A New Video and Digital Camera System for Studies of the Dynamics of Microtopographic Features on Tidal Flats     

LARS CHRESTEN LUND-HANSEN  EINER LARSEN  KURT THOMAS JENSEN  KIM N. MOURITSEN  CHRISTIAN CHRISTIANSEN  THORBJØRN JOEST ANDERSEN 《Marine Georesources & Geotechnology》2013,31(1-2):115-122

A newly developed video and camera system for tidal flat microtopographic studies is presented. It consists of a SONY handy cam placed in an underwater housing mounted on a frame about 70?cm above the sediment surface. A rectangular surface area of 30?×40?cm is imaged by the camera. The camera records video sequences and/or digital images at predetermined time lapses. The total number of images is about 540, and a similar number of 10-second long video sequences can be recorded. The camera is programmed with a PC before deployment, and the total deployment time depends on the time lapse between recordings. The camera is connected to an external power supply (12?volt), and a halogen projector pointing towards the sediment surface ensures that the system is operable on a 24-hours scale. The system has been tested in the Danish Wadden Sea. It has proved to be a very useful tool in studies of topographic effects of erosion and deposition sequences, and for studies of benthic organisms-sediment interactions. The test site was further equipped with sensors for water and seabed measurements, which proved to be indispensable regarding the interpretation of recorded image time-series.  相似文献   

A spatial disaggregation procedure for precipitation     

THOMAS SKAUGEN 《水文科学杂志》2013,58(6):943-956

Abstract

A disaggregation procedure is presented to render forecast values of precipitation from an atmospheric model with spatial resolution of 11 × 11 km suitable as input for a distributed hydrological model with spatial resolution of 1.1 × 1.1 km. Statistical and morphological properties of the input field, such as spatial mean, variance, correlation structure and intermittency, are respected in the disaggregated field. The adopted approach is a combination of interpolation and simulation. The four nodal points of the atmospheric model grid cell are used both for determining the parameters of the exponential distribution for simulating precipitation values, and in a simple interpolation procedure to determine the spatial location of the precipitation values. A shifted distribution with two parameters is used in the case of full coverage of the grid cell, and a one-parameter distribution with a theoretically derived intermittency parameter is used if intermittency is present. The results are promising with respect to the statistical and morphological properties of the disaggregated field.  相似文献   

Sediment suspension dynamics and a new criterion for the maintenance of turbulent suspensions     

MIKE R. LEEDER  THOMAS E. GRAY  JAN ALEXANDER 《Sedimentology》2005,52(4):683-691

The vertical component of the turbulent flow acceleration term, , is used to determine the net positive vertical force that may support a suspended sediment load. A dimensionless criterion, Λ, is proposed for the maintenance of suspension, defined as the ratio of the maximum vertical turbulent stress to immersed weight of the suspended load above a unit bed area. In order that a suspension be maintained: where v ′ is instantaneous vertical turbulent velocity, σ and ρ are solid and fluid densities, respectively and m is the suspended load dry mass. The Λ criterion is dynamic, being a ratio of stresses and is analogous in this respect to Shields dimensionless stress criterion, θ, for the initiation of bedload motion. The new criterion is successful in predicting the maintenance of steady-state suspended sediment transport in open channel shear flow and deposition from non-uniform particulate density flows of wall jet type.  相似文献   

Late Wisconsin glacial advance and retreat patterns in southwestern Ohio, USA     

MICHAEL P. EKBERG  THOMAS V. LOWELL  ROBERT STUCKENRATH 《Boreas: An International Journal of Quaternary Research》1993,22(3):189-204

Despite the application of radiocarbon dating for more than three decades along the southern margin of the Laurentide Ice Sheet, fundamental questions about the timing of glacial advances remain. For one of its sublobes, the Miami, we undertook areal mapping, detailed lithostratigraphic analysis, and radiocarbon dating to interpret four pulses of ice advance. On top of the undated sediments deposited during the first advance is a major unconformity. The second advance occurred about 20,000 BP and marks the beginning of the late Wisconsin glaciation. A minor recession (more than 30 km) ensued, but plants did not reoccupy the landscape. A third advance of the ice margin produced a stone-rich lodgement till to within 20 km of the late Wisconsin maximum. The final ice motion only occurred in the northern part of the study area and may be of local extent. Large accumulations of supraglacial gravity flowtills and outwash mark the final ice-margin retreat. Of these. only the second advance is well dated. This study implies that the number of advances of the ice margin is fewer than previously suggested. Consequently we argue that several of the sublobes across the southern margin of the Laurentide Ice Sheet acted in unison for the interval of 22.000 to 18.000 BP implying ice-sheet external forcing.  相似文献   

Teaching geology with the aid of CCTV     

PETER R. THOMAS 《Geology Today》1992,8(6):222-224

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A Metamorphosed Early Cambrian Crust-Mantle Transition in the Eastern Alps, Austria   总被引：1，自引：0，他引：1  

MELCHER  FRANK; MEISEL  THOMAS 《Journal of Petrology》2004,45(8):1689-1723

In the Speik Complex (Eastern Alps, Austria), highly melt-depleted,metamorphosed harzburgites with abundant pods and layers ofchromitite are interlayered with a suite of metamorphosed orthopyroxenites,clinopyroxenites and gabbros. Coarse-grained orthopyroxenitesoccur as centimetre- to metre-wide veinlets and pods, but alsoas intrusive plugs several tens of metres wide. Intimately associatedmetaclinopyroxenite and metagabbro are present as bodies upto several metres thick at a distinct stratigraphic level withinthe complex. In the ultramafic rocks, relict magmatic olivine,orthopyroxene, clinopyroxene and spinel have been overprintedby a metamorphic assemblage of forsterite, diopside, tremolite,anthophyllite, chlorite, serpentine, talc and Cr–Fe-richspinel. Hornblende, epidote, zoisite and chlorite dominate themetamorphic paragenesis in metagabbros, in addition to rarerelicts of clinopyroxene and two phases of Ca-rich garnet. Thepolymetamorphic evolution of the Speik Complex includes rarelypreserved pre-Variscan (400 Ma) eclogite-facies conditions,Variscan (330 Ma) amphibolite-facies conditions (600–700°C,>5 kbar) and Eoalpine (100 Ma) greenschist- to amphibolite-faciesconditions reaching 550°C and 7–10 kbar. Orthopyroxenitesare characterized by high concentrations of SiO₂, MgO and Cr,and by U-shaped chondrite-normalized rare earth element (REE)patterns similar to those of their harzburgite hosts. The REEpatterns of the clinopyroxenites are flat to slightly enrichedin light REE. Metagabbro compositions are variable, but generallycharacterized by low SiO₂ and high mg-numbers (61–78).Their REE patterns all have Gd_N/Yb_N > 1; some samples havelarge positive Eu anomalies implying the original presence ofcumulus plagioclase. In the orthopyroxenites, clinopyroxenitesand some peridotites, Pt, Pd and Re are distinctly enrichedcompared with Os, Ir and Ru, whereas most harzburgites haveunfractionated to slightly fractionated platinum-group element(PGE) patterns with respect to average upper mantle. The Re–Osisotope compositions of the pyroxenites define an errorchronat 550 ± 17 Ma and a supra-chondritic ¹⁸⁷Os/¹⁸⁸Os of0·179 ± 0·003. An isochron age of 554 ±37 Ma with _Nd(i) +0·7 is indicated by the Sm–Ndisotope compositions of whole-rock pyroxenite and gabbro samples,whereas the harzburgites plot on an errorchron of 745 ±45 Ma and _Nd(i) +6. The pyroxenites and gabbros probably representa cogenetic suite of magmatic dykes intruded into uppermost,highly depleted, suboceanic mantle below the crust–mantletransition zone in an oceanic basin close to the northwesternmargin of Gondwana. KEY WORDS: pyroxenite; metagabbro; geochemistry; Re–Os isotopes; Sm–Nd isotopes  相似文献   

Rapid Change of Lava Composition from 1998 to 2002 at Piton de la Fournaise (Reunion) Inferred from Pb Isotopes and Trace Elements: Evidence for Variable Crustal Contamination     

VLASTELIC  IVAN; STAUDACHER  THOMAS; SEMET  MICHEL 《Journal of Petrology》2005,46(1):79-107

After an unusually long quiet period of nearly 6 years, in 1998the Piton de la Fournaise volcano started a new cycle of intensevolcanic activity. We report geochemical data on the first nineevents (53 samples), from the long-lived initial eruption (sixand a half months) of 1998 to the high-flux picritic eruptionof January 2002. Pb isotopes and trace elements display systematic,coupled variations, which are mostly confined to the beginningand the end of the period. Two well-defined binary mixing trendsare shown by Pb–Pb and Pb–trace element relationships.These trends indicate a change of end-member components betweenMarch and June 2001 that coincides with the transition fromsteady-state basalts to picrites. A three-component mixing modelinvolving a homogeneous plume and two contaminants successfullyexplains the data. The Pb–Pb relationship requires thattwo mixing processes occur successively: plume-derived magmainteracts first with altered oceanic crust, and the resultinghybrid product then interacts at shallower level with the oldlavas constituting the base of the volcanic edifice. Assimilationof edifice material decreased continuously from 1998 to 2002,whereas assimilation of oceanic crust drastically increasedduring the late-stage picritic eruption. These results suggestthat picrites may have resided for an unusually long time atan oceanic crustal level before ascending rapidly through thevolcanic edifice with little interaction with channel walls. KEY WORDS: assimilation; lead isotopes; picrites; Piton de la Fournaise; trace elements  相似文献   

GLACIER FLUCTUATIONS, SUSPENDED SEDIMENT FLUX AND GLACIO-LACUSTRINE SEDIMENTS   总被引：1，自引：0，他引：1  

PETER JANSSON  GUNHILD ROSQVIST  THOMAS SCHNEIDER 《Geografiska Annaler: Series A, Physical Geography》2005,87(1):37-50

ABSTRACT. In order to perform inverse modelling of climate variability based on palaeoclimate proxy records, the complexity of intermediate steps in the chain of processes from the climate forcing to the responding proxy has to be considered. In reconstructing climate-forced glacier fluctuations from proglacial lacustrine sediments it is important to understand how climate affects glacier dynamics. A glacier system is complex with many factors influencing sediment production, transport and deposition. Fluvial and mass movement processes in the proglacial environment may affect lake sedimentation substantially. We argue that it is easy to over-interpret glaciolacustrine sediment variability by ignoring these complications. The sediment records may contain individual layers resulting from single precipitation or melt events, as well as persistent changes in climate-forced glacier dynamics. We conclude that it is necessary to consider all possible influencing factors in order to derive reliable palaeoclimate data from lacustrine sediment sequences.  相似文献   

Card-carrying hunters, rural poverty, and wildlife decline in northern Côte d'Ivoire     

THOMAS J BASSETT 《The Geographical journal》2005,171(1):24-35

This paper examines wildlife decline in northern Côte d'Ivoire with emphasis on the political-economic and cultural dimensions of market hunting. Hunting and the trade in wild animal meat are situated within the economic diversification strategies of impoverished farmers and the flourishing of hunter associations. The role of hunter associations is critical in the rise in the number of hunters and firearms in the case study area of Kakoli. Initiation into the national hunters' association, Binkadi, allows card-carrying members to bear arms without legal repercussions. Higher quality arms and rural poverty motivate many of these recruits to engage in market hunting. Game market surveys for 1981–82 and 1997–99 in Kakoli show tremendous pressure on both vulnerable and robust species alike. Game depletion is conceptualized as the outcome of interacting social and biophysical processes that produce hunting pressure and habitat change. By drawing attention to the political-economic, ecological, and cultural dimensions of wildlife decline, this study implicates a diversity of actors and institutions in the dynamics of game depletion.  相似文献   

MUD BANK SLIDING IN A DAM RESERVOIR     

A. ALEXIS  D. MAROT  P. THOMAS Doctor-Engineer  Laboratoire Genie Vivil Nantes Saint-Nazaire  BP  Saint-Nazaire  France. Doctor  Laboratoire Genie Vivil Nantes Saint-Nazaire  BP  Saint-Nazaire  France. Professor  Laboratoire Genie Vivil 《国际泥沙研究》2001,16(3)

l INTRODUCTIONTo ensure the long-term futhe of dams, the emPtying of storage reservoirs is often realised with tWoobjectives:'-periodic insPection of the sttucthe,-flushing oftalrials Which sedimatd within the reservoir.Both aims are lthely to cause the instability of sedhoent, and these displacements can induceenvirOInnenal distheances that have to be reduced to the Ininimum. This imPlies a good knowledge ofthese maerials and of the processes Which originated the disPlacements.The se…  相似文献