Controlling Sediment Deposition Research Articles

Flow and Sediment Transport on a Tidal Salt Marsh Surface

The physical processes that control mineral sediment deposition on a mesotidal salt marsh surface on the Atlantic Coast of Virginia were characterized through a series of measurements of sediment concentration, flow velocity, turbulence, water surface elevation, marsh topography and particle size distributions of sediment deposited on the marsh surface. The comprehensive nature of the data set allowed assessment of the temporal and spatial variability in marsh surface deposition, the variability in depositional processes among tides of different amplitudes, as well as the specific processes that control deposition on this tidal marsh. Through three different types of measurements, it was found that sediment deposition occurred on the marsh surface during rising tides at tidal elevations ranging from those barely flooding the creek bank to high spring tides, and that sediment was not remobilized by tidal flows after initial deposition. Sediment deposition occurred on this marsh surface largely because fine sediment in suspension formed flocs. Analysis of inorganic grain size distributions of sediment deposited within 8m of the tidal creek indicated that 70–80% of this sediment was deposited in a flocculated form. The rest (particles larger than 20μm) were deposited as individual particles. In the marsh interior, 25m from the tidal creek, single grain settling predominated. Reduction of turbulence levels within the vegetation canopy on the marsh also promoted particle settling. The processes controlling sediment deposition did not vary among tides. However, suspended sediment concentrations near the creek bank increased with increasing tidal amplitude, consequently promoting higher rates of deposition on higher tides.

Current controlled sediment deposition from the shelf to the deep ocean: the Cenozoic evolution of circulation through the SW Pacific gateway

The circulation of cold, deep water is one of the controlling factors of the Earth’s climate. Forty percent of this water enters the world ocean through the Southwest Pacific as a deep western boundary current (DWBC) flowing northwards at bathyal to abyssal depths, east of the New Zealand microcontinent. South of latitude 50°S, the DWBC is intimately linked with the Antarctic circumpolar current (ACC), which is the prominent force for the shallow-water circulation. The Pacific DWBC is presently the largest single contributor of deep ocean water, and deciphering its evolution is of fundamental importance to understanding ocean and climate history, and global ocean hydrography. The evolution of the DWBC system, and of related circum-Antarctic currents, has taken place since 30–25 Ma when plate movements created the first oceanic gaps south of Australia and South America. The stratigraphic record preserved in sediment drifts of the Southwest Pacific, in eastern New Zealand, is the best available for deciphering the Neogene history of Southern Ocean water masses, and of the circulation of the ACC, DWBC and their precursor systems. Major current activity commenced on the New Zealand margin in the late Eocene or early Oligocene (Hoiho Drift; early ACC) and was widespread by the mid-late Oligocene (Marshall Paraconformity and Weka Pass Limestone drift; ACC). During the Neogene the eastern South Island continental shelf built seawards by accretion at its outer edge of large Miocene current drifts up to tens of kilometres long and hundreds of metres thick (Canterbury drifts). Also commencing in the mid-Cenozoic, but in depths >2000 m, the DWBC emplaced large deep-water sediment drifts. Rates of drift deposition accelerated considerably in the late Neogene, when climatic change (and particularly glacial sea-level falls) caused the delivery of large volumes of turbiditic sediment into the path of the DWBC via the Bounty and Hikurangi channels.

Current controlled sediment deposition from the shelf to the deep ocean: the cenozoic evolution of circulation through the SW pacific gateway

Current controlled sediment deposition from the shelf to the deep ocean: the cenozoic evolution of circulation through the SW pacific gateway

The chemistry of lake sediments in time and space

Five short cores (1.0–1.5 m) representing different depositional zones of an isolated bay of Lake Minnetonka, Minnesota (USA), were independently dated by 210Pb and pollen analysis and were analyzed stratigraphically for elemental chemistry (following sediment fractionation) and sedimentary pigments (including myxoxanthophyll and oscillaxanthin). Because of good dating control, short-interval timestratigraphic units could be traced across the basin, and lake-wide accumulation rates and concentrations could be estimated. The accumulation and concentration of each component at a core-site relative to basinwide averages provide new indices, ‘relative accumulation’ and ‘relative concentration’, that are used to evaluate three major processes controlling sediment deposition: (1) the basin-wide flux, (2) the physical redistribution of bulk sediment, and (3) the selective bio-geochemical transport and fossilization of individual sedimentary components within the lake.

On isostasy at Atlantic‐type continental margins

Cross‐spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern North America, southwest Africa, and the Coral Sea/Lord Howe Rise. The resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy. The relatively young Coral Sea/Lord Howe rise margin is associated with the lowest value of the effective elastic thickness (Te < 5 km), while the relatively old eastern North American margin is associated with the highest value (10 < Te < 20 km). Thus Te appears to increase with the age of the margin. These results, which are consistent with flexure studies from the major ocean basins, have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age. The model suggests that a low‐rigidity plate (or Airy‐type model) is most applicable early in rifting history, while a high‐rigidity plate is more applicable later in margin evolution. Thus sediments at a margin initially load a relatively thin and hot plate, while sediments later in margin evolution load a relatively thick and cold plate. The model explains a number of features of the tectonic‐stratigraphic evolution of margins. These include a change in flexural style from a narrow and deep basin to a broad and shallow basin, coastal plain onlap, and uplift in the continental interior. Furthermore, the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins. In particular the model explains the existance of (Airy) isostatic gravity anomalies at margins. The amplitude and wavelength of these anomalies are a function of the sediment supply, the type of mechanisms controlling sediment deposition (up building and/or out building), and the thermal structure, history, and age of the margin.