransportes in der Brandungszone

Zusammenfassung Für den in der Brandungszone unter dem Einfluß der Brandungsenergie und der Brandungsströmungen küstenparallel verlaufenden Materialstrom wird unter Verwendung integraler Größen die Bilanzgleichung aufgestellt. Es zeigt sich, daß der Materialstrom und damit die An- und Ablandung längs der Küste eindeutig durch die Verteilung der Brandungsenergie und die Größe der Brandungsströmung bestimmt wird. Anhand von fünf Beispielen werden die Anwendung der Methode erläutert und typische Vorgänge an der Küste erklärt.

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The balance of littoral transport in the surf zone

Summary By means of integrals the balance equation is set up for the drift of material which under the influence of the surf energy and the longshore current in the surf zone is flowing parallel to the coast. This equation plainly represents the material drift and, hence, accretion and erosion along the coast as a function of the distribution of the surf energy and the strength of the longshore current. Finally, five examples are presented showing how to apply the method and explaining typical shore processes.

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Le bilan du transport des matériaux le long de la côte dans la zone du déferlement

Résumé Au moyen d'intégrales on a établi l'équation de bilan pour le courant des matériaux qui, sous l'influence de l'énergie et des courants de déferlement, est dirigé en sens parallèle à la côte dans la zone du déferlement. Cette équation représente nettement les courants des matériaux dirigés vers le large et vers la plage en fonction de la répartition de l'énergie et de la puissance du courant de déferlement. Enfin, cinq exemples sont présentés pour exposer l'application de la méthode et pour expliquer des procès typiques rencontrés sur les côtes.

     

The solubility control of rare earth elements in natural terrestrial waters and the significance of PO 4 3− and CO 3 2− in limiting dissolved rare earth concentrations: A review of recent information

Rare earth element (REE) concentrations in alkaline lakes, circumneutral pH groundwaters, and an acidic freshwater lake were determined along with the free carbonate, free phosphate, and free sulfate ion concentrations. These parameters were used to evaluate the saturation state of these waters with respect to REE phosphate and carbonate precipitates. Our activity product estimates indicate that the alkaline lake waters and groundwaters are approximately saturated with respect to the REE phosphate precipitates but are significantly undersaturated with respect to REE carbonate and sulfate precipitates. On the other hand, the acidic lake waters are undersaturated with respect to REE sulfate, carbonate, and phosphate precipitates. Although carbonate complexes tend to dominate the speciation of the REEs in neutral and alkaline waters, our results indicate that REE phosphate precipitates are also important in controlling REE behavior. More specifically, elevated carbonate ion concentrations in neutral to alkaline natural waters tend to enhance dissolved REE concentrations through the formation of stable REE-carbonate complexes whereas phosphate ions tend to lead to the removal of the REEs from solution in these waters by the formation of REE-phosphate salts. Removal of REEs by precipitation as phosphate phases in the acid lake (pH=3.6) is inconsequential, however, due to extremely low [PO ₄ ^3– ] _F concentrations (i.e., 10^–23 mol/kg).     

The first multi-model ensemble of regional climate simulations at kilometer-scale resolution part 2: historical and future simulations of precipitation

This paper presents the first multi-model ensemble of 10-year, “convection-permitting” kilometer-scale regional climate model (RCM) scenario simulations downscaled from selected CMIP5 GCM projections for historical and end of century time slices. The technique is to first downscale the CMIP5 GCM projections to an intermediate 12–15 km resolution grid using RCMs, and then use these fields to downscale further to the kilometer scale. The aim of the paper is to provide an overview of the representation of the precipitation characteristics and their projected changes over the greater Alpine domain within a Coordinated Regional Climate Downscaling Experiment Flagship Pilot Study and the European Climate Prediction system project, tasked with investigating convective processes at the kilometer scale. An ensemble of 12 simulations performed by different research groups around Europe is analyzed. The simulations are evaluated through comparison with high resolution observations while the complementary ensemble of 12 km resolution driving models is used as a benchmark to evaluate the added value of the convection-permitting ensemble. The results show that the kilometer-scale ensemble is able to improve the representation of fine scale details of mean daily, wet-day/hour frequency, wet-day/hour intensity and heavy precipitation on a seasonal scale, reducing uncertainty over some regions. It also improves the representation of the summer diurnal cycle, showing more realistic onset and peak of convection. The kilometer-scale ensemble refines and enhances the projected patterns of change from the coarser resolution simulations and even modifies the sign of the precipitation intensity change and heavy precipitation over some regions. The convection permitting simulations also show larger changes for all indices over the diurnal cycle, also suggesting a change in the duration of convection over some regions. A larger positive change of frequency of heavy to severe precipitation is found. The results are encouraging towards the use of convection-permitting model ensembles to produce robust assessments of the local impacts of future climate change.

     

Geochemistry of bedded barite of the Mesoproterozoic Aggeneys-Gamsberg Broken Hill-type district,South Africa

Stratiform and stratabound barite ± magnetite beds are intimately associated with the polymetallic Broken Hill-type (BHT) massive sulfide deposits of the Aggeneys-Gamsberg Pb–Zn–Cu ± Ag–Ba district in the Northern Cape Province, South Africa. Barite samples were collected and studied from four localities in the district. Although metamorphic water–rock interaction processes have partially altered the chemical and to a lesser degree the isotopic composition of barite, samples identified as being the least altered display distinctly different isotopic compositions that are thought to reflect different modes of origin. All barite samples are marked by low concentrations of SrO (0.5 ± 0.2 wt%), highly radiogenic ⁸⁷Sr/⁸⁶Sr ratios, elevated δ ³⁴S and δ ¹⁸O values compared to contemporaneous Mesoproterozoic seawater. Radiogenic ⁸⁷Sr/⁸⁶Sr signatures (0.7164 ± 0.0028) point to an evolved continental crustal source for Sr and Ba, while elevated δ ³⁴S values (27.3 ± 4.9‰) indicate that contemporaneous seawater sulfate, modified by bacterial sulfate reduction, was the single most important sulfur reservoir for barite deposition. Most importantly, δ ¹⁸O values suggest a lower temperature of formation for the Gamsberg deposit compared with the occurrences in the Aggeneys area, i.e. Swartberg-Tank Hill and Big Syncline. The obvious differences in temperature of formation are in good agreement with the Cu-rich, Ba-poor nature of the sulfide mineralization of the Aggeneys deposits vs the Cu-poor, Ba-rich character of the Gamsberg deposit. In conjunction with this, isotopic and petrographic arguments favor a sub-seafloor replacement model for the stratabound barite occurrences of the Aggeneys deposits, while at Gamsberg, deposition at the sediment–water interface as a true sedimentary exhalite appears more likely.     

Modelling mixing and circulation in subglacial Lake Vostok,Antarctica

Lake Vostok, isolated from direct exchange with the atmosphere by about 4 km of ice for millions of years, provides a unique environment. This inaccessibility raises the importance of numerical models to investigate the physical conditions within the lake. Using a three-dimensional numerical model and the best available geometry, we test different parameter settings to define a standard model configuration suitable for studying flow in this subglacial lake. From our model runs we find a baroclinic circulation within the lake that splits into three different parts: Along a topographic ridge in the northern part of Lake Vostok, bottom water masses are transported eastward, diverging away from the ridge. In the lake’s surface layer, the flow in these two vertical overturning cells has opposite directions. In the southern part of the lake, where freezing occurs across about 3,500 km², two opposing gyres split the water column vertically. The general flow is stronger in the southern basin with horizontal velocities in the order of 1 mm/s. The strongest upwelling, found in the eastern part of this basin, is about 25 μm/s. We estimate the lower limit of the overturning timescale to be about 2.5 years vertically and 8.6 years horizontally. The basal mass loss of ice from the ice sheet floating on the lake is 5.6 mm/year (equivalent to a fresh water flux of 2.78 m³/s, or a basal ice loss of 0.09 km³/year). This imbalance indicates either a constant growth of the lake or its continuous (or periodical) discharge into a subglacial drainage system.