Riverine Sediment Input Research Articles

Fate of metals in coastal sediments of a Mediterranean flood-dominated system: An approach based on total and labile fractions

The dynamics of sediment-bound metals (Cs, Cu, Ni, Pb, Ti and Zn) were studied off the Têt River (western Gulf of Lion), a typical Mediterranean coastal river punctuated by short and violent flash-floods. Spatial and temporal sampling strategies were combined to elucidate the fate of these elements in response to both the riverine sediment input and the offshore transport of these sediments through hydrodynamics. Our results show the temporal entrapment of riverborne particles and associated metals, consecutively to a major flood event, in the nearshore sedimentary unit called prodelta. Here, deposition and resuspension mechanisms define a sedimentological cycle that could be followed completely in this study. In terms of speciation between reactive (labile) and residual fractions along the fluvio-deltaic continuum, our results show that Cu, Pb and Zn are the most labile (potentially mobile) metals in the river, in accordance with their contributions from anthropogenic sources. But in the marine surficial sediments, two main behaviours can be discriminated when compared to the riverine suspended particulate matter. While Pb and Zn depict rather a constant labile fraction, Cu is characterized by decreasing levels (up to 50% difference). In terms of environmental impact, these contrasting trends have direct repercussions for the contaminant dispersal in the coastal area. Whereas Pb and Zn conserve their enhanced levels because of their stronger affinity with fine sediments, Cu is marked by the entire loss of its anthropogenic component that is progressively transferred to the dissolved phase, likely mediated by organic ligands. We ascribe these behaviours to different post-depositional partition mechanisms with respect to oxidation of the particulate organic phase at the bottom sediment/water interface. Also, analysis of one sediment core from the prodelta indicates that these early diagenetic processes govern the chemical forms of land-derived contaminants sequestrated in the nearshore sedimentary archives.

Sediment transport pathways in a dredged ria system, southwest England

The Fowey Ria system, southwest England, comprises the River Fowey catchment, the Fowey estuary, the cliffs and bays adjacent to the ria mouth, and part of the inner continental shelf of the English Channel. Previously, large quantities of sediment were introduced into the upper ria by ore mining activity. Today, in common with other rias, the Fowey receives a low riverine sediment input. Material from maintenance dredging in the lower ria is dumped in a spoil ground outside the ria mouth. The sediments of the system are investigated using an integrated approach to determine sediment distribution and sediment transport pathways. Surface sediments are analysed for grain size and mineralogy. Grain size trend analysis is used to examine sediment dispersal patterns away from the locus of deposition in the spoil ground. Archived data are used to investigate the seabed morphology and to determine long-term (100 year) bathymetric changes. Within the ria, mixing of sediment from several sources occurs. In the upper reaches, riverine and locally-eroded sediment is transported seawards towards the main area of commercial activity. Sand and finer-grained material moves into the ria from offshore. The bed of the inner continental shelf comprises interfluves covered by a thin veneer of sediment, with a natural composition of locally-derived lithic fragments and biogenic material. The area is mainly low/non-depositional in character, except within the partially-infilled palaeovalley and its tributaries. Sediments dumped in the spoil ground disperse in a complex pattern: coarse-grained material is moved by the action of waves and tidal currents towards the southwest and northeast; fine-grained material is transported either to the east or the west, depending upon the prevailing wave and tidal current regime. Because of its geomorphology, the lower ria acts as an efficient sediment trap, retaining (a) riverine material and sediment eroded from the upper reaches; and (b) sediment entering the ria from offshore. Despite being subjected to major anthropogenic disturbance from past mining and present-day dredging activities, the Fowey Ria conforms to the general sediment model for southwest England rias [Castaing, P., Guilcher, A., 1995. Geomorphology and sedimentology of rias. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries – Developments in Sedimentology, No. 53. Elsevier Science, Amsterdam, pp. 69–111]. A conceptual model of sediment transport pathways for the Fowey Ria system is presented as the basis for further investigations.

The Effect of Sea-Level and Climate Change on the Development of a Mixed Siliciclastic-Carbonate, Deltaic Coastline: Suwannee River, Florida, U.S.A.

Abstract Florida's mixed siliciclastic–carbonate, northwest-central Gulf of Mexico coastline is characterized by a 300-km-long, low-energy, sediment-starved, open-marine marsh system fronted by a broad, shallow limestone shelf. The only significant point source of sediment to this system is the Suwannee River, which forms a small (20 km2) delta at the center of this coastline. Late Pleistocene eolian dune formation along the Suwannee River coastline reflects slightly drier conditions and local sediment sources influencing the early geologic development of this region. As sea level rose in the early to middle Holocene, dunes on the low-gradient shelf were transgressed and provided the core for the modern offshore sandy shoals. With decelerating sea-level rise in the middle to late Holocene and probable increased riverine sediment input, coastal sedimentation switched from transgressive to aggradational. Paleo-distributary channels seaward of the delta coastline at approximately 4,000 cal yr BP indicate initial deltaic formation. At this same time, oyster reefs from the delta southward were able to keep pace with sea-level rise and began to form large (< 20 km2) offshore oyster bioherms. Away from the river mouth, the coastline continued to transgress until 2,350 cal yr BP to 1350 cal yr BP, when the shoreline stabilized and the modern marsh system, south and north of the river mouth respectively, began to aggrade. The geologic development of this coastline and the relict eolian dunes preserved within the modern marsh system indicates that middle to late Holocene sea level did not exceed current elevations along this portion of the Gulf of Mexico coastline. This finding agrees with other studies from the low-gradient west-central Florida coast but contrasts with evidence for middle to late Holocene highstands along other Gulf of Mexico shorelines.

Geochronology of Lake Baikal from 210Pb and 137Cs radioisotopes

Two box cores of near surface sediments were obtained from Lake Baikal in Southeastern Siberia, Russia. The cores were taken from the northern and southern basins of the lake during a joint American–Russian research expedition in the summer of 1994. The cores were analyzed for 210Pb, 137Cs and total organic carbon (TOC). Organic carbon is an indicator of photosynthetic production by phytoplankton, taking place primarily in the euphotic zone of the water column. Accumulation rates of TOC may be used as indicators of paleo-productivity when sedimentation rates are determined using the 210Pb dating method and combined with both the density of sediment and organic carbon content. Accordingly, the lake is characterized by changes in accumulations of TOC, which may be linked to rates of sedimentation. Accumulations of TOC and sedimentation rates were higher in the southern basin site than in the northern basin site. The southern station core was taken from an area in close proximity to the Selenga River delta, which carries 50% of the water input to Lake Baikal. Productivity should thus be higher in this region due to the high nutrient input and sediment accumulation higher due to influx of riverine sediment input. Traces of 137Cs (an anthropogenic product) were found in both cores. However, activities of 137Cs were significantly higher in the southern basin, likely due to the input of the Selenga River in the southern region, which extends to a region in Mongolia in close proximity to the area of the Chinese atomic-bomb atmospheric testing of the 1970s. Application of a quantitative inverse model to the 210Pb profiles yielded the following results: (i) station 12, near the Selenga Delta, had an accumulation rate of about 0.38 cm/y in 1957 but this rate was halved by 1980 to about 0.22 cm/y and has been roughly steady since that time; 137Cs values are consistent with the age-to-depth determination from 210Pb for station 12; (ii) since about 1960, station 5A in the northern basin had an accumulation rate lower by a factor 2–4 than that of the station near the Selenga Delta; the 137Cs values are consistent with the 210Pb age-to-depth determination for station 5A; (iii) the 137Cs activities for station 12 systematically increase with time towards the present day and are about a factor 6–10 higher than 137Cs activities recorded for station 5A, which do not show a corresponding systematic increase with time.

Variations in sedimentary environments and accretionary patterns in estuarine marshes undergoing rapid submergence, Chesapeake Bay

Major processes controlling sedimentological characteristics and accretionary rates and patterns were evaluated in two of the most common tidal marsh settings (estuarine tributaries and estuarine embayments) in Chesapeake Bay, one of the largest microtidal estuaries in the world. Textural analyses of 29 vibracores show spatial and temporal trends in grain size and organic matter which reflect differences in depositional settings within the marshes and changes in the controlling processes, as well as broader events in the surrounding watersheds. Spatially, the surficial sediments normally follow expected patterns, with coarsest sediments and lowest organic matter contents found in bay margin or channel margin locations, while the finest, organic-rich sediments occur in interior and submerged upland marshes. However, temporally, the marshes do not always show the expected increasing dominance of organic input over time (indicated by increasing % loss on ignition with decreasing depth), indicative of the marsh building process. Sea-level rise, tidal channel migrations, and anthropogenic effects likely contribute to decreases in organic matter contents in the upper sediment column. Furthermore, accretion rates and stratigraphic characteristics can be affected by these same processes, illustrating the importance of understanding temporal changes in marsh depositional environments over various time scales before sedimentologic and accretionary patterns are evaluated. Examination of the vibracores reveals that the marsh is composed of a complex stratigraphy with at least four major stratigraphic sequences occurring: (1) emerging or developing marsh sequences characteristic of channel margin and interior marshes; (2) submerging or mineral matter enriched marshes, also characteristic of channel and interior marsh areas; (3) high-energy marsh sequences characteristic of bay margin environments; and (4) submerged upland marsh sequences. The sedimentological properties of the stratigraphic sequences are controlled by overwash processes, tidal channel migrations, duration of tidal flooding, various anthropogenic effects, and sea-level rise. Accretion rates and patterns over the last ∼200 years, determined from pollen histories, differ between the marshes along the estuarine tributary (Nanticoke River) and those found in the tidal embayment (Monie Bay). Marshes in the upper-estuarine tributary have a larger riverine sediment input, and trapping of the high sediment loads in the upper estuary often yields marsh accretion rates (maximum of 0.74 cm/yr) which are greater than relative sea-level rise (∼0.4 cm/yr), resulting in stable marshes. By comparison, estuarine processes limit penetration of high sediment loads to the lower reaches of marshes along the tributary and, as a consequence, accretion rates are generally lower than the rate of relative sea-level rise, and marsh loss is prevalent. Overall, accretion rates in the marshes in the embayment are close to or less than the local sea-level rise and do not have as distinctive spatial patterns as the tributary marshes. Although marsh loss has yet to be identified as a significant process here, decreases in organic content of the upper sediment column may signal the early stages of submergence due to sea-level rise.

Surface area control of organic carbon accumulation in continental shelf sediments

The relationship between organic carbon (OC) and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments. Cores from many North American shelf environments show downcore decreases in OC to similar refractory background concentrations if expressed relative to the surface area of the sediments. This consistent concentration is 0.86 mg-OC m −2, which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces. A more global collection of sediment-water interface samples show that this relationship is even more extensive, with exceptions occurring in areas of very high riverine sediment input, organic pollution, or low-oxygen water columns. Density separations indicate that organic matter is largely adsorbed to mineral grains. The microtopography of surfaces was examined with N 2 sorption and most surface area was found to be inside pores of <10 nm width. These data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay. Such protection would likely work in concert with other protection mechanisms such as humification. This consistent surface area correlation with OC concentration may explain control of spatial and temporal variations in OC burial rates by sedimentation rates; the pore protection hypothesis provides a causal mechanism for this observed control.

Landward Migration of Isolated Shelly Sand Ridge (Chenier) on the Macrotidal Flat of Gomso Bay, West Coast of Korea: Controls of Storms and Typhoon

ABSTRACT A shelly sand ridge, 860 m long, 30-60 m wide, and up to 1.6 m high, parallels the shoreline on a high tidal mudflat in Gomso Bay, which opens to the eastern Yellow Sea. The ridge is composed entirely of fine to medium sand and shell material, with some subangular gravel, and overlies tidal mud discordantly. The lithostratigraphy of the bay deposits and 14C dates of shells from vibracores suggest that the shelly sand ridge, together with mid to lower sand flats, has accumulated on the tidal mud since 1800 yr B.P. This geologically recent development suggests that ridge evolution has been independent of regional sea-level change. River influence also seems to be negligible because of the apparently meager riverine sediment input into the bay. Judging from a series of aerial photographs spanning the past two decades (1967-1989), broad sand shoals on the mid to lower flat have moved landward and have become the shelly sand ridge. Vertical sections from a trench cut across the ridge also attest to its landward migration, with gently landward-dipping interbeds of sand and shell. Repeated measurements of the morphological change of the ridge over a two-year period (1990-1992) indicate that net transport was landward, on the order of 8 m/yr, except for during the typhoon season. The migration rate in the stormy winter monsoon season was generally two or three times greater than that of the remaining season. However, in the summer of 1992, a typhoon entering the Yellow Sea vigorously shifted the ridge landward, causing a displace ent of up to 11 m in a few days. This suggests that strong waves associated with storms or typhoons have predominantly driven the shelly sand ridge and offshore sands onto the tidal flat.

Synergism of Riverine and Winter Storm-Related Sediment Transport Processes in Louisiana's Coastal Wetlands: ABSTRACT

ABSTRACT The roles of various mechanisms that supply sediments from major sources, including rivers and nearshore shelf, to coastal Louisiana are not well understood or quantified, temporally or spatially. Recent studies reveal that an important association between riverine sediment input and the cyclic passage of winter storms results in a periodic supply of suspended sediments to coastal marshlands. The fact that these two mechanisms coincide in winter and early spring maximizes the availability of particulate matter for counteracting coastal land loss by enhancing substrate accretion and plant productivity. Identification of these optimum periods of suspended sediment activity, when overbank sedimentation, riverine concentrations, flood processes, and cold front passages are synergistic, can maximize the performance of wetland restoration efforts, because mineral matter and nutrients are critical for marsh vitality. When mitigation of coastal land loss is planned, it would be advantageous for weekend restoration to consider this timing when both sediment availability and constructive processes of sediment transport are optimal.