The fate of carbon resulting from pore water exchange in a mangrove and Spartina alterniflora ecozone

Mangrove and salt-marsh wetlands are important coastal carbon sinks.In order to quantify carbon export via pore water exchange and to evaluate subsequent fate of the exported carbon,we carried out continuous observations in a mangrove-Spartina alterniflora ecozone in the Zhangjiang River Estuary,China.The carbon fluxes via pore water exchange were estimated using^(222)Rn and^(228)Ra as tracers to be(2.15±0.63)mol/(m^(2)∙d)for dissolved inorganic carbon(DIC)and(-0.008±0.07)mol/(m^(2)∙d)for dissolved organic carbon(DOC)in the wet season and(3.02±0.65)mol/(m^(2)∙d)for DIC and(-0.15±0.007)mol/(m^(2)∙d)for DOC in the dry season in the mangrove-dominated creek(M-creek),while(2.52±0.82)mol/(m^(2)∙d)for DIC and(0.02±0.09)mol/(m^(2)∙d)for DOC in the dry season in the S.alterniflora-dominated creek(SA-creek).The negative value means that pore water was a sink of DOC in the creek.The total carbon via pore water exchange in the tidal creeks in the mangroves accounted for 41%-55%of the net carbon fixed by mangrove vegetation and was 3-4 times as much as the soil carbon accretion in the mangroves.The exported carbon in the form of DIC contributed all of the carbon outwelling from the M-creek and 79%of the carbon outwelling from the SA-creek,implying effective fixation of carbon by the wetland ecosystem.Moreover,it resulted in 54%in the dry season,75%in the wet season of the carbon dioxide released from the M-creek to the atmosphere,and 84%of the release from the SA-creek.Therefore,quantification of pore water exchange and related soil carbon loss is essential to trace the fate of carbon fixed in intertidal wetlands.

Effect of particle composition and consolidation degree on the wave-induced liquefaction of soil beds

The wave-induced liquefaction of seabed is responsible for causing damage to marine structures.Particle composition and consolidation degree are the key factors affecting the pore water pressure response and liquefaction behavior of the seabed under wave action.The present study conducted wave flume experiments on silt and silty fine sand beds with varying particle compositions.Furthermore,a comprehensive analysis of the differences and underlying reasons for liquefaction behavior in two different types of soil was conducted from both macroscopic and microscopic perspectives.The experimental results indicate that the silt bed necessitates a lower wave load intensity to attain the liquefaction state in comparison to the silty fine sand bed.Additionally,the duration and development depth of liquefaction are greater in the silt bed.The dissimilarity in liquefaction behavior between the two types of soil can be attributed to the variation in their permeability and plastic deformation capacity.The permeability coefficient and compression modulus of silt are lower than those of silty fine sand.Consequently,silt is more prone to the accumulation of pore pressure and subsequent liquefaction under external loading.Prior research has demonstrated that silt beds with varying consolidation degrees exhibit distinct initial failure modes.Specifically,a dense bed undergoes shear failure,whereas a loose bed experiences initial liquefaction failure.This study utilized discrete element simulation to examine the microscopic mechanisms that underlie this phenomenon.

Wave-induced pore water pressure in marine cohesive soils

Cyclic triaxial tests and numerical analyses were undertaken, in order to evaluate the wave-induced pore water pressure in seabed sediments in the Hangzhou Bay. The cyclic triaxial tests indicate that the rate of pore water pressure generation in cohesive soils decreases with time, and the development of the pore water pressure can be represented by a hyperbolic curve. Numerical analyses, taking into account the generation and dissipation of pore water pressure simultaneously, suggest that the pore water pressure buildup in cohesive soils may increase with time continuously until the pore water pressure ratio approaches to 1, or it may decrease after a certain time, which is controlled by drain conditions. These phenomena are different from those in sands. For waves with a retum period of 100 a in the Hangzhou Bay, if the wave duration is more than 60 h, then the pore water pressure ratio will be close to 1 and soil fabric failure will take place.

The numerical study of wave-induced pore water pressure response in highly permeable seabed

The coupling numerical model of wave interaction with porous medium is used to study wave- induced pore water pressure in high permeability seabed. In the model, the wave field solver is based on the two dimensional Reynolds-averaged Navier-Stokes （RANS） equations with a k-s closure, and Forchheimer equations are adopted for flow within the porous media. By introducing a Velocity-Pressure Correction equation for the wave flow and porous flow, a highly efficient coupling between the two flows is implemented. The numerical tests are conducted to study the effects of seabed thickness, porosity, particle size and intrinsic permeability coefficient on regular wave and solitary wave-induced pore water pressure response. The results indicate that, as compared with regular wave-induced, solitary wave-induced pore water pressure has larger values and stronger action on seabed with different parameters. The results also clearly show the flow characteristics of pore water flow within seabed and water wave flow on seabed. The maximum pore water flow velocities within seabed under solitary wave action are higher than those under regular wave action.

Fully Coupled Simulation of Interactions Among Waves, Permeable Breakwaters and Seabeds Based on N−S Equations

Interstitial flows in breakwater cores and seabeds are a key consideration in coastal and marine engineering designs and have a direct impact on their structural safety.In this paper,a unified fully coupled model for wave−permeable breakwater−porous seabed interactions is built based on an improved N−S equation.A numerical wave flume is constructed,and numerical studies are carried out by applying the finite difference method.In combination with a physical model test,the accuracy of the numerical simulation results is verified by comparing the calculated and measured values of wave height at measurement points and the seepage pressure within the breakwater and seabed.On this basis,the characteristics of the surrounding wave field and the internal flow field of the pore structure,as well as the evolution process of the fluctuating pore water pressure inside the breakwater and seabed,are further analyzed.The spatial distribution of the maximum fluctuating pore water pressure in the breakwater is compared between two cases by considering whether the seabed is permeable,and then the effect of seabed permeability on the dynamic pore water pressure in the breakwater is clarified.This study attempts to provide a reference for breakwater design and the protection of nearby seabeds.

Relative roles of resuspended particles and pore water in release of contaminants from sediment

Sediment layers containing contaminants play a significant role in environmental hydrodynamics. Experiments were conducted in order to characterize the relative roles of resuspended particles and pore water under different flow and sediment conditions. A conservative tracer （NaC1） and a reactive tracer （phosphate） were used as contaminants in the bottom sediment in a laboratory flume. The mixing between the overlying water and pore water occurred over a short time while the desorption of contaminants from fine-grained resuspended particles lasted a relatively long time. The effects of resuspended particles and pore water on the variations of release flux and concentration of contaminants in water with time under different hydrodynamic conditions were quantified. The results show that pore water dominated the initial release flux, which could be several orders of magnitude greater than the flux due to molecular diffusion. Flux contribution of desorption from sediment particles in the latter release could be equal to what was seen from pore water in the initial stage.

Experimental study on characteristics of pore water conversion during methane hydrates formation in unsaturated sand

Understanding the pore water conversion characteristics during hydrate formation in porous media is important to study the accumulation mechanism of marine gas hydrate.In this study,low-field NMR was used to study the pore water conversion characteristics during methane hydrate formation in unsaturated sand samples.Results show that the signal intensity of T_(2) distribution isn’t affected by sediment type and pore pressure,but is affected by temperature.The increase in the pressure of hydrogen-containing gas can cause the increase in the signal intensity of T_(2) distribution.The heterogeneity of pore structure is aggravated due to the hydrate formation in porous media.The water conversion rate fluctuates during the hydrate formation.The sand size affects the water conversion ratio and rate by affecting the specific surface of sand in unsaturated porous media.For the fine sand sample,the large specific surface causes a large gas-water contact area resulting in a higher water conversion rate,but causes a large water-sand contact area resulting in a low water conversion ratio(C_(w)=96.2%).The clay can reduce the water conversion rate and ratio,especially montmorillonite(C_(w)=95.8%).The crystal layer of montmorillonite affects the pore water conversion characteristics by hindering the conversion of interlayer water.

Characteristics of Pore Water Pressure of Saturated Silt Under Wave Loading

The characteristics of dynamic stress in the seabed under wave loading are constant principal stress and continuous rotation of the principal stress direction. Cyclic triaxial-torsional coupling shear tests were pefformed on saturated silt by the hollow cylinder apparatus under different relative densities, deviator stress ratios and vibration frequencies to study the development of pore water pressure of the saturated silt under wave loading. It was found that the development of pore water pressure follows the trend of ＂fast - steady - drastic＂. The turning point from fast to steady stage is not affected by relative density and deviator stress ratio. However, the turning point from steady to drastic stage relies on relative density and deviator stress ratio. The vibration cycle for the liquefaction of saturated silt decreases with increasing deviator stress ratio and increases with relative density. The vibration cycle for the liquefaction of the saturated silt increases with vibration frequency and reaches a peak value, after which it decreases with increasing vibration frequency for the relative density of 70%. But the vibration cycle for the liquefaction of saturated silt increases with vibration frequency for the relative density of 30%. The development of pore water pressure of the saturated silt is influenced by relative density and vibration frequency.

Experimental Study of Pore Water Pressure and Bed Profile Change Under Regular Breaking Waves

There lies a close relationship between the seabed destruction and the distribution of pore water pressure under the action of breaking wave. The experiments were carried out in a wave flume with a 1：30 sloping sandy seabed to study regular breaking wave induced pore water pressure. A wide range of measurements from the regular wave runs were reported, including time series of wave heights, pore pressures. The video records were analysed to measure the time development of the seabed form and the characteristics of the orbital motion of the sand in the wave breaking region. The pore water pressure in the breaker zone showed the time variation depending on the wave phases including wave breaking and bore propagation. The time-averaged pore water pressure was higher near the seabed surface. The peak values of pore water pressure increase significantly at the breaking point. The direction of pore water pressure difference forces in the breaker zone is of fundamental importance for a correct description of the sediment dynamics. The upwards- directed pressure differences may increase sand transport by reducing the effective weight of the sediment, thereby increasing the bed form evolution. The seabed configuration changed greatly at the wave breaking zone and a sand bar was generated remarkably. The amplitude of the pore water pressure changed with the seabed surface. The results are to improve the understanding of sand transport mechanisms and seabed responses due to breaking regular waves over a sloping sandy bed.

Understanding the microscopic moisture migration in pore space using DEM simulation

The deformation of soil skeleton and migration of pore fluid are the major factors relevant to the triggeringof and damages by liquefaction. The influence of pore fluid migration during earthquake has beendemonstrated from recent model experiments and field case studies. Most of the current liquefactionassessment models are based on testing of isotropic liquefiable materials. However the recent NewZealand earthquake shows much severer damages than those predicted by existing models. A fundamentalcause has been contributed to the embedded layers of low permeability silts. The existence ofthese silt layers inhibits water migration under seismic loads, which accelerated liquefaction and causeda much larger settlement than that predicted by existing theories. This study intends to understand theprocess of moisture migration in the pore space of sand using discrete element method （DEM） simulation.Simulations were conducted on consolidated undrained triaxial testing of sand where a cylindersample of sand was built and subjected to a constant confining pressure and axial loading. The porositydistribution was monitored during the axial loading process. The spatial distribution of porosity changewas determined, which had a direct relationship with the distribution of excess pore water pressure. Thenon-uniform distribution of excess pore water pressure causes moisture migration. From this, themigration of pore water during the loading process can be estimated. The results of DEM simulationshow a few important observations： （1） External forces are mainly carried and transmitted by the particlechains of the soil sample; （2） Porosity distribution during loading is not uniform due to nonhomogeneoussoil fabric （i.e. the initial particle arrangement and existence of particle chains）; （3）Excess pore water pressure develops differently at different loading stages. At the early stage of loading,zones with a high initial porosity feature higher porosity changes under the influence of external loading,which leads to a larger pore pressure variation （increase or decrease） in such zones. As the axial strainincreases, particle rearrangement occurs and final porosity distribution has minor relationship with theinitial condition, and the pore pressure distribution becomes irregular. The differences in the porepressure development imply that water will migrate in the pore space in order to balance the pore waterpressure distribution. The results of this simulation offer an insight on the microscale water migration inthe soil pore space, which is important for holistic description of the triggering of soil liquefaction in lightof its microstructure. 2015 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.

Pore Pressure Accumulation of Anisotropically Consolidated Soft Clay Subjected to Complex Loads Under Different Stress Paths

Owing to different influence factors of foundation soil,the initial stress state of the soil under various working conditions is complex.To simulate this situation,in this paper,a series of tests on undisturbed soft clay under pure principal stress axis rotation were carried out by using the hollow cylinder apparatus(HCA).The influence of initial consolidation angle ζ(the angle between the vertical direction and direction of the applied load in consolidation)and intermediate principal stress coefficient b on pore water pressure accumulation of undisturbed soft clay were mainly studied.The test results show that,during pure principal stress axis rotation,the pore water pressure accumulation of the undisturbed soft clay fluctuates and increases with the rotation of the major principal stress;the values of major principal stress anglesα,corresponding to the peak value of the pore water pressure in a certain cycle,are different with different initial consolidation angles;the pore water pressure accumulation of soft clay is greatly affected by the intermediate principal stress coefficient b.With the fixed initial consolidation angle ζ,the variation trend of the maximum pore water pressure for each cycle is appropriately the same with different b values.With the increase of cycles,the difference value of pore water pressure between b=0 and b=1 in each cycle increases gradually with different initial consolidation angles ζ.While with different initial consolidation anglesζ,the increase of the pore water pressure when b increases from 0 to 0.5 is different with that when b increases from 0.5 to 1;the variation of maximum pore water pressure withζis significantly affected by the value of b;the value of maximum pore water pressure increases with the cycle number increases under all test conditions,but the growth rate decreases gradually.And the variation of maximum pore water pressure with the cycle number N is obviously influenced by both ζ and b.

Br/Cl,I/Cl and chlorine isotopic compositions of pore water in shallow sediments：implications for the fluid sources in the Dongsha area,northern South China Sea

The Dongsha area is one of the most promising target areas for gas hydrate exploration in the South China Sea（SCS）.The study of pore water geochemistry has played a key role in Chinese gas hydrate exploration.Br/Cl,I/Cl and δ37Cl in pore water were applied here in tracing gas hydrate occurrence,chemical evolution of pore fluids and water/rock interactions in low temperature sediment environments.The samples were collected from Sites HD255 PC and HD309 PC in the Dongsha area in 2004.At Site HD255 PC,we found the elevated Br/Cl,I/Cl and decreased SO_4/Cl at the depth of 4–5 m,suggestive of a laterally migrated fluid probably generated from the gas hydrate occurrence.The range of δ37Cl is –0.54‰ to ＋0.96‰,and positive δ^（37）Cl at 4–5 m interval should be related with different diffusion rates between ^（35）Cl and ^（37）Cl.At Site HD309 PC,a laterally migrated fluid was also found at the depth of 3–4 m,with the Br/Cl two times to that of the seawater and decreased I/Cl,indicating the fluid has no relationship with the gas hydrate.In this site,the chlorine isotopic composition varies from –0.7‰ to＋1.9‰.Extra high Br/Cl might relate with the deep generated fluid.At higher temperature and pressure,the Br/Cl of the fluid is elevated during the hydrous silicate formation,while positive δ37Cl is also associated with the same mechanism.

The effect of pore fluid on seismicity: a computer model

The influence of fluid on seismicity of a computerized system is analyzed in this paper. The diffusion equation of fluid in a crustal fault area is developed and used in the calculation of a spring slide damper model. With mirror imagin boundary condition and three initial conditions, the equation is solved for a dynamic model that consists of six seismic belts and eight seismogenous sources in each belt with both explicit algorithm and implicit algorithm. The analysis of the model with water sources shows that the implicit algorithm is better to be used to calculate the model. Taking a constant proportion of the pore pressure of a broken element to that of its neighboring elements, the seismicity of the model is calculated with mirror boundary condition and no water source initial condition. The results shows that the frequency and magnitude of shocks are both higher than those in the model with no water pore pressure, which provides more complexity to earthquake prediction.

Experiment and analysis of the formation,expansion and dissipation of gasbag in fine sediments based on pore water pressure survey

Deep-seated gas in seabed sediments migrates upwards from effect of external factors,which easily accumulates to form gasbags at interface of shallow coarse-fine sediments.Real-time monitoring of this process is important to predict disaster.However,there is still a lack of effective monitoring methods,so we attempt to apply multi-points pore water pressure monitoring technology when simulating forming and dissipation of gasbags in sediments through laboratory experiment.This study focuses on discussion of sensitivity of pore water pressure monitoring data,as well as typical changing characteristics and mechanisms of excess pore water pressure corresponding to crack generation,gasbag formation and gas release.It was found that the value of excess pore water pressure in sediments is negatively correlated with vertical distance between sensors and gas source,and the evolution of gasbag forming and dissipation has a good corresponding relationship with the change of excess pore water pressure.Gasbag formation process is divided into three stages:transverse crack development,longitudinal cavity expansion,and oblique crack development.Formation of gasbag begins with the transverse crack at the interface of coarse-fine sediments while excess pore water pressure attenuates rapidly and then drops,pressure remains almost unchanged when cavity expanses longitudinally,oblique crack appeared in final stage of gasbag evolution while excess pore water pressure accumulated and dissipated again.The variation curve of excess pore water pressure in gas release stage has saw-tooth fluctuation characteristics,and the value and time of pressure accumulation are also fluctuating,indicating the uncertainty and non-uniqueness of gas migration channels in sediments.

Pore Water Pressure Buildup Under Cyclic Rotation of Principal Stress and Stability Evaluation of Seabed Deposit

The cyclic rotation of principal stress direction with a constant amplitude is the characteristics of cyclic stress in seabed deposit induced by travelling waves. Presented in the paper are the results obtained from tests simulating the cyclic stress characteristics, with emphasis laid on the buildup of pore water pressure in soil samples. Regression analysis of test data shows that the pore water pressure can be expressed as the function of the number of cycles of cyclic loading, or as the function of generalized shear strain. Using the results thus obtained, the possibility of failure of seabed deposit under cyclic loading induced by travelling waves can be evaluated. The comparison with the results of conventional cyclic torsional shear tests shows that neglect of the effect of the cyclic rotation of the principal stress direction will result in considerable over-estimation of the stability of seabed deposit.

Pore-water geochemistry in methane-seep sediments of the Makran accretionary wedge off Pakistan:Possible link to subsurface methane hydrate

Cold seeps are pervasive along the continental margin worldwide,and are recognized as hotspots for elemental cycling pathway on Earth.In this study,analyses of pore water geochemical compositions of one-400 cm piston core(S3)and the application of a mass balance model are conducted to assess methane-associated biogeochemical reactions and uncover the relationship of methane in shallow sediment with gas hydrate reservoir at the Makran accretionary wedge off Pakistan.The results revealed that approximately 77%of sulfate is consumed by the predominant biogeochemical process of anaerobic oxidation of methane.However,the estimated sulfate-methane interface depth is-400 cm below sea floor with the methane diffusive flux of 0.039 mol/(m^(2)·a),suggesting the activity of methane seepage.Based on the δ^(13)C_(DIC) mass balance model combined with the contribution proportion of different dissolved inorganic carbon sources,this study calculated the δ^(13)C of the exogenous methane to be-57.9‰,indicating that the exogenous methane may be a mixture source,including thermogenic and biogenic methane.The study of pore water geochemistry at Makran accretionary wedge off Pakistan may have considerable implications for understanding the specific details on the dynamics of methane in cold seeps and provide important evidence for the potential occurrence of subsurface gas hydrate in this area.

Mechanics of water pore formation in lipid membrane under electric field

Transmembrane water pores are crucial for substance transport through cell membranes via membrane fusion, such as in neural communication. However, the molecular mechanism of water pore formation is not clear. In this study, we apply all-atom molecular dynamics and bias-exchange metadynamics simulations to study the process of water pore formation under an electric field. We show that water molecules can enter a membrane under an electric field and form a water pore of a few nanometers in diameter. These water molecules disturb the interactions between lipid head groups and the ordered arrangement of lipids. Following the movement of water molecules, the lipid head groups are rotated and driven into the hydrophobic region of the membrane. The reorientated lipid head groups inside the membrane form a hydrophilic surface of the water pore. This study reveals the atomic details of how an electric field influences the movement of water molecules and lipid head groups, resulting in water pore formation.

Reservoir Characterization and Pore Type Systems of Carbonate Low Resistivity Pay in Persian Gulf

The main focus of study is to characterize lower and upper cretaceous carbonate deposits with Low Resistivity Pay, in Persian Gulf. Four oil reservoirs in the Cretaceous including the Zubair, Buwaib, Shuaiba and Khatiyah Formations of Southern fields have been analyzed. Here is a look at that to determine main factors on decreasing resistivity in pay zone. In some intervals resistivity responses reach less than 6 to 1 ohm·m. Significant hydrocarbon accumulations are “hidden” in low resistivity Pay zone, (LRPZ). LRPZ reservoirs have been found in some formations in Persian Gulf. Causes of LRPZ reservoirs on the basis of experimental analysis include clay-coated grains, carbonate with interstitial dispersed clay. On the other side Smectite and Kaolinite of main clays types have high CEC and greater impact on lowering resistivity. Micritization and Pyritization of digenetic process have noticeable impact on LRPZ. It is mentioned that Lønøy method applied to address pore throat sizes which contain Inter crystalline porosity, Chalky Limestone, Mudstone micro porosity. Pore systems are classified at class 2 and 3 Lucia and pore size varies from 0.5 to 4 micron. NMR Core and Log results show different pore size distribution. NMR core and MRIL results explain that decreasing of resistivity in pay zone is related to texture and grain size variation not being existence of moved water. Irreducible water estimate for this reservoir was between 30% and 50%. T2 cut off estimates, for defining irreducible water saturation, 115 ms.

Electrical resistivity change of saturated sand during reliquefaction under hammering loading

The electrical resistivity method was verified as an optional technique to monitor the change of mesostructure of saturated soils.To investigate the change laws of resistivity and analyze the reliquefaction meso-mechanism during the consecutive liquefaction process,five successive impact liquefaction tests were performed in a one-dimensional cubical chamber.The resistivity variation and excess pore water pressure(EPWP)were measured.The results indicate that the excess pore water pressure experienced four stages:quick increase stage,slow dissipation stage,rapid dissipation stage,and stability stage.Meanwhile,a swift decrease of resistivity emerged before the start of the rapid dissipation stage of EPWP,and then an increasing trend of resistivity is demonstrated with the densification of soil.It is proved that the vertical pore connectivity of liquefied sand is better than its random deposit state,based on a comparative study of porosity calculated from the settlement and resistivity of sand after each test.

A simple guideline to apply excitation-emission matrix spectroscopy(EEMs)for the characterization of dissolved organic matter(DOM)in anoxic marine sediments

Marine sediments represent a major carbon reservoir on Earth.Dissolved organic matter(DOM)in pore waters accumulates products and intermediates of carbon cycling in sediments.The application of excitation-emission matrix spectroscopy(EEMs)in the analysis of subseafloor DOM samples is largely unexplored due to the redox-sensitive matrix of anoxic pore water.Therefore,this study aims to investigate the interference caused by the matrix on EEMs and propose a guideline to prepare pore water samples from anoxic marine sediments.The parameters determined by fluorescence spectra include 3D-index derived from EEMs after parallel factor analysis(PARAFAC),fluorescence index(FI)(contribution of terrigenous DOM),biological index(BIX)and humification index(HIX)derived from 2D emission spectra.First,we investigated the impacts of extensively-presented ions as typical electron acceptors,which are utilized by anaerobic microbes and stratified in marine sediments:Fe(II),Fe(III),Mn(II)and sulfide in anoxic pore water resulted in biases of fluorescent signals.We proposed threshold concentrations of these ions when the interference on EEMs occurred.Effective removal of sulfide from sulfide-rich samples could be achieved by flushing with N_(2)for 2 min.Second,the tests based on DOM standard were further verified using pristine samples from marine sediments.There was a significant change in the fluorescence spectra of DOM in anoxic sediments from the Rhône Delta.This study demonstrated that the change was caused by oxidation of the matrix rather than the intrinsic alteration of DOM.It was confirmed by extracted DOM via both EEMs analysis and Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR-MS).Slight oxidation of sulfur-containing compounds(e.g.,sulfhydryl)and polyphenol-like compounds occurred.Finally,a sample preparation sequence is proposed for pore water from anoxic sediments.This method enables measurement with small volumes of the sample(e.g.,50µL in this study)and ensures reliable data without the interference of the redox-sensitive matrix.This study provides access to the rapid analysis of DOM composition in marine sediments and can potentially open a window into examining the carbon cycling of the marine deep biosphere.