Lateral Sediment Transport Research Articles

Lateral sediment connectivity by curve number and a proposed approach to soil erodibility at the watershed scale

The flux of suspended sediment and bedload in a watershed is closely linked to the resistance of soil, known as erodibility. However, the relation between the resistance generated by soil to sediment transport from the hillslope to the river flow is still a challenge. Typically, the bedload flux is related to the shear stress of the main channel. However, the link between lateral sediment flux from hillslopes to suspended sediment flux of the main channel and the relation between bedload and suspended sediment flux to soil erodibility need to be developed. For three Brazilian watersheds, suspended sediment and bedload transport, soils, and hydrological data were obtained to determine watershed soil erodibility. The landscape connectivity expressed by hydrological-sediment connectivity was applied to determine the lateral sediment transport to the primary watershed channel. The flow resistance and lateral suspended sediment discharge were represented by the curve number, which summarized the resistance from the soil-group complex leading to soil erodibility. The curve number addressed the lateral flow and suspended sediment transport to represent the lateral connectivity on the landscape. Sediment transport (sand and suspended sediment) from hillslopes was added to the bedload transport along the longitudinal axis of the watershed, and the watershed soil erodibility (KW) was, thus, obtained for each of the three evaluated watersheds.

Analytical studies of the dynamics of pocket beach

The beach site is located in the utmost northeastern part of the Nevskaya Bay. A boulder dam is built on the western side, which, however, fails to protect the beach from the effects of waves that cause longshore sediment transport. The planned construction of transverse beach–retaining structures outlines in the “Water Sports Base in Primorsky District” initiative, mightchange the current situation. This study objective is to predict the evolution of the coastal contour resulting from the planned construction in the next few decades. A natural analogue of such an artificial structure could be a pocket beach located between two natural promontories. The sediment equilibrium at this study location includes three main components: volumes of erosion, accumulation and bypassing. The results include the computed wave patterns and the movement of sediment along the shoreline. The lateral sediment transport diminishes notably over time, driven by the alteration of the shoreline contour and the reduction in the angle between the wave equilibrium and the coastal resultant. The erosion and accumulation volumes increase over time, but their rates slow down. The shoreline displacement becomes more prominent over time; however, the rates of erosion differ from accumulation. The shoreline is moving particularly fast in the first years after construction, and then the process slows down gradually. The introduction of artificial beach protection structures in the 300th Anniversary Park of St. Petersburg will considerably reduce both the pace of beach erosion and the affected area. However, the erosion process will not be completely halted.

Sedimentary molybdenum and uranium cycling under seasonally contrasting redox conditions on the Namibian Shelf

Sedimentary molybdenum (Mo) and uranium (U) enrichments have been widely used as a proxy for redox conditions in oxygen-depleted marine paleo-environments. However, in a dynamic upwelling system the seasonal fluctuations from oxic to completely anoxic-sulfidic bottom waters and lateral sediment transport can modify the primary Mo and U signal of the sediment, which in turn may impact paleo-redox interpretations. In this study we present pore water and solid phase data collected at two cross shelf transects during the ‘more oxygenated’ austral winter and ‘anoxic’ austral summer to study the influence of spatially and seasonally contrasting redox conditions on the formation of authigenic Mo and U enrichments in organic carbon (TOC) rich mud belt sediments on the Namibian shelf. A mass balance was established for each element based on diffusive fluxes and element mass accumulation rates to evaluate the respective mechanisms of trace metal delivery, accumulation and recycling.Mo is delivered to the sediment in its dissolved form via diffusion across the sediment–water interface, especially during austral summer when bottom waters are anoxic and surface sediments are highly sulfidic. In the center of the inner shelf mud belt, the benthic Mo fluxes of up to 37 nmol cm−2 yr−1 into sulfidic surface sediments are the highest ever reported for reducing sulfidic systems and agree with the rate of Mo accumulation in the solid phase. Concurrently, high sedimentation rates and low terrigenous input limit solid phase Mo accumulation on the Namibian shelf. In ancient marine sediments, this mode of Mo cycling can be identified by low Mo/TOC ratios of ∼2 similar to those found in sediments deposited below the perennial oxygen minimum zone on the Peruvian shelf and to those found in deposits of the Cretaceous Oceanic Anoxic Event 2. Diffusive U fluxes into the sediment are generally too low to account for the sedimentary enrichment leading to the conclusion that U is delivered mainly in particulate form. In areas with anoxic bottom water, shallow dissolved U maxima directly below the sediment water interface and rather low sedimentary U content indicate that particulate U is recycled and largely released back into the bottom water. At sites where bottom water oxygen concentrations vary from anoxic to completely oxic on seasonal timescales, the depth at which Mo and U are removed from pore waters moves vertically within the sediment column thus defining a layer between the sediment surface and ∼20 cm depth, in which Mo and U accumulate in the solid phase. Our results emphasize the importance of short-term redox fluctuations in the bottom waters and underlying sediments, as well as lateral sediment transport for the authigenic enrichment of redox-sensitive trace metals in reducing shelf sediments. The relative enrichment patterns identified might be useful for the reconstruction of open marine anoxia in the geological past.

A conceptual-model-based sediment connectivity assessment for patchy agricultural catchments

Abstract. The accelerated sediment supply from agricultural soils to riverine and lacustrine environments leads to negative off-site consequences. In particular, the sediment connectivity from agricultural land to surface waters is strongly affected by landscape patchiness and the linear structures that separate field parcels (e.g. roads, tracks, hedges, and grass buffer strips). Understanding the interactions between these structures and sediment transfer is therefore crucial for minimising off-site erosion impacts. Although soil erosion models can be used to understand lateral sediment transport patterns, model-based connectivity assessments are hindered by the uncertainty in model structures and input data. Specifically, the representation of linear landscape features in numerical soil redistribution models is often compromised by the spatial resolution of the input data and the quality of the process descriptions. Here we adapted the Water and Tillage Erosion Model and Sediment Delivery Model (WaTEM/SEDEM) using high-resolution spatial data (2 m × 2 m) to analyse the sediment connectivity in a very patchy mesoscale catchment (73 km2) of the Swiss Plateau. We used a global sensitivity analysis to explore model structural assumptions about how linear landscape features (dis)connect the sediment cascade, which allowed us to investigate the uncertainty in the model structure. Furthermore, we compared model simulations of hillslope sediment yields from five subcatchments to tributary sediment loads, which were calculated with long-term water discharge and suspended sediment measurements. The sensitivity analysis revealed that the assumptions about how the road network (dis)connects the sediment transfer from field blocks to water courses had a much higher impact on modelled sediment yields than the uncertainty in model parameters. Moreover, model simulations showed a higher agreement with tributary sediment loads when the road network was assumed to directly connect sediments from hillslopes to water courses. Our results ultimately illustrate how a high-density road network combined with an effective drainage system increases sediment connectivity from hillslopes to surface waters in agricultural landscapes. This further highlights the importance of considering linear landscape features and model structural uncertainty in soil erosion and sediment connectivity research.

Investigating the provenances and transport mechanisms of surface sediments in the offshore muddy area of Shandong Peninsula: Insights from REE analyses

The provenances and transport mechanisms of marine sediments are of significance to understanding the oceanic material cycle. Here we examine the rare earth elements (REE) contents and differentiation characteristics as revealed by the analysis of 62 surface sediment samples collected from the offshore muddy area of the Shandong Peninsula. We identify the sediments' provenances and reveal the transport mechanisms based on the REE results and measured temperature and salinity data. The REE concentration of the surface sediments in the study area varies between 153.81 and 205.57 μg/g. Discriminant function and (ΣREE)N*-(Sm/Nd)N scatter diagrams both show that these sediments are mainly from the Huanghe River, and are also impacted upon by the Changjiang River. The North Shandong Peninsula Coastal Current and the Yellow Sea Warm Current, which meet at 37.7°N, drive the lateral sediment transport.

Effects of fluvial instability on the bed morphology in vegetated channels

Flume experiments are conducted to investigate the effect of streambed instability in channels with randomly-distributed vegetation, varying vegetation density and flow conditions, in the absence of upstream sediment supply. The bed morphology is captured with the photogrammetry technique and a Laser Scanner, and its changes with the vegetation and flow conditions are investigated. The results demonstrate that the presence of vegetation contributes in promoting the stability of the streambed and the formation of multiple bars. In runs with low vegetation density, the trajectory of sediment transport is predominantly in the longitudinal direction. However, a slight lateral dispersion of sediments is observed in the run with low flow discharge. By increasing the vegetation density, the bed structures become shorter, with a lower wavelength, than before, but with a similar trend. The analysis of the energy spectra and the high-order generalized structure functions of bed elevation fluctuations demonstrates that the bed surfaces are monofractals and can be described by a single exponent. However, the runs affected also by a lateral dispersion of sediments during the sediment transport phase are characterized by multifractality, which implies that a complex bed morphology at small spatial lags occurs at the end of these runs. The study of the two-dimensional (2D) second-order structure functions demonstrates that the bed is characterized by an anisotropic behavior, with flow-aligned bed structures that reflect the way in which the bed was formed.Article HighlightsVegetation contributes in promoting the stability of the streambedMultiple bars are formed in vegetated channels with different wavelengths, depending on the flow and the vegetation density conditionsBed surfaces in the presence of vegetation are monofractals, except those in which lateral sediment transport occurs

Analysis of interaction between morphology and flow structure in a meandering macro-tidal estuary using 3-D hydrodynamic modeling

This study employs a 3-D numerical model to examine the behavior of primary and secondary flows on a macro-tidal estuarine bend with seasonally evolving morphology. Three scenarios were developed based on actual bathymetric surveys at the meander bend in the upper region of Japan's Chikugo macro-tidal estuary namely, S1: one month post-flood (channel at maximum capacity), S2: two months post-flood (onset of mud bar formation) and S3: nine months post-flood (most stable channel with low capacity). Secondary circulation begins before the center of the curve at −1/3.5 of the radius of curvature (θ), and fully develops between the center until +θ/3.5 for both flood and ebb phases. Primary flow mainly dominates the morphological evolution in the estuary, as secondary flow, which is responsible for the lateral transport of the sediments accounts for 10–15% of the primary flow. The residual currents for one tidal cycle are generally inwards, resulting in net lateral sediment transport near the bottom. Primary current decelerates due to the increase in cross-sectional area at the curve exit while secondary currents exhibit bottom inward flows at the curve center (S1). Since flood currents are larger than ebb due to tidal asymmetry, suspended sediments are transported landward, resulting in the mud bar formation inside the curve upstream (S2). Shear stress (τ) exceeds critical value of 1.8 N/m2 especially at the inner bank during flood tide, but falls below this value during ebb. Since having τ greater than 1.8 N/m2 causes large scouring, it is presumed that erosion, resuspension and transportation occur more actively during flood tide than ebb. The channel becomes more stable as its flow capacity decreases and together, the water flow is mainly directed towards the outer bank during the entire dry season due to further sedimentation at the inner bank (S3). The process is repeated until the occurrence of the next flood, hence giving insight on the seasonal variation in deposition and erosion in the estuarine curve. The overall flow structure and the trend of seasonal flow volume variation in the channel are similar along the streamline from entry to exit for both tidal phases. This study successfully establishes the roles of primary and secondary flows in the evolution of morphology in an actual macro-tidal estuarine setting with strong tidal asymmetry.

Sinuous bar fingers of digitate shallow‐water deltas: Insights into their formative processes and deposits from integrating morphological and sedimentological studies with mathematical modelling

AbstractThis paper helps to address the growing need to resolve the severe loss of deltaic lands by providing a new understanding for shallow‐water digitate delta growth. The integration of satellite image analysis of modern deltas, field studies of the Ganjiang Delta in the Poyang Lake and ‘Delft3D’ simulations further results in improved facies models for shallow‐water digitate deltas. These analyses show that shallow‐water digitate delta bar fingers are sinuous in contrast to the straight deep‐water digitate delta bar fingers. These differences are assigned to the effect of water depth on outflow hydraulics, where friction‐dominated shallow‐water delta effluents promote mouth bar deposition that then divert flow around the mouth bar, resulting in the formation of sinuous bar fingers. These effects are further strengthened by the meandering of the shallow‐water jet that increases lateral sediment transport, and by the higher flow resistance and lower gradient of the shallow‐water outflows. Our data and analyses further show differences in the morphology and deposits of the shallow‐water sinuous bar fingers, where some bar fingers develop sinuous and others meandering (with point bars) distributary channels. Lateral channel migration and point bar formation (meandering) occur as a secondary process that does not change the shape or width of the bar fingers themselves, and is suggested to be a function of slight initial differences in channel sinuosity. These differences in distributary channel morphology have a strong effect on bar finger facies distribution. Sediment cohesion is another important control on bar finger bending processes, because high cohesion promotes formation of enclosed bays, where their bank strength exceeds the centrifugal force of water flow. Lower sediment cohesion results in sinuous bar fingers without formation of enclosed bays. This work provides insights into natural and artificial shallow‐water digitate delta growth and provides new quantitative facies models for shallow‐water digitate deltas.

Oblique subduction and mantle flow control on upper plate deformation: 3D geodynamic modeling

Most subduction zones on Earth are oblique, i.e., the angle between the plate convergence vector and the trench notably differs from 90°. Therefore, modeling and understanding the strain partitioning in the forearc, the development of extensional basins in the back-arc region and the diachronous transition from subduction to collision require a 3D approach. Here, we assess how oblique oceanic subduction and subsequent collision and associated mantle flow around the subducted lithosphere control the thermo-mechanical evolution of active margins. We conducted a series of 3D thermo-mechanical subduction models and discuss the influence of different subduction obliquity angles, the role of mantle flow variations and their connection with sediment transport and back-arc deformation. Numerical models are complemented by scaled analogue models to visualize the mantle flow evolution. Oceanic subduction along an oblique trench results in asymmetric mantle return flow leading to the gradual decrease of the subduction obliquity angle driven by the gradual rotation of the lower plate and the along-trench variation of slab retreat. This creates laterally variable subduction velocities and slab geometries. Back-arc extension is governed by both the toroidal mantle flow along the slab edges and by the oblique subduction induced lateral mantle flow gradient. The diachronous transition from oceanic to continental subduction and collision facilitates the laterally variable trench advance and retreat and back-arc deformation. Tectonically induced lateral sediment transport in the trench and along the subduction interface decreases its strength and viscosity and can alter subduction velocities. Our model results provide critical insights into the evolution of oblique subduction and collisional systems, such as the Arabia-Eurasia convergence zone.

Study of Sediment Transport in a Tidal Channel‐Shoal System: Lateral Effects and Slack‐Water Dynamics

AbstractLateral flows redistribute sediment and influence the morphodynamics of channel‐shoal systems. However, our understanding of lateral transport of suspended sediment during high and low water slack is still fairly limited, especially in engineered estuaries. Human interventions such as dike‐groyne structures influence lateral exchange mechanisms. The present study aims to unravel these mechanisms in a heavily engineered, turbid channel‐shoal system in the Changjiang Estuary, using a high‐resolution unstructured‐grid three‐dimensional model and in situ observations. Analysis of model results reveals two typical transport patterns during slack‐water conditions, that is, shoal‐to‐channel transport during low water slack and channel‐to‐shoal transport during high water slack. A momentum balance analysis is carried out to explain mechanisms driving the lateral transport of suspended sediment during high water slack, revealing the importance of lateral pressure gradients, Coriolis force, and the curvature‐induced term. Groyne fields play a crucial role in sediment transport, especially during low water slack. A model scenario in which one groyne is removed reveals that groyne fields strongly influence lateral sediment transport. The decomposition of the sediment transport flux reveals that the turbidity maximum is shaped by a balance between seaward advection by residual flows, and landward transport by tidal pumping and gravitational circulation. Within the turbidity maximum, sediment is laterally redistributed by lateral flows during slack‐water conditions, greatly influencing estuarine channel morphology.

Acoustic Measurement of Ship Wave–Induced Sediment Resuspension in a Large River

This paper presents a proof of concept that acoustic velocity and backscatter intensity measurements can provide essential information on ship wave–induced sediment transport processes in the littoral zone of a large river. Through a case study in the Hungarian section of Danube River, a combined application of an acoustic Doppler velocimeter (ADV) and a formerly calibrated acoustic backscatter sensor (ABS) is introduced for parallel measurements of 3D flow velocities and suspended sediment concentration (SSC) in a characteristic point of the near-bank zone. SSC times series detected by the ABS supported the calibration of the ADV backscatter, which eventually enabled the analysis of the temporal behavior of flow velocity and SSC conditions altered by ship waves. A suitable method for the separation of primary and secondary waves provided essential information on the turbulence intensities and sediment fluxes, and was found that the low-frequency primary waves are rather responsible for the lateral sediment transport instead of the dynamic secondary waves. The introduced data processing techniques and the main findings of the paper mean an important step toward the better understanding of the impacts of ship waves on the riverbanks, inherently playing a crucial role in the hydro-morphological and habitat conditions of river systems.

Lateral sediment transport and late Quaternary changes of eolian sedimentation in the East Sea (Japan Sea)

• GSDs reveal offshore limit of lateral sediment transport in the East Sea. • Lateral sediment transport from the Korean Peninsula is within ~70 km offshore. • Eolian sediments in the SKP were transported by upper westerlies and winter monsoons. • East Asian winter monsoon intensity controlled eolian sedimentation in the East Sea. We investigated grain size records of a sediment core (05-P21) from the South Korea Plateau in the East Sea, located ca. 270 km east of the east coast of the Korean Peninsula. To verify an exclusive eolian origin for the detrital components of the sediments, we first examined how far offshore lateral-sediment-transport contributions from the eastern margin of the Korean Peninsula could reach, based on spatial changes in the grain size distribution (GSD) of surface sediments. To distinguish grain populations representing different transport processes, we performed end-member analyses (EMAs) of the GSDs of detrital fractions using the AnalySize software. EMA of the surface sediment GSDs showed that they consist of three end-member (EM) grain populations representing contributions from lateral (EM1 and EM2) and eolian transport (EM3). EM contributions by lateral transport reached only ca. 70 km seaward, confirming the exclusive eolian origin of detrital components of the 05-P21 core sediments. EMA of GSDs of the 05-P21 sediments distinguished two EM grain populations with fine (3.6 µm) and coarse (9.5 µm) median sizes, suggesting transport by upper westerlies and winter monsoon northwesterly surface winds, respectively. Variation in the proportions of fine and coarse EMs since ca. 186 ka indicated controls by both orbital and sub-orbital scale changes in intensity of the East Asian winter monsoon in association with global ice volume changes and Dansgaard–Oeschger cycles.

Geochemical signatures of redepositional environments: The Namibian continental margin

Trace metal abundances in marine sediments have been used extensively to interpret periods of elevated primary productivity and ancient ocean redox conditions. However, sediment reworking that results in post-depositional oxidation, such as bio-irrigation or lateral sediment transport through oxic water, can modify the primary geochemical signal of the sediment, which in turn may impact paleo-redox and/or -productivity interpretations. In the case of sediments on the Namibian Continental Margin (NCM), lateral transport and redeposition contribute to the accumulation of organic matter on the margin slope. To better constrain the geochemical effects of lateral transport on the NCM, we examined the trace metal signature (including solid-phase Fe, Mo, V, Ni, Cu and Ag, and pore-water Fe, Mo, and V) in surface sediments (up to 25 cmbsf) along a transect from shelf to slope through the primary (shelf) and secondary (upper slope) depositional zones of the margin. Despite varying bottom water redox conditions ranging from seasonally anoxic (upper shelf), suboxic (shelf break), and oxic (upper slope), each site has elevated total organic carbon contents (average TOC content of 9.1, 3.2 and 6.8 wt%, respectively), due to high surface water primary productivity and lateral transport of organic-rich material from the shelf to the upper slope. Our results show that the solid-phase contents of the productivity proxies Ni, Cu, and Ag parallel the organic carbon accumulations largely irrespective of the local redox conditions. In contrast, the solid-phase contents of V, Mo, and Fe respond to the local bottom water redox conditions at each site, being enriched under strongly reducing conditions and less-enriched under oxic bottom waters despite enhanced TOC at all three investigated sites. Thus, the enrichment of trace metal and TOC at each site on the NCM can not only be used to reconstruct primary depositional bottom water redox conditions but also to identify zones of sediment redeposition. Using an offshore transect allows for the identification of intense lateral transport and redeposition of organic-rich sediments that are occurring along the margin. The relative concentrations of both redox-sensitive and productivity-related trace metals suggest that the decoupling of trace metals and organic carbon enrichments occasionally observed in the geological record could be explained by the process of lateral transport and redeposition.

Sediment exchange between channel and sand ridges in the southern Yellow Sea: The importance of tidal asymmetries

The radial sand ridges found off the coast of Jiangsu (China) are major morphological features in the southern Yellow Sea and undergo rapid changes in morphology. Despite extensive observational and modeling efforts, the physical processes controlling sediment transport and associated morphological evolution over the ridges remain not well understood. Here we analyzed data collected from systematic hydrodynamic and sedimentary surveys including records from moorings and bottom-mounted tripods deployed at channels and adjacent ridges. We explored the role of advection and local resuspension processes in controlling morphological changes and how they are modulated by tidal asymmetry. Bottom shear stresses due to mean tidal currents, waves, and wave-current interactions were estimated to examine the effects of tidal asymmetry, advection and resuspension processes on suspended sediment concentration (SSC) variability. The simplified depth-averaged model of Bass et al. (2002) was used to explain the phase relationships between fine sediment suspensions and tidal currents. We found that resuspension is the major controlling factor of bottom SSC during mean and spring tides, while advection is the dominant process during neap tides. The asymmetries in water level and tidal current lead to a net lateral sediment transport directed from the channel to the sand ridges, which cause erosion in channels and deposition on the sand ridges. This sediment transport pattern likely contributes to the evolution of the sand ridges under fair weather conditions (i.e., excluding storm events).

Late Miocene sediment delivery from the axial drainage system of the East Carpathian foreland basin to the Black Sea

Abstract We describe a late Miocene to early Pliocene axial drainage system in the East Carpathian foreland, which was an important sediment supplier to the Black Sea and the Dacian Basin. Its existence explains the striking progradation of the northwest Black Sea shelf prior to the onset of sediment supply from the continental-scale Danube River in the late Pliocene to Pleistocene. This axial drainage system evolved due to the diachronous along-strike evolution of the Carpathians and their foreland; continental collision, overfilling, slab breakoff, and subsequent exhumation of the foreland occurred earlier in the West Carpathians than in the East Carpathians. After overfilling of the western foreland, excess sediment was transferred along the basin axis, giving rise to a 300-km-wide by 800-km-long, southeast-prograding river-shelf-slope system with a sediment flux of ∼12 × 103 km3/m.y. Such late-stage axial sediment systems often develop in foreland basins, in particular, where orogenesis is diachronous along strike. Substantial lateral sediment transport thus needs to be taken into account, even though evidence of these axial systems is often eroded following slab breakoff and inversion of their foreland basins.

Enhanced Lateral Exchange of Carbon and Nitrogen in a Coastal Wetland With Invasive Spartina alterniflora

AbstractLateral movements of materials and energy in coastal wetlands, due mainly to tidal activities, have been recognized as key processes in understanding the biogeochemical cycles of ecosystems. However, our understanding of the roles of lateral movement in shaping ecosystem functions remains limited. Here we quantified the effects of lateral sediment transport on total carbon (C, inorganic + organic) and nitrogen (N) pools in plants and soils in two dominant wetland types: invasive Spartina alterniflora (Spartina) marshes and native Phragmites australis (Phragmites) marshes in coastal Shanghai of the Yangtze Estuary. We found that the accreted sediments across the water‐marsh gradients caused by lateral movement resulted in contrasting C and N contents between the two communities. The sediment load and C and N pools in the plants and soils of the Spartina marshes were significantly higher than those in the adjacent Phragmites marshes. The shifts in species composition and community structure not only altered the C and N balance but also enhanced the ecosystem net primary productivity. Our findings highlight the importance of lateral transport in altering ecosystem structure after Spartina invasion. The ecosystem C and N pools were significantly higher in the invaded ecosystems than in the native community. Our study also reveals that the plant density and structures can alter tidal hydrodynamics and the lateral transportations of sediments, which in turn influence ecosystem C and N cycle. The C accumulation processes of the native and invaded marshes were further complicated by the contrasting productivities of the ecosystems.

The spatial footprint of hydrothermal scavenging on 230ThXS-derived mass accumulation rates

Mid-ocean ridges are valuable archives of sedimentary flux records used to investigate atmospheric, oceanographic, and solid Earth responses to climate variability. Constant flux proxies, such as extraterrestrial helium-3 (3HeET) and excess thorium-230 (230ThXS), constrain vertical mass accumulation rates independent of the biases associated with lateral sediment transport and age model resolution. However, thorium scavenging by hydrothermal particles can perturb local 230ThXS deposition and complicate its application as a constant flux proxy in near-ridge environments. We characterize the footprint of hydrothermal scavenging on sedimentary 230ThXS using coupled 3HeET-230ThXS analyses in cores from the Mid-Atlantic Ridge and the Juan de Fuca Ridge. Samples deposited >10 km from the Juan de Fuca Ridge indicate reliable off-axis behavior of both constant flux proxies. In contrast, samples deposited <10 km from the Juan de Fuca Ridge axis and within the axial valley of the Mid-Atlantic Ridge suggest 50–80% deficits in sedimentary 230ThXS relative to its production rate. These deficits contrast with sedimentary 230ThXS surpluses recently observed on the East Pacific Rise. The spatial footprint of hydrothermal scavenging varies globally and temporally, likely as a function of the intensity of local hydrothermal activity. The combined ridge data suggest that near-vent sediments (typically within ∼5 km, but variable by ridge) receive relatively high 230ThXS deposition rates as a direct result of hydrothermal particle scavenging, while more distal sediments receive relatively low 230ThXS deposition rates due to diffusive loss of overlying seawater 230ThXS towards the vent. Aside from the East Pacific Rise, far-field sediments are likely to exhibit typical 230ThXS deposition rates at distances greater than ∼10 km of the ridge axis. However, 230ThXS systematics within the axial valleys of slow-spreading ridges may be complicated by other factors. Combined 3HeET-230ThXS studies at multiple ridges are needed to further characterize the nature of hydrothermal scavenging and to evaluate the potential of sedimentary 230ThXS anomalies to record large-scale variability in past hydrothermal activity.

Morphological evolution of a mud-capped dredge pit on the Louisiana shelf: Nonlinear infilling and continuing consolidation

Buried sandy paleo-river channels on the continental shelf can be accessed through dredging for the restoration of beaches and barrier islands which are under the threat of land loss and rising sea levels. However, our knowledge on post-dredging evolution of paleo-river channels is still very limited. In this study we focus on infilling rate, sediment acoustic characteristics, slope change and surficial morphology, all of which have implications for the management of seafloor oil and gas infrastructure. Bathymetric data collected during nine bathymetric surveys over 13 years were used to study the long-term changes of the Peveto Channel dredge pit, which is ~5 km offshore of Holly Beach in western Louisiana, USA. This pit was constructed in 2003 and was completely infilled in or before 2016. Initial pit infilling rates began at 214 cm/year from 2003 to 2004 and then rapidly decreased to an average rate of only 33 cm/year from 2006 to 2016. This corresponds volumetrically to 418,753 and 80,816 m3/year for the same intervals. Pit wall margins displayed small lateral migration, moving <30 m outwards over 13 years, posing minimal threat to adjacent infrastructure. After complete infilling, numerous small mounds were found on 2016 bathymetry, morphologically resembling ‘mud volcanoes’ that have been reported on deltas, continental shelves and slopes around the world. Newly deposited sediment on seafloor surface had a reflectivity lower than the ambient seafloor environment. Massive amounts of sediments were transported to the pit despite no proximity to direct riverine inputs, making the pit act as an efficient sediment trap. Inferred depth-integrated lateral sediment transport rates were 9.91–12.73 g/m/s. Despite the fact that Peveto Channel pit was filled up, consolidation continues and the filled sediment surface hasn't reach a condition in equilibrium with ambient seafloor yet. Our results highlight the continuing dewatering and long consolidation processes at the pit site even after a complete infilling.

Holocene paleoceanography of Bigo Bay, west Antarctic Peninsula: Connections between surface water productivity and nutrient utilization and its implication for surface-deep water mass exchange

Paleoceanographic changes in response to Holocene climate variability in Bigo Bay, west Antarctic Peninsula (WAP) were reconstructed through geochemical, isotopic, sedimentological, and microfossil analysis. Core WAP13-GC47 is composed of 4 lithologic units. Unit 4 was deposited under ice shelf settings. Unit 3 represents the mid-Holocene open marine conditions. Unit 2 indicates lateral sediment transport by a glacier advance during the Neoglacial period. The chronological contrast between the timing of open marine conditions at core WAP13-GC47 (ca. 7060 cal. yr BP at 540 cm) and the ages of calcareous shell fragments (ca. 8500 cal. yr BP) in Unit 2b suggests sediment reworking during the Neoglacial period. Unit 1 was deposited during the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Surface water productivity, represented by biogenic opal and total organic carbon (TOC) concentrations, increased and bulk δ15N (nitrate utilization) decreased during the warmer early to middle Holocene and the MWP. In contrast, surface water productivity decreased with increased bulk δ15N during the colder Neoglacial period and LIA in Bigo Bay. The nitrate utilization was enhanced during cold periods in association with strong surface water stratification resulting from increased sea ice meltwater discharge or proximity to an ice shelf calving front in Bigo Bay. Reduced nitrate utilization during warm periods is related to weak stratification induced by less sea ice meltwater input and stronger Circumpolar Deep Water influence.

Millennial-scale variations in dustiness recorded in Mid-Atlantic sediments from 0 to 70 ka

Sedimentary records of dust deposition in the subtropical Atlantic provide important constraints on millennial- and orbital-scale variability in atmospheric circulation and North African aridity. Constant flux proxies, such as extraterrestrial helium-3, yield dust flux records that are independent of the biases caused by lateral sediment transport and limited resolution that may be associated with age-model-derived mass accumulation rates. However, Atlantic dust records constrained using constant flux proxies are sparsely distributed and generally limited to the past 20 ka. Here we extend the Atlantic record of North African dust deposition to 70 ka using extraterrestrial helium-3 and measurements of titanium, thorium, and terrigenous helium-4 in two sediment cores collected at 26°N and 29°N on the Mid-Atlantic Ridge and compare results to model estimates for dust deposition in the subtropical North Atlantic. Dust proxy fluxes between 26°N and 29°N are well correlated, despite variability in lateral sediment transport, and underscore the utility of extraterrestrial helium-3 for constraining millennial-scale variability in dust deposition. Similarities between Mid-Atlantic dust flux trends and those observed along the Northwest African margin corroborate previous interpretations of dust flux variability over the past 20 ka and suggest that long distance transport and depositional processes do not overly obscure the signal of North African dust emissions. The 70 ka Mid-Atlantic record reveals a slight increase in North African dustiness from Marine Isotope Stage 4 through the Last Glacial Maximum and a dramatic decrease in dustiness associated with the African Humid Period. On the millennial-scale, the new records exhibit brief dust maxima coincident with North Atlantic cold periods such as the Younger Dryas, and multiple Heinrich Stadials. The correlation between Mid-Atlantic dust fluxes and previous constraints on North African aridity is high. However, precipitation exerts less control on dust flux variability prior to the African Humid Period, when wind variability governs dust emissions from consistently dry dust source regions. Thus, the Mid-Atlantic dust record supports the hypothesis that both aridity and wind strength drive dust flux variability across changing climatic conditions.