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921.
Nils Lenhardt Harald Böhnel Klaus Wemmer Ignacio S. Torres-Alvarado Jens Hornung Matthias Hinderer 《Bulletin of Volcanology》2010,72(7):817-832
The volcaniclastic Tepoztlán Formation (TF) represents an important rock record to unravel the early evolution of the Transmexican
Volcanic Belt (TMVB). Here, a depositional model together with a chronostratigraphy of this Formation is presented, based
on detailed field observations together with new geochronological, paleomagnetic, and petrological data. The TF consists predominantly
of deposits from pyroclastic density currents and extensive epiclastic products such as tuffaceous sandstones, conglomerates
and breccias, originating from fluvial and mass flow processes, respectively. Within these sediments fall deposits and lavas
are sparsely intercalated. The clastic material is almost exclusively of volcanic origin, ranging in composition from andesite
to rhyolite. Thick gravity-driven deposits and large-scale alluvial fan environments document the buildup of steep volcanic
edifices. K-Ar and Ar-Ar dates, in addition to eight magnetostratigraphic sections and lithological correlations served to
construct a chronostratigraphy for the entire Tepoztlán Formation. Correlation of the 577 m composite magnetostratigraphic
section with the Cande and Kent (1995) Geomagnetic Polarity Time Scale (GPTS) suggests that this section represents the time
intervall 22.8–18.8 Ma (6Bn.1n-5Er; Aquitanian-Burdigalian, Lower Miocene). This correlation implies a deposition of the TF
predating the extensive effusive activity in the TMVB at 12 Ma and is therefore interpreted to represent its initial phase
with predominantly explosive activity. Additionally, three subdivisions of the TF were established, according to the dominant
mode of deposition: (1) the fluvial dominated Malinalco Member (22.8–22.2 Ma), (2) the volcanic dominated San Andrés Member
(22.2–21.3 Ma) and (3) the mass flow dominated Tepozteco Member (21.3–18.8 Ma). 相似文献
922.
A multi‐method field experiment to determine local groundwater flow in a glacier forefield
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Florian Kobierska Tobias Jonas Nena Griessinger Christian Hauck Stephan Huxol Stefano M. Bernasconi 《水文研究》2015,29(6):817-827
We implemented multiple independent field techniques to determine the direction and velocity of groundwater flow at a specific stream reach in a glacier forefield. Time‐lapse experiments were conducted using two electrical resistivity tomography (ERT) lines installed in a cross pattern. A circular array of groundwater tubes was also installed to monitor groundwater flow via discrete salt injections. Both inter‐borehole and ERT results confirmed this stream section as a losing reach and enabled quantification of the flow direction. Both techniques yielded advection velocities varying between 5.7 and 21.8 m/day. Estimates of groundwater flow direction and velocity indicated that groundwater infiltrates from the stream nearby and not from the adjacent lateral moraine. Groundwater age estimated from radon concentration measurements supported this hypothesis. Despite uncertainties inherent to each of the methods deployed, the combination of multiple field techniques allowed drawing consistent conclusions about local groundwater flow. We thus regard our multi‐method approach as a reliable way to characterize the two‐dimensional groundwater flow at sites where more invasive groundwater investigation techniques are difficult to carry out and local heterogeneities can make single measurements unreliable. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
923.
We performed numerical simulations of groundwater flow and radionuclide transport to study the influence of several factors, including the ambient hydraulic gradient, groundwater pressure anomalies, and the properties of the excavation damaged zone (EDZ), on the prevailing transport mechanism (i.e., advection or molecular diffusion) in a generic nuclear waste repository within a clay‐rich geological formation. By comparing simulation results, we show that the EDZ plays a major role as a preferential flowpath for radionuclide transport. When the EDZ is not taken into account, transport is dominated by molecular diffusion in almost the totality of the simulated domain, and transport velocity is about 40% slower. Modeling results also show that a reduction in hydraulic gradient leads to a greater predominance of diffusive transport, slowing down radionuclide transport by about 30% with respect to a scenario assuming a unit gradient. In addition, inward flow caused by negative pressure anomalies in the clay‐rich formation further reduces transport velocity, enhancing the ability of the geological barrier to contain the radioactive waste. On the other hand, local high gradients associated with positive pressure anomalies can speed up radionuclide transport with respect to steady‐state flow systems having the same regional hydraulic gradients. Transport behavior was also found to be sensitive to both geometrical and hydrogeological parameters of the EDZ. Results from this work can provide useful knowledge toward correctly assessing the post‐closure safety of a geological disposal system. 相似文献
924.
Christian Saad Salaheddine El Adlouni André St-Hilaire Philippe Gachon 《Stochastic Environmental Research and Risk Assessment (SERRA)》2015,29(1):275-294
Floods have potentially devastating consequences on populations, industries and environmental systems. They often result from a combination of effects from meteorological, physiographic and anthropogenic natures. The analysis of flood hazards under a multivariate perspective is primordial to evaluate several of the combined factors. This study analyzes spring flood-causing mechanisms in terms of the occurrence, frequency, duration and intensity of precipitation as well as temperature events and their combinations previous to and during floods using frequency analysis as well as a proposed multivariate copula approach along with hydrometeorological indices. This research was initiated over the Richelieu River watershed (Quebec, Canada), with a particular emphasis on the 2011 spring flood, constituting one of the most damaging events over the last century for this region. Although some work has already been conducted to determine certain causes of this record flood, the use of multivariate statistical analysis of hydrologic and meteorological events has not yet been explored. This study proposes a multivariate flood risk model based on fully nested Archimedean Frank and Clayton copulas in a hydrometeorological context. Several combinations of the 2011 Richelieu River flood-causing meteorological factors are determined by estimating joint and conditional return periods with the application of the proposed model in a trivariate case. The effects of the frequency of daily frost/thaw episodes in winter, the cumulative total precipitation fallen between the months of November and March and the 90th percentile of rainfall in spring on peak flow and flood duration are quantified, as these combined factors represent relevant drivers of this 2011 Richelieu River record flood. Multiple plausible and physically founded flood-causing scenarios are also analyzed to quantify various risks of inundation. 相似文献
925.
926.
The POM-DOM piezophilic microorganism continuum(PDPMC)—The role of piezophilic microorganisms in the global ocean carbon cycle 总被引:1,自引:0,他引:1
FANG JiaSong ZHANG Li LI Jiang Tao Chiaki KATO Christian TAMBURINI ZHANG YuZhong DANG HongYue WANG GuangYi WANG FengPing 《中国科学:地球科学(英文版)》2015,(1):106-115
The deep ocean piezosphere accounts for a significant part of the global ocean,hosts active and diverse microbial communities which probably play a more important role than hitherto recognized in the global ocean carbon cycle.The conventional biological pump concept and the recently proposed microbial carbon pump mechanism provide a foundation for our understanding of the role of microorganisms in cycling of carbon in the ocean.However,there are significant gaps in our knowledge and a lack of mechanistic understanding of the processes of microbially-mediated production,transformation,degradation,and export of marine dissolved and particulate organic matter(DOM and POM)in the deep ocean and the ecological consequence.Here we propose the POM-DOM piezophilic microorganism continuum(PDPMC)conceptual model,to address these important biogeochemical processes in the deep ocean.We propose that piezophilic microorganisms(bacteria and archaea)play a pivotal role in deep ocean carbon cycle where microbial production of exoenzymes,enzymatic breakdown of DOM and transformation of POM fuels the rapid cycling of marine organic matter,and serve as the primary driver for carbon cycle in the deep ocean. 相似文献
927.
Tristan Garcia Gilbert Féraud Christophe Falguères Henry de Lumley Christian Perrenoud David Lordkipanidze 《Quaternary Geochronology》2010,5(4):443-451
Several hominid remains have been discovered in the open-air site of Dmanisi (Georgia), the oldest prehistoric site in Eurasia. Two major arguments prove that this site is close in age to the Plio-Pleistocene boundary: a Villafranchian fauna and the morphological characteristics of hominid remains recently ascribed to Homo georgicus. Direct dating of the lower hominid-bearing level was carried out on volcanic glass and minerals using the 40Ar/39Ar method. The concordant results from two different sampled locations allow the determination of the age of the earliest human presence in Eurasia. This radioisotopic result strengthens the argument that the first dispersal of hominids outside Africa occurred at least 1.8 Ma ago. 相似文献
928.
Stephanie Kastner Christian Ohlendorf Torsten Haberzettl Andreas Lücke Christoph Mayr Nora I. Maidana Frank Schäbitz Bernd Zolitschka 《Journal of Paleolimnology》2010,44(4):887-902
We studied the internal lake processes that control the spatial distribution and characteristics of modern sediments at the
ICDP (International Continental Scientific Drilling Program) deep drilling site in Laguna Potrok Aike, southern Patagonia,
Argentina. Sediment distribution patterns were investigated using a dense grid of 63 gravity cores taken throughout the lake
basin and 40 additional shoreline samples. Analysis of the surficial sediment distribution points to distinct internal depositional
dynamics induced by wind-driven lake internal currents. Distribution maps illustrate that the spatial characteristics of analysed
variables are linked to high erosional wave activity. Persistent wave action and littoral erosion along all shores, especially
the eastern shore, is caused by prevailing Southern Hemispheric Westerlies. Several sediment variables (grain size, benthic
diatoms, total inorganic carbon and calcium) indicate re-suspension of littoral sediment followed by re-distribution to profundal
accumulation areas near the eastern shore. Variations within the catchment influence sediment characteristics in the north-eastern
bay. That area is characterized by different mineralogical and sedimentological conditions as well as greater accumulation
of pollen, inorganic carbon and diatoms. These findings are related to the influence of episodic inflow into this bay. Spatial
differences in stable isotope values throughout the lake suggest that ephemeral tributaries around the lake basin may also
contribute to the detected spatial sediment variations. 相似文献
929.
Snow cover strongly influences plant growth in Arctic and alpine ecosystems. Snow characteristics and snowmelt timing are likely to change in a warmer climate. We studied year rings and shoot growth of the dwarf shrub bilberry ( Vaccinium myrtillus ), and species abundances of the vegetation, in response to early or late snowmelt at a study site in the Central Alps, near Davos, Switzerland. Snowmelt was manipulated on experimental plots for 3 and 30 years. Additional plots were set up along a natural snowmelt gradient, and at high and low elevation. Growth ring data showed an increasing trend in annual growth increment over the last 20 years, especially in the extraordinarily hot summer of 2003. Comparing high and low elevation sites, growth rings were wider at low elevation, but only in cold years. In years with relatively cold summers, however, xylem ring width was greater in plots with late rather than early snowmelt along the natural snowmelt gradient, possibly indicating drought stress in early snowmelt plots. Snow cover had a strong influence on species abundances along the natural snowmelt gradient, and change (not yet significant) was beginning to be seen in plots with 30 years of snow manipulation. Our results indicate that beneficial effects of early snowmelt for shrub growth may be offset in cold summers. Although early snowmelt prolongs the growing season, harsh conditions and frost events early in the growing season may become more likely, and hamper plant growth, and this could affect plant growth in all Arctic and alpine snow-dominated ecosystems. 相似文献
930.
Jonny Rutqvist Deborah Barr Jens T. Birkholzer Kiyoshi Fujisaki Olaf Kolditz Quan-Sheng Liu Tomoo Fujita Wenqing Wang Cheng-Yuan Zhang 《Environmental Geology》2009,57(6):1347-1360
This paper presents an international, multiple-code, simulation study of coupled thermal, hydrological, and mechanical (THM)
processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement
drifts. Simulations were conducted considering two types of repository settings (1) open emplacement drifts in relatively
shallow unsaturated volcanic rock, and (2) backfilled emplacement drifts in deeper saturated crystalline rock. The results
showed that for the two assumed repository settings, the dominant mechanism of changes in rock permeability was thermal–mechanically
induced closure (reduced aperture) of vertical fractures, caused by thermal stress resulting from repository-wide heating
of the rock mass. The magnitude of thermal–mechanically induced changes in permeability was more substantial in the case of
an emplacement drift located in a relatively shallow, low-stress environment where the rock is more compliant, allowing more
substantial fracture closure during thermal stressing. However, in both of the assumed repository settings in this study,
the thermal–mechanically induced changes in permeability caused relatively small changes in the flow field, with most changes
occurring in the vicinity of the emplacement drifts. 相似文献