Mima-type mounds in southwest Missouri: Expressions of point-centered and locally thickened biomantles   总被引：1，自引：0，他引：1  

Jennifer L. Horwath  Donald L. Johnson 《Geomorphology》2006,77(3-4):308

Mima-type mounds, formed in gravelly soils of the Diamond Grove Prairie Natural Area near Joplin, Missouri, are the focus of this study. Emphasis is on the spatial and morphological aspects of the mounds, and more particularly on the analysis of mound soils and gravel distributions as a means for shedding light on mound origins in this region. The results strongly suggest that hierarchically dominant point-centered bioturbation by small vertebrates is the mode of mound genesis. Pocket gophers (Geomys bursarius), aided by other biota, create mounds as they burrow in residual gravelly soils that have evolved dense, relatively impermeable claypans that perch water during wet periods. Although pocket gophers do not presently inhabit the Diamond Grove area, evidence of past occupation, along with laboratory and field data, support them as the dominant role in forming the mounds. We conclude that these mounds are expressions of point-centered and locally thickened biomantles. Various subsidiary processes such as aeolian inputs, water erosion, and physical and chemical weathering also have genetically impacted Diamond Grove mounds.  相似文献   

Spatial and temporal differences of Atlantic menhaden (Brevoortia tyrannus) recruitment across major drainages (1966–2004) of the Chesapeake Bay watershed     

Joseph W. Love  Andrea K. Johnson  Eric B. May 《Estuaries and Coasts》2006,29(5):794-801

Atlantic menhaden (Brevoortia tyrannus) is well known for its commercial and ecological importance and has been historically declining in the Chesapeake Bay (Maryland), one of its principal nursery habitats along the eastern coast. Using data from the Striped Bass Seine Survey of the Maryland Department of Natural Resources (2003), we evaluated how the distribution of Atlantic menhaden has changed from 1966 to 2004 for 12 river drainages. We observed significant or marginally significant declines in 42% of the drainages, with drainages of the northern Bay showing the majority of those declines. Continued recruitment to several drainages of the Bay may partly explain why the adult spawning population is not declining. We determined if temporal changes in abundance were related to changes in salinity or water quality for five major drainages of the watershed. For one of these drainages, the Patuxent River, differences in productivity across sites largely explained differences in abundance. For the four remaining drainages, differences in recruitment could not be explained by productivity or salinity gradients. While reducing nitrogen loading and enhancing water clarity may improve Atlantic menhaden production, we suggest that the role of offshore processes on large-scale declines has been largely neglected and studies on larval ingression are necessary for further elucidation of spatial and temporal patterns of juvenile distribution in the Chesapeake Bay.  相似文献   

Seagrass patch characteristics alter direct and indirect interactions in a tritrophic estuarine food web     

Matthew W. Johnson  Kenneth L. Heck 《Estuaries and Coasts》2006,29(3):499-510

We used a mesocosm approach to examine how_ratch characteristics influenced predation and habitat selection in a tritrophic food web. Our experiments included juvenile red drum (Sciaenops ocellatus; RD), juvenile pinfish (Lagodon rhomboides; PF), and grass shrimp (Palaemonetes sp.; GS), members of a food web common in seagrass meadows of the northern Gulf of Mexico. We added an additional level of complexity to the experiment by including a predator that could feed at two different trophic levels. RD were top predators, PF were both prey items for RD and predators of GS, and GS were prey for RD and PF. We used 4 different artificial seagrass habitats that varied by size (0.049 and 0.203 m²) and shape (circular and stellate) to control for covariation between patch size and seagrass density. Predation on GS was measured in each habitat when PF, RD, and PF+RD were present, and predation on PF was measured when RD and RD+GS were present. Habitat selection by each of these 3 species was measured individually and in the presence of every other combination of the 3 species. Neither predation nor habitat selection were consistently influenced by patch characteristics (size, shape, or perimeter: area ratios) or the number of trophic levels. For GS, there was a significant negative relationship between patch size and predation rates in the GS+PF+RD treatment. Habitat selection by GS without the threat of predation suggested a preference for smaller habitats, but when in the presences of RD or RD+PF, GS preferred larger habitats. In predation experiments, PF predation by RD showed no significant relationships with patch characteristics or trophic structure. For our habitat selection experiments, PF preference was for larger habitats in the PF only and GS+PF+RD treatments. There were no significant relationships between patch size, shape, or trophic structure and RD habitat selection.  相似文献   

Dissolved organic carbon in ridge-axis and ridge-flank hydrothermal systems     

Susan Q. Lang  David A. Butterfield  H. Paul Johnson 《Geochimica et cosmochimica acta》2006,70(15):3830-3842

The circulation of hydrothermal fluid through the upper oceanic crustal reservoir has a large impact on the chemistry of seawater, yet the impact on dissolved organic carbon (DOC) in the ocean has received almost no attention. To determine whether hydrothermal circulation is a source or a sink for DOC in the oceans, we measured DOC concentrations in hydrothermal fluids from several environments. Hydrothermal fluids were collected from high-temperature vents and diffuse, low-temperature vents on the basalt-hosted Juan de Fuca Ridge axis and also from low-temperature vents on the sedimented eastern flanks. High-temperature fluids from Main Endeavour Field (MEF) and Axial Volcano (AV) contain very low DOC concentrations (average = 15 and 17 μM, respectively) compared to background seawater (36 μM). At MEF and AV, average DOC concentrations in diffuse fluids (47 and 48 μM, respectively) were elevated over background seawater, and high DOC is correlated with high microbial cell counts in diffuse fluids. Fluids from off-axis hydrothermal systems located on 3.5-Ma-old crust at Baby Bare Seamount and Ocean Drilling Program (ODP) Hole 1026B had average DOC concentrations of 11 and 13 μM, respectively, and lowered DOC was correlated with low cell counts. The relative importance of heterotrophic uptake, abiotic sorption to mineral surfaces, thermal decomposition, and microbial production in fixing the DOC concentration in vent fluids remains uncertain. We calculated the potential effect of hydrothermal circulation on the deep-sea DOC cycle using our concentration data and published water flux estimates. Maximum calculated fluxes of DOC are minor compared to most oceanic DOC source and sink terms.  相似文献   

MODES-2D—Method of detecting strike-shift shear zones     

Jacob H. Pyper Griffiths  Arvid M. Johnson   《Tectonophysics》2006,422(1-4):129-148

One of the goals of using the Global Positioning System (GPS) and other geodetic survey techniques in tectonics has been to detect boundaries such as faults or shear zones between rigid or mildly deforming crustal masses. The calculation of infinitesimal strains and rotations with GPS data has been widely used to detect shear zones but it has been largely unsuccessful because infinitesimal strain and rotation, although useful in many other ways, is non-diagnostic of shear zones or faults. Our approach is to work with components of deformation, not strain, and to design specifically a diagnostic method of detecting shear zones. This paper introduces the first part of our method, the detection of two-dimensional, strike-shift shear zones (MODES-2D). The MODES-2D method has three elements: (1) determination of the orientation of a suspected strike-shift shear zone by analyzing components of a deformation tensor derived from a data set of displacements in an arbitrary coordinate system; (2) resolution of the deformation tensor into the coordinate system parallel and normal to the detected shear zone; and (3) exploration of the resolved data set for evidence for a belt of inhomogeneous deformation, which is an essential characteristic of a shear zone. The operation of MODES-2D is illustrated herein with a theoretical survey network across an ideal shear zone developed with a buried dislocation-fault and with a survey network afforded by the crossing of the Kaynaşlı viaduct by the 1999 Düzce–Bolu earthquake rupture in Turkey.  相似文献   

Close linkage of copper (and uranium) transport to diagenetic reddening of “upstream” basin sediments for sediment-hosted stratiform copper (and roll-type uranium) mineralization     

Alex C. Brown   《Journal of Geochemical Exploration》2006,89(1-3):23

Copper and uranium may be closely associated metals in sedimentary basins in which “upstream” sediments have been diagenetically reddened (e.g., sediment-hosted stratiform copper (SSC) deposits and roll-type uranium deposits) and the immediate hosts of mineralization are adjacent reduced sediments. The timing of leaching is closely linked to the process and timing of reddening, with descending meteoric water providing the oxygen needed for the reddening process. To leach and transport copper, the low-temperature pore solution must evolve to a brine, and importantly, its Eh must decrease to moderately positive levels (0.1 ± 0.1 V). For uranium, a simple oxidized solution is sufficient. Given the parallel Eh paths for copper and uranium and their close associations with diagenetic reddening, the dominant metal in sediment-hosted stratiform copper or roll-type uranium deposits is probably related closely to the source mineral and/or rock constituents of the reddened sediments.  相似文献   

Source constraints of Tungurahua volcano explosion events     

Mario C. Ruiz  Jonathan M. Lees  Jeffrey B. Johnson 《Bulletin of Volcanology》2006,68(5):480-490

The most recent eruptive cycle of Tungurahua volcano began in May 2004, and reached its highest level of activity in July 2004. This activity cycle is the last one of a series of four cycles that followed the reawakening and major eruption of Tungurahua in 1999. Between June 30 and August 12, 2004, three temporary seismic and infrasonic stations were installed on the flanks of the volcano and recorded over 2,000 degassing events. The events are classified by waveform character and include: explosion events (the vast majority, spanning three orders of pressure amplitudes at 3.5 km from the vent, 0.1–180 Pa), jetting events, and sequences of repetitive infrasonic pulses, called chugging events. Travel-time analysis of seismic first arrivals and infrasonic waves indicates that explosions start with a seismic event at a shallow depth (<200 m), followed ∼1 s later by an out-flux of gas, ash and solid material through the vent. Cluster analysis of infrasonic signals from explosion events was used to isolate four groups of similar waveforms without apparent correlation to event size, location, or time. The clustering is thus associated with source mechanism and probably spatial distribution. Explosion clusters do not exhibit temporal dependence.  相似文献   

Determination of porosity and saturation using seismic waveform inversion     

H. Keers  L. Johnson  D. Vasco 《Studia Geophysica et Geodaetica》2007,51(1):119-140

Characterization of a reservoir model requires determination of its petrophysical parameters, such as porosity and saturation. We propose a new method to determine these parameters directly from seismic data. The method consists of the computation and inversion of seismic waveforms. A high frequency method is presented to model wave propagation through an attenuative and dispersive poroelastic medium. The high frequency approximation makes it possible to efficiently compute sensitivity functions. This enables the inversion of seismic waveforms for porosity and saturation. The waveform inversion algorithm is applied to two laboratory crosswell datasets of a water saturated sand. The starting models were obtained using travel time tomography. The first dataset is inverted for porosity. The misfit reduction for this dataset is approximately 50%. The second dataset was obtained after injection of a nonaqueous-phase liquid (NAPL), possibly with some air, which made the medium more heterogeneous. This dataset was inverted for NAPL and air saturation using the porosity model obtained from the first inversion. The misfit reduction of the second experiment was 70%. Regions of high NAPL and high air saturation were found at the same location. These areas correlate well with the position of one of the injection points as well as regions of higher NAPL concentrations found after excavation of the sand. It is therefore possible to directly invert waveforms for pore fluid saturation by taking into account the attenuation and dispersion caused by the poroelasticity.  相似文献   

Ground Water Flow Modeling with Sensitivity Analyses to Guide Field Data Collection in a Mountain Watershed   总被引：3，自引：0，他引：3  

Raymond H. Johnson 《Ground Water Monitoring & Remediation》2007,27(1):75-83

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Magnetic-field accumulation in supergranules     

Alfred Clark Jr.  H. Kevin Johnson 《Solar physics》1967,2(4):433-440

Some theoretical models illustrating the growth of magnetic fields around the boundaries of supergranulation cells are given. The evolution of a magnetic field in a prescribed velocity field is calculated for the idealized case of hexagonal cells. The calculations are carried out both for a perfect conductor and a fluid of finite conductivity. The theory is developed in such a way as to be virtually free of specific assumptions about the depth dependence of the supergranulation flow. The results demonstrate the tendency of the field to accumulate at those boundary points where the flow is most strongly converging.  相似文献