Sediment Sink Research Articles

Evolution of a tide-dominated abandoned channel: A case of the abandoned Qingshuigou course, Yellow River

Deltaic river systems including both active channels and abandoned channels, are of significant importance to understand the deltaic morphology, as well as to the coastal management and ecosystem maintenance. However, the healing process of the abandoned channel has not yet been well understood. Based on the sedimentary record and hydrographic surveys, the deposit architecture and sediment dynamics of a tide-dominated abandoned channel, the abandoned Qingshuigou channel of the Yellow River, are investigated. The abandoned Qingshuigou channel has been fully disconnected from the main river channel since an artificial diversion completed in 1996. Twenty years after the abandonment, sedimentation with an average thickness of >20 cm was identified in the abandoned channel, illustrating a prominent seaward thickening trend from middle to lower channel. At the present river mouth of the abandoned channel, sedimentation layer with a thickness of 38 cm well matched the sedimentation rates derived from both chronological framework (1.85 cm/yr derived from 210Pb) and sedimentary facies (1.90 cm/yr). These deposits are composed by fine-grained sediment and enriched with organic matters, differing largely from the river-dominated deposits before abandonment. Tidal delivery of the sediment resuspended in the shallow water nearby Qingshuigou river mouth dominated the refilling process of the abandoned channel. Meanwhile, severe erosion near the abandoned river mouth shortened the channel (23.5 km in 1996 versus 14.5 km in 2018) and induced a lateral migration of the tidal inlet. Due to seasonally varying hydrodynamics, the tide-dominated abandoned Qingshuigou channel converts from a sediment sink in summer seasons to both a source and a sink in winter seasons.

Inferring sediment transfers and functional connectivity of rivers from repeat topographic surveys

AbstractHigh‐resolution topographic models have revolutionized monitoring of river changes by comparing sequential river topographic surveys (i.e. change detection). Nevertheless, much more may be obtained from this innovative quantification of changes. In this paper, we enhance the interpretation of geomorphic processes by presenting a new method for understanding of sources and sinks of sediment, river sediment transfers and functional sediment connectivity. Repeat digital elevation models (DEMs) obtained by photogrammetry were used to quantify topographic change after two floods by creating a DEM of difference (DoD) of a 6.5 km‐long reach of Rambla de la Viuda stream, an ephemeral gravel‐bed river in eastern Spain. The proposed method involved dividing the channel into 10 m‐long longitudinal strips that were used to systematically draw boundaries between the erosional and depositional areas of the DoD. The analysis objectively: (i) drew a series of erosional and depositional segments, from 120 to 1360 m in length; (ii) estimated ranges of source‐to‐storage sediment transport distances, 320–670 m in the upstream and middle reaches and up to 2030 m in the lower reach; and (iii) obtained values of functional connectivity (i.e. the ratio between the sediment exported (erosion) and retained (deposition), ranging from 103 to 10−3). The variability in these three parameters along the river was found to be related to the level of channel disturbance by in‐stream mining during the 1990s and 2000s. Additionally, this method indicates that the main process responsible for self‐adjustment of the present morphosedimentary conditions is intra‐reach erosion of banks and channel beds. Thus, this study proposes a new methodology to characterize morphological change, sediment transfer and connectivity that may serve as environmental indicators of the hydromorphological integrity of rivers with potential application to the European Water Framework Directive. © 2019 John Wiley & Sons, Ltd.

Isotope Constraints on the Sources of Particulate Organic Carbon in a Subtropical Deep Reservoir

AbstractReservoirs are active sites in the terrestrial environment in terms of carbon transformation and storage. Continuous reservoir construction reinforces the importance of such artificial storage; however, knowledge pertaining to the sources, cycling, and preservation of autochthonous/allochthonous particulate organic carbon (POC) in reservoirs remains incomplete. We monitored the flux and carbon isotope composition of sinking particles in a subtropical deep reservoir, and the results were compared with the isotopic signatures of potential POC sources (soil) and the sedimentary sink. We found that the typhoon‐induced POC flux was transported laterally at intermediate water depths within the reservoir and that it contributed more than 70% of the annual deposition. An isotope ternary mixing model showed that over 87% of the POC preserved in sediments was allochthonous. Although the autochthonous POC from primary production was preferentially degraded, the preserved autochthonous POC (42 ± 14 gC m−2 yr−1) was comparable to the observed carbon dioxide uptake (35 gC m−2 yr−1) by the reservoir, suggesting a high efficiency of carbon preservation in subtropical deep reservoirs.

Changes in sediment connectivity following glacial debuttressing in an Alpine valley system

Increasing air temperature and declining winter snowfalls are resulting in rapid glacier recession and the expansion of proglacial margins in Alpine regions. Such margins include substantial debris accumulations (e.g. frontal/lateral moraine ridges; till-covered and steep valley sidewalls) which may be unstable due to glacial debuttressing. Rainfall, snowmelt and ice melt out may then cause mass movements. Here, we quantify the decadal-scale erosion and deposition patterns and changes in connectivity for two valley sidewall geomorphological systems following retreat of the Glacier d'Otemma, Switzerland. We apply archival digital photogrammetric methods to the period 1964 to 2009 to determine high resolution digital elevation models. These were differenced to calculate patterns of erosion and deposition and to quantify the evolution of sediment connectivity. We found that gully headward erosion (rates between ca. −10.6 mm/year and −1002.1 mm/year) was the main geomorphological process during glacier thinning but increasing depositional rates downslope of the gullies (ca. +21.3 to +298.5 mm/year) were recorded in the following years associated with significant alluvial fan growth at the slope base. While gullying enhanced connectivity by removing glacially conditioned sediment transfer buffers, so connecting side-slopes to upstream sediment sinks (the upslope contributing area between 1964 and 2009 increased by +73.8% and +195.1% in each subsystem), alluvial fans reduced the rates of sediment transfer to the rapidly enlarging glacial forefields. The detail of these responses is conditioned by three generic processes: (1) the wider geomorphic setting – here, the presence of a moraine bastion as a primary part of the sediment cascade strongly influenced gully morphology evolution and the likely length of the paraglacial period length; (2) the thickness of sediment left by the retreating glacier which controlled the influence of bedrock topographic buffers on connectivity; and (3) the extent to which diffusive drainage systems develop in response to the deposition at the hillslope base, which tend to disconnect sediment flux. Post-glacially, gully development has a self-limiting effect on sediment connectivity in that while gullying increases sediment connectivity, the eroded sediment leads to deposition on the alluvial plain that reduces sediment connectivity.

Spatial and seasonal variability in grain size, magnetic susceptibility, and organic elemental geochemistry of channel-bed sediments from the Mekong Delta, Vietnam: Implications for hydro-sedimentary dynamic processes

Understanding the relationship between depositional processes and their products in the fluvial–marine transition zone in tide-dominated depositional systems is fundamental to improving environmental and stratigraphic interpretations of long-term sedimentary records. The distributary channels of the Mekong Delta represent a typical example of a complex sedimentary environment characterized by strong fluvial and tidal interactions, and seasonal variability in hydro-sedimentary dynamic processes, particularly the migration of a salt wedge front linked to seasonal fluctuations in freshwater discharge. For this study, we analyzed grain size, magnetic susceptibility, and organic elemental geochemistry of channel-bed surficial sediments collected in 2015 from all distributary channels during the dry season and from the Mekong–Co Chien River during the flood season. This paper reports spatial and seasonal differences in the characteristics of channel-bed surficial sediments, which are closely linked to sediment sources and depositional processes during the dry and flood seasons.Distributary channels are characterized by coarse-grained sediments in the upper reaches and fine-grained sediments in the lower reaches, in both dry and flood seasons, reflecting the difference between the upstream fluvial-dominated environment and the downstream tide-dominated environment. Flood season samples from the Co Chien River show higher magnetic mineral contents and a terrestrial source of organic carbon compared with dry season samples, indicating sourcing from the drainage basin and trapping of suspended materials in the distributary channel due to the barrier effect of the saltwater wedge at the river mouth. Compared with the flood season, during the dry season the mud content was lower in the downstream reach close to the river mouth and higher in the upstream reach of the Co Chien River, and the content of magnetic minerals was lower in both reaches, indicating dissolution of magnetic minerals during early diagenesis. Furthermore, the lower C/N ratios (<10) and slightly enriched δ13C values of the dry season samples from the Co Chien River imply an additional contribution of organic carbon from a marine source. These various features imply a landward import of mud into the Co Chien River induced by stronger salinity intrusion during the dry season. Along the downstream tract of the Bassac River, two mud-dominated reaches were identified during the dry season. The magnetic mineral contents, C/N ratios and δ13C values indicate that the mud-dominated reach near the Bassac River mouth is also supplied from resuspended sediments by estuarine circulation and tidal pumping, whereas the mud in the reach that is transitional from fluvial to tide dominance has a riverine supply. The results suggest that the Mekong distributary channels are important sinks for suspended sediments during both the flood and dry seasons, controlled by a sediment trapping mechanism provided by either estuarine processes or the morphological configuration of the channels, both of which are controlled by fluvial–tidal dynamics. These depositional processes may be compensating for the incision of channels induced by sand mining in recent decades.

Phosphorus burial in ferruginous SiO2-rich Mesoproterozoic sediments

Abstract Persistently low atmospheric oxygen requires that net organic carbon burial was muted through much of Earth’s middle age. In order to achieve global mass balance with respect to O2, recent models have suggested that redox-dependent mechanisms, such as Fe(II)-phosphate precipitation, limited phosphate availability in dominantly anoxic and ferruginous oceans, in turn limiting net primary production, and therefore organic carbon burial. Nevertheless, observational constraints on phosphorus cycling in ferruginous Proterozoic systems are rare, leaving these models largely untested. Here, we present high-resolution petrographic and mineralogical data showing that the 1.3 Ga Sherwin Ironstone (Roper Group, Australia) was dominated by syndepositional precipitation of the Fe(II)-silicate minerals greenalite and berthierine, interlaminated with abundant authigenic calcium fluorapatite (CFA). Set in a quantitative geochemical framework, these data reveal that elevated marine SiO2(aq) concentrations facilitated extensive Fe(II)-silicate production, leaving CFA, rather than Fe(II)-phosphate, as the principal inorganic phosphorous sink in shallow-water Roper Group sediments. More broadly, the physical and chemical factors that triggered Fe(II)-silicate and CFA burial in the Roper Seaway highlight semi-restricted basins as important loci of phosphorus removal from the mid-Proterozoic ocean.

Sedimentation Patterns in Sorsogon Bay, Philippines Using 210Pb

210Pb has been used as a tracer to give an insight into the sedimentation process occurring in Sorsogon Bay. The sedimentation process can provide the possible sources, movement, pathways, and sinks of sediments and sediment-associated materials (e.g., chemicals, pollutants) – which are important for a better understanding of the changes happening in the bay and its watershed. The information that will be derived can be used as an input parameter by other researchers doing modeling work (material/water/pollutant flow, movement, dispersion, and residence time dynamics) at Sorsogon Bay. Seven sediment cores have been collected in Sorsogon Bay to determine the 210Pb-derived sedimentation rate estimates across the bay. A sedimentation rate of 1 cm/yr could be estimated for the eastern, central, and western areas of Sorsogon Bay. Areas near Sorsogon City (SO-07), Cadacan River (SO-03), and the area offshore of Buenavista and Rizal (SO-06) have enhanced sediment deposition, which could be due to an area where enhanced erosion from human activities is apparent, proximity to a river system that drains/carries volcanic material and debris from Mount Bulusan, and near an open dumpsite where possible materials (wastes, debris, leachates) could be carried offshore respectively. The sedimentation rates are shown to be higher in the shallower areas of the bay.

Methane emissions from aqueous sediments are influenced by complex interactions among microbes and environmental factors: A modeling study

Methane fluxes from aqueous sediments strongly influence global atmospheric methane. However, many questions still puzzle researchers; for example, why are some unstable sediments atmospheric methane sinks? In this study, a biofilm model originally developed for wastewater treatment was modified to simulate the microbial kinetics and substance conversions in aqueous surface sediments. The model was validated by the experimental data and could predict chemical profiles and microbial distributions in sediments. The model revealed complicated interactions between different microbial communities and environmental factors, including competition between aerobic methane-oxidizing bacteria, nitrite-dependent anaerobic methane-oxidizing bacteria, and anaerobic ammonia-oxidizing bacteria. The results of model simulations showed that the effects of environmental factors, especially dissolved oxygen and ammonia in overlying water, on methane fluxes are very complicated. Rapid environmental changes (which can be caused by tide, day-night alternation, or zoobenthic and human activity) and intensive competition between microbes greatly affected methane fluxes and resulted in alternation between atmospheric methane source and sink in unstable sediments. This study extends the application of a wastewater treatment model to ecological studies of microbial interactions in natural sediments and explains some problems that might be difficult to resolve by using experimental methods.

Using multiple composite fingerprints to quantify source contributions and uncertainties in an arid region

Developing targeted protection measures at a watershed scale requires spatially distributed information of sediment sources. Therefore, the objectives of this study are to (1) test and evaluate the ability of multiple composite fingerprints (MCF) to quantify sediment provenance using multiple particle size classes in an arid region; (2) quantify uncertainty of the estimated proportional contributions of sediment sources; and (3) provide decision support information for sediment control in the Danghe Reservoir Watershed. In total, 66 samples were collected from north alluvial fan, south alluvial fan, and high mountains, and all samples were divided into six particle size groups. A multistep test was used to remove the tracers that were non-conservative, unable to differentiate sources, or highly variable within a source. Based on geochemical properties of distributed source samples and a linear mixing model, a MCF method with multiple particle size tracking was used to estimate proportions of three potential source contributions. More importantly, the uncertainty of sediment source contributions was quantified using the Gaussian first-order approximation. The results showed that the MCF method with multiple particle size tracking could obtain relatively accurate estimates of the contributions with an overall mean absolute relative error of 3.5% and a relatively narrow 95% confidence interval. The major contributions were consistently coming from the high mountains for all six particle groups. During these runoff events, the overall estimated mean proportions were 49.0%, 26.5%, and 24.5% from the high mountains, south alluvial fan, and north alluvial fan, respectively. Furthermore, the Gaussian first-order approximation revealed that more than 60% of the total uncertainty contribution was a byproduct of the downstream sediment mixture, while each individual sediment source produced less than 15% of the absolute uncertainty. Acquiring watershed scale sediment source information is challenging and the MCF method proved accurate. A majority of the contribution uncertainties were associated with the downstream sediment mixture, which is because the sediment sink inherited both spatial and temporal variations of all contributing sources. Consequently, a larger sample size is recommended for sediment mixtures, compared to each sediment source, in order to increase the accuracy of the source proportion estimation.

Assessing the relative impacts of land‐use change and river regulation on Burdekin River (Australia) floodplain wetlands

Abstract The Burdekin River floodplain wetlands are internationally important and act as a sink for sediments and nutrients that would otherwise enter the World Heritage‐listed Great Barrier Reef lagoon. The Burdekin River has the highest natural discharge of any Great Barrier Reef catchment and contributes the greatest mass of sediment to the reef. The river and its catchment have been substantially modified by land clearance, river regulation introduced in the 1960s, the construction of the Burdekin Falls Dam in 1987 and invasive aquatic plants. We hypothesised that the natural variability of Burdekin River discharge would render its wetland ecosystems resistant to human‐induced pressures. Diatoms were analysed from the sediments of two lower Burdekin River floodplain wetlands with contrasting regulation histories. Labatt Lagoon has a long history of flow alteration, whereas Swan's Lagoon has experienced limited regulation. Both wetlands experienced dramatic increases in sedimentation as a result of land clearance, yet the Swan's Lagoon diatom record indicates that the wetland's ecology remains within the range of natural variability. In contrast, Labatt Lagoon is markedly different from its pre‐settlement state. It is now permanent, having been ephemeral for more than 1000 years before European settlement. The conversion to artificial permanence facilitated aquatic plant invasion, resulting in more marked changes than widespread pastoralism. Modern diatom assemblages from 28 lower Burdekin River floodplain wetlands indicate that the contemporary flora of Swan's Lagoon is unusual. Most wetlands have similar assemblages to those in the Labatt Lagoon record in its present, altered state. It is possible, therefore, that the ecological shift registered in Labatt Lagoon is widespread. Diatom records highlight the striking impact that river regulation has had on the trajectory of the Burdekin floodplain wetlands. It is likely that wetland rehabilitation can be facilitated by the re‐introduction of a variable hydrological regime.

Recent morphological changes of the mouth bar in the Modaomen Estuary of the Pearl River Delta: Causes and environmental implications

The Pearl River Delta (PRD) is one of the most economically important and urbanized deltas in Asia. The Modaomen Estuary, being the most important passage of water and sediment through the PRD into the sea, was regulated by estuarine reclamation since the mid-1980s. After the completion of regulation projects in the 1990s, it was transformed from a shallow bay into a series of regulated channels. As a result, the evolution of the mouth bar of the Modaomen Estuary, a key morphological unit determining delta formation, has been altered. In this study, we examine morphological responses of the mouth bar of the Modaomen Estuary to human activities and natural factors (i.e., riverine flood) from 1994 to 2010. As an effect of the estuarine extension towards the sea caused by the regulation projects, the mouth bar has gradually moved towards the sea. From 1994 to 2010, for instance, the west and east branches extended more than 2.1 km further seawards. In addition, flood scouring played an important role in the bar extension and the formation of secondary branches at the mouth bar. Furthermore, the front of the mouth bar was eroded by wave dynamics because of reductions in the sediment load from the Pearl River. In general, the mouth bar deflected southwestward after 1994, indicating an evolution of the sediment sink at the Modaomen estuary in the same direction. Our studies contribute to better understanding of the evolution of large estuaries and provide scientific guidelines for the sustainable management of the PRD.

Fate of cobalt and nickel in mackinawite during diagenetic pyrite formation

AbstractAs iron sulfide mineral phases are important sedimentary sinks for naturally occurring or contaminant metals, it is important to know the fate of metals during the diagenetic transformation of primary sulfide minerals into more stable phases, such as pyrite (FeS2). Furthermore, the trace metal content of pyrite has been proposed as a marine paleoredox proxy. Given the diverse low-temperature diagenetic formation pathways for pyrite, this use of pyrite requires validation. We, therefore, studied nickel (Ni) and cobalt (Co) incorporation into freshly precipitated mackinawite (FeSm), and after experimental diagenesis to pyrite (FeS2) using S0 as an oxidant at 65 °C. Metal incorporation was quantified on bulk digests using ICP-OES or ICP-AES. Bulk mineralogy was characterized with micro-X-ray diffraction (micro-XRD), documenting the transformation of mackinawite to pyrite. Epoxy grain mounts were made anoxically of mackinawite and pyrite grains. We used synchrotron-based micro-X-ray fluorescence (μXRF) to map the distribution of Co and Ni, as well as to collect multiple energy maps throughout the sulfur (S) K-edge. Iron (Fe) and S K-edge micro-X-ray absorption near edge spectroscopy (μXANES) was used to identify the oxidation state and mineralogy within the experimentally synthesized and diagenetically transformed minerals, and map end-member solid phases within the grain mounts using the multiple energy maps. Metal-free FeSm transformed to pyrite, with residual FeSm detectable. Co- and Ni-containing FeSm also transformed to pyrite, but with multiple techniques detecting FeSm as well as S0, implying less complete transformation to pyrite as compared to metal-free FeSm. These results indicate that Co and Ni may inhibit transformation for FeSm to pyrite, or slow it down. Cobalt concentrations in the solid diminished by 30% during pyrite transformation, indicating that pyrite Co may be a conservative tracer of seawater or porewater Co concentrations. Nickel concentrations increased several-fold after pyrite formation, suggesting that pyrite may have scavenged Ni from the dissolution of primary FeSm grains. Nickel in pyrites thus may not be a reliable proxy for seawater or porewater metal concentrations.

Rapid response of the Changjiang (Yangtze) River and East China Sea source-to-sink conveying system to human induced catchment perturbations

In order to investigate human impact in the Changjiang – East China Sea source to sink conveying system, we analyze the siltation and erosion state of the estuary-shelf deposition systems, and discuss the response intensity and timing of the depositional system of the Changjiang subaqueous delta and the related Changjiang distal mud. Results show that after the Three Gorges Dam was put into operation in 2003, the abrupt reduction of the sediment discharge to the estuary disturbed the equilibrium between the estuary-coast-shelf deposition systems, modifying the siltation-erosion balance and seabed sediment types. From the Changjiang subaqueous delta to its distal mud areas, the degree of response to catchment changes diminishes, with a larger response lag from north to south of the distal muds. Influenced by dam emplacements, the change of the Changjiang – East China Sea source to sink conveying system can be divided into three stages, i.e., pre-1950, 1950–2002 and post-2003. Before 1950, the source to sink system remained relatively stable. Between 1950 and 2002, man-made fluvial impoundments created an important sink for terrigenous sediment of the upstream regions, but without profound system changes. In contrast, after 2003, the morphodynamic patterns for the entire mid-lower reaches of the Changjiang River, the subaqueous delta and the distal muds shifted rapidly from siltation to erosion within a short period of time. Furthermore, the upper reaches of the river became disconnected from the source to sink conveying system, and the mid-lower reaches of the river, together with its subaqueous delta and northern distal mud area were converted into net sediment sources. As such, the sink area has shrunk while the source area expanded for the Changjiang – East China Sea source to sink conveying system.

The future of insular beaches: Insights from a past-to-future sediment budget approach

Sustainable management of insular beaches, that are critical to tourism-based economies, depends on sound understanding of coastal evolution drivers. However, interconnections among geological, oceanographical, biological and human key-drivers of coastal change, operating at relevant spatial and temporal scales, remains poorly understood.This work aims at understanding and quantifying the main drivers of insular beaches evolution using a past-to-future sediment budget approach, and to address future coastal impacts raised by anthropogenic interventions and climate change. This approach was developed in Porto Santo's beach from the early 20th century to the middle 21st century.Results show that anthropic activities undertaken during the late 20th century perturbed the existing long-term (natural) coastal stability. They caused significant reduction of the main sediment source (river sediment yield) and increased sediment sinks (e.g. sediment extraction from beach, harbour sedimentation). Altogether, this resulted in the onset of an erosive trend that persists until present. Projecting patterns of coastal change into the forthcoming decades strongly depends on sediment management strategies. We show that the adoption of a neutral strategy (i.e. compensating for anthropogenic-induced losses with beach nourishment) will not be enough to cease beach erosion, given the negative impacts related to acceleration of future sea level rise. Still, maintenance of the socioeconomic values of Porto Santo's beach can be achieved by triggering positive anthropic influences on its sediment budget. The past-to-future sediment budget approach proposed herein provided a unified perspective on the evolution of the main drivers of coastal change and simultaneously offers foundations for adaptations strategies aiming at increasing sustainability of insular beaches.

Morphological response of Lingding Bay in the Pearl River Estuary to human intervention in recent decades

The evolution of the Pearl River Estuary (PRE) in China has been intensely disturbed by diverse human activities in recent decades. In this study, the morphological changes of Lingding Bay in the PRE, consisting of three shoals (West, Middle, and East Shoals) and two channels (West and East Channels), in response to human activities since the 1970s were examined. From 1974 to 1985, the subaqueous delta was dominated by siltation, with siltation in the West and Middle Shoals but erosion in the East Shoal and West Channel. From 1985 to 1999, the subaqueous delta was eroded with increasing water volume of 347.9 × 106 m3, resulting from the combined influences of land reclamation and channel dredging. In addition, water area in the Lingding Bay reduced by 8.0% in 1999 compared with that in 1985. From 1999 to 2011, the water area continued to decrease and water volume still increased, indicating the subaqueous delta was narrowed and deepened. And the Middle Shoal was dominated by siltation, but other geomorphic units were eroded. Furthermore, the subaqueous delta in inner and outer Lingding Bay switched from siltation to erosion and from erosion to siltation during this period compared with the previous period of 1985–1999, respectively. Such phenomenon revealed that the sediment sink had evolved towards outer Lingding Bay (i.e., southern subaqueous delta) during this period. Our study has important implications for understanding the individual and combined effects of human activities on the evolution mode of PRE and provides scientific guidelines for the sustainable managements in the PRE.

Mesoscale Morphological Changes of Nearshore Sand Banks since the Early 19th Century, and Their Influence on Coastal Dynamics, Northern France

Tidal sand banks are common along the coast of northern France facing the North Sea, where they form linear shore-parallel or slightly oblique sand bodies from shallow coastal areas to depths of tens of meters. Hydrographic surveys have been carried out since the 1830s for mapping the seabed of the coastal zone. An analysis of the bathymetry evolution shows significant morphological changes have occurred across the shoreface since the early 19th century, largely due to cross-shore and longshore sand bank migration. Our results show that nearshore sand banks mainly migrated onshore and gained sediment, especially during the 20th century; acting as temporary sediment sinks, which can in turn serve as sand sources for providing sediment to the coast. Alongshore, the migration and elongation of sand banks can be related to tidal asymmetry that is mostly directed to the east-north-east in the region. Shore-perpendicular movement can likely be explained by the action of shore-normal storm-waves in the nearshore zone after their refraction over shallow offshore sand banks. A seaward displacement of sand banks was also observed. This may be related to the combined action of waves and tidal currents which can induce erosion on one side of the bank, decreasing its width, and eventually leading to its seaward migration. Our observations point out that some nearshore sand banks respond to the action of currents and waves, and interact between each other via feedback morphodynamic processes induced by sand bank morphological changes. The substantial morphologic changes that affected the nearshore zone of northern France during the last centuries probably had large impacts on coastal hydrodynamics and associated shoreline evolution.

Yangtze River-derived sediments in the southwestern South Yellow Sea: Provenance discrimination and seasonal transport mechanisms

The southwestern South Yellow Sea (SYS) is influenced by tremendous sediment loads derived from the Yellow River (Huanghe) and the Yangtze River (Changjiang). However, the influence of Yangtze River sediments and their transport mechanism in this area remain unresolved. In this study, we present the grain size, clay minerals, and elemental compositions of surface sediments from the southwestern SYS, together with an analysis of field observation seawater salinity and suspended sediment data, to investigate the provenance and transport mechanism of the sediments. Discrimination diagrams of clay minerals, Cr/Th vs. Sc/Al, and rare earth elements (REEs) indicate that the southwestern SYS sediments are mainly derived from the old Yellow River subaqueous delta and the Yangtze River. Interestingly, Yangtze sediments enter the SYS in ways that differ seasonally. In summer, Yangtze sediments may be transported into the central SYS by the dispersion of Changjiang Diluted Water (CDW) and may also be transported northwestward by the Subei Coastal Current (SCC) to the area off the Jiangsu coast. In winter, the Yangtze-derived sediments deposited off the Jiangsu coast in summer are transported into the central and/or even north SYS under the combined effects of the SCC, Yellow Sea Coastal Current (YSCC), and Yellow Sea Warm Current (YSWC). Throughout the transport processes, there is an alternation between a source and sink of Yangtze sediments off the Jiangsu coast; the coast serves as a sink of Yangtze sediments in summer but as a source of SYS sedimentation in winter. The depositional and transport mechanisms of Yangtze-derived sediments are closely associated with seasonal variations in the circulation system of the SYS.

The evolution and control of detrital sediment provenance in the middle and northern Okinawa Trough since the last deglaciation: Evidence from Sr and Nd isotopes

The Okinawa Trough (OT) is a large sink of sediments supplied by the East Asian continent. Identifying the provenance of the OT sediments is key to reconstructing the temporal and spatial variations of the terrigenous supply to this area and is important for understanding the impact of paleoclimatic and paleoceanographic variability on the sediment supply to this marginal sea over the late Quaternary. In this contribution, we show that radiogenic strontium (Sr) and neodymium (Nd) isotopes allow to efficiently distinguish Yellow and Yangtze/Taiwan River detrital sediments, and can be used to reconstruct distinct changes in the provenance of the detrital fraction of marine sediments from the middle and northern OT since the last deglaciation. The Sr and Nd isotope signatures are compared to those of the potential sediment sources, namely the Yellow and Yangtze Rivers, the Taiwan orogen, and volcanic material from the OT and nearby islands, and the relative contributions of these sources are reconstructed. The Sr and Nd isotope compositions of the detrital fraction in the two sediment cores recovered from the middle and northern OT show that the sediments mainly originated from the Yangtze River between 18 and 10.5 ka, which was caused by low sea level and a widely developed channel system on the continental shelf. During the period between 10.5 and 7.0 ka, the rising sea level resulted in elevated Yangtze and Yellow Rivers sediment input into the OT. Simultaneously, large-scale volcanic activity also contributed significant amounts of material to the OT. During the last 7.0 ka, besides important contributions from the Yellow River, the intensification of the Kuroshio Current resulted in increased delivery of sediment from Taiwan to the OT.

The role of hydrological conditions for riverine Hg species transport in the Idrija mining area

Estimation of mercury (Hg) species fluxes in Hg contaminated rivers is crucial to predict Hg methylation in connected sediment sinks. Cinnabar (HgS) was mined and roasted for ∼500 years in the Idrija mining area, Slovenia, which is drained by the Idrijca River to the Gulf of Trieste (GT), Italy. Mining residues dumped into the Idrijca River caused high proportions of cinnabar in sediments, whereas soils containing high proportions of natural organic matter bound to Hg (NOM-Hg) are attributed to atmospheric Hg deposition. Previous calculations of Hg fluxes have been based on the erosion of cinnabar only, and neglected transport of NOM-Hg derived from soil. Here, we estimated NOM-Hg and cinnabar fluxes in the Idrijca River and evaluated the extent of variability under changing hydrological conditions. We estimated the discharge of NOM-Hg by Idrijca's tributaries and the importance of NOM-Hg fluxes for Hg methylation in the GT. Mass balance calculations reveal that approximately 11.2 Mg y−1 of NOM-Hg and 38.9 Mg y−1 of cinnabar are transported by the Idrijca River to the GT under median-flow conditions. In the past 520 years, a total of 53,000 tons of Hg have been released from the Idrija mining area, of which ∼32,000 tons were NOM-Hg. Under low-flow conditions, Idrijca's tributaries deliver more than 1280 kg y−1 of NOM-Hg. This study highlights the importance of Hg species analyses and their flux calculations to estimate risks of biological Hg uptake in sedimentary Hg sinks connected to Hg mining areas.

Sorption temperature and the stability of iron-bound soil organic matter

Iron oxide surfaces can preserve soil organic matter (SOM) through the formation of mineral–organic associations (MOAs). Warming climates can affect SOM dynamics through altering the formation and stability of MOAs. We conducted batch sorption experiments from 15 to 35 °C using hematite and soil humic and fulvic acids to assess the stability of MOAs formed at each temperature using laboratory incubations and thermal analysis. Isotopic depletion was consistent with selective sorption of lignin- and lipid-derived carbon from humic acid, but fractionation was independent of temperature. Microbial incubations of the MOAs over 45 days showed that the mineralizable carbon (Co) was 0.639 ± 0.064% for humic acid and 1.46 ± 0.13% for fulvic acid, suggesting that the former may be more biologically stable than the latter. Thermogravimetric analyses (TGA) indicated that humic acid-MOAs were also more thermally stable (TGA-T50 ~285 °C) and less exergonic upon decomposition than fulvic acid-MOAs (~270 °C). Sorption temperature did not affect any of the biological (Co, k) or thermal (TGA-T50, ΔE) indices. These findings suggest that sorption temperature may not affect the formation or the stability of iron oxide MOAs. In warming climates, iron oxides might continue to act as a carbon sink in soils and sediments globally.