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1.
《Continental Shelf Research》2007,27(3-4):506-525
Clinoforms on modern shelves and slopes, as well as in ancient rock records, are widely recognized as a fundamental building element of continental margin growth. Regardless of their dominant lithology, clinoforms are composed of three geometric elements: topset, foreset and bottomset. Traditionally, much emphasis in the study of clinoforms was put on the geometry of the topset, viewed as the most energetic portion of a clinoform and studied to discern if aggradation was active rather than erosional truncation, and on the foreset, the area with the highest sediment accumulation rates. Here we focus on the factors forcing clinoforms to taper out and on the inferred mechanisms for bottomset creation. We base our analysis on muddy shelf clinoforms, a particular class of clinoforms that is typical of low-gradient settings and is characterized by a substantial component of shore-parallel sediment transport.This paper is based on a large dataset of CHIRP-sonar profiles, bathymetric and isopach maps of the Late Holocene clinoform on the Adriatic shelf, where integrated stratigraphic studies from sediment cores allow a very high (in some cases century-scale) chronological resolution. Knowledge on the dominant oceanographic regime affecting sediment dispersal and, ultimately, clinoform development, has been recently refined during the EuroSTRATAFORM collaboration. Muddy clinoforms are markedly three-dimensional features that cannot be entirely understood solely on two-dimensional shore normal sections. We suggest that this class of clinoform is advection dominated and that an increase in the energy regime at the toe of the foreset may prevent sediment from reaching beyond the transition to the bottomset region.  相似文献   

2.
科技进步使得地震采集数据量及其精度实现质的飞跃,南沙海域的地震调查,采用国际先进水平的等离子震源,获得地震剖面精度优于3 m局部甚至可在1 m之内,为科学研究、工程、浅层资源、地质灾害预警等研究奠定基础.在南沙第四系识别出了七个地震反射界面,确定500 ms至海底沉积区间内地震层序与三期海平面升降并与冰期、间冰期对应,典型地震剖面展示第四纪至少有三套完整的具有三角洲顶积层、前积层、底积层的地震相证据,与全球的第四纪海平面的变化一致.精细的浅层地震结构表明：南沙的地质现象丰富,浅层断层非常发育,下切河谷、泥石流、滑坡等地质遗迹的形成的地震相清晰,是地质活动频繁的地区.  相似文献   

3.
《Continental Shelf Research》2007,27(3-4):452-474
Crenulated clinoforms of complex and uncertain origin characterize large portions of the Late-Holocene prograding mud wedge in the western Adriatic continental shelf. Sediment failure was originally postulated as the most plausible mechanism for the formation of the crenulations. Subsequent work has shown that, although the origin of the crenulations may have been related to deformation processes, their maintenance through time seems to be better explained by different sediment accumulation rates in the flat and steep flanks. In order to establish relationships between active sediment dynamics, across-shelf transport and sediment accumulation in these crenulated clinoforms, two tripods and a mooring were deployed off the Pescara River during autumn and winter 2002–2003 as part of the EuroSTRATAFORM program, and in combination with the Po and Apennine Sediment Transport and Accumulation (PASTA) study. The tripods were placed on the shallow topset region and close to the clinoform roll-over point (i.e., offlap break), at 12 and 20-m water depth, respectively, and the mooring was located at 50-m depth, in the crenulated foreset region. Several sediment-resuspension events were recorded, mainly related to Bora and Sirocco storms, during which wave–orbital and current velocities increased considerably. Sediment transport in the topset region was predominantly towards the SE, following the direction of the coastal current and the bathymetry, but showing a significant offshore component at the roll-over point that was intensified during storm events. Currents at the foreset region were also directed to the SE. In mid-waters they were clearly aligned with the local bathymetry, whereas near the bottom they had an important and persistent offshore component. This current behavior seems to be associated with an intense bottom Ekman transport that causes the near-bottom current to be deflected to the left (i.e., offshore) with respect to the direction of the surface current. This mechanism enhances the suspended-sediment transport from the topset down the foreset region along the Adriatic prograding mud wedge, contributing to the basinward clinoform progradation and controlling the depth of the clinoform roll-over point. In addition, activity of near-inertial internal waves was also recorded by the near-bottom instrument deployed in the foreset region. During periods characterized by a strong near-inertial signal, increases of the water turbidity clearly coincided with an intensified offshore velocity component, which suggest that this mechanism also contributes to the transport of suspended sediment across the clinoform. Both the bottom Ekman transport and the internal waves are mechanisms that could be responsible for the formation/maintenance of the Adriatic seafloor crenulations until present-day, although several arguments suggest that the latter likely plays the major role.  相似文献   

4.
The Markermeer is a large and shallow man-made freshwater lake in the Netherlands, characterized by its high turbidity. As part of a study aiming to mitigate this high turbidity, we studied the water–bed exchange processes of the lake’s muddy bed. The upper centimeter’s–decimeter’s of the lake bed sediments mainly consists of soft anoxic mud. Recent measurements have proved the existence of a thin oxic layer on top of this soft anoxic mud. This oxic layer, which is much easier to be eroded than the anoxic mud, is believed to be related with Markermeer’s high-turbidity levels. Our hypothesis is that the thin oxic layer develops from the anoxic mud, enhanced by bioturbation. Actually, we will demonstrate that it is the bioturbated state of the bed that increases its erodability, and not the oxidation state of the sediments. In particular, we will refer to bioturbation caused by meiobenthic fauna. The objective of this study is therefore to determine the influence of the development of the thin oxic layer on the water–bed exchange processes, as well as to establish the role of bioturbation on those processes. This is done by quantifying the erosion rate as a function of bed shear stresses, and at different stages of the development of the oxic layer. Our experiments show that bioturbation increases the rate at which Markermeer sediments are eroded by almost an order of magnitude. The short-term fine sediment dynamics in Markermeer are found to be driven by the complex and highly dynamic interactions between physics, chemistry, and biology. Finally, the long-term fine sediment dynamics are driven by the erosion of the historical deposits in the lake’s bed, which is only possible after bioturbation, and which leads to an increase of the stock of sediments in the lake’s muddy bed.  相似文献   

5.
Cohesive sediment dynamics in mountainous rivers is poorly understood even though these rivers are the main providers of fine particles to the oceans. Complex interactions exist between the coarse matrix of cobble bed rivers and fine sediments. Given that fine sediment load in such environments can be very high due to intense natural rainfall or snowmelt events and to man‐induced reservoir or dam flushing, a better understanding of the deposition and sedimentation processes is needed in order to reduce ecohydrological downstream impacts. We tested a field‐based approach on the Arc and Isère alpine rivers combining measurements of erosion and settling properties of river bed deposits before and after a dam flushing, with the U‐GEMS (Gust Erosion Microcosm System) and SCAF (System Characterizing Aggregates and Flocs), respectively. These measurements highlight that critical shears, rates of erosion, settling velocities and propensity of particles to flocculate are highly variable in time and space. This is reflective of the heterogeneity of the hydrodynamic conditions during particle settling, local bed roughness, and nature and size of particles. Generally the deposits were found to be stable relative to what is measured in lowland rivers. It was, however, not possible to make a conclusive assessment of the extent to which the dynamics of deposits after reservoir flushing were different from those settled after natural events. The absence of any relationships between erosion and deposition variables, making it impossible to predict one from another, underlined the need to measure all of them to have a full assessment of the fine sediment dynamics and to obtain representative input variables for numerical models. While the SCAF was found to be effective, an alternative to the U‐GEMS device will have to be found for the erodibility assessment in cobble bed rivers, in order to make more rapid measurements at higher shears. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

6.
Clinoform mechanics in the Gulf of Papua, New Guinea   总被引：1，自引：0，他引：1  
The largest islands of the Indo-Pacific Archipelago are estimated to account for 20–25% of the global sediment discharge to the ocean, and much (>50%) of this sediment is supplied to wide (>150 km) continental shelves. These conditions are conducive to creation of large-scale morphologic features known as clinoforms—sigmoidal-shaped deposits on the continental shelf. The Gulf of Papua (GOP) receives 3.84 ×108 tons of sediment annually from three principal sediment suppliers, the Fly, Kikori and Purari Rivers, and its prograding clinoform is the focus of this study. During three research cruises, 80 cores and 37 CTD/optical backscatter casts were collected, and an instrumented tripod was deployed twice. Sedimentological and radiochemical results indicate that the GOP clinoform has characteristics similar to those seaward of other major rivers (e.g., Amazon, Ganges–Brahmaputra), specifically sand/mud interbedding on the topset, rapidly accumulating muds on the foreset, and siliciclastic mud mixed with carbonate sand on the bottomset.Using core data and field observations, the mechanics of clinoform progradation are examined. Discrete, large sedimentation events are identified as processes building the clinoform feature. X-radiographs from foreset cores reveal thick beds (>5 cm) between bioturbated sections. Detailed 210Pb and grain-size data indicate that low activities and increased clay contents are associated with these beds. They are hypothesized to be formed by fluid–mud deposition in response to periods of large wave-tide bed shear stresses, more likely during the SE-tradewind season, and their regular occurrence produces high rates of mean accumulation (4 cm/y). Bed preservation is determined by the rates of sediment accumulation and bioturbation.To assess the influence of physical oceanographic factors on clinoform shape, bottom shear stresses from tides and surface waves were calculated using available wave and tripod data. This effort reveals that the depth range (25–40 m) of the clinoform rollover point (seaward edge of the topset region) is roughly consistent with the sediment-transport regime. Furthermore, calculations corroborate the core data that suggest possible seasonal sediment storage in the inner topset region (<15-m water depth, during the NW-monsoon winds) with subsequent transfer to foreset beds (more probable during SE-tradewind conditions).A 100-yr sediment budget created with accumulation rate data suggests approximately 20% of the total sediment supplied to the GOP accumulates on the clinoform (creating the clinoform morphology). Less than 5% is believed to escape to the adjacent slope, and much of the remaining 75% is likely trapped on the inner-topset region (<20 m water depth) and within the mangrove forests and flood/delta plains of the northern GOP.  相似文献   

7.
Compared to downstream fining of a gravel‐bedded river, little field evidence exists to support the process of downstream fining in large, fine sand‐bedded rivers. In fact, the typically unimodal bed sediments of these rivers are thought to produce equal mobility of coarse and fine grains that may discourage downstream fining. To investigate this topic, we drilled 200 sediment cores in the channel beds of two fine‐grained sand‐bedded reaches of the Yellow River (a desert reach and a lower reach) and identified a fine surface layer (FSL) developed over a coarse subsurface layer (CSL) in the 3‐m‐thick bed deposits. In both reaches downstream, the thickness of the FSL increased, while that of the CSL decreased. Comparison of the depth‐averaged median grain sizes of the CSL and the FSL separately in both reaches shows a distinct downstream fining dependence to the median grain size, which indicates that at a large scale of 600‐800 km, the CSL shows a significant downstream fining, but the FSL shows no significant trends in downstream variations in grain size. This result shows that fine sediment supply (<0·08 mm median grain size) from upstream, combined with lateral fine sediment inputs from tributaries and bank erosion, can cause a rapid fining of the downstream channel bed surface and can develop the FSL layer. However, in the desert reach, lateral coarse sediment supply (>0·08 mm median grain size) from wind‐borne sediments and cross‐desert tributaries can interrupt the FSL and coarsen the channel bed surface locally. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

8.
1 INTRODUCTIONNdsral rivers entering reservoirs carry also sediment, partly as bedload and patly in suspension. At theentrance of the reservoir flow velocity is considerably reduced and the capability of sediment transport isdrashcally fading. Bedload maerial is deposited totally at the reservoir entrance fondng a delta, material...in suspension is cAned further intO the reservoir ulh.. it is partly deposited and partly dischargedthrough the dam towards downstreaxn (depending on the trapp…  相似文献   

9.
Ten large volume water samples were taken from the Rhône River (Switzerland-France) in November, 1989 for recovery of total suspended sediment by continuous flow centrifugation. the samples were freeze-dried and analysed for particle size, organic carbon, total nitrogen, and carbonate. for comparative purposes, four bed sediments collected in July, 1989 are also described. the Rhône can be subdivided into three sections on the basis of the origins of the water. the first section is the Upper Rhône River draining into Lake Geneva. Waters are derived from glaciers, with low temperature and conductivity and high turbidity. Suspended sediment is coarse, has a bimodal distribution, and is low in both organic matter and carbonate. the second reach is from Lake Geneva to the confluence with the Saône at Lyon and has warmer water with higher conductivity and very low turbidity. Suspended sediment is higher in organic matter, with high carbonate originating from the lake. the final section is from Lyon to Arles, with warmer water and higher conductivity and turbidity due to modification by the Saône. Sediment is rich in organic matter, which May, account for an observed decline in oxygen in the river waters downstream from Lyon. Carbonate in these sediments also decreases due to increased turbidity from the Saône. Suspended sediments other than from the Upper Rhône show a remarkable consistency in grain size, predominantly in the fine silts (mode 9-11 μm). This consistency indicates a high degree of suitability for geochemical analysis. Bed sediments were bimodal throughout, with a dominant coarse population in two out of the four samples. Grain size statistical parameters could be easily explained by application of the theory of mixing of two major populations in the sand size (bed traction load) and the fine silt/clay size (suspended sediment load).  相似文献   

10.
《国际泥沙研究》2020,35(2):115-124
Bed-load transport plays a critical role in river morphological change and has an important impact on river ecology.Although there is good understanding of the role of the variation of river bed grain size on transport dynamics in equilibrium conditions,much less is understood for non-equilibrium conditions when the channel is either aggrading or degrading.In particular,the relative role of different grain sizes in the promotion and hindering of the transport of coarse and fine fractions in a degrading channel has yet to be investigated.The current study attempts to provide new understanding through a series of flume experiments done using uniform and graded sediment particles.The experiments revealed coarser grain-size fractions for a poorly-sorted sediment,relative to uniform-sized sediment,reduced the transport of finer grains and finer fractions enhanced the transport of coarse grains.This hinderingpromotion effect,caused by relative hiding and exposure of finer and coarse fractions,increased with bed slope and decreased with relative submergence.In particular,as relative submergence increased,the graded fractions tended towards behaving more like their unifo rm-sized counterparts.Also,the bed-load parameter of the graded fractions increased more with a rise in bed slope than observed for the uniformsized counterparts.These results revealed,for degrading channel conditions,such as downstream of a dam,bed-load equations developed for uniform bed sediment are inappropriate for use in natural river systems,particularly in mountain streams.Furthermore,changes in river bed composition due to activities that enhance the input of hill-slope sediment,such as fire,logging,and agricultural development,are likely to cause significant changes in river morphology.  相似文献   

11.
Many aquatic environments exhibit soft, muddy substrates, but this important property has largely been ignored in process-based models of Earth-surface flow. Novel laboratory experiments were carried out to shed light on the feedback processes that occur when particulate density currents (turbidity currents) move over a soft mud substrate. These experiments revealed multiple types of flow-bed interaction and large variations in bed deformation and bed erosion, which are interpreted to be related to the interplay between the shear forces of the current and the stabilising forces in the bed. Changes in this force balance were simulated by varying the clay concentrations in the flow and in the bed. Five different interaction types are described, and dimensional and non-dimensional phase diagrams for flow-bed interaction are presented.  相似文献   

12.
A model was developed and analyzed to quantify the effect of graded sediment on the formation of tidal sand ridges. Field data reveal coarse (fine) sediment at the crests (in the troughs), but often phase shifts between the mean grain-size distribution and the bottom topography occur. Following earlier work, this study is based on a linear stability analysis of a basic state with respect to small bottom perturbations. The basic state describes an alongshore tidal current on a coastal shelf. Sediment is transported as bed load and dynamic hiding effects are accounted for. A one-layer model for the bed evolution is used and two grain size classes (fine and coarse sand) are considered. Results indicate an increase in growth and migration rates of tidal sand ridges for a bimodal mixture, whilst the wavelength of the ridges remains unchanged. A symmetrical externally forced tidal current results in a grain-size distribution which is in phase with the ridges. Incorporation of an additional external M4 tidal constituent or a steady current results in a phase shift between the grain-size distribution and ridge topography. These results show a general agreement with observations. The physical mechanism responsible for the observed grain-size distribution over the ridges is also discussed.Responsible Editor: Jens Kappenberg  相似文献   

13.
Despite increasing recognition of the potential of aquatic biota to act as ‘geomorphic agents’, key knowledge gaps exist in relation to biotic drivers of fine sediment dynamics at microscales and particularly the role of invasive species. This study explores the impacts of invasive signal crayfish on suspended sediment dynamics at the patch scale through laboratory and field study. Three hypotheses are presented and tested: (1) that signal crayfish generate pulses of fine sediment mobilisation through burrowing and movement that are detectable in the flow field; (2) that such pulses may be more frequent during nocturnal periods when signal crayfish are known to be most active; and (3) that cumulatively the pulses would be sufficient to drive an overall increase in turbidity. Laboratory mesocosm experiments were used to explore crayfish impacts on suspended sediment concentrations for two treatments: clay banks and clay bed substrate. For the field study, high frequency near‐bed and mid‐flow turbidity time series from a lowland river with known high densities of signal crayfish were examined. Laboratory data demonstrate the direct influence of signal crayfish on mobilisation of pulses of fine sediment through burrowing into banks and fine bed material, with evidence of enhanced activity levels around the mid‐point of the nocturnal period. Similar patterns of pulsed fine sediment mobilisation identified under field conditions follow a clear nocturnal trend and appear capable of driving an increase in ambient turbidity levels. The findings indicate that signal crayfish have the potential to influence suspended sediment yields, with implications for morphological change, physical habitat quality and the transfer of nutrients and contaminants. This is particularly important given the spread of signal crayfish across Europe and their presence in extremely high densities in many catchments. Further process‐based studies are required to develop a full understanding of impacts across a range of river styles. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

14.
A controlled reservoir release from Llyn Celyn to the Afon Tryweryn, Wales, U.K., has been used to study suspended load and turbidity variations. Turbidity was monitored continuously at two sites and 235 suspended solids samples were obtained at these and three additional sites during the passage of the release wave. The results are compared with data for a natural tributary flood event. The reservoir release data relate to sediment source depletion and reflects changing sources along the channel. Close to the dam, fine organic matter dominates the seston which scanning electron microscopy revealed to be predominantly allochthonous organic matter, with algal fragments and inorganic diatom frustules, derived from the periphyton of the channel bed. Coulter Counter analysis showed the seston to be relatively coarse with a median particle-size of 20 μm. Within 3 km of the dam, however, minerogenic particles dominate the sediment load of which more than 90 per cent is finer than 10 μm. This represents the flushing of channel-bed accumulations derived from tributary sources. The relationships between suspended sediment concentration and turbidity during the release are characterized by a marked, anticlockwise hysteresis. This contrasts with the clockwise hysteresis for the tributary flood event, but the different relationships cannot be explained by particle-size variations alone; seston composition also appears to be an important control.  相似文献   

15.
I. INTRODUCTIONWhen a sediment--laden flow reaches the backwater zone of a reservoir, the suddenreduction of flow velocity causes sediment particles to settle toward the river bed. Undercertain circumstsnces, it will plunge and form a layer of sediment--water mixture flowingbeneath the water surface. This flowing layer is called the turbidity current. A turbiditycurrent is relatively stable and has important impacts on reservoir sedimentation.In the case of deep reservoirs, due to temper…  相似文献   

16.
Tsunami Sediment Characteristics at the Thai Andaman Coast   总被引：1，自引：0，他引：1  
This paper describes and summarizes the 2004 Indian Ocean tsunami sediment characteristics at the Thai Andaman coast. Field investigations have been made approximately 3 years after the 2004 Indian Ocean tsunami event. Seven transects have been examined at five locations. Sediment samples have been collected for grain-size analyses by wet-sieve method. Tsunami sediments are compared to three deposits from coastal sub-environments. The mean grain-size and standard deviation of deposits show that shoreface deposits are fine to very fine sand, poorly to moderately well sorted; swash zone deposits are coarse to fine sand, poorly to well sorted; berm/dune deposits are medium to fine sand, poorly to well sorted; and tsunami deposits are coarse to very fine sand, poorly to moderately well sorted. A plot of deposit mean grain-size versus sorting indicates that tsunami deposits are composed of shoreface deposits, swash zone deposits and berm/dune deposits as well. The tsunami sediment is a gray sand layer deposited with an erosional base on a pre-existing soil (rooted soil). The thickness of the tsunami sediment layer is variable. The best location for observation of the recent tsunami sediment is at about 50–200 m inland from the coastline. In most cases, the sediment layer is normally graded. In some cases, the sediment contains rip-up clasts of muddy soils and/or organic matter. The vertical variation of tsunami sediment texture shows that the mean grain-size is fining upward and landward. Break points of slope in a plot of standard deviation versus depth mark a break in turbulence associated with a transition to a lower or higher Reynolds number runup. This can be used to evaluate tsunami sediment main layer and tsunami sediment sub layers. The skewness of tsunami sediment indicates a grain size distribution with prominent finer-grain or coarse-grain particles. The kurtosis of tsunami sediment indicates grain-size distributions which are flat to peak distribution (or multi-modal to uni-modal distribution) upward. Generally, the major origins of tsunami sediment are swash zone and berm/dune zone sands where coarse to medium sands are the significant material at these locations. The minor origin of tsunami sediment is the shoreface where the significant materials are fine to very fine sands. However, for a coastal area where the shoreface slope is mild, the major origin of tsunami sediment is the shoreface. The interpretation of runup number from tsunami sediment characteristics gets three runups for the 2004 Indian Ocean tsunami at the Thai Andaman coast. It corresponds to field observations from local eyewitnesses. The 1st runup transported and deposited more coarse particles than the following runups. Overall, the pattern of onshore tsunami sediment transportation indicates erosion at swash zone and berm/dune zone, followed by dynamic equilibrium at an area behind the berm/dune zone and after that deposition at inland zone until the limit of sediment inundation. The total deposition is a major pattern in onshore tsunami sediment transportation at the deposition zone which the sediment must find in the direction of transport.  相似文献   

17.
Dam break flows and resulting river bed erosion can have disastrous impacts on human safety,infrastructure,and environmental quality.However,there is a lack of research on the mobility of non-uniform sediment mixtures resulting from dam break flows and how these differ from uniform sized sediment.In this paper,laboratory flume experiments revealed that coarse and fine fractions in non-uniform sediment had a higher and a lower bed-load parameter,respectively,than uniform sediments of the same size.Thus,the finer fractions were more stable and the coarser fractions were more erodible in a nonuniform bed compared to a uniform-grained bed.These differences can be explained by the hiding and protrusion of these fractions,respectively.By investigating changes in mobility of the mixed-size fractions with reservoir water levels,the results revealed that at low water levels,when the coarser fractions were only just mobile,the bed-load parameter of the finer fractions was higher than the coarser fractions.The opposite was observed at a higher water level,when a significant proportion of the coarsest fractions was mobilized.The higher protrusion of these grains had an important effect on their mobility relative to the finer grains.The transported sediment on these mixed-sized beds was coarser than the initial bed sediment,and became coarser with an increase in reservoir water level.  相似文献   

18.
Observations on phreatomagmatic ash deposits of Phlegraean Fields and Vesuvius supply evidence for the origin of vesiculated tuff in a cool environment. Early deposition by fallout of a matrix-free bed of damp accretionary lapilli is followed by deposition of cohesive mud or a mud rain. The lapilli bed becomes partly or completely transformed into a vesiculated tuff by mud percolation and eventual coalescence of accretionary lapilli with consequent trapping of air originally contained in the interstices. The proposed mechanism accounts for vesiculated tuff formation in distal deposits beyond limits commonly attained by pyroclastic surges. This same mechanism may, nevertheless, also operate in proximal tuff-ring and cone deposits during fallout of phreatomagmatic ash separating bed sets in surge-dominated successions. The sequence of events in the proposed model fits well with the evolution of a cooling phreatomagmatic ash cloud in which early ash aggregation (accretionary lapilli fallout) is followed closely by steam condensation (mud or muddy rainfall). This new model invoking a cool-temperature origin is intended to be complementary to previously proposed theories. Although difficult to assess because of the often complete obliteration of original lapilli, the process is believed to be relatively common in the generasion of vesiculated tuffs within phreatomagmatic deposits.  相似文献   

19.
Tropical mobile mud belts represent a major class of biogeochemical and diagenetic systems characterized by extensive and frequent physical reworking of fine-grained, organic-rich deposits underlying oxygenated waters. Large regions of the Gulf of Papua, Papua New Guinea deltaic complex are dominated by such conditions. A reworked mud belt lies within the inner shelf between 10 and 20 m depth on a sedimentary clinoform derived from coalescing deltas. Deposits across the topset are typically suboxic, nonsulfidic over the upper 0.5–1 m, and have low to moderate maximum pore water concentrations of dissolved Fe(II) and Mn(II) (100–200, but up to 800 μM). Sediments are reactive, with surficial ΣCO2 production 0.1–0.3 mM d−1 and benthic O2 fluxes 23±15 mmol m−2 d−1 (upper 20 cm). The highest rates occur within inner topset deposits (10–20 m) and near the high accumulation rollover region of the topset–foreset beds (40–50 m). Lower rates are found inshore along intertidal channels—mangrove fringe and within scoured or exposed consolidated deposits of the middle topset region. Remineralization rate patterns are independent of relative dominance by terrestrial or marine carbon in sediments. Dissolved O2 usually penetrates 2–5 mm into surface sediments when macrofaunal burrows are absent. More than 75% of the highly reactive sedimentary Fe(III) pool (350–400 μmol g−1) is typically diagenetically reduced in the upper 0.5 m. Pore water can be measureably depleted at depths >0.5 m, but dissolved H2S generally remains below detection over the upper 1–2 m. As in other deltaic topset regions, concentration gradients often indicate that compared to many marine deposits of similar sediment accumulation rates, relatively refractory Corg is supplied to the SO4 reducing zone. Sedimentary C/S ratios are 4–6 within the suboxic topset regions but decrease to <3 in offshore foreset beds where sulfidic diagenesis dominates. Only 15–20% of the diagenetically reduced Fe(II) is pyritic and a maximum of 10–25% is carbonate, implying that most Fe(II) is associated with authigenic or lithogenic silicates or oxides. The dominance of suboxic, nonsulfidic diagenetic processes reflect coupling between delivery of oxide-rich terrestrial debris, remobilization and reoxidation of deposits, and repetitive entrainment/remineralization of both labile and refractory organics. Distinct sedimentary indicators of reactive, suboxic mobile mud belts within tropical climatic zones are: abundant total highly reactive Fe (ΣFeR )>300 μmol g−1; most reactive Fe is diagenetically reduced (ΣFe(II)/ΣFeR0.7–0.8); the proportion of diagenetically reduced Fe present as pyrite is low (Py–Fe(II)<0.2); C/S 4–8; and Corg/particle surface area <0.4 (mg C m−2). These depositional environments must be most common in tropical climates during high sea stand.  相似文献   

20.
Secondary flows induced by the blocking effect of a river plume on a transverse upwelling are investigated in a microtidal region of freshwater influence (ROFI). A nested version of the SYMPHONIE primitive-equation free-surface model for 3-D baroclinic coastal flows has been developed for the Rhône ROFI. The main characteristics of the model are a generalized sigma coordinate system in finite differences, using a time splitting for external and internal modes and high-order numerical advection schemes for density fields in combination with an modified turbulence closure scheme. The nesting system consists of two grids forced by the high-resolution ALADIN model atmospheric data. The coarse grid of 3 km resolution for the whole Gulf of Lions allows the forcing of the Liguro-Provençal large-scale current when the fine mesh of 1-km resolution is centred on the river mouth of the Grand Rhône. Documented field experiments from the Biodypar 3 field campaign performed during March 1999 are used for validation. Numerical results, CTD profiles and a SPOT TSM visible image are in good agreement concerning the shape and structure of the river plume. Other coastal flow features can be observed from satellite imagery. Computations of realistic situations recover these main secondary structures. Complementary process-oriented runs give an explanation of how the coastal upwelling induced by an inhomogeneous offshore wind is destabilized by the combination of the river plume and along-shelf current-blocking effects. In the end, a factor-separation analysis provides evidence that the locally non-linear effects in momentum contribute to the occurrence of secondary vortices.Responsible Editor: Phil Dyke  相似文献   

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