Sediment Dispersal Research Articles

River mouths are hotspots for terrestrial organic carbon burial on the Sunda Shelf: Implications for tropical coastal carbon sequestration

Tropical small mountainous rivers (SMRs), characterized by extensive basin weathering, discharge high amounts of sediments and particulate terrestrial organic carbon (OCterr) into the ocean. Burial of OCterr in marine sediments is important for atmospheric CO2 sequestration, yet its rate and efficiency remain not fully understood. Previous studies indicated generally low burial rates and burial efficiencies of OCterr in tropical coastal ocean settings influenced by SMRs but lacked detailed insights into specific processes and quantitative estimates of burial efficiencies. This study investigated δ13C values of bulk OC (δ13COC) and OCterr biomarker proxies (BIT, #ringstetra, ΣIIIa/ΣIIa and %C32 1,15-diol indices) in river surface sediments on the Malay Peninsula and marine surface sediments on the adjacent Sunda Shelf to quantify sedimentary OCterr contents, burial rates, and burial efficiencies along the river-estuary-ocean continuum. The substantial variation in δ13COC and OCterr biomarker proxies between riverbed and shelf sediments revealed a strong decrease in fractional OCterr contribution offshore and alongshore. Using a two-endmember mixing model based on δ13COC, we determined average OCterr contents of 1.40, 0.80, and 0.05 wt% in riverbed, river-mouth, and shelf sediments, respectively. The low OCterr contents in offshore sediments are attributed to physical mechanisms (such as local hydrodynamic regime and clay mineral flocculation) that affect sediment dispersal. This process leads to notably high burial rates of terrigenous sediment (11.5 ± 2.1 kg/m−2(−|-) yr−1) and OCterr (99.2 ± 29.0 g/m−2(−|-) yr−1) at river mouths. In comparison, burial rates of OCterr were significantly lower (0.1–0.5 g/m−2(−|-) yr−1) offshore due to strong degradation of OCterr, resulting in a lower overall OCterr burial efficiency across the study area (18.2 %) compared to the global marginal sea average (21 %). These findings highlight the significance of carbon burial at the mouths of SMRs as CO2 sinks, whereas degradation of OCterr during offshore transport is a potentially large CO2 source in the tropical coastal ocean.

River plume identification through a deep-learning model: an innovative approach

ABSTRACT River plumes are complex physical phenomena that occur at the interface between riverine and marine systems. The lack of direct measurements of sediment concentration complicates the study of these geomorphological features, since the boundaries of river plumes are often gradual and unclear. Therefore, the identification and digitalization of river plumes is not simple and the methods applied in different study areas are not always objective and replicable. The aim of this work is to provide a valid approach based on a deep-learning model that uses Convolution Neural Network (CNN) layers for the digitalization of river plumes. We describe the methodology applied to implement the input dataset used for training the model, the errors obtained, and an application for a study area of about 300 km located in the Mediterranean. The model uses Sentinel-2 Level-1C images. The application of the model to a specific study area allowed us to understand the possibility of investigating these geomorphological features to obtain results in agreement with previous works. As a matter of fact, by using the red band as a proxy of sediment concentration, we were able to investigate the average behaviours of sediment dispersion along the coast and to extract innovative data related to specific events for the study of morphological characteristics such as dimension and direction.

Provenance and sediment dispersal in Pearl River Estuary, southern China unraveled by magnetic properties

The Pearl River Estuary (PRE), with eight outlets supplied by multiple tributaries, is an excellent study area for reconstructing sediment provenance and transport pathways of a multi-sourced sediment-routing system. However, it less known the sediment source to sink dispersal model. This study aimed to estimate the PRE sediment budget by comparing the magnetic characteristics of estuarine surface sediments with those of all potential sources. In the bottom sediments of PRE and adjacent region, magnetic minerals, mainly magnetite and hematite, have a characteristic terrigenous signature. Source and sink comparison for magnetic parameters shows that the possible input of southwestern Taiwan rivers can be excluded, whereas the possibility of sediment contribution from nearby rivers, including Han, Rong, and Jiulong rivers, to the estuary could not be ruled out. Quantitative provenance analysis indicates that West River-sourced sediment dominates the western side of the estuary, the East River-sourced sediment is most important in the eastern side of the estuary, whereas sediment supplied by the North River is high only close to the outlets. A mixed-sediment contribution to the estuary is suggested, with a fluvial-dominated sediment input around the west river outlets and a tide-dominated sediment dispersal in the eastern side of the estuary. Overall, our study demonstrates the importance of fingerprinting changes in sediment supply using sediment magnetic properties and provides new insights into the PRE sediment-routing system.

Unsteady Secondary Flow Structure at a Large River Confluence

AbstractRiver confluences, which are characterized by complex hydrodynamics, are key nodes for flood control and environmental protection. Two field surveys were carried out at the confluence of the Yangtze River and Poyang Lake to investigate the transient character of flow structures, which are often assumed steady. Repeat‐transect acoustic Doppler current profile measurements were processed and analyzed, adopting a new method to separate mean flow from turbulence and measurement error based on physics‐informed generalized Tikhonov regularization. The two field surveys were characterized by two distinct mixing interface modes: A so‐called Kelvin–Helmholtz (KH) mode, and a wake mode. In KH mode, large‐scale flow fluctuations were observed. These flow fluctuations exert a substantial influence on the alternation of secondary flows by changing the water surface pressure gradient, affecting the intensity of secondary flows rather than their spatial structure. We infer that temperature‐induced stratification is the main cause of this. In the wake mode, multiple vortices in the wake region at the confluence apex also produced flow fluctuations, directly related to the primary velocity gradient. We argue that even for constant incoming flows, secondary flow at river confluences can exhibit channel‐scale unsteadiness related to migration of the turbulent mixing interface. Our findings highlight the crucial role of density effects in regulating secondary flow unsteadiness, which is essential for understanding contaminant and sediment dispersal in river systems.

Detrital zircon U-Pb geochronology of the Moatize and N'Condédzi coalfields, Zambezi Karoo Basin of Mozambique: Implications for provenance, sediment dispersal and basin evolution

Detrital zircon U-Pb geochronology of the Moatize and N'Condédzi coalfields in the Zambezi Karoo Basin of Mozambique provides key insights into the regional provenance, sediment dispersal pathways and basin evolution. Borehole cores from the two coalfields reveal a stratigraphy spanning the early Roadian (middle Permian) to the Carnian (Upper Triassic). The Permian sandstones of the Moatize Coalfield (MC) yield three detrital zircon populations, with ages ranging from 1150 to 950 Ma, 900–780 Ma and 650–490 Ma. In contrast, the Permian sandstones of the N'Condédzi Coalfield (NC) have only one population, which ranges from 1150 to 950 Ma. During the Permian, the provenance area for the NC was the Tete-Chipata Terrane and Malawi Complex (1150–950 Ma) to the north-northeast. In the MC, the detrital zircon populations of the early lake delta depositional setting (Roadian to Wordian) indicate a main provenance in the Zambezi Belt (900–780 Ma) located to the south of the MC, with minor sourcing from the Nampula Block (1150–950 Ma and 650–490 Ma) to the east. The transition from a lake delta to an alluvial depositional setting is attributed to a major tectonic event in the MC, which involved the formation of a braided channel belt. Sandstones from this braided channel belt yield three detrital zircons populations (650–490 Ma, 900–780 Ma and 1150–950 Ma), indicating provenance from both the Zambezi Belt and the Nampula Block. The overlying sandstones in the MC show only a minor population from the Zambezi Belt (900–780 Ma), implying a shift in provenance to the Nampula Block that was likely induced by tectonics. The absence of detrital zircon populations of 900–780 Ma and 650–490 in the Permian sandstones of NC implies that the two coalfields were not connected during the mid to late Permian. It is likely that an intra-rift horst (the Mesoproterozoic Gabbro-Anorthosite Tete Suite) separated the two coalfields. The Lower Triassic sandstones of the NC yield a main detrital zircon population indicating provenance from the Tete-Chipata Terrane and Malawi Complex (1150–950 Ma). A minor population at 650–490 Ma is linked to increased aridity at the Permian – Triassic boundary, which caused expansion of the watershed across the Tete-Chipata Terrane and the Malawi Complex. The Upper Triassic sandstones in the NC yield a 1150–950 Ma detrital zircon population, indicating provenance from the Tete-Chipata Terrane and Malawi Complex and a return to the source-to-sink conditions seen in the mid to late Permian.

Continental-scale sediment mixing and dispersal across northern Gondwana: detrital zircon U-Pb-O-Hf isotopic evidence from the Cambro-Ordovician sandstones overlying the Arabian Shield

ABSTRACT The juvenile Neoproterozoic basement of the Arabian Shield is overlain with angular unconformity by a voluminous Cambro-Ordovician cover sequence known as the Saq Formation and Wajid Group. Provenance studies of this vast siliciclastic cover over the Saudi Arabian part of the Arabian-Nubian Shield (ANS) have to date been based solely on U-Pb zircon data and heavy minerals. We present the first combined in-situ U-Pb, δ18O and Lu-Hf isotopic data for detrital zircon from the Saq and Wajid units exposed along the northeastern margin and southern part of the Arabian Shield. U-Pb age spectra reveal prominent age peaks at ca. 0.8–0.55 Ga and 1.1–0.9 Ga with subordinate peaks at ca. 2.2–1.7 Ga and 2.7–2.5 Ga. The δ18O secular variation mirrors global compilations with Archaean zircon defining a restricted δ18O range of ca. 4.0–8.0 ‰ and younger zircon showing wide variation in δ18O up to ca. 14 ‰. The ca. 0.8–0.55 Ga age peak has the largest variation in εHf(t) with about half of all these Neoproterozoic zircon grains being juvenile (εHf(t) > 5), which are interpreted to be sourced from the juvenile terranes of the ANS. Neoproterozoic zircon with evolved εHf(t) signatures require a more distal source beyond the ANS. As extensive ca. 1.1–0.9 Ga crust is lacking in the vicinity of the ANS, the ca. 1.1–0.9 Ga age peak is interpreted to be derived from either contemporaneous orogenic belts in Central Africa or recycled from older sedimentary rocks containing these age components. Extreme variations in δ18O of post – Archaean zircon, together with the evolved εHf(t), indicate crustal thickening and increased incorporation of supracrustal material associated with collisional orogenesis. The remarkable similarities in age spectra and isotopic compositions of the Saq Formation and Wajid Group sandstones with those from other regions in northern Gondwana indicate a continental–scale homogenization and dispersion process.

Early extensional salt tectonics controls deep-water sediment dispersal

Predicting the distribution of sedimentary facies during the early stages of deformation of salt-detached continental margins is key to constraining the location and stratigraphic architecture of hydrocarbon and CO2 reservoirs, as well as to understanding the oceanic carbon cycle. Despite its importance, we still have a relatively poor understanding of salt-sediment interactions during the early phases of extensional salt tectonics, mainly because subsequent salt-related deformation and/or deep burial of the related stratigraphic succession means the related deposits are poorly imaged in seismic reflection data and/or not penetrated by borehole data. We investigated the interplay between early extension-related salt deformation and deep-water sediment dispersal using 3-D seismic reflection data from the northern Levant Basin offshore Lebanon. Our results indicate that salt tectonics has two contrasting impacts: Whereas slope-parallel faults favor early sediment transfer along downslope-oriented corridors to the abyssal plain, slope-normal faults and ramp-syncline basins trap land-derived sediments, hampering or delaying their transport to the abyssal plain. These results help refine source-to-sink models of turbidite systems developing in young salt basins, highlighting the crucial role of extensional tectonics in controlling sediment dispersal and the development of intra-slope depocenters and emphasizing the impact of fault strike, ramp-syncline basin evolution, and salt thinning. Our study has significant implications for predicting the location of deep-water coarse-grained sediment and the preservation of land-derived organic carbon in mature, more structurally complex, salt basins.

Impact of seasonal variations in fronts on suspended sediments transport off the coastal area of Fujian Province

The Zhejiang-Fujian (Zhe-Min) coastal muddy area plays a crucial role in facilitating sediment exchange through a cross-front. The mud depocenter off the Zhe-Min coastal area is a source of suspended sediment that can be transported to the continental shelf of the East China Sea (ECS). Although the front of the inner shelf of the ECS has been extensively reported, the cross-front material transport off the coastal area of Zhe-Min in summer has not been well studied, especially using measured data. To reveal how the front controls the transport of suspended sediment, this study focuses on the impact of fronts on the dispersion of suspended sediment off the coastal area of Fujian Province in different seasons. The results indicate that the front acts as a barrier, inhibiting the dispersion of suspended sediment into the sea. The high-concentration suspended sediment is mainly found to the northwest of the front, with an average SSC of 8.5 mg/L in winter and 3.1 mg/L in summer. The suspended sediment concentration (SSC) follows a V-shaped distribution along the cross-isobath transects, with lower SSC observed at the 50-m isobath compared with the shallow water area and the deep water area. The SSC at the front was the lowest, with an average concentration of 2.3 mg/L in winter and 1.9 mg/L in summer. The front is crucial for the development of the Zhe-Min coastal muddy area. The winter monsoon is strong, resulting in a sufficient supply of suspended sediments in the muddy area and a high transport flux of suspended sediments in the nearshore. The front hinders the dispersion of high-concentrated sediment from the nearshore to the offshore, resulting in the deposition of fine-grained sediments in the nearshore and the formation of an inner shelf muddy sedimentary zone. The findings of this study will help improve our understanding of the sediment source-to-sink processes in the ECS and land–sea interactions.

New understanding on the interplay of longitudinal and transverse sediment dispersal: Combination of 3D seismic profile and u-pb ages of detrital zircons of Paleogene sedimentary rocks in the northern South China Sea

The northern South China Sea (SCS) experienced a significant transition from a lacustrine to marine environment in the Paleogene, and its deep-water basin of Zhu-II Depression is in particular extensively studied due to its richness in oil and gas resources. However, limited number of boreholes in the deep-water area has long constrained a better understanding on the provenance of the Paleogene strata and the associated sediment transport processes. In this study, detrital zircon U-Pb ages and three-dimensional (3D) seismic-reflection data were systematically employed to investigate the “source-to-sink” pathways and interplay of longitudinal and transverse sediment dispersal. Our results indicate that the Zhu-II Depression sediments of the northern SCS were predominantly derived from the surrounding nearby paleo-uplifts in the early and middle Eocene. A significant provenance shift took place in the late Eocene, when the local paleo-uplift source was replaced by a distant source from the western SCS. Sediments were transported from west to east by the “Kontum-Ying-Qiong River” as a longitudinal dispersal. In the Oligocene, the “Kontum-Ying-Qiong River” delivered large amounts of sediments from Central Vietnam to the eastern part of the northern SCS. Meanwhile, the Pearl River gradually evolved through regional tectonic processes and influenced the deep-water area of Zhu-II Depression as a transverse dispersal. Sediments from both “Kontum-Ying-Qiong” and Pearl Rivers converged and deposited as deep-water deltas in the Zhu-II Depression. This dual provenance system in the northern SCS deep-water area was featured by the interplay between longitudinal and transverse sediment dispersal. It was largely controlled by the tectonic-palaeogeographic pattern inherited from the Mesozoic arc system.

Cenozoic sedimentary archives of a strike-slip fault zone in the Tanhai Region, Bohai Bay Basin, Northeastern China

The Changdi fault, also known as the Changdi fault zone, is commonly proposed to be a branch of the Tan-Lu fault zone in the Tanhai region of the Bohai Bay Basin. Using three-dimensional seismic data, borehole data, isopach maps, and RMS maps, we attempted to provide a comprehensive understanding of the structural properties and sediment dispersal patterns occurring within the Changdi area from a source-to-sink perspective. Two distinct geomorphological factors were identified in the Changdi area and four types of strike-slip fault-controlled alluvial channels were proposed for the first time. Additionally, three types of sediment dispersal systems, characterized by various sediment infillings, source inputs, and source areas, were reported. The spatial and temporal variations in the geometry and kinematics of the Changdi fault zone were observed to regulate sediment dispersal within the subsag. Sediment dispersal systems in the Changdi area were regulated by sediment dispersal associated with faults resulting from strike-slip movement. The movement of the input port in the provenance area was controlled by strike-slip movements. The movement of the sedimentary center indicated that there was a dextral strike-slip distance of 3.24 km during the E2s3L period. This study suggests that dextral strike-slip movements occurred in the Changdi fault zone, forming a strike-slip fault-controlled basin. The findings of this study may prove useful in developing models to predict the patterns of sediment dispersal controlled by strike-slip faults as well as the location of areas rich in sand.

Factors influencing the variation of the Sepik-Ramu River system's sediment plume off the north coast of New Guinea

The Sepik-Ramu River system (SRRs) is a principal contributor of terrestrial materials to the pelagic waters of the tropical Western Pacific, boasting the highest sediment discharge in New Guinea. However, this system's sediment outflow and dispersal patterns are not well-documented. This study seeks to address this gap by analyzing the estuarine sediment plumes, which are the key indicators of the marine dispersion of terrestrial materials. Utilizing Moderate Resolution Imaging Spectroradiometer (MODIS) images from 2010 to 2020 and applying the Novoa17 algorithm, we examined the distribution and temporal evolution of the sediment plume of the SRRs estuary. The sediment plume was categorized into two distinct zones: core and diffuse, based on suspended sediment concentration (SSC) levels. Our findings reveal significant seasonal variations of these sediment plumes in the nearshore. The plume area expands during the wet season and contracts during the dry season, respectively, with core and diffuse plumes sharing similar change characteristics. The diffuse plume primarily extends northeast in the wet season and subsequently shifts northwest in the dry season, and the core plume consistently points to the northeast. The study further explores the driving factors and mechanisms behind these patterns, identifying river discharge as the primary influence on the core plume and sea surface winds and coastal currents as determinants of the diffuse plume's behavior. Interannual variations in the plume characteristics were found to be influenced by the El Niño-Southern Oscillation (ENSO), which indirectly affects plume dynamics through its impact on regional precipitation. During El Niño events, characterized by drier conditions and reduced discharge, the plume SSC and area typically decrease. During La Niña events, the plume's SSC and area typically increase, except for the 2011 super La Niña's negative anomaly, caused by the rain belt's excessive westward shift.

Multiscale ocean dynamics control the sedimentary source-to-sink processes in the southwestern Okinawa Trough (East China Sea)

The sedimentary source-to-sink processes remain under debate in the southwestern Okinawa Trough due to its complex oceanographic context. We employe a multidisciplinary approach combining marine sedimentology and physical oceanography to address the sedimentary source-to-sink processes. This study shows that the East China Sea shelf has been the provenance of the southwestern Okinawa Trough during the past 3000 a, judged from the detrital zircon U-Pb geochronology. Numerical simulations indicated that the bottom currents are the primary drivers that transport sediments from the East China Sea shelf continuously entering the southwestern Okinawa Trough. This study confirms that the source-to-sink system in the southwestern Okinawa Trough is controlled by multiscale ocean processes, and verifies that the sediments are mainly sourced from the East China Sea shelf in the past 3000 a. These arguments greatly improve our understanding on the sediment dispersal and have important implications on the climatic and oceanographic reconstructions.

Influence of sedimentary processes and fault tectonics on the evolution of submarine canyons in the East Andaman Basin: Insights from high-resolution seismic data analysis

The Andaman Sea Basin, a well-known back-arc spreading centre and a mature petroliferous basin, presents a unique and complex scenario for understanding the interplay of sedimentary processes and fault tectonics on seafloor geomorphology evolution, particularly in the eastern shelf region. This study, using high-resolution 3D seismic data and published exploratory well-seismic interpretation results, aims to establish a comprehensive stratigraphic framework for the Tanintharyi region, spanning the Lower Miocene, Middle Miocene, Upper Miocene, Pliocene, and Quaternary. A submarine canyon system was identified and investigated along the Tanintharyi continental slope within the East Andaman Basin, comprising thirteen slope-confined canyons arranged predominantly in an east-west vertical slope orientation. By analysing seismic reflection features, five distinct seismic facies have been identified, including basal lag (BL), slump and debris-flow deposits (SDFDs), canyon confined sheets (CCSs), laterally inclined packages (LIPs), and channel-levees (CLs). The evolution of this canyon system, a vital focus of this study, can be categorised into three phases based on the morphology and sedimentary structures observed: canyon initiation, canyon extension, and erosion-sedimentation. The findings of this study highlight the critical controlling role of faults in canyon evolution, with the intensity of fault activity displaying a negative correlation with canyon size, indicating the direct impact of faults on geomorphological changes. In addition, the geomorphological changes induced by fault activity have resulted in changes in sediment supply, dispersal, and sedimentation rates, thus significantly impacting the spatial distribution and dimensions of slope-confined canyons within the East Andaman Basin, with far-reaching implications.

Physico-chemical alteration of pyroxene in coastal environments: Empirical constraints from New Zealand and the Azores

Developing approaches to determine the modes of transport and weathering of mafic detrital minerals in natural sedimentary environments is critical to our understanding of sediment production, dispersal and provenance, as well as carbon capture under natural and enhanced weathering regimes. We integrated the characterisation of morphological and surface textures with the surface composition of recent detrital clinopyroxene grains concentrated in a sandy coastal area from the western North Island of New Zealand and a rocky shore area of Santa Maria Island in the Azores Archipelago. Using the compactness shape descriptor, 341 grains were subdivided into elongated, elongated angular, euhedral, angular and subangular groups, with each group further characterised using morphological and microtextural indicators of abrasion, breakage and dissolution/chemical weathering. In both studied environments, the clinopyroxenes are dominated by elongated to angular shapes with flat cleavage surfaces and conchoidal fractures. This is consistent with dominant subaqueous transport by rivers and longshore currents for New Zealand, and dominant wave action with limited sediment supply by rivers along the rocky shore of Santa Maria. More abundant subabraded and abraded shapes with bulbous and elongated depression microtextures are observed in New Zealand, which indicates additional effects by aeolian transport that are not seen in Santa Maria. Consistent with semi-quantitative EDS spot analyses that document fresh clinopyroxenes surfaces, chemical weathering textures are very rare to absent. In Santa Maria, sampling of recent beach sand was complemented by ∼125 kyr-old palaeobeach clinopyroxenes, but these yielded similar textural results without evidence for chemical weathering. However, the surface of these older clinopyroxenes includes small adhering smectite (typically <50 μm in width and a few μm in thickness) interpreted to reflect incipient cementation with buffering of acidic fluids by more reactive lithic fragments in the deposits. The lack of chemical weathering but pervasive evidence for mechanical breakage of clinopyroxenes indicate that, at the sand size, these minerals break faster than they dissolve due to frequent subaqueous and/or aeolian reworking in the studied high-energy environments and temperate climatic conditions.

Assessing the impact of an offshore longline mussel farm on local water circulation in a highly hydrodynamic energetic bay

Thought to be a sustainable choice, molluscs are the most consumed aquaculture foods after finfish. The expansion of the mussel aquaculture industry offshore reports lower environmental impacts compared to inshore farms. Although an offshore location has the potential to reduce a farm's ecological impacts, the effect of large developments on water currents is still not fully understood. High hydrodynamic regimes can influence the dispersion of farm biodeposits, organic loading, flow alterations with the potential to impact water residence time, particle and sediment dispersal (including larvae and biodeposits) and seabed sediment resuspension, which in turn, can have ecological impacts. Farm-induced flow changes of the UK's first large scale suspended longline mussel farm were assessed by a combination of oceanographic mooring and vessel-mounted Acoustic Doppler Current Profiler (ADCP) measurements. Data was separated by upstream and downstream according to the direction of the flow in relation to the farm. M-ADCP results showed a 28% decrease current velocities within the farm's boundaries, demonstrating within-farm current attenuation produced by mussel ropes drag. Flow was then redistributed above and beneath the farm showing velocity increases of 66% and 7% respectively, resulting in above-farm flow acceleration and downwelling (with up to 171% increase in near-seabed velocities compared to sea surface velocities). However, overall mean downstream sea surface velocities decreased by 63%. An overall 21% increase in near-seabed velocities showed the farm's effect on near-bed currents, the opposite to what is naturally achieved by seabed friction. This was further measured by VM-ADCP results also showing secondary flow acceleration at the farm's flanks and increased near-headline flow perturbations of up to 80% mean u velocities over a small horizontal scale (0.5 km), further demonstrating the effects of the farm on the local circulation. Through-farm surface current velocities (waves) were reduced by up to 72%. Flow changes were localised and dependent on the different tidal phases, the farm's design and, the abundance of mussel ropes providing drag.

Longshore sediment transport rate in Formosa Bay, Rio de Janeiro State - Southeast Brazil

An approach involving data on wave-induced sediment transport estimates is essential for managing coastal environments. Numerical models are powerful tools to understand and investigate many processes responsible for longshore sediment transport (LST) in a systematic way, since direct measurement of LST is a difficult task. This paper presents estimates of longshore sediment transport through numerical modeling of wave-induced sediment dispersion patterns in the Formosa Bay, State of Rio de Janeiro - Southeast Brazil. The analysis was conducted using the computational package Coastal Modeling System/SMC-Brazil and mathematical formulations proposed by Cerc (1984). The results suggest average residual solid discharge (Qsravg) in the order of 131,442.745 m3/yr in northern sector, −151,908.32 m3/yr in central sector, and 71,987.69 m3/yr in southern sector of investigated area. In addition, it is important to highlight that minor variations in the wave incidence angle may cause changes in the longshore sediment drift, promoting the convergence and divergence zones throughout the investigated area. The results presented contribute to the advancement of knowledge on the topic addressed here and will be useful in future engineering projects related to beach nourishment and the management of coastal areas subject to erosion processes.

Short-Term Sediment Dispersal on a Large Retreating Coastal River Delta via 234Th and 7Be Sediment Geochronology: The Mississippi River Delta Front

Many Mississippi River Delta studies have shown recent declines in fluvial sediment load from the river and associated land loss. In contrast, recent sedimentary processes on the subaqueous delta are less documented. To help address this knowledge gap, multicores were collected offshore from the three main river outlets at water depths of 25–280 m in June 2017 just after the peak river discharge period, with locations selected based on 2017 U.S. Geological Survey seabed mapping. The coring locations included the undisturbed upper foreset, mudflow lobes, gullies, and the undisturbed prodelta. Nine multicores were analyzed for Beryllium-7 activity, and four cores were analyzed for excess Thorium-234 activity via gamma spectrometry, granulometry and X-radiography. Our results indicate a general trend of declining 7Be and 234Th activities and inventories with increasing distance from sources and in deeper water. The core X-radiographs are graded from the predominantly physically stratified nearshore to the more bioturbated offshore, consistent with the sedimentation patterns. Sediment focusing assessed via the 7Be and 234Th sediment inventories shows preferential sedimentation in gully and lobe environments, whereas the upper foreset and prodelta focusing factors are relatively depleted. Overall, short-term sediment deposition from the main fluvial source remains active offshore from all three major river outlets, despite the overall declining river load.

Valley‐bottom wetlands as temporary buffers for source‐to‐sink dispersal of sediment and associated phosphorus in dryland landscapes

AbstractSediment trapping in wetlands is an essential ecosystem service, with implications for downstream ecosystems and water users. There is however limited empirical evidence of the contemporary rates and magnitude of sediment trapping in valley‐bottom wetlands. Time‐averaged suspended sediment samples from the inlets and outlets of forestry‐ and agriculturally‐impacted valley‐bottom wetlands with contrasting morphometric characteristics were compared in terms of suspended sediment and associated total phosphorous (total P) fluxes over annual scales, a dataset that was limited by Covid travel constraints. Although both wetlands were net depositional, contemporary suspended sediment mass balances for the agriculturally‐impacted wetland revealed a temporal change in the amount of sediment trapped over two water years (2019/2020 and 2020/2021), with trapping efficacies of 91% and 24%, respectively. The proportion of sediment trapped in the water year of 2020/2021 within the adjacent wetland, with a small commercially forested catchment, was up to 4 times higher than the agriculturally‐impacted wetland, which drained a larger catchment. Rates of total P retention showed that the agriculturally‐impacted wetland was a net sink for phosphorus in 2019/2020, but shifted to a source of phosphorus in 2020/2021 as the export of suspended sediment was enhanced. However, this contrasts with the forestry‐impacted wetland, which was a net sink of sediment and associated phosphorus during the one‐year study period of 2020/2021. Overall, despite data constraints, this study suggests that the efficacy of valley‐bottom wetlands in the delivery of sediment trapping and phosphorus removal ecosystem services varies temporally and spatially. This variability is potentially related to the interaction between annual rainfall regimes, catchment size and wetland geomorphic character. The temporary nature of sediment recycling processes could serve to balance wetland dynamics by regulating vertical growth of valley floors and longitudinal slope stability and should be considered in catchment management and wetland restoration planning strategies.

Magmatic and rare Eocene to Precambrian xenocrystic zircons in Dominica reveal discrete crystallization episodes and multiple sediment sources

The effects of sediment contribution to the Lesser Antilles island arc have been well explored with whole-rock trace-element chemistry and isotopic studies. To better understand the source of these sediments, we analyzed &amp;gt;400 zircons for U-Pb ages and trace-element chemistry in eight andesitic-dacitic ignimbrites and lavas younger than 100 ka from the island of Dominica in the central Lesser Antilles arc. The overwhelming majority of the zircons analyzed were magmatic in origin, with U-Pb ages younger than 10 Ma, but predominantly younger than 300 ka. Zircon trace-element chemistry is consistent with derivation from an oceanic-island arc, and positive εHf(t) values (+5 to +15) support a juvenile depleted mantle source. Rare Precambrian to Eocene xenocrystic zircons (36) were also found in the Dominican volcanics and record sediment dispersal from several different terranes as the Caribbean plate migrated eastward along the northern margin of South America. Although some previous detrital zircon studies in the region suggested zircons younger than 100 Ma were derived from the Great Arc of the Caribbean, the younger Dominican zircon xenocrysts (300−50 Ma) have elevated Th/Yb and U/Yb, as well as variable positive/negative εHf(t) values, inconsistent with an oceanic arc origin. These zircons in Dominica were most likely derived from the Eastern and Central Cordillera of the Andes, which experienced a flare-up in magmatism ca. 65−45 Ma. As the Great Arc of the Caribbean traversed along the South American margin, terrigenous sediments transported via river systems and turbidites accumulated in the forearc basin. Older zircons (1800−300 Ma) have a slightly different chemistry and equivocal source(s), including the Andes, northern Venezuelan coastal ranges, and/or the Guyana Shield.

Stratigraphic evolution and sediment dispersal in a marine rift-basin: The case of the Lewis trough, Eastern Dampier sub-basin (Australia)

The understanding of the stratigraphic evolution of marine rift basins is important for both “pure” scientific and economic reasons. From a hydrocarbon exploration point of view the main goal is to develop tools to predict reservoir location, quality, and extent. Stratigraphic models try to do that, and only historical cases can be used to test those models. On the other hand, depositional model cannot precisely predict reservoir location as factors unique to the specific basin under analysis affect sediment distribution (basin physiography, type of flows, among others). This aspect can be assessed only on a case-by-case basis and can be used for initial hypothesis generation based on analogues. This paper shows a case history study based on the integration of well logs, and 3D seismic data across the Eastern Dampier sub-basin, Northwester Shelf of Australia. Advanced seismic interpretation techniques, calibrated to well data, enable the definition of a chrono-stratigraphic framework of the late syn-rift sequence (Late Jurassic) of the Lewis Trough, and the description of the sediment distribution through the interpretation of geomorphological features. During the Late Jurassic three different channel systems deliver sandstones to the deep-water part of the basin through multiple entry points. The timing of the sediment input seems to be linked to variation of sediment input as during that time relative sea-level was slowly rising. The location of the entry points is related to local physiography and defies any attempt of prediction based on general “rules.” From a stratigraphic point view, the findings of the study support the idea that, in rift basin, mechanical subsidence is the primary control for accommodation space, with episodic pulses of extension followed by longer periods of tectonic quiescence. It is during these episodic movements along the basin's bounding faults that space for sedimentation is created. This interpretation is in striking contrast with the basic assumptions of the conventional sequence stratigraphy model.