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61.
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63.
Contrasting rainfall generated debris flows from adjacent watersheds at Forest Falls, southern California, USA 总被引:1,自引:0,他引:1
Debris flows are widespread and common in many steeply sloping areas of southern California. The San Bernardino Mountains community of Forest Falls is probably subject to the most frequently documented debris flows in southern California. Debris flows at Forest Falls are generated during short-duration high-intensity rains that mobilize surface material. Except for debris flows on two consecutive days in November 1965, all the documented historic debris flows have occurred during high-intensity summer rainfall, locally referred to as ‘monsoon’ or ‘cloudburst’ rains. Velocities of the moving debris range from about 5 km/h to about 90 km/h. Velocity of a moving flow appears to be essentially a function of the water content of the flow. Low velocity debris flows are characterized by steep snouts that, when stopped, have only small amounts of water draining from the flow. In marked contrast are high-velocity debris flows whose deposits more resemble fluvial deposits. In the Forest Falls area two adjacent drainage basins, Snow Creek and Rattlesnake Creek, have considerably different histories of debris flows. Snow Creek basin, with an area about three times as large as Rattlesnake Creek basin, has a well developed debris flow channel with broad levees. Most of the debris flows in Snow Creek have greater water content and attain higher velocities than those of Rattlesnake Creek. Most debris flows are in relative equilibrium with the geometry of the channel morphology. Exceptionally high-velocity flows, however, overshoot the channel walls at particularly tight channel curves. After overshooting the channel, the flows degrade the adjacent levee surface and remove trees and structures in the immediate path, before spreading out with decreasing velocity. As the velocity decreases the clasts in the debris flows pulverize the up-slope side of the trees and often imbed clasts in them. Debris flows in Rattlesnake Creek are relatively slow moving and commonly stop in the channel. After the channel is blocked, subsequent debris flows cut a new channel upstream from the blockage that results in the deposition of new debris-flow deposits on the lower part of the fan. Shifting the location of debris flows on the Rattlesnake Creek fan tends to prevent trees from becoming mature. Dense growths of conifer seedlings sprout in the spring on the late summer debris flow deposits. This repeated process results in stands of even-aged trees whose age records the age of the debris flows. 相似文献
64.
Jian-Yang Li Lucille Le Corre Stefan E. Schröder Vishnu Reddy Brett W. Denevi Bonnie J. Buratti Stefano Mottola Martin Hoffmann Pablo Gutierrez-Marques Andreas Nathues Christopher T. Russell Carol A. Raymond 《Icarus》2013
Dawn spacecraft orbited Vesta for more than one year and collected a huge volume of multispectral, high-resolution data in the visible wavelengths with the Framing Camera. We present a detailed disk-integrated and disk-resolved photometric analysis using the Framing Camera images with the Minnaert model and the Hapke model, and report our results about the global photometric properties of Vesta. The photometric properties of Vesta show weak or no dependence on wavelengths, except for the albedo. At 554 nm, the global average geometric albedo of Vesta is 0.38 ± 0.04, and the Bond albedo range is 0.20 ± 0.02. The bolometric Bond albedo is 0.18 ± 0.01. The phase function of Vesta is similar to those of S-type asteroids. Vesta’s surface shows a single-peaked albedo distribution with a full-width-half-max ∼17% relative to the global average. This width is much smaller than the full range of albedos (from ∼0.55× to >2× global average) in localized bright and dark areas of a few tens of km in sizes, and is probably a consequence of significant regolith mixing on the global scale. Rheasilvia basin is ∼10% brighter than the global average. The phase reddening of Vesta measured from Dawn Framing Camera images is comparable or slightly stronger than that of Eros as measured by the Near Earth Asteroid Rendezvous mission, but weaker than previous measurements based on ground-based observations of Vesta and laboratory measurements of HED meteorites. The photometric behaviors of Vesta are best described by the Hapke model and the Akimov disk-function, when compared with the Minnaert model, Lommel–Seeliger model, and Lommel–Seeliger–Lambertian model. The traditional approach for photometric correction is validated for Vesta for >99% of its surface where reflectance is within ±30% of global average. 相似文献
65.
Leah E. Morgan Madicken Munk Brett Davidheiser‐Kroll Nicholas H. Warner Sanjeev Gupta Rachel Slaybaugh Patrick Harkness Darren F. Mark 《Geostandards and Geoanalytical Research》2017,41(3):381-396
The chronology of the Solar System, particularly the timing of formation of extra‐terrestrial bodies and their features, is an outstanding problem in planetary science. Although various chronological methods for in situ geochronology have been proposed (e.g., Rb‐Sr, K‐Ar), and even applied (K‐Ar), the reliability, accuracy, and applicability of the 40Ar/39Ar method makes it by far the most desirable chronometer for dating extra‐terrestrial bodies. The method however relies on the neutron irradiation of samples, and thus a neutron source. Herein, we discuss the challenges and feasibility of deploying a passive neutron source to planetary surfaces for the in situ application of the 40Ar/39Ar chronometer. Requirements in generating and shielding neutrons, as well as analysing samples are described, along with an exploration of limitations such as mass, power and cost. Two potential solutions for the in situ extra‐terrestrial deployment of the 40Ar/39Ar method are presented. Although this represents a challenging task, developing the technology to apply the 40Ar/39Ar method on planetary surfaces would represent a major advance towards constraining the timescale of solar system formation and evolution. 相似文献
66.
Adrian Spence David J. Mcnally Margaret V. McCaul Brett Paull 《Geochimica et cosmochimica acta》2011,75(10):2571-2581
Soil microbial biomass is a primary source of soil organic carbon (SOC) and therefore plays a fundamental role in carbon and nitrogen cycling. However, little is known about the fate and transformations of microbial biomass in soil. Here we employ HR-MAS NMR spectroscopy to monitor 13C and 15N labeled soil microbial biomass and leachate degradation over time. As expected, there is a rapid loss of carbohydrate structures. However, diffusion edited HR-MAS NMR data reveals that macromolecular carbohydrates are more resistant to degradation and are found in the leachate. Aromatic components survive as dissolved species in the leachate while aliphatic components persist in both the biomass and leachate. Dissolved protein and peptidoglycan accumulate in the leachate and recalcitrant amide nitrogen and lipoprotein persists in both the degraded biomass and leachate. Cross-peaks that appear in 1H-15N HR-MAS NMR spectra after degradation suggest that specific peptides are either selectively preserved or used for the synthesis of unknown structures. The overall degradation pathways reported here are similar to that of decomposing plant material degraded under similar conditions suggesting that the difference between recalcitrant carbon from different sources is negligible after decomposition. 相似文献
67.
Rates of erosion and landscape change along the Blue Ridge escarpment,southern Appalachian Mountains,estimated from in situ cosmogenic 10Be 下载免费PDF全文
Colleen L. Linari Paul R. Bierman Eric W. Portenga Milan J. Pavich Robert C. Finkel Stewart P.H.T. Freeman 《地球表面变化过程与地形》2017,42(6):928-940
The Blue Ridge escarpment, located within the southern Appalachian Mountains of Virginia and North Carolina, forms a distinct, steep boundary between the lower‐elevation Piedmont and higher‐elevation Blue Ridge physiographic provinces. To understand better the rate at which this landform and the adjacent landscape are changing, we measured cosmogenic beryllium‐10 (10Be) in quartz separated from sediment samples (n = 50) collected in 32 streams and from three exposed bedrock outcrops along four transects normal to the escarpment, allowing us to calculate erosion rates integrated over 104–105 years. These basin‐averaged erosion rates (5.4–49 m Myr?1) are consistent with those measured elsewhere in the southern Appalachain Mountains and show a positive relationship between erosion rate and average basin slope. Erosion rates show no relationship with basin size or relative position of the Brevard fault zone, a fundamental structural element of the region. The cosmogenic isotopic data, when considered along with the distribution of average basin slopes in each physiographic province, suggest that the escarpment is eroding on average more rapidly than the Blue Ridge uplands, which are eroding more rapidly than the Piedmont lowlands. This difference in erosion rates by geomorphic setting suggests that the elevation difference between the uplands and lowlands adjacent to the escarpment is being reduced but at extremely slow rates. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
68.
Brett A. Yuskiewicz F. Stratmann W. Birmili A. Wiedensohler E. Swietlicki O. Berg J. Zhou 《Atmospheric Research》1999,50(3-4)
Direct physical measurements of particle mass and number concentration indicate an increase in overall aerosol mass resulting from cloud processing, most likely through aqueous-phase chemistry (e.g., SO2 oxidation). Measurements conducted in the Pennines of Northern England reveal an average increase of 14 to 20% in dry aerosol mass (0.003<particle diameter<0.9 μm) after aerosol passage through an orographic cloud. The rate of in-cloud mass production is most sensitive to changes in upwind particle size distributions, SO2 concentration, and cloud water acidity. Newly-formed mass appears in size range between 200 and 600 nm and enhances the bimodality of the particle number distribution after cloud processing. Furthermore, the cloud-produced mass is estimated to increase total light scattering, bsp, by 18 to 24%. The scattering efficiency of the dry, cloud-generated aerosol is 5.0±0.3 m2 g−1 and increases to 7.4±0.7 m2 g−1 when adjusted to 90% relative humidity by incorporating particle hygroscopicity data. 相似文献
69.
Clues from Current High CO2 Environments on the Effects of Ocean Acidification on CaCO3 Preservation
Andreas J. Andersson Nicholas R. Bates Marlene A. Jeffries Kyra Freeman Charles Davidson Shaun Stringer Evan Betzler Fred T. Mackenzie 《Aquatic Geochemistry》2013,19(5-6):353-369
Acidification of surface seawater owing to anthropogenic activities has raised serious concerns on its consequences for marine calcifying organisms and ecosystems. To acquire knowledge concerning the future consequences of ocean acidification (OA), researchers have relied on incubation experiments with organisms exposed to future seawater conditions, numerical models, evidence from the geological record, and recently, observations from aquatic environments exposed to naturally high CO2 and low pH, e.g., owing to volcanic CO2 vents, upwelling, and groundwater input. In the present study, we briefly evaluate the distribution of dissolved CO2–carbonic acid parameters at (1) two locations in the Pacific and the Atlantic Ocean as a function of depth, (2) a mangrove environment in Bermuda, (3) a seasonally stratified body of water in a semi-enclosed sound in Bermuda, and (4) in temporarily isolated tide pools in Southern California. We demonstrate that current in situ conditions of seawater pCO2, pH, and CaCO3 saturation state (Ω) in these environments are similar or even exceed the anticipated changes to these parameters in the open ocean over the next century as a result of OA. The observed differences between the Pacific and Atlantic Oceans with respect to seawater CO2–carbonic acid chemistry, preservation of CaCO3 minerals, and the occurrence and distribution of deep-sea marine calcifiers, support the hypothesized negative effects of OA on the production and preservation of CaCO3 in surface seawater. Clues provided from shallow near-shore environments in Bermuda and Southern California support these predictions, but also highlight that many marine calcifiers already experience relatively high seawater pCO2 and low pH conditions. 相似文献
70.
Cross-sections of the Hartley-Huggins bands of 03 at the temperature 195 K have been obtained from photoabsorption measurements at column densities in the range 2 × 1017?1 × 1021 cm?2 throughout the wavelength region 240–350 nm with a 6.65 m photoelectric scanning spectrometer equipped with a 2400 lines mm?1 grating and operated at an instrumental width (FWHM) of 0.003 nm. The assumptions made in putting the measured relative cross-sections on an absolute basis are discussed. Fine structure in the cross-section observed in the Huggins bands is illustrated in the region 323–327 nm where shallow features of width 0.01–0.02 nm occur superposed on a stronger apparent continuum exhibiting broader wavy structure. 相似文献