Variations In Sediment Flux Research Articles

How plausible are high-frequency sediment supply-driven cycles in the stratigraphic record?

This paper is an attempt to quantify the plausible time scales of clastic sediment supply variations at the entrance of sedimentary basins. Our approach is based on the sedimentary system concept, which simplifies natural systems by dividing them into three zones of dominant processes: the erosion, the transfer, and the sedimentation subsystems. We examine recent results from geomorphology, which show that frequent climate changes can induce high-frequency sediment flux variations at the outlet of the source area. We put forward the crucial role of the transfer subsystem, which conveys sediment from the erosion zone to the basin. By applying a diffusive model to a number of worldwide rivers, we extend from large (>1000 km) to intermediate (>300 km) rivers the previous finding that the transfer subsystem acts as a buffer for short periods sediment pulses (tens to hundreds of kiloyears). This implies that high-frequency stratigraphic cycles in clastic accumulations fed by large drainage systems are unlikely to reflect sediment supply cycles of tens to hundreds of thousands of years of periodicities.

A 900 k.y. record of strath terrace formation during glacial-interglacial transitions in northwest China

The timing of the development of strath terraces with respect to climatic variability remains equivocal. Previous studies attribute strath-terrace formation to glacial or interglacial climates or to variations in sediment and water fluxes that cause lateral erosion followed by vertical incision. A chronology of strath-terrace formation spanning similar to900 k.y. has been generated on the basis of loess-paleosol couplets and paleomagnetic, thermoluminescence, and radiocarbon dating of strath terraces in the Qilian Shan of northeastern Tibet. Repetitive stratigraphic and geomorphic patterns on each terrace indicate that they formed during glacial-interglacial transitions. Long-term bedrock incision rates and inferred rock uplift rates appear steady and unrelated to strath formation over the past 900 k.y.

Varved lacustrine sediments of Lake Tougou-ike, Western Japan, with reference to Holocene sea-level changes in Japan

Eighteen metres of laminated sediments were recovered in a Holocene sediment core taken from Lake Tougou-ike in 1992. The lamination, evident from the alternation of light- and dark-coloured laminae, records cyclic deposition of biochemical sediments. Annual environmental changes in and around the lake caused seasonal variations in sediment flux to the lake floor, explaining these cyclic variations in sedimentary facies. AMS 14 C dating of organic remains from 40 horizons in the succession supports the interpretation of each light-to-dark couplet as an annual increment. Each laminated couplet is identified as a non-glacial varve, formed mainly by seasonal changes in diatom productivity. Varve counting and 14 C dating provide a detailed Holocene chronological framework, with time resolution on the order of individual years, within which to reconstruct paleoenvironmental changes recorded in the sediment core. Holocene relative sea-level changes at Lake Tougou-ike were estimated with ±10 yr precision using the variable authigenic iron mineral content of the sediment as a proxy. The results demonstrate that the relative sea level at Lake Tougou-ike rose abruptly at about 8900, 7700, 5600, 4000, 3600 and 2700 cal yr BP, and decreased gradually in the intervening intervals. These relative rises in sea level are demonstrated to be coeval with ice-sheet discharge events recorded in sediment cores from the North Atlantic Ocean. Abrupt coastal environmental changes caused by sea-level changes likely took place throughout the Japanese islands, and will probably be identified in both the Holocene and the last glacial interval, the latter being a time when Dansgaard-Oeschger cycles were clearly active in the North Atlantic region.

Modeling erosion of sedimentary coasts in the western Russian Arctic

Morphodynamic modeling is employed in the present work to predict the long-term evolution (over the next 100 years) of typical sedimentary coasts in the western Russian Arctic. The studied objects are the coasts of Varandey (the Barents Sea), Baydaratskaya Bay and Harasavey (the Kara Sea). The model developed takes into account both the short-term processes (storm events) and long-term factors (for example, changes in sea level, inter-annual variations in gross sediment flux, lack or excess of sediment supply). Predicted and observed morphological changes in coastal profiles are shown to agree well for time scales ranging from weeks to decades. It is revealed that under given environmental conditions, the morphological evolution is strongly influenced by storm surges and associated wind-driven circulation. The water level gradient created by a surge generates a seaward flow at the bed. This outflow is shown to be an important destructive mechanism contributing to the erosion and recession of Arctic coasts. The rate of change is found to depend on both the exposure of the coast (relative to the direction of dominant winds) and its height above the sea. The open coast of Varandey is expected to retreat as much as 300–500 m over 100 years, while recession of the less exposed coasts of Baydaratskaya Bay would not exceed about 100 m/century. If long-term sediment losses are insignificant, the rate of erosion decays with time and the morphodynamic system may tend toward equilibrium. It is concluded that the expected relative sea-level rise (up to 1 m over the nearest 100 years) is non-crucial to the future coastal evolution if an erosion activity is already high enough.

A circumpolar perspective on fluvial sediment flux to the Arctic ocean

Quantification of sediment fluxes from rivers is fundamental to understanding land‐ocean linkages in the Arctic. Numerous publications have focused on this subject over the past century, yet assessments of temporal trends are scarce and consensus on contemporary fluxes is lacking. Published estimates vary widely, but often provide little accessory information needed to interpret the differences. We present a pan‐arctic synthesis of sediment flux from 19 arctic rivers, primarily focusing on contributions from the eight largest ones. For this synthesis, historical records and recent unpublished data were compiled from Russian, Canadian, and United States sources. Evaluation of these data revealed no long‐term trends in sediment flux, but did show stepwise changes in the historical records of two of the rivers. In some cases, old values that do not reflect contemporary fluxes are still being reported, while in other cases, typographical errors have been propagated into the recent literature. Most of the discrepancy among published estimates, however, can be explained by differences in years of records examined and gauging stations used. Variations in sediment flux from year to year in arctic rivers are large, so estimates based on relatively few years can differ substantially. To determine best contemporary estimates of sediment flux for the eight largest arctic rivers, we used a combination of newly available data, historical records, and literature values. These estimates contribute to our understanding of carbon, nutrient, and contaminant transport to the Arctic Ocean and provide a baseline for detecting future anthropogenic or natural change in the Arctic.

Introduction

Abstract The stratigraphic record stored in sedimentary basins had traditionally been interpreted in terms of tectonic subsidence, climate change and sea-level rise or fall. During the last decade, however, the fundamental control of sediment flux in creating sedimentary sequences, and thus the need to understand variations in sediment flux in order to interpret the sedimentary record, has been recognized. Sediment flux is a first-order control on the pattern and distribution of sedimentary facies in depositional basins. Consequently, basin fills reflect the sediment flux across their margins and the hinterland conditions driving the sediment flux provide evidence of dynamic geomorphic processes. In context, the drainage systems and sedimentary basin can be regarded as a ‘production line’ with the sedimentary record giving valuable insight into long-term landscape evolution and geomorphological processes illuminating the evolution of sedimentary basins. This special publication contains a set of papers on sediment supply to basins, with a focus on clastic sediments. It presents a mix of hinterland and sedimentary basin studies with a gradation from orogenic belts to the deep marine. The papers present a current perspective on controls and constraints on sediment supply, a model and empirically based driven understanding of sediment flux and the interaction of geomorphology, landscape evolution and sedimentary geology to provide a more complete picture of the Earth system.

Sediment Transport and Trapping in the Hudson River Estuary

The Hudson River estuary has a pronounced turbidity maximum zone, in which rapid, short-term deposition of sediment occurs during and following the spring freshet. Water-column measurements of currents and suspended sediment were performed during the spring of 1999 to determine the rate and mechanisms of sediment transport and trapping in the estuary. The net convergence of sediment in the lower estuary was approximately 300,000 tons, consistent with an estimate based on sediment cores. The major input of sediment from the watershed occurred during the spring freshet, as expected. Unexpected, however, was that an even larger quantity of sediment was transported landward into the estuary during the 3-mo observation period. The landward movement was largely accomplished by tidal pumping (i.e., the correlation between concentration and velocity at tidal frequencies) during spring tides, when the concentrations were 5 to 10 times higher than during neap tides. The landward flux is not consistent with the long-term sediment budget, which requires a seaward flux at the mouth to account for the excess input from the watershed relative to net accumulation. The anomalous, landward transport in 1999 occurred in part because the freshet was relatively weak, and the freshet occurred during neapetides when sediment resuspension was minimal. An extreme freshet occurred during 1998, which may have provided a repository of sediment just seaward of the mouth that re-entered the estuary in 1999. The amplitude of the spring freshet and its timing with respect to the spring-neap cycle cause large interannual variations in estuarine sediment flux. These variations can result in the remobilization of previously deposited sediment, the mass of which may exceed the annual inputs from the watershed.

The link between abrupt climate change and basin stratigraphy: a numerical approach

To use basin stratigraphy for studying past climate change, it is important to understand the influence of evolving boundary conditions (river discharge and sediment flux, initial bathymetry, sea level, subsidence) and the complex interplay of the redistribution processes (plumes, turbidity currents, debris flows). To provide understanding of this complexity, we have employed source to sink numerical models to evaluate which process dominates the observed variability in a sedimentary record of two coastal Pacific basins, Knight Inlet in British Columbia and the Eel Margin of northern California. During the last glacial period, the Eel River supplied comparatively more sediment with a less variable flux to the ocean, while today the river is dominated by episodic events. Model results show this change in the variability of sediment flux to be as important to the deposit character as is the change in the volume of sediment supply. Due to the complex interaction of flooding events and ocean storm events, the more episodic flood deposits of recent times are less well preserved than the flood deposits associated with an ice-age climate. In Knight Inlet, the evolving boundary conditions (rapidly prograding coastline, secondary transport by gravity flows from sediment failures) are a strong influence on the sedimentary record. The delta and gravity flow deposits punctuate the sedimentary record formed by hemipelagic sedimentation from river plumes. Missing time intervals due to sediment failures can take away the advantage of the otherwise amplified lithologic record of discharge events, given the enclosed nature of the fjord basin.

Estimation of temporally averaged sediment delivery ratio using aggradational terraces in headwater catchments of the Waipaoa River, North Island, New Zealand

The sediment delivery ratio was estimated for two periods (28 years and eight years) following reforestation of seven tributary catchments (0·33 to 0·49 km2) in the headwaters of the Waipaoa River basin, North Island, New Zealand. In these catchments, gully erosion, which largely resulted from clearance of the natural forest between 1880 and 1920, is the main source of sediment to streams. Reforestation commenced in the early 1960s in an attempt to stabilize hillslopes and reduce sediment supply. Efforts have been partially successful and channels are now degrading, though gully erosion continues to supply sediment at accelerated rates in parts of the catchment. Data from the area indicate that the sediment delivery ratio (SDR) can be estimated as a function of two variables, ψ (the product of catchment area and channel slope) and A g (the temporally averaged gully area for the period). Sediment input from gullies was determined from a well defined relationship between sediment yield and gully area. Sediment scoured from channels was estimated from dated terrace remnants and the current channel bed. Terrace remnants represent aggradation during major floods. This technique provides estimates of SDR averaged over periods between large magnitude terrace-forming events and with the present channel bed. The technique averages out short-term variability in sediment flux. Comparison of gully area and sediment transport between two periods (1960–1988 and 1988–1996) indicates that the annual rate of sediment yield from gullies for the later period has decreased by 77 per cent, sediment scouring in channels has increased by 124 per cent, and sediment delivered from catchments has decreased by 78 per cent. However, average SDR for the tributaries was found to be not significantly different between these periods. This may reflect the small number of catchments examined. It is also due to the fact that the volume of sediment scoured from channels was very small relative to that produced by gullies. According to the equation for SDR determined for the Waipaoa headwaters, SDR increases with increasing catchment area in the case where A g and channel slope are fixed. This is because the amount of sediment produced from a channel by scouring increases with increasing catchment area. However, this relationship does not hold for the main stem of the study catchments, because sediment delivered from its tributaries still continues to accumulate in the channel. Higher order channels are, in effect, at a different stage in the aggradation/degradation cycle and it will take some time until a main channel reflects the effects of reforestation and its bed adjusts to net degradation. Results demonstrate significant differences among even low order catchments, and such differences will need to be taken into consideration when using SDR to estimate sediment yields. Copyright © 2001 John Wiley & Sons, Ltd.

Hillslope evolution by nonlinear creep and landsliding: An experimental study

Landscape evolution models are widely used to explore links between tectonics, climate, and hillslope morphology, yet mecha- nisms of hillslope erosion remain poorly understood. Here we use a laboratory hillslope of granular material to experimentally test how creep and landsliding contribute to hillslope erosion. In our experimental hillslope, disturbance-driven sediment transport rates increase nonlinearly with slope due to dilation-driven gran- ular creep, and become increasingly episodic at steep slope angles as creep gives way to periodic landsliding. We use spectral analysis to quantify the variability of sediment flux and estimate the slope- dependent transition from creep to landsliding. The power spec- trum of sediment flux steepens with hillslope gradient, exhibiting fractal 1/f scaling just below the creep-landsliding transition. By evolving the experimental hillslope under fixed base-level boundary conditions, we demonstrate how disturbance-driven transport gen- erates hillslope convexity. The transient evolution is consistent with numerical predictions derived from a recently proposed nonlinear transport model, as initially steep hillslopes are lowered rapidly by landsliding before slopes decay slowly by creep-dominated transport.

Bed-material transport estimated from channel morphodynamics: Chilliwack River, British Columbia

This study investigates the relation between channel changes, as mapped from aerial photography, and bed-material transport along Chilliwack River, British Columbia. Detailed mapping of channel features was completed for five dates between 1952 and 1991 using an analytical stereoplotter. Data were transferred to a geographic information system (GIS) to analyse changes during four consecutive periods. Erosion and deposition volumes along channel reaches were estimated by multiplying measured areal changes by the bed-material depth along each reach. Bed-material transport rates are related to morphologic changes using a sediment budget approach. The highest rate of transport for the four study periods is estimated as 55 000 ± 10 000 m3 a−1 between 1983 and 1991. These rates are compared with estimates from short-term (1–2 year) changes along the lower reach to investigate variations in sediment flux that may otherwise remain undetected. Significant morphologic change occurs roughly once every 5 years when flows are large enough to erode and entrain large volumes of bed material stored within the contemporary floodplain. In the absence of large floods, transport rates decline and vegetation begins to establish new floodplain. Copyright © 2000 John Wiley & Sons, Ltd.

Sediment Flux and Bed Level Measurements from a Macro Tidal Mudflat

A detailed analysis is presented of the variations in suspended sediment flux and bed level, measured for one year in the Spurn Bight, Humber Estuary (U.K.). Suspended sediment concentrations were correlated to flow speed, and wave activity dominated the erosion processes and governed the residual tidal flux. An onshore flux and gradual accretion of a few millimetres of sediment were typical of calm hydrodynamic conditions. In contrast, large waves caused the erosion of several centimetres of sediment and prevented deposition over slack water, producing significant seaward transport. This eroded material could be replenished in just a few days under calm conditions. The mudflat level was continually changing in order to reach some equilibrium in balance with the wave climate, tidal cycle, river runoff and seasonal effects. Large differences in the mudflat morphology rapidly occurred when there were significant deviations from the modal forcing conditions. These relatively short term events tended to cancel out with time, and the net seasonal changes in bed level variations were about a few millimetres. Multiple regression analysis was used to produce empirical expressions predicting the residual sediment flux and bed level from readily observable environmental parameters. The results quantified the dominant physical processes affecting the residual tidal flux and established some site-specific relationships that provide a basis for more general flux predictions.

The role of sediment supply in controlling alluvial architecture: an example from the Spanish Pyrenees

This study isolates the controls on the alluvial architecture of two interfingering late Lutetian fluvial systems preserved at the northern margin of the Tremp-Graus Basin, in the south central Pyrenees. One of these, a northerly, mountain-belt-derived, high-gradient, bedload-dominated gravelly system, displays marked variations in its alluvial architecture, with systematic changes in the width, depth, internal facies arrangement and stacking pattern of its channel bodies. The other, an easterly, basin-axis-derived, lower gradient, poorly drained alluvial system, displays no discernible systematic variations in alluvial architecture. A common control that would similarly affect both of the interfingering fluvial systems cannot be responsible for their contrasting architectural patterns. Variations in base level, despite the contemporaneous shoreline being only 18 km downstream, climate, or the tectonic rotation of the study area are therefore discounted. Fluctuations in sediment supply to the northerly derived system resulting from the episodic uplift of its drainage basin are instead proposed, although intrinsic controls cannot be totally discounted. Similar sediment flux variations in the headwaters of the easterly derived system are envisaged, but its longer, lower-gradient transport path was effective in dampening out these fluctuations upstream of the study area.

Sediment ages and flux variations from depth profiles of 210Pb: lake and marine examples

Sediment accumulation rates are redetermined for several freshwater and marine settings to demonstrate the application of a new methodology for interpreting sediment core profiles of radioisotope activity versus depth. Depositional histories were reconstructed using the sediment isotope tomography (SIT) methodology, which abandons the stringent requirement of current methods that sediment accumulation rates and isotopic fluxes may not both vary with time. Using inverse numerical analysis techniques in conjunction with a predictive model, the SIT method is able to separate the effects of sediment accumulation from flux variations to determine age-to-depth relationships. Cumulative probability procedures provide a framework for interpreting the sensitivity and accuracy of the model-determined age-to-depth relationships. Restrictions applied to previous methods on interpreting sediment profiles affected by mixing must also be adhered to when using the SIT method. Previously published data from two Estonian lakes, Kurtna and Nômmejärv, from Rockwell Reservoir in Northeastern Ohio and from Cape Lookout Bight, NC, are examined to demonstrate the broad applicability of the methodology. With the exception of one location, the resultant age-to-depth relationships determined by the model agree with earlier interpretations. In the Estonian Lake Nômmejärv, previous use of the Constant Flux model overestimates sediment ages for two known time horizons, giving ages of 43 yr at 22 cm and 111 yr at 45 cm. The age-to-depth relationship determined by the SIT method reproduces satisfactorily the time horizons of 16±1 yr at 22 cm and 35±2 yr at 45 cm, in agreement with the onset of industrial activities in the region. These examples illustrate the suitability of this new methodology for application in a variety of complex depositional environments.

Seasonal and annual variability in particle fluxes in the Gulf of California: A response to climate forcing

A six-year time series of sediment trap samples is used to document seasonal and interannual changes in particle fluxes in Guaymas Basin, Gulf of California. Sedimentation in this region of the Gulf of California is dominated by two seasonally varying components: biogenic silica during the late fall–spring and lithogenic material during the summer. This variability in sediment fluxes is a direct response to the seasonally reversing monsoon climate in this region. In addition, this seasonal variability in particle flux to the seafloor combined with the lack of bioturbation results in the formation of sediment laminae or varves in Guaymas Basin. Observed interannual differences in the magnitude of sediment fluxes appear to be related to ENSO. These findings indicate that biogenic sediment fluxes, and hence primary productivity, are higher during non-ENSO years in the central Gulf of California. A decrease in the carbonate: opal flux ratio from 1991 to 1996 indicates that diatoms became an increasingly important component of the plankton during this period and further supports our findings that productivity in Guaymas Basin is higher during non-ENSO years.

Relations between dune morphology, air flow, and sediment flux on reversing dunes, Silver Peak, Nevada

ABSTRACTRelations between wind speed, sediment flux and dune morphology were measured for two reversing dunes situated in the south‐western part of the Silver Peak dunefield in Clayton Valley, west‐central Nevada. The larger dune was 120 m in length with a height of 12.5 m and the smaller dune 80 m long and 6 m high. Both dunes were sharp crested, aligned approximately E‐W perpendicular to the dominant wind direction, and had slightly concave stoss profiles. Twenty‐seven rotating cup anemometers were placed (0.3 m elevation) along N‐S transects on each of the dunes. At each anemometer site a passive wedge‐shaped sediment trap was used to measure sediment flux. Amplification of wind speed was observed towards the crest on the stoss side of both dunes with speed‐up factors (ucrest/Ubase) ranging from 1·50–3·19, with a corresponding increase in sediment flux by 1–2 orders of magnitude. In general, the ratio of crest flux to base flux (qc/qt,) increased with increasing incident basal wind speed on both dunes. Direct measurements of the stoss slope variation in sediment flux relative to the dune crest are in good agreement with Owen's transport model. Friction speed (u) was approximated from near surface (0·3 m) point wind speed. Although not all assumptions of the Owen model are upheld, the modified model performance is sufficiently robust to predict short‐term variation in stoss sediment flux on the study dunes.Improved models that adequately account for variation in sediment flux under changing air flow and transport conditions are necessary for the prediction of longterm evolution of dunes. In this regard, further progress in model development will require increased understanding of the spatial and temporal variability of airflow and the short term response of sediment flux to these flow conditions.

Utilizing microcomputer-based models to simulate changes in the nearshore environment

This paper demonstrates that changes in the dynamic nearshore environment could be successfully simulated by utilizing a combination of appropriate wave and sediment models. With the use of two microcomputer-based simulation models (WAVE and NSCWIS), nearshore sedimentary fluxes, and eventual beach and nearshore changes have been simulated. The ‘WAVE’ model was modified by the Canada Center for Inland Waters, and the ‘NSCWIS’ model was put forward by the US Army Engineer Waterways Experiment Station, Coastal Engineering Research Center. The ‘WAVE’ model is a parametric-type numerical wave prediction model, and uses as input a standard bathymetric grid and meteorological wind data. When initialized with input wind data the ‘WAVE’ FORTRAN '77 program calculates the wave advection terms at user-specified grid locations. The outputs from the ‘WAVE’ model are used as inputs into the ‘NSCWIS’ model in order to simulate shallow water wave and current conditions, and volumetric sediment transport rate. The ‘NSCWIS’ FORTRAN '77 program provides results on volumetric sediment transport rates. The immersed weight sediment transport rates have been calculated after obtaining estimates of the volumetric sediment transport rate. The model results have been compared with data collected from the Northeast, Northwest and East Beaches of Point Pelee, the southernmost promontory of Canada, on the north shore of Lake Erie. Total sediment flux data were collected each October from 1984–1987 at the East and Northwest Beaches, and from 1982–1989 along the Northeast Beach. To compare the actual total sediment flux data obtained from the study beaches with the simulated immersed transport rate, simulation runs were executed for both models (WAVE and NSCWIS). The simulated net annual immersed sediment flow outputs were then compared to the annual total sediment flux observed in the topographic bathymetric surveys using standard regression methods. Approximately 85.0% of the total sediment flux variations could be accounted for by variations in the estimated net annual immersed sediment flow associated with longshore currents.

Predicting sediment flux from fold and thrust belts

The rate of sediment influx to a basin exerts a first‐order control on stratal architecture. Despite its importance, however, little is known about how sediment flux varies as a function of morphotectonic processes in the source terrain, such as fold and thrust growth, variations in bedrock lithology, drainage pattern changes and temporary sediment storage in intermontane basins. In this study, these factors are explored with a mathematical model of topographic evolution which couples fluvial erosion with fold and thrust kinematics. The model is calibrated by comparing predicted topographic relief with relief measured from a DEM of the Central Zagros Mountains fold belt. The sediment‐flux curve produced by the Zagros fold belt simulation shows a delay between the onset of uplift and the ensuing sediment flux response. This delay is a combination of the natural response time of the geomorphic system and a time lag associated with filling, and then subsequently uplifting and re‐eroding, the proximal part of the basin. Because deformation typically propagates toward the foreland, the latter time lag may be common to many ancient foreland basins. Model results further suggest that the response time of the bedrock fluvial system is a function of rock resistance, of the width of the region subject to uplift and erosion, and, assuming a nonlinear dependence of fluvial erosion upon channel gradient, of uplift rate. The geomorphic response time for the calibrated Zagros model is on the order of a few million years, which is commensurate with, or somewhat larger than, typical recurrence intervals for episodes of thrusting. However, model experiments also highlight the potential for significant variations in both geomorphic response time and in sediment flux as a function of varying rock resistance. Given a reasonable erodibility contrast between resistant and erodible lithologies, model sediment flux curves show significant sediment flux variations that are related solely to changes in rock resistance as the outcrop pattern changes. An additional control on sediment flux to a basin is drainage diversion in response to folding or thrusting, which can produce major shifts in the location and magnitude of sediment source points. Finally, these models illustrate the potential for a significant mismatch between tectonic events and sediment influx to a basin in cases where sediment is temporarily ponded in an intermontane basin and later remobilized.

Modelling sedimentation in ocean trenches: the Nankai Trough from 1 Ma to the present

Abstract Sediment accumulation within ocean trenches located at actively accreting convergent margins is determined by an interplay between sediment supply, sediment subduction/accretion at the toe of the overriding accretionary complex and the rate of subduction. Modelling trench sedimentation provides insight into the principal controlling factors, and a means of deriving, from the pattern of sedimentation, how factors, such as the rates of sediment supply and subduction, have varied over the period of accumulation of the trench sediments. Two DSDP‐ODP drill sites within the Nankai Trough reveal a coarsening‐upward megasequence, indicating a progressive facies transition from abyssal muds to outer‐trench silts to inner‐trench sands. The changing geometry of the trench‐wedge over the past 1 Myr has been determined by modelling variations in net sediment flux for two trench‐perpendicular profiles. The models were constrained to fit the stratigraphy at the drill sites, and the simulated present‐day geometries of the trench were matched with those shown on seismic reflection profiles by successive adjustment of the model. Results from both sites confirm a ‘slow’ subduction rate of <20 km Myr‐1. At the south‐western site (582), the width of the trench‐wedge has ranged from 13 to 21 km over the past 1 Myr. To the north‐east, at Site 808, the width has ranged from 7 to 13 km over the past 0.5 Myr. These changes in trench‐wedge width are primarily the result of large changes in sediment supply rate. The subduction of the Shikoku Ridge, a fossil spreading centre adjacent to Site 808, has had a major influence on the style of sedimentation within the trench.The style of accretion from the trench to the toe of the accretionary complex has important implications for geometrical adjustment of the trench‐wedge. Thrust displacement lifts the protothrust region out of the trench, resulting in a decreased width. This is followed by a phase of increasing width as the trench‐wedge adjusts towards a new equilibrium. The cyclical, episodic accretion process results in a periodic second‐order variation in trench‐fill size that is superimposed on primary trends determined by variations in sediment supply rates and subduction rates over time.

Sediment flux distribution in the Southern Brazil Basin during the late Quaternary: the role of deep‐sea currents

ABSTRACTDetailed sedimentological and stratigraphic analyses were carried out on seven Kullenberg cores collected across the Brazilian continental margin during the French cruises Byblos and Apsara III, in order to highlight the factors controlling the sediment flux distribution in the Southern Brazil Basin during the late Quaternary. On the continental slope and upper continental rise above 3000 m depth, sediment fluxes are important and highly variable (4·2–14·2 g cm−2 10−3 yr). The sediments show a pelagic or turbiditic character, depending on the width of the shelf and proximity of canyons. The material is characterized by high kaolinite contents, and originates from the coastal rivers draining the South American continent north of Rio de Janeiro. On the middle continental rise between 3000 and 4000 m depth, sediment fluxes are the lowest observed in the area (0·9 g cm−2 10−3 yr), because terrigenous input is trapped at shallower depths on the São Paulo Plateau. Pelagic settling is the dominant process. In the deep domains, below 4000 m depth, contouritic accumulations are developed on the path of the northwards moving Antarctic bottom water (AABW) currents. The deposits consist of fine‐grained silty‐clayey muds with very low carbonate contents. The sediment fluxes (1·45 g cm−2 10−3 yr) are higher than on the middle continental rise, as a consequence of fine‐grained terrigenous supply derived from higher latitudes (Argentine Basin and Southern Ocean), and transported in the basin through the Vema Channel by the AABW currents. This material is characterized by high smectite and chlorite contents. These data reveal large sediment flux variations which are linked to distinct depth‐related domains. Such a distribution is the consequence of the presence of two available sources of terrigenous sediments: (1) the Brazilian continental areas with a downslope material transport and a sediment distribution controlled by the morphology of the margin, and (2) the Argentine Basin with an alongslope material transport by deep‐sea currents which dominate the sedimentation in the abyssal domains.