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31.
32.
Seawater is constantly circulating through oceanic basement as a low-temperature hydrothermal fluid (<150°C). In cases when
ultramafic rocks are exposed to the fluids, for instance during the initial phase of subduction, ferromagnesian minerals are
altered in contact with the water, leading to high pH and formation of secondary magnesium hydroxide, among other – brucite,
that may scavenge borate and phosphate from seawater. The high pH may promote abiotic formation of pentoses, particularly
ribose. Pentoses are stabilized by borate, since cyclic pentoses form a less reactive complex with borate. Analyses have shown
that borate occupies the 2' and 3' positions of ribose, thus leaving the 5' position available for reactions like phosphorylation.
The purine coding elements (adenine, in particular) of RNA may be formed in the same general hydrothermal environments of
the seafloor. 相似文献
33.
Olgeir Sigmarsson Didier Laporte Marion Carpentier Bertrand Devouard Jean-Luc Devidal Joan Marti 《Contributions to Mineralogy and Petrology》2013,165(3):601-622
Phonolite and trachyte are the felsic magmas of the alkaline magma suites, which characterize the Canary Islands. The October 2011 submarine eruption off El Hierro, the westernmost island, nevertheless, produced a small volume of rhyolitic magma. The rhyolite occurred as highly vesicular, white coloured pumices enveloped in and mingled with darker coloured basanitic pumice. The basanitic pumice is relatively crystal poor with a few euhedral olivines (mostly Fo77–79), clinopyroxenes and Fe-rich spinels, whereas very rare olivine of same composition is found together with equally rare Fe-sulphide and FeTi-rich oxides in the rhyolite. The Fe–Mg exchange equilibrium in the oxides permits to calculate an equilibrium temperature of 970–890 °C for the rhyolite, in agreement with quartz-melt equilibrium at ca. 930 °C. A striking mineralogical feature of the rhyolite is the presence of rounded to contorted grains of milky quartz, which are xenocrysts incorporated and partly dissolved into the magma. Analyses of residual volatile concentrations in the glasses show that the rhyolite melt was highly degassed, whereas the basanitic glass still has important halogen concentrations. Trace element patterns of the mafic glasses and their elevated incompatible element concentrations are typical of the western Canary Island basanites. In contrast, the trace element composition of the rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (e.g. Rb, Ba, K and Th). All other measured LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart that can be explained by fractionation of accessory phases (1 % apatite, 1 % sphene and 0.1 % zircon). Surprisingly, low U concentration is presumably related to elevated oxygen fugacity in the rhyolite, causing U to be in a hexavalent state, and fluxing of F-rich gas leading to volatilization of UF6, known to emanate at low temperature. The results suggest that a gas-rich basanitic melt remobilized a small volume of stagnant rhyolitic melt formed by incorporation of approximately 10 % quartz-rich sediment into a late differentiate of trachytic composition. Sediments at the interface of an old oceanic crust adjacent to a continental shield and younger volcanic island are likely to act as magma traps were sediment assimilation may alter the mantle-derived magma composition. Quartz assimilation thus explains the production of rhyolite magma in a volcanic island characterized by an alkaline magma series from primitive basanites to trachytes. 相似文献
34.
Luisa Patiño Maria Isabel Velez Marion Weber César A. Velásquez-r Santiago David Manuela Rueda Ivonne Castañeda Diana Arboleda 《第四纪科学杂志》2020,35(8):1046-1056
This study presents the first multi-proxy palaeoenvironmental and palaeoclimatic history for northern South America based on the palaeolimnological reconstruction of a pond located in a dry paramo at 3570 masl. During the Last Glacial Maximum (LGM), the study area was under glacial conditions, then during global events Heinrich Stadial 1 (HS1), Bølling–Allerød (BA), and the Younger Dryas (YD), the pond expanded, accumulation rates and proxies for erosion reached the highest values, indicative of humid conditions, with maxima in humidity during the BA and YD. Dry conditions and pond desiccation occurred in the Greenlandian–Northgrippian and by 6010 cal a bp the area was transformed into the mire of today. Comparisons with records from other sites in South America indicate that changes in humidity are most likely controlled by the Intertropical Convergence Zone, mainly during the glacial and postglacial, and by changes in the Pacific Ocean, more pronounced after the YD. 相似文献
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36.
The resolution of the sea-ice component of a coarse-resolution global ocean general circulation model (GCM) has been enhanced to about 22 km in the Southern Ocean. The ocean GCM is designed for long-term integrations suitable for investigations of the deep-ocean equilibrium response to changes in southern hemisphere high-latitude processes. The space and time scales of the high-resolution sea-ice component are commensurate with those of the resolution of satellite passive-microwave sea-ice data. This provides the opportunity for a rigorous evaluation of simulated sea-ice characteristics. It is found that the satellite-derived continuous high ice concentration of the interior winter ice pack can only be captured when vertical oceanic mixing is modified in a way that less local, intermittent convection occurs. Furthermore, the width and the variability of the coastal polynyas around the Antarctic continent and its ice shelves are best captured when some form of ice-shelf melting is accounted for. The width of the wintertime ice edge is reasonably reproduced, while its variability remains underestimated, closely following the coarse-grid pattern of the ocean model due to its high dependence on ocean temperature. Additional variability besides daily winds, e.g. in form of idealized tidal currents, improves the temporal and spatial ice-edge variability, while leads in the interior ice pack become more abundant, more in line with the fine-scale satellite-derived texture. The coast- or ice-shelf line is described on the fine grid based on satellite passive-microwave data. This method requires parts of a coarse coastal ocean grid cell to be covered by an inert layer of “fast ice” or “ice shelf”. Reasonable long-term global deep-ocean properties can only be achieved when these areas are not inert, i.e. are exposed to heat flux and ice growth, or when the vertical mixing parameterization allows for excessive open-ocean convection. The model area exposed to cold high-latitude atmospheric conditions thus being most decisive for a realistic representation of the long-term deep-ocean properties, suggests that high-latitude coastlines are definitely in need of being represented at high resolution, including ice sheets and their effects on the heat and freshwater flux for the ocean. 相似文献
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38.
Hydrogeology Journal - Increasing population growth and global climatic changes threaten water security in semiarid regions such as Northern Ghana. The Tamnean Plutonic Suite aquifer is the main... 相似文献
39.
Enrico Kallmeier Christoph Breitkreuz Hubert Kiersnowski Marion Geißler 《Sedimentary Geology》2010,223(1-2):15-34
Facies analysis focussing on coarse-grained sediments has been carried out on more than 2500 m of drill cores from seven wells from southern margins of the North German Basin (NGB). The NGB forms a central element of the Southern Permian Basin (SPB). The wells exposed conglomerates and sandstones of the Rotliegend Grüneberg and Parchim Formations deposited in the Kotzen Basin and the Barnim Basin.17 lithofacies types have been grouped into six lithofacies associations. The studied successions are dominated by fluid gravity flow deposits (hyperconcentrated flows and stream flows) of alluvial fan and alluvial plain systems. Maximum particle size/bed thickness plots (MPS/BTh) support the interpretation as fluid gravity flow deposits. The MPS and BTh data have also been used to differentiate coarsening–thickening and fining–thinning trends of the fan systems.The dominance of water-rich mass flow processes together with sedimentary structures such as dewatering structures and outwashed tops suggests the presence of wet-type fans and plains under semi-humid to semi-arid seasonal climates in the central SPB. The investigated sediments show variation in clast composition subsequent to deep erosion processes on basin margins and changes of source areas. Synsedimentary normal faults and clastic dykes have been interpreted as indicators of tectonic activity of grabens itself and its frames. On a larger scale, then evolution from a half-graben to a graben is apparent for the Tuchen Sub-basin at least. The progradational/retrogradational cycles of the studied alluvial fan systems document combined local tectonic movements and influences of climatic changes. However, our data did not allow for a clear distinction between climatic and tectonic signals. Furthermore, a one-to-one correlation of fan cycles with depositional trends in the NGB basin centre would appear to be oversimplistic. 相似文献
40.
Environmental conservation and the production of new territories: the example of French départements
Pech Pierre Dizière Sophie Gillet Anne-Gabrielle Mamder Julie Tichit Marion 《GeoJournal》2010,75(2):149-161
Very important initiatives have been taken and policies have been adopted in the European Union to protect areas of great
importance for threatened species and habitats. Protected areas differ broadly in terms of category, natural conditions and
administrative organisation, from international initiative such as Biosphere Reserves, European ones with Natura 2000 network
until the institution of national and regional protected areas. In France, the administrative subdivisions known as “départements” were created with the French Revolution in the end of the eighteenth century; in 1985 an original and autonomous procedure
to establish special protected areas, called “Sensitive Natural Spaces” (SNS) was devolved to them. The scope of this paper
is to present an overview of these devolved powers which enable French départements to create protected areas and to levy a departmental tax on sensitive natural spaces (DTSNS). We statistically studied some
parameters by multivariate methods in order to explain the choices of this policy by the départements. The huge variations in the way these powers are implemented prove the development of new environmental territories. 相似文献