Gully cut‐and‐fill cycles as related to agro‐management: a historical curve number simulation in the Tigray Highlands          下载免费PDF全文

Sil Lanckriet  Amaury Frankl  Gebrekidan Mesfin  Katrien Descheemaeker  Jan Nyssen 《地球表面变化过程与地形》2015,40(6):796-808

Gully cut‐and‐fill dynamics are often thought to be driven by climate and/or deforestation related to population pressure. However, in this case‐study of nine representative catchments in the Northern Ethiopian Highlands, we find that neither climate changes nor deforestation can explain gully morphology changes over the twentieth century. Firstly, by using a Monte Carlo simulation to estimate historical catchment‐wide curve numbers, we show that the landscape was already heavily degraded in the nineteenth and early twentieth century – a period with low population density. The mean catchment‐wide curve number (> 80) one century ago was, under the regional climatic conditions, already resulting in considerable simulated historical runoff responses. Secondly, twentieth century land‐cover and runoff coefficient changes were confronted with twentieth century changing gully morphologies. As the results show, large‐scale land‐cover changes and deforestation cannot explain the observed processes. The study therefore invokes interactions between authigenic factors, small‐scale plot boundary changes, cropland management and sociopolitical forces to explain the gully cut processes. Finally, semi‐structured interviews and sedistratigraphic analysis of three filled gullies confirm the dominant impact of (crop)land management (tillage, check dams in gullies and channel diversions) on gully cut‐and‐fill processes. Since agricultural land management – including land tenure and land distribution – has been commonly neglected in earlier related research, we argue therefore that it can be a very strong driver of twentieth century gully morphodynamics. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

High concentrations of amorphous,biogenic Si (BSi) in the sediment of a small high-latitude lake: implications for biogeochemical Si cycling and for the use of BSi as a paleoproxy     

Petra Tallberg  Sophie Opfergelt  Jean-Thomas Cornelis  Anne Liljendahl  Jan Weckström 《Aquatic Sciences - Research Across Boundaries》2015,77(2):293-305

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Geoarchaeological prospection of a medieval manor in the dutch polders using an electromagnetic induction sensor in combination with soil augerings     

David Simpson  Alexander Lehouck  Marc Van Meirvenne  Jean Bourgeois  Erik Thoen  Jelier Vervloet 《Geoarchaeology》2008,23(2):305-319

In archaeological prospection, geophysical sensors are increasingly being used to locate buried remains within their natural context. To cover a large area in sufficient detail, an electromagnetic induction sensor can be very useful, measuring simultaneously the electrical conductivity and the magnetic susceptibility of the soil (e.g., Geonics EM38DD). In this study, an 8 ha field containing a Medieval manor was mapped in a submeter resolution, using a mobile sensor configuration equipped with a GPS. As different soil features can yield analogous responses, the interpretation of geophysical maps can be ambiguous. Therefore, soil auger observations were laid out along two perpendicular transects to provide vertical profiles across the sensor measurements. This information greatly enhanced the interpretation of the anomalies obtained by the sensor. Both natural and anthropogenic features were delineated, which clearly presented a moated site along a former tidal channel. © 2008 Wiley Periodicals, Inc.  相似文献   

Application of Multiple Point Geostatistics to Non-stationary Images   总被引：5，自引：2，他引：3  

Luis Manuel de Vries  Jesus Carrera  Oriol Falivene  Oscar Gratacós  Luit Jan Slooten 《Mathematical Geosciences》2009,41(1):29-42

Simulation of flow and solute transport through aquifers or oil reservoirs requires a precise representation of subsurface heterogeneity that can be achieved by stochastic simulation approaches. Traditional geostatistical methods based on variograms, such as truncated Gaussian simulation or sequential indicator simulation, may fail to generate the complex, curvilinear, continuous and interconnected facies distributions that are often encountered in real geological media, due to their reliance on two-point statistics. Multiple Point Geostatistics (MPG) overcomes this constraint by using more complex point configurations whose statistics are retrieved from training images. Obtaining representative statistics requires stationary training images, but geological understanding often suggests a priori facies variability patterns. This research aims at extending MPG to non-stationary facies distributions. The proposed method subdivides the training images into different areas. The statistics for each area are stored in separate frequency search trees. Several training images are used to ensure that the obtained statistics are representative. The facies probability distribution for each cell during simulation is calculated by weighting the probabilities from the frequency trees. The method is tested on two different object-based training image sets. Results show that non-stationary training images can be used to generate suitable non-stationary facies distributions.  相似文献   

Three-dimensional regional-scale hydrostratigraphic modeling based on sequence stratigraphic methods: a case study of the Miocene succession in Denmark   总被引：1，自引：0，他引：1  

Peter B. Scharling  Erik S. Rasmussen  Torben O. Sonnenborg  Peter Engesgaard  Klaus Hinsby 《Hydrogeology Journal》2009,17(8):1913-1933

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Detrital Cr-spinel in the Šambron–Kamenica Zone (Slovakia): evidence for an ocean-spreading zone in the Northern Vardar suture?     

Davide Lenaz  Claudio Mazzoli  Jan Spišiak  Francesco Princivalle  Lara Maritan 《International Journal of Earth Sciences》2009,98(2):345-355

The Šambron–Kamenica Zone is situated on the northern margin of the Levočské vrchy mountains and Šarišskà vrchovina Highland, where the Central Carpathian Paleogene joins the Pieniny Klippen Belt. Sandstone outcrops in this area. From Cretaceous to Late Oligocene in age, these sediments suggest transport directions from S and SE. The heavy mineral assemblages of this sandstone include Cr-spinel grains, mainly displaying types II and III alpine-peridotite affinities, and are representative of Ocean Island Basalt volcanism. A sample from Upper Eocene sediments at Vit’az shows a clear change in Cr-spinel composition, which turns out to have types I and II peridotite affinities, and to derive from arc and Middle Ocean Ridge Basalt volcanism, with sediment transport directions from SW and WSW. These data indicate major variations in the Upper Eocene tectonic setting, giving constraints to paleogeographic reconstruction of the Slovak Central Carpathians.  相似文献   

Accuracy of Laser Ablation U-Pb Zircon Dating: Results from a Test Using Five Different Reference Zircons   总被引：2，自引：0，他引：2  

Urs Klötzli  Eva Klötzli  Zekeriya Günes  Jan Kosler 《Geostandards and Geoanalytical Research》2009,33(1):5-15

Using a state‐of‐the‐art 193 nm‐LA‐MC‐ICP‐MS system and with careful control of analytical procedures, the long term external reproducibility and accuracy of the ages Phanerozoic zircons measured over a period of months using calibrator bracketing for the ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb ages were ca. 1% (2 RSD) if a single reference zircon was used for the matrix‐matched calibration. When different reference zircons were used for the calibration, suspicious systematic shifts in the obtained ages were observed and thus a reduction in the overall accuracy of the dating method became obvious. Such shifts were within a few percent range of the U‐Pb and Pb/Pb ages and seemed to vary independently of zircon age and composition. A “test of accuracy” experiment was conducted reducing instrumental effects as far as possible by analysing five different reference zircons mounted on a single mount eight times during the same session. An identical protocol was used for all analyses, with unchanged instrument parameters and with ion beam intensities kept as identical as possible. For data reduction, every zircon served consecutively as the reference zircon for calibration, with the others in the batch treated as unknowns. The known reference age and the four calculated ages obtained using the four other RMs for calibration were then compared. Even using such a strict analytical protocol, shifts in ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U and ²⁰⁷Pb/²⁰⁶Pb ratios were still present. They varied non‐systematically and ranged from ?4.35% to 3.08% for the investigated age range (1065 Ma to 226 Ma). Assuming the absence of instrumental effects (i.e., memory, dead‐time correction, non‐linearity of ion counters and interdetector calibration, crystallographic orientation, ablation cell geometry and setup, gas flows), the observed shifts were attributed to matrix and/or ablation related effects. It is proposed that non‐spectral matrix effects in the Ar plasma torch resulted in non‐uniform signal enhancement (or depression?) leading to shifts both in elemental and Pb isotopic ratios. Additionally, the ablated particle size distribution could be an important factor controlling plasma conditions and thus mass bias and fractionation. Until such effects are well understood and controlled, it would seem that any LA‐ICP‐MS zircon U‐Pb and ²⁰⁷Pb/²⁰⁶Pb age determination cannot be meaningfully interpreted at below a ca. 3% to 4% (2 RSD) confidence level.  相似文献   

Biomarker lipids of the freshwater fern Azolla and its fossil counterpart from the Eocene Arctic Ocean     

Eveline N. Speelman  Gert-Jan Reichart  Jan W. de Leeuw  W. Irene C. Rijpstra  Jaap S. Sinninghe Damsté 《Organic Geochemistry》2009,40(5):628-637

Eocene sediments recovered from the Lomonosov Ridge in the central Arctic Ocean during Integrated Ocean Drilling Program Expedition 302 contain high amounts of fossil remains of the free floating freshwater fern Azolla. Both extant Azolla and the sediments from the Arctic Azolla interval were found to contain relatively high quantities of compounds identified as 1,ω20 C₃₀–C₃₆ diols. Furthermore, structurally related mid-chain hydroxy fatty acids, long chain n-alkanols and keto-ols were discovered. The different series have a common feature: there is always a hydroxy group at the ω20 position. In addition, structurally related C₂₉ ω20,ω21 diols, C₂₉ 1,20,21 triols, C₂₉ dihydroxy fatty acids as well as a series of wax esters containing these mono- and dihydroxy lipids are reported. Selective extraction of Azolla surface lipids revealed that these compounds are most likely present in the leaf waxes of the fern. The suite of long-chain, mid-chain ω20 hydroxy wax constituents is described for the first time. As they are well preserved in the Eocene sediments, these lipids may serve as palaeo-environmental indicators of the Arctic Azolla interval and as markers for the past occurrence of Azolla in general.  相似文献   

Lower Cretaceous Mesoeucrocodylians from Scandinavia (Denmark and Sweden)     

Daniela Schwarz-Wings  Jan Rees  Johan Lindgren 《Cretaceous Research》2009,30(5):1345-1355

The crocodyliform faunas of the lowermost Cretaceous Rabekke and Jydegård Formations on the Baltic island of Bornholm, Denmark, and the Annero Formation of Skåne, southernmost Sweden, are represented by isolated teeth, osteoderms, and vertebrae. The rich Berriasian assemblage of the Rabekke Formation includes at least three distinctive taxa: Bernissartia sp., Theriosuchus sp., and Goniopholis sp., an association that is also known from several other contemporaneous European vertebrate localities. In contrast to this fauna, the Jydegård and Annero Formations have yielded only rare mesoeucrocodylian remains, which are assigned to Theriosuchus sp. and an undetermined mesoeucrocodylian taxon, possibly Pholidosaurus. Geographically, the Scandinavian localities represent the easternmost and northernmost distribution of typical continental Jurassic-Cretaceous crocodyliform communities in Europe.  相似文献   

Copper and sewage inputs from recreational vessels at popular anchor sites in a semi-enclosed Bay (Qld, Australia): estimates of potential annual loads     

Leon LM  Warnken J 《Marine pollution bulletin》2008,57(6-12):838-845

Environmental impacts of vessels are well documented; Cu pollution as result of Cu based antifouling paints and nutrient pollution (such as N) from marine sewage are two examples of such disturbances. Understanding environmental impacts as well as the use of coastal waterways by recreational vessels is of concern to regulatory authorities, waterway users and local residents. In this study more than 55 aerial surveys were conducted of selected popular anchorages in eastern Moreton Bay, Queensland, Australia. Numbers of recreational vessels at certain times during the year were used in multiple linear regression analyses to develop predictive models for recreational vessel numbers. Over one year approximately 10,000 locally registered recreational craft (>6m length overall) generated an estimated 59,000 vessel nights. With Cu leaching rates from the literature, and estimates of sewage inputs (assuming little or no use of pump-out facilities), load estimates associated with overnight use of 20 popular anchor sites were calculated as 141+/-46 kg of Cu and 1.17+/-0.38 t of nitrogen (N) annually. More importantly, the models showed vessel activity to be highly variable, and focused at peak holiday times, with 14% of vessel activity and associated pollutant loads entering the environment during Christmas and Easter. This study highlighted the inherent difficulties in managing a popular maritime amenity and Marine Parks such as the Moreton Bay Marine Protected Area, Queensland, Australia with its variety of stakeholders and types and intensities of uses.  相似文献