Sediment oxygen consumption and benthic nutrient fluxes on the Louisiana continental shelf: A methodological comparison     

Tina Miller-Way  Gregory S. Boland  Gilbert T. Rowe  Robert R. Twilley 《Estuaries and Coasts》1994,17(4):809-815

There has been considerable discussion but little experimental evidence regarding the comparability of in-situ and remote (shipboard or laboratory) incubations for the determination of sediment oxygen consumption and benthic nutrient flux rates. This paper presents the results of such a comparison, using in situ chamber and shipboard chemostatic systems, for a shallow station on the Louisiana, continental shelf during April 1992. Results indicated no methodological differences between rates of sediment oxygen consumption and nutrient flux (NH₄ ⁺, NO₅ ^?, NO₂ ^?, PO₄ ^3?, and SiO₂/Si(OH)₂) that could be attributed to the removal of cores from shelf sediments. This conclusion implies that subcoring from box cores is no more destructive of sediment structure and salient environmental characteristics than chamber emplacement. Differences between the methods occurred when ambient oxygen concentrations were low (<2 ml l^?1). These differences were caused by initial reaeration of bottom water in the shipboard system and reflect the sensitivity of heterotrophic metabolism, dissolution kinetics, and diffusive fluxes to low oxygen concentrations. The differences in exchange rates observed in this study reiterate the importance in maintaining ambient conditions in the experimental apparatus. The results of this study corroborate the small body of, data that addresses this issue and extends methodological similarities to include nutrient exchanges. Given the comparability of rates, use of remote chemostatic systems is more advantageous for work in shelf environments than in-situ batch methods due to increased statistical rigor, logistical convenience, and the ability to minimize changes in experimental conditions during incubations.  相似文献   

True-amplitude migration of 2D synthetic data1     

C. Hanitzsch  J. Schleicher  P. Hubral 《Geophysical Prospecting》1994,42(5):445-462

True-amplitude (TA) migration, which is a Kirchhoff-type modified weighted diffraction stack, recovers (possibly) complex angle-dependent reflection coefficients which are important for amplitude-versus-offset (AVO) inversion. The method can be implemented using existing prestack or post-stack Kirchhoff migration and fast Green's function computation programs. Here, it is applied to synthetic single-shot and constant-offset seismic data that include post-critical reflections (complex reflection coefficients) and caustics. Comparisons of the amplitudes of the TA migration image with theoretical reflection coefficients show that the (possibly complex) angle-dependent reflection coefficients are correctly estimated.  相似文献   

New constraints on the sources and behavior of neodymium and hafnium in seawater from Pacific Ocean ferromanganese crusts     

Tina van de Flierdt  Martin Frank  Alex N. Halliday  James R. Hein 《Geochimica et cosmochimica acta》2004,68(19):3827-3843

The behavior of dissolved Hf in the marine environment is not well understood due to the lack of direct seawater measurements of Hf isotopes and the limited number of Hf isotope time-series obtained from ferromanganese crusts. In order to place better constraints on input sources and develop further applications, a combined Nd-Hf isotope time-series study of five Pacific ferromanganese crusts was carried out. The samples cover the past 38 Myr and their locations range from sites at the margin of the ocean to remote areas, sites from previously unstudied North and South Pacific areas, and water depths corresponding to deep and bottom waters.For most of the samples a broad coupling of Nd and Hf isotopes is observed. In the Equatorial Pacific ε_Nd and ε_Hf both decrease with water depth. Similarly, ε_Nd and ε_Hf both increase from the South to the North Pacific. These data indicate that the Hf isotopic composition is, in general terms, a suitable tracer for ocean circulation, since inflow and progressive admixture of bottom water is clearly identifiable.The time-series data indicate that inputs and outputs have been balanced throughout much of the late Cenozoic. A simple box model can constrain the relative importance of potential input sources to the North Pacific. Assuming steady state, the model implies significant contributions of radiogenic Nd and Hf from young circum-Pacific arcs and a subordinate role of dust inputs from the Asian continent for the dissolved Nd and Hf budget of the North Pacific.Some changes in ocean circulation that are clearly recognizable in Nd isotopes do not appear to be reflected by Hf isotopic compositions. At two locations within the Pacific Ocean a decoupling of Nd and Hf isotopes is found, indicating limited potential for Hf isotopes as a stand-alone oceanographic tracer and providing evidence of additional local processes that govern the Hf isotopic composition of deep water masses. In the case of the Southwest Pacific there is evidence that decoupling may have been the result of changes in weathering style related to the buildup of Antarctic glaciation.  相似文献   

Late Quaternary deformation and slip rates in the northern San Andreas fault zone at Olema Valley, Marin County, California     

Karen Grove  Tina M. Niemi 《Tectonophysics》2005,401(3-4):231-250

Quaternary sedimentary deposits along the structural depression of the San Andreas fault (SAF) zone north of San Francisco in Marin County provide an excellent record of rates and styles of neotectonic deformation in a location near where the greatest amount of horizontal offset was measured after the great 1906 San Francisco earthquake. A high-resolution gravity survey in the Olema Valley was used to determine the depth to bedrock and the thickness of sediment fill along and across the SAF valley. In the gravity profile across the SAF zone, Quaternary deposits are offset across the 1906 fault trace and truncated by the Western and Eastern Boundary faults, whose youthful activity was previously unknown. The gravity profile parallel to the fault valley shows a basement surface that slopes northward toward an area of present-day subsidence near the head of Tomales Bay. Surface and subsurface investigations of the late Pleistocene Olema Creek Formation (Qoc) indicate that this area of subsidence was located further south during deposition of the Qoc and that it has migrated northward since then. Localized subsidence has been replaced by localized contraction that has produced folding and uplift of the Qoc. This apparent alternation between transtension and transpression may be the result of a northward-diverging fault geometry of fault strands that includes the valley-bounding faults as well as the 1906 SAF trace. The Vedanta marsh is a smaller example of localized subsidence in the fault zone, between the 1906 SAF trace and the Western Boundary fault. Analyses of Holocene marsh sediments in cores and a paleoseismic trench indicate thickening, and probably tilting, toward the 1906 trace, consistent with coseismic deformation observed at the site following the 1906 earthquake.New age data and offset sedimentary and geomorphic features were used to calculate four late Quaternary slip rate estimates for the SAF at this latitude. Luminescence dates of 112–186 ka for the middle part of the Olema Creek Formation (Qoc), the oldest Quaternary deposit in this part of the valley, suggest a late Pleistocene slip rate of 17–35 mm/year, which replaces the unit to a position adjacent to its sediment source area. A younger alluvial fan deposit (Qqf; basal age 30 ka) is exposed in a quarry along the medial ridge of the fault valley. This fan deposit has been truncated on its western side by dextral SAF movement, and west-side-down vertical movement that has created the Vedanta marsh. Paleocurrent measurements, clast compositions, sediment facies distributions, and soil characteristics show that the Bear Valley Creek drainage, now located northwest of the site, supplied sediment to the fan, which is now being eroded. Restoration of the drainage to its previous location provides an estimated slip rate of 25 mm/year. Furthermore, the Bear Valley Creek drainage probably created a water gap located north of the Qqf deposit during the last glacial maximum 18 ka. The amount of offset between the drainage and the water gap yields an average slip rate of 21–30 mm/year. Finally, displacement of a 1000-year-old debris lobe approximately 20 m from its hillside hollow along the medial ridge indicates a minimum late Holocene slip rate of 21–25 mm/year. Similarity of the late Pleistocene rates to the Holocene slip rate, and to previous rates obtained in paleoseismic trenches in the area, indicates that the rates may not have changed over the past 30 ka, and perhaps the past 200–400 ka. Stratigraphic and structural observations also indicate that valley-bounding faults were active in the late Pleistocene and suggest the need for further study to evaluate their continued seismic potential.  相似文献   

ModTech: a GIS-enabled Ground Water Modeling Program   总被引：1，自引：0，他引：1  

Tina Pint  Shu-Guang Li 《Ground water》2006,44(4):506-508

  相似文献   

Migration velocity analysis by depth image-wave remigration: first results   总被引：2，自引：0，他引：2  

J. Schleicher  A. Novais  F.P. Munerato 《Geophysical Prospecting》2004,52(6):559-573

The image‐wave equation for depth remigration is a partial differential equation that is similar to the acoustic wave equation. In this work, we study its finite‐difference solution and possible applications. The conditions for stability, dispersion and dissipation exhibit a strong wavenumber dependence. Where higher horizontal than vertical wavenumbers are present in the data to be remigrated, stability may be difficult to achieve. Grid dispersion and dissipation can only be reduced to acceptable levels by the choice of very small grid intervals. Numerical tests demonstrate that, upon reaching the true medium velocity, remigrated images of curved reflectors propagate to the correct depth and those of diffractions collapse to single points. The latter property points towards the method's potential for use as a tool for migration velocity analysis. A first application to inhomogeneous media shows that in a horizontally layered medium, the reflector images reach their true depth when the remigration velocity equals the inverse of the mean medium slowness.  相似文献   

Ground-penetrating radar investigation of active faults along the Dead Sea Transform and implications for seismic hazards within the city of Aqaba, Jordan   总被引：1，自引：0，他引：1  

Lee Slater  Tina M. Niemi   《Tectonophysics》2003,368(1-4):33

Ground-penetrating radar (GPR) was used in an effort to locate a major active fault that traverses Aqaba City, Jordan. Measurements over an exposed (trenched) cross fault outside of the city identify a radar signature consisting of linear events and horizontal offset/flexured reflectors both showing a geometric correlation with two known faults at a control site. The asymmetric linear events are consistent with dipping planar reflectors matching the known direction of dip of the faults. However, other observations regarding this radar signature render the mechanism generating these events more complex and uncertain.GPR measurements in Aqaba City were limited to vacant lots. Seven GPR profiles were conducted approximately perpendicular to the assumed strike of the fault zone, based on regional geological evidence. A radar response very similar to that obtained over the cross fault was observed on five of the profiles in Aqaba City, although the response is weaker than that obtained at the control site. The positions of the identified responses form a near straight line with a strike of 45°. Although subsurface verification of the fault by trenching within the city is needed, the geophysical evidence for fault zone location is strong. The location of the interpreted fault zone relative to emergency services, military bases, commercial properties, and residential areas is defined to within a few meters. This study has significant implications for seismic hazard analysis in this tectonically active and heavily populated region.  相似文献   

Constraining sediment provenance for tsunami deposits using distributions of grain size and foraminifera from the Kujukuri coastline and shelf,Japan     

Jessica E. Pilarczyk  Yuki Sawai  Dan Matsumoto  Yuichi Namegaya  Naohisa Nishida  Ken Ikehara  Osamu Fujiwara  Chris Gouramanis  Tina Dura  Benjamin P. Horton 《Sedimentology》2020,67(3):1373-1392

Tsunami deposits preserved in the geological record provide a more comprehensive understanding of their patterns of frequency and intensity over longer timescales; but recognizing tsunami deposits can prove challenging due to post-depositional changes, lack of contrast between the deposits and surrounding sedimentary layers, and differentiating between tsunami and storm deposition. Modern baseline studies address these challenges by providing insight into modern spatial distributions that can be compared with palaeotsunami deposits. This study documents the spatial fingerprint of grain size and foraminifera from Hasunuma Beach and the Kujukuri shelf to provide a basis from which tsunami deposits can be interpreted. At Hasunuma Beach, approximately 50 km east of Tokyo, the spatial distribution of three common proxies (foraminiferal taxonomy, foraminiferal taphonomy and sediment grain size) for tsunami identification were mapped and clustered using Partitioning Around Medoids cluster analysis. Partitioning Around Medoids cluster analysis objectively discriminated two coastal zones corresponding to onshore and offshore sample locations. Results show that onshore samples are characterized by coarser grain sizes (medium to coarse sand) and higher abundances of Pararotalia nipponica (27 to 63%) than offshore samples, which are characterized by finer grain sizes (fine to medium sand), lower abundances of Pararotalia nipponica (2 to 19%) and Ammonia parkinsoniana (0 to 10%), higher abundances of planktonics (15 to 58%) and species with fragile tests including Uvigerinella glabra. When compared to grain-size and foraminiferal taxonomy, foraminiferal taphonomy; i.e. surface condition of foraminifera, a proxy not commonly used to identify tsunami deposits, was most effective in discriminating modern coastal zones (identified supratidal, intertidal and offshore environments) and determining sediment provenance for tsunami deposits at Kujukuri. This modern baseline study assists the interpretation of tsunami deposits in the geological record because it provides a basis for sediment provenance to be determined.  相似文献   

Analyses of dust coma morphology of Comet Hyakutake (1996 B2) near perigee: outburst behavior, jet motion, source region locations, and nucleus pole orientation     

David G Schleicher  Laura M Woodney 《Icarus》2003,162(1):190-213

We present inner-coma dust imaging of Comet Hyakutake (1996 B2) obtained on 11 consecutive nights in late March 1996, an interval including a major outburst and the comet’s closest approach to Earth. The evolution of the outburst morphology is followed, along with the motion along the tail of several outburst fragments. Two spiral dust jets—a primary jet, along with a much weaker secondary jet—are visible throughout the interval and are produced by two source regions on a rotating nucleus. These are examined as a function of rotational phase and viewing geometry, with their appearance changing from a nearly face-on view on March 18 to side-on by March 28. The dust outflow velocity as a function of distance from the nucleus is derived, with the dust continuing to accelerate to a distance of 4000 km or more and reaching an average outflow velocity of 0.38 km s⁻¹ between 3000 and 8000 km. We present details of our Monte Carlo modeling of the jets and our methodology of fitting the model to the images. The modeling yields the pole orientation of the nucleus, with an obliquity of approximately 108°, corresponding to an RA of 13^h41^m and a Dec of −1.1°. For an assumed spherical nucleus, the primary active region is centered at approximately −66° latitude, has a radius of about 56°, and therefore covers about 22% of the surface. The source of the secondary jet is at a latitude of −28°, has a radius of about 16°, and is located at a longitude nearly 180° away from the primary source. Estimated uncertainties for the pole orientation and the source locations and sizes are each about 3°. This solution for the nucleus orientation and source locations explains the strong asymmetry in measured production rates before and after perihelion in radio observations (Biver et al., 1999, Astron. J. 118, 1850-1872). The modeling also tightly constrains the sidereal rotation period as 0.2618 ± 0.0001 day, completely consistent with the expected +0.0003 day difference from the observed solar rotation period of 0.2614 ± 0.0004 day determined by Schleicher and Osip (2002, Icarus 159, 210-233), given the pole orientation and position of the comet in its orbit.  相似文献   

Photometry and Imaging of Comet C/2000 Wm1 (Linear)     

Schleicher  David G.  Woodney  Laura M.  Birch  Peter V. 《Earth, Moon, and Planets》2002,90(1-4):401-403

Earth, Moon, and Planets -  相似文献