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171.
Beate Orberger Christiane Wagner Alina Tudryn Richard Wirth Rachael Morgan José D. Fabris Jean Marc Greneche Carlos Rosière 《Physics and Chemistry of Minerals》2014,41(9):651-667
The pseudomorphic transformation of magnetite into hematite (martitization) is widespread in geological environments, but the process and mechanism of this transformation is still not fully understood. Micro- and nano-scale techniques—scanning electron microscopy, focused ion bean transmission electron microscopy, and Raman spectroscopy—were used in combination with X-ray diffraction, Curie balance and magnetic hysteresis analyses, as well as Mössbauer spectroscopy on martite samples from a banded iron formation (2.9 Ga, Dharwar Craton, India), and from lateritic soils, which have developed on siliciclastic and volcanic rocks previously affected by metamorphic fluids (Minas Gerais, Brazil). Octahedral crystals from both samples are composed of hematite with minor patches of magnetite, but show different structures. The Indian crystals show trellis of subhedral magnetite hosting maghemite in sharp contact with interstitial hematite crystals, which suggests exsolution along parting planes. Grain boundary migrations within the hematite point to dynamic crystallization during deformation. Dislocations and fluid inclusions in hematite reflect its precipitation related to a hydrothermal event. In the Brazilian martite, dislocations are observed and maghemite occurs as Insel structures and nano-twin sets. The latter, typical for the hematite, are a transformation product from maghemite into hematite. For both samples, a deformation-induced hydrothermally driven transformation from magnetite via maghemite to hematite is proposed. The transformation from magnetite into maghemite comprises intermediate non-stoichiometric magnetite steps related to a redox process. This study shows that martite found in supergene environment may result from earlier hypogene processes. 相似文献
172.
Comte Valentin Schneider Léonard Calanca Pierluigi Rebetez Martine 《Theoretical and Applied Climatology》2022,147(1-2):423-436
Theoretical and Applied Climatology - In this study, we investigated the consequences of climate change on bioclimatic indices in vineyards along the edge of Lake Neuchatel in Switzerland. Like in... 相似文献
173.
Bohumil M. Svoma Daniel S. Krahenbuhl Chad E. Bush Jonny W. Malloy Joshua R. White Melissa A. Wagner 《自然地理学》2013,34(3):174-187
A critical issue in research concerning long-term climate change is the relationship between circulation features and global temperature variations. We establish that the annual areal size of the northern hemisphere subtropical high pressure belt (SHPB), as defined by seven 500-hPa height isohypses, shares over 70% of the variability with global annual near-surface air temperature since 1948. The area enclosed by the 5850-m isohypse of the 500-hPa surface in the northern hemisphere has more than doubled since the 1950s, with greatest increases over northern Africa, the Middle East and India. A long-term historical run of a coupled global climate model shows rapidly increasing SHPB annual sizes, since the mid-1970s. Since the SHPB’s descending air produces increased aridity, SHPB expansion may transition humid regions to more arid lands. To examine this aspect, first, variations in recorded precipitation using a gridded database for the region experiencing expansion of the SHPB show a decrease in precipitation (though significant only at the 88% confidence level) over the last 60?years. Second, variations in a 0.5° spatial resolution monthly drought index, the Standardized Precipitation Evapotranspiration Index, are highly correlated (+0.78) with annual variations in the area enclosed by the 500-hPa height isohypses. These results support those of previous investigations that suggest further northward expansion of the northern hemisphere subtropical dry zones with continued global climate change. 相似文献
174.
Constantin Athanassas Yannis Bassiakos Günther A. Wagner Michael E. Timpson 《Geoarchaeology》2012,27(3):237-258
In this paper, we employed optically stimulated luminescence (OSL) dating of sediments from two archaeological sites located in Navarino, Messenia, southwestern Greece, to deduce a chronology for the archaeological sites. Archaeological surveys identified two Paleolithic sites on fossilized coastal dunes. Chipped stone tool assemblages were identified eroding out of paleosols developed in the dunes. The assemblage from one site lacked distinct typological features and hence it was difficult to assign to a chronological period. The lithic assemblage from the other site contained artifacts that typologically can be assigned to the Levallois‐Mousterian. Previous efforts to date the artifact‐bearing sediments at these sites were unsuccessful. Using newer OSL dating methods (i.e., the Single‐Aliquot‐Regenerated Dose protocol and thermally transferred‐OSL[TT‐OSL]), we attempted to construct a chronological framework for Late Pleistocene human activity in the southwest Peloponnese. The revised OSL chronology for the first site is 28 ± 5 ka, while a luminescence age of 8 ± 1 ka for the second site only represents a later deflation event. Within the framework of Quaternary environmental change, the location of Paleolithic sites relative to the coast would have changed during the course of the Pleistocene. As a result, Paleolithic exploitation strategies would have been strongly influenced by the changing coastal geomorphology, encouraging hominids to adapt to new distributions of resources. OSL dating of the archaeological sites allowed us to connect traces of hominid activity with climatic stadials/interstadials of the later Pleistocene derived from existing relative sea‐level curves. Ultimately, these data permitted the reconstruction of regional Late Pleistocene paleogeography. © 2012 Wiley Periodicals, Inc. 相似文献
175.
Yoann Le Bars Valentin Vallaeys Éric Deleersnijder Emmanuel Hanert Loren Carrere Claire Channelière 《Ocean Dynamics》2016,66(4):589-603
With the second largest outflow in the world and one of the widest hydrological basins, the Congo River is of a major importance both locally and globally. However, relatively few studies have been conducted on its hydrology, as compared to other great rivers such as the Amazon, Nile, Yangtze, or Mississippi. The goal of this study is therefore to help fill this gap and provide the first high-resolution simulation of the Congo river-estuary-coastal sea continuum. To this end, we are using a discontinuous-Galerkin finite element marine model that solves the two-dimensional depth-averaged shallow water equations on an unstructured mesh. To ensure a smooth transition from river to coastal sea, we have considered a model that encompasses both hydrological and coastal ocean processes. An important difficulty in setting up this model was to find data to parameterize and validate it, as it is a rather remote and understudied area. Therefore, an important effort in this study has been to establish a methodology to take advantage of all the data sources available including nautical charts that had to be digitalized. The model surface elevation has then been validated with respect to an altimetric database. Model results suggest the existence of gyres in the vicinity of the river mouth that have never been documented before. The effect of those gyres on the Congo River dynamics has been further investigated by simulating the transport of Lagrangian particles and computing the water age. 相似文献
176.
177.
Chad W. Higgins Martin Froidevaux Valentin Simeonov Nikki Vercauteren Caitlin Barry Marc B. Parlange 《Boundary-Layer Meteorology》2012,143(2):379-391
Taylor’s frozen turbulence hypothesis is the central assumption invoked in most experiments designed to investigate turbulence
physics with time resolving sensors. It is also frequently used in theoretical discussions when linking Lagrangian to Eulerian
flow formalisms. In this work we seek to quantify the effectiveness of Taylor’s hypothesis on the field scale using water
vapour as a passive tracer. A horizontally orientated Raman lidar is used to capture the humidity field in space and time
above an agricultural region in Switzerland. High resolution wind speed and direction measurements are conducted simultaneously
allowing for a direct test of Taylor’s hypothesis at the field scale. Through a wavelet decomposition of the lidar humidity
measurements we show that the scale of turbulent motions has a strong influence on the applicability of Taylor’s hypothesis.
This dependency on scale is explained through the use of dimensional analysis. We identify a ‘persistency scale’ that can
be used to quantify the effectiveness of Taylor’s hypothesis, and present the accuracy of the hypothesis as a function of
this non-dimensional length scale. These results are further investigated and verified through the use of large-eddy simulations. 相似文献
178.
179.
Hendrik Vogel Bernd Wagner Giovanni Zanchetta Roberto Sulpizio Peter Rosén 《Journal of Paleolimnology》2010,44(1):295-310
Lake Ohrid is probably of Pliocene age, and the oldest extant lake in Europe. In this study climatic and environmental changes
during the last glacial-interglacial cycle are reconstructed using lithological, sedimentological, geochemical and physical
proxy analysis of a 15-m-long sediment succession from Lake Ohrid. A chronological framework is derived from tephrochronology
and radiocarbon dating, which yields a basal age of ca. 136 ka. The succession is not continuous, however, with a hiatus between
ca. 97.6 and 81.7 ka. Sediment accumulation in course of the last climatic cycle is controlled by the complex interaction
of a variety of climate-controlled parameters and their impact on catchment dynamics, limnology, and hydrology of the lake.
Warm interglacial and cold glacial climate conditions can be clearly distinguished from organic matter, calcite, clastic detritus
and lithostratigraphic data. During interglacial periods, short-term fluctuations are recorded by abrupt variations in organic
matter and calcite content, indicating climatically-induced changes in lake productivity and hydrology. During glacial periods,
high variability in the contents of coarse silt to fine sand sized clastic matter is probably a function of climatically-induced
changes in catchment dynamics and wind activity. In some instances tephra layers provide potential stratigraphic markers for
short-lived climate perturbations. Given their widespread distribution in sites across the region, tephra analysis has the
potential to provide insight into variation in the impact of climate and environmental change across the Mediterranean. 相似文献
180.
Eleanor Donoghue Valentin R. Troll Chris Harris Aoife O'Halloran Thomas R. Walter Francisco J. Pérez Torrado 《Journal of Volcanology and Geothermal Research》2008
The Miocene Tejeda caldera on Gran Canaria erupted ~ 20 rhyolite–trachyte ignimbrites (Mogán Group 14–13.3 Ma), followed by ~ 20 phonolitic lava flows and ignimbrites (Fataga Group 13–8.5 Ma). Upper-Mogán tuffs have been severely altered immediately within the caldera margin, whereas extra-caldera Mogán ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid–rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar, plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid–rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The δ18O values of the altered intra-caldera tuffs are significantly higher than in unaltered extra-caldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extra-caldera ignimbrites have δD values between − 110‰ and − 173‰, which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The δD values of the altered intra-caldera tuffs range from − 52‰ to − 131‰, with ambient meteoric water at the alteration site estimated at ca. − 15‰. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock δD values of around − 45‰, given that ?Dclay–water is ca. − 30‰ at 100 °C, and decreases in magnitude at higher temperatures. All altered tuff samples have δD values that are substantially lower than − 45‰, indicating interaction with a meteoric water source with a δD value more negative than − 15‰, which may have been produced in low-temperature steam fumaroles. Supported by numerical modeling, our Gran Canaria data reflect the near-surface, epithermal part of a larger, fault-controlled hydrothermal system associated with the emplacement of the high-level Fataga magma chamber system. In this near-surface environment, fluid temperatures probably did not exceed 200–250 °C. 相似文献