Beaver dam analogues drive heterogeneous groundwater–surface water interactions     

Jeffrey Wade  Laura Lautz  Christa Kelleher  Philippe Vidon  Julianne Davis  Julio Beltran  Casey Pearce 《水文研究》2020,34(26):5340-5353

Beaver dam analogues (BDAs) are a cost-effective stream restoration approach that leverages the recognized environmental benefits of natural beaver dams on channel stability and local hydrology. Although natural beaver dams are known to exert considerable influence on the hydrologic conditions of a stream system by mediating geomorphic processes, nutrient cycling, and groundwater–surface water interactions, the impacts of beaver-derived restoration methods on groundwater–surface water exchange are poorly characterized. To address this deficit, we monitored hyporheic exchange fluxes and streambed porewater biogeochemistry across a sequence of BDAs installed along a central Wyoming stream during the summer of 2019. Streambed fluxes were quantified by heat tracing methods and vertical hydraulic gradients. Biogeochemical activity was evaluated using major ion porewater chemistry and principal component analysis. Vertical fluxes of approximately 1.0 m/day were observed around the BDAs, as was the development of spatially heterogeneous zones of nitrate production, groundwater upwelling, and anaerobic reduction. Strong contrasts in hyporheic zone processes were observed across BDAs of differing sizes. This suggests that structures may function with size-dependent behaviour, only altering groundwater–surface water interactions after a threshold hydraulic step height is exceeded. Patterns of hyporheic exchange and biogeochemical cycling around the studied BDAs resemble those around natural beaver dams, suggesting that BDAs may provide comparable benefits to channel complexity and near-stream function over a 1-year period.  相似文献   

Where does water go when it rains? Moving beyond the variable source area concept of rainfall‐runoff response     

Jeffrey J. McDonnell 《水文研究》2003,17(9):1869-1875

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Towards a free–free template for CMB foregrounds     

C. Dickinson  R. D. Davies  R. J. Davis 《Monthly notices of the Royal Astronomical Society》2003,341(2):369-384

A full-sky template map of the Galactic free–free foreground emission component is increasingly important for high-sensitivity cosmic microwave background (CMB) experiments. We use the recently published Hα data of both the northern and southern skies as the basis for such a template.
The first step is to correct the Hα maps for dust absorption using the 100-μm dust maps of Schlegel, Finkbeiner & Davis. We show that for a range of longitudes, the Galactic latitude distribution of absorption suggests that it is 33 per cent of the full extragalactic absorption. A reliable absorption-corrected Hα map can be produced for ∼95 per cent of the sky; the area for which a template cannot be recovered is the Galactic plane area  | b | < 5°, l = 260°–0°–160°  and some isolated dense dust clouds at intermediate latitudes.
The second step is to convert the dust-corrected Hα data into a predicted radio surface brightness. The free–free emission formula is revised to give an accurate expression (1 per cent) for the radio emission covering the frequency range 100 MHz–100 GHz and the electron temperature range 3000–20 000 K. The main uncertainty when applying this expression is the variation of electron temperature across the sky. The emission formula is verified in several extended H  ii regions using data in the range 408–2326 MHz.
A full-sky free–free template map is presented at 30 GHz; the scaling to other frequencies is given. The Haslam et al. all-sky 408-MHz map of the sky can be corrected for this free–free component, which amounts to a  ≈6  per cent correction at intermediate and high latitudes, to provide a pure synchrotron all-sky template. The implications for CMB experiments are discussed.  相似文献   

Amino acid analyses of Antarctic CM2 meteorites using liquid chromatography‐time of flight‐mass spectrometry     

Daniel P. Glavin  Jason P. Dworkin  Andrew Aubrey  Oliver Botta  James H. Doty  Zita Martins  Jeffrey L. Bada 《Meteoritics & planetary science》2006,41(6):889-902

Abstract— Amino acid analyses of the Antarctic CM2 chondrites Allan Hills (ALH) 83100 and Lewis Cliff (LEW) 90500 using liquid chromatography‐time of flight‐mass spectrometry (LC‐ToF‐MS) coupled with UV fluorescence detection revealed that these carbonaceous meteorites contain a suite of indigenous amino acids not present in Antarctic ice. Several amino acids were detected in ALH 83100, including glycine, alanine, β‐alanine, γ‐amino‐n‐butyric acid (γ‐ABA), and α‐aminoisobutyric acid (AIB) with concentrations ranging from 250 to 340 parts per billion (ppb). In contrast to ALH 83100, the CM2 meteorites LEW 90500 and Murchison had a much higher total abundance of these amino acids (440–3200 ppb). In addition, ALH 83100 was found to have lower abundances of the α‐dialkyl amino acids AIB and isovaline than LEW 90500 and Murchison. There are three possible explanations for the depleted amino acid content in ALH 83100: 1) amino acid leaching from ALH 83100 during exposure to Antarctic ice meltwater, 2) a higher degree of aqueous alteration on the ALH 83100 parent body, or 3) ALH 83100 originated on a chemically distinct parent body from the other two CM2 meteorites. The high relative abundance of ?‐amino‐n‐caproic acid (EACA) in the ALH 83100 meteorite as well as the Antarctic ice indicates that Nylon‐6 contamination from the Antarctic sample storage bags may have occurred during collection.  相似文献   

Retardation of Tritium and Cesium in Brine-Saturated Mudstone, Halite, and Carbonate Porous Media     

Evangelos A. Voudrias  Jeffrey L. Means  Jeffrey Kittel 《Ground water》1993,31(4):605-615

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Complex structure associated with the Wolf-Rayet star WR147     

R. J. Davis 《Astrophysics and Space Science》1994,216(1-2):67-68

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Imaging the surface of Mercury using ground-based telescopes     

Michael Mendillo  Johan Warell  Sanjay S. Limaye  Jeffrey Baumgardner  Ann Sprague  Jody K. Wilson 《Planetary and Space Science》2001,49(14-15)

We describe and compare two methods of short-exposure, high-definition ground-based imaging of the planet Mercury. Two teams have recorded images of Mercury on different dates, from different locations, and with different observational and data reduction techniques. Both groups have achieved spatial resolutions of <250 km, and the same albedo features and contrast levels appear where the two datasets overlap (longitudes 270–360°). Dark albedo regions appear as mare and correlate well with smooth terrain radar signatures. Bright albedo features agree optically, but less well with radar data. Such confirmations of state-of-the-art optical techniques introduce a new era of ground-based exploration of Mercury's surface and its atmosphere. They offer opportunities for synergistic, cooperative observations before and during the upcoming Messenger and BepiColombo missions to Mercury.  相似文献   

Alkali elemental and potassium isotopic compositions of Semarkona chondrules     

Conel M. O'D  Jeffrey N. GROSSMAN 《Meteoritics & planetary science》2005,40(4):541-556

Abstract— We report measurements of K isotope ratios in 28 Semarkona chondrules with a wide range of petrologic types and bulk compositions as well as the compositions of CPX‐mesostasis pairs in 17 type I Semarkona chondrules, including two chondrules with radial alkali zonation and 19 type II chondrules. Despite the wide range in K/Al ratios, no systematic variations in K isotopic compositions were found. Semarkona chondrules do not record a simple history of Rayleigh‐type loss of K. Experimentally determined evaporation rates suggest that considerable alkali evaporation would have occurred during chondrule formation. Nevertheless, based on Na CPX‐mesostasis distribution coefficients, the alkali contents of the cores of most chondrules in Semarkona were probably established at the time of final crystallization. However, Na CPX‐mesostasis distribution coefficients also show that alkali zonation in type I Semarkona chondrules was produced by entry of alkalis after solidification, probably during parent body alteration. This alkali metasomatism may have gone to completion in some chondrules. Our preferred explanation for the lack of systematic isotopic enrichments, even in alkali depleted type I chondrule cores, is that they exchanged with the ambient gas as they cooled.  相似文献   

Late Quaternary submarine bedforms and ice‐sheet flow in Gerlache Strait and on the adjacent continental shelf,Antarctic Peninsula     

Jeffrey Evans  Julian A. Dowdeswell  Colm Cofaigh 《第四纪科学杂志》2004,19(4):397-407

Geophysical data from Gerlache Strait, Croker Passage, Bismarck Strait and the adjacent continental shelf reveal streamlined subglacial bedforms that were produced at the bed of the Antarctic Peninsula Ice Sheet (APIS) during the last glaciation. The spatial arrangement and orientation of these bedforms record the former drainage pattern and flow dynamics of an APIS outlet up‐flow, and feeding into, a palaeo‐ice stream in the Western Bransfield Basin. Evidence suggests that together, they represent a single ice‐flow system that drained the APIS during the last glaciation. The ice‐sheet outlet flowed north/northeastwards through Gerlache Strait and Croker Passage and converged with a second, more easterly ice‐flow tributary on the middle shelf to form the main palaeo‐ice stream. The dominance of drumlins with low elongation ratios suggests that ice‐sheet outlet draining through Gerlache Strait was comparatively slower than the main palaeo‐ice stream in the Western Bransfield Basin, although the low elongation ratios may also partly reflect the lack of sediment. Progressive elongation of drumlins further down‐flow indicates that the ice sheet accelerated through Croker Passage and the western tributary trough, and fed into the main zone of streaming flow in the Western Bransfield Basin. Topography would have exerted a strong control on the development of the palaeo‐ice stream system but subglacial geology may also have been significant given the transition from crystalline bedrock to sedimentary strata on the inner–mid‐shelf. In the broader context, the APIS was drained by a number of major fast‐flowing outlets through cross‐shelf troughs to the outer continental shelf during the last glaciation. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Deep-ocean vertical noise directionality     

Carey  W.M. Evans  R.B. Davis  J.A. Botseas  G. 《Oceanic Engineering, IEEE Journal of》1990,15(4):324-334

The structure of beam noise measured at the output of a vertical array in a range dependent ocean basin was investigated using the modified wide-angle parabolic equation (PE). Noise sources were distributed throughout the basin, and the field due to each noise source at an array located in the midbasin was calculated. The response of the array to the superposition of the noise sources was found by beamforming. An efficient and direct approach that superimposes the noise sources on the PE field as the field is marched toward the array was developed. Downslope calculations of the midbasin vertical directionality were made between 50 and 400 Hz with this technique. Use of a geoacoustic model shows that the bottom behaves as a low-pass filter  相似文献