Tidal resuspension and deposition of particulate matter in the Oyster Grounds, North Sea

Moored current meters, fluorom eters and transmissometers were used in combination with sediment traps (aspect ratio \u3e4) and shipborne sampling to determine fluxes of deposition and resuspension of total suspended matter (TSM) under tidal action in the 45 m deep Oyster Grounds, North Sea. Here, we present data from the mixed layer below the major thermocline at about 20 m above the bottom (mab) as obtained during a 14-day period of calm weather in July 1994. Around neap tide near-bottom current velocities remained smaller than 0.15 m s-1 and TSM was dominated by particles advected from a relatively turbid area to the southeast of the study site. At the onset of spring tide, current speeds increased with maximum values greater than 0.20 m s-1 and seabed friction velocities exceeding the threshold value for resuspension. Particles resuspended were strongly enriched with organic carbon compared to the bulk sediment, suggesting that not the bed proper but a fine-grained fluff fraction was eroded. This resuspended fluff was by far the dominant source for the mass fluxes in the sediment trap (at 3.2 mab), which showed a distinct tidal cycle with highest fluxes directly after low water slack tide and lowest fluxes during maximum ebb current. This pattern was caused by variations in apparent settling velocity of TSM, presumably due to floc formation during periods in the tidal cycle when current speeds were low and relatively high concentrations of both chlorophyll-a and TSM were found. From a simple model on advection, deposition and resuspension of TSM, we calculated a net accumulation on the sediment of 75 g m-2 during the 14-day study period, which is the difference between gross fluxes of deposition and resuspension. Upon deposition, the average retention time of particles until their next resuspension is calculated at 1-2 weeks, which may be sufficient for substantial decomposition of organic matter associated with TSM. This implies that, upon resuspension, particles transported further along the shelf are relatively poor in organic carbon. It is concluded that the Oyster Grounds serve as a mid-shelf temporary depocenter and that mineralization in this and similar areas may play a crucial role in the carbon budget of the North Sea

Absorbed Mn2+ and Mn redox cycling in Iberian continental margin sediments (northeast Atlantic Ocean)

Although Mn2+ sorption has been investigated extensively in the laboratory, the role of Mn2+ sorption in natural marine sediments remains speculative. Our objectives were to study (1) the role of Mn2+ sorption in the redox cycling of Mn, (2) to quantify Mn cycling and (3) to identify its rate-determining factors at the Iberian margin. Profiles of pore water Mn2+, adsorbed Mn2+ and solid phase Mn were measured together with benthic oxygen fluxes along three transects across the margin from the shelf to the deep sea as well as in the Nazeré Canyon. In the profiles, peaks of adsorbed Mn2+ were observed in-between those of solid phase Mn and pore water Mn2+. We propose that upon Mn reduction, the produced Mn2+ is adsorbed onto adjacent Mn oxide or oxyhydroxide surfaces. Available adsorption-sites diminish and/or saturate as Mn reduction continues, upon which Mn2+ is released into the pore water. Mn redox chemistry is controlled by the organic carbon flux to the sediment. A simple steady state model was formulated that includes Mn2+ sorption as a combination of an instantaneous reversible equilibrium process and a first-order kinetic reaction. Model derived, depth integrated rates of Mn reduction as well as Mn2+ desorption and oxidation rates range between 1 and 35 µmoles m-2 d-1. Mn cycling is most intense at moderate carbon fluxes. Moreover, Mn cycling is enhanced at higher deposition fluxes of Mn oxide in the canyon. Budgets based on the model indicate that adsorbed Mn2+ is an important redox intermediate between Mn oxide and pore water Mn2+ in the reduced sediment layer. Adsorption of Mn2+ restrains the efflux of dissolved Mn2+ into the water column, by lowering the pore water gradient at stations with a thin oxidation zone. There, adsorbed Mn2+ enhances the retention of Mn2+ in the sediment column

A key role for iron-bound phosphorus in authigenic apatite formation in North Atlantic continental platform sediments

A combination of pore water and solid phase analysis was used to determine whether authigenic carbonate fluorapatite (CFA) is currently forming in the sediment at two locations (OMEX I and II) on the North Atlantic continental platform Goban Spur (southwest of Ireland). Results of selective P extractions suggest that an early diagenetic redistribution of Fe-bound P to an authigenic P phase may be occurring at both stations. A steady-state diagenetic model describing the depth profiles of pore water HPO42− and three solid phase forms of P (organic P, Fe-bound P and authigenic P) was developed and applied to the data of station OMEX-I. The model results indicate that CFA formation can account for the observed increase of authigenic P with depth at this station. Furthermore, the results show that an intense cycling of P between Fe-bound P and pore water HPO42− at the redox interface can create conditions beneficial for CFA formation. This internal P cycle is driven by downward, bioturbational transport of mainly in-situ-formed Fe-bound P into the reduced sediment zone. Losses from the internal P cycle due to CFA formation and HPO42− diffusion are compensated for by sorption of HPO42− released from organic matter to Fe oxides in the oxidized surface sediment. Fe-bound P thus acts as an intermediate between organic P and CFA. CFA can account for between 25 and 70% of the total burial flux of reactive P at station OMEX-I and thus may act as an important sink for P in this low sedimentation, continental margin environment

Early diagenesis of silica in sandy North sea sediments: quantification of the solid phase

The suitability of selected alkaline leaching techniques for the quantification of reactive solid phase silica in sandy North Sea sediments was assessed. A single leach in a 2 M Na2CO3 solution for 5 h at 85°C resulted in an overestimation of the biogenic silica fraction owing to an increased leachout of silica from clays. Sequential leaching in 0.1 M Na2CO3 at 85°C for 5 h yielded two to ten times lower contents. Levels down to 3 μmol g-1 dry sediment were determined. The obtained contents ranged from below 10 μmol g-1 for the erosion dominated stations of the Dogger Bank proper to contents exceeding 20 μmol g-1 for locations north and south of the Dogger Bank. Dissolution of natural sediments in silica-poor seawater allowed for a characterization of the most labile pool of silica. Marked differences between the individual stations in readily soluble silica contents were only observed in the upper layers. The stations at the border of the area had maximal contents exceeding 1 μmol g-1, as compared with below 0.2 μmol g-1 for the others. The consistency between the results of the dissolution experiments, pore-water data and exchange fluxes indicates a control of the pore-water silica concentration by the availability of reactive solid silica in the sediments studied. © 1993.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Manganese oxide reactivity in North Sea sediments

Bulk Mn oxide dissolution rates were determined in dilute acid (HCl, pH 3, proton-assisted dissolution) and in saturating ascorbic acid (pH 3, proton-assisted plus reductive dissolution) for North Sea sediments from three sites characterised by different energy regimes, one at the Frisian Front, one in the German Bight, and one in the Skagerrak. Profiles of extractable manganese (0.1 N hydrochloric acid, 1 N hydrochloric acid, citrate-dithionite-bicarbonate buffer and hydroxylamine hydrochloride in acetic acid) are presented for the three sites. The assemblage of sedimentary Mn oxides was described as a reactive continuum with a gamma distribution of Mn oxide reactivities. Initial reduction rates were highest for the oxidised sediment samples. The reactivity of the Mn assemblages during reductive dissolution in ascorbic acid decreased by up to ∼9 orders of magnitude. The largest heterogeneity in Mn reactivity of the oxidised sediment samples was found at the German Bight site, while the most homogeneous Mn assemblage was found at the Skagerrak site. The large range in sedimentary Mn oxide reactivity may explain the great variability in values for kinetic coefficients used to describe Mn reduction. Considering the broad spectrum of Mn oxide reactivities, overlap of the Mn reduction zone with that of Fe oxides is expected. © 2004 Elsevier B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Kinetics of silica sorption on North Sea sediments

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Spatial and temporal variability of benthic silica fluxes in the southeastern North Sea

Sediment-water exchange fluxes and pore water profiles of Si(OH)4 were determined in August 1991 and February 1992 for a total of 16 stations located along the northeastward transport path of organic matter in the North Sea. The shape of Si(OH)4 profiles indicated that at several stations mass transport is controlled by turbulent diffusion induced by wave and current mixing in the upper, perturbated centimetres of the sedimentary column. The spatial distribution of silica effluxes clearly reflected the depositional environment, with highest exchange rates linked to areas of recent deposition. Outside the main deposition areas, transient deposition of fresh planktonic material is a key process in explaining observed silica effluxes. The temporal variability of silica effluxes followed the annual cycle of pelagic primary production. During August 1991, measured fluxes ranged from 0.18 to 8.90 mmoles Si m-2 day-1. Fluxes obtained during February 1992 were decreased by a factor between 2 and 8. Fluxes measured before and after inactivation of fauna with N2-flushing permitted an estimation of the bioirrigation to be made. The latter accounted for an enhancement of solute exchange ranging from 1.1 to 3.4. © 1995 Elsevier Science Ltd.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Visualization, participation and rhetoric: The discursive power of landscape design representations in participatory processes

Visual landscape design representations facilitate communication and knowledge exchange during participatory planning and design processes. The production of representations is considered to be a discursive act: actors and institutions construct knowledge with a certain authority and credibility through the use of visual expression. We aim to study the context in which the production of representations is embedded and how this context manifests itself in the communicative qualities of design representations. We present a visual discourse analysis of landscape design representations, employing empirical examples from the transdisciplinary design competition Rebuild by Design. The analysis uncovers interdependencies among three components of the visual discourse: the arrangement of participatory processes, media interactivity and the visual rhetoric embedded in the composition and style of the image. A conscious use of these discursive components could help prevent miscommunication, manage participant expectations and increase the validity of participatory design process outcomes.</p

A Mössbauer spectroscopic study of the iron redox transition in eastern Mediterranean sediments

Fe cycling at two sites in the Mediterranean Sea (southwest of Rhodes and in the North Aegean) has been studied, combining the pore water determination of nutrients, manganese, and iron, citrate-bicarbonate-dithionite (CDB) and total sediment extractions, X-ray diffraction, and 57Fe Mössbauer spectroscopy (MBS). At the Rhodes site, double peaks in the CDB-extractable Mn and Fe profiles indicate non-steady-state diagenesis. The crystalline iron oxide hematite, identified at both sites by room temperature (RT) MBS, appears to contribute little to the overall Fe reduction. MBS at liquid helium temperature (LHT) revealed that the reactive sedimentary Fe oxide phase was nanophase goethite, not ferrihydrite as is usually assumed. The pore water data at both sites indicates that upon reductive dissolution of nanophase goethite, the upward diffusing dissolved Fe2+ is oxidized by Mn oxides, rather than by nitrate or oxygen. The observed oxidation of Fe2+ by Mn oxides may be more common than previously thought but not obvious in sediments where the nitrate penetration depth coincides with the Mn oxide peak. At the Rhodes site, the solid-phase Fe(II) increase occurred at a shallower depth than the accumulation of dissolved Fe2+ in the pore water. The deeper relict Mn oxide peak acts as an oxidation barrier for the upward diffusing dissolved Fe2+, thereby keeping the pore water Fe2+ at depth. At the North Aegean site, the solid-phase Fe(II) increase occurs at approximately the same depth as the increase in dissolved Fe2+ in the pore water. Overall, the use of RT and cryogenic MBS provided insight into the solid-phase Fe(II) gradient and allowed identification of the sedimentary Fe oxides: hematite, maghemite, and nanophase goethite. Copyright © 2005 Elsevier Ltd.SCOPUS: ar.jinfo:eu-repo/semantics/publishe