Near-bottom Sediments Research Articles

A radiation safety assessment in aquatic ecosystems formed by self-discharging wells: a case study of the watercourse: the well No. 36-RG — a stream — the Aremzyanka River

This article investigates the impact of geothermal water from the Cherkashinskaya No. 36-RG self-injection exploration well on ecosystem watercourses. The well was drilled in 1965 to investigate deposits of iodine-bromine water. This article presents estimations of the acceptable levels of radiation hazard content in the water and near-bottom sediments of gravity-flow hydrogeological wells. The radiation safety of working with these materials for personnel in geological exploration and national economic enterprises is also evaluated. The article provides information on the study of the natural radiation background, as well as results on the natural radioactivity of the bottom sediments and the volumetric radium content of the water. Natural radioactivity in the bottom sediments of potassium (40K) and thorium (232Th) is analyzed in this study. The relevance of this research originates from over fifty years of pollution by highly mineralised iodine-bromine water within the catchment area of one of Western Siberia's largest transboundary rivers, the Irtysh River. The Aremzyanka River is first-order tributary of the Irtysh River, and it receives water from a stream originating from exploration well No. 36-RG.

Опасные газонасыщенные объекты на акваториях Мирового океана: море Бофорта, шельф Северного склона Аляски

The article is devoted to the study of gas saturation of the upper part of the sedimentary cover in the western sector of the Beaufort Sea adjacent to Alaska. For the first time, the interpretation of the upper part of the seismic sections of CDP (common depth point) seismic survey was performed for 52 seismic lines of the United States Geological Survey (USGS) with a total length of 4050 km. 184 anomalous objects were found in near-bottom sediments, potentially associated with shallow gas accumulations (gas pockets). The average distance between these objects along the seismic lines was 22 km, which is 80% more than in the Chukchi Sea. This is apparently due to less tectonic activity in the Beaufort Sea. The statistically established similarity of anomalous objects in these seas in terms of depths and lengths is due to similar geological conditions for the formation of sedimentary deposits. In addition, 60 seismic lines with a total length of 4390 km show a wide distribution of potential gas hydrate deposits on the continental slope of the Beaufort Sea, which is consistent with the earlier conclusions of the US and Norwegian scientists (K. Andreassen, P. E. Hart, A. Grantz and others).

Опасные газонасыщенные объекты на акваториях Мирового океана: Восточно-Сибирское море

For the first time, the researchers performed the interpretation of the upper part of the common depth point (CDP) seismic sections in the northwestern part of the East Siberian Sea (ESS) along the 44 lines of JSC MAGE in the amount of 8200 km. They revealed 129 anomalous objects in near-bottom sediments, potentially associated with gas accumulations and channels of its subvertical migration. The average distance between these objects along the lines was 63.6 km — 5.2—6.2 times less than in the Chukchi, Laptev and Bering seas. The authors substantiate that the ESS is characterized by a significantly smaller number of gas migration channels — active faults reaching the near-bottom sediments, compared to the seas mentioned above. This is consistent with the lower neotectonic activity of the ESS and the absence of significant seismic events. In the study area, the researchers revealed a large number of depressions in the bottom relief, which are associated with the furrows of ice gouging during the transgressions-regressions of the sea and at the present stage. Significant errors in the GEBCO bathymetry database were also revealed.

Draft Genome Sequences of Six Bacteria Isolated from the Benham Bank, Philippine Rise, Philippines.

We report here the draft genome sequences of six bacteria isolated from the near-bottom waters and surface sediments of the Benham Bank, Philippine Rise, Philippines. These genome sequences represent candidate novel species and/or strains from the families Flavobacteriaceae and Dermacoccaceae and the genera Idiomarina, Bacillus, and Vibrio.

Authigenic rhodochrosite from a gas hydrate-bearing structure in Lake Baikal

Early diagenetic carbonates are rare in Lake Baikal. Siderite (Fe carbonate) concretions in the sediments were discovered only recently. Here, we discuss the first finding of rhodochrosite concretions (Mn carbonate) discovered in the near-bottom sediments of the gas hydrate-bearing seepage structure St. Petersburg-2 in the deep water environment of the Central Baikal Basin. The crystal lattice of rhodochrosite contains iron and calcium substituting to manganese. Based on pore water geochemistry and of δ13C values of rhodochrosite (− 23.3 and − 29.4‰), carbon dioxide (+ 3.8 to − 16.1‰) and methane (− 63.2 to − 67.8‰), we show that carbonate crystallization most likely occurred during microbial anaerobic oxidation of organic matter, and that part of the oxygen making up the rhodochrosite seems to be derived from the 18O-rich water released from dissociating gas hydrates.

Sampling surface particle size distributions and stability analysis of deep channel in the Pearl River Estuary

Particle size distributions (PSDs) of bottom sediments in a coastal zone are generally multimodal due to the complexity of the dynamic environment. In this paper, bottom sediments along the deep channel of the Pearl River Estuary (PRE) are used to understand the multimodal PSDs′ characteristics and the corresponding depositional environment. The results of curve-fitting analysis indicate that the near-bottom sediments in the deep channel generally have a bimodal distribution with a fine component and a relatively coarse component. The particle size distribution of bimodal sediment samples can be expressed as the sum of two lognormal functions and the parameters for each component can be determined. At each station of the PRE, the fine component makes up less volume of the sediments and is relatively poorly sorted. The relatively coarse component, which is the major component of the sediments, is even more poorly sorted. The interrelations between the dynamics and particle size of the bottom sediment in the deep channel of the PRE have also been investigated by the field measurement and simulated data. The critical shear velocity and the shear velocity are calculated to study the stability of the deep channel. The results indicate that the critical shear velocity has a similar distribution over large part of the deep channel due to the similar particle size distribution of sediments. Based on a comparison between the critical shear velocities derived from sedimentary parameters and the shear velocities obtained by tidal currents, it is likely that the depositional area is mainly distributed in the northern part of the channel, while the southern part of the deep channel has to face higher erosion risk.

Механизм формирования аутигенного родохрозита в приповерхностных осадках газогидратоносной структуры Санкт-Петербург-2 в центральной котловине озера Байкал

Механизм формирования аутигенного родохрозита в приповерхностных осадках газогидратоносной структуры Санкт-Петербург-2 в центральной котловине озера Байкал

The distribution of heavy metals in the near-bottom particulate matter, fluffy layer, and bottom sediments over the Yenisei profile in the Kara Sea

The chemical composition and concentrations of metals (Fe, Mn, Cu, Zn, and Cr) were determined for the first time in the marine basin at the water-bottom geochemical interface, including the particulate matter, fluffy layer (0.0–0.2 cm), and the surface layer of the sediments (0–2 cm). It was found that the content of metals in the aquatic particulate matter depended on the concentration of organic matter. The fluffy layer was enriched, compared to the underlying layers, by 5–10% in organic matter including the biomass of microorganisms, and by up to 50% in metals (to the maximum of doubled values for Zn). It is supposed that this low enrichment of the fluffy layer in metals was caused by the active near-bottom currents of marine basins.

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ13C values of DIC become heavier with increasing subbottom depth, and vary between −9.5 and +21.4‰ PDB. Enrichment of DIC in 13C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO2-reduction) were revealed by determining the offset of δ13C between dissolved CH4 and CO2, and also by using δ13C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.

Assemblages of benthic foraminifers in the White Sea

Based upon the composition of the dominant species of benthic foraminifers in the bottom sediments sampled at 50 sampling stations in the White Sea, their six assemblages (communities) were defined and mapped. The ranges of these communities depend on the environmental factors determined by the bathymetric, latitudinal, and circum-continental sea zonalities, the sea currents, the bottom upwellings, the continental runoff, the productivity of the waters, and the CaCO3 content in the near-bottom water layers and bottom sediments.

First discovery and formation process of authigenic siderite from gas hydrate–bearing mud volcanoes in fresh water: Lake Baikal, eastern Siberia

[1] We report on the first authigenic siderite (FeCO3) concretions recovered from near-bottom sediments at gas hydrate-bearing mud volcanoes in fresh water (Lake Baikal, Eastern Siberia). The carbonates appear as firm ‘plate-type’ formations at the Malenky mud volcano (Southern Baikal Basin) and as soft nodules at the K-2 mud volcano (Central Baikal Basin). Calcium is the main divalent component which substitutes iron in the carbonate lattice (7 to 20 mol%). The δ13C values of the carbonates (+3.3 to +6.8‰ at Malenky, and +16.5 to +21.9‰ PDB at K-2) indicate that their formation is due to methanogenesis. The latter was most likely caused by the microbial methyl-type (acetate) fermentation that is suggested from the isotopic composition of the accompanying methane hydrates and dissolved methane. General depletion of the siderites in 18O (−11.6 to −9.9 ‰ at Malenky, and −13.9 to −12.3‰ PDB at K-2) is mainly inherited from the isotope composition of pore water (−15.2 to −15.4‰ SMOW) at ambient temperature (3.5°C).

Microbiological changes in the environment caused by deep dredging. A case study: post-dredging pit Kuźnica II (Puck Bay)

Abstract In this study psychrophilic, mesophilic and denitrifying bacterial abundances were studied seasonally (summer, autumn, winter and spring) in the water column and surface sediment layer (0-5 cm) in the post dredging pit Kuźnica II and natural areas of Puck Bay. The research was conducted between VI 2001 and III 2003. In the pit area an increase in mesophilic bacteria and a decrease in denitrifying bacteria numbers were observed, when compared to the natural areas. In the case of the mesophilic bacteria, the increase was visible in the near-bottom waters and surficial sediments during the period of well developed vegetation - in summer and autumn. In the case of denitrifying bacteria, the decrease of number concerned the sediments. Numbers of psychrophilic bacteria in both the natural and dredged areas did not differ significantly over the course of the study. These results suggest that deep dredging can cause the self-purification potential of the ecosystem to be diminished and induce strong bacteriological pollution.

Estimation of sea‐floor wave velocities and density from pressure and particle velocity by AVO analysis

Sea‐bottom properties play an important role in fields as diverse as underwater acoustics, earthquake and geotechnical engineering, and marine geophysics. Water‐column acousticians study shear and interface waves in the nearbottom sediments with the aim of inferring sea‐bed geoacoustic parameters for predicting reflection and absorption of waves at the sea floor. On the other hand, geotechnical engineers working on design and siting of offshore structures focus on these waves to characterize soil and rock properties. In the field of geophysics, sea‐bottom parameters are of interest for several reasons. In conventional marine acquisition, these parameters determine the partitioning of the incident P‐wave energy from the source into transmitted P‐waves and mode‐converted S‐waves (Tatham and Goolsbee, 1984; Kim and Seriff, 1992). The sea‐floor P‐ and S‐wave velocities and density are also necessary inputs for decomposing multicomponent sea‐floor data into P‐ and S‐waves (Amundsen and Reitan, 1995a and b), as well as in the numerical study of wave propagation phenomena.

Remote determination of in situ sediment parameters using Love waves

In situ properties of near-bottom sediments are determined using an experimental technique based on the dispersive properties of horizontally polarized surface waves generally known as Love waves. This technique basically consists of generating the Love waves, analyzing the recorded seismic signals to determine dispersion, and, finally, mathematically inverting the dispersion data to obtain a geoacoustic model of the sediment. In the field experiments, Love waves are generated using a torsional source and the generated signals are recorded with a linear array of gimbaled geophones that measure transverse horizontal motion. Experimental dispersion curves (wave velocity versus frequency), are then obtained from the recorded wavelets. These velocity waveforms are usually composed of overlapping modes and in order to reduce the influence of modal interference on the experimental phase velocity dispersion curves, a phase-matched filter is used to extract a smoothed phase velocity dispersion curve for the fundamental mode. The dispersion curve is then mathematically inverted to obtain a sediment model using a constrained least-squares method with singular-value decomposition and it is shown that the maximum parameter resolution depth can be estimated from the generalized inverse. Practical use of the technique is demonstrated by an analysis of the results of a field experiment carried out in a marine sand sediment.

Simulation of the transmitted pressure field inside a water-saturated porous (sand) sediment

As part of a study of the effects of porosity on reflectivity the influence of an underlying solid basement on the reflection from the interface between the water and a porous sedimentary structure has been examined analytically and numerical calculations made for various versions of this model. This paper presents the results of two applications of the codes developed. In the first the transmitted pressure field inside a sand sediment, resulting from a source above the water–sediment interface, is calculated and compared with that measured in a recent experiment which appears to demonstrate the Biot slow wave [Boyle and Chotiros, J. Acoust. Soc. Am. 91, 2615–2619 (1992)]. The simulation appears to confirm the demonstration of the slow wave in the tank experiment but suggests it is identified with a different feature in the experimental contour plot. In the second example signatures of the vertical component of the seabed displacement are calculated at various ranges from a very low pressure source, in a simulation of the potential use of both the shear and slow wave in the study of the geoacoustic properties of near-bottom sediments. [Work supplied by ONR.]

Role of Naturally Occurring Gas Hydrates in Sediment Transport

Naturally occurring gas hydrates have the potential to store enormous volumes of both gas and water in semi-solid form in ocean-bottom sediments and then to release that gas and water when the hydrate's equilibrium conditions are disturbed. Therefore, hydrates provide a potential mechanism for transporting large volumes of sediments. Under the combined low bottom-water temperatures and moderate hydrostatic pressures that exist over most of the continental slopes and all of the continental rises and abyssal plains, hydrocarbon gases at or near saturation in the interstitial waters of the near-bottom sediments will form hydrates. The gas can either be autochthonous, microbially produced gas, or allochthonous, catagenic gas from deeper sediments. Equilibrium conditions that stabilize hydrated sediments may be disturbed, for example, by continued sedimentation or by lowering of sea level. In either case, some of the solid gas-water matrix decomposes. Released gas and water volume exceeds the volume occupied by the hydrate, so the internal pressure rises--drastically if large volumes of hydrate are decomposed. Part of the once rigid sediment is converted to a gas- and water-rich, relatively low density mud. When the internal pressure, due to the presence of the compressed gas or to buoyancy, is sufficiently high, the overlying sediment may be lifted and/or breached, and the less dense, gas-cut mud may break through. Such hydrate-related phenomena can cause mud diapirs, mud volcanos, mud slides, or turbidite flows, depending on s diment configuration and bottom topography.

Evidence of Migrating Liquid Hydrocarbons in Deep Sea Drilling Project Cores

Cores from the Deep Sea Drilling Project contain evidence that liquid hydrocarbons are migrating in or into near-bottom sediments at three widely separate locations. The first occurrence noted was the highly publicized, visually observed accumulation of immature petroleum in sediments on the Challenger Knoll in the Gulf of Mexico. Later, systematic chemical analyses revealed two more possible examples of migrated hydrocarbons. The first of these was a low-grade bitumen enrichment in a thin porous zone in Pleistocene sediments on the Shatsky Rise in the western Pacific Ocean. The second was a small but geochemically significant quantity of gasoline-range hydrocarbon and wet gas that apparently has seeped upward into Miocene sediments in the Balearic basin of the western Me iterranean Sea. These migrated hydrocarbons, in addition to the methane gas frequently found in the deep-ocean cores, reveal that hydrocarbon source rocks must be present, at least locally, in deep-ocean sediments, because liquid as well as gaseous hydrocarbons have begun to migrate.