Investigating the response of Cladocera to a major saltwater intrusion event in an Arctic lake from the outer Mackenzie Delta (NT, Canada)     

Kayla Deasley  Jennifer B. Korosi  Joshua R. Thienpont  Steven V. Kokelj  Michael F. J. Pisaric  John P. Smol 《Journal of Paleolimnology》2012,48(2):287-296

An increase in the frequency and intensity of marine storm surges is a predicted consequence of climate warming, and therefore it is important to better understand the biological responses to such events in coastal regions. In late September 1999, a major storm surge resulted in a saltwater intrusion event over a large area of the Mackenzie Delta (NT, Canada) front, causing rapid salinization of lakes on the alluvial plain. Due to a lack of long-term ecological monitoring data in the region, the impacts that the saltwater intrusion event had on the biota of affected lakes were unknown. We used high-resolution paleolimnological approaches to reconstruct past assemblage changes in Cladocera from impacted Lake DZO-29 (unofficial name) in order to determine how different cladoceran species responded to a major increase in lake salinity following the 1999 storm surge. Camptocercus were extirpated from the lake following the saltwater intrusion and have not recovered. We also observed an initial decrease in Alona relative abundance following the marine flooding, likely reflecting a loss of A. quadrangularis, A. barbulata, and A. costata from the lake. A. circumfimbriata, Chydorus biovatus, C. brevilabris, and Bosmina spp. were abundant both before and after the saltwater intrusion, and Paralona pigra was present following the storm surge, but not prior to it. The most notable shift in Cladocera in the recent sedimentary record, however, occurred much earlier, with an increase in pelagic Bosmina taxa and a subsequent decrease in the benthic/littoral taxa Chydorus and Camptocercus, an assemblage shift that is consistent with a response to climate warming in this region, and strongly correlated to other changes in the lake inferred to be as a result of regional warming.  相似文献   

Mineralogical characterization of Mars Science Laboratory candidate landing sites from THEMIS and TES data     

A. Deanne Rogers  Joshua L. Bandfield 《Icarus》2009,203(2):437-453

Data from the Mars Global Surveyor Thermal Emission Spectrometer (TES) and the Mars Odyssey Thermal Emission Imaging System (THEMIS) instruments are used to assess the mineralogic and dust cover characteristics of landing regions proposed for the Mars Science Laboratory (MSL) mission. Candidate regions examined in this study are Eberswalde crater, Gale crater, Holden crater, Mawrth Vallis, Miyamoto crater, Nili Fossae Trough, and south Meridiani Planum. Compositional units identified in each region from TES and THEMIS data are distinguished by variations in hematite, olivine, pyroxene and high-silica phase abundance, whereas no units are distinguished by elevated phyllosilicate or sulfate abundance. Though phyllosilicate minerals have been identified in all sites using near-infrared observations, these minerals are not unambiguously detected using either TES spectral index or deconvolution analysis methods. For some of the sites, small phyllosilicate outcrop sizes relative to the TES field of view likely hinder phyllosilicate mineral detection. Porous texture and/or small particle size (<∼60 μm) associated with the phyllosilicate-bearing surfaces may also contribute to non-detections in the thermal infrared data sets, in some areas. However, in Mawrth Vallis and Nili Fossae, low phyllosilicate abundance (<10-20 areal %, depending on the phyllosilicate composition) is the most likely explanation for non-detection. TES data over Mawrth Vallis indicate that phyllosilicate-bearing surfaces also contain significant concentrations (>15%, possibly up to ∼40%) of a high-silica phase such as amorphous silica or zeolite. High-silica phase abundance over phyllosilicate-bearing surfaces in Mawrth Vallis is higher than that of surrounding surfaces by 10-15%. With the exception of these high-silica surfaces in Mawrth Vallis, regions examined in this study exhibit similar bulk mineralogical compositions to that of most low-albedo regions on Mars; the MSL scientific payload will thus be able to provide important information on surface materials typical of low-albedo regions in addition to investigating the origin of phyllosilicate and/or sulfate deposits. With the exception of Gale crater, all of the landing sites have relatively low dust cover compared to classic high-albedo regions (Tharsis, Arabia and Elysium) and to previous landing sites in Gusev Crater, Utopia Planitia, and Chryse Planitia.  相似文献   

Evidence of correlations between nuclear decay rates and Earth–Sun distance     

Jere H. Jenkins  Ephraim Fischbach  John B. Buncher  John T. Gruenwald  Dennis E. Krause  Joshua J. Mattes 《Astroparticle Physics》2009,32(1):42-46

Unexplained periodic fluctuations in the decay rates of ³²Si and ²²⁶Ra have been reported by groups at Brookhaven National Laboratory (³²Si), and at the Physikalisch–Technische–Bundesanstalt in Germany (²²⁶Ra). We show from an analysis of the raw data in these experiments that the observed fluctuations are strongly correlated in time, not only with each other, but also with the time of year. We discuss both the possibility that these correlations arise from seasonal influences on the detection system, as well as the suggestion of an annual modulation of the decay rates themselves which vary with Earth–Sun distance.  相似文献   

Emission of fine organic aerosol from traditional charcoal broiling in China     

Ximei Hou  Guoshun Zhuang  Yanfen Lin  Juan Li  Yilun Jiang  Joshua S. Fu 《Journal of Atmospheric Chemistry》2008,61(2):119-131

The indoor PM_2.5 aerosol samples for charcoal broiling source under Chinese traditional charbroiling and the ambient fine aerosols samples (PM_2.5) were collected in Beijing to investigate the characteristics of the charcoal broiling source and its impact on the fine organic aerosols in the atmosphere. The concentrations of 20 species of the trace organic compounds, including polycyclic aromatic hydrocarbons (PAHs), fatty acids, levoglucosan, and cholesterol in PM_2.5 were identified and quantified by GC/MS. The total PAHs and fatty acids emitted from charcoal broiling to PM_2.5 were 8.97 and 87,000 ng mg⁻¹ respectively. The concentrations of the light molecular weight (LMW) 3- and 4-ring PAHs were much higher than those of the high molecular weight (HMW) 5- and 6-ring PAHs. Fatty acids were the most abundant species in source profile, accounting for over 90% of all identified organic compounds. More polyunsaturated fatty acid (linoleic acids) than the saturated fatty acid (stearic acids) emitted in the cooking. Charcoal broiling is a minor source of PAHs compared to the source of biomass burning. Comparing the ratios of levoglucosan/fatty acid and levoglucosan/cholesterol in the charcoal broiling samples to the ambient samples, it is evident that meat cooking is an important source of fatty acids, but a less important source of cholesterol. Cooking, as one of the source of fine organic particles, plus other anthropogenic sources would be related to the formation of the severe haze occurred and spread over the urban atmosphere in most of the cities of China in the past several years.  相似文献   

Eclipsing binary Trojan asteroid Patroclus: Thermal inertia from Spitzer observations     

Michael Mueller  Franck Marchis  Joshua P. Emery  Stefano Mottola  Jérome Berthier 《Icarus》2010,205(2):505-515

We present mid-infrared observations of the binary L5-Trojan system (617) Patroclus-Menoetius before, during, and after two shadowing events, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. For the first time, we effectively observe changes in asteroid surface temperature in real time, allowing the thermal inertia to be determined very directly. A new detailed binary thermophysical model is presented which accounts for the system’s known mutual orbit, arbitrary component shapes, and thermal conduction in the presence of eclipses.We obtain two local thermal-inertia values, representative of the respective shadowed areas: and . The average thermal inertia is estimated to be , potentially with significant surface heterogeneity. This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith. Independently, we establish the presence of fine-grained (<a few μm) silicates on the surface, based on emissivity features near 10 and similar to those previously found on other Trojans.We also report V-band observations and report a lightcurve with complete rotational coverage. The lightcurve has a low amplitude of peak-to-peak, implying a roughly spherical shape for both components, and is single-periodic with a period equal to the period of the mutual orbit, indicating that the system is fully synchronized.The diameters of Patroclus and Menoetius are 106±11 and , respectively, in agreement with previous findings. Taken together with the system’s known total mass, this implies a bulk mass density of , significantly below the mass density of L4-Trojan asteroid (624) Hektor and suggesting a bulk composition dominated by water ice.All known physical properties of Patroclus, arguably the best studied Trojan asteroid, are consistent with those expected in icy objects with devolatilized surface (extinct comets), consistent with what might be implied by recent dynamical modeling in the framework of the Nice Model.  相似文献   

Out-of-plane librations of spinning tethered satellite systems     

Joshua R. Ellis  Christopher D. Hall 《Celestial Mechanics and Dynamical Astronomy》2010,106(1):39-67

We analyze the out-of-plane librations of a tethered satellite system that is nominally rotating in the orbit plane. To isolate the librational dynamics, the system is modeled as two point masses connected by a rigid rod with the system mass center constrained to an unperturbed circular orbit. For small out-of-plane librations, the in-plane motion is unaffected by the out-of-plane librations and a solution for the in-plane motion is determined in terms of Jacobi elliptic functions. This solution is used in the linearized equation for the out-of-plane librations, resulting in a Hill’s equation. Floquet theory is used to analyze the Hill’s equation, and we show that the out-of-plane librations are unstable for certain ranges of in-plane spin rate. For relatively high in-plane spin rates, the out-of-plane librations are stable, and the Hill’s equation can be approximated by a Mathieu’s equation. Approximate solutions to the Mathieu’s equation are determined, and we analyze the dominant characteristics of the out-of-plane librations for high in-plane spin rates. The results obtained from the analysis of the linearized equations of motion are compared to numerical simulations of the nonlinear equations of motion, as well as numerical simulations of a more realistic system model that accounts for tether flexibility. The instabilities discovered from the linear analysis are present in both the nonlinear system and the more realistic system model. The approximate solutions for the out-of-plane librations compare well to the nonlinear system for relatively high in-plane rotation rates, and also capture the significant qualitative behavior of the flexible system.  相似文献   

Simulated impacts of artificial groundwater recharge and discharge on the source area and source volume of an Atlantic Coastal Plain stream, Delaware, USA     

Joshua W. Kasper  Judith M. Denver  Thomas E. McKenna  William J. Ullman 《Hydrogeology Journal》2010,18(8):1855-1866

A numerical groundwater-flow model was used to characterize the source area and volume of Phillips Branch, a baseflow-dominated stream incising a highly permeable unconfined aquifer on the low relief Delmarva Peninsula, USA. Particle-tracking analyses indicate that the source area (5.51 km²) is ～20% smaller than the topographically defined watershed (6.85 km²), and recharge entering ～37% of the surface watershed does not discharge to Phillips Branch. Groundwater residence time within the source volume ranges from a few days to almost 100 years, with 95% of the volume “flushing” within 50 years. Artificial discharge from groundwater pumping alters the shape of the source area and reduces baseflow due to the interception of stream flow paths, but has limited impacts on the residence time of groundwater discharged as baseflow. In contrast, artificial recharge from land-based wastewater disposal substantially reduces the source area, lowers the range in residence time due to the elimination of older flow paths to the stream, and leads to increased discharge to adjacent surface-water bodies. This research suggests that, in this and similar hydrogeologic settings, the “watershed” approach to water-resource management may be limited, particularly where anthropogenic stresses alter the transport of soluble contaminants through highly permeable unconfined aquifers.  相似文献   

Continuum deformation and stability analyses of a steep hillside slope under rainfall infiltration   总被引：6，自引：4，他引：2  

Ronaldo I. Borja  Joshua A. White 《Acta Geotechnica》2010,5(1):1-14

Rainfall weakens an earth slope in a number of ways. It increases the degree of saturation of the soil, thereby breaking the bonds created by surface tension between the soil particles. When the volume of infiltrating water is large enough to mobilize fluid flow inside the soil matrix, the fluid exerts a downhill frictional drag on the slope, creating a destabilizing effect. When excess fluid can no longer infiltrate the slope due to increased saturation in the soil, it is discharged as a surface runoff and erodes the slope. In this paper, we present a physics-based framework for continuum modeling of a hydrologically driven slope failure similar to what occurred in a steep experimental catchment CB1 near Coos Bay, Oregon. We quantify the rainfall-induced slope deformation and assess the failure potential of the slope using finite element modeling that couples solid deformation with fluid pressure in an unsaturated soil. Results of the studies suggest that for a steep hillside slope underlain by a shallow bedrock similar to the CB1 site, failure would occur by multiple slide blocks with the failure surfaces emerging on the slope face. These results suggest that an infinite slope mechanism would be insufficient to represent the failure kinematics for a slope similar to CB1.  相似文献   

Coefficients for Penman-based estimation of daily potential evapo-transpiration in Botswana     

N. Persaud  D. Lesolle  X. Zhou  H. Wang  W. Joshua 《Journal of Arid Environments》2006,66(4):764-772

Reference potential evapo-transpiration (PET) for Botswana was estimated using a Penman-based equation in order to determine the appropriateness of five different sets of values for the 10 input coefficients. Calculated estimates were compared to the only existing set of observed PET values from two free drainage lysimeters at the headquarters of the Department of Meteorological Services (DMS) in Gaborone, Botswana. These values consisted of 36 five-day mean values of PET for grass cover during the period March 21, 1989–20 March, 1990 distributed as follows: fall 9, winter 8, spring 10, summer 9. Estimates were most sensitive to the wind coefficient in the aerodynamic term of the Penman-based equations. Analysis of the residual error between observed and estimated PET showed that the coefficients currently used by DMS best matched the lysimeter observations over the 4 seasons. However, only further comparisons between estimates and long-term, accurate, and multi-locational lysimeter measurements of reference crop PET would establish the set of coefficients appropriate for Penman-based PET estimation across Botswana.  相似文献   

Calculating the effective permeability of sandstone with multiscale lattice Boltzmann/finite element simulations     

Joshua A. White  Ronaldo I. Borja  Joanne T. Fredrich 《Acta Geotechnica》2006,1(4):195-209

The lattice Boltzmann (LB) method is an efficient technique for simulating fluid flow through individual pores of complex porous media. The ease with which the LB method handles complex boundary conditions, combined with the algorithm’s inherent parallelism, makes it an elegant approach to solving flow problems at the sub-continuum scale. However, the realities of current computational resources can limit the size and resolution of these simulations. A major research focus is developing methodologies for upscaling microscale techniques for use in macroscale problems of engineering interest. In this paper, we propose a hybrid, multiscale framework for simulating diffusion through porous media. We use the finite element (FE) method to solve the continuum boundary-value problem at the macroscale. Each finite element is treated as a sub-cell and assigned permeabilities calculated from subcontinuum simulations using the LB method. This framework allows us to efficiently find a macroscale solution while still maintaining information about microscale heterogeneities. As input to these simulations, we use synchrotron-computed 3D microtomographic images of a sandstone, with sample resolution of 3.34 μm. We discuss the predictive ability of these simulations, as well as implementation issues. We also quantify the lower limit of the continuum (Darcy) scale, as well as identify the optimal representative elementary volume for the hybrid LB–FE simulations.  相似文献