Ikaite: enigmatic crystals of cold waters

Ikaite, found as a constituent of tufa chimneys and mounds in Ikka Fjord, Greenland, is only formed in waters close to freezing point. At higher temperatures it inverts to calcite, forming impressive pseudomorphs which have been found at a large number of locations world‐wide of varying ages, sometimes in association with glacial deposits. The Ikka Fjord deposits, first described by the Danish geologist Hans Pauly, were key to understanding the nature of these widely reported pseudomorphs.     

Oxygen‐18 dynamics in precipitation and streamflow in a semi‐arid agricultural watershed,Eastern Washington,USA

Understanding flow pathways and mechanisms that generate streamflow is important to understanding agrochemical contamination in surface waters in agricultural watersheds. Two environmental tracers, δ¹⁸O and electrical conductivity (EC), were monitored in tile drainage (draining 12 ha) and stream water (draining nested catchments of 6‐5700 ha) from 2000 to 2008 in the semi‐arid agricultural Missouri Flat Creek (MFC) watershed, near Pullman Washington, USA. Tile drainage and streamflow generated in the watershed were found to have baseline δ¹⁸O value of ?14·7‰ (VSMOW) year round. Winter precipitation accounted for 67% of total annual precipitation and was found to dominate streamflow, tile drainage, and groundwater recharge. ‘Old’ and ‘new’ water partitioning in streamflow were not identifiable using δ¹⁸O, but seasonal shifts of nitrate‐corrected EC suggest that deep soil pathways primarily generated summer streamflow (mean EC 250 µS/cm) while shallow soil pathways dominated streamflow generation during winter (EC declining as low as 100 µS/cm). Using summer isotopic and EC excursions from tile drainage in larger catchment (4700‐5700 ha) stream waters, summer in‐stream evaporation fractions were estimated to be from 20% to 40%, with the greatest evaporation occurring from August to October. Seasonal watershed and environmental tracer dynamics in the MFC watershed appeared to be similar to those at larger watershed scales in the Palouse River basin. A 0·9‰ enrichment, in shallow groundwater drained to streams (tile drainage and soil seepage), of δ¹⁸O values from 2000 to 2008 may be evidence of altered precipitation conditions due to the Pacific Decadal Oscillation (PDO) in the Inland Northwest. Copyright © 2009 John Wiley & Sons, Ltd.     

Sirius Passet,Greenland and the Cambrian Explosion

Throughout Earth history there have been many important milestones: e.g. the emergence of life, the rise of oxygen in the atmosphere, snowball Earth events. One of these major events was the emergence of multicellular life, which, as we are all told in Palaeontology lectures, took place in the Cambrian, when a sudden flowering of life forms emerged, including all of the major groups we have today: the ‘Cambrian explosion’. Two great questions emerge: what happened before this (a problem which worried Darwin as it seemed to threaten his thesis of steady evolution) and how, in detail did this ‘explosion’ take place?     

From dog sledge to satellite: Knud Ellitsgaard‐Rasmussen (1923–2009), geologist

Knud Ellitsgaard‐Rasmussen's career spanned an astounding period of change in the science. He was not a geologist whose name was widely known and it is probably true to say that few students today, even in his native Denmark, have heard of him. He was nevertheless a figure of huge importance to geological research in his role as someone who directed geology along very productive lines, creating an environment which was highly conducive to the successful careers of many talented researchers. He saw and presided over geology as it evolved from the heroic days of an individual with a dog‐sledge and a microscope to later days with armies of personnel, aircraft, marine geophysical vessels, the GPS and the ICP‐MS. He led the mapping of a subcontinent.     

HUMAN GEOGRAPHY OF THE AMERICAN TROPICS: A FORTY-YEAR REVIEW

For a decade or so after World War II, human geographers working in the American tropics found socio-political conditions and resultant research topics little changed from those before the war. This was in contrast to the Old World tropics where decolonization processes and the demands of economic development and new nation building produced divergent research currents, For the first half of the period under review, the American tropics continued to be the province largely of geographers with culturalhistorical questions grounded in natural historical bases. The legacy is Humboldt's; the practitioners most notably students of Carl Sauer or German counterparts such as Carl Troll. French and British regionalist approaches, strong in the region before World War II, survived less successfully. By the 1980s, however, broad pan-tropical currents of geographic discourse and debate had become established. New practices and theories were formulated and tested as North Atlantic geographers borrowed from antipodean innovators and others working in the Asian and African tropics. Since the 1980s, there have been greater efforts at dialogue and collaboration with host country colleagues. As might be expected in this era of ‘globalization’, national research styles and agendas have become less evident. This paper offers a highly selective map of research nodes within tropical Americanist geography since the early 1950s. The selection of examples includes qualitative criteria, but more importantly, research that signify stasis or intensification as well as turning points and departures in the overall development of this literature. The contours of this history suggest some highly evolved, even idiosyncratic enterprises, but in the main, it is an unfolding that suggests broad congruencies with human geographic work elsewhere in the tropics.     

Mg2+ removal in the system Mg2+—amorphous SiO2—H2O by adsorption and Mg-hydroxysilicate precipitation

Two chemical processes can remove Mg²⁺ from suspensions containing amorphous silica (am-SiO₂) at low temperatures: adsorption and precipitation of a Mg-hydroxysilicate resembling sepiolite. Mg²⁺ removal from am-SiO₂ suspensions was investigated, and the relative role of the two removal processes evaluated, as a function of: pH, ionic strength, Mg²⁺ concentration, and temperature.The extent of Mg²⁺ adsorption onto am-SiO₂ decreases with increasing NaCl concentration due to displacement of Mg²⁺ by Na⁺. At NaCl concentrations of 0.05 M and above, adsorption occurs only at pH values above 8.5, where rapid dissolution of am-SiO₂ gives rise to high concentrations of dissolved silica, resulting in supersaturation with respect to sepiolite. Removal of Mg²⁺, at concentrations of 40 to 650 μM, from am-SiO₂ suspensions in 0.70 M NaCl at 25 °C occurs at pH 9.0 and above. Experiments show that under these conditions adsorption and Mg-hydroxysilicate precipitation remove Mg²⁺ at similar rates. For 0.05 M Mg²⁺, at 0.70 M ionic strength and 25 °C, measurable Mg²⁺ removal occurs down to ca. pH 7.5 but is primarily due to Mg-hydroxysilicate precipitation. For the same solution conditions at 5°C, Mg²⁺ removal occurs above pH 8.0 and is primarily due to adsorption.Assuming that increasing pressure does not greatly enhance adsorption, Mg²⁺ adsorption onto am-SiO₂ is an insignificant process in sea water. The surface charge of pristine am-SiO₂ in sea water is primarily controlled by interactions with Na⁺. The principal reaction between Mg²⁺ and am-SiO₂ in marine sediments is sepiolite precipitation.The age distribution of sepiolite in siliceous pelagic sediments is influenced by temperatures of bottom waters and by geothermal gradients.     

Solution chemistry studies and flotation behaviour of apatite,calcite and fluorite minerals with sodium oleate collector

The maximum flotation response for three naturally occurring calcium minerals, apatite, calcite and fluorite with sodium oleate collector correlated directly with the minimum interfacial tension of the air/solution interface. For fluorite and apatite the minimum surface tension occurred about the mid-pH region and was attributed to the formation of pre-micellar associated species in solution. In the case of calcite the minimum was observed at high pH since the presence of high concentrations of calcium ions in solution appeared to reduce the concentration of amphililic species in the low and mid-pH regions.Microelectrophoresis data demonstrated that the three minerals acquired a negative charge in sodium oleate solution, resulting from adsorption of oleate species on the mineral surfaces.The flotation behaviour of the systems were shown to be related to the species distribution diagrams suggesting that the role of the acid soap dimer, soap dimer, molecular and lattice species could make a significant contribution to the character and composition of the interfacial films.High flotation response was explained by strong adhesion between the hydrophobic particle and bubble. It was suggested that the reduction in surface tension may not be the major factor contributing to the flotation efficiency but indicated the presence of associated surfactant species in solution which could also synergistically adsorb at the solid/liquid interface, increasing the hydrophobic character of the mineral surface. This would maximize the magnitude of the contact angle and hence the strength of the adhesion between particle and bubble. This adsorption behaviour is not in general agreement with conventionally non-hydrolyzable collector theory which is usually based on electrostatic models.