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Tafoni development in a cryotic environment: an example from Northern Victoria Land, Antarctica
No full textTafoni are a type of cavernous weathering widespread around the world. Despite the extensive distribution of the tafoni, their genesis is not clear and is still a matter of debate, also because the), occur in such different climatic conditions and on so many different types (if substrate. Geomorphological characterization of more than 60 tafoni in three different Antarctic sites (two coastal and one inland) between 74 and 76 degrees S with sampling of weathering products and salt occurrences are described together with thermal data (on different surfaces) and wind speed recorded in different periods of the year in a selected tafone close to the Italian Antarctic station. The aim of this present study is to provide further information to help understand the processes involved in the growth of tafoni in a cryotic environment, and the relationship of these processes to climate, with particular attention to the thermal regime and the role of wind. The new data presented in this paper suggest that there is no single key factor that drives the tafoni development, although thermal stress seems the most efficient process, particularly if we consider the short-term fluctuations. The data also confirm that other thermal processes, such as freezing-thawing cycles and thermal shock, are not really effective for the development of tafoni in this area. The wind speed measured within the tafoni is half that recorded outside, thus favouring snow accumulation within the tafoni and therefore promoting salt crystallization. On the other hand, the wind effect on the thermal regime within the tafoni seems negligible. While both salt weathering and thermal stress appear active in this cryotic environment, these are azonal processes and are therefore active in other climatic areas where tafoni are widespread (such as the Mediterranean region)
Weathering of granite in Antarctica, I: Light penetration in to rock and implications for rock weathering and endolithic communities.
No full textWhere rocks are composed of translucent minerals, light penetrates the rock and, in so doing, impacts on the thermal conditions. Where minerals are not translucent all the heat transformation must be at the rock surface, and steep thermal gradients can occur. Where light does penetrate, a component of the incoming radiation is transformed to heat at differing depths within the rock, thereby decreasing the thermal gradient. Equally, light transmissive minerals facilitate endolithic communities, which can also play a role in rock weathering. The attribute of light transmission within rock and the impact this has on the resulting thermal conditions has not been considered within rock weathering studies. An attempt was made to monitor the amount of light penetrating the outer 2 mm of coarse granite under Antarctic summer conditions and to evaluate the thermal impact of this. It was found that the amount of light penetration at this site exceeded modeled or postulated values from biological studies and that it could significantly impact the thermal conditions within the outer shell of the rock. Although the resulting data highlighted a number of flaws in the experimental procedure, sufficient information was generated to provide the first assessment of the range of thermal responses due to light transmissive minerals in rock
Weathering of granite in Antarctica, II: thermal stress at the grain scale
No full textGranular disintegration has long been recognized and referred to in weathering texts from all environments, including the Antarctic. Despite this universal identification and referral, few to no data exist regarding thermal conditions at this scale and causative mechanisms remain little more than conjecture. Here, as part of a larger weathering study, thermal data of individual grains (using infrared thermometry and ultra-fine thermocouples) composing a coarse granite, as well as the thermal gradients in the outer 10 cm (using thermistors), were collected from a north-facing exposure. Measurements were also made regarding the surface roughness of the rock. Based on recorded temperatures, the nature of the rock surface and the properties of the minerals, an argument is made for complex stress fields that lead to granular disintegration. Mineral to mineral temperature differences found to occur were, in part, due to the changing exposure to solar radiation through the day (and through seasons). Because the thermal conductivity and the coefficient of thermal expansion of quartz are not equal in all directions, coupled with the vagaries of heating, this leads to inter-granular stresses. Although fracture toughness increases with a decrease in temperature, it is suggested that the tensile forces resulting from falling temperatures are able to exceed this and produce granular disassociation. The lack of equality with respect to crystal axis of both thermal conductivity and expansion in quartz further exacerbates the propensity to failure. Grain size and porosity also influence the thermal stresses and may help explain why some grains are held in place despite disassociation near the surface. While the data presented here appear to beg more questions than providing answers, they do provide a basis for better, more detailed studies of this important weathering scale
Biotic and Abiotic Processes on Granite Weathering Landforms in a Cryotic Environment, Northern Victoria Land, Antarctica
No full textA multidisciplinary study was carried out to understand the interactions between biotic and abiotic processes in granite weathering in ice-free areas of Northern Victoria Land, Antarctica. Examples of tafoni, pits and grooves were analyzed, focusing on their morphometry, infills, weathering rind types and vegetation patterns. Surface and subsurface temperatures and incoming radiation were measured to characterize microclimatic conditions. In addition, microscopic, SEM and X-ray diffraction analyses of granite were carried out. These analyses indicate that, under present conditions, mechanical weathering is the main process active in the formation of tafoni, which post-date pits and grooves. In these forms, granular disintegration is mainly induced by chasmoendolithic lichens, salt and thermal stress associated with the dilatation coefficients of different granite-forming minerals. The overall morphology of pits and grooves indicates that they originate from water erosion. In the former, mechanical weathering prevails, caused by epilithic lichens, by freeze-thaw events, and by salt, while only the first two processes are active in the grooves. The intensity of these processes is less effective than in tafoni and on the outer surfaces, suggesting that pits and grooves are inherited features, possibly generated in the same way as landforms occurring on granite in the humid tropic
Promoting Earth Science topics in the primary schools: future teachers at work in the university lab
No full textBiotic and Abiotic Processes on Granite Weathering Landforms in cryotic environment, Northern Victoria Land, Antarctica
No full textA multidisciplinary study was carried out to understand the interactions between biotic and abiotic
processes in granite weathering in ice-free areas of northern Victoria Land, Antarctica. Examples of
tafoni, pits and grooves were analyzed, focusing on their morphometry, infills, weathering rind types
and vegetation patterns. Surface and subsurface temperatures and incoming radiation were measured
to characterize microclimatic conditions. In addition, microscopic, SEM and X-ray diffraction
analyses of granite were carried out. These analyses indicate that, under present conditions,
mechanical weathering is the main process active in the formation of tafoni, which post-date pits
and grooves. In these forms, granular disintegration is mainly induced by chasmoendolithic lichens,
salt and thermal stress associated with the dilatation coefficients of different granite-forming minerals.
The overall morphology of pits and grooves indicates that they originate from water erosion. In the
former, mechanical weathering prevails, caused by epilithic lichens, by freeze-thaw events, and by
salt, while only the first two processes are active in the grooves. The intensity of these processes is less
effective than in tafoni and on the outer surfaces, suggesting that pits and grooves are inherited
features, possibly generated in the same way as landforms occurring on granite in the humid tropics
Chloride penetration resistance in sound and micro-cracked concretes through different experimental techniques
Full text linkConcrete resistance to chloride penetration is one of the main design parameters for the assessment of reinforced concrete structures durability in chloride-contaminated environments, and it is usually determined through one of the accredited accelerated tests in uncracked configuration. In this study, the resistance to chloride penetration was evaluated on six different concrete types, in uncracked and load-induced micro-cracked configurations, subject to pure diffusion and considering two different analysis techniques, colorimetric and potentiometric titration. Results showed that in uncracked conditions, good correlation subsisted between the diffusion coefficients evaluated through the two techniques. In cracked configuration (micro-cracks 10-75 mu m wide and 5-45 mm deep) with both techniques a significant increase in chloride diffusion coefficient was detected for concretes with lower w/c ratio, suggesting that the effect of cracks may be more pronounced for more impervious concretes
LAKE-ICE BLISTERS, TERRA NOVA BAY AREA, NORTHERN VICTORIA LAND, ANTARCTICA
No full textIce blisters, typically 0.2-0.8 m high and 5-20 m long, develop annually on perennially frozen lakes in Northern Victoria Land. They are believed to be caused by hydrostatic pressures generated through progressive freezing of solute-rich water beneath the lake-ice cover during winter. Lake-ice blisters in the study area differfrom icing blisters described from the northern hemisphere. The latter are caused by hydraulic pressures and are found at locations such as river beds or spring sites on sloping terrain. The Antarctic lake-ice blisters reflect the occurrence of dry-based perennially frozen lakes with high salt contents in an extremely cold and arid environment
Preliminary assessment on the effects of longitudinal cracks on carbonation-induced corrosion
Full text linkMulti-step fractionation as a tool for enhanced valorization of technical lignins: a model study
No full textThe valorisation of lignin obtained as a by-product of the pulping and biofuel industries is one of the most promising topics in the bioresource field. Despite its potential value as the only massively available aromatic biopolymer feedstock, technical lignin is nowadays mostly burnt as low cost energy source because of its chemical recalcitrance. The high heterogeneity of this material, largely dependent on the different vegetal sources and the specific biomass recovery methods, restricts its direct use and hinders also the optimization of depolymerisation approaches. The development of effective technical lignin fractionation strategies is therefore today one of the most challenging topic in the green chemistry field.
In this study, the fractionation of an industrial commercial lignin was developed by a three step procedure set-up either in aqueous or in an environmentally friendly organic solvent in order to obtain sustainable and scalable processes.1,2 The first step consisted in a microfiltration or a Soxhlet extraction, depending on the type of solvent used. Then a cascade membrane-mediated ultrafiltration allowed to obtain at the end three refined lignin fractions. The parent lignin and the different lignin fractions were fully characterized.
The two-step process reported here allows accessing lignin fractions with well-defined physico-chemical properties (including mass distribution, glass transition temperature, aliphatic and phenolic hydroxyl groups concentration, syringyl/guaiacyl unit ratio) and represents a valuable approach towards the development of bio-based polymers and the preparation of key platform chemicals, thereby paving the way for an effective exploitation and valorization of this remarkable resource.
[1] Allegretti, C.; Fontanay, S.; Krauke, Y.; Luebbert, M.; Strini, A.; Troquet, J.; Turri, S.; Griffini, G.; D’Arrigo, P. ACS Sustainable Chem. Eng. 2018, 6, 9056-9064; DOI: 10.1021/acssuschemeng.8b01410.
[2] Allegretti, C.; Fontanay, S.; Rischka, K.; Strini, A.; Troquet, J.; Turri, S.; Griffini, G.; D’Arrigo P. ACS Omega 2019, in press; DOI: 10.1021/acsomega.8b02851
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