Mud Wedge Research Articles

Provenance discrimination of the clay sediment in the western Taiwan Strait and its implication for coastal current variability during the late-Holocene

This study aims to quantify the contribution of Yangtze clays to the sediment accumulation in the western Taiwan Strait and reconstruct the strength of Chinese Coastal Current (CCC) since middle-Holocene driven by East Asian Winter Monsoon (EAWM). Both down-core and surficial sediment samples were collected for grain size, radiocarbon, and clay mineral analyses. One 250-cm-long core was collected from the southern Yangtze distal mud wedge in western Taiwan Strait which receives Yangtze-derived clays transported by the Zhejiang-Fujian Coastal Current (ZFCC), the southern part of CCC. Clay minerals were examined in surficial sediment samples which were influenced by the Yangtze, Zhejiang-Fujian, western Taiwanese rivers, and the inner-shelf mud wedge. Ternary diagrams of smectite–kaolinite–chlorite revealed that three endmembers represented the Yangtze, Min, and western Taiwanese rivers, respectively. The estuaries seaward of the tidal current limits of Zhejiang-Fujian rivers, especially the Qiantang and Ou, were influenced by Yangtze-derived sediments through energetic tidal mixing. It was found that smectite can be used as a fingerprint of the Yangtze fine-grained sediment because among all the studied rivers, the Yangtze is the only one supplying smectite. Clay mineral results in core sediments revealed a dramatic provenance change at the depth of 113 cm, dated at ~4.0 cal. kyr BP. Smectite disappeared in the upper core, suggesting decreased contribution of Yangtze clays to the southern distal mud wedge. Decreased grain size of the fine population in the upper core also indicated that the ZFCC weakened during the late-Holocene. Such a decline also occurred in Subei Coast Current (northern part of CCC), revealed by the previous studies. The decline of CCC was related to the decreased EAWM of the late-Holocene, and it resulted in decreased sediment accumulation rate of the inner-shelf mud.

Seasonal transportation and deposition of the suspended sediments in the Bohai Sea and Yellow Sea and the related mechanisms

The Yellow River is well known for high concentration of suspended particulate matter (SPM), which is one of the most important SPM sources in the Bohai Sea and Yellow Sea, China. The mechanisms of long-distance transport and deposition of the Yellow River sediment discharges are hot topics. Based on field observations in four different seasons of 2010–2011 and 15 numerical experiments, this work studied the seasonal sediment transportation and deposition in the Bohai and Yellow Seas. Results show that the horizontal distribution of suspended sediment concentration in the Bohai Sea and Yellow Sea is neither closely related to the Yellow River discharge in flood and dry seasons nor to the temperature and salinity distributions in winter and summer because most of the Yellow River-discharged sediments deposited near the river mouth. However, the winter northerly wind events, especially the winter storm events with high waves, are the major factors inducing the long-distance transport of the sediments from the Bohai Sea to the Yellow Sea. The net SPMs are transported to the Yellow Sea from the Bohai Sea in both winter and summer. The net SPM flux at the Bohai Strait is 14.0 Mt/year by the combined effects of tides, wind, and waves. The SPM is transported to the Shandong Peninsula in the Yellow Sea for the generation of the mud wedge off the peninsula. The northern part of the mud wedge is related to the southerly wind in summer, and the southeastern part resulted from the winter northerly wind, especially the strong wind.

DDTs and HCHs in sediment cores from the coastal East China Sea

Four sediment cores were collected along the Yangtze-derived sediment transport pathway in the inner shelf of the East China Sea (ECS) for OCP analysis. The sediment records of HCHs and DDTs in estuarine environment reflected remobilization of chemicals from enhanced soil erosion associated to extreme flood events or large scale land use transformation. The sediment records in the open sea, instead, reflected long-term historical trends of OCP application in the source region. Unlike the so-called mud wedge distribution of sediment, inventories of HCHs and DDTs slightly increased from the mouth of Yangtze River alongshore toward south, suggesting the sediment deposition rate was one of factors on the exposure of chemicals within the inner shelf of the ECS. Re-suspension and transport of the Yangtze-derived sediment and consequent fractionation in grain size and TOC were also responsible for the spatial variation of inventories of catchment derived OCPs in a major repository area of the Yangtze suspended sediment. The total burdens of HCHs and DDTs in the inner shelf of the ECS were 35tons and 110tons, respectively. After 1983 (year of the official ban in China), those values were 13tons and 50tons, respectively. It appears that the Yangtze still delivers relatively high inputs of DDTs more than 30years after the official ban. High proportions of DDD+DDE and β-HCH suggested those OCPs mainly originated from historical usage in the catchment recent years.

Late Quaternary sedimentary environmental evolution offshore of the Hangzhou Bay, East China—implications for sea level change and formation of Changjiang alongshore current

This study focuses on sedimentary environmental changes offshore of Hangzhou Bay, East China, since the Late Quaternary. AMS C-14 ages from core CJK10, lithologies, distribution of foraminifera, heavy minerals, and S and Cl elements show a fluvial terrace environment during similar to 23.2-11.0 cal ka BP; a littoral to tidal-flat environment during 11.0-10.2 cal ka BP; and a shallow marine environment with a relatively low sedimentation rate (0.1-0.22 cm/a) since 4.3 cal ka BP. High depositional rates (similar to 1.6 cm/a) from 10.9 to 10.2 cal ka BP resulted from sufficient accommodation space created by rapid sea level rise from -44 m to -33 m, from high sediment delivery by local rivers, and effective trapping of sediments by tidal-flat vegetation. The rate of sea level rise was variable; relatively high from 10.9 to 10.6 cal ka BP (2.1 cm/a), and lower since 10.6 cal ka BP (1.2 cm/a). The Changjiang alongshore current crossed the Hangzhou Bay to form the mud wedge on the inner shelf of the East China Sea later than 9.4 cal ka BP. The CJK10 site was a tide-dominated shelf environment and experienced erosion from approximately 9.4-9.2 cal ka BP to 4.3 cal ka BP. The depositional hiatus was caused by the Changjiang alongshore current, which was relatively weak during 9.4-7.5 cal ka BP and increased in strength during similar to 7.5-4 cal ka BP. From similar to 4.3 cal ka BP, a large amount of sediment from the Changjiang River was partly deposited on the continental shelf of Hangzhou Bay with some transported southward. Therefore, this study clarifies the history of Changjiang-derived sediment dispersal and deposition, although a detailed record of the changes in the Changjiang alongshore current since 4.3 cal ka BP is difficult to obtain because of the scarcity of evidence.

Pleistocene shelf-margin delta: Intradeltaic deformation and sediment bypass, northern Gulf of Mexico

Recent studies of shelf-margin deltas in the Gulf of Mexico lack three-dimensional (3-D) seismic interpretation that allows prediction of lithology distribution, deformation features, and overall reservoir heterogeneity, and only a few 3-D studies of shelf-margin deltas in other parts of the world exist. This paper examines the 3-D geomorphology and internal facies architecture within a sequence-stratigraphic framework of a shelf-margin delta in a salt dome minibasin, within the Gulf of Mexico, offshore Louisiana, using 3-D seismic data. The 3-D data also allow an evaluation of the potential pathways for sediment bypass from delta front to slope and deep-water depositional environments. The delta consists of a series of offlapping sandy clinoforms, interpreted as being associated with a prolonged forced regression and ensuing lowstand of eustatic sea level, related to oxygen isotope stage 6, which lasted for about 45 k.y. The lowstand delta is onlapped by a transgressive mud wedge and is underlain and capped by regionally persistent highstand mudstones. The central part of the delta shows intradeltaic syndepositional growth faults, likely triggered by deposition of heavy delta-front sands over mobile prodeltaic muds, reflecting the river-dominated nature of the delta. A postdepositional shallow-water mass transport complex (MTC), which moved almost perpendicular to the direction of the regional delta progradation, remobilizes the deltaic deposits. The MTC was likely induced by uplift of the adjacent western salt dome. We hypothesize that the high degree of syndepositional and postdepositional deformation is related to the prolonged nature of the eustatic lowstand, as well as the salt tectonics. Tributive submarine slope channels form downdip of the delta front and may also reflect incision during the prolonged eustatic lowstand. The salt tectonics caused focusing of the dispersed delta-front sediments, mostly originating from terminal distributary channels, into the convergent tributive slope-channel system. Although this is an unstable supply-dominated shelf-margin delta, the salt tectonics likely prevented the delta from avulsing once it reached the shelf edge and also caused the focusing of dispersed sediment effectively into a point source for delivery to deep-water systems. This contrasts with less confined high-sediment supply delta systems farther along the coast, which lack linked submarine deposits because they reach the shelf margin before maximum eustatic lowstand and avulse before building significant deep-water systems.

Rare earth element geochemistry in the inner shelf of the East China Sea and its implication to sediment provenances

Gravity core EC2005, located in the mud wedge off the Zhejiang-Fujian coast in the inner shelf of the East China Sea (ECS), was analyzed for its lithology, grain size, rare earth elements (REE) and AMS 14C dating. Results revealed high-resolution paleoenvironmental evolution and multiple switches of sediment provenances in the inner shelf of ECS. The lithology, grain size and REE concentrations of sediments varied significantly down the core. Mean grain size ranged from 8 to 121 μm, and the values of ΣREE were 152.8–227.9 μg/g. The chondrite- and shale-normalized REE patterns indicated their similarity to the terrigenous sediments in the study area. Between 17.3–12.3 ka BP the sediments in the inner shelf of ECS were mainly originated from local drainage basins. The source then gradually switched to the Yangtze River between 12.3 and 9.8 ka BP. Since about 10–9.8 ka BP, the sediments in the study area were dominantly derived from the Yangtze River. The continuous rising of sea level and formation of coastal current along the inner shelf played a key role in the switches of sediment provenances.

Provenance, structure, and formation of the mud wedge along inner continental shelf of the East China Sea: A synthesis of the Yangtze dispersal system

Surficial grain-size and down-core clay mineralogical data show that sediment along the inner-most part of the continental shelf in East China Sea is mainly derived from the Yangtze River (Changjiang), spanning from the Yangtze mouth (33°N) ~1000km southward to the southwestern corner of the Taiwan Strait (24°N). High-resolution CHIRP seismic profiles reveal an elongated mud wedge extending along the inner shelf, with a northern depocenter on the modern Yangtze delta and a southern depocenter at 27.5°N. Four distinct acoustic units are delineated within the mud wedge (from bottom up): unit I (late-Pleistocene, mainly valley fills), unit II (formed by transgressions, thin strata), unit III (11–2kyr BP, downlapping strata) and unit IV (2–0kyr BP, flat and opaque strata). Incised valleys, up to 15-m deep, are filled by flat-lying or inclined strata in unit I. The thin (<3m) and acoustically transparent unit II is only seen between 30 and 26°N in water depths between 40 and 90m. Separated by acoustically opaque strata or unconformities, units III and IV are widely distributed. During the past 11kyr Yangtze sediment accumulation has been unsteady, showing two high and one low accumulation-rate periods. The high-accumulation period at 5–8kyr BP may be related to maximum East Asian summer monsoon precipitation in the Yangtze basin; the other high-accumulation period, 0–2kyr BP, probably reflects intensive human activities in the river's watershed. The low-accumulation-rate period at 2–5kyr BP, which is seen in both northern and southern Yangtze depocenters, is probably related to low river discharge and/or intensified Taiwan Warm Current and China Coastal Current.

Sediment-budget modelling of multi-sourced basin fills: application to recent deposits of the western Adriatic mud wedge (Italy)

We analysed modern mass-accumulation patterns on the western Adriatic mud wedge (Italy), an elongated belt of shelf mud formed by coalesced prodeltas of the Adige, Po, and Apennine rivers, as part of an integrated strategy aimed at producing a quantitative sediment budget model for muddy continental shelves sourced by multiple compositionally distinct fluvial systems. Sediment provenance and source-specific accumulation rates of surface sediments were quantified by combining results of grain-size analysis and geochemical analysis of specific size fractions with bulk mass accumulation rates. Statistical classification algorithms adapted to compositional data were used to partition the total (geochemical) variation of sediment properties into size-related and provenance-specific factors. We identified geochemically distinct fluvial end-member sediment types in two different grain-size fractions, which were grouped into sediments derived from the Apennine rivers, and sediments derived from the Po and Adige rivers. Compositional fingerprints (end-member compositions) of each source area were estimated by taking into account relative rates of fluvial sediment supply from rivers as predicted by numerical modelling. The end members allow us to explain geochemical compositional variation of mud-wedge surface sediments in terms of provenance and size-selective dispersal, and map mass accumulation rates of sediments from individual source areas (grain size<63 μm), as well as bulk sand accumulation rates (grain size>63 μm) across the western Adriatic mud wedge. The source-specific rates of fine-grained sediment supply derived from geostatistical estimates of mass-accumulation rates were used to calibrate the numerical model of sediment supply to present-day conditions.

Environmental evolution and impact of the Yellow River sediments on deposition in the Bohai Sea during the last deglaciation

Samples of one core in the Bohai Sea mud area (BSMA) were analyzed in grain size, magnetic susceptibility (MS), major minerals, foraminifera, ostracod, stable isotopes, and 14C dating to reconstruct sedimentary environment and impact of the Yellow River sediments on deposition in the Bohai Sea during the last deglaciation. The central Bohai Sea was river mouth or swamp environment before 12.9 ka BP, when sediments with low MS values had high sand content. Appearance of two peat layers at 11.2 and 12.9 ka BP at Core M7-6 showed that seawater had not yet entered the central Bohai Sea, where the limnetic ostracod was frequently existed and foraminifera were hardly seen. Abundance and diversity of foraminifera began to increase sharply, while their oxygen and carbon isotopes became heavier from ∼9.3 ka BP due to seawater entrance into the central Bohai Sea. Related to intrusion of the Yellow Sea Warm Current (YSWC) extension into the central Bohai Sea, oxygen and carbon isotopes reached the maximum at ∼6.5 ka BP. MS values between 2.4 and 8.3 ka BP increased continually together with slow sedimentation rate, revealing that the impact of the Yellow River sediments was limited because of strong hydrodynamic condition and small sediment discharge. At the last 2.4 ka, the influence of the Yellow River reached the maximum due to large sediment discharge into the Bohai Sea. However, the BSMA had slow sedimentation rate compared with other mud areas in the Chinese marginal seas despite of its close distance from the Yellow River mouth, indicating that although the BSMA was the primary sink of the Yellow River sediments, most of the Yellow River sediments were deposited at other areas (e.g. deltaic areas and Shandong mud wedge).

Cold event at 5 500 a BP recorded in mud sediments on the inner shelf of the East China Sea

A 700-year record (1.0–1.5 a resolution) of the East Asian winter monsoon (EAWM), based on grain-size analysis and AMS14C dating of Core EC2005 from the inner-shelf mud wedge of the East China Sea (ECS), was compared with the Dongge stalagmite δ18O record during the mid-Holocene. The upper muddy section of Core EC2005 has been formed mainly by suspended sediments derived from the Changjiang (Yangtze) River mouth since 7.3 ka BP. High precipitation and a strengthened EAWM might have played key roles in the high sedimentation rate (1 324–1 986 cm/ka) between 5.9–5.2 ka BP. The EAWM strengthened when the Asian summer monsoon weakened, especially around 5 500 a BP, which corresponded to a worldwide cold event. The EAWM during the mid-Holocene shows statistically significant solar periodicities at 62 and 11 a. The 5 500 a BP cold event might be resulted from orbital forcing and changes in solar activity.

Holocene depocenter shift in the middle-lower Changjiang River basins and coastal area in response to sea level change

Authors collected 38 sedimentary boreholes and numerous seismic profiles from previous publications to delineate the Holocene sedimentation rate of six major depositional sinks in the middle-lower Changjiang River basins and its river coast. The results demonstrate that the highest sedimentation rate of ca. 15 m/ka occurred in the mono-depositional sink of the former Changjiang River mouth during 10 000–8000 aBP, when post-glacial transgression happened and the Changjiang water level remained at lower stand. With the rising of the Changjiang water level in response to sea level rise, Jianghan Basin of the middle Changjiang River becomes the other important depositional sink with highest sedimentation rate of 10 m/ka since 7000 aBP. As Jianghan Basin was mostly filled up at ca. 4000 aBP, Dongting Basin and the lower Changjiang valley trapped sediments in great amounts like in the river mouth. A considerable amount of Changjiang sediments has been delivered, both eastward and southward, to the inner continental shelf of the East China Sea, especially after 2000 aBP. This indicates reduced sediment storage capacity of the middle-lower Changjiang valley and the river mouth. In total, ca. 1307.4 billion tons of sediment have deposited in the middle-lower Changjiang floodplain since 7000 aBP. In the meantime, ca. 947 billion tons of sediment have been deposited in the river coast to form the Changjiang subaqueous delta and the Zhejiang-Fujian along-shelf mud wedge. Our result also reveals two time stages with lower sedimentation rates(< 4 m/ka) in all basins during 8000–7000 aBP and in the estuarine area during 4000–2000 aBP, probably owing to stengthened chemical weathering of decline of monsoon precipitation.

Clay minerals and geochemistry of the bottom sediments in the northwestern East China Sea

Clay minerals of 34 sediments collected from the northwestern continental shelf of the East China Sea have been determined by X-ray diffraction analysis. The clay mineral distribution is mainly controlled by the sediment source and the dominant circulation pattern. The predominant clay mineral in our study area is illite comprising more than 67% of the whole clay fraction. The highest concentration of illite (>68%) is found in the southeastern offshore parts beyond the reach of terrigenous input from the Jeju Island. It means that these illites are largely transported by the Kuroshio Current from the South China Sea (SCS). Smectite is highly concentrated in the northwest middle part and in the outer-shelf mud patch. It seems to be due to the high supply of smectite transported from China where fine-grained sediments are discharged from modern and ancient Huanghe (Yellow) River. The relatively high abundant kaolinite is likely derived from the Changjiang (Yangtze) River via the Taiwan Warm Current. In contrast, large amounts of chlorite and high chlorite/kaolinite ratios occur in the northwestern area, reflecting the transportation by the Yellow Sea Coastal Current from the southern Yellow Sea. The discrimination diagrams clearly show that the sediments in the northwestern East China Sea are ultimately sourced from Chinese rivers, especially from the Huanghe River, whereas the sediment in the northeast part might come from the Jeju Island. The muddy sediments of the Changjiang River’s submerged delta have much lower 87Sr/86Sr ratios (0.716 2–0.718 0) than those of the Shandong Peninsular mud wedge (0.721 6–0.724 9), which are supposed to be originated from the Huanghe River, suggesting the distribution pattern of 87Sr/86Sr ratios as a new tracer to discriminate the provenance of shelf sediments in the study area. The 87Sr/86Sr ratios of the outer-shelf muddy sediments ranged from 0.7169 to 0.7216 in a wide range and was between those of the Huanghe River and Changjiang River sediments, suggesting multiple sources of the sediment in the area.

Sediment undulations on the Llobregat prodelta: Signs of early slope instability or sedimentary bedforms?

A field of sediment undulations has been mapped by means of high‐resolution multibeam bathymetry and seismic reflection profiles in the Llobregat River prodelta, off the city of Barcelona, Catalonia, Spain. Similar features had previously been recognized in other prodelta environments and interpreted either as downslope sediment deformation or sedimentary structures induced by bottom currents or hyperpycnal flows. Since the study area is undergoing significant offshore development, proper interpretation of such sediment undulations is needed for a correct risk assessment. The occurrence of the sediment undulations is restricted to the prodelta front on slope gradients between 3 and 0.2°. The undulations have developed at the edge and atop an area of gas bearing sediments within the Late Holocene high‐stand mud wedge. An evaluation is made of the characteristics of the sediment undulations in order to determine the most likely process for the origin of these structures. Among these characteristics are the continuity of the reflections and lack of diffractions in between different undulations, their size distribution (large to small) both from shallow to deep and with depth in section, the asymmetry (decreasing from proximal to distal), the crest to trough vertical distance on the landward side of the undulations (up to 0.5 m), and the lack of features that could indicate a progressive movement such as growth structures and drag folds. These characteristics indicate that the sediment undulations on the Llobregat River prodelta do not result from sediment deformation but rather from the interaction of bottom currents generated by hyperpycnal flows from the Llobregat River with regional sea water circulation. Their identification as sediment waves implies that such features do not pose a major hazard for further offshore development.

Sediment transport processes from the topset to the foreset of a crenulated clinoform (Adriatic Sea)

Crenulated clinoforms of complex and uncertain origin characterize large portions of the Late-Holocene prograding mud wedge in the western Adriatic continental shelf. Sediment failure was originally postulated as the most plausible mechanism for the formation of the crenulations. Subsequent work has shown that, although the origin of the crenulations may have been related to deformation processes, their maintenance through time seems to be better explained by different sediment accumulation rates in the flat and steep flanks. In order to establish relationships between active sediment dynamics, across-shelf transport and sediment accumulation in these crenulated clinoforms, two tripods and a mooring were deployed off the Pescara River during autumn and winter 2002–2003 as part of the EuroSTRATAFORM program, and in combination with the Po and Apennine Sediment Transport and Accumulation (PASTA) study. The tripods were placed on the shallow topset region and close to the clinoform roll-over point (i.e., offlap break), at 12 and 20-m water depth, respectively, and the mooring was located at 50-m depth, in the crenulated foreset region. Several sediment-resuspension events were recorded, mainly related to Bora and Sirocco storms, during which wave–orbital and current velocities increased considerably. Sediment transport in the topset region was predominantly towards the SE, following the direction of the coastal current and the bathymetry, but showing a significant offshore component at the roll-over point that was intensified during storm events. Currents at the foreset region were also directed to the SE. In mid-waters they were clearly aligned with the local bathymetry, whereas near the bottom they had an important and persistent offshore component. This current behavior seems to be associated with an intense bottom Ekman transport that causes the near-bottom current to be deflected to the left (i.e., offshore) with respect to the direction of the surface current. This mechanism enhances the suspended-sediment transport from the topset down the foreset region along the Adriatic prograding mud wedge, contributing to the basinward clinoform progradation and controlling the depth of the clinoform roll-over point. In addition, activity of near-inertial internal waves was also recorded by the near-bottom instrument deployed in the foreset region. During periods characterized by a strong near-inertial signal, increases of the water turbidity clearly coincided with an intensified offshore velocity component, which suggest that this mechanism also contributes to the transport of suspended sediment across the clinoform. Both the bottom Ekman transport and the internal waves are mechanisms that could be responsible for the formation/maintenance of the Adriatic seafloor crenulations until present-day, although several arguments suggest that the latter likely plays the major role.

What controls the distribution of shallow gas in the Western Adriatic Sea?

In this paper, we discuss what controls the occurrences of shallow gas off the Western Adriatic coast. This region of the Western Adriatic, including the Po delta and the late-Holocene mud wedge, was sampled with the R/V Seward Johnson II within the framework of the EuroSTRATAFORM project. As a part of the program, we were evaluating the causes of anomalous sub-surface and seafloor features, and specifically, quantifying shallow gas and identifying its impact on geophysical properties. The highest gas concentration off the Po delta region yielded 4.1×10 4 ppm, where as offshore Tronto River, the highest methane value was 7.9×10 4 ppm. Headspace gas analyses match well with acoustic anomalies along the Western Adriatic coast. Shallow biogenic gas off the Po delta (NW Adriatic) is spatially correlated with organic matter deposition driven by rapid and thick flood deposition. In contrast to the Po Delta, where sub-surface gas is associated with the flood deposition of organic matter, shallow gas along the Central Adriatic occurs deeper in cores, and appears to be related to methanogenesis of organic matter present in deeper deposits.

Sedimentary evolution of the Holocene subaqueous clinoform off the Shandong Peninsula in the Yellow Sea

A subaqueous clinoform wraps around the end of the Shandong Peninsula in the Yellow Sea. The clinoform, previously called the Shandong mud wedge, is up to 40 m thick nearshore and thins seaward. Its origin has been controversial since it was first noted in the 1980s. For our recent investigation offshore of the northeastern Shandong Peninsula, high-resolution shallow seismic profiles covering about 1700 km were obtained, and two drilling cores penetrating the clinoform were recovered. We described the lithofacies characteristics, identified benthic foraminifera and ostracoda, measured grain size, analyzed the mineral components of the cores, and obtained AMS 14C dates on 28 samples, to decipher the sedimentary evolution of the clinoform in response to postglacial sea-level changes. The shallow seismic profiles show that the clinoform comprises three units: a lowermost seismic unit (SU 3, less than 2–3 m thick) showing a retrogradational/aggradational stacking pattern with subhorizontal internal reflectors, a middle seismic unit (SU 2, up to 35 m thick) showing a prograding reflection pattern with mostly seaward-stepping reflectors separated by several erosive surfaces, and an uppermost seismic unit (SU 1, maximum thickness 18 m) exhibiting an aggradational reflection pattern with subparallel reflectors. The three seismic units are bounded on the bottom by distinct erosion surfaces. Analyses of the two cores indicate that the clinoform comprises three depositional units, DU 1, DU 2, and DU 3 in descending order, corresponding respectively to SU 1, SU 2, and SU 3. The depositional units are distinguished by lithofacies characteristics and the downcore distributions of benthic foraminiferal and ostracod assemblages. Mineral components of the cores suggest that the clinoform's provenance is predominantly Yellow River-derived sediments. AMS 14C dates suggest that DU 3 was formed from about 11.6 to 9.6 cal kyr BP, DU 2 from 9.6 to 6.5 cal kyr BP, and DU 1 after ca. 6.5 cal kyr BP. Some major meltwater pulse (MWP) events during the deglacial transgression were registered in the clinoform stratigraphy, with the basal surfaces/sediments of DU 3 and DU 2 corresponding respectively to MWP-1B (ca. 11.6–11.3 cal kyr BP) and MWP-1C (ca. 9.6–9.2 cal kyr BP) events, and with DU 2 being largely formed after the 8.4–8.2 cal kyr meltwater event. Rapid centennial- to multidecadal-scale flooding events were presumably responsible for the erosive surfaces separating the downlapping internal reflectors in SU 2. The change of depositional pattern from progradational in SU 2 to aggradational in SU 1 is attributed to a change from strong, dominantly to-and-fro tidal currents to the modern circulation in the Bohai Sea as well as to the weakening of the tidal-current field during the highstand after about 6500 cal yr BP. As a product of complex interactions among sediment supply, sea-level changes, and hydrodynamic regimes, the clinoform can be regarded as the distal part of the tidal depositional system in the eastern Bohai Sea, which was largely derived from the Yellow River.

Sedimentary structures offshore Ortona, Adriatic Sea — Deformation or sediment waves?

The late Holocene mud wedge on the Adriatic shelf offshore Ortona, Italy, shows undulating sub-parallel seismic reflector sequences which extend several kilometres along strike and 100–200 m down-dip in water depth between 20 and 80 m. The amplitude of such undulations is up to 5 m and the undulations continue as stacked sediment packages downwards throughout the 35 m thick mud wedge. The undulations are separated by 4° to 5° dipping boundary zones and at first glance these sediment undulations resemble the seafloor sedimentary structures visible in the Humboldt Feature offshore California. There is an ongoing debate whether seafloor undulations are the result of deformation processes or sediment deposition and/or reworking due to submarine shelf currents. A dense net of recently reprocessed and digitally interpreted high-resolution Chirp seismic data on the Adriatic shelf favours an interpretation that these undulations developed, in the upper part of the stratigraphic section through sediment reworking rather than through deformation. There are three lines of evidence for this: (1) the spatial extent of the undulations coincides with higher seabed reflector amplitudes than found both on the shelf and in the distal part of the shelf. If the features were solely caused by slope failure there should be no change of amplitudes, (2) the seabed reflector amplitude is generally higher on the gently dipping or flat upslope limbs than on the steeper downslope limbs, supporting a current origin that causes preferred deposition of sediments with higher acoustic impedance on the top, (3) the boundaries between the undulations are dipping at angles that are much lower than the angle of internal friction of these sediments excluding that a simple model like the Mohr–Coulomb under gravitational loading could describe the undulations as sediment deformation and failures.

Flux and fate of Yangtze River sediment delivered to the East China Sea

Numerous cores and dating show the Yangtze River has accumulated about 1.16 × 10 12 t sediment in its delta plain and proximal subaqueous delta during Holocene. High-resolution seismic profiling and coring in the southern East China Sea during 2003 and 2004 cruises has revealed an elongated (∼ 800 km) distal subaqueous mud wedge extending from the Yangtze River mouth southward off the Zhejiang and Fujian coasts into the Taiwan Strait. Overlying what appears to be a transgressive sand layer, this distal clinoform thins offshore, from ∼ 40 m thickness between the 20 and 30 m water depth to < 1–2 m between 60 and 90 m water depth, corresponding to an across shelf distance of less than 100 km. Total volume of this distal mud wedge is about 4.5 × 10 11 m 3, equivalent to ∼ 5.4 × 10 11 t of sediment. Most of the sediment in this mud wedge comes from the Yangtze River, with some input presumably coming from local smaller rivers. Thus, the total Yangtze-derived sediments accumulated in its deltaic system and East China Sea inner shelf have amounted to about 1.7 × 10 12 t. Preliminary analyses suggest this longshore and across-shelf transported clinoform mainly formed in the past 7000 yrs after postglacial sea level reached its mid-Holocene highstand, and after re-intensification of the Chinese longshore current system. Sedimentation accumulation apparently increased around 2000 yrs BP, reflecting the evolution of the Yangtze estuary and increased land erosion due to human activities, such as farming and deforestation. The southward-flowing China Coastal Current, the northward-flowing Taiwan Warm Current, and the Kuroshio Current appear to have played critical roles in transporting and trapping most of Yangtze-derived materials in the inner shelf, and hence preventing the sediment escape into the deep ocean.

Pleistocene “muddy” forced-regression deposits on the Adriatic shelf: A comparison with prodelta deposits of the late Holocene highstand mud wedge

The upper Pleistocene–Holocene stratigraphy of the Adriatic margin consists of a succession of depositional sequences separated by shelf-wide unconformities. The most recent of these depositional sequences encompasses the last ca. 25 kyr, and consists of lowstand, transgressive and highstand units formed, respectively, during the Last Glacial Maximum lowstand, the subsequent sea level rise and the modern highstand. Older deposits are grouped into four depositional sequences, each recording ca. 100 kyr glacio-eustatic cycles during an interval between ca. 450 and 25 kyr BP. Each of these Pleistocene sequences is composed dominantly by muddy progradational units deposited during prolonged phases of sea level fall; therefore, these sequences represent a unique example of well-preserved muddy forced-regression deposits. Detailed seismic–stratigraphic analyses show that forced-regression deposits composing the four sequences are not directly connected to nearshore and/or fluvio-deltaic environments. In contrast, typical forced-regression deposits defined in outcrop and reported in the literature consist of sand-rich, nearshore/deltaic facies preserved as patchy remnants of ancient shorelines. We suggest that muddy forced-regression deposits on the Adriatic shelf represent the offshore counterpart of shoreface deposits recognised in outcrop as the product of typical forced regressions. Interestingly, muddy forced-regression deposits on the Adriatic shelf bear clear similarities with the late Holocene highstand mud wedge, suggesting a common depositional mechanism through lateral advection of fine-grained sediment. In this view, subaqueous progradation of muddy deposits occurs below storm wave base, with a subaqueous roll-over point that, during proceeding sea level fall, likely positioned at decreasing distance from the time-equivalent shoreline.

Fine-sediment mass balance in the western Adriatic continental shelf over a century time scale

To obtain a quantitative estimate of the sediment accumulating on the muddy belt extending along the western Adriatic continental shelf, a budget was calculated based on the mass accumulation rates measured from more than one hundred activity–depth profiles of 210Pb. 137Cs depth-distributions were used, when possible, to support 210Pb results. 210Pb profiles show quasi-monotonic declines in the foreset region of the central Adriatic clinoform. In the prodelta of the rivers Po and Isonzo, 210Pb profiles are characterized by intervals of low and uniform activity, which are thought to be produced by flood deposition. Since the north Adriatic is shallow and occasionally affected by storms from Bora wind, sediments are not in steady-state and their accumulation is the net result of recurring deposition, resuspension and winnowing events. In the western Adriatic, mass accumulation rates range between 0.04 and 6.6 g cm − 2 yr − 1 . Peak values were recorded in the Po and Isonzo prodeltas and along a narrow belt between Pescara and the Gargano promontory. Furthermore, the latter depocentre is located immediately seaward of the offlap break (40–50 m water depth) and parallels that of the most recent sigmoid of the HST. The integration of mass accumulation rates throughout the mud wedge of the north and central Adriatic gave a value of ∼42.0 Tg yr − 1 of sediment accumulating along the Italian coast. The balance between sediment accumulation and riverine sediment supply (∼46.6 Tg yr − 1 ) suggests an export of 4.6 Tg yr − 1 to the south Adriatic basin and/or the Mediterranean Sea. The along-shore sediment transport is very effective and, in particular, 16.2 Tg yr − 1 of sedimentary material (∼35% of the riverine supply) is transferred from north to central Adriatic, through the Ancona section.