Tracing the re-ionization-epoch intergalactic medium with metal absorption lines     

Benjamin D. Oppenheimer  Romeel Davé  Kristian Finlator 《Monthly notices of the Royal Astronomical Society》2009,396(2):729-758

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Modified water regime and salinity as a consequence of climate change: prospects for wetlands of Southern Australia     

Daryl L. Nielsen  Margaret A. Brock 《Climatic change》2009,95(3-4):523-533

The natural Australian landscape sustains a mosaic of wetlands that range from permanently wet to temporary. This diversity of wetland types and habitats provides for diverse biotic communities, many of which are specific to individual wetlands. This paper explores the prospects for southern Australian wetlands under modified water regime and salinity induced by climatic changes. Extended droughts predicted as a consequence of climate change (lower rainfall and higher temperatures) combined with human-induced changes to the natural hydrological regime will lead to reductions in the amount of water available for environmental and anthropogenic uses. Reduced runoff and river flows may cause the loss of some temporary wetland types that will no longer hold water long enough to support hydric communities. Species distributions will shift and species extinctions may result particularly across fragmented or vulnerable landscapes. Accumulation of salts in wetlands shift species-rich freshwater communities to species-poor salt tolerant communities. Wetlands will differ in ecological response to these changes as the salinity and drying history of each wetland will determine its resilience: in the short term some freshwater communities may recover but they are unlikely to survive and reproduce under long term increased salinity and altered hydrology. In the long term such salinized wetlands with altered hydrology will need to be colonized by salt tolerant species adapted for the new hydrological conditions if they are to persist as functional wetlands. As the landscape becomes more developed, to accommodate the need for water in a warmer drying climate, increasing human intervention will result in a net loss of wetlands and wetland diversity.  相似文献   

Internal architecture of a raised beach ridge system (Anholt,Denmark) resolved by ground-penetrating radar investigations     

Lars B. Clemmensen  Lars Nielsen 《Sedimentary Geology》2010,223(3-4):281-290

The internal architecture of raised beach ridge and associated swale deposits on Anholt records an ancient sea level. The Holocene beach ridges form part of a progradational beach ridge plain, which has been interpreted to have formed during an isostatic uplift and a relative fall in the sea level over the past 7700 years. The ridges are covered by pebbles and cobbles and commonly show evidence of deflation. Material presumably removed from the beach ridges and adjacent swales form the present dune forms on Anholt. Ground-penetrating radar (GPR) reflection lines have been collected with 250 MHz shielded antennae across the fossil ridge and swale structures. The signals penetrate the subsurface to a maximum depth of ～ 10 m below the fossil features. The GPR data resolve the internal architecture of the beach ridges and swales with a vertical resolution of about 0.1 m. GPR mapping indicates that the Holocene beach ridges are composed of seaward-dipping beachface deposits as well as minor amounts of inland dipping deposits of wash-over origin. The beachface deposits downlap on underlying shoreface deposits, and we use these surfaces as markers of a relative palaeo-sea level. The new data indicate that the highest relative sea level at about 8.5 m was reached 6500 years ago; 700 years later the relative sea level had dropped 0.7 m indicating a change in the relative sea level around 1 mm/year. This fall in the relative sea level most likely records the influence of an isostatic rebound causing younger beach ridge deposits to indicate lower sea levels.  相似文献   

Reconstruction of nonmonotonic electron density profiles of the Martian topside ionosphere     

H. Zou  E. Nielsen  X.-D. Wang 《Planetary and Space Science》2010,58(10):1391-1399

One of the problems in reconstructing the real ionosphere from an ionogram is the occurrence of a ‘valley,’ where electron density decreases with altitude and make a non-monotonic profile. For the case of the Earth ionosphere, the ordinary and extraordinary ray data, accompanied with an empirical model, based on the observations, are necessary to obtain a mathematical solution for a ‘valley,’ such as the region between the E and F layers. MARSIS/MEX is a topside sounder designed to observe the ionosphere of Mars. Some ‘valley’ structures were found in the ionograms measured by MARSIS. The echoes of the extraordinary ray are not available owing to the absence of the strong magnetic field on Mars. Therefore, it is difficult to have a mathematical solution for the valleys in the Martian ionosphere. In this paper, a least square method with a simple model is presented to solve the ‘valley’ problem in the topside ionosphere of Mars. The electron density profiles with ‘valleys’ observed by the Radio Occultation experiment onboard MGS are used to rebuild the virtual depths at MARSIS frequencies. The reconstructed electron density profile by the least square method with a simple model from the rebuilt virtual depth curve is compared with the original electron density profile. It is proved that this method can reproduce small valleys in the profile of the Martian ionosphere well.  相似文献   

Suspended Matter,Chl-a,CDOM, Grain Sizes,and Optical Properties in the Arctic Fjord-Type Estuary,Kangerlussuaq, West Greenland During Summer     

Lars Chresten Lund-Hansen  Thorbjørn Joest Andersen  Morten Holtegaard Nielsen  Morten Pejrup 《Estuaries and Coasts》2010,33(6):1442-1451

Optical constituents as suspended particulate matter (SPM), chlorophyll (Chl-a), colored dissolved organic matter (CDOM), and grain sizes were obtained on a transect in the arctic fjord-type estuary Kangerlussuaq (66°) in August 2007 along with optical properties. These comprised diffuse attenuation coefficient of downwelling PAR (K _d(PAR)), upwelling PAR (K _u(PAR)), particle beam attenuation coefficient (c _p), and irradiance reflectance R(−0, PAR). PAR is white light between 400 and 700 nm. The estuary receives melt water from the Greenland Inland Ice and stations covered a transect from the very high turbid melt water outlet to clear marine waters. Results showed a strong spatial variation with high values as for suspended matter concentrations, CDOM, diffuse attenuation coefficient K _d(PAR), particle beam attenuation coefficients (c _p), and reflectance R(−0, PAR) at the melt water outlet. Values of optical constituents and properties decreased with distance from the melt water outlet to a more or less constant level in central and outer part of the estuary. There was a strong correlation between inorganic suspended matter (SPMI) and diffuse attenuation coefficient K _d(PAR) (r ² = 0.92) and also for particle beam attenuation coefficient (c _p; r ² = 0.93). The obtained SPMI specific attenuation—K _d^*(PAR) = 0.13 m² g⁻¹ SPMI—and the SPMI specific particle beam attenuation—c _p^* = 0.72 m² g⁻¹—coefficients were about two times higher than average literature values. Irradiance reflectance R(−0, PAR) was comparatively high (0.09−0.20) and showed a high (r ² = 0.80) correlation with K _u(PAR). Scattering dominated relative to absorption—b(PAR)/a(PAR) = 12.3. Results strongly indicated that the high values in the optical properties were related to the very fine particle sizes (mean = 2–6 μm) of the suspended sediment. Data and results are discussed and compared to similar studies from both temperate and tropical estuaries.  相似文献   

Effects of projected climate change on energy supply and demand in the Pacific Northwest and Washington State     

Alan F. Hamlet  Se-Yeun Lee  Kristian E. B. Mickelson  Marketa M. Elsner 《Climatic change》2010,102(1-2):103-128

Climate strongly affects energy supply and demand in the Pacific Northwest (PNW) and Washington State (WA). We evaluate potential effects of climate change on the seasonality and annual amount of PNW hydropower production, and on heating and cooling energy demand. Changes in hydropower production are estimated by linking simulated streamflow scenarios produced by a hydrology model to a simulation model of the Columbia River hydro system. Changes in energy demand are assessed using gridded estimates of heating degree days (HDD) and cooling degree days (CDD) which are then combined with population projections to create energy demand indices that respond both to climate, future population, and changes in residential air conditioning market penetration. We find that substantial changes in the amount and seasonality of energy supply and demand in the PNW are likely to occur over the next century in response to warming, precipitation changes, and population growth. By the 2040s hydropower production is projected to increase by 4.7–5.0% in winter, decrease by about 12.1–15.4% in summer, with annual reductions of 2.0–3.4%. Larger decreases of 17.1–20.8% in summer hydropower production are projected for the 2080s. Although the combined effects of population growth and warming are projected to increase heating energy demand overall (22–23% for the 2020s, 35–42% for the 2040s, and 56–74% for the 2080s), warming results in reduced per capita heating demand. Residential cooling energy demand (currently less than one percent of residential demand) increases rapidly (both overall and per capita) to 4.8–9.1% of the total demand by the 2080s due to increasing population, cooling degree days, and air conditioning penetration.  相似文献   

Atlantic surface water inflow to the Nordic seas during the Pleistocene–Holocene transition (mid–late Younger Dryas and Pre‐Boreal periods, 12 450–10 000 a BP)     

Tine L. Rasmussen  Erik Thomsen  Tove Nielsen  Stefan Wastegård 《第四纪科学杂志》2011,26(7):723-733

The inflow of Atlantic Water to the Nordic seas from mid–late Younger Dryas to earliest Holocene (12 450–10 000 a BP) is reconstructed on the basis of a high‐resolution core (LINK14) from 346 m water depth on the east Faroe shelf. We have analysed the distribution of planktic and benthic foraminifera, stable isotopes and ice‐rafted debris (IRD), and calculated absolute temperatures and salinities by transfer functions. During the investigated time period there was almost continuous inflow of Atlantic Water to the Nordic seas. Deposition of IRD during the mid–late Younger Dryas and Pre‐Boreal coolings indicates the presence of melting icebergs and that summer sea surface temperatures were low. The east–west temperature gradient across the Faroe–Shetland Channel was much steeper than today. The cold conditions around the Faroe Islands are attributed to stronger East Greenland and East Icelandic currents than at present. The near‐continuous inflow of Atlantic Water is consistent with published evidence suggesting that deep convection took place in the Nordic seas, although the convection sites probably had shifted to a more easterly position than at present. Around the time of deposition of the Saksunarvatn Tephra c. 10 350 a BP, sea surface temperatures increased to the present level. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

ROBUST: The ROle of BUffering capacities in STabilising coastal lagoon ecosystems     

Rutger de Wit  Lucas J. Stal  Bente Aa. Lomstein  Rodney A. Herbert  Hans van Gemerden  Pierluigi Viaroli  Victor-Ugo Cecherelli  Francisco Rodríguez-Valera  Marco Bartoli  Gianmarco Giordani  Roberta Azzoni  Bart Schaub  David T. Welsh  Andrew Donnelly  Ana Cifuentes  Josefa Antn  Kai Finster  Lise B. Nielsen  Anne-Grethe Underlien Pedersen  Anne Turi Neubauer  Marina A. Colangelo  Sander K. Heijs 《Continental Shelf Research》2001,21(18-19)

“Buffer capacities” has been defined in ecology as a holistic concept (e.g., Integration of Ecosystem Theories: A Pattern, second ed. Kluwer, Dordrecht, 1997, 388pp), but we show that it can also be worked out in mechanistic studies. Our mechanistic approach highlights that “buffering capacities” can be depleted progressively, and, therefore, we make a distinction between current and potential “buffering capacities”. We have applied this concept to understand the limited “local stability” in seagrass ecosystems and their vulnerability towards structural changes into macro-algal dominated communities. We explored the following processes and studied how they confer buffering capacities to the seagrass ecosystem: (i) net autotrophy is persistent in Zostera noltii meadows where plant assimilation acts as a sink for nutrients, this contrasted with the Ulva system that shifted back and forth between net autotrophy and net heterotrophy; (ii) the Z. noltii ecosystem possesses a certain albeit rather limited capacity to modify the balance between nitrogen fixation and denitrification, i.e., it was found that in situ nitrogen fixation always exceeded denitrification; (iii) the nitrogen demand of organoheterotrophic bacteria in the sediment results in nitrogen retention of N in the sediment and hence a buffer against release of nitrogen compounds from sediments, (iv) habitat diversification in seagrass meadows provides shelter for meiofauna and hence buffering against adverse conditions, (v) sedimentary iron provides a buffer against noxious sulfide (note: bacterial sulfide production is enhanced in anoxic sediment niches by increased organic matter loading). On the other hand, in the coastal system we studied, sedimentary iron appears less important as a redox-coupled buffer system against phosphate loading. This is because most inorganic phosphate is bound to calcium rather than to iron. In addition, our studies have highlighted the importance of plant–microbe interactions in the seagrass meadows.  相似文献   

Melt inclusions in high-An plagioclase from the Gorda Ridge: an example of the local diversity of MORB parent magmas   总被引：2，自引：0，他引：2  

R. L. Nielsen  Jennifer Crum  Rene Bourgeois  Kaylea Hascall  Lance M. Forsythe  Martin R. Fisk  David M. Christie 《Contributions to Mineralogy and Petrology》1995,122(1-2):34-50

 The use of ocean floor basalt chemistry as a tool to investigate mantle composition and processes requires that we work with basalts that have been modified little since leaving the mantle. One source of such basalts is melt inclusions trapped in primitive crystals. However, obtaining information from these melt inclusions is complicated by the fact that melt inclusions in natural basalts are essentially always altered by post-entrapment crystallization. This requires that we develop techniques for reconstructing the original trapped liquid compositions. We conducted a series of experiments to reverse the effects of post-entrapment crystallization by re-heating the host crystals to their crystallization temperature. For these experiments we used plagioclase crystals separated from a single Gorda Ridge lava. The crystallization temperature for these crystals was determined by a set of incremental re-heating experiments to be ∼1240–1260° C. The inclusions are primitive, high Ca-Al basaltic melts, saturated with plagioclase, olivine and Al-rich chromite at low pressure. The inclusion analyses can be linked to the host lava composition by low pressure fractionation. The major element composition of the re-homogenized melt inclusions within each crystal is relatively constant. However, the incompatible element analyses have extremely wide ranges. The range of La/Sm and Ti/Zr from inclusions analyzed from a single sample from the Gorda Ridge exceeds the range reported for lavas sampled from the entire ridge. The pyroxene compositions predicted to be in equilibrium with the melt inclusion trace element signature cover much of the range represented by pyroxenes from abyssal peridotites. The volumetric proportions of the magmas entering the base of the crust can be evaluated using frequency distribution of melt inclusion compositions. This distribution suggests that the array of magmas was skewed towards the more depleted compositions, with little evidence for an enriched component in this system. This pattern is more consistent with a dynamic flow model of the mantle or of a passive flow model where the melts produced in the peripheral areas of the melting regime were not focused to the ridge. Received: 5 January 1995 / Accepted: 13 June 1995  相似文献   

Using the pseudospectral technique on curved grids for 2D acoustic forward modelling1     

Per Nielsen  Flemming If  Per Berg  Ove Skovgaard 《Geophysical Prospecting》1994,42(4):321-341

The Fourier pseudospectral method has been widely accepted for seismic forward modelling because of its high accuracy compared to other numerical techniques. Conventionally, the modelling is performed on Cartesian grids. This means that curved interfaces are represented in a ‘staircase fashion‘causing spurious diffractions. It is the aim of this work to eliminate these non-physical diffractions by using curved grids that generally follow the interfaces. A further advantage of using curved grids is that the local grid density can be adjusted according to the velocity of the individual layers, i.e. the overall grid density is not restricted by the lowest velocity in the subsurface. This means that considerable savings in computer storage can be obtained and thus larger computational models can be handled. One of the major problems in using the curved grid approach has been the generation of a suitable grid that fits all the interfaces. However, as a new approach, we adopt techniques originally developed for computational fluid dynamics (CFD) applications. This allows us to put the curved grid technique into a general framework, enabling the grid to follow all interfaces. In principle, a separate grid is generated for each geological layer, patching the grid lines across the interfaces to obtain a globally continuous grid (the so-called multiblock strategy). The curved grid is taken to constitute a generalised curvilinear coordinate system, where each grid line corresponds to a constant value of one of the curvilinear coordinates. That means that the forward modelling equations have to be written in curvilinear coordinates, resulting in additional terms in the equations. However, the subsurface geometry is much simpler in the curvilinear space. The advantages of the curved grid technique are demonstrated for the 2D acoustic wave equation. This includes a verification of the method against an analytic reference solution for wedge diffraction and a comparison with the pseudospectral method on Cartesian grids. The results demonstrate that high accuracies are obtained with few grid points and without extra computational costs as compared with Cartesian methods.  相似文献