The potential of site-specific and local chironomid-based inference models for reconstructing past lake levels     

Joshua Kurek  Les C. Cwynar 《Journal of Paleolimnology》2009,42(1):37-50

We examined the relationship between three key environmental variables (water depth, loss-on-ignition, and bottom-water temperature) and fossil chironomid distributions sampled from within-lake gradients in three small, moderately deep (18–35 m), maar lakes on St Michael Island, western Alaska. Site-specific (one lake, 29 samples) and local (three lakes, 87 samples) inference models for reconstructing water depth were developed using partial least squares regression and calibration. These models and a previously published regional model (136 lakes, one central-lake sample from each) are used to infer water depths from 78 fossil samples spanning the last ~30,000 ¹⁴C years B.P. at Zagoskin Lake. Although the site-specific [r ² _boot = 0.90, root mean square error of prediction (RMSEP) = 1.76] and local (r _boot² = 0.68, RMSEP = 4.36) inference models have better performance statistics than the regional model, few clear trends among all three models exist in the lake-level reconstruction. We propose that multiple, within-lake sampling of gradients can be used to improve the performance statistics of water-depth transfer functions and ultimately reconstruct paleohydrology in regions known to exhibit large fluctuations in moisture balance through time given that: (1) adequate analogs are established and (2) taphonomic processes important to benthic invertebrate remains are more fully understood.  相似文献   

A fully coupled method for massively parallel simulation of hydraulically driven fractures in 3‐dimensions          下载免费PDF全文

Randolph R. Settgast  Pengcheng Fu  Stuart D.C. Walsh  Joshua A. White  Chandrasekhar Annavarapu  Frederick J. Ryerson 《国际地质力学数值与分析法杂志》2017,41(5):627-653

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Joint isotopic mass balance: a novel approach to quantifying channel bed to channel margins sediment transfer during storm events          下载免费PDF全文

John W. Underwood  Carl E. Renshaw  Francis J. Magilligan  W. Brian Dade  Joshua D. Landis 《地球表面变化过程与地形》2015,40(12):1563-1573

The important role of floodplains and the broader riparian zone in providing temporary storage for a large fraction of the annual sediment load of rivers is well established, but this understanding is largely based on observations of the long‐term average behavior of the catchment. Here we combine measurements of the fallout radionuclides ⁷Be and ²¹⁰Pb and the stable isotopes of hydrogen in water to quantify fine sediment mobilization and storage in a stream and its channel margins during individual intermediate‐sized storm events with recurrence intervals of a few months or less. We demonstrate this method using five storm events in a small (~15 km²), undeveloped, gravel‐bedded tributary of the Connecticut River (USA). We estimate that in each storm, the mass of sediment deposited onto the margins accounts for almost 90% of the sediment mobilized from the bed, with the remainder of the mobilized bed sediment transported downstream as suspended load. The result that the bed is a net source of sediment to the stream and the margins a net sink is robust, but estimates of the mass of material eroded from the bed and deposited on the margins are less certain. The source of sediment to the bed remains unclear as, consistent with earlier studies, we observe only limited deposition of sediment to the bed during the storm events. The suspended sediment is organic‐rich and thus its source may be associated with in‐channel organic decay between storm events. Understanding the coupled interactions between discharge magnitude and frequency and sediment resupply at the event time scale has important implications for stream restoration efforts seeking to connect the channel and the broader riparian zone, and for the development of accurate sediment budgets and predictions of sediment flux from a watershed. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

On the occurrence,trace element geochemistry,and crystallization history of zircon from in situ ocean lithosphere   总被引：10，自引：2，他引：8  

Craig B. Grimes  Barbara E. John  Michael J. Cheadle  Frank K. Mazdab  Joseph L. Wooden  Susan Swapp  Joshua J. Schwartz 《Contributions to Mineralogy and Petrology》2009,158(6):757-783

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a _TiO2 ≈ 0.7, a _SiO2 ≈ 1.0, pressure ≈ 2 kbar); intra-sample variation is typically ~60–150°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680–740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)_N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.  相似文献   

Electrostatic dust transport on Eros: 3-D simulations of pond formation     

Anna L.H. Hughes  Joshua E. Colwell 《Icarus》2008,195(2):630-648

NEAR-Shoemaker spacecraft images of the surface of the near-Earth Asteroid 433 Eros reveal that more than 200 craters on Eros are partially filled with smooth deposits, termed ponds [Veverka, J., and 32 colleagues, 2001a. Science 292, 484-488]. These ponds appear smooth even at a high resolution of 1.2 cm/pixel and spectral analysis suggests that they may be made up of particles ?50 μm in size [Robinson, M.S., Thomas, P.C., Veverka, J., Murchie, S., Carcish, B., 2001. Nature 413, 396-400; Riner, M.A., Eckart, J.M., Gigilio, J.G., Robinson, M.S., 2006. Lunar Planet. Sci. XXXVII. Abstract 2291]. Coupled with the concentration of ponds at low latitudes, the possible small particle size suggests that these deposits might be related to electrostatic transport of dust near the local terminator [Robinson, M.S., Thomas, P.C., Veverka, J., Murchie, S., Carcish, B., 2001. Nature 413, 396-400]. The work presented here incorporates the precise lighting geometry within a crater at a specified latitude into two models for electrostatic transport of dust grains in order to explore dust deposition and pond formation via this mechanism, particularly as a function of latitude. We find that micrometer-sized dust particles are preferentially transported into craters at latitudes where solar illumination angles are often low. In addition we find that if particles are electrostatically lifted off the surface they are preferentially transported into topographic depressions independent of whether the particles undergo stable levitation. The primary limiting factor for our model is uncertainty concerning the dust launching mechanism. Despite that, and though it does not match the observed north-south asymmetry in pond distribution, our model demonstrates potential for good general agreement between future predictions of pond formation via electrostatic transport of dust and observations of pond locations on the surface of Eros.  相似文献   

Structural health monitoring of bridges in the State of Connecticut     

Joshua Olund  Alan Cardini  Paul D''Attilio  Eric Feldblum  John DeWolf 《地震工程与工程振动(英文版)》2008,7(4):427-437

A joint effort between the Connecticut Department of Transportation and the University of Connecticut has been underway for more than 20 years to utilize various structural monitoring approaches to assess different bridges in Connecticut. This has been done to determine the performance of existing bridges, refine techniques needed to evaluate different bridge components, and develop approaches that can be used to provide a continuous status of a bridge＇s structural integrity. This paper briefly introduces the background of these studies, with emphasis on recent research and the development of structural health monitoring concepts. This paper presents the results from three different bridge types： a post-tensioned curved concrete box girder bridge, a curved steel box-girder bridge, and a steel multi-girder bridge. The structural health monitoring approaches to be discussed have been successfully tested using field data collected during multi-year monitoring periods, and are based on vibrations, rotations and strains. The goal has been to develop cost-effective strategies to provide critical information needed to manage the State of Connecticut＇s bridge infrastructure.  相似文献   

Hydrochemical evidence for mixing of river water and groundwater during high-flow conditions, lower Suwannee River basin, Florida, USA     

Christy A. Crandall  Brian G. Katz  Joshua J. Hirten 《Hydrogeology Journal》1999,7(5):454-467

 Karstic aquifers are highly susceptible to rapid infiltration of river water, particularly during periods of high flow. Following a period of sustained rainfall in the Suwannee River basin, Florida, USA, the stage of the Suwannee River rose from 3.0 to 5.88 m above mean sea level in April 1996 and discharge peaked at 360 m³/s. During these high-flow conditions, water from the Suwannee River migrated directly into the karstic Upper Floridan aquifer, the main source of water supply for the area. Changes in the chemical composition of groundwater were quantified using naturally occurring geochemical tracers and mass-balance modeling techniques. Mixing of river water with groundwater was indicated by a decrease in the concentrations of calcium, silica, and ²²²Rn; and by an increase in dissolved organic carbon (DOC), tannic acid, and chloride, compared to low-flow conditions in water from a nearby monitoring well, Wingate Sink, and Little River Springs. The proportion (fraction) of river water in groundwater ranged from 0.13 to 0.65 at Wingate Sink and from 0.5 to 0.99 at well W-17258, based on binary mixing models using various tracers. The effectiveness of a natural tracer in quantifying mixing of river water and groundwater was related to differences in tracer concentration of the two end members and how conservatively the tracer reacted in the mixed water. Solutes with similar concentrations in the two end-member waters (Na, Mg, K, Cl, SO₄, SiO₂) were not as effective tracers for quantifying mixing of river water and groundwater as those with larger differences in end-member concentrations (Ca, tannic acid, DOC, ²²²Rn, HCO₃). Received, March 1999 / Revised, July 1999 / Accepted, July 1999  相似文献   

Weak gravitational lensing by voids     

Luca Amendola  Joshua A. Frieman  Ioav Waga 《Monthly notices of the Royal Astronomical Society》1999,309(2):465-473

We consider the prospects for detecting weak gravitational lensing by underdensities (voids) in the large-scale matter distribution. We derive the basic expressions for magnification and distortion by spherical voids. Clustering of the background sources and cosmic variance are the main factors that limit in principle the detection of lensing by voids. We conclude that only voids with radii larger than ∼100  h ⁻¹ Mpc have lensing signal-to-noise ratio larger than unity.  相似文献   

Urban Perennials: How Diversification has Created a Sustainable Community Garden Movement in The United States     

Joshua Birky  Elizabeth Strom 《Urban geography》2013,34(8):1193-1216

Our research seeks to understand how the contemporary community garden movement in the United States differs from its predecessors and whether its new foundation increases the political and financial sustainability of today's gardens. To this end, this article reviews historical and contemporary literature, and surveys participants in three distinctive community gardens, to answer several related research questions. First, what is the current state of the US community garden movement, and how have its historical roots shaped its response to current policy concerns? Second, which populations comprise today's community gardeners, and what motivates them to participate? After considering the findings from our three study gardens, and from a review of secondary and “gray” literature, we conclude by positing that a greater diversity of gardeners and gardening motivations, accompanied by changing leading views on urban land uses, will lead to such urban gardens remaining a more lasting feature of city neighborhood landscapes.  相似文献   

Place Perception and Social Interaction on an Exurban Landscape in Central Colorado*   总被引：1，自引：0，他引：1  

Soren C. Larsen  Curt Sorenson  Dave McDermott  Joshua Long  Chris Post 《The Professional geographer》2007,59(4):421-433

An assumption commonly made in research on exurbanization is that the subdivision of land ownership necessarily creates a socially divisive landscape devoid of shared place perceptions and concerns. This article presents the results of an ethnography project undertaken in a former Colorado ranching valley that is currently exurbanizing. The study revealed not only the emergence of social networks and groups among new residents, but also the presence of distinct themes in place perception and assessment within these assemblages. In particular, the results indicated that concerns about natural hazards and environmental limitations have motivated new residents to interact with one another.  相似文献