New magnitude scales M L and spectrum-based M w for the area around Shanxi Rift System,North China     

Bin Li  Jens Havskov  Lars Ottemöller  Mathilde Bøttger Sørensen 《Journal of Seismology》2015,19(1):141-158

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Particle tracking in the vicinity of Helgoland,North Sea: a model comparison     

Ulrich?CalliesEmail author  Andreas?Plü?  Jens?Kappenberg  Hartmut?Kapitza 《Ocean Dynamics》2011,61(12):2121-2139

Station Helgoland Roads in the south-eastern North Sea (German Bight) hosts one of the richest long-term time series of marine observations. Hydrodynamic transport simulations can help understand variability in the local data brought about by intermittent changes of water masses. The objective of our study is to estimate to which extent the outcome of such transport simulations depends on the choice of a specific hydrodynamic model. Our basic experiment consists of 3,377 Lagrangian simulations in time-reversed mode initialized every 7 h within the period Feb 2002–Oct 2004. Fifty-day backward simulations were performed based on hourly current fields from four different hydrodynamic models that are all well established but differ with regard to spatial resolution, dimensionality (2D or 3D), the origin of atmospheric forcing data, treatment of boundary conditions, presence or absence of baroclinic terms, and the numerical scheme. The particle-tracking algorithm is 2D; fields from 3D models were averaged vertically. Drift simulations were evaluated quantitatively in terms of the fraction of released particles that crossed each cell of a network of receptor regions centred at the island of Helgoland. We found substantial systematic differences between drift simulations based on each of the four hydrodynamic models. Sensitivity studies with regard to spatial resolution and the effects of baroclinic processes suggest that differences in model output cannot unambiguously be assigned to certain model properties or restrictions. Therefore, multi-model simulations are needed for a proper identification of uncertainties in long-term Lagrangian drift simulations.  相似文献   

Comment on the “Joint determination of K decay constants and Ar/K for the Fish Canyon sanidine standard, and improved accuracy for Ar/Ar geochronology” by Paul R. Renne et al. (2010)     

Winfried H. Schwarz  Karsten Kossert  Jens Hopp 《Geochimica et cosmochimica acta》2011,75(17):5094-5100

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Age structure and recharge conditions of a coastal aquifer (northern Germany) investigated with 39Ar, 14C, 3H, He isotopes and Ne     

J��rgen S��ltenfu��  Roland Purtschert  Jens F. F��hrb?ter 《Hydrogeology Journal》2011,19(1):221-236

A tide-influenced two-layer aquifer system in northern Germany was investigated using environmental dating tracers (³H, ³⁹Ar, ¹⁴C), the noble gas isotopes ³He, ⁴He and Ne. The study area is a marshland at the River Ems estuary, exposed to regular flooding until?AD 1000. The construction of dykes, artificial land drainage and groundwater abstraction define the hydraulic gradient. The aquifer depicts a pronounced age stratification with depth. Tritium concentrations above 0.03 TU are found only in the top 30?m. Two tritium-free samples between 20 and 30?m depth show ³⁹Ar ages of 130 and 250?years. Below a clay layer?Cabout 50?m below surface level (mbsl)?Call analysed samples are ³⁹Ar free and, thus, older than 900?years. The initial ¹⁴C activities were about 70 pmC. Resulting ¹⁴C ages increase with depth and increase up to 9,000?years, in agreement with minimal ³⁹Ar ages. Concentrations of radiogenic ⁴He correlate with ¹⁴C ages. Samples in a mid-depth range (20?C70 mbsl) show significant gas loss. The gas loss is assigned to recharge in a methane producing environment. Deduced ⁴He ages were used to assign this water to a infiltration period of about AD 1000.  相似文献   

Deep-sea benthic foraminifera,carbonate dissolution and species diversity in Hardangerfjord,Norway: An initial assessment     

Elisabeth Alve  John W. Murray  Jens Skei 《Estuarine, Coastal and Shelf Science》2011

This is the first record of live (stained) deep-sea benthic foraminifera in the 850 m deep silled Hardangerfjord, the second deepest fjord in Western Norway. Estimates of organic carbon flux (∼2.5 g Cm⁻² y⁻¹) show that the fjord-values are comparable to similar depths on the continental slope. Accordingly, although these first samples only provide relative abundance data, the low proportion of live to dead individuals in the top cm of the sediment suggests a low foraminifera biomass. Another similarity with the deep sea is that the abiotic environment of the deep basins is stable even though the deepest basins are isolated from the open deep sea by the continental shelf and sills in the outer parts of the fjord suggesting that the deep-sea species are introduced as propagules during deep-water renewals. There is evidence of an increase in dissolution of fragile calcareous tests (e.g., Nonionella iridea) especially in the innermost part of Hardangerfjord since the 1960s and this has led to a relative increase in dead agglutinated assemblages. The presence of larger forms with tests >1 mm provides substrata for the attachment of smaller forms and therefore an increase in species diversity. Indeed, the diversity is comparable both to that of the open deep sea and that of reported macrofauna from the same sites, reflecting similar ecological status. Holtedahl (1965) suggested that there may be some down-slope transport of sediment into the deep basins with the deposition of turbidites. Despite some evidence of transport, no major recent disturbance due to turbidite deposition seems to have occurred and hence Hardangerfjord presents a unique environment with elements of deep-sea faunas in a land-locked setting.  相似文献   

Fine-scale population structure in a deep-sea teleost (orange roughy, Hoplostethus atlanticus)     

Jens Carlsson  Samuel Shephard  James CoughlanClive N. Trueman  Emer RoganTom F. Cross 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(6):627-636

Microsatellite and otolith chemistry variability were analysed to assess fine scale genetic structure in the deep-sea teleost orange roughy (Hoplostethus atlanticus). The Porcupine Bank located on the continental shelf west of Ireland, comprises a complex system of mounds and flat areas that are broken up by canyons. Orange roughy form spawning aggregations on mounds and flat areas, and were heavily fished until the resource was depleted. By analysing adults in spawning condition and juvenile orange roughy from six mounds and one flat area, shallow but significant genetic population structure was evident (F_ST=0.0031, D_est across loci=0.0306 and G-test). Most of the structure was accounted for by inclusion of a sample from the flats (six of ten significant pairwise F_ST estimates and G-tests, and five of the highest D_est estimates included the flat sample). While the flat sample contributed most to the genetic structure, there was still significant (albeit weaker) structure among mound samples. The observed structure was supported by otolith analyses. Fish caught as late juveniles in either the flat or mound areas showed consistent differences in chemistry at the otolith core throughout the initial 10 years of growth, which could indicate site fidelity. We hypothesise that seafloor topographic structures (mounds and flats) may provide discrete spawning areas for orange roughy and that the limited gene flow between these spawning areas is insufficient to counteract genetic drift.  相似文献   

Desertification in reverse? Observations from northern Burkina Faso   总被引：1，自引：0，他引：1  

Kjeld Rasmussen  Bjarne Fog  Jens E. Madsen 《Global Environmental Change》2001,11(4):1707

The idea of degradation of arid and semi-arid lands, often termed desertification in its irreversible form, due to human impact and/or climatic change has been much debated since the mid-1970s. From the time of the United Nation's Conference On Desertification in Nairobi, 1976, certain areas of northern Burkina Faso have been pointed out as examples of severe desertification. Several studies demonstrated that revitalization of a series of E–W oriented fossille dunes in the Oudalan province was ongoing. The present study includes an analysis of the trends of vegetation development in the region, covering the period 1955 to 1994, with emphasis on the fossile dunes. It is demonstrated that desertification and revitalization of dunes were phenomena associated with the period between the early 1970s and the mid-1980s, and that the decline in vegetation cover on the dunes seems to have been reversed in recent years. The analysis is based upon time series of aerial photos and satellite images, field studies of vegetation, interviews with local people and review of relevant literature. The findings are discussed with reference to the debate concerning desertification and land degradation, as well as to the current revisions of the ‘range management paradigm’. The observations indicate that the environmental history of the region is complex and cannot be boiled down to ‘human-induced irreversible degradation’. Rather they support the idea of semi-arid cultural landscapes undergoing constant change in response to both human impact and climatic trends and fluctuations.  相似文献   

Mineral paragenesis of the Kalahari managanese field, South Africa     

Jens Gutzmer  Nicolas J. Beukes 《Ore Geology Reviews》1996,11(6):405-428

The mineralogy of the managanese ores of the giant Paleoproterozoic Kalahari manganese deposit of the Transvaal Supergroup has been subject of many studies and up to now 135 different ore and gangue minerals have been described, of which eight represent new mineral species. Through correlation of different mineral assemblages with specific geological events and by determining relative ages of minerals in outcrop, hand specimen and under the microscope an attempt is made in this publication to construct a paragenetic sequence for this complex mineral assemblage. Sedimentation and early diagenesis of the Hotazel Formation, composed of interbedded iron formation and braunite lutite, was followed by low-grade metamorphism and associated stratabound metasomatism. Braunite lutite of sedimentary origin, comprising 97% of the total ore reserve, is composed of braunite, hematite and kutnahorite, and abundant early diagenetic kutnahorite and manganoan calcite forming laminae and ovoids. Fluid flow during late diagenesis or lower greenschist facies metamorphism led to stratabound metasomatic oxidation of Mn-bearing carbonates to hausmannite and Mn-poor calcite. Three structurally controlled hydrothermal alteration events succeed metamorphism. These events are referred to as Wessels, Mamatwan and Smartt events. The Wessels alteration event is the oldest of the three events and it is of great economic importance because virtually all of the high-grade ore (> 42% Mn), 3% of the total ore reserve, formed during this event through alteration of carbonate-rich low-grade Mamatwan-type ore (braunite lutite) to high-grade, carbonate-poor Wessels-type manganese ore. This Wessels hydrothermal alteration event took place in the northwestern part of the Kalahari deposit, associated with a system of major north-south- and minor east-west-striking normal faults. The Mamatwan alteration event is observed throughout the Kalahari manganese deposit but alteration is very localized. Reduction halos and discolouration of braunite lutite around fracture or joint-hosted sulphide-carbonate mineralization are typical of the Mamatwan alteration event. In contrast, the Smartt alteration event is characterized by oxidation of braunite lutite and the formation of todorokite and manganomelane. Syn- and Post-Kalahari supergene alteration has taken place below the suboutcrop of the Hotazel Formation against the calcretized sediments of the Cenozoic Kalahari Formation. Cryptomelane and pyrolusite are the predominant products of surficial weathering.  相似文献   

Post-drought increase in regional-scale groundwater nitrate in southwest Germany     

Karuna Jutglar  Jost Hellwig  Michael Stoelzle  Jens Lange 《水文研究》2021,35(8):e14307

Elevated nitrate concentrations in groundwater are a common challenge for water management. One important factor in this context is higher frequencies and intensities of wet-dry cycles that may cause increased nitrate concentrations in groundwater due to nitrate flushes after drought termination. Yet systematic studies on regional-scale impacts of droughts on groundwater nitrate concentrations are missing so far. Here we analyzed time series of 44 shallow groundwater wells and 41 springs all across the German Federal State Baden-Wuerttemberg from 2000 to 2018 to characterize patterns of post-drought nitrate increase in groundwater. In general, half of the exceptional nitrate concentrations, which exceeded the 80th percentile of long-term nitrate measurements, could be related to droughts in the research timeframe. The 2003 drought event stood out in terms of drought severity and post-drought nitrate concentration increases in our data. The great majority (91%) of all monitoring sites showed at least one exceptionally high nitrate concentration in the 4 years following the 2003 drought event. Springs were mainly located in forests of steep low mountain ranges and wells in cropland of flat river valleys. Therefore, delay times between drought intensity and nitrate concentration increases as well as magnitudes of nitrate concentration increase were diverse among wells and springs. We derived two distinct nitrate response patterns: (i) nitrate increases immediately following drought events (more common for springs and fractured rock aquifers) and (ii) delayed nitrate increases (more common for wells and porous aquifers). Springs generally showed quicker (median of 101 days) but weaker (median of +1.3 mg/L) post-drought nitrate increases than wells (185 days, +3.4 mg/L). Only few sites exhibited no post-drought nitrate increase and post-drought mean-nitrate concentrations of groundwater reservoirs were extraordinarily high in 2006. Overall, we demonstrate that post-drought nitrate increase in groundwater is omnipresent, while different landscapes and hydrogeological characteristics create a diverse regional pattern. As severe droughts become more frequent in a changing climate, post-drought nitrate increase may intensify problems regarding water quality and supply.  相似文献   

Finding Science with Science: Evaluating a Domain and Scientific Ontology User Interface for the Discovery of Scientific Resources     

Kristin Stock  Vera Karasova  Anne Robertson  Guillaume Roger  Mark Small  Mohamed Bishr  Jens Ortmann  Tim Stojanovic  Femke Reitsma  Lukasz Korczynski  Boyan Brodaric  Zoe Gardner 《Transactions in GIS》2013,17(4):612-639

Current approaches to the discovery of scientific resources (publications, data sets and web services) are dominated by keyword search. These approaches do not allow scientists to search on the deeper semantics of scientific resources, or to discover resources on the basis of the scientific approaches taken. This article evaluates a user interface that allows users to discover scientific resources through structured knowledge in the form of ontologies describing the domain and the scientific knowledge inherent within the scientific resource, and also through informal user tags. These combined capabilities provide scientists with new and powerful options for resource discovery. A qualitative user evaluation explored how scientists felt about the approach for resource discovery in the context of their scientific work. The study showed that marine scientists were enthusiastic about the capabilities of such an approach and appreciated the ability to browse the visual structure of the knowledge and query on scientific method but, overall, preferred the use of tags over ontologies. The exploratory nature of the user study was used to identify future directions for such improvements.  相似文献