Wind erosion of the wind-deposited Navajo Sandstone, USA     

David B. Loope  & Joseph A. Mason 《Geology Today》2009,25(2):65-70

Outcrops of the Early Jurassic Navajo Sandstone in southern Utah and northern Arizona, south-western USA are being actively eroded by sand-laden, south-westerly winds. Small-scale stepped topography with risers facing into the wind develops even on steep canyon walls when wind-swept grains strike the rock at a low angle. Photosynthetic, endolithic microbes directly underlie most outcrop surfaces; the crusts formed by these organisms are essential to formation of the small-scale steps. Wind erosion of highlands also forms troughs and pits that are tens of metres across. The pits have deeply scalloped, overhanging walls, and contain central domes surrounded by 'moats' filled with dune sand. Wind erosion of aeolian sandstone is favoured by a positive feedback mechanism in which grains that are liberated from outcrops by impacting particles become a fresh supply of pre-sorted abrasive particles for further attack.  相似文献   

Holocene history of lacustrine and marsh sediments in a dune-blocked drainage, Southwestern Nebraska Sand Hills, U.S.A.   总被引：1，自引：0，他引：1  

Jon P. Mason  James B. Swinehart  David B. Loope 《Journal of Paleolimnology》1997,17(1):67-83

As many as 2500 interdune lakes lie within the Nebraska Sand Hills, a 50000 km stabilized sand sea. The few published data on cores from these lakes indicate they are typically underlain by less than two m of Holocene lacustrine sediments. However, three lakes in the southwestern Sand Hills, Swan, Blue, and Crescent, contain anomalously thick marsh (peat) and lacustrine (gyttja) sediments. Swan Lake basin contains as much as 8 m of peat, which was deposited between about 9000 and 3300 years ago. This peat is conformably overlain by as much as 10.5 m of gyttja. The sediment record in Blue lake, which is 3 km downgradient from Swan lake, dates back to only about 6000 years ago. Less than two m of peat, which was deposited from 6000 to 5000 years ago, is overlain by 12 m of gyttja deposited in the last 4300 years. Crescent Lake basin, one km downgradient from Blue Lake, has a similar sediment history except for a lack of known peat deposits. Recently, a 8-km long segment of a paleovalley was documented running beneath the three lakes and connecting to the head of Blue Creek Valley. Blockage of this paleovalley by dune sand during two arid intervals, one shortly before 10500 yr BP and one in the mid-Holocene, has resulted in a 25 m rise in the regional water table. This made possible the deposition of organic-rich sediment in all three lakes. Although these lakes, especially Swan, would seem ideal places to look for a nearly complete record of Holocene climatic fluctuations, the paleoclimatic record is confounded by the effect dune dams have on the water table. In Swan Lake, the abrupt conversion from marsh to lacustrine deposition 3300 years ago does not simply record the change to a wetter regional climate; it reflects the complex local hydrologic changes surrounding the emplacement and sealing of dune dams, as well as regional climate.  相似文献   

Summer temperature variability across four urban neighborhoods in Knoxville,Tennessee, USA     

Kelsey?N.?EllisEmail author  Jon?M.?Hathaway  Lisa?Reyes?Mason  David?A.?Howe  Thomas?H.?Epps  Vincent?M.?Brown 《Theoretical and Applied Climatology》2017,127(3-4):701-710

The urban heat island (UHI) is a well-documented effect of urbanization on local climate, identified by higher temperatures compared to surrounding areas, especially at night and during the warm season. The details of a UHI are city-specific, and microclimates may even exist within a given city. Thus, investigating the spatiotemporal variability of a city’s UHI is an ongoing and critical research need. We deploy ten weather stations across Knoxville, Tennessee, to analyze the city’s UHI and its differential impacts across urban neighborhoods: two each in four neighborhoods, one in more dense tree cover and one in less dense tree cover, and one each in downtown Knoxville and Ijams Nature Center that serve as control locations. Three months of temperature data (beginning 2 July 2014) are analyzed using paired-sample t tests and a three-way analysis of variance. Major findings include the following: (1) Within a given neighborhood, tree cover helps negate daytime heat (resulting in up to 1.19 ^°C lower maximum temperature), but does not have as large of an influence on minimum temperature; (2) largest temperature differences between neighborhoods occur during the day (0.38–1.16 ^°C difference), but larger differences between neighborhoods and the downtown control occur at night (1.04–1.88 ^°C difference); (3) presiding weather (i.e., air mass type) has a significant, consistent impact on the temperature in a given city, and lacks the differential impacts found at a larger-scale in previous studies; (4) distance from city center does not impact temperature as much as land use factors. This is a preliminary step towards informing local planning with a scientific understanding of how mitigation strategies may help minimize the UHI and reduce the effects of extreme weather on public health and well-being.  相似文献   

Soft X-ray spectral variations of the narrow-line Seyfert 1 galaxy Markarian 766     

M. J. Page  F. J. Carrera  J. P. D. Mittaz  K. O. Mason 《Monthly notices of the Royal Astronomical Society》1999,305(4):775-794

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A 5000 year record of intertidal peat stratigraphy and sea level change from northwest Alaska     

J. W. Jordan  O. K. Mason 《Quaternary International》1999,60(1):1750

Coastal environments of northwest Alaska preserve a detailed record of sea level change during the past 5000 ¹⁴C yr. Rapid burial of intertidal marshes by storm overwash processes, a short open water period, and the arctic maritime climate contribute to the preservation of marine and eolian facies. Eustatic and storm-controlled changes in sea level can be identified from interbedded sequences of marsh peat and coastal flood deposits on barrier islands and estuaries along northwest Seward Peninsula. Mean eustatic sea level has risen about 1.5 m during the last 5000 years, at an average of ca. 0.028 cm yr⁻¹, based on the depth and age relationship of a suite of 23 peat horizons sampled from a 140 km-long reach of coast. Nearshore erosion and sedimentation are controlled by secular variations in wave climate. Peaks in tidal amplitude and storminess correlate with transgressive sedimentary regimes and occurred during the periods 3300–1700, 1200–900, and since about 400 ¹⁴C yr BP. Short-term fluctuations in relative sea level are superimposed on the long-term regional eustatic trend, and record the coastal response to variable rates of sea-level change during the late Holocene.  相似文献   

Groundwater Pumping Is a Significant Unrecognized Contributor to Global Anthropogenic Element Cycles     

Mason O. Stahl 《Ground water》2019,57(3):455-464

Quantifying anthropogenic contributions to elemental cycles provides useful information regarding the flow of elements important to industrial and agricultural development and is key to understanding the environmental impacts of human activity. In particular, when anthropogenic fluxes reach levels large enough to influence an element's overall cycle the risk of adverse environmental impacts rises. While intensive groundwater pumping has been observed to affect a wide-range of environmental processes, the role of intensive groundwater extraction on global anthropogenic element cycles has not yet been characterized. Relying on comprehensive datasets of groundwater and produced water (groundwater pumped during oil/gas extraction) chemistry from the U.S. Geological Survey along with estimates of global groundwater usage, I estimate elemental fluxes from global pumping, consumptive use, and depletion of groundwater. I find that groundwater fluxes appreciably contribute to a number of elements overall cycles and thus these cycles were underestimated in prior studies, which did not recognize groundwater pumping's role. I also estimate elemental loadings to agricultural soils in the United States and find that in some regions, groundwater may provide a significant portion (more than 10%) of crop requirements of key nutrients (K, N). With nearly 40% of globally irrigated land under groundwater irrigation, characterizing nutrient and toxic element fluxes to these soils, which ultimately influence crop yields, is important to our understanding of agricultural production. Thus, this study improves our basic understanding of anthropogenic elemental cycles and demonstrates that quantification of groundwater pumping elemental fluxes provides valuable information about the potential for environmental impacts from groundwater pumping.  相似文献   

Preparation and Measurement of Cassiterite for Sn Isotope Analysis          下载免费PDF全文

Ryan Mathur  Wayne Powell  Andrea Mason  Linda Godfrey  Junming Yao  Mark E. Baker 《Geostandards and Geoanalytical Research》2017,41(4):701-707

Increased interest in the fractionation of Sn isotopes has led to the development of several techniques for preparing cassiterite (SnO₂, the primary ore of Sn) for isotopic analysis. Two distinct methods have been applied in recent isotopic studies of cassiterite: (a) reduction to tin metal with potassium cyanide (KCN) at high temperature (800 °C), with subsequent dissolution in HCl, and (b) reduction to a Sn solution with hydriodic acid (HI) at low temperature (100 °C). This study compares the effectiveness and accuracy of these two methods and contributes additional methodological details. The KCN method consistently yielded more Sn (> 70% in comparison with < 5%), does not appear to fractionate Sn isotopes at high temperatures over a 2‐hour period and produced consistent Sn isotope values at flux mass ratios of ≥ 4:1 (flux to mineral) with a minimum reduction time of 40 min. By means of a distillation experiment, it was demonstrated that HI could volatilise Sn, explaining the consistently low yields by this method. Furthermore, the distillation generated Sn vapour, which is up to 0.38‰ per mass unit different from the starting material, the largest induced Sn fractionation reported to date. Accordingly, the HI method is not recommended for cassiterite preparation for Sn isotopic analysis.  相似文献   

The statistics of a passive scalar in field-guided magnetohydrodynamic turbulence     

J. Mason  S. Boldyrev  F. Cattaneo  J.C. Perez 《地球物理与天体物理流体动力学》2014,108(6):686-695

A variety of studies of magnetised plasma turbulence invoke theories for the advection of a passive scalar by turbulent fluctuations. Examples include modelling the electron density fluctuations in the interstellar medium, understanding the chemical composition of galaxy clusters and the intergalactic medium, and testing the prevailing phenomenological theories of magnetohydrodynamic turbulence. While passive scalar turbulence has been extensively studied in the hydrodynamic case, its counterpart in MHD turbulence is significantly less well understood. Herein we conduct a series of high-resolution direct numerical simulations of incompressible, field-guided, MHD turbulence in order to establish the fundamental properties of passive scalar evolution. We study the scalar anisotropy, establish the scaling relation analogous to Yaglom’s law, and measure the intermittency of the passive scalar statistics. We also assess to what extent the pseudo Alfvén fluctuations in strong MHD turbulence can be modelled as a passive scalar. The results suggest that the dynamics of a passive scalar in MHD turbulence is considerably more complicated than in the hydrodynamic case.  相似文献   

Submicron-sized fluid inclusions and distribution of hydrous components in jadeite,quartz and symplectite-forming minerals from UHP jadeite–quartzite in the Dabie Mountains,China: TEM and FTIR investigation     

Dawei Meng  Xiuling Wu  Xiaoyu Fan  Xin Meng  Jianping Zheng  Roger Mason 《Applied Geochemistry》2009

Fluid inclusions and clusters of water molecules at nanometer-to submicron-scale in size have been investigated using transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) in jadeite, quartz and symplectite aegirine–augite, albite, taramite and magnetite corona minerals from ultrahigh-pressure (UHP) jadeite–quartzite at Shuanghe, the Dabie Mountains, China. Fluid inclusions from 0.003 μm to 0.78 μm in size occur in jadeite and quartz crystals, and a small number of fluid inclusions from 0.001 μm to 0.25 μm have also been detected in symplectite-forming minerals. Most of the fluid inclusions have round or negative crystal morphology and contain aqueous fluids, but some contain CO₂-rich fluids. They are usually connected to dislocations undetectable at an optical scale. The dislocations represent favorable paths for fluid leakage, accounting for non-decrepitation of most fluid inclusions when external pressure decreased at later stages, although there was partial decrepitation of some fluid inclusions unconnected to defect microstructures resulting from internal overpressure. Non-decrepitation and partial decrepitation of fluid inclusions resulted in changes of original composition and/or density. It is clear that identification of hidden re-equilibration features has significant implications for the petrological interpretation of post-peak metamorphic processes. Micro-FTIR results show that all jadeite and quartz samples contain structural water occurring as hydroxyl ions (OH⁻) and free water (H₂O) in the form of clusters of water molecules. The H₂O transformed from OH⁻ during exhumation and could have triggered and enhanced early retrograde metamorphism of the host rocks and facilitated plastic deformation of jadeite and quartz grains by dislocation movement, and thus the H₂O released during decompression might represent early-stage retrograde metamorphic fluid. The nominally anhydrous mineral (NAM) jadeite is able to transport aqueous fluids in concentrations of at least several hundred ppm water along a subduction zone to mantle depths in the form of clusters of water molecules and hydroxyl ions within crystals.  相似文献   

Ejecta fragmentation in impacts into gypsum and water ice     

K. Miljkovi?  N.J. Mason 《Icarus》2011,214(2):739-747

Using the light gas gun at the Open University’s Hypervelocity Impact facility, a series of impact experiments exploring impacts into water ice and gypsum have been performed. Fragmentation of solid ejecta was recorded using two different methods, analysed and compared with the total ejecta. Preliminary results show that the size distribution of the ejecta fragments from water ice is very similar to those from gypsum. These results also represent a step towards a better understanding of ejecta fragmentation in geological materials, including icy surfaces in the Solar System.  相似文献