Studies on settling, traction and entrainment of larger benthic foraminiferal tests: implications for accumulation in shallow marine sediments   总被引：1，自引：0，他引：1  

ELZA K. YORDANOVA  JOHANN HOHENEGGER 《Sedimentology》2007,54(6):1273-1306

Settling and traction velocities were measured on optimally preserved tests of larger foraminifera using a settling tube and flume tank. Within larger foraminifera with porcelaneous tests, the peneroplids, Peneroplis antillarum, P. planatus, P. pertusus and Dendritina cf. D. zhengae, are distinguished by low test densities (ca 1·2) that do not change with growth. Buoyancy is high because of low Reynolds numbers and increases in large individuals because of the allometric change of test shape. The fusiform Alveolinella quoyi, with test densities ca 1·6, is characterized by high Reynolds numbers, inducing the weakest buoyancy within porcelaneous larger foraminifera. The highest buoyancy was recorded for the three soritids, Parasorites orbitolitoides, Sorites orbiculus and Amphisorus hemprichii, because of their low test densities (ca 1·25) and the extremely flat, biconcave, plate‐like shape. Flat tests, however, reduce traction and entrainment from smooth surfaces. Within hyaline larger foraminiferat, the amphisteginids show thick‐lenticular (Amphistegina lobifera, A. radiata) to thin‐lenticular tests (A. bicirculata, A. papillosa), influencing buoyancy. Here, high test densities (ca 1·8) decrease with growth in A. lobifera, A. lessonii and A. bicirculata, and remain constant in A. radiata and A. papillosa. Minimum velocities required for entrainment are lower for thick‐lenticular tests and higher for thin‐lenticular tests. Test densities remain constant with growth in the calcarinid Baculogypsina sphaerulata (ρ ∼ 1·78) and decrease slightly in Calcarina gaudichaudii and Neorotalia calcar (starting at ρ ∼ 1·85), all living under extreme hydrodynamic conditions. Density decreases the most in Baculogypsinoides spinosus (starting at ρ ∼ 1·8), resulting in higher buoyancy through low Reynolds numbers. Traction is promoted in spherical tests of Baculogypsina and Baculogypsinoides. Within nummulitids, the thick‐lenticular Palaeonummulites venosus (test density decreasing with size; starting at 1·78) is less buoyant, expressed in high Reynolds numbers, but easily entrained. Thick‐lenticular juveniles and extremely flat adults distinguish Operculinella cumingii, Heterostegina depressa and the giant Cycloclypeus carpenteri. Test densities increase during growth, starting from ca 1·6 and attaining a maximum of 1·8. Buoyancy is low in small tests and high in large tests, while entrainment velocities are reduced as the tests flatten. High buoyancy is also a characteristic of the entirely flat tests in Operculina ammonoides (from deeper regions) and Planostegina operculinoides, which is expressed in the lowest Reynolds numbers within larger foraminifera.  相似文献   

Gas retention in fine-grained pyroclastic flow materials at high temperatures     

T. H. Druitt  G. Avard  G. Bruni  P. Lettieri  F. Maez 《Bulletin of Volcanology》2007,69(8):881-901

The ability of a dense pyroclastic flow to maintain high gas pore pressure, and hence low friction, during runout is determined by (1) the strengths and longevities of gas sources, and (2) the ability of the material to retain residual gas once those sources become ineffective. The latter is termed the gas retention capacity. Gas retention capacity in a defluidizing granular material is governed by three timescales: one for the evacuation of bubbles (t _be ; brief and not considered in this paper), one for hindered settling from the expanded state (t _sett), and one for diffusive release of residual pore pressure from the non-expanded state (t _diff). The relative magnitides of t _sett and t _diff depend on bed thickness, t _sett dominating in thin systems and t _diff in thick ones. Three pyroclastic flow materials, two ignimbrites and a block-and-ash flow sample, were studied experimentally to investigate expansion behaviour under gas flow and to determine gas retention times. Effects of particle size were evaluated by using two size cuts (<4 mm and <250 μm) from each sample. Careful drying of the materials was necessary to avoid effects of humidity-related cohesion. Two sets of experiments were carried out: (1) expansion in the non-bubbling regime at 50–200°C, (2) bed collapse tests from the initially bubbling state at 50–550°C. Provided that gas channelling was avoided by gentle stirring, all the samples exhibited a regime of uniform expansion prior to the onset of bubbling. Fine particle size (in particular high fines content), low particle density and high temperature all favoured smoother fluidization by increasing the maximum expansion possible in the non-bubbling state. An empirical equation describing the uniform expansion of the materials was determined. High temperature also favoured greater gas partitioning into the dense phase of the bubbling bed, as well (in finer-grained samples) as higher voidage in the settled bed. Large values of t _sett and t _diff were favoured by fine particle size. Temperature had less influence, suggesting that experimental results at low temperatures (50–200°C) can be extrapolated to higher temperatures. Gas retention times provide insight into the ability of pyroclastic flows in expanded (t _sett) or non-expanded (t _diff) flow states to retain gas once air ingestion or gas production have become ineffective. Finer-grained pyroclastic flows are expected to retain gas longer, and hence to have higher apparent ‘mobilities’, than coarser-grained ones of comparable volume, as has been observed on Montserrat.  相似文献   

The annual variations on land surface energy in the northern Tibetan Plateau   总被引：3，自引：0，他引：3  

MA Wei-qiang  MA Yao-ming 《Environmental Geology》2006,50(5):645-650

It is vital to study the regional heat fluxes in the Tibetan Plateau Area. In this paper, the characteristics of down- and upward short wave radiation fluxes, down- and upward long wave radiation fluxes, net radiation flux, soil heat flux, sensible heat flux, and latent heat in the areas of CAMP/Tibet [coordinated enhansive observating period (CEOP) Asian-Australia Monsoon Project (CAMP) in Tibetan Plateau] are analyzed. Some new concepts about the characteristics of radiation flux budget and land surface energy budget are obtained. An erratum to this article can be found at  相似文献   

Mechanism of Crystal Redistribution in a Sheet-like Magma Body: Constraints from the Nosappumisaki and Other Shoshonite Intrusions in the Nemuro Peninsula, Northern Japan     

SIMURA  RAYKO; OZAWA  KAZUHITO 《Journal of Petrology》2006,47(9):1809-1851

Processes of crystal separation in a magma heavily laden withcrystals without phase change are investigated from observationson frozen magma systems: Nosappumisaki and other shoshoniteintrusions in the Nemuro peninsula, Japan, for which the originof the crystals and the initial conditions are well constrained.The Nosappumisaki intrusion is 120 m in thickness and extendsfor more than 1·5 km. It exhibits a wide range of lithologicalvariation, principally as a result of crystal redistributionafter intrusion. Crystals in each lithology can be clearly dividedinto two kinds according to their composition and texture: thosepresent before the intrusion of the magma (‘phenocrysts’)and those that crystallized in situ after intrusion. From thevertical change in mode and size of ‘phenocrysts’,it is shown that (1) augite ‘phenocrysts’ were rapidlydeposited, with little overgrowth after intrusion, by significantcoagulation or clustering on a time-scale of more than a fewyears, and (2) plagioclase ‘phenocrysts’, definitelydenser than the melt but concentrated in the upper level, floatedby counter flow of massive deposition of augite ‘phenocrysts’.These results indicate that in a magma heavily laden with crystalsof a few millimeters in size (>20 vol. %), crystal–crystaland crystal–melt interaction play an important role inthe separation of crystals from the host melt. KEY WORDS: magma chamber; sill; crystal settling; plagioclase flotation; Nosappumisaki  相似文献   

A Note On The Contribution Of Dispersive Fluxes To Momentum Transfer Within Canopies   总被引：5，自引：12，他引：5  

D. Poggi  G. G. Katul  J. D. Albertson 《Boundary-Layer Meteorology》2004,111(3):615-621

Dispersive flux terms are formed when the time-averaged meanmomentum equation is spatially averaged within the canopy volume.These fluxes represent a contribution to momentum transfer arisingfrom spatial correlations of the time-averaged velocity componentswithin a horizontal plane embedded in the canopy sublayer (CSL).Their relative importance to CSL momentum transfer is commonlyneglected in model calculations and in nearly all fieldmeasurement interpretations. Recent wind-tunnel studies suggestthat these fluxes may be important in the lower layers of thecanopy; however, no one study considered their importance acrossall regions of the canopy and for a wide range of canopy roughnessdensities. Using detailed laser Doppler anemometry measurementsconducted in a model canopy composed of cylinders within a largeflume, we demonstrate that the dispersive fluxes are onlysignificant (i.e., >10%) for sparse canopies. These fluxes arein the same direction as the turbulent flux in the lower layers ofthe canopy but in the opposite direction near the canopy top. Fordense canopies, we show that the dispersive fluxes are <5% atall heights. These results appear to be insensitive to theReynolds number (at high Reynolds numbers).  相似文献   

长江流域一次暴雨过程的模拟对行星边界层参数化的敏感性研究     

左志燕  康红文 《沙漠与绿洲气象(新疆气象)》2004,27(2):4-6,10

用MM5对长江流域的一次暴雨进行模拟考察其对行星边界层参数化的敏感性。不同的边界层参数化表现在不同的地表通量和垂直混合设计上，本文分析了MM5中4个主要方案地表通量和垂直混合参数化方案的不同以及它们对降水强度、落区和时间的影响。研究还发现，地表通量对暴雨模拟结果的影响比垂直混合方案要大。  相似文献   

Fluxes of material in the Arabian Sea and Bay of Bengal — Sediment trap studies     

V. Ramaswamy  R. R. Nair 《Journal of Earth System Science》1994,103(2):189-210

In order to investigate how monsoons influence biogeochemical fluxes in the ocean, twelve time-series sediment traps were deployed at six locations in the northern Indian Ocean. In this paper we present particle flux data collected during May 1986 to November 1991 and November 1987 to November 1992 in the Arabian Sea and Bay of Bengal respectively. Particle fluxes were high during both the SW and NE monsoons in the Arabian Sea as well as in the Bay of Bengal. The mechanisms of particle production and transport, however, differ in both the regions. In the Arabian Sea, average annual fluxes are over 50gm^-2y^-1 in the western Arabian Sea and less than 27gm^-2 y^-1 in the central part. Biogenic matter is dominant at sites located near upwelling centers, and is less degraded during peak flux periods. High particle fluxes in the offshore areas of the Arabian Sea are caused by injection of nutrients into the euphotic zone due to wind-induced mixed layer deepening. In the Bay of Bengal, average annual fluxes are highest in the central Bay of Bengal (over 50gm^-2y^-1) and are least in the southern part of the Bay (37gm^-2y^-1). Particle flux patterns coincide with freshwater discharge patterns of the Ganges-Brahmaputra river system. Opal/carbonate and organic carbon/carbonate carbon ratios increase during the SW monsoon due to variations in salinity and productivity patterns in the surface waters as a result of increased freshwater and nutrient input from rivers. Comparison of S years data show that fluxes of biogenic and lithogenic particulate matter are higher in the Bay of Bengal even though the Arabian Sea is considered to be more productive. Our results indicate that in the northern Indian Ocean interannual variability in organic carbon flux is directly related to the strength and intensity of the SW monsoon while its transfer from the upper layers to the deep sea is partly controlled by input of lithogenic matter from adjacent continents.  相似文献   

干旱半干旱区非均匀地表区域能量通量的卫星遥感参数化   总被引：13，自引：11，他引：2  

马耀明  戴有学  马伟强  李茂善  王介民  文军  孙方林 《高原气象》2004,23(2):139-146

卫星遥感在估算非均匀地表区域能量通量时有其独到的作用。文中介绍了利用Landsat TM资料估算非均匀地表区域地表能量通量和蒸发(蒸散)量的参数化方案、研究结果和存在的难点问题。并提出了解决问题的可能途径。  相似文献   

Lateral and vertical juxtaposition of matrix‐rich and matrix‐poor lithologies caused by particle settling in mixed mud–sand deep‐marine sediment suspensions     

Katrina Angus  Robert William Charles Arnott  Viktor Terlaky 《Sedimentology》2019,66(3):940-962

Sandy sedimentary rocks rich in detrital matrix (>10% silt/clay) have long been recognized in the ancient sedimentary record, and nowhere more commonly than in deep‐marine turbidite systems. Despite this, their depositional mechanisms remain poorly understood, in part because these rocks, which are enriched in fine‐grained sediment, are often poorly exposed in outcrop or are confined to observation in core. Matrix‐rich strata in the Neoproterozoic Windermere Supergroup, in contrast, are very well‐exposed and show systematic changes in lithofacies over distances of several tens to a few hundreds of metres along‐strike. Notably, these strata are observed in both basin floor and continental slope deposits, suggesting that their occurrence and systematic lithological arrangement is related to mechanistic, rather than palaeogeographic, controls. Specifically, the facies transect consists of structureless, clayey sandstone that transforms along‐strike to a two‐layer deposit with the development of an upper, planar‐based, markedly more matrix‐rich layer. Further along‐strike, the basal clayey sandstone thins and eventually pinches out, leaving only the (upper) sandy claystone layer, which in turn thins along‐strike and then pinches out. These systematic changes in lithology, but more specifically the distribution of clay, is interpreted to form a depositional continuum related to particle settling in a horizontally advecting, high concentration particle suspension formed along the margins of an avulsion‐related high‐energy turbulent suspension.  相似文献   

The sediment budget and dynamics of a delta‐canyon‐lobe system over the Anthropocene timescale: The Rhone River delta,Lake Geneva (Switzerland/France)     

Tiago A. Silva  Stphanie Girardclos  Laura Stutenbecker  Maarten Bakker  Anna Costa  Fritz Schlunegger  Stuart N. Lane  Peter Molnar  Jean‐Luc Loizeau 《Sedimentology》2019,66(3):838-858

Deltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deep‐sea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon‐levée system and the distal lobe. Mean sedimentation rates in these areas vary from 0·0246 m year^?1 (distal lobe) to 0·0737 m year^?1 (delta front). Although the delta front–levées–distal lobe complex only comprises 17·0% of the analysed area, it stored 74·9% of the total deposited sediment. Results show that 52·5% of the total sediment stored in this complex was transported toward distal locations through the sublacustrine canyon. Namely, the canyon–levée complex stored 15·9% of the total sediment, while 36·6% was deposited in the distal lobe. The results thus show that in deltaic systems where density currents can occur regularly, a significant proportion of riverine sediment input may be transferred to the canyon‐lobe systems leading to important distal sediment accumulation zones.  相似文献