Sediment Supply Research Articles

U–Pb detrital zircon ages from late Eocene to early Oligocene Pinghu formation of the East China Sea Shelf Basin: Inferences on synergistic effects between East Asia offshore sedimentation and major rivers supply

Our understanding of the late Eocene to early Oligocene provenance evolution of the East China Sea Shelf Basin has been limited, which hampers our study of the synergistic effects between East Asia offshore sedimentation and major rivers. To address this, we compared newly acquired detrital zircon chronological data from the East China Sea with published detrital U–Pb ages of major rivers in East Asia. Through this analysis, we have been able to reconstruct the provenance changes of sediments in the East China Sea during this crucial period. We have identified two large-scale and three small-scale denudation surfaces that developed in the Pinghu and lower Huagang Formations. These surfaces indicate that sediments were also derived from underlying strata and bedrock, particularly in the lower Pinghu and lower Huagang Formations. The newly acquired detrital zircon U–Pb age range was found to be roughly similar, ranging from 36.2 ± 0.7 to 2954.0 ± 30.3 Ma. The age peaks of the lower Pinghu Formation were more consistent, while the middle, upper Pinghu Formation, and lower Huagang Formation exhibited multiple age peaks. Comparing the compiled zircon U–Pb age peaks of the Chuanjiang segment of the upper Yangtze River with those of the East China Sea Shelf Basin, we found relative similarity. Additionally, the younger zircon grains with ages <500 Ma in the east Cathaysia catchment showed similarities with the East China Sea Shelf Basin. In terms of provenance contribution, the sediments of the lower Pinghu Formation were predominantly provided by the adjacent uplifts in the east Cathaysia catchment, with the Chuanjiang segment also playing a significant role. From the middle Pinghu Formation onwards, the provenance from the Chuanjiang segment became dominant, accounting for over 90 % of the sediments. However, this influence was limited to the northern part of the study area. This suggests a substantial increase in the Yangtze River catchment during this period, potentially related to the complete connection of the modern Yangtze River.

Characterizing Erosion and Deposition in and Around Riparian Vegetation Patches: Complex Flow Hydraulics, Sediment Supply, and Morphodynamic Feedbacks

AbstractRiparian vegetation plays a fundamental role in alluvial channel evolution by modifying flow and sedimentation dynamics. To elucidate the roles of sediment supply, flow‐dependent transport capacity, and morphodynamic feedbacks on the evolution of emergent vegetation patches over a single hydrograph, we conducted two experiments with different reach‐scale sediment supplies: a high‐supply experiment (HSE) and low‐supply experiment (LSE). We measured flow velocities, bedload transport, and topographic changes around a full‐scale patch of live emergent willows in an outdoor laboratory flume. Erosion occurred in the patch‐adjacent channel areas irrespective of the sediment supply, whereas deposition within the patch interior was suppressed in the LSE and enhanced in the HSE. The magnitude of patch deposition in each experiment was controlled by the local sediment supply to the patch and local sediment mobility during each discharge in the hydrograph. The local sediment supply was affected by bed morphodynamics at the patch head, which modulated the reach‐scale sediment supply by redistributing the bedload in the channel. Sediment mobility within the patch was flow‐dependent and a function of velocity and turbulent kinetic energy. For different discharges, the velocity in the patch changed proportionally with the freestream velocity; however, the turbulent kinetic energy was more sensitive, being elevated compared with the freestream during high flows and inhibited at low flows. Therefore, deposition and erosion within vegetation patches are not simply functions of the reach‐scale sediment supply or patch flow characteristics, as is often assumed, but additionally depend on the local sediment supply to the patch interior.

Surviving from transgression to regression of Lake Pannon: Fan deltas of the Nemčiňany Fm. persisted across the rifting until the post-rift stage of the Danube Basin, western Slovakia

Successions deposited under rifting and post-rift settings of an isolated epicontinental basin often exhibit contrasting characteristics. Facies linked to transgression during basin rifting are typically locally sourced fan deltas transporting coarse-grained sediment, whereas the post-rift setup generally involves a normal regression marked by more extensive catchments, sediment sourced from greater distances, and an overall decrease in average grain size compared to the transgressive facies. In this study, we present a specific scenario based on a sedimentological, stratigraphic, and authigenic 10Be/9Be analysis of the Nemčiňany Formation, an Upper Miocene fan deltaic succession in the eastern Danube Basin, Slovakia. Deposition of the Nemčiňany Fm. was initiated shortly after ~11.6 Ma by the fourth rifting phase of the Pannonian Basin System, triggering the transgression of Lake Pannon. The depositional system persisted until ~9.6 Ma, likely balancing the increased accommodation rate with sediment supply in the shallow lake area situated on a basement high. Subsequently, a relative decrease in the accommodation rate prompted the progradation of shelf-slope scale clinoforms, originating from the Nemčiňany depositional system, toward the center of the Komjatice depression at ~9.4 Ma, resulting in a normal regression of Lake Pannon. The persistence of the Nemčiňany fan delta system, unlike common stratigraphic patterns, was associated to the high sediment supply yielded by the paleo-Hron river, which entered the basin in the study area. These observations underscore the need for caution in predicting provenance shifts during changes in geodynamic stages of a basin.

Elevation Changes in Restored Marshes at Poplar Island, Chesapeake Bay, MD: I. Trends and Drivers of Spatial Variability

Tidal marshes provide numerous ecosystem services, but are threatened by recent increases in global sea level rise (SLR). Marsh restoration and creation are important strategies for mitigating marsh loss, restoring ecosystem services, increasing coastal community resilience, and providing much needed habitat for threatened species. Dredged material resulting from navigation channel maintenance can provide a substrate for these restoration projects. Few studies, however, have addressed the sustainability of these marshes. The Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island, where fine-grained, nutrient-rich dredged material from upper Chesapeake Bay is being used to create > 302 ha of tidal marshes, provides a case study. The low supply of inorganic sediment is counteracted by abundant nutrients, stimulating high rates of organic matter production and elevation change. Using > 10 years of data from 39 surface elevation tables, we found that the mean low marsh rate of elevation change (7.7 ± 3.21 mm year−1) was double the mean high marsh rate (3.6 ± 0.47 mm year−1) and exceeded the natural reference marsh (3.0 ± 2.28 mm year−1) and relative SLR (5.7 mm year−1). By stimulating organic matter production, the high nutrient substrate appears to offset the low inorganic sediment inputs in mid-Chesapeake Bay. Spatial variability was correlated with initial elevation, but was also influenced by local factors that may affect sediment redistribution within the marshes.

Identifying and Constraining Marsh-Type Transitions in Response to Increasing Erosion over the Past Century

Marsh environments, characterized by their flora and fauna, change laterally in response to shoreline erosion, water levels and inundation, and anthropogenic activities. The Grand Bay coastal system (USA) has undergone multiple large-scale geomorphic and hydrologic changes resulting in altered sediment supply, depositional patterns, and degraded barrier islands, leaving wetland salt marshes vulnerable to increased wave activity. Two shore-perpendicular transect sites, one along a low-activity shoreline and the other in a high activity area of the same bay-marsh complex, were sampled to investigate how the marshes within 50 m of the modern shoreline have responded to different levels of increased wave activity over the past century. Surface sediments graded finer and more organic with increased distance from the shoreline while cores generally exhibited a coarsening upwards grain-size trend; all cores contained multiple large sedimentological shifts. 210Pb-based mass accumulation rates over the last two decades were greater than the long-term (centurial) average at each site with the fastest accumulation rates of 7.81 ± 1.58 and 7.79 ± 1.63 kg/m2/year at the sites nearest the shoreline. A shoreline change analysis of three time-slices (1848–2017, 1957–2017, 2016–2017) shows increased erosion at both sites since 1848 with modern rates of −0.95 and −0.88 m/year. Downcore sedimentology, mass accumulation rates, and shoreline change rates paired with foraminiferal biofacies and identification of local estuarine indicator species, Paratrochammina simplissima, aided in identifying paleo marsh types, their relative proximity to the shoreline, and sediment provenance. The high-energy marsh site transitioned from middle marsh to low marsh in the 1960s, and the low-energy marsh site transitioned later, at the end of the twentieth and early twenty-first century, due to its more protected location. Marsh type transition corresponds chronologically with the coarsening upwards grain-size trend observed and the degradation of Grand Batture Island; since its submergence, signatures of multiple storm event have been preserved downcore.

Responses of Coastal Wetlands to Rising Sea-Level Revisited: The Importance of Organic Production

A network of 15 Surface Elevation Tables (SETs) at North Inlet estuary, South Carolina, has been monitored on annual or monthly time scales beginning from 1990 to 1996 and continuing through 2022. Of 73 time series in control plots, 12 had elevation gains equal to or exceeding the local rate of sea-level rise (SLR, 0.34 cm/year). Rising marsh elevation in North Inlet is dominated by organic production and, we hypothesize, is proportional to net ecosystem production. The rate of elevation gain was 0.47 cm/year in plots experimentally fertilized for 10 years with N&P compared to nearby control plots that have gained 0.1 cm/year in 26 years. The excess gains and losses of elevation in fertilized plots were accounted for by changes in belowground biomass and turnover. This is supported by bioassay experiments in marsh organs where at age 2 the belowground biomass of fertilized S. alterniflora plants was increasing by 1,994 g m−2 year−1, which added a growth premium of 2.4 cm/year to elevation gain. This was contrasted with the net belowground growth of 746 g m−2 year−1 in controls, which can add 0.89 cm/year to elevation. Root biomass density was greater in the fertilized bioassay treatments than in controls, plateauing at about 1,374 g m−2 and 472 g m−2, respectively. Growth of belowground biomass was dominated by rhizomes, which grew to 3,648 g m−2 in the fertilized treatments after 3 years and 1,439 g m−2 in the control treatments after 5 years. Depositional wetlands are limited by an exogenous supply of mineral sediment, whereas marshes like North Inlet could be classified as autonomous because they depend on in situ organic production to maintain elevation. Autonomous wetlands are more vulnerable to SLR because their elevation gains are constrained ultimately by photosynthetic efficiency.

Fossilized autogenic responses of grain‐size transition to sediment supply and water discharge: Alluvial fan experiments

ABSTRACTAutogenic feedbacks can produce large‐scale, organized stratigraphic patterns in alluvial fans, but autogenic depositional signatures of specific upstream boundary conditions remain challenging to interpret. Here, a combination of theory, experiment and field application is used to explore how autogenic lithofacies changes can be interpreted as stratigraphic indicators of upstream boundary conditions. Six experiments were conducted to test the effects of sediment supply and water discharge rates on autogenic advance and retreat of the lithofacies boundary (grain‐size transition) in an alluvial fan with two dominant grain sizes. Migration of the grain‐size transition caused a short‐term zigzag pattern in the grain‐size transition position in the dip‐directional deposit section. For each experiment, time‐lapse images and laser topographic scans of the fan surface and stratigraphic cross‐sections of the final deposits were used to quantify characteristic timescales of autogenic processes. Timescales for fan‐margin migration, surface wet‐fraction change and grain‐size transition migration generally shorten as sediment supply rate increases and water discharge rate decreases. Increasing the sediment supply rate shortens the duration of the fluvial sediment storage and release cycle, producing higher frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge tends to widen channels and lengthens the duration of the fluvial sediment storage and release cycle, constructing lower frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge also enables more sediment to transport further downstream during release events, leading to higher magnitude zigzags in the grain‐size transition trajectory. These relationships between upstream boundary conditions and the grain‐size transition trajectory demonstrate how autogenic stratigraphic signals could be used as a tool to infer relative changes in boundary conditions.

Multi-criteria assessment of factors affecting the reduction of retention capacity of dam reservoirs

Maintaining the appropriate capacity of a retention reservoir is necessary for the optimal performance of the functions for which it was built, including flood and drought protection. Therefore, to properly manage individual reservoirs and sediments within their catchments, it is necessary to analyze the factors affecting the reduction of the retention capacity of reservoirs. Our study proposes a methodology for conducting a multi-criteria assessment of anthropogenic pressures and natural impacts affecting the reduction of reservoir capacity, such as land use, hydrographic network density, hydraulic development, and land slopes. For this purpose, geospatial analyses were applied to a grid of basic fields (hexagons). The research procedure showed that land slopes in the catchment area are the key factor determining the supply of sediment to the reservoir. Our study focused on the basins of reservoirs located in the southern part of Poland: Goczałkowice on the Vistula, Rożnów on the Dunajec, and Tresna on the Soła. However, our proposed new approach to multi-criteria assessment of reservoirs can be applied to and implemented in other catchments. The application of solutions based on our study may contribute to maintaining or potentially increasing the level of water retention in reservoirs and their catchment areas.

Channel-reach morphometric analysis on a headwater stream in a low-mountainous region: a case study from Mecsek Hills

Small catchments in mountainous regions affect downstream rivers as a primary source of sediment supply and also generate flash swasfloods, especially during extreme events. These floods have significantly shaped the catchments of small streams in the Mecsek Hills and some rural areas over the past two decades. However, there has been no previous study examining the hydromorphology of headwater catchments in low mountain environments in Hungary. The present study was meant to investigate the fundamental hydrogeomorphological properties of a first-order catchment. A customary and detailed GIS survey of 50-metre sections was aimed at deciphering flash flood vulnerability and geomorphic interrelations within a micro watershed. We found moderate susceptibility to flash floods compared to the whole Mecsek Hills. Stable large woody debris jams were identified during the field survey as major geomorphic channel features functioning as natural barriers which drive channel evolution and reduce flood hazards.

Quantifying the consequences of unsustainable sand mining and cascade dams on aspects in a tropical river basin

Human interventions at the river basin scale, such as sand mining and hydropower dam construction, have profoundly affected hydrological and hydraulic alteration regimes, sediment budgets, and morphological changes worldwide. Quantifying the consequences of unsustainable ongoing sand mining and hydropower is crucial for obtaining sediment load data and managing hydrogeomorphology. In this study, comprehensive long-term consecutive four-field monitoring, statistical methods, and hydrological models (SWAT) were applied to quantify the spatiotemporal changes in long-term discharge and sediment load from 1996 to 2020 for the tropical river of the Vu Gia Thu Bon (VGTB) in the central region of Vietnam. The SWAT model was calibrated from 1996 to 2010, validated from 2011 to 2020 and showed good performance for daily discharge and monthly sediment. The evolution of river bathymetric data (2010, 2015, 2018, and 2021) was analysed to clarify the upstream sediment supply trapped in the riverbed and how the sand mining volume was removed. The results showed that the mean annual sediment in the Vu Gia and Thu Bon Rivers decreased by 57.3% and 23.8%, respectively, in the postdam period compared with the predam period. The thalweg elevation decreased at the Ai Nghia and Giao Thuy stations from 2010 to 2021 by 1.8 m and 3.9 m, respectively. The water level decreased by 21.1% at Ai Nghia and 44.3% at Giao Thuy. Dam development, sand mining, and changes in land use are the main factors responsible for flow discharge and sediment morphodynamic alterations. Morphological change have increased the water transfer rate from the Vu Gia River to the Thu Bon River through the Quang Hue channel. Downstream of the Vu Gia River, water transfer and riverbed incision have decreased flow discharge and water level and increased saltwater intrusion in recent years. As a result, water shortages induced by saltwater intrusion during drought periods have emerged as a significant constraint in hindering the domestic water supply and agricultural production.

Stratigraphic-sedimentary evolution of a mixed siliciclastic-carbonate system in the Huizhou Sag of the Pearl River Mouth Basin, Northern South China Sea

Mixed siliciclastic-carbonate systems are regulated interactively by factors such as siliciclastic sediment supply, carbonate production, sea-level change, tectonism, and climate conditions. These systems record vital information that aids in understanding ancient environments. This study used a merged 3D seismic volume, in conjunction with over 100 industrial wells, to systematically investigate the stratigraphic-sedimentary evolution of such a system within the Huizhou Sag of the Pearl River Mouth Basin, located on the northern continental shelf of the South China Sea. In total, six major sequence boundaries were identified for the Zhujiang Formation within the area, thus subdividing the interval into five typical third-order depositional sequences. Each of these sequences can be divided into a transgressive and a highstand systems tract. Lowstand or falling stage systems tracts were also recognized, the deposition of which was potentially in response to the uplifting process of the Dongsha Rise. During the deposition of the Zhujiang Formation, the Huizhou Sag may have undergone a sequential evolutionary history from delta-shore, to delta-shore-tidal-lagoon, to delta-shore-carbonate, and finally to delta-shore-shallow marine systems. This evolution responded to a varying degree of mixing processes, which was mainly regulated by siliciclastic sediment supply, confined paleomorphology, and local oceanic currents. Furthermore, the deposition of the Zhujiang Formation in the Huizhou Area was time-equivalent with the spreading process after the ridge jump of the South China Sea (23–16.5 Ma), providing valuable insights into sea-level fluctuations, provenance changes, and tectonic evolution. Our results may also shed light on the evaluation of lithologic traps and hydrocarbon sweet spots within mixed siliciclastic-carbonate systems.

Holocene climate regulates multiple sulfur isotope compositions of pyrite in the East China sea via sedimentation rate

Marine authigenic pyrite sulfur isotope (δ34Spyr) is one of the most important proxies for reconstructing remarkable changes of Earth's surface in deep time. Recent observations of unexpectedly large δ34Spyr variations in Quaternary sediments complicate the application of this isotopic proxy, and a mechanistic understanding of such variabilities is needed. In this study, we measured multiple sulfur isotopic compositions of pyrite in well dated sediments spanning the last 7500 years from the East China Sea (ECS) inner shelf to study the mechanism how Holocene climate controls pyrite sulfur isotopic compositions. We found that variabilities of pyrite multiple sulfur isotopes, δ34Spyr and Δ33Spyr (ranging from −38.3‰ to 2.3‰ and from −0.01‰ to 0.21‰, respectively) are significantly correlated with large fluctuations of sedimentation rates (0.03–2.09 cm/yr). In tandem with box model calculations and compiled data, the large δ34Spyr variations up to 40‰ are primarily ascribed to fluctuated anaerobic oxidation of methane coupled sulfate reduction (AOM-SR). Further analyses suggest that heightened distillation of interstitial sulfate, likely propelled by high methane flux coupled with increased sedimentation rates, can produce such negatively correlated δ34Spyr-Δ33Spyr pattern and large variations in δ34Spyr. Possible changes in isotope fractionation factors caused by varied cell-specific sulfate reduction rates result in a maximum δ34Spyr variation of <20‰ and therefore play a minor role in the large variation in δ34Spyr (>40‰). The major and minor isotopic compositions of pyrite align with sulfide produced through pure sulfate reduction, indicating little evidence of additional oxidative sulfur cycling. Sedimentation rates in the ECS are controlled by the East Asian winter monsoon that influences the sediment transport capacity from longshore currents on a multicentennial timescale, and also by the coupled East Asian summer monsoon that affects the sediment availability from the Yangtze River on a millennial timescale. Holocene climate changes alter sediment supplies at the ECS and therefore play a vital role in controlling multiple sulfur isotope compositions of sedimentary pyrite in this region. This study has the potential to provide new insights into δ34S excursions in seawater sulfate that occurred during periods of notable acceleration in sedimentation rates throughout the geological history.

Controls on mechanical properties of a carbonate mudstone: Insights from non-destructive techniques and geochemical data

This study quantifies geomechanical properties of carbonate mudrocks in the Arabian intra-shelf basin, focusing on different sedimentologic zones (proximal to distal). Core-based mechanical properties were obtained through unconfined conditions. Micro Rebound Testing (MRT) was conducted on 700′ ft of core at 0.5 ft intervals to capture high-resolution rock hardness (HLD), unconfined compressive strength (UCS), and static Young's Modulus (E). Additional tests validated log-based parameters, including impulse hammer, acoustic velocity testing for (E), P-wave and S-wave velocities (Vp and Vs), scratch testing, and XRF/XRD analyses for mineralogical and elemental variability impact on mechanical properties.Multivariate statistical analysis correlates mechanical properties with elemental data, showing direct proportionality to calcium percentage, (E), Vp, and Vs, while inversely correlated to aluminum, iron, sulfur, and total organic carbon (TOC), especially in distal zones. Porosity, as measured by neutron porosity log, is a significant control on mechanical properties, with higher porosity resulting in lower mechanical properties at any given calcium percentage. Variations in geomechanical properties within each zone are principally controlled by depositional environments, sediment supply, diagenesis, and a pycnocline dividing the water column into anoxic, dysoxic (or suboxic), and oxic conditions.Zone 1A and 4 exhibit high brittleness characterized by high Ca-calcite, HLD, (E), Vp, Vs., low terrigenous elements/minerals, and porosity. Zone 1B and 2A are moderately brittle, showing a slight decrease in brittleness and a slight increase in terrigenous elements/minerals, porosity, and organic matter, particularly in the distal zone (Zone 1B). Zone 2B, 3A, and 3B represent moderate to highly ductile zones with low levels of Ca-calcite, HLD, (E), Vp, Vs, and an increase in terrigenous elements/minerals, porosity, and TOC, especially in the distal zone (Zone 2B and 3B). This study highlights the value of high-resolution mechanical data from cores with applications for improved stress estimation and hydraulic fracture stimulation in unconventional reservoirs.

Propagation Velocity of Excitation Waves Caused by Turbidity Currents

Turbidity currents are important carriers for transporting terrestrial sediment into the deep sea, facilitating the transfer of matter and energy between land and the deep sea. Previous studies have suggested that turbidity currents can exhibit high velocities during their movement in submarine canyons. However, the maximum vertical descent velocity of high-concentration turbid water simulating turbidity currents does not exceed 1 m/s, which does not support the understanding that turbidity currents can reach speeds of over twenty meters per second in submarine canyons. During their movement, turbidity currents can compress and push the water ahead, generating propagating waves. These waves, known as excitation waves, exert a force on the seafloor, resuspending bottom sediments and potentially leading to the generation of secondary turbidity currents downstream. Therefore, the propagation distance of excitation waves is not the same as the initial journey of the turbidity currents, and the velocity of excitation waves within this journey has been mistakenly regarded as the velocity of the turbidity currents. Research on the propagation velocity of excitation waves is of great significance for understanding the sediment supply patterns of turbidity currents and the transport patterns of deep-sea sediments. In this study, numerical simulations were conducted to investigate the velocity of excitation waves induced by turbidity currents and to explore the factors that can affect their propagation velocity and amplitude. The relationship between the velocity and amplitude of excitation waves and different influencing factors was determined. The results indicate that the propagation velocity of excitation waves induced by turbidity currents is primarily determined by the water depth, and an expression (v2 = 0.63gh) for the propagation velocity of excitation waves is provided.

Delft3D model-based estuarine suspended sediment budget with morphodynamic changes of the channel-shoal complex in a mega fluvial-tidal delta

ABSTRACT Reduced riverine sediment supply and sea-level rise (SLR) threaten land building and ecosystem in deltas. However, the sediment-morphodynamic processes in a channel-shoal complex are not well understood. Here, based on bathymetry and the Delft3D model, geomorphic changes and suspended sediment budgets in the South Passage, Nanhui and Jiuduansha Shoal in the mega-Changjiang Delta were examined. Results reveal that with riverine suspended sediment concentration (SSC) decreased by 75%, the net sediment deposition rate was reduced from 4.20 cm/yr in 1979–1990 to 3.21 cm/yr in 1990–2003, and further declined to 2.21 cm/yr in 2003–2013 and 0.40 cm/yr in 2013–2020. Severe erosion occurred along the upper South Passage and extended toward the mouth bar. Strong accretions accumulated in the Nanhui and Jiuduansha Shoal. After river SSC declined from 0.53 kg/m3 to 0.35 kg/m3, 0.16 kg/m3, and 0.12 kg/m3, net suspended sediment deposition was lowered by 3.13%, 7.35% and 8.67%, respectively. Moreover, SLR of 5 cm, 15 cm, 25 cm, and 50 cm resulted in a further 1.11%, 4.18%, 4.16%, and 14.79% reduction in sediment trapping efficiency. Our findings highlight the strong likelihood that reduced river sediment input, SLRs and intensified anthropogenic effects will exacerbate sediment deficit and erosion in mega fluvial-tidal deltas.

Sediment fluxes within salt marsh tidal creek systems in the Yangtze Estuary

Creeks are essential for salt marshes by conveying water and sediment through this geomorphic system. In this paper, we investigate the mechanisms that determine the residual sediment flux using measurements conducted in tidal creeks in salt marshes of the Yangtze Estuary. A main creek and a secondary creek were studied to explore whether the mechanisms determining residual sediment fluxes through the main creek differ from those in the secondary creek. Measurements in creeks were carried out over 5 years, spanning different months. Sediment import was found during most tides, both in the main creek and the secondary creek, implying that creeks in Chongming generally function as a conveyor belt of sediment into the marsh. However, sediment export can occur during certain overbank tides. When comparing the role of creeks in drainage and sediment delivery, the main creek functions more in delivering sediment while the secondary creek primarily serves as a drainage conduit. To better understand the mechanisms behind sediment fluxes, the residual sediment flux was compared with the residual discharge and the sediment differential (differences in sediment concentration between flood and ebb). Overbank tides generally lead to a net outward discharge as more water from saltmarshes can be concentrated into the marsh creek during ebb tides. This net outward discharge tends to export more sediment during ebb tides. However, due to the sediment abundance during the flood phase in the turbid environment, sediment import can be expected even with the residual export of water. Export of sediment was only found for the few tides with a net outward discharge and a small positive sediment concentration differential. Large negative sediment differentials (larger averaged suspended sediment concentration during ebb tides) have not been observed because the sediment supply during ebb is limited. This paper unravels how the sediment differential and residual discharge contribute to the residual sediment flux, providing a better understanding of sediment dynamics in marsh creek systems.

Patterns of Alluviation in Mixed Bedrock‐Alluvial Channels: 2. Controls on the Formation of Alluvial Patches

AbstractUnderstanding the development and spatial distribution of alluvial patches in mixed bedrock‐alluvial rivers is necessary to predict the mechanisms of the interactions between sediment transport, alluvial cover, and bedrock erosion. This study aims to analyze bedrock alluviation patterns using a 2D morphodynamic model, and to use the model results to better understand the mechanisms responsible for alluvial patterns observed experimentally. A series of simulations are conducted to explore how alluvial patterns in mixed bedrock‐alluvial channels form and evolve for different channel slopes and antecedent sediment layer thicknesses. In initially bare bedrock low‐slope channels, sediment cover increases with increasing sediment supply because areas of Froude‐subcritical flow enable sediment deposition, while in steep‐slope channels the flow remains fully supercritical and the model predicts so‐called runaway alluviation. For channels initially covered with sediment, the model predicts a slope‐dependent sediment supply threshold above which sediment cover increases with increasing sediment supply, and below which the bedrock becomes fully exposed. For a given sediment supply, the fraction of bedrock exposure and average alluvial thickness converge toward the equilibrium value regardless of the initial cover thickness as long as it exceeds a minimum threshold. Steep channels are able to maintain a continuous strip of sediment under sub‐capacity sediment supply conditions by achieving a balance between increased form drag as bedforms develop and reduced surface roughness as the portion of alluvial cover decreases. In lower‐slope channels, alluvial patches are distributed sporadically in regions of the subcritical flow.

Sensitivity of modelled passive margin stratigraphy to variations in sea level, sediment supply and subsidence

AbstractWe produced a 10 Myr synthetic stratigraphic section using a forward stratigraphic model that generates marine deltaic stratigraphy over geological timescales. We recursively fit the model using a Bayesian inversion algorithm to test: (1) if it could be accurately reconstructed; (2) if the parameters used to create it could be recovered; and (3) the sensitivity of the model output to given model parameters and the attendant physical processes. The original synthetic stratigraphic section was produced with cyclical sea‐level variations of 40 and 30 m with 2.4 and 10 Myr periods respectively. Sediment was also supplied cyclically, in 2.4 and 10 Myr cycles with amplitudes of 30 and 80 tons/100 kyr, respectively, varying from a mean of 232 tons/100 kyr. Parameter values were sampled to fit the model using a Markov chain Monte Carlo algorithm, resulting in a ±5 m (1σ) variation between the experimental output and the original. Sea level varied by ±7 m (1σ) within the posterior distribution of parameters. As a result, both the 10 Myr and 2.4 Myr sea‐level cycles could be extracted from the original output. The variation in sediment supply was approximately ±38 tons/100 kyr (1σ) and, as a result, only the larger long‐term supply variations could be accurately recovered in refitting the model. The variation in thermal, flexural and total subsidence across those parameter sets is less than ±10 m (1σ). The original section experienced 150 m of total subsidence at the depocentre. Our results demonstrate the distinct and interpretable imprint of sea level and subsidence on continental margin stratigraphy can be quantified. Moreover, we conclude that sea‐level change produces a defined effect on the geometries of stratigraphic architecture, and that techniques applied for the purpose of delineating sea‐level variation from continental margin strata have a well‐founded conceptual basis.

Uncertainties of Annual Suspended Sediment Transport Estimates Driven by Temporal Variability

AbstractThe majority of sediment transported from rivers to the global oceans is moved in suspension as fine particles. Thereby, the transported sediment shapes the physical environment regarding erosive and accumulative processes. Temporal variations in sediment supply and transport lead to unquantified uncertainties in annual load estimates, requiring high‐resolution data sets and a sound understanding of site‐specific catchment characteristics. We investigate the temporal variability of suspended sediment transport in four catchments in Germany with highly different discharge regimes and catchment sizes (&lt;1,000 km2 to &gt;100,000 km2). The data set consists of high‐resolution 15‐min turbidity measurements with daily discharge and frequent manual sampling. Utilizing a bootstrap approach based on the 15‐min time series, we assessed the impact of the sampling interval on annual load estimates for less frequent data sets. We use the sediment load exceedance time (Ts80%) as a measure of variability and relate it to uncertainties in annual load estimates. Since low‐frequency data sets rely on sediment rating curves, we performed a sensitivity analysis of the rating parameters a, b, and ε. Our results indicate a negative exponential relationship between Ts80% and uncertainties in annual load estimates. Based on the Ts80%, we can derive the shortest sampling frequency necessary to obtain annual load estimates with an error of &lt;20% over varying discharge regimes. Additionally, Ts80% is linked to rating exponent b, with low b‐values indicating high Ts80%‐values and lower variability, and high b‐values indicating higher variability.

The Signature of Climate in Fluvial Suspended Sediment Records

AbstractArid regions are often characterized by exceptionally high rates of fluvial sediment transport, but the processes responsible for this apparent connection between climate and sediment transport remain unclear. We examined decades of continuous flow records and suspended sediment concentrations from 71 rivers across the United States by comparing the suspended sediment rating curve behavior, quantified using power law coefficients and exponents, to an aridity index. Results indicate that higher aridity correlates with both greater overall suspended sediment concentration and lower sensitivity of concentration to changes in discharge, demonstrating that rivers in arid locations on average have greater suspended sediment transport efficiency across most discharges, and achieve high transport rates at a relatively lower discharge than rivers in temperate climates. Furthermore, based on additional analyses of the Normalized Difference Vegetation Index, specific suspended sediment yield, and a hydrograph flashiness index, we attribute the relationships between sediment transport and climate primarily to differences in vegetation density, precipitation, and runoff, variables that all influence both sediment supply and riverbed grain sorting. Finally, we note that the observed contrasts in sediment transport behavior likely represent climate‐driven differences in the magnitude and frequency of sediment supply rather than annual suspended sediment load, which does not depend significantly on climate. This study highlights a critical connection between multiple interrelated climatic factors and sediment transport, an important finding for future hazard mitigation in a changing climate with rapidly shifting vegetation patterns and hydrology.