Vertical Sediment Research Articles

Seafloor cratering and sediment remolding at sites of fluid escape

Episodic fluid escape from marine sediments results from overpressure development and pressure release, and can occur slowly through geologic time or catastrophically. Morphological features in regions of fluid seepage include doming, mud volcanism, cratering, and pockmark formation. Vertical sediment mobilization and surface erosion are considered the principal mechanisms for these topographic changes. However, the impact of mobilization on the geotechnical properties of sediments has not been explicitly considered. Here we develop a one-dimensional numerical subsidence model that incorporates the well-established behavior of remolded fine-grained cohesive sediments. We use this to show that if subsurface overpressure results in the mobilization of sediments, large settlements (20%–35% reduction in volume) can occur when overpressure dissipates. This presents a novel mechanism to explain changes in seafloor and subsurface topography in areas of fluid escape, while highlighting an important interplay between subsurface fluid flow and the geotechnical properties of fine-grained cohesive sediments.

Deformed microbial mat structures in a semiarid temperate coastal setting

This study focuses on sedimentary structures formed by microbial consortia, in a particular coastal setting, an ancient tidal channel, separated from the ocean by a sandy spit and connected by a blind tidal channel at the opposite end. Most studies in modern and ancient environments consider water movement as the triggering mechanism acting in the formation and deformation of sedimentary structures. As such, the paper documents the presence of several microbial structures such as shrinkage cracks, flip-over mats, microbial chips, and multidirectional ripples which are related to tidal processes, while bulges and gas domes structures are formed after occasional inundation events. However, the more conspicuous structures covering a great area at the study site are folds and roll-ups, the product of deformation of microbially induced structures by the action of sporadic spring-tidal currents due to strong winds. Therefore, the objective of this research is to document modern sedimentary structures in a coastal area and to provide a mechanistic explanation for their formation, based on the interplaying effects of the moisture variation and high shear stress. Also, several microbial sedimentary structures are distinguished throughout vertical sediment cores, such as microbial chips, detached mat, sponge fabrics, tears, and concentric structures, which are identified in a sedimentary profile. Through the recognition and interpretation of modern sedimentary deformation structures, this study contributes empirical tools for the reconstruction of analogous paleoenvironments in fossil studies.

Effects of common carp (Cyprinus carpio) on sediment mixing depth and mobile phosphorus mass in the active sediment layer of a shallow lake

The common carp (Cyprinus carpio) is a globally distributed, benthivorous fish that is capable of substantially altering lake ecology. Although dozens of studies have demonstrated that it has the potential to negatively affect submersed vegetation and water quality, little attention has been paid to effects of carp on sediment mixing and sediment phosphorus (P) in lakes. This study examined carp effects on sediment mixing and mobile (mainly iron bound and pore water) sediment P in a shallow, Midwestern lake (Kohlman Lake, MN, USA) undergoing restoration with aluminum (Al) to reduce internal P loading. Using Al as a tracer, we determined that the sediment mixing depth was at least 2.5 times greater in areas with carp (13.0 ± 3.7 cm) than in areas from which carp had been excluded (5.0 ± 1.2 cm) using exclosures. Vertical sediment profiles of P mass suggested that the increase in sediment mixing depth caused by carp increased the amount of mobile P potentially available for release by 55–92%, depending on location within the lake. The increase in sediment mixing depth is likely to have a negative effect on the efficacy of management methods designed to reduce mobile sediment P availability in lakes.

Methane production potentials, pathways, and communities of methanogens in vertical sediment profiles of river Sitka.

Biological methanogenesis is linked to permanent water logged systems, e.g., rice field soils or lake sediments. In these systems the methanogenic community as well as the pathway of methane formation are well-described. By contrast, the methanogenic potential of river sediments is so far not well-investigated. Therefore, we analyzed (a) the methanogenic potential (incubation experiments), (b) the pathway of methane production (stable carbon isotopes and inhibitor studies), and (c) the methanogenic community composition (terminal restriction length polymorphism of mcrA) in depth profiles of sediment cores of River Sitka, Czech Republic. We found two depth-related distinct maxima for the methanogenic potentials (a) The pathway of methane production was dominated by hydrogenotrophic methanogenesis (b) The methanogenic community composition was similar in all depth layers (c) The main TRFs were representative for Methanosarcina, Methanosaeta, Methanobacterium, and Methanomicrobium species. The isotopic signals of acetate indicated a relative high contribution of chemolithotrophic acetogenesis to the acetate pool.

Contribution of Methane Benthic and Atmospheric Fluxes of an Experimental Area (Sitka Stream)

Contributions of methane benthic fluxes and methane emissions to the total methane amount were studied during the summer season for two consecutive years. Considering the fact that methane is produced in great amount within hyporheic sediments, we have expected that both methane released by the benthic fluxes into the overlying water and methane emitted to the atmosphere should significantly affect the total methane budget of the stream water within the experimental area. In order to asses this assumption, we measured methane concentrations within the vertical sediment profile of a hyporheic zone, direct benthic methane fluxes by static chambers, methane concentrations in the surface water, and methane emissions into the atmosphere simultaneously. The methane concentrations in interstitial water tended sharply to increase with sediment depth (0.54–734.99 μmol L−1) and were strongly correlated with benthic fluxes (r = 0.88, p < 0.05). We also found a considerable variability in the benthic methane fluxes (0–12.59 mmol m−2 day−1) and the spatial variability (0–71.43%) of the experimental area, suggesting that there may be various factors that affect the rate of benthic fluxes including methanotrophy in the top sediment layers. Generally, the contribution of benthic fluxes and methane emissions to the methane budgets of the experimental area was negligible (0.034 (0.45%) and 0.074 mol day−1 (0.98%), respectively).

Sediment transport dynamics near a river inflow in a large alpine lake

AbstractSediment dynamics were investigated in Lake Maggiore, Italy, with field observations from October to mid‐December 2012. Three moorings were deployed in Pallanza Bay, a small embayment on the western side of the lake near the Toce River inflow, to measure temperature and currents throughout the water column and suspended sediment concentration (SSC) was estimated with acoustic instrumentation. River intrusions are shown to dominate observed SSC, although a small amount of sediment resuspension was observed at the site of the shallowest mooring during a large wind event that produced strong upwelling of the thermocline followed by downwelling. Although vertical turbulent sediment flux is typically assumed to indicate resuspension and the upward transport of sediment ( ), downward turbulent sediment flux was observed ( ) near the bed during the largest observed intrusion event. The downward turbulent sediment flux significantly contributes to net deposition rates, which are one order of magnitude larger than rates of erosion measured during the two major events observed. Horizontal transport of sediment occurs in vertically confined layers due to buoyancy‐driven intrusions. Beneath the intrusions, sediment settles out of the water column at settling rates that appear to be constant with depth based on acoustic Doppler current profiler backscatter measurements. The effective settling velocities needed to produce the observed vertical transport of SSC during an inflow intrusion are one order of magnitude larger than those due to the Stokes settling velocity ( ) alone. Particle flocculation and possible convective instabilities may play a role in generating the large observed effective settling rates.

INHERITED FLOODPLAINS AND REGIME OF SEDIMENTATION IN RIVERBED – FLOODPLAIN COMPLEXES

During circumpolar distribution of the large rivers with large streamflow the wide floodplains with high amplitude relief were formed in their valleys. While the discharge of the river diminished this relief was partly reformed in the course of the lateral reformations of the river channel and ancient floodplains on the large areas took on inherited character. Due to their wideness inherited floodplains serve as a effective barriers between the erosion part of the river system (erosion furrows on the slopes and gullies) and balka-river part where transit and accumulation of the sediments by the most part take place. The most part of the sediments from gullies and slopes accumulates on the inherited floodplain. In the result there is very low correlation of sediment transport regimes even between close ranges of the river. Dating of the sediments proves that as largest basins on the floodplain are being infilled and the level of the floodplain rises, the rate of accumulation and barrier effect diminish. At last the inherited floodplain relief changes totally due to lateral and vertical river bed deformations and sediment transport in this river range is being retrieved.

Identifying spatial and temporal dynamics of proglacial groundwater–surface-water exchange using combined temperature-tracing methods

The effect of proglacial groundwater systems on surface hydrology and ecology in cold regions often is neglected when assessing the ecohydrological implications of climate change. We present a novel approach in which we combined 2 temperature-tracing techniques to assess the spatial patterns and short-term temporal dynamics of groundwater–surface-water exchange in the proglacial zone of Skaftafellsjökull, a retreating glacier in southeastern Iceland. Our study focuses on localized groundwater discharge to a surface-water environment, where high temporal- and spatial-resolution mapping of sediment surface and subsurface temperatures (10–15 cm depth) were obtained by Fiber-Optic Distributed Temperature Sensing (FO-DTS). The FO-DTS survey identified temporally consistent locations of temperature anomalies at the sediment–water interface, indicating distinct zones of cooler groundwater upwelling. The high-resolution FO-DTS surveys were combined with calculations of 1-dimensional groundwater seepage fluxes based on 3 vertical sediment temperature profiles, covering depths of 10, 25, and 40 cm below the lake bed. The calculated groundwater seepage rates ranged between 1.02 to 6.10 m/d. We used the combined techniques successfully to identify substantial temporal and spatial heterogeneities in groundwater–surface exchange fluxes that have relevance for the ecohydrological functioning of the investigated system and its potential resilience to environmental change.

Spatial distribution of bacterial communities driven by multiple environmental factors in a beach wetland of the largest freshwater lake in China.

The spatial distributions of bacterial communities may be driven by multiple environmental factors. Thus, understanding the relationships between bacterial distribution and environmental factors is critical for understanding wetland stability and the functioning of freshwater lakes. However, little research on the bacterial communities in deep sediment layers exists. In this study, thirty clone libraries of 16S rRNA were constructed from a beach wetland of the Poyang Lake along both horizontal (distance to the water-land junction) and vertical (sediment depth) gradients to assess the effects of sediment properties on bacterial community structure and diversity. Our results showed that bacterial diversity increased along the horizontal gradient and decreased along the vertical gradient. The heterogeneous sediment properties along gradients substantially affected the dominant bacterial groups at the phylum and species levels. For example, the NH+4 concentration decreased with increasing depth, which was positively correlated with the relative abundance of Alphaproteobacteria. The changes in bacterial diversity and dominant bacterial groups showed that the top layer had a different bacterial community structure than the deeper layers. Principal component analysis revealed that both gradients, not each gradient independently, contributed to the shift in the bacterial community structure. A multiple linear regression model explained the changes in bacterial diversity and richness along the depth and distance gradients. Overall, our results suggest that spatial gradients associated with sediment properties shaped the bacterial communities in the Poyang Lake beach wetland.

Intertidal landscape response time to dike breaching and stepwise re-embankment: A combined historical and geomorphological study

Intertidal flats and marshes provide important ecosystem services to coastal and estuarine societies, and have therefore been subject to extensive research. Most of these studies, however, have one thing in common: intertidal landscape response has been studied from a rather short-term perspective, mostly less than 100years. Furthermore, the impact of embankment practices on the development of the remaining intertidal area and its stability has hardly been studied at all. In this paper, a longer term perspective (sixteenth to twenty-first centuries) is used in order to (1) reconstruct intertidal landscape evolution, using both historical and soil maps and (2) assess the effect of stepwise embankment on the intertidal area, both in terms of the surface proportions of the different components of an intertidal area and the vertical and lateral sediment elevation and sedimentation rates. The reconstruction and embankment effect assessment will be applied to the Land of Saeftinghe, an intertidal area in the Western Scheldt estuary that was partially re-embanked in a stepwise manner into an embankment landscape called the Waasland polder area (Belgium/The Netherlands). Important outcomes of this study are: (1) a combination of historical–geographical methods proves to be a valuable tool for past intertidal landscape reconstruction; (2) dike breaching and partial (stepwise) re-embankment are important driving mechanisms for the evolution of the remaining intertidal landscape towards new equilibrium conditions, though the new landscape equilibria can only be reached with a certain time lag; (3) subsequent embankments with a short time interval (15 to 21years) prevent tidal marsh surface areas from reaching an equilibrium state; and (4) between 60 and 96years are needed for tidal marsh elevation to reach an equilibrium.

Ice-sheet grounding-zone wedges (GZWs) on high-latitude continental margins

Abstract Grounding-zone wedges (GZWs) are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line-source at the grounding zone of marine-terminating ice sheets during still-stands in ice-sheet retreat. GZWs form largely through the delivery of deforming subglacial sediments. The presence of GZWs in the geological record indicates an episodic style of ice retreat punctuated by still-stands in grounding-zone position. Moraine ridges and ice-proximal fans may also build up at the grounding zone during still-stands of the ice margin, but these require either considerable vertical accommodation space or sediment derived from point-sourced subglacial meltwater streams. By contrast, GZWs form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding-zone. An inventory of GZWs is compiled from available studies of bathymetric and acoustic data from high-latitude continental margins. The locations and dimensions of GZWs from the Arctic and Antarctic, alongside a synthesis of their key architectural and geomorphic characteristics, are presented. GZWs are only observed within cross-shelf troughs and major fjord systems, which are the former locations of ice streams and fast-flowing outlet glaciers. Typical high-latitude GZWs are less than 15 km in along-flow direction and 15 to 100 m thick. GZWs possess a transparent to chaotic acoustic character, which reflects the delivery of diamictic subglacial debris. Many GZWs contain seaward-dipping reflections, which indicate sediment progradation and wedge-growth through continued delivery of basal sediments. GZW formation is inferred to require high rates of sediment delivery to a fast-flowing ice margin that is relatively stable for probably decades to centuries. Although the long-term stability of the grounding zone is controlled by ice-sheet mass balance, the precise location of any still-stands is influenced strongly by the geometry of the continental shelf. The majority of high-latitude GZWs occur at vertical or lateral pinning points, which encourage grounding-zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone.

Vertical sediment distribution of methanogenic pathways in two shallow Arctic Alaskan lakes

Methanogenesis (MG) occurs in anaerobic lake sediments during the terminal step of organic matter degradation. Methane is typically produced via two primary catabolic pathways (acetoclastic or hydrogenotrophic) in which the primary substrates are acetate or H2/CO2, respectively. The acetoclastic pathway has been shown to dominate in a 2:1 ratio over the hydrogenotrophic pathway in freshwater sediments. Rates of methane production from each pathway are regulated primarily by the quantity and quality of organic matter. As acetate and H2 are produced through decomposition of organic matter, increased terrestrially derived organic matter loading can fuel sediment MG. Increased delivery of terrestrially derived organic matter to arctic lakes is expected under future climate change scenarios. Therefore, we compared unamended rates of MG in anaerobic sediment slurries to those amended with acetate or hydrogen. We also evaluated the vertical sediment distribution of MG pathways in 1-cm increments to a final depth of 5 cm using an inhibitor for the acetoclastic pathway, methyl fluoride. In both lakes, unamended rates of MG decreased with increasing sediment depth. Additions of acetate or hydrogen stimulated rates of MG at all depths in both lakes resulting in rates 1–3 orders of magnitude greater than MG rates in unamended slurries. The ratio of the acetoclastic to the hydrogenotrophic pathway decreased with increasing sediment depth in both lakes. Our findings suggest that increased delivery of terrestrial organic matter to shallow arctic lakes may increase sediment methane production.

On luminescence bleaching of tidal channel sediments

We investigate the processes responsible for bleaching of the quartz OSL signal from tidal channel sediment. Tidal dynamics are expected to play an important role for complete bleaching of tidal sediments. However, no studies have examined the amount of reworking occurring in tidal channels and on tidal flats due to the mixing caused by currents and waves. We apply bed level data to evaluate the amount of vertical sediment reworking in modern tidal channels and at a tidal flat. Cycles of deposition and erosion are measured with a bed level sensor, and the results show that gross sedimentation was several times higher than net sedimentation. We propose that tidal channel sediment is bleached either on the tidal flat before it is transported to the tidal channels and incorporated in channel-fill successions or, alternatively, on the shallow intertidal part of the channel banks. Based on the quantitative measures of sediment reworking, we suggest that repeated depositional and erosional cycles allow for complete bleaching of the tidal flat sediment, and we infer that quartz OSL dating can provide accurate ages for shallow tidal channel fill.

Study on the Vertical Distribution of Cr in Jiaozhou Bay

Based on investigation dada on Cr in 1982 in Jiaozhou Bay, this paper analyzed the seasonal and spatial distribution of Cr, and revealed the transfer process of Cr. Results showed that horizontal diffusion and vertical sedimentation were major transfer processes of Cr, which were mainly determined by water exchange and gravity settling. These findings were essential to pollution control and environmental remediation in Jiaozhou Bay. From the point of view of pollution control, reducing the discharge of Cr from the sources was the major countermeasure.

Particle inertia effect on sediment dispersion in turbulent open-channel flows

Particle inertia effect plays a significant role in sediment dispersion which has not been fully elucidated. In this paper, the profound effect of particle inertia on the sediment dispersion was analyzed. The theoretical expression for the drift velocity based on the two-phase mixture theory in turbulent open channels is firstly introduced. The influence of particle inertia on sediment dispersion was investigated through three different aspects including vertical dispersion, motion, and flux properties based on the drift velocity. Results show that the dispersion of suspended sediment in turbulent open-channel flows is affected by three major processes including turbulence of water sediment mixtures, particle random motion, and collisions among particles, of which the contributions of particle turbulence and collisions to the sediment dispersion are remarkable for particles of high inertia. With respect to the vertical mean velocity and sediment flux, it shows that the predictive results agree well with the measurements when the term of particle inertia is considered. As a result, particle inertia considerably affects the behavior of suspended sediment. In particular, the influence of inertia must be accounted for in circumstances of flows laden with high-inertia particles.

The role of soil in vegetated gravelly river braid plains: more than just a passive response?

AbstractThis paper reviews the role of alluvial soils in vegetated gravelly river braid plains. When considering decadal timescales of river evolution, we argue that it becomes vital to consider soil development as an emergent property of the developing ecosystem. Soil processes have been relatively overlooked in accounts of the interactions between braided river processes and vegetation, although soils have been observed on vegetated fluvial landforms. We hypothesize that soil development plays a major role in the transition (speed and pathway) from a fresh sediment deposit to a vegetated soil‐covered landform. Disturbance (erosion and/or deposition), vertical sediment structure (process history), vegetation succession, biological activity and water table fluctuation are seen as the main controls on early alluvial soil evolution. Erosion and deposition processes may not only act as soil disturbing agents, but also as suppliers of ecosystem resources, because of their role in delivering and changing access (e.g. through avulsion) to fluxes of water, fine sediments and organic matter. In turn, the associated initial ecosystem may influence further fluvial landform development, such as through the trapping of fine‐grained sediments (e.g. sand) by the engineering action of vegetation and the deposit stabilization by the developing aboveground and belowground biomass. This may create a strong feedback between geomorphological processes, vegetation succession and soil evolution which we summarize in a conceptual model. We illustrate this model by an example from the Allondon River (Switzerland) and identify the research questions that follow. Copyright © 2014 John Wiley & Sons, Ltd.

Effect of self-stratification on sediment diffusivity in channel flows and boundary layers: a study using direct numerical simulations

Abstract. Sediment transport in nature comprises of bedload and suspended load, and precise modelling of these processes is essential for accurate sediment flux estimation. Traditionally, non-cohesive suspended sediment has been modelled using the advection–diffusion equation (Garcia, 2008), where the success of the model is largely dependent on accurate approximation of the sediment diffusion coefficients. The current study explores the effect of self-stratification on sediment diffusivity using suspended sediment concentration data from direct numerical simulations (DNS) of flows subjected to different levels of stratification, where the level of stratification is dependent on the particle size (parameterized using particle fall velocity Ṽ and volume-averaged sediment concentration (parameterized using shear Richardson number Riτ. Two distinct configurations were explored, first the channel flow configuration (similar to flow in a pipe or a duct) and second, a boundary-layer configuration (similar to open-channel flow). Self-stratification was found to modulate the turbulence intensity (Cantero et al., 2009b), which in turn was found to reduce vertical sediment diffusivity in portions of the domain exposed to turbulence damping. The effect of particle size on vertical sediment diffusivity has been studied in the past by several authors (Rouse, 1937; Coleman, 1970; Nielsen and Teakle, 2004); so in addition to the effect of particle size, the current study also explores the effect of sediment concentration on vertical sediment diffusivity. The results from the DNS simulations were compared with experiments (Ismail, 1952; Coleman, 1986) and field measurements (Coleman, 1970), and were found to agree qualitatively, especially for the case of channel flows. The aim of the study is to understand the effect of stratification due to suspended sediment on vertical sediment diffusivity for different flow configurations, in order to gain insight of the underlying physics, which will eventually help us to improve the existing models for sediment diffusivity.

Hydrodynamic evolutions at the Yangtze Estuary from 1998 to 2009

Over the past decade, the Yangtze Estuary has witnessed an unprecedented scale of human interventions and modifications through extensively varied resource utilizations. During the processes, mankind has obtained various resources and benefits via the “golden waterway”, such as navigation channel, harbor, shipping industry, shoreline, reclaimed land, freshwater and fishery resource. At the same time, the estuary and coast have also experienced a series of gradual changes in characteristics, such as sedimentation, erosion, sand hungry, water pollution, intertidal area loss, self-purification capacity decrease, and biological reduction. With the help of measurement data and numerical modeling, this study analyzed the response and feedback mechanisms between hydrodynamic evolutions and morphological processes in the Yangtze Estuary from 1998 to 2009. The results of this study indicate the following. (i) The water level along the main outlet of the Yangtze Estuary increased from 1998 to 2009. This increase was induced by the variation of the whole river regime (including natural geomorphodynamic process and local topography feedbacks from extreme metrological events and human activities). (ii) The decrease of the flow partition ratio at the 3rd bifurcation is directly induced by the Deepwater Navigational Channel (DNC) project and the corresponding morphological changes in the North Passage. (iii) The estuarine environmental gradients (salinity and suspended sediment concentration) were compressed, and the fresh-salt gradient became steeper. This has the indirect effect of backfilling in the waterway, i.e., strengthening the stratification effect near the ETM area and enhancing the tendency of up-estuary sediment transport. The results of this study give insights into explaining other phenomena such as deposition in the middle reach of the DNC, bathymetry evolutions, variations in vertical velocity and sediment concentration profiles, waterway backfilling and delta reclamation.

Late Pleistocene glaciofluvial sedimentation in Gariūnai–Pagiriai proglacial valley, SE Lithuania

The results of studies of glaciofluvial sediments and their sedimentation in a progla cial valley in the northern part of SE Lithuanian outwash plain are presented. The aim of the study was to compare the features of sediments in the geomorphologically well expressed Gariūnai–Pagiriai proglacial valley with the rest part of the SE Lithuanian Sandy Plain in which the complexity of proglacial fluvial sedimentation was revealed during the resent investigations. The change of depositional structures of sandy sediments and grain size distribution in vertical sediment sequences was studied for lithofacies analysis. The studies carried out let to interpret the sedimentation conditions of deposits in the proglacial valley as a meandering stream of SE direction, which downstream turns SW and intersects the SE Lithuanian Sandy Plain composed of braided stream outwash sediments. Lithofacies of proglacial valley deposits were distinguished in the deposit sequences due to a qualitative sedimentological research. Mid-channel and side-channel bar deposits of the proglacial valley were described and conditions of their sedimentation were evaluated. It was interpreted that formation of glaciofluvial terraces in the proglacial valley took place in several stages under the changing mode of ice meltwater flow and the steady source of clastic sediment material.

Vertical and lateral flux on the continental slope off Pakistan: correlation of sediment core and trap results

Abstract. Due to the lack of bioturbation, the varve-laminated muds from the oxygen minimum zone (OMZ) off Pakistan provide a unique opportunity to precisely determine the vertical and lateral sediment fluxes in the nearshore part of the northeastern Arabian Sea. West of Karachi (Hab area), the results of two sediment trap stations (EPT and WPT) were correlated with 16 short sediment cores on a depth transect crossing the OMZ. The top of a distinct, either reddish- or light-gray silt layer, 210Pb-dated as AD 1905 ± 10, was used as an isochronous stratigraphic marker bed to calculate sediment accumulation rates. In one core, the red and gray layer were separated by a few (5–10) thin laminae. According to our varve model, this contributes < 10 years to the dating uncertainty, assuming that the different layers are almost synchronous. We directly compared the accumulation rates with the flux rates from the sediment traps that collected the settling material within the water column above. All traps on the steep Makran continental slope show exceptionally high, pulsed winter fluxes of up to 5000 mg m−2 d−1. Based on core results, the flux at the seafloor amounts to 4000 mg m−2 d−1 and agrees remarkably well with the bulk winter flux of material, as well as with the flux of the individual bulk components of organic carbon, calcium carbonate and opal. However, due to the extreme mass of remobilized matter, the high winter flux events exceeded the capacity of the shallow traps. Based on our comparisons, we argue that high-flux events must occur regularly during winter within the upper OMZ off Pakistan to explain the high accumulations rates. These show distribution patterns that are a negative function of water depth and distance from the shelf. Some of the sediment fractions show marked shifts in accumulation rates near the lower boundary of the OMZ. For instance, the flux of benthic foraminifera is lowered but stable below ~1200–1300 m. However, flux and sedimentation in the upper eastern Makran area are dominated by the large amount of laterally advected fine-grained material and by the pulsed nature of the resuspension events at the upper margin during winter.