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Methane (CH 4) is a potent greenhouse gas that has a major impact on Earth's climate. CH 4 is accommodated in discrete bubbles in aquatic muds, whose sizes greatly exceed the pore size of the hosting sediment. This critical review examines the mechanics of CH 4 gas in consolidated aquatic muds at the scale of a single bubble and at a macroscale of gassy sediments, obtained from lab experiments, field observations, and numerical and analytical modeling. Linear elastic fracture mechanics (LEFM) theory is shown to control the single bubble shape, size, morphology, and inner pressure evolution over its entire life cycle. Reviewed implications focus on the effects of the inner bubble pressure on its solute exchange with ambient pore waters; on the dynamic water load effect (e.g., waves, tides) on the bubble growth rate and its release from sediment into the water column; and on competitive bubble pair growth in the aquatic muds, the process that presumably shapes the bubble size distribution pattern in muds. Alternatively, gassy sediment effective mechanical and physical characteristics and effective gassy media theories are examined at the macroscale, which makes them suitable for remote sensing acoustic applications. This review indicates, however, that most of the developed macroscale effective medium theories rely on the cumulative sediment gas content. Moreover, no theory for proper upscaling of the entire set of the microscale single bubble descriptors addressed in this review – bubble size distribution, their orientations and spatial locations, and inner bubble pressures – to the effective medium mechanical properties of gassy muds, exists. This review will serve, therefore, as a basis for the improved upscaling, while preserving the basic microscale bubble descriptors, their growth physics, and controls. Laying this foundation will enhance the accuracy of the acoustic applications. Improved assessment of sediment gas retention based on this upscaling will contribute to geohazard prediction and should reduce a long-persisting uncertainty related to CH 4 fluxes from the aquatic sediments. [ABSTRACT FROM AUTHOR]

The objectives of this study are to better understand controls on bed erodibility in muddy estuaries, including the roles of both sediment properties and recent hydrodynamic history. An extensive data set of erodibility measurements, sediment properties, and hydrodynamic information was utilized to create statistical models to predict the erodibility of the sediment bed. This data set includes >160 eroded mass versus applied stress profiles collected over 15 years along the York River estuary, a system characterized by “depth-limited erosion,” such that the critical stress for erosion increases rapidly with depth into the bed. For this study, erodibility was quantified in two ways: the mass of sediment eroded at 0.2 Pa (a stress commonly produced by tides in the York), and the normalized shape of the eroded mass profile for stresses between 0 and 0.56 Pa. In models with eroded mass as the response variable, the explanatory variables with the strongest influence were (in descending order) tidal range squared averaged over the previous 8 days (a proxy for recent bottom stress), salinity or past river discharge, sediment organic content, recent water level anomalies, percent sand, percent clay, and bed layering. Results support the roles of 1) recent deposition and bed disturbance increasing erodibility and 2) cohesion/consolidation and erosion/winnowing of fines decreasing erodibility. The most important variable influencing the shape of the eroded mass profile was eroded mass at 0.2 Pa, such that more (vs. less) erodible cases exhibited straighter (vs. more strongly curved) profiles. Overall, hydrodynamic variables were the best predictors of eroded mass at 0.2 Pa, which, in turn, was the best predictor of profile shape. This suggests that calculations of past bed stress and the position of the salt intrusion can serve as useful empirical proxies for muddy bed consolidation state and resulting erodibility of the uppermost seabed in estuarine numerical models. Observed water content averaged over the top 1 cm was a poor predictor of erodibility, likely because typical tidal stresses suspend less than 1 mm of bed sediment. Future field sampling would benefit from higher resolution observations of water content within the bed’s top few millimeters.

Methane (CH4) transport from organic-rich fine-grained (muddy) shallow aquatic sediments to water column is mediated dominantly by discrete bubbles, which is an important natural source of greenhouse CH4. The lifespan of these bubbles within the sediment comprises two successive stages: growth from nucleation up to a mature size and then buoyant ascent toward the sediment–water interface. Bubbles often experience an oscillating overburden load due to the passage of winds and/or storm-induced short period surface waves or long-period tides, which can potentially affect both stages of the bubble’s lifespan. However, little is known about the wave effects over bubble growth phase. In the present work, this subject is investigated using a numerical single-bubble mechanical/solute transport model, which quantifies the effects of different parameters (amplitude and period) of the wave loading and of the water depth, over the bubble growth pattern in sediments and its specific characteristics. It was found that bubbles induce early sediment fracturing in the presence of waves, attributed to the low overburden load appearing at wave troughs. Bubbles at shallow depth rapidly grow at wave troughs by inducing multiple intense fracturing events. However, this ability decreases with an increasing water depth because of a slower solute influx. In the presence of waves, bubbles mature in shorter time, whose contrast to the no wave case is controlled by the ratio of wave amplitude to equilibrium water depth. Due to the higher frequency of occurrence of wave troughs for shorter-period waves, bubble growth is accelerated compared with the case of longer-period waves. Overall, our modeling suggests that the fastest bubble growth can be predicted for higher amplitude, short-period waves traveling in shallow water. We further infer that accelerated bubble growth, along with subsequent wave-induced ascent can sufficiently shorten the bubble’s total lifespan in sediment, which explains the observed episodic in situ ebullitions correlated with wind- or storm-induced waves.

The occurrence of hypoxic muddy sediments on shallow beaches and other sheltered areas is a well-known environmental problem, which negatively affects coastal areas, tourism potential, the public use of beaches and sediment biodiversity. The usual solution is limited to dredging and removal of sludge to a landfill site. In this study, a laboratory-scale experiment was performed to determine the effectiveness of two technologies: a modification of air sparging and a new approach based on injecting oxygen-saturated seawater in hypoxic muddy sediments (oxygen-saturated seawater injections method), for remediating sludge in coastal sediments, minimizing environmental impact respect to dredging. Our results showed that both technologies significantly increased dissolved oxygen content in pore water, facilitating the oxidation of more than 90% of the organic matter, and other reduced inorganic compounds such as sulphide, with the consequent increase in sulphate concentration from 0.3 to 3.0 g·L−1. Moreover, a rise of redox potential from − 258 mV to above 200 mV, and a dramatic drop in chemical oxygen demand were also indicators that oxic conditions had been restored. After 65 days, soft, black, muddy and hypoxic sediment with high organic matter content and a characteristic foul odour was transformed into well-oxygenated sediment, which had a low organic matter content and had lost its initial shiny black colour and odour. The main difference between both technologies was the depth influenced by sediment remediation; oxygen-saturated seawater injections affected deeper areas than clean pressurized air injections. [ABSTRACT FROM AUTHOR]

Abstract Conservation and restoration of salt marsh ecosystems are becoming increasingly important because of the many ecosystem services they provide. However, the processes controlling salt marsh establishment and persistence, especially on bare tidal flats in muddy areas, remain unclear. As muddy sediments typically experience a restriction of soil drainage, we expect that a surface drainage relief due to a heterogeneity topography, as might occur on the edge of tidal channels, can facilitate the establishment of salt marsh vegetation on muddy tidal flats. By means of a manipulative field experiment, using “Mega‐Marsh Organ” mesocosms, we investigated the impact of surface drainage and elevation relative to mean sea level on (1) the survival of Spartina anglica seedlings from three different age classes: 1‐yr, 3‐month, and 1‐week; and (2) the growth performance of mature S. anglica marsh tussocks. S. anglica seedling survival, especially in the establishment phase, was positively affected by better surface drainage, increases of seedling age, and higher elevation relative to mean sea level. That is, the survival rate of S. anglica seedlings at the end of 6th week increased from 0% (at surface water undrained, 1‐week, 0 cm elevation) to 94.44% (at surface water drained, 1‐yr, 90 cm elevation). In contrast, surface drainage did not affect the performance of large S. anglica marsh tussocks, as only increased elevation relative to mean sea level was shown to affect S. anglica tussock growth in terms of plant height, shoot numbers, and dry biomass. Based on our findings, we proposed a conceptual model to understand how surface drainage‐driven feedbacks in a heterogeneous topography may be reinforced to induce salt marsh establishment in muddy systems. Further testing of present hypothesized model would be beneficial for insights into salt marsh establishment on tidal mudflats.

Conservation and restoration of salt marsh ecosystems are becoming increasingly important because of the many ecosystem services they provide. However, the processes controlling salt marsh establishment and persistence, especially on bare tidal flats in muddy areas, remain unclear. As muddy sediments typically experience a restriction of soil drainage, we expect that a surface drainage relief due to a heterogeneity topography, as might occur on the edge of tidal channels, can facilitate the establishment of salt marsh vegetation on muddy tidal flats. By means of a manipulative field experiment, using "Mega‐Marsh Organ" mesocosms, we investigated the impact of surface drainage and elevation relative to mean sea level on (1) the survival of Spartina anglica seedlings from three different age classes: 1‐yr, 3‐month, and 1‐week; and (2) the growth performance of mature S. anglica marsh tussocks. S. anglica seedling survival, especially in the establishment phase, was positively affected by better surface drainage, increases of seedling age, and higher elevation relative to mean sea level. That is, the survival rate of S. anglica seedlings at the end of 6th week increased from 0% (at surface water undrained, 1‐week, 0 cm elevation) to 94.44% (at surface water drained, 1‐yr, 90 cm elevation). In contrast, surface drainage did not affect the performance of large S. anglica marsh tussocks, as only increased elevation relative to mean sea level was shown to affect S. anglica tussock growth in terms of plant height, shoot numbers, and dry biomass. Based on our findings, we proposed a conceptual model to understand how surface drainage‐driven feedbacks in a heterogeneous topography may be reinforced to induce salt marsh establishment in muddy systems. Further testing of present hypothesized model would be beneficial for insights into salt marsh establishment on tidal mudflats. [ABSTRACT FROM AUTHOR]

Whereas there have been numerous theoretical and experimental studies on the properties of marine granular sands, there are significantly fewer studies on sediments classified as muds. The validity of geoacoustic models for muddy sediments has not been successfully tested due to the lack of inverted low-frequency sound speed and attenuation values. The geoacoustic properties of a surface fine-grained mud layer, overlaying three sand transition layers, and a half-space basement within the New England Mud Patch, were studied using explosive signals from long-range along-shelf sound propagation tracks. The sound-speed profile of the mud layer in the low-frequency band (100–500 Hz) was estimated using acoustic normal mode characteristics including the mode shapes and the modal dispersion curves of low-order modes, which mainly propagated in the water column and the surface mud layer. The ambiguity of sound speed at the top of the mud layer and sound-speed gradient was approximately removed. It was found that the resultant sound-speed ratio at the water-sediment interface was close to unity and the sound-speed gradient was 1.8 1/s with a standard deviation of 1.0 1/s. The attenuation in the mud layer was inverted using the attenuation coefficient of the first mode extracted from explosive signals at three source locations. The estimated attenuation at 150 Hz had a mean of 0.006 dB/m and a standard deviation of 0.003 dB/m. [ABSTRACT FROM AUTHOR]

Ebullition of greenhouse methane (CH4) from the aquatic sediments is often observed at various hydrostatic pressure drops: at low tides, waves, and even at atmospheric pressure drops. It is especially pronounced at the different vent structures, for example, pockmarks, mud volcanoes, and cold seeps. The modeling conducted in the current study suggests that long timescale (glacial to centennial frequency) sea level drops may induce "stable" bubble ascent and control the position of the gas horizon in muddy aquatic sediment. Bubbles escape in the "dynamic" regime from the shallow gas horizon and subsequently to the water column is more feasible under shorter‐period waves of higher amplitude travelling in shallow water. These findings are illustrated by examples of various vent structures (e.g., pockmarks), pronounced in shallow straits and bays, described in the literature. Plain Language Summary: Release of greenhouse methane bubbles is often observed at various hydrostatic pressure drops: at low tides, waves, and even at atmospheric pressure drops in marine and lacustrine settings. It is pronounced at the different vent structures, for example, pockmarks, mud volcanoes, and cold seeps. It has been shown that long timescale (glacial to centennial frequency) sea level drops induce a stable bubble ascent and control a position of the gas horizon in muddy aquatic sediment. Bubble escape from the gas horizon is more feasible under shorter‐period (internal and surface) waves of higher amplitude travelling in shallow water. In this case the bubble starts a rapid unrestricted ascent and will ultimately be released into the water column. Otherwise, the bubble will migrate in the stable regime remaining close to the gas horizon. These findings are illustrated by examples of various vent structures (e.g., pockmarks), especially emphasized in shallow straits and bays, described in the literature. These important insights improve our understanding of the global carbon cycle in general, and of bubble ascent from the gas horizon in aquatic sediment, in particular. Key Points: CH4 bubbles will escape from gas horizon in the dynamic regime under shorter‐period waves with higher amplitude travelling in shallow waterLong timescale (glacial to centennial frequency) sea level drops induce a stable bubble ascent and control gas horizon positionThe findings can explain dynamics of various vent structures, e.g., pockmarks and cold seeps, pronounced in shallow straits and bays [ABSTRACT FROM AUTHOR]

Kobashi, D. and Jose, F., 2018. Potential impacts of sand mining on hydrodynamics and fine sediment suspension and deposition on an inner-shelf shoal. In: Almar, R.; Almeida, L.P.; Trung Viet, N., and Sall, M. (eds.), Tropical Coastal and Estuarine Dynamics. Journal of Coastal Research, Special Issue No. 81, pp. 76–85. Coconut Creek (Florida), ISSN 0749-0208. Wave and hydrodynamic models are implemented to investigate whether sand mining on a transgressive sand shoal could influence hydrodynamic processes and sedimentary environment of the shoal covered with fine sediments. An effort is made to evaluating shoal hydrodynamic response to a hypothetical dredging scenario under a host of probable storm conditions. The model results show that changes in hydrodynamic and sedimentary environment strongly depend on the storm conditions and the shoal bathymetry. Wave dissipation, flow conditions and sediment re-suspension are significantly higher on the shallower western shoal and gradually decrease toward the deeper eastern section of the shoal. The finding suggests that the deeper eastern shoal can be exposed to frequent deposition of fine sediments, as opposed to the western shoal, despite its location farther away from the nearby major river mouth. It is concluded that large-scale sand mining, in particular from the shallower western shoal, is expected to profoundly alter the hydrodynamics and sediment suspension and deposition processes, which may also influence the benthic ecology of the shoal. The mining from the deeper eastern shoal may result in lesser impacts than the mining from the ecologically sensitive western shoal. [ABSTRACT FROM AUTHOR]

Marine ecosystems are exposed to a range of human-induced climate stressors, in particular changing carbonate chemistry and elevated sea surface temperatures as a consequence of climate change. More research effort is needed to reduce uncertainties about the effects of global-scale warming and acidification for benthic microbial communities, which drive sedimentary biogeochemical cycles. In this research, mesocosm experiments were set up using muddy and sandy coastal sediments to investigate the independent and interactive effects of elevated carbon dioxide concentrations (750 ppm CO2) and elevated temperature (ambient +4°C) on the abundance of taxonomic and functional microbial genes. Specific quantitative PCR primers were used to target archaeal, bacterial, and cyanobacterial/chloroplast 16S rRNA in both sediment types. Nitrogen cycling genes archaeal and bacterial ammonia monooxygenase (amoA) and bacterial nitrite reductase (nirS) were specifically targeted to identify changes in microbial gene abundance and potential impacts on nitrogen cycling. In muddy sediment, microbial gene abundance, including amoA and nirS genes, increased under elevated temperature and reduced under elevated CO2 after 28 days, accompanied by shifts in community composition. In contrast, the combined stressor treatment showed a non-additive effect with lower microbial gene abundance throughout the experiment. The response of microbial communities in the sandy sediment was less pronounced, with the most noticeable response seen in the archaeal gene abundances in response to environmental stressors over time. 16S rRNA genes (amoA and nirS) were lower in abundance in the combined stressor treatments in sandy sediments. Our results indicated that marine benthic microorganisms, especially in muddy sediments, are susceptible to changes in ocean carbonate chemistry and seawater temperature, which ultimately may have an impact upon key benthic biogeochemical cycles.

Marine ecosystems are exposed to a range of human-induced climate stressors, in particular changing carbonate chemistry and elevated sea surface temperatures as a consequence of climate change. More research effort is needed to reduce uncertainties about the effects of global-scale warming and acidification for benthic microbial communities, which drive sedimentary biogeochemical cycles. In this research, mesocosm experiments were set up using muddy and sandy coastal sediments to investigate the independent and interactive effects of elevated carbon dioxide concentrations (750 ppm CO2) and elevated temperature (ambient +4°C) on the abundance of taxonomic and functional microbial genes. Specific quantitative PCR primers were used to target archaeal, bacterial, and cyanobacterial/chloroplast 16S rRNA in both sediment types. Nitrogen cycling genes archaeal and bacterial ammonia monooxygenase (amoA) and bacterial nitrite reductase (nirS) were specifically targeted to identify changes in microbial gene abundance and potential impacts on nitrogen cycling. In muddy sediment, microbial gene abundance, including amoA and nirS genes, increased under elevated temperature and reduced under elevated CO2 after 28 days, accompanied by shifts in community composition. In contrast, the combined stressor treatment showed a non-additive effect with lower microbial gene abundance throughout the experiment. The response of microbial communities in the sandy sediment was less pronounced, with the most noticeable response seen in the archaeal gene abundances in response to environmental stressors over time. 16S rRNA genes (amoA and nirS) were lower in abundance in the combined stressor treatments in sandy sediments. Our results indicated that marine benthic microorganisms, especially in muddy sediments, are susceptible to changes in ocean carbonate chemistry and seawater temperature, which ultimately may have an impact upon key benthic biogeochemical cycles. [ABSTRACT FROM AUTHOR]

Mud cracks are common in nature, but the genetic mechanism is not clear. The influence factors of mud cracks were summarized, and their significance was concluded, and the problems in the research of mud cracks were discussed. The results show that the internal influence factors of mud cracks include the composition and thickness of muddy sediments, and the contact relationship between the upper and lower layers; the change of thickness and composition leads to different fracture degrees; the smaller friction between the upper and lower sediments is, the more polygons of mud cracks are, and the easier warping-up of the edge is; the external influence factors of mud cracks include temperature, alternation of dry and wet environments, water and salinity and biological perturbation; the range of single mud crack is larger in high temperature; the crossing angle of mud crack tends to 120° under the condition of continuous alternation of dry and wet environments; the moisture influences the size of polygon of mud crack; the salinity influences the morphology of the edge of mud crack; the morphology of mud crack can be transformed by animals and plants; the thickness is an important factor for preserving mud cracks as well as the rapid cover of later sediments. The study on genetic mechanism of mud cracks contributes to solve the engineering problems caused by mud cracks. The appearance of mud cracks indicates that the sediments have been exposed on the surface, and the mud cracks preserved can be indicative for the ancient shoreline, the regional uplift of strata, the climate changes of cold-warm and dry-wet, and so on. [ABSTRACT FROM AUTHOR]

While mean grain size of sensitive component in muddy sediments from Chinese coastal seas was widely used as a proxy for reconstruction of the East Asian winter monsoon (EAWM) strength, many intractable problems still remain concerning the discrepancies in different studies. This paper provides a comprehensive overview of recent researches on the reconstruction of Holocene EAWM strength using sensitive grain size (SGS) in muddy sediments from the Chinese coastal seas. In the present study, 15 time-series of SGS in sediments from six mud areas are included. These records are summarized and compared during the last 8000 years, 3000 years and 150 years, respectively. The results show that trends of SGS time-series are inconsistent at millennial, multi-centennial and decadal time scales. The inconsistencies could be partly caused by age uncertainties and chosen SGSs. More important, grain size characteristics in sediments from some mud areas could be influenced by factors other than coastal currents driven by EAWM, such as sedimentary environments, riverine runoff and storms. Thus SGS of muddy sediments should be used with caution when reconstructing winter monsoon strength, especially those in the northern Yellow Sea and near the Yangtze River estuary. [ABSTRACT FROM AUTHOR]

The East Asian winter monsoon (EAWM) transports cold air from the high northern latitudes to the mid- and low-latitudes. Thus investigations on its variation pattern and mechanism are essential for acquiring further knowledge on the interplay across the high and low latitudes. Proxies such as grain size and foraminiferal δ 18 O in marine sediments and elemental ratios in the Chinese loess have been used for the reconstruction of EAWM strength. However, more high-resolution reconstructions for EAWM are required for further understanding the mechanism of climate systems and predicting climate change. In this study, we present a high-resolution time-series of the late Holocene EAWM strength reconstructed by element method. Comparisons of element concentrations, elemental ratios and mean grain size (MGS) of environment-sensitive components in a sediment core from the shelf of the East China Sea indicate that K/Ti ratio is an appropriate proxy for the reconstruction of the EAWM strength. K/Ti ratio exhibits similar trends with proxies of sea surface temperature and EAWM strength in different regions influenced by the EAWM. Warm and cold periods such as the Roman Warm Period, Medieval Warm Period and Little Ice Age are identified in the reconstructed results. Total solar irradiance anomaly (ΔTSI) and K/Ti ratios exhibit comparable tendencies in the last two millennia and frequency spectrum analysis exhibits similar periodicities in the time series of ΔTSI and K/Ti, which confirm that solar irradiance has a significant impact on the EASM on centennial time scale. [ABSTRACT FROM AUTHOR]

This study aimed to conduct the isolation, screening and identification of bacteria with a high level of cellulolytic activity from the muddy sediments of mangrove swamps in Thailand. One hundred and ninety aquatic bacterial isolates were isolated from different muddy sediments and eighty one isolates were determined to be cellulolytic bacteria. The cellulolytic bacterium identified as Bacillus cereus JD0404 showed maximum hydrolysis activity on carboxymethylcellulose agar plates. Its cellulolytic performance for CMCase activity, Avicelase activity and β-glucosidase activity was 1.778 ± 0.003 U/mL, 0.079 ± 0.001 U/mL and 0.048 ± 0.002 U/mL, respectively. The optimum temperature and pH for the enzyme activity were determined to be 50 °C and 7.0 respectively. The cellulolytic activity was greatly enhanced by Mn 2+ and considerably inhibited by EDTA and toluene. Preliminary bioconversion application showed that the B. cereus JD0404 could be used for the hydrolysis of cellulose-based biomass. This study demonstrated a feasible bacterium for environmentally friendly industries and biotechnology. [ABSTRACT FROM AUTHOR]

Sulfate reduction is considered the primary pathway for organic carbon remineralization on the northern Gulf of Mexico Louisiana continental shelf (LCS) where bottom waters are seasonally hypoxic, yet limited information is available on the importance of iron and manganese cycling in the region. Sedimentary manganese, iron, and sulfur cycling were investigated on the LCS using a combined chemical analysis and sediment diagenesis modeling approach. Three stations situated 320 km across the LCS along the 20 m isobath were sampled up to five times between the spring of 2006 and summer of 2007. Bottom water oxygen levels at the stations ranged from 203 mmol m −3 in spring to 2.5 mmol m −3 in summer. Porewater Mn and Fe 2+ concentrations (up to 275 and 300 μmol L −1 , respectively), sulfate reduction rates (1.0–8.4 mmol m −2 d −1 ), and the fraction of total oxalate extracted iron obtained as Fe(II) (0.25–0.52) differed between station and season. Sediments at station Z02 on the eastern LCS, south of Terrebonne Bay, had higher organic matter content and sulfate reduction rates than sediments at Z03, 160 km further west. Sulfate reduction rates were higher in summer than spring at station Z02 but not at Z03 where porewater Mn and Fe concentrations were highest in summer. Porewater Fe 2+ concentrations, solid phase oxalate-extractable Fe concentrations, and sediment incubation experiments suggested iron reduction at Z03 may account for 20% or more of organic carbon remineralization. LCS Fe(III) concentrations decreased and sulfate reduction rates increased in model simulations by lowering interfacial dissolved oxygen levels and increasing the rates of organic matter deposited on the sediment surface. Results from this study demonstrate that LCS sedimentary metal oxide cycling may be more important in organic carbon mineralization pathways than previously recognized. [ABSTRACT FROM AUTHOR]

Gassy sediments contribute to destabilization of aquatic infrastructure, air pollution, and global warming. In the current study a precise shape and size of the buoyant mature methane bubble in fine-grained muddy aquatic sediment is defined by numerical and analytical modeling, their results are in a good agreement. A closed-form analytical solution defining the bubble parameters is developed. It is found that the buoyant mature bubble is elliptical in its front view and resembles an inverted tear drop in its cross-section. The size and shape of the mature bubble strongly correlate with sediment fracture toughness. Bubbles formed in the weaker sediments are smaller and characterized by a larger surface-to-volume ratio that induces their faster growth and may lead to their faster dissolution below the sediment–water interface. This may prevent their release to the water column and to the atmosphere. Shapes of the bubbles in the weaker sediments deviate further from the spherical configuration, than those in the stronger sediments. Modeled bubble characteristics, important for the acoustic applications, are in a good agreement with field observations and lab experiments. [ABSTRACT FROM AUTHOR]

Methane (CH 4) bubbles in muddy aquatic sediments threaten climate sustainability and sediment mechanical stability. Mechanical response of muddy sediment to bubble growth is described by Linear Elastic Fracture Mechanics (LEFM). Minor roles of mechanical sediment characteristics in CH 4 bubble solute supply and growth rates, were quantified by preceding studies, compared to biogeochemical controls. We investigate them using a coupled single-bubble mechanical/reaction-transport numerical and analytical models. We demonstrate that inner pressure of the growing bubble at fracturing, concentration at its surface, bubble size and spatial location, are uniquely defined by the Fracture Toughness of muddy sediment. However, a temporal evolution of the bubble inner pressure at expansion between the fracturing events depends on the Young's modulus. Fracture Toughness and Young's modulus thus play complementary, spatial and temporal, roles in the bubble inner pressure evolution. The proportionality suggested by LEFM manages the bubble solute exchange and growth rates. Higher Fracture Toughness controls development of longer flatter bubbles in the deeper sediments. A substantial role of the mechanical muddy sediment characteristics in the CH 4 bubble growth dynamics and solute exchange is demonstrated, comparable to the role of the biogeochemical controls. Their contributions to development of no-growth and competitive bubble growth conditions, affecting a gas dynamics at a macro-scale, are discussed. [Display omitted] • Fracture Toughness-to-Young's modulus ratio controls solute supply and bubble growth rate. • Bubble pressure, concentration, size and spatial location are defined by Fracture Toughness. • Temporal evolution of bubble inner pressure depends on Young's modulus. • Flat bubble shape in deep sediments is due to bigger contribution of Fracture Toughness. [ABSTRACT FROM AUTHOR]

Abstract: Although magnetic susceptibility (MS) of sediments is widely used as an indicator in paleoclimate reconstructions, its significance is unclear in short-term environmental change in muddy sediments on the continental shelves of the Yellow Sea and the East China Sea. In the present study, we found positive and negative correlations between MS and the median grain size (MZ) of fine sediments near the Yangtze estuary and in the North Yellow Sea, respectively. These distinct relationships between MS and MZ depend on sources of materials thus can be used to study changes in material sources of sediments in the South Yellow Sea. According to these relationships, the positive correlation between MS and MZ in the central South Yellow Sea Mud before 1920 AD revealed the impact of materials from the Yangtze River; and the negative correlation after 1920 AD could be attributed to the changes of location of the Yangtze trunk stream to the sea. The inference of the geological change around 1920 AD was also supported by the abrupt change in trends of TOC/TN ratio. [Copyright &y& Elsevier]

Large areas of muddy sediments on the coastal shelves of China provide important samples for studying climate and ecological changes. Analysis of a large number of such samples, which is essential for systematic study on environmental information recorded in mud areas because of complicated sedimentary environment and variable sedimentary rate, requires a fast and economical method. In this study, we investigated the potential of X-ray fluorescence core scanner (XRFS), a fast analytical instrument for measuring the elemental concentrations of muddy sediments, and observed a significant correlation between the element concentrations of muddy sediments determined by regular X-ray fluorescence spectrometer (XRF) and XRFS, respectively. The correlations are mainly determined by excitation energy of elements, but also influenced by solubility of element ions. Furthermore, we found a striking link between Al concentrations and marine-originated organic carbon (MOC), a proxy of marine primary productivity. This indicates that MOC is partly controlled by sedimentary characteristics. Therefore, XRFS method has a good potential in fast analysis of a large number of muddy sediment samples, and it can also be used to calibrate MOC in ecological study of coastal seas. [ABSTRACT FROM AUTHOR]

Abstract: Methane (CH4) is the most abundant hydrocarbon and one of the most important greenhouse gases in the atmosphere. CH4 bubble growth and migration within muddy aquatic sediments are closely associated with sediment fracturing. In this paper we present the modeling of buoyancy-driven CH4 bubble growth in fine-grained muddy aquatic sediment prior to the beginning of its rise. We designed a coupled mechanical/reaction-transport numerical model that enables a differential fracturing over the bubble front (as it occurs in nature), when the fracturing increment stays constant at the bubble head and subsides towards bubble tail during bubble growth. We show that this differential fracturing over the bubble front controls the bubble shape and size temporal evolution, and is significantly affected by the critical stress intensity factor of the muddy sediment. The intercalated stages of elastic expansion and fracturing during the bubble growth shorten with time as the bubble approaches its terminal size (prior to its ascent). Our simulations reveal a high asymmetry in the bubble shape growing with time, with respect to its initial symmetric penny-shaped configuration. It was found that the bubble grows allometrically, while the importance of the bubble surface area growth with time. We also confirmed the earlier predictions about the ”inverted tear-drop” bubble cross-section just prior to the beginning of its rise. Modeling of the terminal bubble characteristics will permit prediction of the delivery of gaseous methane from the sediment to the atmosphere via the water column. [Copyright &y& Elsevier]

To investigate seasonal variations of nutrient distribution in the mudflat-shallow water system, we conducted field surveys once a month from August 2007 to July 2008 in the inner area of Ariake Bay (IAB), Japan. The NH-N concentration of the water column increased in autumn because of the high NH release from the sediments, ranging from 850 to 3,001 μmol m day. The NO-N concentration was maximal in January, which was thought to be caused by NO release from the oxic sediments and by NO regeneration due to water column nitrification. The PO-P concentration of the water column was high in summer-autumn due to the high PO release from the reduced sediments, ranging from 22 to 164 μmol m day. We estimated the total amounts of DIN and PO-P release ( R and $$ {R_{{\mathrm{P}{{\mathrm{O}}_4}}}} $$, respectively) from the muddy sediment area of the IAB. In summer-autumn, R and $$ {R_{{\mathrm{P}{{\mathrm{O}}_4}}}} $$ corresponded to about 47.7 % of DIN input and about 116.6 % of PO-P input from the river, respectively. Thus, we concluded that the muddy sediments were an important source of nutrients for the water column of the IAB during summer-autumn. In addition, we found that phosphorus necessary for the growth of Porphyra ( Porphyra yezoensis, Rhodophyceae) would be insufficient in the water column when phosphorus during the Porphyra aquaculture period is supplied only from the river. Therefore, the phosphorus release from the muddy sediments was thought to play an important role in the sustainable production of Porphyra in Ariake Bay. [ABSTRACT FROM AUTHOR]

The concentrations of nine heavy metals (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pd and Zn) in the labile and total fractions of muddy and sandy sediment samples collected from twelve sites in Suez Gulf during April 1999 were studied to evaluate the pollution status of the Suez Gulf. The enrichment factors (EF) for each element were calculated. There are extremely high concentrations of Cd, Ni, Pb and slightly concentration of Cr and Cu in both muddy and sandy sediments. The concentration of Zn was moderately high and can be considered as seriously contaminate Metal pollution index (MPI) shows high values ranged between 46 to 156 and 40 to 232 for both sandy and muddy sediments, respectively. Concentrations of heavy metals were normalized against iron for total fraction in both of sandy and muddy sediments. Principal component analysis (PCA) was studied on the data matrix obtained and represented three-factor model explaining 92.22% for labile and 88.82% for total fractions of muddy sediment. The main source of contamination is the offshore oil fields and industrial wastes. This is largely a result of ineffective and inefficient operation equipment, illegal discharge of dirty ballast water from tankers and lack of supervision and prosecution of offenders. [ABSTRACT FROM AUTHOR]

Extraction of large numbers of living and debris-free nematodes is based on the active downward migration of nematodes through muddy sediments. This study is an assessment of how representative the extracted nematode population is in comparison to the in situ population. Nematode extraction efficiencies ranged from 20-29%; however, the method yielded 153 000 and 223 000 debris-free nematodes in 48 h (per m2 of mud with nematode population density of 776 000 individuals m–2). Sieve mesh size, migration device mesh size, and size distribution and behaviour of nematode species all influenced the extraction results. The species composition of the extracted nematode populations was significantly different from the in situ population assemblage. Metachromadora remanei and M. vivipara migrated less efficiently than Ptycholaimellus jacobi, Chromadora macrolaima and Daptonema oxycerca. This bias should be monitored in experimental studies using nematode populations extracted by means of sieving and downward migration. [ABSTRACT FROM AUTHOR]

Microbial spoilage of olive fruits is among the most frequent reasons for two types of off-flavors in olive oils, assigned as musty and fusty/muddy sediment. To characterize both off-flavors on a molecular level by means of the sensomics approach, the key aroma compounds in a premium extra virgin olive oil ( PreOO 1) eliciting the typical sensory properties were compared to those identified in two off-flavor olive oils obtained from the International Olive Council (IOC). A comparative aroma extract dilution analysis (cAEDA) followed by identification experiments revealed 53 odorants in the musty reference olive oil ( MusOO 1) and 48 odorants in the fusty/muddy sediment one ( FusOO 1). Odorants, differing in flavor dilution (FD) factors or showing a high FD factor in at least one of the olive oils, were quantitated by stable isotope dilution analysis (SIDA), followed by the calculation of odor activity values (OAVs; ratio of concentration of an odorant to the respective odor threshold in odorless refined sunflower oil). Aroma recombination and omission experiments revealed 13 key aroma compounds in MusOO 1 and 12 in FusOO 1. To demonstrate the importance of volatile phenols, 10 phenolic smelling odorants were quantitated in further 13 extra virgin olive oils, in 3 musty and in 13 fusty/muddy sediment defective olive oils, and in 8 olive oils with other off-flavors. Both sensory defects could successfully be discriminated from extra virgin olive oils by applying either a principal component analysis or a hierarchical cluster analysis. Considering possible reaction pathways and all results obtained including Pearson coefficients between the odorant concentrations and the intensities of the defects, specifically 2-methoxyphenol and 4-ethylphenol were proposed as marker compounds for the quality assignment of both types of off-flavors induced by microbial spoilage among the identified key aroma compounds.