Estuarine sediments structure

Pigments bound to structural compounds such as lignins or surface waxes in higher plants have decay rate constants slower than those of similar pigments from nonvascular sources (Webster and Benfield, 1986 Bianchi and Findlay, 1990). Other work has shown the importance of pigment decay in the free versus bound state in estuarine sediments (Sun et al 1994). 

To overcome the bias resulting from uneven dispersal of tracer or inhibitor, sediment rate measurements are often made in slurries, which destroy the gradient structure of sediments, which is essential to the in situ fluxes. Slurries may provide useful information on potential rates, but not in situ rates. Potential nitrification rates and rates measured in intact cores were not correlated in estuarine sediments (Caffrey et ah, 2003). The lack of correlation was explained by the inclusion of variable amounts of anoxic sediments in the slurries from which the potential rates were derived. 

Structural Interrelationships among Humic Substances in Marine and Estuarine Sediments... 

Fulvic acids. Marine sedimentary humic substances soluble in base and acid (fulvic acids) have previously been examined by and NMR (12). The dominant structural components were postulated to be polysaccharide - like substances, probably polyuronic acids. Solid-state NMR spectra of fulvic acids isolated from a number of marine and estuarine sediments are shown in Figure 1. Major peaks at 72 and 106 ppm betray the overwhelming presence of polysaccharide -like substances, and, as shown by Hatcher and others (12.), the moderate peak for carboxyl or amide carbon at 175 ppm suggests that these polysaccharides are more like polyuronides. Aromatic carbons (110 to 160 ppm) are decidedly minor components. Aliphatic carbons (0-50 ppm) are also minor components. H NMR spectra shown by Hatcher and others (12) indicate that these aliphatic structures are highly branched. 

Humic acids of marine and estuarine sediments are characterized by major amounts of paraffinic structures that previous studies have shown to be highly branched and to contain significant quantities of carboxyl/amide and alcohol/ether carbon. Some humic acids, namely those from well preserved sapropelic marine sediments show significant quantities of carbohydrate-like structures incorporated. This, no doubt, is a reflection of the solubility characteristics of polysaccharides which may have some carboxyl functionalities (uronic acid groups). 

Sediments are currently analysed in environmental monitoring since they provide the history of contaminations in their stratigraphic structure, they are necessary for calculations of the total load of a river as most of the polluting inorganic or organic material are adsorbed to the solid particles and, prior to a decision on the possible deposition of a sediment, the contents have to be determined to estimate the effect of disposal on groundwater and (later) plant life. For this latter purpose very often the aqua regia soluble heavy metal contents are used instead of the total contents. To respond to the needs of sediment CRMs, three materials have been prepared, namely estuarine sediment (CRM 277), lake sediment (CRM 280) and river sediment (CRM 320). 

The high fluvial nutrient supply to the estuaries supports high organic productivity, but the brackish waters, whose salinity frequently fluctuates, restrict the number of species. Consequently, estuarine sediments contain fairly large quantities of organic matter, and their structures are often completely or partly obliterated by burrowing animals and rootlets. 

Rohrlich, et al.. 1969 Gaertner and Schellmann, 1965 Leclalre, 1968 Giresse and Odin, 1973) at fairly shallow depths (< 80 meters). These reactions take place under saline or estuarine conditions. The transformation of sediment into berthierine is apparently progressive the initial sedimentary material found in shell tests becomes gradually transformed into a single phase, 7 chlorite structure. 

Estuarine A term used to describe structures, organisms, sediments, and other features associated with estuaries. 

The content of aromatic carbon varies considerably in marine and estuarine humic acids, but is, in all cases, greater than that of Mangrove Lake humic acids. This is probably a reflection of the greater contribution of vascular plant-derived material which can be expected to provide lignin-like components rich in aromatic structures. Note that the humic acids from fluvial sediments of the Potomac River are the most aromatic. Peaks at 150 and 55 ppm are characteristic of oxygen-substituted aromatic carbons typically associated with lignin of vascular plants. Humic acids from New York... 

Humin varies widely in composition. Sediments derived from algal/microbial biomass have humin with paraffinic structures resembling those of corresponding humic acids. Estuarine or coastal marine sediments examined in this study have humin with highly aromatic structures which resemble coal-like materials rather than modern plant residues. In these latter sediments no structural correspondence exists between humin and humic acids which appear to more nearly reflect the nature of modern plant... 

SO that the variation of sand content may be represented by a single east-west section. The mean water depth decreases to the west so that the muddy floor is at a higher elevation than the sand to the east. This is the reverse of the usual estuarine structure where a shallow sandy margin surrounds a deeper mud bottom. The sedimentation rate is taken to be 1 mm/yr in the study area. This corresponds to a value of g equal to 3 X 10" sec... 

We are still a long way from establishing the kind of general quantitative theory of the estuarine variables that is necessary for the understanding and effective management of these coastal environments. The articles in this volume expose the rich variety of phenomena and interactions that will have to be included in such a theory. Although particular emphasis is placed here on the transport, physicochemical structure, and evolution of the bottom sediments, the relationship of these factors to their broader geological and hydrodynamical contexts is also considered and clarified. 

M. A. Ross, Vertical structure of estuarine fine sediment suspensions, PhD thesis, University of Florida, Gainesville (1988). 

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