Transition from arid to hyper-arid environment in the southern Levant deserts as recorded by early Pleistocene cummulic Aridisols     

Rivka Amit  Ori Simhai  Avner Ayalon  Yehouda Enzel  Ari Matmon  Onn Crouvi  Naomi Porat  Eric McDonald 《Quaternary Science Reviews》2011,30(3-4):312-323

The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ～2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ～1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (10⁵–10⁶ years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ¹⁸O and relatively high δ¹³C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ～1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 10⁶ yr.  相似文献   

Reverse currents in fast magnetic reconnection     

Mira Jardine  Eric R Priest 《地球物理与天体物理流体动力学》2013,107(1-2):163-168

Abstract

It is suggested that reverse currents seen in recent numerical reconnection experiments (Biskamp, 1986; Forbes and Priest, 1983) are caused by the choice of outflow boundary conditions. The specification of the normal velocity at the outflow boundary may result in a mismatch in velocity at the diffusion region which is manifested as a spike of reverse current.  相似文献   

Middle to early Late Ordovician hydrothermal veining in the Molong Volcanic Belt,northeastern Lachlan Fold Belt: Sedimentological evidence     

G. H. Packham  J. B. Keene  L. M. Barron 《Australian Journal of Earth Sciences》2013,60(2):257-269

Detrital volcanic and vein quartz, accompanied by felsic volcanic debris, occur as minor constituents in the Ordovician subduction‐related mafic volcanics of the Molong Volcanic Belt. In the western province of the Molong Volcanic Belt, detrital quartz is present in the three episodes of the mafic Volcanics. Volcanic quartz occurs in allochthonous limestone blocks in the Bendigonian Hensleigh Siltstone overlying the Mitchell Formation. The second volcanic episode (the Fairbridge Volcanics) commenced after a hiatus of approximately 20 million years and lasted around 10 million years from Darriwilian to Gisbornian time. Locally derived vein quartz, volcanic quartz and felsic detritus are concentrated at the bases of autochthonous Wahringa and Yuranigh Limestone Members of the volcanics and are extensive and abundant in basal beds of the regional Eastonian limestone body that transgressed over an eroded volcanic centre at Cargo. This early Eastonian debris, deposited early in an 8 million‐year volcanic hiatus preceding the final Ordovician Bolindian volcanism, establishes a pre‐Eastonian age for mineralisation at Cargo. It is inferred that the pauses in volcanism were preceded by magmatic fractionation, intrusion and hydrothermal activity and followed by erosion, subsidence and deposition of autochthonous limestones. Minor occurrences of vein and volcanic quartz are found in Bolindian volcanogenic sediments of the third volcanic phase. It is concluded that hydrothermal vein formation (and mineralisation by inference) was associated with pauses in volcanic activity throughout the Middle to early Late Ordovician over a wide area in the western province, culminating in the mineralisation at Cargo and Copper Hill near Molong. Volcanism in the eastern province of the Molong Volcanic Belt was continuous from at least Darriwilian to latest Ordovician time. Here, detrital hydrothermal vein quartz and volcanic quartz and felsic detritus are distributed through late Middle and early Late Ordovician turbidites of the Weemalla Formation. The possible existence of cycles in the source area like those of the Fairbridge Volcanics is masked by the distal nature of these deposits. Vein formation occurred in both provinces from late Middle Ordovician to early Late Ordovician, long before the formation of the world‐class mineral deposit at Cadia associated with the latest Ordovician Cadia Monzonite.  相似文献   

IGU Commission on Health and Developement     

Helen Roy  Eric Wajnberg 《GeoJournal》1989,18(4):451-451

The International Geographical Union Reports

IGU Commission on Population Geography  相似文献   

The French experience in low level radioactive waste management     

Raymonde Andre-Jehan  Eric Molinas 《Engineering Geology》1993,34(3-4):219-227

The Radioactive Waste Management Agency (ANDRA-FRANCE) is now operating a new facility in the eastern part of the Paris basin which is designed to dispose of one million cubic meters of waste.

The safety of the waste disposal is based on a multibarrier concept including waste packages, concrete disposal modules, site and closure operations.

Under normal conditions, confinement is guaranteed by the waste packages and the disposal modules, as they prevent the waste from being leached by rainfall or underground water over a certain period of time.

The site must bring an additional guarantee concerning the isolation of waste from water. Consequently, the chosen site must be located in an area where no natural disasters (landslides, earthquakes, etc.) can harm the isolating barriers. The geological, hydrogeological and chemical characteristics must allow us to minimize and control the transport of radionuclides within the ground. Finally, the chosen site must be in an area where it is easy to implement a system to monitor the environment.

A set of criteria guides the choice of site. The criteria include such factors as low seismicity, geotechnical stability, a hydrogeology that is simple to model, a location sufficiently above the water table and safe from the threat of flooding, good radionuclide sorption and the absence of any mineral or other natural resources of economic interest.

At the time of the closure of the disposal facility, the entire collection of modules will be covered by an impervious cap composed of clayey layers interbedded by sandy layers and overlain by humus to promote the growth of grass. The facility will then look like a succession of undulating green mounds.

A 300-year monitoring period will follow the closure. During this period, the water collecting networks and cap will be maintained and radioactivity in underground and stream water will be controlled.

We have selected the AUBE site as a case study to illustrate the French waste management experience. We will report on how the site characterization program has been calTied out, including the hydrogeological modelling which is being applied to both the operating and post-closure periods.  相似文献   

Geomorphological and historical changes on the Argentina delta,Ligurian coast,Italy     

Eric Bird  Paolo Fabbri 《GeoJournal》1993,29(4):428-429

A preliminary investigation of the geomorphological evolution of the small delta of the Argentina river (Riviera di Ponente, Italy) shows that it has largely been affected by anthropic processes. Erosion is undermining land and structural developments not only in the delta, but also along the nearby shores and a thorough knowledge of them seems necessary prior to management applications. In fact, processes affecting the evolution of the investigated area could well be regarded as a good example of the evolution of many other deltaic areas in the Northern Mediterranean.  相似文献   

Dynamics of epiphytic photoautotrophs and heterotrophs inZostera marina (eelgrass) microcosms: Responses to nutrient enrichment and grazing     

Hilary A. Neckles  Eric T. Koepfler  Leonard W. Haas  Richard L. Wetzel  Robert J. Orth 《Estuaries and Coasts》1994,17(3):597-605

The combined effects of nutrient enrichment and grazing by isopods and amphipods on abundances of seagrass epiphytes were tested inZostera marina L. (eelgrass) microcosms. Using epifluorescence microscopy, densities of epiphytic diatoms, cyanobacteria, heterotrophic flagellates, and heterotrophic bacteria were enumerated after 1 mo and 2 mo of treatment. In general, numbers of diatoms decreased, in the presence of grazers and showed little response to nutrient enrichment, whereas numbers of cyanobacteria increased with nutrient enrichment and showed little response to grazing. Thus, macrofaunal grazing maintained a photoautotrophic community domainated by cyanobacteria, particularly under nutrient enriched conditions. Following 2 mo of treatment, dense macroalgal growth under nutrient-enriched conditins with grazers absent appeared to limit populations of both epiphytic autotrophs. Patterns of abundance of heterotrophic bacteria suggested that the original bacteria population was nutrient limited. Bacteria populations may have been limited by organic carbon supplies at the end of the experiment. Abundances of heterotrophic flagellates and bacteria were strongly correlated on both sampling dates. Results suggest that heterotrophic flagellates might serve as a link between heterotrophic bacterial production and higher trophic levels in seagrass epiphyte food webs.  相似文献   

Rock-magnetic proxies of climate change in the loess-palaeosol sequences of the western Loess Plateau of China   总被引：5，自引：0，他引：5  

Christopher P. Hunt  Subir K. Banerjee  Jiamao Han  Peter A. Solheid  Eric Oches  Weiwei Sun  Tungsheng Liu 《Geophysical Journal International》1995,123(1):232-244

Results of the first detailed study of the climate proxy record in the loess-palaeosol sequence at Xining-one of the few palaeoclimate sites in the currently arid western Loess Plateau of China-illustrate the importance of making many types of rock-magnetic measurements other than susceptibility. A multiparameter approach yielded confirmation that here, as elsewhere in the Loess Plateau, the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite. Nevertheless, magnetic enhancement was caused not exclusively by changes in relative grain size, but also by variations in concentration and mineralogy of the magnetic fraction.
The effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence, respectively. the resulting signal was ascribed more confidently to variation in magnetic grain size, which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility. Variations in magnetic mineralogy were also determined to constrain interpretations further. the data were then used to discuss climate history at Xining. Finally, results from Xining were compared with other western sites and contrasted with eastern sites.
In summary: (1) data is presented from a new Loess Plateau site which also appears to yield a global climate signal; (2) a demonstration is made of a more rock-magnetically robust way to separate concentration, composition and grain-size controls on susceptibility and other magnetic parameters; and (3) models are provided for inter-regional comparisons of palaeoclimate proxy records.  相似文献   

Large-scale deformation associated with ridge subduction   总被引：1，自引：0，他引：1  

Eric L. Geist  Michael A. Fisher  David W. Scholl 《Geophysical Journal International》1993,115(2):344-366

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Variability of dissolved organic carbon in sediments of a seagrass bed and an unvegetated area within an estuary in Southern Texas     

Eric T. Koepfler  Ronald Benner  Paul A. Montagna 《Estuaries and Coasts》1993,16(3):391-404

Pore-water dissolved organic carbon (PWDOC) concentrations were examined in vegetated and bare sediments of aHalodule wrightii seagrass bed, and in a mud bottom sediment of a southern Texas estuary. Temporal variability was examined at diel (dawn and noon) and bimonthly time scales. Distribution patterns of PWDOC were compared with physical, chemical, and biological factors thought to exert control on PWDOC. Concentration of PWDOC, bacterial production, and resultant PWDOC turnover times displayed statistically significant spatial and temporal variability. Concentration of PWDOC ranged from 14 mg C 1^?1 to 107 mg C 1^?1 of pore water, or 9–71 μg C cm^?3 wet sediment. PWDOC was more variable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 μg C cm^?3) and high bacterial production rates (mean = 1.92 μg C cm^?3 h^?1) were observed at the mud station, whereas PWDOC concentrations were high (mean = 24.6 μg C cm^?3) and bacterial production rates were low (mean = 0.43 μg C cm^?3 h^?1) at the bare station. PWDOC turnover times (T_t), assuming 50% bacterial growth efficiency (1–840 h) were shortest at the mud station (mean=13 h) and longest at the bare station (mean=180 h). In the overlying water column, T_t values were longer, ranging from 1,000–10,000 h. PWDOC concentrations were 25% higher in vegetated sediments than in neighboring bare sediments. This difference was probably due to inputs of labile photosynthetic excretia, since bacterial production rates in vegetated sediments displayed significant diel variability and were 4 times greater than that of bare sediments. Based upon the entire data set, PWDOC was significantly related to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxygen metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that organism activities and detrital quality are the major determinants controlling variability in PWDOC.  相似文献