Structural softening of the lithosphere

Structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant. Structural softening is fundamentally different than material softening, where the decrease in stress is generated by a change in rheological properties with progressive deformation, such as grain size reduction resulting from large shearing strain. We study structural softening generated by folding of the crust-mantle boundary, which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere. For ductile rheologies, the stress decrease represents a decrease of the effective lithospheric viscosity, which is proportional to the ratio of stress to lithospheric shortening strain rate. We present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate. The decrease in effective viscosity can be up to 10-fold.     

Ground Thermal Profiles from Mount Kenya, East Africa

This paper presents and compares ground thermal regimes at 4200 and 4800 m a.s.l. on Mount Kenya's southern aspect. Temperatures were recorded using Tinytalk? data loggers, installed at the ground surface and at depths of 1 cm, 5 cm, 10 cm and 50 cm. Temperatures were logged at 2‐hour intervals over a period of 12 months (August 1998 to July 1999). The study is designed to demonstrate near‐surface freeze conditions, which would have implications for contemporary periglacial landform production. Although ground freeze at 4200 m a.s.l. occurs during most nights (c. 70% at 1 cm depth), freeze penetration is restricted to the top 2 to 3 cm, such that no freeze was recorded at 5 cm depth. At 4800 m a.s.l., the diurnal frost frequency at the surface is 365 days (100%), whilst that at 10 cm depth is 165 days (45%). The paper demonstrates that a greater longevity of contemporary thin snow cover at 4800 m a.s.l. permits progressive sub‐surface cooling with depth. However, the near‐surface ground temperature profiles suggest that conditions are not conducive to permafrost development at the sites.     

Degradation of methyl bromide and methyl chloride in soil microcosms: Use of stable C isotope fractionation and stable isotope probing to identify reactions and the responsible microorganisms

Bacteria in soil microcosm experiments oxidized elevated levels of methyl chloride (MeCl) and methyl bromide (MeBr), the former compound more rapidly than the latter. MeBr was also removed by chemical reactions while MeCl was not. Chemical degradation dominated the early removal of MeBr and accounted for more than half of its total loss. Fractionation of stable carbon isotopes during chemical degradation of MeBr resulted in a kinetic isotope effect (KIE) of 59 ± 7‰. Soil bacterial oxidation dominated the later removal of MeBr and MeCl and was characterized by different KIEs for each compound. The KIE for MeBr oxidation was 69 ± 9‰ and the KIE for MeCl oxidation was 49 ± 3‰. Stable isotope probing revealed that different populations of soil bacteria assimilated added ¹³C-labeled MeBr and MeCl. The identity of the active MeBr and MeCl degrading bacteria in soil was determined by analysis of 16S rRNA gene sequences amplified from ¹³C-DNA fractions, which identified a number of sequences from organisms not previously thought to be involved in methyl halide degradation. These included Burkholderia, the major clone type in the ¹³C-MeBr fraction, and Rhodobacter, Lysobacter and Nocardioides the major clone types in the ¹³C-MeCl fraction. None of the 16S rRNA gene sequences for methyl halide oxidizing bacteria currently in culture (including Aminobacter strain IMB-1 isolated from fumigated soil) were identified. Functional gene clone types closely related to Aminobacter spp. were identified in libraries containing the sequences for the cmuA gene, which codes for the enzyme known to catalyze the initial step in the oxidation of MeBr and MeCl. The cmuA gene was limited to members of the alpha-Proteobacteria whereas the greater diversity demonstrated by the 16S rRNA gene may indicate that other enzymes catalyze methyl halide oxidation in different groups of bacteria.     

Global environmental consequences of tourism

In 2000, almost 700 million international tourist arrivals were counted worldwide. Even though a global activity of this scale can be assumed to have a substantial impact on the environment, its consequences have never been assessed and quantified. In this contribution, five major aspects of the leisure-related alteration of the environment are investigated: (1) the change of land cover and land use, (2) the use of energy and its associated impacts, (3) the exchange of biota over geographical barriers and the extinction of wild species, (4) the exchange and dispersion of diseases, and (5), a psychological consequence of travel, the changes in the perception and the understanding of the environment initiated by travel.     

Computations of the tropospheric radiation budget in the Caribbean and Gulf of Mexico area for clear sky conditions

Summary Computations of solar heating and infrared cooling for clear sky conditions in the area of the Central American Seas are presented, as based on conventional radiosondes in 1960. Results are discussed with regard to latitude mean data available in the literature.Both solar heating and infrared cooling display the most pronounced latitudinal variation in winter, while the distribution is rather uniform in summer. Large heating rates are found in the lower layers over the Caribbean Sea. The infrared cooling shows maxima around 700 mb and below the 950 mb level, in both summer and winter. This contrasts withLondon's result, but agrees with a more recent study byDavis. Infrared cooling rates computed for clear sky conditions up to 500 mb are somewhat larger than the results form radiometer soundings in fall 1960 as reported byRiehl. Discrepancies betweenRiehl's data andLondon's latitude mean values appear to result essentially from different cloud conditions. The pattern of net cooling is in general consistent withLondon's latitude mean cross-sections for average cloudiness.Effective outgoing radiation in winter displays a decrease from the Caribbean Sea towards the cold interior of the North American Continent, while latitudinal contrasts vanish during the summer season. Computations compare well withBudyko's table andRiehl's estimate for fall 1960, while they are substantially lower thanLondon's data and estimates fromSverdrup's graph.

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Zusammenfassung Absorption von Sonnenstrahlung und langwellige Ausstrahlung für wolkenfreien Himmel wurden für den Bereich des Amerikanischen Mittelmeers berechnet, auf Grund herkömmlicher Radiosondenaufstiege im Jahre 1960. Die Ergebnisse werden mit Hinblick auf die in der Literatur verfügbaren Breitenkreismittelwerte diskutiert.Absorption von Sonnenstrahlung und langwellige Ausstrahlung ändern sich mit der geographischen Breite besonders stark im Winter, wogegen horizontale Unterschiede im Sommer weniger ausgeprägt sind. Eine relativ starke Absorption von Sonnenstrahlung scheint in den unteren Schichten über der Karibischen See stattzufinden. Die langwellige Ausstrahlung ist besonders stark um 700 mb und unterhalb des 950 mb-Niveaus, und zwar sowohl im Sommer wie im Winter. Das steht im Gegensatz zu den Ergebnissen vonLondon, stimmt aber mit einer neueren Studie vonDavis überein. Die für wolkenfreien Himmel berechnete langwellige Ausstrahlung bis hinauf zum 500 mb-Niveau ist etwas grösser als die aus Aufstiegen von Radiometersonden im Herbst 1960 gewonnenen Werte vonRiehl. Unterschiede zwischen den Daten vonRiehl und den Breitenkreismittelwerten vonLondon scheinen im wesentlichen aus unterschiedlichen Bewölkungsverhältnissen herzurühren. Die aus Absorption und langwelliger Ausstrahlung resultierenden Abkühlungsbeträge entsprechen im allgemeinen den vonLondon für mittlere Bewölkungsverhältnisse mitgeteilten mittleren Vertikalschnitten.Die effektive Ausstrahlung der Erdoberfläche zeigt im Winter eine Abnahme von der Karibischen See auf den dann kalten Nordamerikanischen Kontinent hin; Unterschiede mit der geographischen Breite verschwinden hingegen im Sommer. Die Berechnungen entsprechen den Tabellenwerten vonBudyko und den Abschätzungen vonRiehl für den Herbst 1960;London's Daten sowie Abschätzungen nach dem Nomogramm vonSverdrup hingegen sind deutlich höher.