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Crust Formation (crust + formation)

Distribution by Scientific Domains

Earth and Environmental Science	80%

Selected Abstracts

Sources of sulphur in gypsiferous sediments and crusts and pathways of gypsum redistribution in southern Tunisia

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2004
Nick A. Drake
Abstract Southern Tunisia contains one of the most extensive gypsum accumulations in Africa comprising Triassic, Cretaceous, Eocene and Mio-Pliocene marine evaporites, spring deposits, playa sediments, aeolian sands and gypsum crusts. Sulphur isotope analysis (,34S) of bedrock samples, groundwater, playa brines, playa sediments, and gypsiferous crusts provides insight into the sources of gypsum in the region and sheds light on the processes that lead to gypsum crust formation. Results suggest that recycling of marine gypsum is the most likely source of the sulphate in the groundwater, playa sediments and crusts. The low ,34S values found in Eocene and Mio-Pliocene samples suggest that this recycling has been going on for millions of years. Though bedrock appears to be the ultimate source of the gypsum in the crusts, transport of this sulphate to playas, concentration therein, and subsequent dispersal across the landscape by aeolian processes provides the most likely pathway for sur,cial gypsum crust formation. Comparison of these results with those from Australia, Chile and Namibia suggests that, although the source of the sulphur varies from region to region, the processes of sur,cial crust formation appear to be similar. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Elementary processes of soil,water interaction and thresholds in soil surface dynamics: a review

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2004
Richard S. B. Greene
Abstract Elementary processes of soil,water interaction and the thresholds to these processes are important to understand as they control a range of phenomena that occur at the soil surface. In particular processes involved with wetting by rainfall that lead to particle breakdown are critical. This breakdown causes soil detachment and crust formation, which are both key elements in erosion. This paper reviews the range of approaches that have been taken in describing the processes associated with the wetting of a soil surface by rainfall. It assembles the studies that emphasize soil physics, soil chemistry, and erosion mechanics in a framework to enable a balanced consideration of important processes and management strategies to control erosion for a particular situation. In particular it discusses the factors associated with the two basic processes of soil structural breakdown, i.e. slaking and dispersion, and how these processes are critical in particle detachment, transport and surface crust formation. Besides the balance between the exchangeable cation composition and electrolyte concentration (measured as the sodium adsorption ratio (SAR) and total cation concentration (TCC) respectively) of the soil, the importance of energy input and soil organic matter content in controlling clay dispersion is emphasized. Based on the balance between these factors, the soil can be in one of three different regions, i.e. a dispersed region, a ,occulated region and one where the resilience of the soil is variable. The implications of each of these regions to soil erosion management are brie,y outlined, as are the critical roles that soil cover levels and organic matter have in controlling erosion. Finally, the relationship between various laboratory measures of aggregate stability, and corresponding ,eld erosion characteristics, is discussed. Copyright © 2004 John Wiley & Sons, Ltd. [source]

A quantitative study of epidermal Langerhans cells in cutaneous leishmaniasis caused by Leishmania tropica

INTERNATIONAL JOURNAL OF DERMATOLOGY, Issue 11 2004
Simin Meymandi MD
Objective, The purpose of this study was to characterize the number and distribution of epidermal Langerhans cells in different clinical forms of dry-type cutaneous leishmaniasis (CL). Methods, Sixteen cases of dry-type cutaneous leishmaniasis caused by Leishmania tropica were studied. These cases were classified clinically as five cases of acute leishmaniasis with indurated papules, nodules and plaques with central crust formation and duration < 2 years, six cases of lupoid leishmaniasis with characteristic papules around previous scars of cutaneous leishmaniasis with duration > 2 years, and five cases of chronic nonlupoid type with nonhealing lesions of duration > 2 years. Paraffin-embedded blocks were stained with hematoxylin and eosin (H&E) and stained immunohistochemically for CD1a. Results, The number of Langerhans cells per millimeter length of epidermis was increased in acute cases compared to chronic and lupoid cases. Conclusions, Lesions of acute leishmaniasis contain the greatest amounts of antigen for presentation, so Langerhans cells increase in number and in trafficking to present antigens derived from Leishman bodies to the cellular immune system. In chronic leishmaniasis, the Langerhans cell population is reduced, perhaps because of exhaustion of the source of Langerhans cells, or because of reduced response to modified antigen. [source]

Taphonomy and compositional fidelity of Quaternary fossil assemblages of terrestrial gastropods from carbonate-rich environments of the Canary Islands

LETHAIA, Issue 3 2008
YURENA YANES
Quaternary aeolian deposits of the Canary Islands contain well-preserved terrestrial gastropods, providing a suitable setting for assessing the taphonomy and compositional fidelity of their fossil record over ~13 kyr. Nine beds (12, 513 shells) have been analysed in terms of multivariate taphonomic and palaeoecological variables, taxonomic composition, and the stratigraphic and palaeontological context. Shells are affected by carbonate coatings, colour loss and fragmentation. Shell preservation is size-specific: juveniles are less fragmented and show colour preservation more commonly than adults. In palaeosols, the adult shell density correlates negatively with the proportion of fragmented adults, negatively with the proportion of juveniles, and positively with the proportion of adults with coatings. High bioturbation intensity in palaeosols is associated with low shell fragmentation and high proportion of shells with coatings. These relationships imply that high adult density in palaeosols was driven by an increase in shell production rate (related to a decrease in predation rates on adults and a decrease in juvenile mortality) and a decrease in shell destruction rate (related to an increase in durability enhanced by carbonate precipitation). In dunes, the relationships between taphonomic alteration, shell density and bioturbation are insignificant. However, dune assemblages are characterized by a lower frequency of shells with coatings and higher rates of colour loss, indicating lower shell durability in dunes than in palaeosols. Additionally, non-random differences in the coating proportion among palaeosols imply substantial temporal variation in the rate of carbonate crust formation, reflecting long-term changes in bioturbation intensity that covaries positively with shell preservation. Dunes and palaeosols do not differ in species abundances despite differences in the degree of shell alteration, suggesting that both weakly and strongly altered assemblages offer data with a high compositional fidelity. Carbonate-rich terrestrial deposits originating in arid conditions can enhance the preservation of gastropods and result in fossil assemblages that are suitable for palaeoecological and palaeoenvironmental studies of terrestrial ecosystems. [source]