Different Types Of Sediments Research Articles

The relationship between plantar pressure and footprint shape

Fossil footprints preserve the only direct evidence of the external foot morphologies and gaits of extinct hominin taxa. However, their interpretation requires an understanding of the complex interaction among foot anatomy, foot function, and soft sediment mechanics. We applied an experimental approach aimed at understanding how one measure of foot function, the distribution of plantar pressure, influences footprint topography.Thirty-eight habitually unshod and minimally shod Daasanach individuals (19 male, 19 female) walked across a pressure pad and produced footprints in sediment directly excavated from the geological layer that preserves 1.5 Ma fossil footprints at Ileret, Kenya. Calibrated pressure data were collected and three-dimensional models of all footprints were produced using photogrammetry. We found significant correlations (Spearman's rank, p < 0.0001) between measurements of plantar pressure distribution and relative footprint depths at ten anatomical regions across the foot. Furthermore, plantar pressure distributions followed a pattern similar to footprint topography, with areas of higher pressure tending to leave deeper impressions. This differs from the results of experimental studies performed in different types of sediment, supporting the hypothesis that sediment type influences the relationship between plantar pressure and footprint topography. Our results also lend support to previous interpretations that the shapes of the Ileret footprints preserve evidence of a medial transfer of plantar pressure during late stance phase, as seen in modern humans. However, the weakness of the correlations indicates that much of the variation in relative depths within footprints is not explained by pressure distributions under the foot when walking on firm ground, using the methods applied here. This warrants caution when interpreting the unique foot anatomies and foot functions of extinct hominins evidenced by their footprint structures. Further research is necessary to clarify how anatomical, functional, and sedimentary variables influence footprint formation and how each can be inferred from footprint morphology.

Influence of sieving time on the efficiency and accuracy of grain‐size analysis of beach and dune sands

AbstractGrain size is a fundamental property of sediments and is commonly used to describe sedimentary facies and classify sedimentary environments. Among the various conventional techniques utilized to determine grain‐size frequency distributions, sieving is the most widely applied procedure. The accuracy of such analyses is, among other factors, strongly dependent on the sieving time. However, despite a substantial amount of research in this field, optimal sieving times for different types of sediments have, to date, not been established. In this article, the influence of sieving time on grain‐size analyses of medium‐grained microtidal and mesotidal beach and dune sands has been determined. To assess the precision of important textural parameters, such as median grain size, sorting, skewness and kurtosis, an error analysis was carried out for different sieving times (2, 5, 10, 15 and 20 minutes). After calibrating the analytical and sampling methodologies, significant deviations were registered when sieving time was less than 10 minutes. However, such deviations were very small and grain‐size distributions remained almost identical for sieving times of 10 minutes and longer, relative errors being as low as 0% in some cases.

Degradation of rizazole in water–sediment systems

This study investigated the degradation of rizazole in water-sediment systems (West Lake system, WL; Beijing–Hangzhou Grand Canal system, BG) with two different types of sediments under aerobic and anaerobic conditions. The half-lives of rizazole in the WL water phase (14.59–15.13 d) were similar to those in the BG water phase (15.90–16.46 d). Within 3–7 d, the rizazole concentration in the sediments reached the maximum values, i.e., equilibrium. Rizazole dissipation was faster in the WL sediment phase with higher organic matter content (T1/2 = 18.99–19.09 d) compared with the BG sediment phase (T1/2 = 31.08–33.32 d). Rizazole degradation was slightly faster in the West Lake water-sediment system (WL system) (T1/2 = 17.11–18.05 d) than in the Beijing–Hangzhou Grand Canal water—sediment system (BG system) (T1/2 = 20.51–25.02 d). The aerobic degradation of rizazole was similar to its anaerobic degradation in the water-sediment system. The findings are useful to understand the behavior of pesticide in environment.

Hydraulic conditions at the source zone and their impact on plume behavior

A laboratory sandbox model (105 × 60 × 15 cm) was filled with different types of sediments to arrange different hydraulic conditions at the tracer’s injection zone. Tracer-input functions were monitored at the injection location and tracer breakthrough curves (BTCs) were measured at a downgradient detection plane. Numerical transport simulations analogous to the sandbox experiments were conducted in a numerical model parameterized with the properties of the sediments used in the sandbox. The results of the study demonstrate that the hydraulic conditions at the source zone have a major influence on plume development. Tracer injection into high-permeability zones causes early arrival, and enhanced transverse and reduced longitudinal spreading. Tracer injection into low-permeability zones causes late arrival, and reduced transverse and enhanced longitudinal spreading. It was further found that using simplified tracer-input functions in the analysis of BTCs can lead to biased transport parameters. The results suggest that transport parameter estimation based on tracer experiments can benefit from monitoring of the tracer-input function and subsequent consideration of the measured tracer-input function in the analysis of BTCs.

Seismic Microzonation of Breginjski Kot (NW Slovenia) Based on Detailed Engineering Geological Mapping

Breginjski kot is among the most endangered seismic zones in Slovenia with the seismic hazard assessed to intensity IX MSK and the design ground acceleration of 0.250 g, both for 500-year return period. The most destructive was the 1976 Friuli Mw = 6.4 earthquake which had maximum intensity VIII-IX. Since the previous microzonation of the area was based solely on the basic geological map and did not include supplementary field research, we have performed a new soil classification of the area. First, a detailed engineering geological mapping in scale 1 : 5.000 was conducted. Mapped units were described in detail and some of them interpreted anew. Stiff sites are composed of hard to medium-hard rocks which were subjected to erosion mainly evoked by glacial and postglacial age. At that time a prominent topography was formed and different types of sediments were deposited in valleys by mass flows. A distinction between sediments and weathered rocks, their exact position, and thickness are of significant importance for microzonation. On the basis of geological mapping, a soil classification was carried out according to the Medvedev method (intensity increments) and the Eurocode 8 standard (soil factors) and two microzonation maps were prepared. The bulk of the studied area is covered by soft sediments and nine out of ten settlements are situated on them. The microzonation clearly points out the dependence of damage distribution in the case of 1976 Friuli earthquake to local site effects.

The idea of simultaneous batch in-situ extraction and arsenic and antimony hydride generation atomic absorption spectrometry determination directly in the solid (tsunami and lake sediments) samples

The article presents the application of in-situ extraction from a solid sample in order to determine metalloids: arsenic and antimony. The reaction vessel, in which hydride generation followed the extraction, was connected with the atomic absorption spectrometer (AAS) in a fast sequential mode. Deionised water, phosphatic buffer (pH = 6) and hydrochloric acid (2 mol L−1) were used as the extractants to determine the concentration of metalloids in the following fractions: easily (water) leachable, exchangeable, and acid leachable. Two different types of sediments were used while developing the application: lake bottom sediments and tsunami deposits. Both types of the sediments samples (5–20 mg) were placed directly in the reaction vessel and after in-situ extraction the determination of the metalloids was conducted, what allowed to assess concentration of arsenic and antimony during single analysis. The results obtained from the analyses of both sediments types were compared with the results from traditional off-line extraction. As a result a good correspondence with both hydrochloric acid and phosphatic buffer was found. The methodology of solid samples analysis was developed with detection limits of 50 ng g−1 (for As) and 30 ng g−1 (for Sb) for 10 mg of a solid sample.

A study on Isatis suspension trap efficiency; Advantages and disadvantages

Different types of sediment trap have been introduced so far to be used in erosion measuring stations, each with its own advantages and disadvantages in trapping aeolian particles. These traps have different collection and retention efficiency due to aerodynamic conditions. Some characteristics such as isokinetic feature of the trap, collection efficiency, time needed to download sediment, capability of installation in different altitudes and capability to collect sediment from different geographical directions are the criteria used to assess the efficiency of different types of sediment traps. In the present study Isatis Suspension Trap (Isatis SUSTRA) was tested in a wind tunnel and wind erosion measuring station. This is a kind of passive trap and its sediment collecting efficiency varies between 55% to 88% for 5 to 9 m/s velocities respectively in the height of 15cm. Although the efficiency of the trap in low wind velocities is less than 55%, it is remarkable comparing with other traps. Larger mouth of the trap provides it with vaster opportunity to trap sediment. Isatis SUSTRA has the capability to segregate sediments in different geographical directions. This privilege leads to 1) possibility to analyze horizontal distribution in different geographical directions, 2) determining the most sediment producing fraction and easy recognition of the source, 3) estimating moved materials such as dissolved salts or heavy elements in mine range) through separating sediment samples and 4) easy installation in different altitudes and enough capacity to reserve aeolian sediment. Generally speaking, the results from indoor and outdoor tests indicate that Isatis SUSTRA has appropriate capabilities with regard to collecting efficiency, keeping, sediment reserve capacity and easy download in hard conditions.

The stable carbon isotopic composition of organic material in platform derived sediments: implications for reconstructing the global carbon cycle

AbstractIn pelagic carbonate sediments, the degree to which the δ13C values of inorganic and organic fractions co‐vary has been used to interpret rates of production, burial and decomposition of organic carbon. This relationship is relatively consistent through time, permitting estimates of organic carbon production and preservation. However, as the majority of pelagic sediments older than 200 Myr have been subducted, carbonate sediments deposited in epeiric seas and platforms are often substituted for pelagic carbonates in analyses of ancient global carbon cycling. There are well‐known pitfalls to using shallow marine carbonate materials, including diagenesis, semi‐isolation of depositional environments and input of different types of sediments with varying inorganic δ13C (δ13Cinorganic) values, which can obscure any global signatures. One method used to assess whether global changes in δ13C are accurately represented by δ13Cinorganic records is to examine variations in the δ13C of co‐occurring organic material (δ13Corganic). If a δ13Corganic record co‐varies with a co‐occurring δ13Cinorganic record, it is argued that the signals must be related to variations in the global carbon cycle. This assumption has been investigated by analysing the isotopic composition of the organic carbon preserved in the uppermost 150 m of periplatform sediments recovered during ODP Leg 166 from the western margin of Great Bahama Bank. The δ13Corganic values measured in this study were compared to previously published δ13Cinorganic records measured on identical samples, thus allowing a study of the correlation between the two records through time. These analyses showed that the correlation coefficient between δ13Cinorganic and δ13Corganic increased from the proximal location (Site 1005, r2 = 0·1), to the distal site (Site 1006, r2 = 0·63). The importance of platform‐derived carbonate and organic material at the proximal location, Site 1005, is reflected in the absence of a co‐variation between inorganic and organic δ13C records, which exhibit no correlation on the platform itself. In contrast, the co‐variance in δ13C values at the basinal location, Site 1006, is explained by a two‐point mixing model, which demonstrates the importance of both pelagic and platform‐derived carbonate and organic carbon in generating the positive correlation between the organic and inorganic δ13C values; this results in a correlation between δ13Cinorganic and δ13Corganic records at Site 1006 that is unrelated to global carbon cycling. Such data question the applicability of using δ13Corganic values to support the ability of δ13Cinorganic values to record global carbon cycling in carbonates recovered from environments where multiple sources of carbonate and organic carbon contribute to the bulk δ13C signal.

Elemental concentration of U, Th and K in tectonically active regions of Khari River basin, Kachchh, Western India

Elemental concentrations of U, Th and K were determined using alpha counter and gamma-ray spec- trometer to study the natural environmental radiation over a wide tectonically active region of Khari river basin in Kachchh, western India. Activity concentrations of 238 U, 232 Th and 40 K estimated in different types of sediments deposited/relocated in the Khari river cliff due to neotec- tonic activity near Bharasar (2311 0 36.5 00 N, 6935 0 22.6 00 E) and Kodki transverse fault (2314 0 37.37 00 N, 6934 0 59. 99 00 E) near Bhuj in the Khari River basin. Distribution of these radioactive elements on the earth's surface is con- trolled by geological features like faults, shear zones, metamorphism etc. Mainland Kachchh has two E-W trending major faults viz. Kachchh Mainland Fault and Katrol Hill Fault, which exhibits evidences of active tec- tonics during Quaternary. Kodki transverse fault is one of several NNE-SSW and NNW-SSE trending transverse faults affecting the Katrol Hill Range. The River Khari flows from the Katrol Ridge and traverses through Creta- ceous sediments of Bhuj Formation (fluvio-deltaic, com- prised dominantly of sandstones) in its course. All the three tectonically relocated sediments in the Khari river cliff have similar 238 U and much lower 232 Th activity concen- tration values corresponding to the worldwide median values of 35 and 30 Bqkg -1 , respectively, while 40 K activity concentrations were quite lower for two sediments and almost similar for the scarp-derived colluvium com- pared to the worldwide median value of 400 Bqkg -1 . Kodki transverse fault, however, has higher 238 U and 232 Th and almost half of 40 K activity concentration values rela-

Assessment of an Automation Algorithm for TDR Bridge Scour Monitoring System

Bridge scour is the number one cause of bridge failures. There are high risks associated with scour-induced damages due to their catastrophic nature. Developing real time bridge scour monitoring systems is a pressing task for the research community. Such systems need to be accurate, rugged, and field worthy. Implementing automatic signal interpretation is important to ensure the success of a long term scour monitoring instrument. This paper is part of the efforts to develop a TDR system to monitor the development of scour and sedimentation process. The focus of this paper is on an automation algorithm for TDR signal analyses. It discusses the theoretical foundation of this TDR algorithm for scour depth estimation. The influence of different types of sediments and the conditions of river (including the salinity and the effects of high air entrainment and suspended sediments) on the accuracy are assessed. The study indicates that the scour monitoring algorithm based on extending a physics-based general design equation is accurate under these different conditions. This will help implement a robust algorithm to automate the TDR bridge scour monitoring system.

Development and evaluation of an automation algorithm for a time-domain reflectometry bridge scour monitoring system

Bridge scour is a major threat to the safety of bridges. There is a high risk of scour-induced damage due to the catastrophic nature of bridge foundation failure. The development of an innovative bridge scour monitoring system is a pressing task for the research community. Such a system needs to be fieldworthy, which is a characteristic assessed in terms of accuracy, ruggedness, and automation. Among these criteria, an automatic signal analysis algorithm is generally a prerequisite for deploying a long-term field monitoring program. This paper describes the development and validation of an algorithm for a scour monitoring system based on the principles of guided radar: time-domain reflectometry (TDR). This algorithm is based on the extension of the classic dielectric mixing model to layered systems. The performance of this algorithm is evaluated using experiments designed to simulate different field scour conditions. These include different types of sediments and the variation of river conditions (i.e., salinity of river water, air entrainment, and amount of suspended sediments). The experiment results indicate that the developed analyses algorithm is robust and accurate for scour-depth estimation under these investigated conditions.

Sediment preference and burrowing behaviour in the sympatric brittlestars Ophiura albida Forbes, 1839 and Ophiura ophiura (Linnaeus, 1758) (Ophiuroidea, Echinodermata)

Ophiura albida and Ophiura ophiura are widespread and highly abundant brittlestar species occurring sympatrically on soft bottoms along the western European coasts. Laboratory choice experiments revealed that O. albida preferred staying on fine rather than on coarse sediments, whereas O. ophiura did not distinguish between these types of sediment. Sediment-specific burrowing behaviour of the two species was investigated under different stress and food conditions in order to evaluate relations of predator avoidance and feeding strategies with the observed sediment preference. In the presence of a predator, O. albida burrowed preferentially in fine sediment while coarse sediment did not seem to support quick burrowing for efficient escape. Conversely, O. ophiura tended to escape the predator by fleeing across the sediment surface rather than by burrowing, reflecting its unselectivity towards different sediment types. For O. albida, stationary burrowing behaviour suggests deposit feeding, predating and/or scavenging on infaunal organisms to be the predominant feeding behaviour rather than hunting for epibenthic prey organisms; more so, as this foraging strategy reduces the species' exposure to predators. In contrast, O. ophiura seems to be a true hunter and predator for epibenthic prey which was reflected in the experiments by low burrowing activities in presence of food enriched sediments. We suggest that in O. albida and O. ophiura the evolution of different mechanisms of predator avoidance has been associated with the evolution of contrasting foraging strategies, supporting the species' coexistence in broadly overlapping habitats and explaining their distribution on different types of sediment in the German Bight (North Sea).

Chemical speciation of polycyclic aromatic hydrocarbons in sediments: Partitioning and extraction of humic substances

The primary objective of this study was to examine the chemical speciation of polycyclic aromatic hydrocarbons (PAHs) found in sediments endowed with different characteristics. To achieve this purpose and because the nature of the sediments influences the distribution of PAHs, we have analyzed four different types of sediments. To study the role of organic matter in the sequestration of PAHs, we fractionated humic substances into humic acids and humin–mineral fractions. After their separation and purification, the humic components were examined for their sorptive reactivity by extracting them with organic solvents; these extracts were subsequently subjected to GC/MS analysis. Our results show that PAHs are distributed between labile and sequestered fractions in sediments. A slower uptake of PAHs occurs when the sequestered fraction is formed, and this process can be prolonged and may be influenced by the characteristics if the sediment. Our study suggests that organic contaminants are available in muddy sediments for a longer period of time than in sandy sediments.

Particle-specific sorption/desorption properties determine test compound fate and bioavailability in toxicity tests with Chironomus riparius-high-resolution studies with lindane

We studied the sorption (batch equilibrium experiments) and desorption (consecutively harsher supercritical fluid extractions) of lindane to different types of sediment and food particles, as well as larval uptake in standardized peat-based artificial sediment toxicity tests with the midge Chironomus riparius. Lindane sorption to organic particles was fast and efficient, reaching 98+/-0.1 and 97+/-0.1% of added compound in 48 h for peat and Tetraphyll(R), respectively, and 77+/-0.2% in whole sediment. Sorption to inorganic particles, that is, sand and kaolin clay, was much lower, 9.6+/-1.3% and 8.3+/-0.8%, respectively. Supercritical fluid extractions showed that most of the lindane sorbed to organic particles and sediment was loosely bound, as only 9 to 14% remained associated with particles after weak and intermediate extractions strengths. Larval uptake of dissolved lindane was 4.9+/-0.71 and 10.8+/-1.2 microg/g wet weight in 22 and 68 microg/L treatments, respectively, and four to five times higher than that of particle-associated lindane, ranging 1.0+/-0.15 to 2.7+/-0.21 microg/g in the above treatments. Surprisingly, larval uptake of lindane was similar from refractory peat and the more labile Tetraphyll particles. Despite an efficient larval uptake of dissolved lindane, sorption/desorption of lindane to/from Tetraphyll particles will facilitate digestive uptake in toxicity tests, particularly in spiked-water scenarios where food particles may act as vectors. Our results show that the exposure scenario is an important determinant for the behavior and bioavailability of test compounds in standardized toxicity tests.

Influence of Salinity in the Bioavailability of Zn in Sediments of the Gulf of Cádiz (Spain)

This study assesses the effect of salinity in bioavailability and toxicity of Zn by means of laboratory bioassays by observing contamination in both sediment and water, accumulation of Zn in biological tissues, and histopathological damage in the gills and guts tissues of Ruditapes philippinarum clams, which were exposed to different types of sediments from the Gulf of Cadiz (SW Spain) as well as two dilutions of toxic mud coming from an accidental mining spill. With this objective, the coefficients of distribution (K D) for Zn between overlying water and sediments were calculated, the histopathological frequencies in the tissues of the gills and guts of clams were determined, and the biota-sediment bioaccumulation factors as well as the bioaccumulation factors were quantified in the different stations. Results showed that the greatest histopathological damages appeared when the salinity values decreased. Statistical results showed that salinity was inversely correlated with histopathological damage (p < 0.01) for the lesion index for gills. The most outstanding results were observed in the two dilutions of toxic mud (0.3% and 7.9%) at a salinity value of 10. Salinity was inversely correlated with the concentration of Zn in biological tissues (p < 0.05) and inversely correlated with the concentration of Zn in water and sediment. Zn mobilization to the overlying water is produced when salinity values decrease.

The interactive effects of petroleum-hydrocarbon spillage and plant rhizosphere on concentrations and distribution of heavy metals in sediments in the Yellow River Delta, China

In order to understand how petroleum-hydrocarbon spillage and plant rhizosphere interact to affect concentrations and distribution of heavy metals, nine metals were analyzed from four different types of sediments (i.e. petroleum-hydrocarbon spilled rhizosphere, pristine rhizosphere, petroleum-hydrocarbon spilled unvegetated, and pristine unvegetated) in the Yellow River Delta, China. Our results showed that petroleum-hydrocarbon spillage together with rhizosphere effects were responsible for the significantly higher levels of metals in these four types of sediments. Compared to unvegetated sediments, rhizosphere sediments were well grouped into petroleum-hydrocarbon spilled and pristine sites on the basis of the concentrations of heavy metals by the correspondence analysis (CA). Furthermore, analysis of the transfer factors indicated that the capacity of rhizosphere sediments to stabilize heavy metals increased with increasing petroleum-hydrocarbon spillage, which might be due to the changes of sediment profiles by plant rhizosphere. Our results suggested that interactive effects of petroleum-hydrocarbon spillage and plant rhizosphere played an important role in determining concentrations and spatial distribution pattern of heavy metals.

A new approach for the isolation of cellulose from aquatic plant tissue and freshwater sediments for stable isotope analysis

Isotope studies of the cellulose of plant tissue and sediments are increasingly important in ecological and palaeoenvironmental studies. The reliability of analytical results in these investigations depends on the ability to separate pure cellulose from the organic and inorganic matrix. This study evaluates the performance of a new approach for the isolation of cellulose based on wet oxidation and cellulose dissolution in cuprammonium solution (CUAM) with respect to conventional techniques using solely wet oxidation or wet oxidation in combination with density separation. All the methods led to identical changes in isotopic composition of standard cellulose powder samples, providing evidence that CUAM-treated cellulose is isotopically indistinguishable from cellulose treated with conventional methods. The performance of CUAM with aquatic plant tissue was at least equal to those of the other methods used and in numerous cases achieved better results in terms of cellulose purification and precision of the isotope signal. Compared to conventional methods, contamination of several samples with small amounts of minerogenic matter and biogenic opal could be completely removed from the extraction residue with CUAM. While conventional methods failed to result in isolation of cellulose from two typical fine-grained lacustrine sediments, extraction residues from CUAM treatments revealed infrared (IR) spectra resembling those of standard cellulose powder, without evidence of minerogenic matter, biogenic opal, chitin or refractory organic matter. A series of sedimentary materials ranging from soil, to sediment trap and sediment core material were extracted with CUAM. IR spectra of all materials tested correspond to cellulose, and carbon and oxygen isotope compositions of the extracted cellulose fractions agreed with previous knowledge about the samples. Our results prove that CUAM is a reliable method that yields clean and pure cellulose for high quality isotope analysis from plant tissue and different types of sediments alike.

Role of Microbial Fe(III) Reduction and Solution Chemistry in Aggregation and Settling of Suspended Particles in the Mississippi River Delta Plain, Louisiana, USA

AbstractRiver-dominated delta areas are primary sites of active biogeochemical cycling, with productivity enhanced by terrestrial inputs of nutrients. Particle aggregation in these areas primarily controls the deposition of suspended particles, yet factors that control particle aggregation and resulting sedimentation in these environments are poorly understood. This study was designed to investigate the role of microbial Fe(III) reduction and solution chemistry in aggregation of suspended particles in the Mississippi Delta. Three representative sites along the salinity gradient were selected and sediments were collected from the sediment-water interface. Based on quantitative mineralogical analyses 88–89 wt.% of all minerals in the sediments are clays, mainly smectite and illite. Consumption of SO42−\$\end{document} and the formation of H2S and pyrite during microbial Fe(III) reduction of the non-sterile sediments by Shewanella putrefaciens CN32 in artificial pore water (APW) media suggest simultaneous sulfate and Fe(III) reduction activity. The pHPZNPC of the sediments was ⩽3.5 and their zeta potentials at the sediment-water interface pH (6.9–7.3) varied from −35 to −45 mV, suggesting that both edges and faces of clay particles have negative surface charge. Therefore, high concentrations of cations in pore water are expected to be a predominant factor in particle aggregation consistent with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Experiments on aggregation of different types of sediments in the same APW composition revealed that the sediment with low zeta potential had a high rate of aggregation. Similarly, addition of external Fe(II) (i.e. not derived from sediments) was normally found to enhance particle aggregation and deposition in all sediments, probably resulting from a decrease in surface potential of particles due to specific Fe(II) sorption. Scanning and transmission electron microscopy (SEM, TEM) images showed predominant face-to-face clay aggregation in native sediments and composite mixtures of biopolymer, bacteria, and clay minerals in the bioreduced sediments. However, a clear need remains for additional information on the conditions, if any, that favor the development of anoxia in deep- and bottom-water bodies supporting Fe(III) reduction and resulting in particle aggregation and sedimentation.

The effects of sediment variability on reflection coefficient measurements

The statistical distribution of specularly scattered acoustic energy, commonly known as the reflection coefficient, is an important parameter when developing models for shallow water propagation and acoustic communications. The distribution of measured reflection coefficient data from 5 to 50 kHz and 10 to 70 degrees grazing angles was taken from a sea bottom recently perturbed by hurricane off the coast of Florida at the Sediment Acoustic Experiment 2004 (SAX04). The width and shape of the distributions are attributed to varying sediments in the experimental area and interface roughness. These distributions are analyzed to determine the scattering from different types of sediments including sand and mud. The effects of both roughness scattering and sediment variability on the mean value and distribution of the measured reflection coefficient will be explored. [Work sponsored by ONR Ocean Acoustics.]

Motility of bacteria in sediments

Motility and chemotactic responses to dissolved organic matter of bacteria living within different types of sediments were investigated down to 35 cm beneath the sediment surface. At least 20 % of bacteria swim, although they often attach temporarily to solid surfaces, and they respond to point sources of organic matter. Altogether their motile behaviour and their relative densities are very similar to what has previously been found for planktonic bacteria. Bacterial motility in sand and silt is about half that of motility in moving water.