Mud Belts Research Articles

Changjiang coastal mud‐belt deposits in Taiwan Strait: Controls on its distribution and facies

ABSTRACTCoastal mud belts, which lie parallel to the coast just seaward of the shoreface, are one of the most important settings where shallow‐marine muddy deposits accumulate. However, sedimentary processes and facies distributions of coastal mud belts remain largely uninvestigated. This study uses process‐oriented sedimentology, coupled with provenance analysis and organic geochemistry of sedimentary records at the northern entrance to the Taiwan Strait, to investigate one of the largest such systems in the world, the Changjiang coastal mud belt. Carbon‐14 dates indicate that deposition has been intermittent throughout Marine Isotope Stages 1, 2 and 3, with several cryptic gaps in the muddy succession as a result of exposure or transgressive erosion during periods of lower or rising sea level, respectively. The palaeowater depth of each depositional unit was roughly estimated based on a global sea‐level curve and the elevation and age of the depositional units. Integrated provenance and sedimentology studies suggest that an incised valley most likely cut by the Minjiang, a nearby river draining the Chinese mainland, probably during Marine Isotope Stage 6, was filled with shelf mud during Marine Isotope Stage 3. These results demonstrate that deposition of mud on the Changjiang coastal mud belt was influenced by sea‐level changes that controlled water depth and flow patterns through Taiwan Strait. The Changjiang coastal mud belt migrates landward when sea level rises and moves seaward when sea level drops. In addition, changes in oceanic circulation associated with opening and closing of Taiwan Strait due to sea‐level changes causes significant changes in the source of the mud, with an alternation between sediment contributions from the Changjiang in the north, the nearby Minjiang, and the island of Taiwan to the south‐east. This study also reveals that the Changjiang coastal mud belt deposits are composed of both shallower‐water and deeper‐water facies. The shallower‐water facies predominantly comprises abundant fluid‐mud beds generated by waves and tidal currents. The deeper‐water facies contains shell‐rich mud and bioturbated mud, which were likely deposited by the Chinese coastal current and Taiwan Warm current. The bed types and interpretation of sedimentary processes of the Changjiang coastal mud belt are useful for recognition of ancient deposits of coastal mud belts elsewhere.

Bioturbation, sedimentation rates, and preservation of flood events in deltas

We examine the nature and number of event beds preserved in heterolithic prodelta and delta-front strata in five proximal-accumulation-dominated (PAD) North American delta systems including the Quaternary Gulf of Mexico and the Late Cretaceous Western Interior Seaway. We also explore the continuum between PAD, more distal deposits of mud belts, and thicker subaqueous-delta-clinoforms (SDC). Sediment accumulation rates in modern PAD and SDC systems are commonly on the order of several to severaltens of centimeters per year. They also show that several event beds may be created annually, with preservation depending on supply and reworking. The PAD heterolithics commonly comprise interbedded sand, silt and clay. These show normal and inverse grading and Bouma sequences, indicating deposition from turbidity currents and hyperpycnal flows that are interpreted as the products of river plumes generated during flood episodes. Re-equilibrated Rosselia socialis in stacked delta front turbidite bedsets in the Cretaceous examples show that floods may occur as a cluster of events over several months. Shelf areas adjacent to river mouths may experience high rates of sediment delivery from rivers, but oceanographic processes are instrumental in moving much of the silt and clay farther along the shelf. Direct, hyperpycnal delivery from a river mouth on a low gradient shelf (i.e., typically <0.05°) is rare, owing to the difficulty of maintaining a high suspended sediment concentration, but sedimentation rates are nevertheless high. Shelf facies show evidence for extensive and frequent remobilization during storms and other meteorological events. Storm-wave-enhanced sediment-gravity flows (WESGFS) may form under favorable conditions, such as strong waves, high suspended sediments, and convergent currents, and can be significant in building SDC and other shelf deposits. The proximal portions of PAD and SDC systems typically show low intensity and diversity of bioturbation (BI 0–1). The bioturbation styles encountered in these deposits record: 1) high sedimentation rates, which preclude wholesale biogenic reworking; 2) high-frequency river floods and storm reworking, expressed by reduced colonization windows; 3) soupy substrates, likely reflecting formation of a permanent fluid-mud lutocline; and 4) elevated turbidity near the sediment-water interface, which may cause further suppression of trace-making.

Dynamical analysis of the mud-belt formation in the Bohai, Yellow and East China seas

Mud belts commonly occur on the coast and shelf environments worldwide. It requires a point source (river) and a residual current, all together constituting a tripartite configuration of the mud-belt system. This mud-belt system is analyzed for its basic sedimentological and hydrodynamical parameters to be interrelated under the steady-state conditions. The parameters include the length (L) and sedimentation rate (A) of a mud belt, water depth (h), residual current velocity (ur), and settling velocity of suspended particles (ws). The analysis shows that L is proportional to h, ur and ln A but inversely proportional tows. In addition, the mean sedimentation rate of a mud belt (A¯) is a function of only the sedimentation rates at its head and toe.To test the analytical relationships, three mud belts studied extensively are chosen from the Bohai, Yellow and East China seas. Each of these mud belts follows the ideal tripartite configuration. For all the mud belts, residual currents have been reported to be the strongest during winter seasons. The calculated A¯ gives a constraint to the otherwise wide range of the previously estimated riverine contributions to each mud belt. The two mud belts in the East China Sea and the Bohai Sea are analyzed to receive 30 and 7% of riverine sediment discharges from the Yangtze and Yellow rivers, respectively. In comparison, the Huksan Mud Belt in the southeastern Yellow Sea, on debate concerning its origin, analytically proves to be in a source-sink relationship with the Geum River. The calculated ws suggests that suspended particles are composed dominantly of fine-silt-sized single grains or micro-flocs for the three mud belts. As a result, the derived relationships may be a useful tool to understand dynamical processes of the mud belt. They also may be instrumental in anticipating the possible changes in the mud-belt evolution with A and ur that are likely to vary with the globally warming climate.

Controls On Deep-Water Sand Delivery Beyond the Shelf Edge: Accommodation, Sediment Supply, and Deltaic Process Regime

ABSTRACT Stratigraphic models typically predict accumulation of deep-water sands where coeval shelf-edge deltas are developed in reduced-accommodation and/or high-sediment-supply settings. On seismic data, these relationships are commonly investigated on a small number of clinothems, with a limited control on their lateral variability. Advanced full-volume seismic interpretation methods now offer the opportunity to identify high-order (i.e., 4th to 5th) seismic sequences (i.e., clinothems) and to evaluate the controls on shelf-to-basin sediment transfer mechanisms and deep-water sand accumulation at these high-frequency scales. This study focuses on the Lower Barrow Group (LBG), a shelf margin that prograded in the Northern Carnarvon Basin (North West Shelf, Australia) during the Early Cretaceous. Thanks to high-resolution 3D seismic data, 30 clinothems (average time span of ∼ 47,000 years) from the D. lobispinosum interval (142.3–140.9 Ma) are used to establish quantitative and statistical relationships between the shelf-margin architecture, paleoshoreline processes, and deep-water system types (i.e., quantitative 3D seismic stratigraphy). The results confirm that low values of rate of accommodation/rate of sediment supply (δA/δS) conditions on the shelf are associated with sediment bypass, whereas high δA/δS conditions are linked to increasing sediment storage on the shelf. However, coastal process regimes at the shelf edge play a more important role in the behavior of deep-water sand delivery. Fluvial-dominated coastlines are typically associated with steep slope gradients and more mature, longer run-out turbidite systems. In contrast, wave-dominated shorelines are linked to gentle slope gradients, with limited development of turbidite systems (except rare sheet sands and mass-transport deposits), where longshore drift currents contributed to shelf-margin accretion through the formation of extensive strandplains. In this context, reduced volumes of sand were transported offshore and mud belts were accumulated locally. This study highlights that variations from fluvial- to wave-dominated systems can result in significant lateral changes in shelf-margin architecture (i.e., slope gradient) and impact the coeval development of deep-water systems (i.e., architectural maturity). By integrating advanced tools in seismic interpretation, quantitative 3D seismic stratigraphy represents a novel approach in assessing at high resolution the controls on deep-water sand delivery, and potentially predicting the type and location of reservoirs in deep water based on the shelf-margin architecture and depositional process regime.

The role of saltwater and waves in continental shelf formation with seaward migrating clinoform

Continental shelves have generally been interpreted as drowned coastal plains associated with the allogenic effect of sea-level variation. Here, without disputing this mechanism we describe an alternative autogenic mechanism for subaqueous shelf formation, driven by the presence of dissolved salt in seawater and surface waves. We use a numerical model describing flow hydrodynamics, sediment transport, and morphodynamics in order to do this. More specifically, we focus on two major aspects: 1) the role of saltwater in the subaqueous construction of continental shelves and 2) the transformation of these shelves into seaward-migrating clinoforms under the condition of repeated pulses of water and sediment input and steady wave effects, but no allogenic forcing such as sea-level change. In the case for which the receiving basin contains fresh water of the same density as the sediment-laden river water, the hyperpycnal river water plunges to form a turbidity current that can run out to deep water. In the case for which the receiving basin contains sea water but the river contains sediment-laden fresh water, the hypopycnal river water forms a surface plume that deposits sediment proximally. This proximate proto-shelf can then grow to wave base, after which wave-supported turbidity currents can extend it seaward. The feature we refer to is synonymous with near-shore mud belts.

Records of Holocene climatic fluctuations and anthropogenic lead input in elemental distribution and radiogenic isotopes (Nd and Pb) in sediments of the Gulf of Lions (Southern France)

Marine mud belts represent potential continuous high-resolution climatic, environmental and anthropogenic archives. In this study, a geochemical record of the Gulf of Lions mud belt, which receives sediments from the Rhône watershed and to a lesser extent from the Languedoc region, is reported from Core KSGC-31. The effects of natural climatic changes and possible anthropogenic disturbances on Holocene sedimentation were ascertained by analysing sedimentation rates, chemical weathering (Al2O3/K2O) and sediment-source shifts (neodymium isotopic ratios; εNd). Measurements of elemental and isotopic lead were used to trace the source and determine the potential vectors of anthropogenic contaminations over the Holocene. High εNd values, recorded from 9000 to 3000 calibrated annum before present (cal. a BP) and around 1500 and 600 cal. a BP, are interpreted as an increase in sediment transport from the Alpine crystalline massifs to the sea induced by enhanced hydro-sedimentary conditions upstream. During the early and middle Holocene, low and stable weathering conditions were persistent, while the late Holocene was characterized by higher and more fluctuating weathering conditions. Sudden changes in the 206Pb/207Pb ratio observed during the Roman and Medieval periods suggest clear shifts in lead source from a natural Holocene background to late Holocene anthropogenic contaminations. Even though those shifts are coeval with atmospheric lead contaminations from Spain and Germany recorded in several sediment and ice archives, the local origin (the Cévennes) and the fluvial contamination is more likely in these cases. Those findings are contemporaneous with historical mining records in the Cévennes and point to an intensification of the merchant shipping.

Mud‐rich delta‐scale compound clinoforms in the Triassic shelf of northern Pangea (Havert Formation, south‐western Barents Sea)

AbstractIn recent years it has become clear that many shallow‐marine heterolithic and mudstone‐dominated successions are deposited as mud belts forming part of subaqueous deltas that are related to major fluvial sources either upstream or along shore. Here the Havert Formation is presented as an ancient example of this kind of system. The Havert Formation in the south‐western Barents Sea represents shelf margin clinoforms consisting predominantly of heterolithic deposits. Sediments were mainly derived from the east (Ural Mountains), but a smaller system prograded northward from Fennoscandia. The Havert Formation holds a lot of interest due to: (i) its stratigraphic position, directly above the Permo–Triassic boundary and contemporaneous to the emplacement of the Siberian Traps; (ii) the fact that it represents the first siliciclastic input in the south‐western Barents Sea and it shows interaction between Uralian‐derived and Fennoscandian‐derived sediments; and (iii) its hydrocarbon potential. This study is focused on a detailed sedimentological analysis of cored intervals of the (Ural‐derived) Havert Formation, in combination with seismic interpretation, well‐log correlations and palynological analysis of the Havert and overlying Klappmyss formations. The cored intervals belong to the shelf environment of the Havert shelf‐margin clinoforms (300 to 500 m thick). This sedimentological analysis distinguishes six facies associations, spanning from tidally‐influenced channels at the shoreline to mud‐rich subaqueous platform and foresets of the subaqueous delta. Seismic lines and well‐log correlations show the larger‐scale evolution of the Ural‐derived Havert Formation, characterized by episodes of low‐accommodation and high‐accommodation. The palynological analyses provide the first detailed study of the Havert Formation in the Nordkapp Basin, revising its depositional age in the region as Induan to early Olenekian (Smithian). Furthermore, they strengthen the environmental interpretation; palynofacies present on the shelf record flora of tidally‐influenced coastal plains, whereas the palynofacies in the deep‐water slope contain only amorphous organic matter.

The iron limitation mosaic in the California Current System: Factors governing Fe availability in the shelf/near‐shelf region

AbstractThe California Current System is a productive eastern boundary region off the coasts of Washington, Oregon, and California. There is strong seasonality to the region, with high levels of rainfall and river input to the coastal ocean during the winter season, and coastal and Ekman upwelling during the spring and summer. Iron (Fe) input to the coastal ocean during the winter months can be stored in the continental shelf mud belts and then be delivered to the surface ocean by upwelling in the spring and summer. There have been a number of studies providing strong evidence of Fe‐limitation of diatom growth occurring in regions of the California Current System off of California, and the occurrence of Fe‐limitation has been linked with narrow continental shelf mud belt width and low river input. We provide evidence for potential Fe‐limitation of diatoms off the southern coast of Oregon in July 2014, just off the shelf break near Cape Blanco in a region with moderate shelf width and river input. Since eastern boundary regions account for a disproportionally large amount of global primary production, this observation of potential Fe‐limitation in an unexpected near‐shore region of the California Current System has implications for global models of primary productivity. In order to re‐evaluate the factors impacting Fe availability, we utilize satellite imagery to compare with historical datasets, and show that unexpected levels of Fe can often be explained by eddies, plumes of upwelled water moving offshore, or lack of recent upwelling.

Sulfur and iron diagenesis in temperate unsteady sediments of the East China Sea inner shelf and a comparison with tropical mobile mud belts (MMBs)

AbstractRedox cycling of iron (Fe) and sulfur (S) exerts profound influences on fates of numerous elements in coastal marine sediments. In this study, S and Fe cycling and its geochemical expressions in the East China Sea (ECS) inner shelf, a representative of temperate mobile mud belts (MMBs), were characterized and compared with tropical counterparts (the Amazon shelf and the Gulf of Papua). Fe and S speciation consistently points to the dominance of authigenic nonsulfidized Fe(II) phases (i.e., poorly crystalline clays (PCCs) and carbonates) and the prevalence of Fe redox cycling in the suboxic or weakly sulfidic regimes of the ECS‐MMBs. High contents of authigenic magnetite may be a common diagenetic expression in all MMBs. Compared to the tropical MMBs, three main differences of diagenetic expressions in the ECS‐MMBs are (i) light 34Spyrite in the ECS‐MMB versus characteristically heavy 34Spyrite in the Amazon shelf MMBs; (ii) lower total reactive Fe (FeTR), total diagenetic Fe(II), and ratio of FeTR to total Fe in the ECS‐MMBs; and (iii) Fe(II) carbonates and PCCs are equally important sinks for nonsulfidized Fe(II) in the ECS‐MMBs, whereas PCCs are the predominant sink in the tropical counterparts. These differences are ascribable to factors including low degradability of organic matter, small diffusion scales, less intense chemical weathering in the drainage basin, and/or weaker reverse weathering in the ECS‐MMBs. Despite the differences above, Fe and S diagenetic expressions that characterize the prevalence of Fe redox cycling in the unsteady suboxic regimes are shared by the ECS‐MMBs and tropical MMBs.

Holocene hydrological changes in the Rhône River (NW Mediterranean) as recorded in the marine mud belt

Abstract. Expanded marine Holocene archives are relatively scarce in the Mediterranean Sea because most of the sediments were trapped in catchment areas during this period. Mud belts are the most suitable targets to access expanded Holocene records. These sedimentary bodies represent excellent archives for the study of sea–land interactions and notably the impact of the hydrological activity on sediment accumulation. We retrieved a 7.2 m long sediment core from the Rhône mud belt in the Gulf of Lions in an area where the average accumulation rate is ca. 0.70 m 1000 yr−1. This core thus provides a continuous and high-resolution record of the last 10 ka cal BP. A multiproxy dataset (XRF core scan, 14C dates, grain size and organic-matter analysis) combined with seismic stratigraphic analysis was used to document decadal to centennial changes in the Rhône hydrological activity. Our results show that (1) the early Holocene was characterized by high sediment delivery likely indicative of local intense (but short-duration) rainfall events, (2) important sediment delivery around 7 ka cal BP presumably related to increased river flux, (3) a progressive increase in continental/marine input during the mid-Holocene despite increased distance from river outlets due to sea-level rise possibly related to higher atmospheric humidity caused by the southward migration of the storm tracks in the North Atlantic, (4) multidecadal to centennial humid events took place in the late Holocene. Some of these events correspond to the cold periods identified in the North Atlantic (Little Ice Age, LIA; Dark Ages Cold Period) and also coincide with time intervals of major floods in the northern Alps. Other humid events are also observed during relatively warm periods (Roman Humid Period and Medieval Climate Anomaly).

Tidally dominated sediment dispersal offshore of a small mountainous river: Elwha River, Washington State

Sediment supplied by small mountainous rivers (SMRs) represents a major fraction of the global ocean sediment budget. Studies from the past two decades have shown that much of this sediment is dispersed by episodic wind and wave energy along storm-dominated coasts. In tidally dominated environments, however, different transport styles and deposits may result from persistent tidal dispersal. This study investigates episodic sediment releases generated by dam removal from a SMR in Washington State, in order to evaluate the mechanics of tidally dominated sediment dispersal in an energetic marine environment. The results indicate that asymmetric tidal currents with peak magnitudes of ∼50 to >80cm/s produce daily sediment export in the direction of the dominant tidal phase (i.e., the semi-diurnal phase with faster currents and longer duration), resulting in dispersal of fluvially derived fine sediment to distal sinks. These effects are observed throughout all seasons in the presence or absence of wave events. During the first two years of dam removal, more than 8 million tonnes of sediment were discharged to the coast. The net result was little to no change in grain size at 10–60m water depth across >70% of the seabed offshore of the river mouth. Over the remaining ∼2 to 3km2 of the subaqueous delta, several cm of mud and sand accumulated in a sheltered coastal embayment adjacent to the river mouth. These results demonstrate that SMR discharge events may form patchy, isolated deposits—or even no deposits—along coastlines with strong tidal currents, in contrast to the mid-shelf mud belts formed on storm-dominated shelves. Over longer timescales, knowledge of the erosional capacity of local and regional tidal currents may be key to interpreting the terrestrial event record preserved in (or possibly excluded from) marine SMR deposits.

Mud depocenters on continental shelves—appearance, initiation times, and growth dynamics

Mud accumulates on continental shelves under a variety of environmental conditions and results in a diverse formation of mud depocenters (MDCs). Their three-dimensional architectures have been in the focus of several recent studies. Due to some terminological confusion concerning MDCs, the present study sets out to define eight individual MDC types in terms of surface sediment distribution and internal geometry. Under conditions of substantial sediment supply, prodeltas (distal zones off river deltas; triangular sheets), subaqueous deltas (disconnected from deltas by strong normal-to-shore currents; wedge-like clinoforms), and mud patches (scattered distribution) and mud blankets (widespread covers) are formed. Forced by hydrodynamic conditions, mud belts in the strict sense (detached from source; elongated bodies), and shallow-water contourite drifts (detached from source; growing normal to prevailing current direction; triangular clinoforms) develop. Controlled by local morphology, mud entrapments (in depressions, behind morphological steps) and mud wedges (triangular clinoforms growing in flow direction) are deposited. Shelf mud deposition took place (1) during early outer-shelf drowning (~14 ka), (2) after inner-shelf inundation to maximum flooding (9.5–6.5 ka), and (3) in sub-recent times (<2 ka). Subsequent expansion may be (1) concentric, in cases where the depocenter formed near the fluvial source, (2) uni-directional, extending along advective current transport paths, and (3) progradational, forming clinoforms that grow either parallel or normal to the bottom current direction. Classical mud belts may be initiated around defined nuclei, the remote sites of which are determined by seafloor morphology rather than the location of the source. From a stratigraphic perspective, mud depocenters coincide with sea-level highstand-related, shelf-wide condensed sections. They often show a conformable succession from transgressive to highstand systems tract stages.

Chapter 10 Recent sedimentation in the NW African shelf

Abstract A review of the data gathered over the last five decades on the NW African continental shelf within numerous research programmes shows that the sedimentary processes along the shelf are driven by long-term factors such as Quaternary glacial–interglacial periods and shelf morphology, and by short-term factors such as fluvial and aeolian sediment supply, local climate (temperature, rainfall and wind) and hydrodynamic conditions (tides, swell, longshore current, the Canary Current and upwelling). Based on the sedimentary characteristics, the margin has been subdivided into four segments: northern Morocco (30–36°N) and the Grande Côte of Senegal (15–16°30′N) show long mid-shelf mud belts controlled by fluvial input and low hydrodynamic energy; southern Morocco–Mauritania (16°30′N–30°N) and the Petite Côte south of Cape Vert, Gambia and Casamance (12–15°N) are dominated by biogenic and aeolian sands or no sediment input. In this chapter, we present a synopsis of the state of knowledge on the NW African shelf sediments and processes based on various published and unpublished documents.

Seismic stratigraphy and geomorphology of a tide or wave dominated shelf-edge delta (NW Australia): Process-based classification from 3D seismic attributes and implications for the prediction of deep-water sands

Shelf-edge deltas (SEDs) forming during periods of relative sea level fall and lowstand are generally efficient in transferring sediments to the slope and basins, and their identification in subsurface data is often considered a good indication of coeval development of slope and basin-floor turbidite reservoirs. This study investigates the seismic stratigraphic evolution of a forced-regressive and normal regressive shelf-edge delta (Bonaparte SED) that accumulated on the edge of the NW Australian margin during the late Quaternary. High resolution 2D and 3D reflection seismic data allow reconstruction of the main episodes of delta progradation and understanding of the extrinsic and intrinsic controls on their deposition. The lack of a significant turbidite system forming off the shelf-edge delta throughout the Quaternary is a striking feature of the Bonaparte SED. Instead, slope sedimentation is dominated by the accumulation of plume-derived mud belts and their reworking through mass-transport processes. Seismic geomorphology permits interpretation of the process regime of the youngest shelf-edge depocentre by applying a new process-based shallow-marine classification scheme to the 3D seismic attribute data. Results suggest either a tide or wave dominated delta with fluvial processes being of tertiary significance. A tide or wave-dominated, fluvial-affected shelf-edge delta classification is consistent with the paleogeographical reconstruction of the margin during the last glacial maximum (ca. 25 ka BP). The comparison of this mixed-process shelf-edge delta and starved slope system with a fluvial-dominated counterpart with significant sandy slope deposits emphasizes the potential of assessing the process regime of shelf-edge deltas as a rapid, first approach for predicting the presence or absence of coeval slope and basin-floor reservoirs.

Suspended particulate layers and internal waves over the southern Monterey Bay continental shelf: An important control on shelf mud belts?

Physical and optical measurements taken over the mud belt on the southern continental shelf of Monterey Bay, California documented the frequent occurrence of suspended particulate matter features, the majority of which were detached from the seafloor, centered 9–33 m above the bed. In fall 2011, an automated profiling mooring and fixed instrumentation, including a thermistor chain and upward‐looking acoustic Doppler current profiler, were deployed at 70 m depth for 5 weeks, and from 12 to 16 October a long‐range autonomous underwater vehicle performed across‐shelf transects. Individual SPM events were uncorrelated with local bed shear stress caused by surface waves and bottom currents. Nearly half of all observed SPM layers occurred during 1 week of the study, 9–16 October 2011, and were advected past the fixed profiling mooring by the onshore phase of semidiurnal internal tide bottom currents. At the start of the 9–16 October period, we observed intense near‐bed vertical velocities capable of lifting particulates into the middle of the water column. This “updraft” event appears to have been associated with nonlinear adjustment of high‐amplitude internal tides over the mid and outer shelf. These findings suggest that nonlinear internal tidal motions can erode material over the outer shelf and that, once suspended, this SPM can then be transported shoreward to the middle and shallow sections of the mud belt. This represents a fundamental broadening of our understanding of how shelf mud belts may be built up and sustained.

Coastal iron and nitrate distributions during the spring and summer upwelling season in the central California Current upwelling regime

Distributions of iron and nitrate in the central California Current System upwelling regime (cCCS) from 34 to 41°N were determined during cruises in May 2010 and August 2011. High spatial and temporal resolution data for dissolved Fe and NO3− (nitrate+nitrite) in the cCCS from this study greatly expands upon previous studies that were narrower in scope (e.g., focused on just the Monterey Bay region). Shelf sediments from mid-shelf mud belts in this region provide the dominant source of Fe, and there are areas in the cCCS where insufficient Fe is upwelled to accompany elevated levels of other macronutrients (nitrate, phosphate, silicate) to fuel extensive diatom blooms. Surface dissolved Fe concentrations were related to continental shelf width and upwelling strength, and surface Fe concentrations tended to be lower in the late summer than early spring. We present extensive benthic boundary layer (BBL) dissolved and leachable particulate Fe data from both seasons in the mid-shelf region along the central California coast. Leachable particulate Fe concentrations were strongly related to the width of the mid-shelf mud belts (i.e., the continental shelf between the 50 and 90m isobaths). Dissolved Fe concentrations in the BBL over the mid-shelf were generally highest in wide mud belt areas as well as in areas with very low dissolved oxygen concentrations but did not show a clear seasonal trend. Evidence for probable Fe limitation in upwelled waters was found by using surface dissolved Fe:NO3− ratios and the estimated specific growth rate of coastal diatoms based on either Fe or NO3− concentrations. Several coastal upwelling regions with only moderate to narrow continental shelves (Pt. Arena to Cape Mendocino and the Big Sur Coast) exhibited evidence for Fe limitation in both the spring and summer upwelling seasons.

Deposition fluxes and fate of polycyclic aromatic hydrocarbons in the Yangtze River estuarine‐inner shelf in the East China Sea

Surface sediments were obtained from a matrix of 76 sample sites in the inner shelf mud belts of the East China Sea (ECS) for a comprehensive study of the distribution, composition, deposition flux, and fate of polycyclic aromatic hydrocarbons (PAHs). The sampling sites covered an area of ~80,000 km2 extending ~1000 km from the mouth of the Yangtze River to the Min River in the inner shelf. The total deposition flux of the 16 USEPA priority PAHs (16 PAHs) of the Yangtze estuarine‐inner shelf was estimated to be 152 t/yr, accounting for ~38% of the total annual input of the 16 PAHs into the ECS. This indicates that the Yangtze estuarine‐inner shelf is one of the largest sinks of land‐based PAHs in the world. Principal component analysis indicated that the 16 PAHs in the northern Yangtze estuarine mud area were mostly phenanthrene while shifting to high‐molecular‐weight PAHs in the southern Min‐Zhe coastal mud area. The positive matrix factorization model revealed that the deposition flux of low‐molecular‐weight (LMW) PAHs decreased from north to south, most likely due to the mass transfer between the resuspended sediments triggered by the East Asian monsoon and the water columns, as the resuspended sediments are transported southward. This release of LMW PAHs from the sediments to the water columns could become an important secondary PAH source in ECS.

Mid to late Holocene constraints for continental shelf mud deposition in association with river input: the Guadiana Mud Patch (SW Iberia)

This study correlates Holocene sedimentation with regional environmental changes for the Guadiana middle shelf, SW Iberia, based on a detailed sedimentological analysis combined with radiocarbon and amino acid racemization dating. The sedimentary record of vibrocore CRIDA16 (307 cm) from the mud patch off the Guadiana River is characterized by a transgressive signature. Terrigenous sand–gravel dominates the core base up to ca. 5,000 cal. years b.p., enriched in schist/greywackes and non-reworked quartz. The shelf area was under the direct influence of Guadiana bed load discharges, as sea level was lower than today but rising, favouring the entrapment of fines during the Guadiana estuary infilling in the early Holocene. Core upwards, fines dominate the record. Sea-level stabilization at ca. 5,700–3,700 cal. years b.p. led to estuary infilling by coarser sediments and export of suspended sediment to the middle shelf. After ca. 900–700 cal. years b.p., the formation of the Guadiana Mud Patch signifies deposition in a marine environment dominated by suspended material from the Guadiana River. Comparison with earlier publications showed that a similar evolutionary mechanism of middle shelf mud deposition occurred in other parts of the Iberian Margin, pointing to a generalized establishment of this type of sedimentary body in Iberia since the mid to late Holocene. This suggests that the formation of mud belts on the north and south Iberian Margin was to a large extent synchronous. It is likely that the evolution of the Guadiana Mud Patch on the shelf was controlled foremost by the overall climate trends identified for several parts of Iberia during the Holocene (i.e. increased aridity punctuated by significant humidity stages), and less by human occupation of the region. Enhanced humidity stages led to higher rainfall and intensified floods, with a consequent increase of discharges to the shelf which ultimately were responsible for the Guadiana Mud Patch formation. It is expected that this pattern of sedimentary dynamics has been strongly disturbed since 2002, when the closing of the Alqueva Dam (the biggest on the Iberian Peninsula) was completed in the Guadiana River basin.

Coupling of early diagenetic processes and sedimentary dynamics in tropical shelf environments: the Gulf of Papua deltaic complex

Tropical mobile mud belts represent a major class of biogeochemical and diagenetic systems characterized by extensive and frequent physical reworking of fine-grained, organic-rich deposits underlying oxygenated waters. Large regions of the Gulf of Papua, Papua New Guinea deltaic complex are dominated by such conditions. A reworked mud belt lies within the inner shelf between ∼10 and 20m depth on a sedimentary clinoform derived from coalescing deltas. Deposits across the topset are typically suboxic, nonsulfidic over the upper ∼0.5–1m, and have low to moderate maximum pore water concentrations of dissolved Fe(II) and Mn(II) (∼100–200, but up to ∼800μM). Sediments are reactive, with surficial ΣCO2 production ∼0.1–0.3mMd−1 and benthic O2 fluxes ∼23±15mmolm−2d−1 (upper ∼20cm). The highest rates occur within inner topset deposits (∼10–20m) and near the high accumulation rollover region of the topset–foreset beds (∼40–50m). Lower rates are found inshore along intertidal channels—mangrove fringe and within scoured or exposed consolidated deposits of the middle topset region. Remineralization rate patterns are independent of relative dominance by terrestrial or marine carbon in sediments. Dissolved O2 usually penetrates ∼2–5mm into surface sediments when macrofaunal burrows are absent. More than 75% of the highly reactive sedimentary Fe(III) pool (∼350–400μmolg−1) is typically diagenetically reduced in the upper ∼0.5m. Pore water SO42- can be measureably depleted at depths >0.5m, but dissolved H2S generally remains below detection over the upper ∼1–2m. As in other deltaic topset regions, SO42- concentration gradients often indicate that compared to many marine deposits of similar sediment accumulation rates, relatively refractory Corg is supplied to the SO4 reducing zone. Sedimentary C/S ratios are ∼4–6 within the suboxic topset regions but decrease to <3 in offshore foreset beds where sulfidic diagenesis dominates. Only ∼15–20% of the diagenetically reduced Fe(II) is pyritic and a maximum of ∼10–25% is carbonate, implying that most Fe(II) is associated with authigenic or lithogenic silicates or oxides. The dominance of suboxic, nonsulfidic diagenetic processes reflect coupling between delivery of oxide-rich terrestrial debris, remobilization and reoxidation of deposits, and repetitive entrainment/remineralization of both labile and refractory organics. Distinct sedimentary indicators of reactive, suboxic mobile mud belts within tropical climatic zones are: abundant total highly reactive Fe (ΣFeR )>300μmolg−1; most reactive Fe is diagenetically reduced (ΣFe(II)/ΣFeR∼0.7–0.8); the proportion of diagenetically reduced Fe present as pyrite is low (Py–Fe(II)<0.2); C/S ∼4–8; and Corg/particle surface area <0.4 (mgCm−2). These depositional environments must be most common in tropical climates during high sea stand.

Production, preservation and prediction of source-rock facies in deltaic settings

This article hypothesizes that production and preservation of source-rock type facies in deltaic systems, both landward and seaward of a coastline, is mutually exclusive, time-successive and related to the dynamics of relative sea level. Sedimentologic research in the Holocene Mississippi and Rhône delta complexes suggests that rheotrophic peats and associated organic-rich beds preferably accumulate in the accommodation space created behind landward stepping shorelines in a transgressive systems tract (TST). Such a setting also allows for a sufficient supply of recharging fresh nutrient-rich groundwater into the peat forming mires [Kosters, E.C., Suter, J.R., 1993. Facies relationships and systems tracts in the late Holocene Mississippi Delta Plain. Journal of Sedimentary Petrology 63 (4) 727–733.]. Independently carried-out quantitative paleoecological studies in the same delta systems (and in addition in the Orinoco and Po deltas) suggest that in a progradational setting (highstand systems tract/HST), seasonally discharged nutrient- and sediment-laden river waters on the shelf may give rise to anoxia or dysoxia. Subsequent overfertilization of the shelf leads to accumulation of organic-rich mud belts on the shelf [VanderZwaan, G.J., Jorissen, F.J., 1991. Biofacial patterns in river-induced anoxia. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Continental Shelf Anoxia. Geological Society of London, Special Publication no. 58, pp. 65–82.]. Thus, production and preservation of source-rock type facies landward of a shoreline (as peats and related sediments) is preferred in a TST, when accommodation space and nutrient supply are landward of the shoreline. Vice versa, production and preservation of such facies seaward of a shoreline (as organic-rich shelf muds) occurs preferentially in a highstand systems tract. In that situation, accommodation space is on the shelf, where river-fed nutrients are supplied as well. This hypothesis suggests further potential for application of sequence stratigraphic concepts for improved understanding of the occurrence of source-rock type facies.