Structure and Stability of Non-Transform Discontinuities on the Mid-Atlantic Ridge between 24° N and 30° N     

Sara Spencer  Deborah K. Smith  Johnson R. Cann  Jian Lin  Edward McAllister 《Marine Geophysical Researches》1997,19(4):339-362

Observations of the median valley within the 24–30° N area ofthe Mid-Atlantic Ridge (MAR), using the IOSDL high resolutionside-scan sonar instrument TOBI, image four separate areas of themedian valley, containing part or all of nine spreading segments, and fivenon-transform discontinuities between spreading segments (NTDs).These high resolution side scan images were interpreted in parallel withmultibeam bathymetry (Purdy et al., 1990), giving a greater degree ofstructural precision than is possible with the multibeam data alone. Threedistinct types of NTD were identified, corresponding in part to typespreviously identified from the multibeam bathymetric survey of the area.Type 1 NTDs are termed septal offsets, and are marked by a topographic ridgeseparating the two spreading segments. The offset between the spreadingsegments ranges from 9 to 14 km. These can be further subdivided into Type1A in which the septa run parallel to the overall trend of the MAR and Type1B in which the septa lie at a high angle to the bulk ridge trend. Type 1ANTDs are characterised by overlap of the neovolcanic zones of the segmentson each side, and strong offaxis traces, while Type 1B NTDs show no overlapof neovolcanic zones, and weak offaxis traces. Type 2 NTDs arebrittle/ductile extensional shear zones, marked by oblique extensionalfractures, and associated with rotation of tectonic and volcanic structuresaway from the overall trend of the MAR. Type 3 NTDs are associated withoffsets of less than 5 km, and show no sign of any accommodating structure.In this type of NTD, the offset zone is covered with undeformed volcanics.The type of NTD developed at any locality along the ridge axis appears todepend on the amount of segment offset and segment overlap, the overalltrend of the mid-ocean ridge, the width of the zone of discontinuity, themedian valley offset and the longevity of the offset. These factorsinfluence the mechanical properties of the lithosphere across thediscontinuity, and ultimately the tectonic style of the NTD that can besupported. Thus brittle/ductile extensional shear zones are long-livedstructures favoured by large segment offsets, and small or negative segmentoverlaps. Septa can be short or long lived, and are associated with largesegment offsets. Segment overlaps vary from negative (an along axis gap) tozero, for Type 1B septal offsets, or positive to zero for Type 1A septaloffsets. Non-tectonised NTDs are generally short lived structures,characterised by small segment offsets and zero or positive overlaps.  相似文献   

Fault scarp identification in side-scan sonar and bathymetry images from the Mid-Atlantic Ridge using wavelet-based digital filters     

Sarah A. Little  Deborah K. Smith 《Marine Geophysical Researches》1996,18(6):741-755

Digital filters designed using wavelet theory are applied to high resolution deep-towed side-scan sonar data from the median valley walls, crestal mountains, and flanks of the Mid-Atlantic Ridge at 29°10 N. With proper tuning, the digital filters are able to identify the location, orientation, length, and width of highly reflective linear features in sonar images. These features are presumed to represent the acoustic backscatter from axis-facing normal faults. The fault locations obtained from the digital filters are well correlated with visual geologic interpretation of the images. The side-scan sonar images are also compared with swath bathymetry from the same area. The digitally filtered bathymetry images contain nine of the eleven faults identified by eye in the detailed geologic interpretation of the side-scan data. Faults with widths (measured perpendicular to their strike) of less than about 150 m are missed in the bathymetry analysis due to the coarser resolution of these data. This digital image processing technique demonstrates the potential of wavelet-based analysis to reduce subjectivity and labor involved in mapping and analyzing topographic features in side-scan sonar and bathymetric image data.  相似文献   

Cadmium, copper, and zinc interactions with marine humus as a function of ligand structure     

Stephen R. Piotrowicz  George R. Harvey  Deborah A. Boran  Clifford P. Weisel  M. Springer-Young 《Marine Chemistry》1984,14(4):333-346

The interactions in seawater of cadmium, copper, and zinc with natural levels of marine humus (fulvic and humic acids) were studied using differential pulse anodic stripping voltammetry (DPASV). Interactions with Cu were also examined by ultraviolet spectrophotometry. The degree of interaction relates to the structure of the particular fulvic or humic acid tested. Marine fulvic and humic acids with different equivalent weights and varying degrees of aromaticity and heteroatom substitution were tested to assess the relative importance of these fundamental structural features in determining the degree of interaction. An index of heteroatom substitution, derived from proton magnetic resonance spectra of these materials, when combined with equivalent weights correlated with the nature and degree of metal interaction. Both marine fulvic and humic acids appreciably interacted with Zn. In contrast, the extent of interaction of fulvic acids with Cu appears to be related to their structure while humic acids appear to interact with Cu regardless of structure. Cadmium interacts only with mature, highly crosslinked humic acids.  相似文献   

Temporal and spatial dynamics of urea uptake and regeneration rates and concentrations in Chesapeake Bay     

Michael?W.?LomasEmail author  T.?Mark Trice  Patricia?M.?Glibert  Deborah?A.?Bronk  James?J.?McCarthy 《Estuaries and Coasts》2002,25(3):469-482

We examined the temporal and spatial variability of urea concentrations and urea uptake and regeneration rates collected on cruises along the longitudinal axis of the Chesapeake Bay between 1972 and 1998. Interannually, mean Bay-wide surface urea concentrations ranged between 0.49 and 0.91 μg-at N l^?1 with a nearly 50% decrease in surface concentrations observed between 1988 and 1998. Concentrations of urea from samples collected within ～1 m of the bottom were generally higher and much more varable than surface samples. Seasonally, two different patterns were observed in mean Bay-wide surface urea concentrations. Urea concentrations from near surface waters exhibited a clear summer peak for 1988 through 1994, while for 1973 and 1996 to 1998 a distinct winter-spring peak in concentration was observed. Urea concentrations from deeper waters showed a similar seasonal trend each year with peak concentrations measured in spring. Spatially, urea concentrations in the surface waters decreased in a conservative-type pattern from 0.91 μg-at N I^?1 at the freshwater end member to 0.46 μg-at N I^?1 at the ocean end member. Mean Bay-wide surface urea uptake rates displayed a seasonal pattern throughout the data set with maximum uptake rates (up to 0.33 μg-at N I^?1 h^?1) consistently observed during summer. Mean Bay-wide surface regeneration rates were highest but most variable during fall (1.63±0.82 μg-at N I^?1 h^?1). Mean urea uptake and regeneration rates displayed opposing spatial trends along the axis of the Bay with uptake rates being lowest in the North Bay where regeneration rates were highest. The average temporal and spatial patterns of urea concentration in Chesapeake Bay appear to reflect a balance between external inputs and internal biological recycling.  相似文献   

Geostandards and Geoanalytical Research     

Philip J. Potts  Mireille Polv 《Geostandards and Geoanalytical Research》2003,27(3):213-213

  相似文献   

International Association of Geoanalysts'Protocol for the Certification of Geological and Environmental Reference Materials   总被引：2，自引：1，他引：2  

Jean S. Kane  Philip J. Potts  Michael Wiedenbeck  Jean Carignan  Stephen Wilson 《Geostandards and Geoanalytical Research》2003,27(3):227-244

This protocol has been developed by the International Association of Geoanalysts to demonstrate procedures for the certification of geological and environmental reference materials to comply to the fullest extent possible with recommendations of the International Organisation for Standardisation (ISO Guide 35). A practical approach is described on the assumption that certifications will be normally be based on collaborative analysis programmes in which participating laboratories are preselected on the basis of performance in a proficiency testing programme or on the basis of other criteria of merit.  相似文献   

Evolution of the modern Nile delta promontories: development of accretional features during shoreline retreat   总被引：1，自引：0，他引：1  

Omran Frihy  Deborah Lawrence 《Environmental Geology》2004,46(6-7):914-931

The active accretional features that have developed along the modern Nile Delta promontories during shoreline retreat are analysed using topographic maps, remote imagery, ground and hydrographic surveys, together providing 15 time-slice maps (1922–2000) at Rosetta and 14 time-slice maps (1909–2000) at Damietta. Small double sandy spits developed and persisted at Rosetta between 1986 and 1991. At Damietta, a much larger single spit, 9 km long, formed approximately east of the mouth of the Damietta Nile branch between 1955 and 1972, although its source has now been depleted. Both the Rosetta and Damietta inlets are associated with submerged mouth bars that accumulated prior to the damming of the Nile, but that continue to contribute to local sedimentation problems, particularly at Rosetta. The development of the active accretional features along the Nile promontories reflects a combination of factors including sediment availability, transport pathways from source areas, a decrease in the magnitude of Nile flood discharges, as well as the impact of protective structures at the river mouths.  相似文献   

Capitalism, masculinity and whiteness in the dialectical landscape: The case of Tarzan and the Tycoon     

Deborah Dixon  John Grimes 《GeoJournal》2004,59(4):265-275

The intent of this paper is to explore how ‘reel’ and ‘real’ landscapes can be rendered dialectic. The analysis focuses on the filming of Tarzan's Secret Treasure (1941) at the wildlife preserve of Wakulla Springs in Florida, which was administered by the financial tycoon Ed Ball. Using Mitchell's recent commentaries on ‘good’ and ‘bad’ dialectics as our entry point, we offer two readings of this cinematic process. The first renders the on and off screen Wakulla Springs as ideological fodder, serving to valorise capital, masculinity and whiteness. The second reading addresses how this place has been actively constructed from discourses pertaining to class, gender and race, such that Wakulla exists as a series of fully relational spaces within diverse contexts, both ‘real’ and ‘reel’. In structuring our analysis in this manner, we hope to make clear why it does indeed matter for film geographers to acknowledge the conceptual presumptions brought to bear in the framing of the real and the reel as objects of analysis. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

A Regionally Based Energy End‐Use Strategy: Case Studies from Centre County,Pennsylvania*     

Deborah Feder 《The Professional geographer》2004,56(2):185-200

The production and use of fossil fuels and nonrenewable electricity creates many forms of environmental degradation. To reduce degradation, this research suggests an energy strategy based on energy end‐use analysis and regional geography. Energy end‐use analysis and regional geography are used to match renewable energy resources with site‐specific, end‐use needs. Fieldwork conducted within Centre County, Pennsylvania, demonstrates that small‐scale solar, wind, and micro‐hydropower resources could displace a proportion of household electricity use. Such an approach meets energy end‐use needs, while conserving fossil fuels and reducing environmental degradation.  相似文献   

Comparison of precipitation and land runoff as sources of estuarine nitrogen     

David L. Correll  Deborah Ford 《Estuarine, Coastal and Shelf Science》1982,15(1):45-56

The seasonal and year-to-year variation of bulk precipitation as a source of total N, nitrate and ammonia is reported for a 7-year period at the Rhode River, a subestuary of Chesapeake Bay, U.S.A. These values are placed in perspective by comparison with analogous nitrogen loading due to watershed discharges.Although year-to-year variations were important, in an average year about the same amount of readily available nitrogen entered the system via precipitation as entered in land runoff and, during the summer and fall, precipitation was the largest source.  相似文献