Composition of the water accommodated fractions as a function of exposure times and temperatures

The water accommodated fractions (WAFs) of nine oils in seawater have been studied. The oils range from light condensate to heavy crude, and include one highly biodegraded oil and one very wax rich oil. This study has identified large variations in the chemical composition of WAFs, depending on oil type, temperature, and mixing time. Experiments at different temperatures (2-13 °C) showed that it takes longer time to reach equilibrium at the lowest temperatures, and that this varies for the different oil types. Oils with higher pour point (wax rich oils) need a longer time to establish WAF in equilibrium than oils with lower pour points (naphthenic oils). At 13 °C a mixing time of 48 h, as recommended in standard procedures, seems to be sufficient for asphalthenic and paraffinic oils. The results demonstrated that for WAF prepared from an unknown oil, or at lower temperatures, different mixing times should be tested. Since the WAF often is used in toxicity testing, the toxicity might be underestimated if the mixing time is too short.     

Contour scaling of a sandstone by salt weathering under simulated hot desert conditions

A laboratory study has been used to investigate relationships between salts and contour scaling—a weathering feature commonly observed on rock surfaces in salt-rich environments. Surface disaggregation and essentially surface-parallel cracks were produced in sandstone blocks using 10 per cent solutions of sodium sulphate and magnesium sulphate applied daily to single exposed surfaces for sixty days. A control block soaked once in saturated magnesium sulphate and subsequently wetted daily with distilled water showed extensive surface disaggregation, but no cracking. Both surface disaggregation and subsurface cracking were associated with relative concentrations of microcrystalline salt. A tentative model of contour scaling is proposed, which involves linking together potential cracks by salt-induced fracturing of intervening, crack-stopping grains. Further control blocks treated respectively with 10 per cent and saturated sodium chloride showed no evidence of subsurface cracking and only limited surface disaggregation of the ‘saturated’ block.     

The importance of non-carbonate mineral weathering as a soil formation mechanism within a karst weathering profile in the SPECTRA Critical Zone Observatory,Guizhou Province,China

Soil degradation, including rocky desertification,of the karst regions in China is severe. Karst landscapes are especially sensitive to soil degradation as carbonate rocks are nutrient-poor and easily eroded. Understanding the balance between soil formation and soil erosion is critical for long-term soil sustainability, yet little is known about the initial soil forming processes on karst terrain. Herein we examine the initial weathering processes of several types of carbonate bedrock containing varying amounts of non-carbonate minerals in the SPECTRA Critical Zone Observatory, Guizhou Province, Southwest China. We compared the weathering mechanisms of the bedrock to the mass transfer of mineral nutrients in a soil profile developed on these rocks and found that soil formation and nutrient contents are strongly dependent upon the weathering of interbedded layers of more silicate-rich bedrock(marls). Atmospheric inputs from dust were also detected.     

Patterns of damage in igneous and sedimentary rocks under conditions simulating sea‐salt weathering

A saline‐spray artificial ageing test was used to simulate the effects produced in granites and sedimentary rocks (calcarenites, micrites and breccia) under conditions in coastal environments. Three main points were addressed in this study: the durability of the different kinds of rock to salt decay, the resulting weathering forms and the rock properties involved in the weathering processes. For this, mineralogical and textural characterization of each of the different rocks was carried out before and after the test. The soluble salt content at different depths from the exposed surfaces was also determined. Two different weathering mechanisms were observed in the granite and calcareous rocks. Physical processes were involved in the weathering of granite samples, whereas dissolution of calcite was also involved in the deterioration of the calcareous rocks. We also showed that microstructural characteristics (e.g. pore size distribution), play a key role in salt damage, because of their influence on saline solution transport and on the pressures developed within rocks during crystallization. Copyright © 2003 John Wiley & Sons, Ltd.     

Rock temperatures as an indicator of weathering processes affecting rock art

To aid rock art conservation, rock temperatures have been monitored at different depths and at low (30 min) and high (1 min) acquisition rates in a painted rock shelter in the uKhahlamba‐Drakensberg Park (South Africa). Preliminary data for winter (cold and dry) show that in that season cryoclasty is unlikely to occur (rare subzero thermal events and probable reduced moisture availability) and thermal shocks are improbable (highest measured ΔT/Δt < 2 °C min^?1). High amplitude (about 30 °C) rock temperature cycles accompanied by reversals of the thermal gradient have been observed to occur almost daily and hint at the possibility of thermal stress fatigue. Copyright © 2006 John Wiley & Sons, Ltd.     

Rock albedo and monitoring of thermal conditions in respect of weathering: some expected and some unexpected results

Broadly speaking, there is, at least within geomorphic circles, a general acceptance that rocks with low albedos will warm both faster and to higher temperatures than rocks with high albedos, reflectivity influencing radiative warming. Upon this foundation are built notions of weathering in respect of the resulting thermal differences, both at the grain scale and at the scale of rock masses. Here, a series of paving bricks painted in 20 per cent reflectivity intervals from black through to white were used to monitor albedo‐influenced temperatures at a site in northern Canada in an attempt to test this premise. Temperatures were collected, for five months, for the rock surface and the base of the rock, the blocks being set within a mass of local sediment. Resulting thermal data did indeed show that the dark bricks were warmer than the white but only when their temperatures were equal to or cooler than the air temperature. As brick temperature exceeded that of the air, so the dark and light bricks moved to parity; indeed, the white bricks frequently became warmer than the dark. It is argued that this ‘negating’ of the albedo influence on heating is a result of the necessity of the bricks, both white and black, to convect heat away to the surrounding cooler air; the darker brick, being hotter, initially convects faster than the white as a product of the temperature difference between the two media. Thus, where the bricks become significantly hotter than the air, they lose energy to that air and so their respective temperatures become closer, the albedo influence being superceded by the requirement to equilibrate with the surrounding air. It is argued that this finding will have importance to our understanding of weathering in general and to our perceptions of weathering differences between different lithologies. Copyright © 2005 John Wiley & Sons, Ltd.     

Radiogenic argon in glauconites as a function of mineral recrystallization

Natural glauconitic grains were subjected to hydrothermal treatment at a pressure of 2 kbars and temperatures of 200–414°C for the period of one month.The measurement of radiogenic argon before and after treatment shows that the mica-type glauconite mineral contains 92–96% of the argon initially present up to temperatures as high as 320°C.After recrystallization of the mixed-layered glauconitic minerals the new phases still contain significant amounts of argon in the structure. The mixed-layered glauconitic minerals are much more sensitive to temperature treatment at elevated pressure than glauconitic-type mica, they, however, still contain up to 84% of the initial argon of the untreated material at temperatures as high as 320°C, i.e., just below the thermal stability of glauconite mica. It is evident that low-temperature metamorphism will affect the apparent radiogenic age of glauconitic grains to an extent which depends upon their composition and the temperature of metamorphism.     

Structure function as a characteristic of geophysical fields

Summary Facts about the statistical properties of random fields may be obtained by analysing their increments (i.e. the differences of values at two points). The properties of structure functions are described and the possibility of testing the statistical homogeneity of geophysical fields and their possible homogeneization are discussed.     

巢湖浮游藻类功能群的组成特性及其影响因素

2015年1-12月逐月采集了巢湖9个断面的浮游植物,并对所采植物进行功能群划分,结果显示,巢湖的浮游植物可以划分为15个功能群:C、N、P、MP、S2、X1、X3、Y、F、J、H1、H2、LM、M、W1.不同时期调查的植物优势功能群存在差异,其中H2在12次调查中的优势度均≥0.02,成为巢湖的绝对优势功能群,H2功能群的组成藻类为鱼腥藻,次要的功能群为M,主要组成藻类为微囊藻,两个功能群均为富营养化功能类群.巢湖植物优势功能群不同时期的演替规律为:1月H2+Y经2月H2+J,3月H2,4月H2+F,5月H2,6月M,7月M+H2,8、9月M,11月H2+M转变为12月H2.RDA分析结果显示,巢湖植物功能群分布受水环境因子影响较为明显,整体上,水温、溶解性总磷、溶解性总氮、高锰酸盐指数和水深是影响巢湖植物功能群分布格局的主要因素.     

Chemical weathering in a drainage basin underlain by old red sandstone

A hydrochemical budget is used to quantify the rate of chemical weathering and solutional denudation on Old Red Sandstone in East Twin drainage basin (0.2 km²) on the Mendip Hills for the 1972–73 Water Year. Net nutrient uptake by the biomass and precipitation inputs are subtracted from stream solute outputs to give an estimate of the solutes released to the system by weathering. The mineralogies of the sandstone and the soil are compared to predict possible weathering reactions for the primary and secondary minerals. Tentative estimates of primary mineral alteration and secondary mineral formation are then made by substituting the hydrochemical balances into the formulated weathering reactions. Finally the rate of solutional denudation (0.8 tonnes/a or 1.6 mm/100 a) is compared with other estimates of solutional and mechanical denudation at East Twin and with similarly derived results for other lithologies.     

Laboratory simulation of salt weathering under moderate ageing conditions: Implications for the deterioration of sandstone heritage in temperate climates

Salt weathering is a significant process affecting the deterioration and conservation of stone-built heritage in many locations and environments. While much research has focused on the impact of salt weathering under arid or coastal conditions with characteristic climatic conditions and salt types, many sites found to be experiencing salt-induced deterioration, such as sandstone rock-hewn cave temples in Gansu Province, China and sandstone buildings in the northern UK, experience high humidities, moderate temperature ranges, and different salt types. To evaluate the impact of salt weathering on sandstone-built heritage under such mild humid environmental conditions, a lab simulation experiment was designed. The experiment was carried out on three types of sandstone (used in the northern UK and Gansu Province, China) and utilized a realistic diurnal humidity and temperature cycle (85% RH/16°C + 60% RH/22°C), and three widespread damaging salts, that is, Na₂SO₄, MgSO₄, and the mixture of Na₂SO₄–MgSO₄. The nature and extent of deterioration was monitored by photography, weight loss, and the changes in petrophysical properties measured using hardness, ultrasonic pulse velocity (P-wave velocity), water absorption coefficient by capillarity, open porosity, and apparent density. All three sandstones were found to be susceptible to MgSO₄ and the mixture of Na₂SO₄–MgSO₄, but weakly affected by Na₂SO₄ under mild humid environmental conditions.     

Plagioclase weathering in the groundwater system of a sandy,silicate aquifer

The weathering rate of plagioclase was estimated in the groundwater system of a sandy, silicate aquifer formed after the Wisconsin Glacial Stage. The study area is an isthmus lying between Crystal and Big Muskellunge Lakes in northern Wisconsin, USA. Plagioclase occupies 3% of the quartz and K‐feldspar dominated sediments. Groundwater in the study area is recharged in part by precipitation through the isthmus soils and in part by seepage from Crystal Lake, which is of low ionic strength and chemically in steady state. Water analysis revealed that the chemistry of groundwater recharged from Crystal Lake is regulated by mineral dissolution reactions. The rate constant for plagioclase was estimated using mass balances for sodium concentrations along a groundwater flowline from Crystal Lake. For this calculation, various kinds of hydrological/mineralogical information were used: groundwater flow path from oxygen isotope analysis, groundwater travel times from flow modelling, mineral composition from microprobe analysis and surface area of minerals from BET (Brunauer–Emmett–Teller) analysis. The overall range of the estimation was less than an order of magnitude (3·5 × 10^?16 to 3·4 × 10^?15 mol/m²/s). The result is up to three orders of magnitude slower than the previous field estimates, which applied geometric methods in measuring mineral surface areas. However, this result is somewhat higher than the estimates reported by other BET area‐based studies, which were undertaken on soil profiles having different hydrological conditions. This rate difference is interpreted as a result of higher mineral reactivity owing to younger sediment age. The rate difference is smaller when this result is compared with the estimates from the soils of similar age, indicating that the differences in hydrological condition are not sufficient to explain the weathering rate discrepancy between the laboratory and field studies, which is up to five orders of magnitude. Copyright © 2002 John Wiley & Sons, Ltd.     

Mass-production of Cambro–Ordovician quartz-rich sandstone as a consequence of chemical weathering of Pan-African terranes: Environmental implications

A vast sheet of mature quartz sand blanketed north Africa and Arabia from the Atlantic coast to the Persian Gulf in Cambro–Ordovician times. U–Pb geochronology of a representative section of Cambrian sandstone in southern Israel shows that these sediments are dominated by 550–650 Ma detrital zircons derived from Neoproterozoic Pan-African basement. The short time lag between magmatic consolidation of a Pan-African source and deposition of its erosional products indicates that, despite their significant mineralogical maturity, the voluminous quartz-rich sandstones on the northern margin of Gondwana are essentially first-cycle sediments.

Mass production of these voluminous first-cycle quartz-rich sandstones resulted from widespread chemical weathering of the Pan-African continental basement. We suggest that conditions favoring silicate weathering, particularly a warm and humid climate, low relief and low sedimentation rates prevailed over large tracts of Gondwana in the aftermath of the Pan-African orogeny. An unusually corrosive Cambro–Ordovician atmosphere and humid climate enhanced chemical weathering on the vegetation-free landscape. We infer that late Neoproterozoic–Cambro–Ordovician atmospheric pCO₂ rose as a consequence of widespread late Neoproterozoic volcanism, followed by an uptake of CO₂ by chemical weathering to produce the Cambro–Ordovician sandstone as a negative feedback.     

Neoformation of montmorillonite by post-depositional subsurface weathering in a slope deposit in central France

Observations on an Early Quaternary slope deposit in the Massif Central indicated that the texture and mineralogy of this deposit have been seriously altered by post-depositional subsurface weathering. Analyses of clay fractions and water samples show that the montmorillonite present is a stable mineral in this environment and is, and has been, newly formed. As shown by water analyses from nearby sites, the chemical environment is strongly dependent on local factors. Consequently it can be seen that great care should be taken firstly in correlating different slope deposits on the basis of texture and clay mineralogy and secondly in using these parameters as (paleo) climatic indicators.     

Rock warming and drying under simulated intertidal conditions,part II: weathering and biological influences on evaporative cooling and near‐surface micro‐climatic conditions as an example of biogeomorphic ecosystem engineering

The way in which rocks and engineering materials heat‐up and dry‐out in the intertidal zone is of relevance to both weathering and ecology. These behaviours can be measured in the laboratory under controlled conditions designed to replicate those occurring in the field. Previous studies have demonstrated differences in thermal behaviours between rock types and through time as a result of soiling in terrestrial environments, but the influence of weathering and colonization on rock behaviours in the intertidal zone has not been previously assessed. We measured the warming and drying of blocks of rock (limestone and granite) and marine concrete during ‘low‐tide’ events simulated in the laboratory, before and after a period of exposure (eight months) on rock platforms in Cornwall, UK. As well as differences between the material types, temperatures of control (unexposed) and field‐exposed blocks differed in the order of 1 to 2 °C. Drying behaviours were also different after field exposure. Differences during the first few hours of exposure to air and heat were attributed to discolouration and albedo effects. Over longer periods of time, changes in the availability of near‐surface pore water as a result of micro‐scale bioerosion of limestone and the development of bio‐chemical crusts on marine concrete [observed using scanning electron microscopy (SEM)] are suggested as mechanisms enhancing and reducing, respectively, the efficiency of evaporative cooling. The retention of moisture by epilithic biofilms may also influence thermal and drying behaviours of granite. These observations represent one of the first examples of cross‐scalar biogeomorphic linkages in the intertidal zone. The significance of the results for the subsequent efficiency of weathering, and near‐surface micro‐climatic conditions experienced by colonizing organisms is discussed. The involvement of microorganisms in the creation of more (or less) ecologically stressful conditions through the alteration of substratum geomorphic properties and behaviours is suggested as an example of ‘biogeomorphic ecosystem engineering’. Copyright © 2011 John Wiley & Sons, Ltd.     

Exploring geochemical controls on weathering and erosion of convex hillslopes: beyond the empirical regolith production function

Landscape curvature evolves in response to physical, chemical, and biological influences that cannot yet be quantified in models. Nonetheless, the simplest models predict the existence of equilibrium hillslope profiles. Here, we develop a model describing steady‐state regolith production caused by mineral dissolution on hillslopes which have attained an equilibrium parabolic profile. When the hillslope lowers at a constant rate, the rate of chemical weathering is highest at the ridgetop where curvature is highest and the ridge develops the thickest regolith. This result derives from inclusion of all the terms in the mathematical definition of curvature. Including these terms shows that the curvature of a parabolic hillslope profile varies with distance from the ridge. The hillslope model (meter‐scale) is similar to models of weathering rind formation (centimeter‐scale) where curvature‐driven solute transport causes development of the thickest rinds at highly curved clast corners. At the clast scale, models fit observations. Here, we similarly explore model predictions of the effect of curvature at the hillslope scale. The hillslope model shows that when erosion rates are small and vertical porefluid infiltration is moderate, the hill weathers at both ridge and valley in the erosive transport‐limited regime. For this regime, the reacting mineral is weathered away before it reaches the land surface: in other words, the model predicts completely developed element‐depth profiles at both ridge and valley. In contrast, when the erosion rate increases or porefluid velocity decreases, denudation occurs in the weathering‐limited regime. In this regime, the reacting mineral does not weather away before it reaches the land surface and simulations predict incompletely developed profiles at both ridge and valley. These predictions are broadly consistent with observations of completely developed element‐depth profiles along hillslopes denuding under erosive transport‐limitation but incompletely developed profiles along hillslopes denuding under weathering limitation in some field settings. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical simulation of sedimentation processes in reservoirs as a function of outlet location

This paper builds on a recently published one-dimensional moving-boundary model of the coevolution of topset, foreset and bottomset in a reservoir that captures the dynamics of the internal muddy pond typical to reservoirs. This model was modified to account for different outlet locations at the reservoir＇s downstream end. This model considers a river carrying two sustained phases of sediments： coarse （sand） and fine （mud）. The coarse phase deposits in the topset and delta foreset, while the fine phase forms a dilute suspension of wash load in the river. As the river enters the reservoir, the muddy water plunges on the foreset to form a Froude-supercritical （purely depositional） turbidity current. This turbidity current emplaces the bottomset. The modified numerical model was tested against five laboratory experiments previously reported by the author. The model successfully locates the muddy-water/clear-water interface. In addition, modeled and measured bed deposits are in good agreement. Results clearly indicate that the location of the internal hydraulic jump plays a key role in the final bed deposit.     

Role of fractures in weathering of solid rocks: narrowing the gap between laboratory and field weathering rates

A weathering study of a fractured environment composed of granites and metasediments was conducted in Trás-os-Montes and Alto Douro (north of Portugal) and covered the hydrographic basin of Sordo river. Within the basin, a number of perennial springs were monitored for discharge rate, which allowed for the estimation of annual recharges. A small area of the basin was characterized for parameters such as hydraulic conductivity and effective porosity, which, in combination with the previously calculated recharges, allowed for the calculation of a fracture surface area. The monitored springs were also sampled and analyzed for major inorganic compounds, and using a mole balance model the chemistry of the water samples was explained by weathering to kaolinite of albite–oligoclase plus biotite (granites) or of albite plus chlorite (metasediments). The number of moles of dissolved primary minerals (e.g. albite) could be calculated using this method. These mass transfers were then multiplied by the spring's median discharge rate and divided by the fracture surface area to obtain a weathering rate. Another weathering rate was determined, but using a BET surface area as normalizing factor. Comparing both rates with a representative record of laboratory as well as of field-based weathering rates, it has been noted that rates normalized by the BET were, as expected, similar to commonly reported field-based rates, whereas rates normalized by the fracture surface area were unexpectedly relatively close to laboratory rates (one order of magnitude smaller). The monitored springs are of the fracture artesian type, which means that water emerging at the spring site flowed preferentially through joints and fractures and that weathering took place predominantly at their walls. Consequently, it was concluded that the most realistic weathering rates are those normalized by the fracture surface area, and as a corollary that the gap between laboratory and field weathering rates might not be as wide as usually is reported to be.     

Compressional- and shear-wave velocities as a function of confining stress in dry sandstones

A laboratory study was carried out to investigate the influence of confining stress on compressional- and shear-wave velocities for a set of rock samples from gas-producing sandstone reservoirs in the Cooper Basin, South Australia. The suite of samples consists of 22 consolidated sublitharenites with helium porosity ranging from 2.6% to 16.6%. We used a pulse-echo technique to measure compressional- and shear-wave velocities on dry samples (cylindrical 4.6 × 2 cm) at room temperature and at elevated confining stress (≤ 60 MPa). Compressional- and shear-wave velocities in samples increase non-linearly with confining stress. A regression equation of the form V = A ? Be^?DP gives a good fit to the measured velocities with improved prediction of velocities at high confining stresses compared with equations suggested by other studies. The predicted microcrack-closure stresses of the samples show values ranging from 70 MPa to 95 MPa and insignificant correlation with porosity, permeability or clay content. There is a positive correlation between change in velocity with core porosity and permeability, but this association is weak and diminishes with increasing confining stress. Experimental results show that pore geometry, grain-contact type, and distribution and location of clay particles may be more significant than total porosity and clay content in describing the stress sensitivity of sandstones at in situ reservoir effective stress. The stress dependence of Cooper Basin sandstones is very large compared with data from other studies. The implication of our study for hydrocarbon exploration is that where the in situ reservoir effective stress is much less than the microcrack-closure stress of the reservoir rocks, the variation of reservoir effective stress could cause significant changes in velocity of the reservoir rocks. The velocity changes induced by effective stress in highly stress-sensitive rocks can be detected at sonic-log and probably surface-seismic frequencies.     

Iron content of synthetic phlogopite as a function of growth rate

During monocrystalline growth experiments of Fe-poor phlogopite from hydrothermal potash aqueous solutions, the iron content of the mica has been found to be growth-rate-dependent. It is shown that in our experimental conditions this kinetic effect dominates possible changes of equilibrium iron content connected with variations of solution composition, oxygen fugacity and growth temperature. To a lesser extent, titanium is found to follow the same trend.