1,220 research outputs found

    Constraining the role of early land plants in Palaeozoic weathering and global cooling

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    How the colonization of terrestrial environments by early land plants over 400 Ma influenced rock weathering, the biogeochemical cycling of carbon and phosphorus, and climate in the Palaeozoic is uncertain. Here we show experimentally that mineral weathering by liverworts—an extant lineage of early land plants—partnering arbuscular mycorrhizal (AM) fungi, like those in 410 Ma-old early land plant fossils, amplified calcium weathering from basalt grains threefold to sevenfold, relative to plant-free controls. Phosphate weathering by mycorrhizal liverworts was amplified 9–13-fold over plant-free controls, compared with fivefold to sevenfold amplification by liverworts lacking fungal symbionts. Etching and trenching of phyllosilicate minerals increased with AM fungal network size and atmospheric CO2 concentration. Integration of grain-scale weathering rates over the depths of liverwort rhizoids and mycelia (0.1 m), or tree roots and mycelia (0.75 m), indicate early land plants with shallow anchorage systems were probably at least 10-fold less effective at enhancing the total weathering flux than later-evolving trees. This work challenges the suggestion that early land plants significantly enhanced total weathering and land-to-ocean fluxes of calcium and phosphorus, which have been proposed as a trigger for transient dramatic atmospheric CO2 sequestration and glaciations in the Ordovician

    Reframing the contribution of pelagic Sargassum epiphytic N2 fixation

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    Though nitrogen fixation by epiphytic diazotrophs on pelagic Sargassum has been recognized for decades, it has been assumed to contribute insignificantly to the overall marine nitrogen budget. This six-year study reframes this concept through long-term measurements of Sargassum community nitrogen fixation rates, and by extrapolating mass-specific rates to a theoretical square meter portion of Sargassum mat allowing for comparison of these rates to those of other marine and coastal diazotrophs. On 24 occasions from 2015 to 2021, rates of nitrogen fixation were measured using whole fronds of Sargassum collected from the western edge of the Gulf Stream off Cape Hatteras, North Carolina. Across all dates, mass-specific rates ranged from 0 to 37.77 ÎŒmol N g-1 h-1 with a mean of 4.156 ÎŒmol N g-1 h-1. Extrapolating using a mat-specific density of Sargassum, these rates scale to a range of 0 to 30,916 ÎŒmol N m-2 d-1 and a mean of 3,697 ÎŒmol N m-2 d-1. Quantifying this community’s rates of nitrogen fixation over several years captured the sometimes-extreme variability in rates, characteristic of marine diazotrophs, which has not been reported in the literature to date. When these measurements are considered alongside estimates of the density of pelagic Sargassum, rates of nitrogen fixation by Sargassum’s epiphytic diazotrophs rival that of their coastal macrophyte and planktonic counterparts. Given Sargassum’s wide and expanding geographic range, the results of this study suggest this community may contribute reactive nitrogen on a meaningful, basin-wide scale, which merits further study

    Long‐term Production and Profitability from Grazing Cattle in the Northern Mixed GrassPrairie

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    Conventional wisdom among rangeland professionals has been that for long‐term sustainability of grazing livestock operations, rangeland should be kept in high good to low excellent range condition. Our objective was to analyze production parameters, economic costs, returns, and profit using data generated over a thirty‐four year period (1969‐2002) from grazing a Clayey range site in the mixed‐grass prairie of western South Dakota with variable stocking rates required to maintain pastures in low‐fair, good, and excellent range condition classes. Cattle weights were measured at turnout and at the end of the grazing season. Gross income per acre was calculated by multiplying total gain per acre times price using historical National Agricultural Statistics Services feeder cattle prices. Annual variable costs were estimated from a historical yearling cattle budget developed by South Dakota State University (SDSU) agricultural economists. All economic values were adjusted to a constant dollar using the Bureau of Labor Statistics’ Consumer Price Index. Stocking rate, average daily gain, total gain, net profit, gross revenue, and annual costs per acre varied among range condition classes. Net income for low‐fair range condition (11.18peracre)andgoodrangecondition(11.18 per acre) and good range condition (11.86 per acre) were not different, but both were greater (P \u3c 0.01) than excellent range condition ($ 9.31 per acre). Over the life of the study, real profit (adjusted for inflation) steadily increased (P \u3c 0.01) for the low‐fair and good treatments while it remained level for the excellent treatment. Neither drought nor wet springs impacted profit differently for the three treatments. These results support generally observed rancher behavior regarding range condition: to maintain their rangeland in a lower range condition than would be normally recommend by rangeland professionals. Ecosystem goods and services of increasing interest to society and associated with high range condition, such as floristic diversity, hydrologic function, and wildlife cover, come at an opportunity cost to the rancher

    Trapping of Projectiles in Fixed Scatterer Calculations

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    We study multiple scattering off nuclei in the closure approximation. Instead of reducing the dynamics to one particle potential scattering, the scattering amplitude for fixed target configurations is averaged over the target groundstate density via stochastic integration. At low energies a strong coupling limit is found which can not be obtained in a first order optical potential approximation. As its physical explanation, we propose it to be caused by trapping of the projectile. We analyse this phenomenon in mean field and random potential approximations. (PACS: 24.10.-i)Comment: 15 page

    Read-It: A Multi-modal Tangible Interface for Children Who Learn to Read

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    Multi-modal tabletop applications offer excellent opportunities for enriching the education of young children. Read-It is an example of an interactive game with a multi-modal tangible interface that was designed to combine the advantages of current physical games and computer exercises. It is a novel approach for supporting children who learn to read. The first experimental evaluation has demonstrated that the Read-It approach is indeed promising and meets a priori expectations

    Advanced X-Ray Timing Array Mission: Conceptual Spacecraft Design Study

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    The Advanced X-Ray Timing Array (AXTAR) is a mission concept for submillisecond timing of bright galactic x-ray sources. The two science instruments are the Large Area Timing Array (LATA) (a collimated instrument with 2-50-keV coverage and over 3 square meters of effective area) and a Sky Monitor (SM), which acts as a trigger for pointed observations of x-ray transients. The spacecraft conceptual design team developed two spacecraft concepts that will enable the AXTAR mission: A minimal configuration to be launched on a Taurus II and a larger configuration to be launched on a Falcon 9 or similar vehicle

    Dense, viscous brine behavior in heterogeneous porous medium systems

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    The behavior of dense, viscous calcium bromide brine solutions used to remediate systems contaminated with dense nonaqueous phase liquids (DNAPLs) is considered in laboratory and field porous medium systems. The density and viscosity of brine solutions are experimentally investigated and functional forms fit over a wide range of mass fractions. A density of 1.7 times, and a corresponding viscosity of 6.3 times, that of water is obtained at a calcium bromide mass fraction of 0.53. A three-dimensional laboratory cell is used to investigate the establishment, persistence, and rate of removal of a stratified dense brine layer in a controlled system. Results from a field-scale experiment performed at the Dover National Test Site are used to investigate the ability to establish and maintain a dense brine layer as a component of a DNAPL recovery strategy, and to recover the brine at sufficiently high mass fractions to support the economical reuse of the brine. The results of both laboratory and field experiments show that a dense brine layer can be established, maintained, and recovered to a significant extent. Regions of unstable density profiles are shown to develop and persist in the field-scale experiment, which we attribute to regions of low hydraulic conductivity. The saturated-unsaturated, variable-density ground-water flow simulation code SUTRA is modified to describe the system of interest, and used to compare simulations to experimental observations and to investigate certain unobserved aspects of these complex systems. The model results show that the standard model formulation is not appropriate for capturing the behavior of sharp density gradients observed during the dense brine experiments

    Self-similar solutions of viscous and resistive ADAFs with thermal conduction

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    We have studied the effects of thermal conduction on the structure of viscous and resistive advection-dominated accretion flows (ADAFs). The importance of thermal conduction on hot accretion flow is confirmed by observations of hot gas that surrounds Sgr A∗^* and a few other nearby galactic nuclei. In this research, thermal conduction is studied by a saturated form of it, as is appropriated for weakly-collisional systems. It is assumed the viscosity and the magnetic diffusivity are due to turbulence and dissipation in the flow. The viscosity also is due to angular momentum transport. Here, the magnetic diffusivity and the kinematic viscosity are not constant and vary by position and α\alpha-prescription is used for them. The govern equations on system have been solved by the steady self-similar method. The solutions show the radial velocity is highly subsonic and the rotational velocity behaves sub-Keplerian. The rotational velocity for a specific value of the thermal conduction coefficient becomes zero. This amount of conductivity strongly depends on magnetic pressure fraction, magnetic Prandtl number, and viscosity parameter. Comparison of energy transport by thermal conduction with the other energy mechanisms implies that thermal conduction can be a significant energy mechanism in resistive and magnetized ADAFs. This property is confirmed by non-ideal magnetohydrodynamics (MHD) simulations.Comment: 8 pages, 5 figures, accepted by Ap&S

    Hadronic properties of the S_{11}(1535) studied by electroproduction off the deuteron

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    Properties of excited baryonic states are investigated in the context of electroproduction of baryon resonances off the deuteron. In particular, the hadronic radii and the compositeness of baryon resonances are studied for kinematic situations in which their hadronic reinteraction is the dominant contribution. Specifically, we study the reaction d(e,eâ€ČS11)Nd(e,e'S_{11})N at Q2≄1GeV2Q^2\ge 1 GeV^2 for kinematics in which the produced hadronic state reinteracts predominantly with the spectator nucleon. A comparison of constituent quark model and effective chiral Lagrangian calculations of the S11S_{11} shows substantial sensitivity to the structure of the produced resonance.Comment: 24 pages, 5 figure
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