Evidence for freeze–thaw events and their implications for rock weathering in northern Canada: II. The temperature at which water freezes in rock

Many undertakings have used either a single value or a narrow window of temperatures as a threshold for the freezing of water within rock. These temperatures vary from 0 to ?5 C, with most windows being in the range ?1 to ?4 C. Based on thermal data, these thresholds are commonly used to ‘count’ the number of freeze–thaw events as a basis for determining freeze–thaw weathering. Data collected from northern Canada indicate that the temperature at which freezing occurs can vary substantially, even for the same site. Using exotherm and zero curtain observations from bricks, at angles of 90 and 45, aligned to the four cardinal aspects, the various temperatures at which water froze are shown. Bricks on the north and east commonly did exhibit freezing, based on exotherms, within the window ?1 to ?5 C, while data for the south and west aspects showed substantial variation, with freezes sometimes between ?6·4 and ?8·9 C. The data were evaluated for evidence of zero curtain effects (indicative of water freezing), but no unequivocal events could be found, and it is suggested that, at the scale of observation used here, they are unlikely. It would therefore appear that the use of thermal thresholds may not be meaningful for evaluation of freeze–thaw events. The available data also indicate many instances when temperatures went substantially sub‐zero (e.g. ?20 C) and yet no indication of water freezing occurred – most likely because there was no water available to freeze. This indicates that any form of freeze–thaw event counting, in the absence of some indicator of the presence of water and that it actually froze, is flawed, as thermal conditions alone are not adequate to indicate the occurrence or not of actual freeze–thaw weathering events. These data suggest that evaluations of freeze–thaw occurrence based simply on thermal thresholds may be substantially in error. Copyright © 2006 John Wiley & Sons, Ltd.     

Defect distribution and dissolution morphologies on low-index surfaces of α-quartz

The dissolution of prismatic and rhombohedral quartz surfaces by KOH/H₂O solutions was investigated by atomic force microscopy. Rates of dissolution of different classes of surface features (e.g., steps, voids, and dislocation etch pits) were measured. The prismatic surface etched almost two orders of magnitude faster than the rhombohedral surface, mostly due to the difference in the number and the rate of dissolution of extended defects, such as dislocations. Because of the presence of imperfect twin boundaries, defect densities on the prismatic surface were estimated at 50-100 μm⁻², whereas the rhombohedral surface possessed only ∼0.5-1.0 μm⁻², mostly in the form of crystal voids. Crystal voids etched almost one order of magnitude faster on the prismatic surface than on the rhombohedral surface due to differences in the number and the density of steps formed by voids on the different surfaces. In the absence of extended defects, both surfaces underwent step-wise dissolution at similar rates. Average rates of step retreat were comparable on both surfaces (∼3-5 nm/h on the prismatic surface and ∼5-10 nm/h on the rhombohedral surface). Prolonged dissolution left the prismatic surface reshaped to a hill-and-valley morphology, whereas the rhombohedral surface dissolved to form coalescing arrays of oval-shaped etch pits.     

Large-scale Vertical Momentum,Kinetic Energy and Moisture Fluxes in the Antarctic Sea-ice Region

There are very strong thermal gradients between the Antarctic continent and the sea-ice zone, and between that zone and the ocean to the north. As a result of these contrasts the sea-ice domain is one of strong cyclogenesis and high cyclone frequency. In this study we explore many aspects of that cyclonic behaviour and investigate the manner in which these systems influence, and are influenced by, the sea ice. Using the NCEP-DOE re-analyses (1979–2002) we have determined variables that are proportional to the mean of the wind stress and the mean rate at which mechanical energy is imparted to the surface. Using two decompositions of the wind field we have obtained estimates of how much of these fluxes are contributed to by the transient eddies. We find these to be significant over the sea ice and the ocean to the north, particularly when a new decomposition is used. The presence of frequent and vigorous cyclones is a central factor that determines the positive mean freshwater flux over the sea-ice zone in all seasons. This transfer to the ocean is smallest in summer (0.49 mm day⁻¹) and assumes a maximum of 1.27 mm day⁻¹ in winter.     

The Near Infrared Spectrum of Miranda: Evidence of Crystalline Water Ice

A spectrum from 1.2 to 2.5 μm of Uranus' small satellite Miranda obtained in June 1999 reveals strong water-ice signatures. It confirms the existence of a 2.0-μm water feature previously detected on Miranda and shows a strong second broad 1.5-μm water-ice absorption feature. The spectra also reveal a weak absorption band at 1.65 μm that is indicative of crystalline water ice. Reflectance models which combine the new spectra with new photometry indicate that the spectra are characteristic of a mostly water-ice surface, with a large fraction of carbonaceous or silicate contaminates, and the possible presence of ammonia hydrate, as implied by an apparent weak feature near 2.2 μm. The possible presence of other volatiles is also investigated.     

Oxyanion Concentrations in Eastern Sierra Nevada Rivers – 3. Boron, Molybdenum, Vanadium, and Tungsten

Water samples were collected from 10 locations along the Truckee River system, 14 locations along the Walker River system, and 12 locations along the Carson River, and analyzed for B, Mo, V, W, Na, Cl, and pH. Boron concentrations ranged from approximately 2 mol/kg in the upper reaches of the Truckee River to almost 1,200 mol/kg in Pyramid Lake. Molybdenum, V, and W had concentrations in the nanomolal range; Mo varied from a low of about 12 nmol/kg to a high of 3,200 nmol/kg (Walker Lake); V ranged from 9 nmol/kg to approximately 470 nmol/kg; and W varied from a low value around 0.8 nmol/kg (West Walker River) to 1,030 nmol/kg. The high concentrations of these oxyanion-forming trace elements in the rivers reflects (1) the relative stability of these oxyanions (e.g., MoO₄ ^2-, HVO₄ ^2-, WO₄ ^2-, B(OH)₃, and/or B(OH)₄ ^-) in the alkaline, well oxygenated river and lake waters, (2) contributions of hydrothermal waters (especially for B), and (3) weathering of rocks/regolith with high concentrations of these elements. In the case of Mo, V, and W, each exhibited relatively conservative behavior in the upper, oxygenated reaches of all three rivers. During the study period the region experienced a prolonged drought such that the lower reaches of each river were typified by no flow or stagnant waters and probably low oxygen and/or anoxic conditions (although not measured). Reductive processes occurring in the low flow to stagnant reaches of each river could have led to removal of Mo, V, and W from solution as coprecipitates with Fe monosulfides, or via sorption to Fe oxides/oxyhydroxides and/or organic matter. Boron, however, exhibited essentially no or minor removal from these rivers, and instead was added to each river via B-rich hydrothermal waters (e.g., Steamboat Creek from Steamboat Hot Springs), or by B-rich groundwaters via base-flow during the extensive drought.     

Major ion geochemistry of groundwaters from southern Nevada and eastern California, USA

The dissolved ionic constitutents of groundwaters are,in part,a recored of the minerals and rocks in aquifers through which the water has flowed.The chemical composition and association of these major ions in groundwaters have been used to trace groundwater flow paths and sources,In general,the chemical compostion of water in carbonate-rock aquifers in dominated by calcium,magnesium,and bicarbonate,whereas sodium,chloride,and sulfate can be dominant ions in the water that comes from volcanic aquifers or clay minerals.Since the 1990‘s,we have dealt with the geochemistry of groundwaters from more than 100 springs and wells in southern Nevada and eastrn california ,USA for major solutes and trace elements.This paper compiles the hydrochemical data of major ions of these groundwaters.Based on major ion geochemistry,groundwaters from southern Nevada and eastern California can be classified as carbonate aquifer water,volcanic aquifer water,and mixing water (either mixing of cabonate and volcanic aquifer waters or mixing with local recharges),Piper and stiff diagrams of major ions have graphically shown the general chemical characteristics,classification,and mixing relationships of groundwaters from southern Nevada and eastern California.     

TRAVEL TO WORK AND LABOR FORCE PARTICIPATION OF MEN AND WOMEN IN AN AUSTRALIAN METROPOLITAN AREA

For a number of social and cultural reasons, labor force participation rates of Australian women are low. One reason not previously investigated is the problem of geographic accessibility to employment, especially for suburban women. Results from journey-to-work studies show that females either work locally or in the central-city area, whereas males travel to more dispersed locations. Further investigation of female travel patterns from each of fifty-five suburban local government areas shows that the socioeconomic status of an area, together with its job mix, explains much variety in female work travel and suggests that workforce participation rates depend in part on local job availability. The research contributes to an understanding of the nature of occupational and geographical disaggregation of the Melbourne metropolitan labor market.