Search

A significant contourite depositional system (CDS) on the continental slope of the southern Argentine margin is described here for the first time. This system contains both erosive and depositional features that have resulted from several factors, including topographic intensification of the Antarctic-sourced water masses, the systematic northward decrease in speed of these water masses, a northward increase of downslope sedimentary processes, and local tectonic influences. This system is an exceptional example of a CDS that started to develop at the time of the Eocene-Oligocene boundary, potentially coeval with the opening of the Drake Passage. However, a new margin morphology, characterized by a complex terraced slope lacking any continental rise, developed after a major paleoeeanographic change in the middle to late Miocene. We infer that this change resulted from the extension of North Atlantic Deep Water circulation into the Southern Hemisphere and the deepening of Antarctic Bottom Water circulation in the Argentine Basin.

The paper presents a case study of sediment, termed soft lodgement till (a product of subglacial deposition by active ice), described previously on the basis of macro-evidence from Poland and Canada but not yet studied in detail in one particular site. The till at Anielinek (about 7 m thick) was examined by macroscopic observation, thin section description, and qualitative and quantitative scanning electron microscopy (SEM) analyses. It shows a macroscopic massive structure and overlies a flat top of glaciolacustrine sediments. These sediments form upward intrusions within the till--owing to the existence of reversed density gradients among the deposits--and the upper parts of the intrusions are tilted in consistent directions within the successive parts of the till. Moreover, SEM images also record uniformly oriented microintrusions within this diamictic material. The occurrence of these macro- and microstructures within the successive parts of the till point to an almost continuous process of intrusion during ongoing deposition of the till and to water saturation of the subglacial environment. Such conditions reduced friction of the glacier bed against its substratum, and glacial debris must have been mainly melt-released from the moving glacier base. This process was responsible for deposition of sediment (soft lodgement till) that was immediately and continuously deformed by glacial stress during the ongoing accretion of debris from the glacier base. Cet article presente une etude de cas de sediments, classes en tant que till de fond mou (un produit de la deposition sous la glace par des glaces en mouvement), lesquels avaient ete anterieurement decrits en se basant sur des macroe vidences provenant de la Pologne et du Canada, et qui n'avaient pas encore ete etudies en detail a un site en particulier. Le till d'Anielinek (d'une epaisseur d'environ 7 m) a ete etudie par des observations macroscopiques, par des descriptions de lames minces et par des analyses qualitatives et quantitatives sous un microscope electronique. Ce till presente une structure macroscopique massive et il repose sur une surface plane de sediments glacio-lacustres. Ces sediments forment des intrusions montantes a l'interieur du till--en raison de l'existence de gradients de densite inverses entre ces depots--et les parties superieures des intrusions sont inclinees dans des directions concordantes a l'interieur des parties successives du till. De plus, des images du microscope electronique a balayage montrent aussi des micro-intrusions a orientation uniforme a l'interieur de ce materiel diamictique. L'occurrence de ces macro- et microstructures dans diverses parties successives du till indiquent un processus presque continuel d'intrusion durant le cours de la deposition du till ainsi qu'une saturation en eau de l'environnement sous-glaciaire. De telles conditions diminuent le frottement du lit du glacier contre le substrat et les debris glaciaires devaient surement provenir de la fonte de la base du glacier en mouvement. Ce processus a ete responsable de la deposition de sediments qui sont classes en tant que till de fond mou et qui sont immediatement et continuellement deformes par les contraintes glaciaires durant l'accretion de debris de la base du glacier. [Traduit par la Redaction], Introduction The problem of the origin of subglacial sediments has been the subject of thorough studies and discussion (e.g., Elson 1961; Flint 1971; Boulton 1975; Dreimanis 1989; Ruszczynska-Szenajch 2001; van [...]

A mass balance was constructed quantifying all known sources and sinks for the metals Ag, Cd, Cu, and Pb in New Haven Harbor, Connecticut, USA. Sources included direct atmospheric deposition, rivers, treated sewage effluent, combined sewer overflows, and permitted industrial discharge. Sinks were burial in sediments, tidal exchange with Long Island Sound, removal in salt marshes, and dredging. All of these fluxes were measured directly, rather than estimated, and uncertainties were quantified. The mass balance closed successfully within the uncertainty of the measurements. Riverine inputs account for most of the total yearly metal flux. Metal concentrations in the river can be approximated as a simple linear function of discharge. Salt marshes remove an amount of metal equivalent to 20%-30% of the flux from the river before it reaches the harbor. Burial in sediments is the major sink for all metals examined, but dredging acts as a substantial short-circuit of this sink. Tidal exchange appears to be a relatively small term; however, it is also the least well quantified. Sewage treatment plant (STP) effluent and combined sewer overflow discharge are minor contributors to the overall metal balance, except in the case of Ag. Metal concentrations in STP effluent are a linear function of discharge. Atmospheric deposition is of minor importance but is comparable to sewage effluent. Lakes can be used as natural collectors and indicators of atmospherically deposited metals.

Sinking aggregates are the major component of the vertical particulate flux in most regions of the ocean. Controlling factors for aggregate remineralization rates and solute exchange with the surrounding water, however, are poorly quantified because of few empirical data. To study the role of flow and diffusion on aggregate remineralization rates, oxygen distributions were mapped within and around aggregates by use of microelectrodes in a flow field similar to that experienced by sinking aggregates. The oxygen distribution was asymmetrical with a wake of undersaturated water at the rear (downstream) of the aggregates. Oxygen concentrations within the aggregates were >80% of air saturation. The diffusive fluxes of oxygen at the aggregate-water interface were similar along the equator and at the downstream pole for a wide range of different aggregate sources (field-sampled diatom aggregates, lab-made diatom aggregates, aggregates formed from freeze-thawed diatoms, and zooplankton detritus aggregates) measured at various temperatures. Remineralization rates were reaction limited and, hence, determined by substrate quality and quantity rather than by transport-limited oxygen supply during sedimentation at ambient oxygen concentrations above ~25 [micro]M.

`Les Sillons' (meaning `furrows') is the local name given to an outstanding example of a relict foredune plain on Les Iles de la Madeleine, Quebec. This series of shore-parallel dune [...]

Research indicates that nitrogen export in Finland's Oulujoki river basin is primarily affected by agriculture, constituting 17% of total nitrogen export with a range of 8% in the uppermost region of the basin and 38% in the lowermost region. Forestry management contributes 16% of the total export, with 11% in the uppermost and 24% in the lowermost regions.

1 Artificial environmental gradients were established in a series of pot experiments to investigate the effect of salinity, sediment type and waterlogging on the growth, and interactions between Spartina anglica and Puccinellia maritima. In each experiment, one environmental variable was manipulated and plants grown in pairwise combinations to examine the effect of the environmental factor on the intensity of intra- and interspecific interactions, quantified using the Relative Neighbour Effect (RNE) index. 2 Puccinellia was found to exert an asymmetric, one-way competitive dominance above ground over Spartina in experiments where gradients of sediment type and waterlogging were established. The intensity of the competition was highest in conditions with the least abiotic stress and lower or non-existent where stress was increased. 3 The intensity of the above-ground competition was greatest in loam and least in sand sediments. Reduction in competitive intensity in sand was accompanied by an increase in below-ground Spartina biomass and it is suggested that the production of rhizomes is a potential mechanism by which this species can expand vegetatively into areas without competition. 4 Interspecific competition on Spartina from Puccinellia also varied in intensity in the waterlogging experiment, being more intense in non-immersed treatments, where abiotic stress was reduced. 5 The competitive dominance of Puccinellia and the competition avoidance mechanism shown by Spartina in these experiments help to explain the successional interactions between the species along environmental gradients in natural salt marsh communities. Keywords: competitive intensity, interspecific competition, marsh elevation gradient, Puccinellia maritima, Relative Neighbour Effect, Spartina anglica

The Channeled Scabland, Washington State, United States, is only partly the result of erosion by the catastrophic drainage of Glacial Lake Missoula: there were other sources of meltwater. Recent sedimentary investigations of some sites in the Missoula basin, and in the Channeled Scabland, support a single large late Wisconsin flood, as opposed to multiple floods proposed for this time period. Sediment in the Glacial Lake Missoula basin records rapid infill by jokulhlaups draining into Lake Missoula from upstream, punctuating a long period of normal varve sedimentation. This was independent of sedimentation in the main Scabland tract, where proximal and distal rhythmic beds are explained as resulting from multiple pulses, or surges, within a single flood. Geomorphic and sedimentary evidence supports the conclusion that drainage from the Cordilleran trunk valleys was important, and pulses were probably related to the drainage of these valleys.

Hybrid fall deposits in the Bishop Tuff show features common to both archetypal fall and surge deposits. Like normal-fall deposits, they have an overall plane-parallel bedding and flat-lying pumice clasts but also, like surge deposits, they show variable development of cross-bedding, some crystal and pumice sorting, and some rounding of pumice clasts. All variations exist from normal-fall deposits, through streaky material with incipient development of cross-bedding, to the hybrid fall deposits with well-developed cross-bedding. The streaky and hybrid deposits are interpreted as fall material contemporaneously redeposited by strong (up to 40 m/s) swirling winds, comparable to firestorm whirlwinds, generated by air currents associated with coeval emplacement of pyroclastic flows. Recognition of hybrid fall deposits is important in interpreting the dynamics of explosive eruptions and correctly assessing volcanic hazards. However, although such deposits may be commonly produced by explosive eruptions, especially where pyroclastic flows accompany fall activity, they are likely to be overlooked, or wrongly interpreted as surge deposits or secondary, reworked material.

An analysis of the Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain Formations of Utah and Colorado has resulted in a general sequence-stratigraphic model for nonmarine rocks. In this model, nonmarine deposition is governed by changes in basin accommodation development and corresponding shifts in depositional base level. These fluctuations result in deposition of systematically varying facies and architectural elements that allow nonmarine depositional sequences to be recognized. Internally, nonmarine depositional sequences comprise three systems tracts - degradational, transitional, and aggradational - which are analogous to the low-stand, transgressive, and high-stand systems tracts of marine depositional sequences. Degradational systems tracts overlie sequence-bounding unconformities and consist of relatively coarse-grained, low-sinuosity fluvial deposits that are either contained within incised valleys or deposited as widespread, thin sheets above shallow erosion surfaces. Transitional systems tracts represent an increase in basin accommodation development following degradational systems tract deposition. They are marked by the transition from laterally continuous, low-sinuosity fluvial channel sandstones and conglomerates to lenticular and ribbon-like, meandering and anastomosing channel sandstones. Aggradational systems tracts are characterized by meandering-anastomosing channel sandstones and abundant fine-grained overbank and lacustrine deposits. The Upper Jurassic-Lower Cretaceous nonmarine rocks of the study area contain three depositional sequences. The first of these, the UJ-1 sequence, consists primarily of an aggradational systems tract overlain by a sequence-bounding unconformity. However, the lower parts of this sequence are transitional with older marine rocks and can be considered the late stages of a marine high-stand systems tract. The upper Morrison sequence (UJ-2) consists of degradational, transitional, and aggradational systems tracts. Above the UJ-2 sequence are a sequence-bounding unconformity and degradational and transitional systems tracts of the LK-1 sequence represented by the Buckhorn Conglomerate. The Buckhorn is overlain by a sequence-bounding unconformity and transitional-aggradational systems tracts of the LK-2 sequence that is composed of the upper part of the Cedar Mountain Formation. The Upper Jurassic-Lower Cretaceous sequences in Utah and Colorado can be traced regionally and correlated with nonmarine depositional sequences in central Utah and sequences that contain nonmarine, transitional, and marine rocks in central Wyoming. These sequences were deposited in the back-bulge, forebulge, and distal foredeep depozones of the Late Jurassic-Early Cretaceous foreland-basin system.

A dynamic model is developed to simulate the sedimentary facies on the Northern California Shelf. In the model, dispersal mechanisms of small time scale are described by deterministic sediment-transport equations. However, at large time scale, the shelf dispersal system is described in probabilistic terms owing to its stochastic behavior. The probability of storm-bed generation is described by the bed generation potential, [P.sub.g], which is determined by the probability density distribution function of the local storms. The bed preservation potential, [P.sub.p], valued between 0 and 1, is a parameter that describes the probable truncation a storm bed that will be buried in a bed succession. The truncation depends on the erosion and deposition caused by subsequent random storms. By estimating the preservation potentials of all generated storm beds, the most probable storm-bed succession is predicted. The computations support the hypothesis that sedimentary facies are produced by the coupled mechanisms of progressive sorting and stratal condensation on continental margins.

The primary function of a drainage culvert - to convey the design flow effectively - is often greatly impaired or completely lost due to the presence of deposited sediments. The effect of sediments on the total head loss within the culvert may be quite significant. A case study is presented that describes the performance of a roadway drainage culvert designed for clear-water flow conditions in an alluvial stream carrying sediments. The actual capacity of the culvert is approximately only 20% of the presumed design capacity, as a result of sediment deposition not accounted for in the design. The case study reviews design errors and demonstrates the necessity of sediment-transport calculations when designing roadway drainage culverts in ephemeral alluvial streams. Ignoring sediment transport may have adverse effects, including significant road and adjacent-property flooding as well as continuous and costly maintenance problems. It is more economical to undertake a complete sediment-transport study before design than to deal with continuous maintenance after the project is built.

This paper explores how suspended clays alter the trophic interactions that govern composition and density of the zooplankton community in a small eutrophic North Carolina (USA) lake. Enclosures (12 m cube) were used in a complete, triplicated, cross-classified design. The four treatments were; BASS (1 piscivorous fish/enclosure), BASS + CLAY, SUNFISH (16 planktivorous fish/enclosure), SUNFISH + CLAY. The +CLAY treatments received 100 g.m to the -2.d to the -1 of montmorillonitic clay. It was anticipated that turbidity from clay would release crustacean zooplankton from visual planktivores. Contrary to this expectation, crustacean zooplankton populations in the SUNFISH + CLAY were fourfold less than in the SUNFISH treatment. The reduction was not linked to deleterious effects of clay, since zooplankton populations were higher in the BASS + CLAY than the BASS treatment. Instead, crustacean zooplankton appeared to be governed primarily by Chaoborus predation. Suspended clay weakened the link between visually predaceous fish and Chaoborus, which in turn strengthened the effect of Chaoborus predation on crustacean zooplankton. Since fish selectively cropped larger instars of Chaoborus, rotifers (primarily Keratella cochlearis), which are mostly eaten by smaller instars of Chaoborus, were unaffected by clay.

Newly identified radiolarians from ribbon chert of the Bridge River complex in the southeastern Canadian Coast Mountains range in age from Mississippian to late Middle Jurassic. The Bridge River complex and the associated Cadwallader arc and Tyaughton and Methow basins lie between the Intermontane superterrane to the east and the Insular superterrane to the west. Triassic-Middle Jurassic development of the Bridge River subduction-accretion complex records an important component of convergence between these superterranes. The time span represented in the Bridge River complex (approximately 170 m.y.), one of the longest known age ranges for chert sedimentation, suggests that the Bridge River complex contains remnants of a long-lived, potentially far-traveled Panthalassic oceanic domain.

We investigated how particle size influences deposition and transport of fine particulate organic matter in streams. Field additions of very fine (VFPOM, 15-52 [micro]m), fine (FPOM, 53-106 [micro]m), and medium (MPOM, 107-250 [micro]m) detritus and live diatoms (Asterionella sp.) were used to quantify the longitudinal loss rate ([k.sub.P]) of each material type and to derive estimates of mean transport distance ([S.sub.P]) and field deposition velocity ([v.sub.dep]). In all experiments, smaller particles deposited more slowly, and thus traveled farther, than larger size classes. Significant differences in [k.sub.P] were detected in four of seven paired FPOM and VFPOM particle additions. [v.sub.dep] estimates were neither equivalent nor closely associated with calculated quiescent water fall velocities ([v.sub.fall]) for all size classes. Variation in [S.sub.P] and [v.sub.dep] of FPOM and VFPOM were strongly correlated across hydrological conditions (r = 0.94 and 0.92, respectively). Variation in [v.sub.dep] was poorly associated with physical attributes of the stream. Transport distances were positively associated with the cross-sectional area of the transient storage zone ([A.sub.S]) and the uptake length of water ([S.sub.W]) for both size classes. We argue that local hydrological and benthic conditions establish a minimum rate of particle deposition and that departures from this rate due to gravitational forces begin to occur at particle diameters similar to the larger size classes used in this study (50-100 [micro]m).

EVIDENCE OF STORM AND HUMAN IMPACTS ON ESTUARINE SEDIMENTATION, WEEKS BAY, BALDWIN COUNTY, ALABAMA. Lauree Stober, Vince Beebe, Doug Haywick and David Allison, Department of Earth Sciences, University of South [...]

Samples of vent gas and gas hydrate on the Gulf of Mexico slope were collected by research submersible (~540 m water depth) and by piston coring (~1060-1070 m water depth). Although gas hydrate that crops out is transiently unstable, the larger volume of structure II gas hydrate in the gulf is stable or increasing in volume because gas from the subsurface petroleum system is venting prolifically within the gas hydrate stability zone. Vent gas from gas hydrate shows no meaningful molecular evidence of gas hydrate decomposition. Gas hydrate fabrics, mainly vein fillings, are typical of ongoing crystallization. Once crystallized, most hydrocarbons are protected from bacteria within the crystal lattice of gas hydrate. A leaky petroleum system is proposed to be the main source of thermogenic greenhouse gases in the central gulf. Stable gas hydrate sequesters large volumes of greenhouse gases, suggesting that gas hydrate may not be a significant factor in models of climate change at present. Keywords: gas hydrates, methane, hydrocarbons, recent sedimentation, climate effects, Gulf of Mexico.

Rivers around the world are being regulated by dams to accommodate the needs of a rapidly growing global population. These regulatory efforts usually oppose the natural tendency of rivers to flood, move sediment, and migrate. Although an economic benefit, river regulation has come at unforeseen and unevaluated cumulative ecological costs. Historic and contemporary approaches to remedy environmental losses have largely ignored hydrologic, geomorphic, and biotic processes that form and maintain healthy alluvial river ecosystems. Several commonly known concepts that govern how alluvial channels work have been compiled into a set of 'attributes' for alluvial river integrity. These attributes provide a minimum checklist of critical geomorphic and ecological processes derived from field observation and experimentation, a set of hypotheses to chart and evaluate strategies for restoring and preserving alluvial river ecosystems. They can guide how to (i) restore alluvial processes below an existing dam without necessarily resorting to extreme measures such as demolishing one, and (ii) preserve alluvial river integrity below proposed dams. Once altered by dam construction, a regulated alluvial river will never function as before. But a scaled-down morphology could retain much of a river's original integrity if key processes addressed in the attributes are explicitly provided. Although such a restoration strategy is an experiment, it may be the most practical solution for recovering regulated alluvial river ecosystems and the species that inhabit them. Preservation or restoration of the alluvial river attributes is a logical policy direction for river management in the future.

Reefs of Isla Colon, Bocas del Toro are subjected to extensive sedimentation with reduced water clarity in many sites. Yet, recent surveys have revealed new coral growth in some areas. We measured sedimentation rates at three transect sites with high, intermediate and low turbidity levels. Open-cell sediment traps were placed at 15 m intervals on 160 m transect lines perpendicular to shoreline. We collected samples every three days and made horizontal Secchi disk readings. Wind velocity and direction as well as weather conditions were also noted and a coral survey was made. A two-way ANOVA with three replicates revealed significant differences in sedimentation rate and some regions of the high-sediment transect paradoxically had living coral while intermediate-level was devoid of coral. High sedimentation if intermittent does not seem to prevent coral growth.

The sediment cores from the pristine systems in the Amazon basin are examined in relation to the floodplains in northern Bolivia. The flood plain depositional layers are linked to the rising floods that occur during La Nina event and it is found that the storm rainfall is significantly heavier in the mountains than on the flood plain.

The eastern continental margin of Baffin Island around Ocean Drilling Program (ODP) Site 645 was surveyed by using single-channel airgun, high-resolution boomer systems and piston cores. The data show that much of the upper slope between the 300 and 1200 m isobaths is erosional. Major sliding and rotational slumping has removed several hundred metres of sediment from the upper slope, giving the sea bed a steplike morphology. From 1200 m to the 2300 m isobath, the slope is constructional and is characterized by abundant acoustically transparent lenses, some of which are traced upslope into acoustically transparent to internally deformed wedge-shaped bodies. These lenses are interpreted to be debris-flow deposits and their abudnance in the lower slope indicates frequent upper slope failures. The wedge-shaped bodies are much less common and are interpreted to be larger slides and/or slumps. Near the base of the slope, fields of diapiric structures pierce the acoustically well stratified section and locally produce small mounds on the sea floor. On the basis of correlation with ODP Site 645, they are interpreted as mud diapirs. The combined data show that the lower slope of Baffin Island is constructed predominantly of shingled lenses of debris-flow deposits and rotated slump blocks that originated from major erosion of the upper slope.

The pollution of rivers, lakes, and sea by sedimentation is increasing on a worldwide scale. Sedimentation pollution of coastal marine areas is an especially serious problem in Southeast Asia, where fish harvested from coastal waters serve both as a major source of protein for human consumption and a significant source of foreign exchange through exports. One major cause of sedimentation is logging, and in particular the building of the roads and other infrastructure needed to get the logs onto the international market.

S.L. O'Hara and colleague's suggestion that prehispanic agriculture eroded the land upstream from Mexico's Lake Patzcuaro ignores climatic changes and the displacement of high-pressure cells. Reply: Deposition changes following the introduction of maize were much greater than ever before, indicating that climate was not the decisive factor. Erosion also appeared greatest during wet periods, more consistent with agricultural than natural erosion.