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61.
Assemblage structure is related to slope and depth on a deep offshore Pacific seamount chain 
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下载免费PDF全文 Scientific study has generated a range of hypotheses about the ecological structure and function of seamounts. Interpretations of these ideas and data are vital to understanding how seamount communities will respond to anthropogenic impacts. Here, we examine how diversity and structure of seamount assemblages vary with depth and slope of the sea floor. We conducted ROV video transects on three seamounts of the Taney Seamount Chain in the Northeast Pacific Ocean. Depth and slope were both related to assemblage structure on the Taney seamounts. Depth differences were seen in alpha‐ and beta‐diversity but not density. Beta‐diversity and density but not alpha‐diversity varied with slope. Overall, slope and depth together explained 14–31% of beta‐diversity. The findings suggest that differences in beta‐diversity as related to depth gradients may differ among onshore and offshore and/or between shallow and deep summit seamounts. Specifically, we hypothesize that differences in productivity and depth gradients among seamounts may generate different patterns of beta‐diversity. 相似文献
62.
Changes in the composition of planktonic ostracod populations across a range of latitudes in the North-east Atlantic 总被引：2，自引：0，他引：2
A large database representing the bathymetric distribution of 117 species of halocyprid ostracods has been compiled from seven stations forming a transect from the equator to 60°N along 20°W, plus an additional station at 32°N, 65°W. This data base is analysed to examine the latitudinal and bathymetric changes in species composition and diversity of assemblages of this important, yet neglected, holoplanktonic group. At each station stratified sampling of the complete water column from the surface down mostly to 2000 m was carried out both day and night. Each sample resulted from the filtration of at least 2500 m3 of water and was analysed using a consistent protocol. The differences between the day and night profiles are attributable to diel vertical migrations, to local-scale heterogeneity, and possibly to a degree of net avoidance. There is a gradient of increasing species richness and diversity from high to low latitudes. By day, halocyprids are either infrequent or absent from the upper 50 m of the water column, but at night after diel vertical migration they become quite abundant in the epipelagic zone, particularly at low latitudes. Bathymetric profiles show ostracod abundances increase rapidly below the thermocline, reaching maxima at 200-400 m and then declining by at least an order of magnitude at 2000 m. Diversity (both species richness, H′ and evenness, J) also increases below the thermocline and thereafter is either maintained or declines only slightly to 2000 m. There are no relationships among diversity, abundance and productivity, but analysis of the whole database shows that the changes in community structure are consistent with Longhurst’s [Longhurst, A.R., 1998. Ecological Geography of the Sea. Academic Press, San Diego, pp. xiv, 398.] biogeochemical provinces. 相似文献
63.
This article introduces a technique for using a combination of remote sensing imagery and open-channel flow principles to estimate depths for each pixel in an imaged river. This technique, which we term hydraulically assisted bathymetry (HAB), uses a combination of local stream gage information on discharge, image brightness data, and Manning-based estimates of stream resistance to calculate water depth. The HAB technique does not require ground-truth depth information at the time of flight. HAB can be accomplished with multispectral or hyperspectral data, and therefore can be applied over entire watersheds using standard high spatial resolution satellite or aerial images. HAB also has the potential to be applied retroactively to historic imagery, allowing researchers to map temporal changes in depth.We present two versions of the technique, HAB-1 and HAB-2. HAB-1 is based primarily on the geometry, discharge and velocity relationships of river channels. Manning's equation (assuming average depth approximates the hydraulic radius), the discharge equation, and the assumption that the frequency distribution of depths within a cross-section approximates that of a triangle are combined with discharge data from a local station, width measurements from imagery, and slope measurements from maps to estimate minimum, average and maximum depths at a multiple cross-sections. These depths are assigned to pixels of maximum, average, and minimum brightness within the cross-sections to develop a brightness–depth relation to estimate depths throughout the remainder of the river.HAB-2 is similar to HAB-1 in operation, but the assumption that the distribution of depths approximates that of a triangle is replaced by an optical Beer–Lambert law of light absorbance. In this case, the flow equations and the optical equations are used to iteratively scale the river pixel values until their depths produce a discharge that matches that of a nearby gage.R2 values for measured depths versus depths estimated by HAB-1 and HAB-2 are 0.51 and 0.77, respectively, in the relatively simple Brazos River, Texas. R2 values for HAB-1 and HAB-2 are 0.46 and 0.26, respectively, in the Lamar River, a complex mountain river system in Yellowstone National Park. Although the R2 values are moderate, depth maps and cross-sections derived from the HAB techniques are consistent with typical stream geomorphology patterns and provide far greater spatial coverage and detail than could be achieved with ground-based survey techniques. Improved depth estimates can be achieved by stratifying the river into different habitat types that normalize for differences in turbulence and substrate. 相似文献
64.
Bathymetric surveys during the 1991–2000 decade in two ice-contact, proglacial lakes on the eastern sector of Bering piedmont lobe captured the buildup effects of the 1993–1995 surge. Following ice-front advance of 1.0–1.5 km into Tsivat and Tsiu Lakes, the basins were significantly altered by surge-related sedimentation including the impact of a subglacial outburst into Tsivat Lake. The subsequent changes in basin shape, size, and morphology were monitored by six bathymetric surveys. Measured changes in water depth serve as a proxy for determining increments of sediment accumulation.
Upwelling, ice-front vents fed by subglacial tunnels transported suspended fine sediment directly into the lake system. The rate of suspension settling within both lakes varied from 0.6 to 1.2 m year−1 prior to the surge. Suspended load during surge years increased sixfold from 1.7 to 13.9 g l−1, accompanied by increased sediment accumulation of 2.2–3.1 m year−1. Vent-related aggradation and subsequent filling of Tsivat Lake caused sediment bypassing to Tsiu Lake, where encroachment by delta growth contributed to a postsurge rate of bottomset accumulation of 3.0 m year−1.
The total sediment influx from subglacial sources is represented by the sum of bathymetrically determined accumulation, plus an estimated volume of sediment that remained suspended, thus passing through the lake system. Total sediment flux along the eastern Bering piedmont lobe from 1991 to 2000 is approximately 227 million cubic meters. 相似文献
65.
Fabrizio Felletti 《Sedimentology》2002,49(4):645-667
ABSTRACT This paper details the influence of syndepositional tectonics in controlling the architecture of a well‐exposed confined turbiditic sandbody, which crops out in the eastern part of the Tertiary Piedmont Basin (Castagnola Basin, northern Italy). The Castagnola Basin was tectonically active during sedimentation of the sandbody, and the lateral distribution of turbidity‐current deposits has been used to constrain both how the basin subsided and the impact of basin topography on flow behaviour and deposition. The sandbody occurs in the lower member of an Upper Oligocene–Lower Miocene turbidite system (the Castagnola Formation). The sandbody is ≈30 m thick and can be followed laterally for ≈1·8 km; it shows onlap terminations onto both northern and southern basin margins. The outcrop is sufficiently large to allow a detailed analysis of the facies and geometrical heterogeneity, as viewed approximately parallel to the average palaeocurrent trend (SW–NE). Correlation between 41 sedimentological logs reveals the diachronous development of a succession of sandstone packages (subunits). Nine vertically stacked and laterally juxtaposed packages have been recognized (subunits B to I from oldest to youngest), which reflect changes in basin floor accommodation as a result of synsedimentary tectonism. Each package shows the development of different vertical stacking patterns with thinning‐ and ‐fining‐upward small‐scale sequences and variable lateral facies arrangements, as a consequence of the position relative to the basin margins. The geometry, stratigraphic relationships, facies distribution and palaeocurrent directions indicate that turbidite deposition during accumulation of most of the sandbody was controlled by (1) synsedimentary tilting of the basin slopes; (2) the distribution of structural and depositional relief within the basin; (3) the thickness and volume of the turbidite flows; and (4) the angle of impingement of turbidity currents against the basin slopes. 相似文献
66.
Monte Kietpawpan Parichart Visuthismajarn Charlchai Tanavud Mark G. Robson 《Natural Hazards》2008,46(1):89-106
A new model to calculate tsunami travel times in the Andaman Sea region has been developed. The model specifically provides
more accurate travel time estimates for tsunamis propagating to Patong Beach on the west coast of Phuket, Thailand. More generally,
the model provides better understanding of the influence of the accuracy and resolution of bathymetry data on the accuracy
of travel time calculations. The dynamic model is based on solitary wave theory, and a lookup function is used to perform
bilinear interpolation of bathymetry along the ray trajectory. The model was calibrated and verified using data from an echosounder
record, tsunami photographs, satellite altimetry records, and eyewitness accounts of the tsunami on 26 December 2004. Time
differences for 12 representative targets in the Andaman Sea and the Indian Ocean regions were calculated. The model demonstrated
satisfactory time differences (<2 min/h), despite the use of low resolution bathymetry (ETOPO2v2). To improve accuracy, the
dynamics of wave elevation and a velocity correction term must be considered, particularly for calculations in the nearshore
region. 相似文献
67.
Michael Davis Ari Matmon Ezra Zilberman Naomi Porat Daniel Gluck Yehouda Enzel 《Geomorphology》2009,106(3-4):352-362
This paper examines the millennial-scale evolution of the longitude profile of Nahal (Wadi) Zin in the Dead Sea basin in the northern Arava valley, Israel. Nahal Zin has incised ~ 50 m into relatively soft late Pleistocene Lake Lisan sediments. Incision was forced by the regressive (> 10 km) lake level fall of a total of > 200 m of Lake Lisan from its highest stand at ~ 25 ka and exposure of the lake-floor sediments to fluvial and coastal processes. Alluvial cut terraces of the incising channel are well preserved along the 17.5 km of the lowermost reach of Nahal Zin. At its outlet into the Dead Sea basin, Nahal Zin deposited a Holocene alluvial fan at the base of a 10–80 m high escarpment in unconsolidated sediments. The escarpment is associated with the Amazyahu fault, which forms the southern structural boundary of the present Dead Sea basin. Geomorphic mapping, optically stimulated luminescence (OSL) ages, and soil stratigraphy allowed correlation of terrace remnants and reconstruction of several past longitudinal profiles of Nahal Zin and its incision history. Together with the published lake level chronology, these data provide an opportunity to examine stream incision related to base level lowering at a millennial scale. OSL ages of the terraces fit relatively well with the established lake level chronology and follow its regression and fall. For a few thousands of years the longitudinal profile response to the lake level fall was downstream lengthening onto the exposed former lake bed. Most of the incision (~ 40 m) occurred later, when the lake level reached the top of the Amazyahu fault escarpment and continued to drop. The incision was a relatively short episode at about 17 ka and cut through this escarpment almost to its base. The fast incision, its timing, and the profiles of the incising channels indicate that the escarpment was an underwater feature and was not formed after the lake retreated.This fairly simple scenario of regressive lake level fall and knickpoint exposure and incision is modeled here using a one-dimensional numerical incision model based on a linear diffusion equation. The calculated diffusion coefficient fits earlier results and data obtained from other streams in the area and confirms the upscaling of this simple model to the millennial scale. 相似文献
68.
The present study offers a two-dimensional horizontal wave propagation and morphodynamic model for muddy coasts. The model can be applied on a general three-dimensional bathymetry of a soft muddy coast to calculate wave damping, fluid mud mass transport and resulting bathymetry change under wave actions. The wave propagation model is based on time-dependent mild slope equations including the wave energy dissipation due to the wave-mud interaction of bottom mud layers as well as the combined effects of the wave refraction, diffraction and breaking. The constitutive equations of the visco-elastic–plastic model are adopted for the rheological behavior of fluid mud. The mass transport velocity within the fluid mud layer is calculated combining the Stokes’ drift, the mean Eulerian velocity and the gravity-driven mud flow. The results of the numerical model are compared against a series of conducted wave basin experiments, wave flume experiments and field observations. Comparisons between the computed results with both the field and laboratory data reveal the capability of the proposed model to predict the wave transformation and mud mass transport. 相似文献
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70.
The maximum error in ocean depth measurement as specified by the International Hydrographic Organization is 1% for depth greater than 30m. Current acoustic multibeam bathymetric systems used for depth measurement are subject to errors from various sources which may significantly exceed this limit. The lack of sound speed profiles may be one significant source of error. Because of the limited ability of sound speed profile measurement, depth values are usually estimated using an assumed profile. If actual sound speed profiles are known, depth estimate errors can be corrected using ray-tracing methods. For depth measurements, the calculation of the location at which a sound pulse impinges on the sea bottom varies with the variation of the sound speed profile. We demonstrate that this location is almost unchanged for a family of sound speed profiles with the same surface value and the same area under them. Based on this observation, we can construct a simple constant-gradient equivalent sound speed profile to correct errors. Compared with ray-tracing methods, the equivalent sound speed profile method is more efficient. If a vertical depth is known (or independently measured), then depth correction for a multibeam system can be accomplished without knowledge of the actual sound speed profile. This leads to a new type of precise acoustic multibeam bathymetric system. 相似文献