Showing papers on "Pore water pressure published in 2003"
TL;DR: In this paper, a new elasto-plastic framework for unsaturated soils is proposed, involving coupling of hydraulic hysteresis and mechanical behaviour, and a specific constitutive model for isotropic stress states is proposed.
Abstract: Consideration of the different roles of pore air pressure, pore water pressure within bulk water and pore water pressure within meniscus water suggests that the degree of saturation will have a significant influence on the stress–strain behaviour of an unsaturated soil, in addition to any influence of suction. This suggestion is supported by experimental evidence. In the light of this, a new elasto-plastic framework for unsaturated soils is proposed, involving coupling of hydraulic hysteresis and mechanical behaviour. Within the proposed framework, plastic changes of degree of saturation influence the stress–strain behaviour, and plastic volumetric strains influence the water retention behaviour. A specific constitutive model for isotropic stress states is proposed, and model predictions are compared with experimental results, in order to demonstrate some of the capabilities of the new framework. Forms of behaviour that can be represented include proper transitions between saturated and unsaturated types ...
719 citations
TL;DR: Brodsky et al. as discussed by the authors used high sampling rates at a well near Grants Pass, Oregon, to perform the first simultaneous analysis of both the dynamic response of water level and sustained changes.
Abstract: [1] Large, sustained well water level changes (>10 cm) in response to distant (more than hundreds of kilometers) earthquakes have proven enigmatic for over 30 years. Here we use high sampling rates at a well near Grants Pass, Oregon, to perform the first simultaneous analysis of both the dynamic response of water level and sustained changes, or steps. We observe a factor of 40 increase in the ratio of water level amplitude to seismic wave ground velocity during a sudden coseismic step. On the basis of this observation we propose a new model for coseismic pore pressure steps in which a temporary barrier deposited by groundwater flow is entrained and removed by the more rapid flow induced by the seismic waves. In hydrothermal areas, this mechanism could lead to 4 � 10 � 2 MPa pressure changes and triggered seismicity. INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 7209 Seismology: Earthquake dynamics and mechanics; 7212 Seismology: Earthquake ground motions and engineering; 7260 Seismology: Theory and modeling; 7294 Seismology: Instruments and techniques; KEYWORDS: earthquakes, triggering, time-dependent hydrology, fractures Citation: Brodsky, E. E., E. Roeloffs, D. Woodcock, I. Gall, and M. Manga, A mechanism for sustained groundwater pressure changes induced by distant earthquakes, J. Geophys. Res., 108(B8), 2390, doi:10.1029/2002JB002321, 2003.
517 citations
TL;DR: This work explored the use of a continuous radon monitor to measure radon concentrations in coastal zone waters over time periods from hours to days and showed that the mixing loss derived from the 223Ra gradient agreed very favorably to the estimated range based on the calculated radon fluxes.
514 citations
TL;DR: In this paper, a series of tests were conducted to trigger rainfall-induced landslides using a small flume, and the effects of grain size on pore-pressure generation and failure behavior of a landslide mass were analyzed.
372 citations
TL;DR: In this paper, an 11 m high cut slope in a typical expansive clay with medium plasticity in Zaoyang, close to the middle route in Hubei, was selected for a comprehensive and well-instrumented field study of the effects of rainfall infiltration.
Abstract: A major infrastructure project, the South-to-North Water Transfer Project, has been proposed to carry potable water from the Yangtze River region in the south to many arid and semi-arid areas in the northern regions of China, including Beijing. The proposed 1200 km ‘middle route’ of the project is likely to be an open channel with a trapezoidal cross-section. At least 180 km of the proposed excavated canal will pass through areas of unsaturated expansive soils. An 11 m high cut slope in a typical expansive clay with medium plasticity in Zaoyang, close to the ‘middle route’ in Hubei, was selected for a comprehensive and well-instrumented field study of the effects of rainfall infiltration. Two artificial rainfall events were created during a month of field investigation and monitoring. Prior to the rainfall events, high soil suction was measured within the top 1 m of soil. This high initial soil suction created an upward flux of water and moisture. The abundant cracks and fissures near the ground surface p...
213 citations
TL;DR: In this paper, the effects of surface gravity waves on pore-water release from permeable sediment (k = 1.3-1.8 × 10-11 m 2 ) in shallow water were studied in a wave tank and tracer experiments demonstrated that shallow-water waves can increase fluid exchange between sandy sediment and overlying water 50-fold relative to the exchange by molecular diffusion.
Abstract: The effects of surface gravity waves on pore-water release from permeable sediment (k = 1.3-1.8 × 10 -11 m 2 ) in shallow water were studied in a wave tank Our tracer experiments demonstrated that shallow-water waves can increase fluid exchange between sandy sediment and overlying water 50-fold, relative to the exchange by molecular diffusion. The main driving force for this increased exchange are the pressure gradients generated by the interaction of oscillating boundary flows and sediment wave ripples. These gradients produce a pore-water flow field, with a regular pattern of intrusion and release zones, that migrates with ripple propagation. The ensuing topography-related filtering rates in the wave tank ranged from 60 to 590 L m -2 d -1 and exceeded the solute exchange rates caused by hydrostatic wave pumping (38 L m -2 d -1 ) and initial molecular diffusion (corresponding to 10-12 L m -2 d -1 ). Wave-induced filtration is ecologically relevant because permeable sandy sediments are very abundant on the continental margins and can be converted into effective filter systems, which suggests that these sediments are sites for rapid mineralization and recycling. We propose that the wave influenced continental shelf may be subdivided into two zones; a shallow zone (water depth < wavelength/2), where wave orbital motion at the sea floor creates ripples and causes topography related advective filtering; and a deeper zone (wavelength/2 < water depth < wavelength), where wave pumping enhances interfacial exchange by hydrostatic pressure oscillations.
210 citations
TL;DR: In this paper, the effects of mechanical forces and poroelasticity on shale behavior are included, as well as chemical and thermal effects, for determining wellbore stability for oil and gas drilling operations.
190 citations
TL;DR: In this paper, the authors examined the relationship between stream-subsurface exchange fluxes, delivery of suspended sediments to the hyporheic region, fine particle accumulation in the streambed, and alteration of sedimentary properties.
Abstract: [1] Hyporheic exchange (mixing of stream water with pore water beneath the stream) is responsible for the transport of many ecologically relevant substances across the stream-subsurface interface. This process is dependent on both the streamflow conditions and sedimentary properties, but the complex nature of fluvial systems has presented a barrier to the development of mechanistic understanding of the dynamics of stream-subsurface interactions. This work presents the results of a controlled study to examine in detail the relationship between stream-subsurface exchange fluxes, delivery of suspended sediments to the hyporheic region, fine particle accumulation in the streambed, and alteration of sedimentary properties. Laboratory flume experiments were used to observe kaolinite clay deposition in a sand bed and the resulting alteration of hyporheic exchange fluxes. Solute and suspended sediment exchange with clean sand beds is predicted well by a fundamental model for bed form-driven advective pumping exchange. However, substantial accumulation of clay in the bed causes an alteration of the pore water flow environment, which reduces both water flux across the stream-subsurface interface and subsequent particle deposition. Measurement of bulk solute exchange and direct observation of clay accumulation in the bed both indicate that transported fine particles are preferentially removed near the stream-subsurface interface. Clogging of inflow regions produces heterogeneous subsurface clay deposits even when the bed is initially homogeneous. This behavior is also predicted by the fundamental colloid pumping model. These results contradict the accepted view that suspended sediments will generally deposit in the deepest regions of the bed and thus clog the bed from the bottom upward. Our results indicate that clogging of the streambed surface will often isolate deeper regions of the bed from the streamflow, so that even relatively low amounts of suspended sediments can substantially degrade streambed habitat.
184 citations
TL;DR: In this paper, a new formulation of general constitutive equations for unsaturated soils is first presented in the incremental stress-strain equations, the suction or pore water pressure is treated as a strain variable instead of a stress variable, and the discretized governing equations are then solved using an adaptive time-stepping scheme which automatically adjusts the time-step size so that the integration error in the displacements and pore pressures lies close to a specified tolerance.
Abstract: This paper presents a complete finite-element treatment for unsaturated soil problems A new formulation of general constitutive equations for unsaturated soils is first presented In the incremental stress–strain equations, the suction or the pore water pressure is treated as a strain variable instead of a stress variable The global governing equations are derived in terms of displacement and pore water pressure The discretized governing equations are then solved using an adaptive time-stepping scheme which automatically adjusts the time-step size so that the integration error in the displacements and pore pressures lies close to a specified tolerance The non-linearity caused by suction-dependent plastic yielding, suction-dependent degree of saturation, and saturation-dependent permeability is treated in a similar way to the elastoplasticity An explicit stress integration scheme is used to solve the constitutive stress–strain equations at the Gauss point level The elastoplastic stiffness matrix in the Euler solution is evaluated using the suction as well as the stresses and hardening parameters at the start of the subincrement, while the elastoplastic matrix in the modified Euler solution is evaluated using the suction at the end of the subincrement In addition, when applying subincrementation, the same rate is applied to all strain components including the suction Copyright © 2003 John Wiley & Sons, Ltd
184 citations
TL;DR: Uranium (U) concentrations and activity ratios (δ 234U) of authigenic carbonates are sensitive recorders of different fluid compositions at submarine seeps of hydrocarbon-rich fluids ("cold seeps") at Hydrate Ridge, off the coast of Oregon, USA as discussed by the authors.
183 citations
TL;DR: In this article, the authors analyzed the spatio-temporal distribution of the late aftershocks of the Antofagasta (northern Chile) 1995 earthquake and showed that the aftershock were concentrated in a plane, an approximately 3 km-thick spatial zone.
Abstract: — We consider various cases of seismicity, induced by artificial fluid injections in boreholes. Like many other authors, we support the hypothesis that to a large extent the triggering of this seismicity is caused by a diffusive process of the pore pressure relaxation in porous (or fractured), saturated rocks. We show that if this hypothesis is correct, then the spatio-temporal distributions of the seismic events must have several specific features related to the effective permeability of the rock. As a rule the fluid injection-induced seismicity obeys such features. These features can be indications of the diffusive and even hydraulic nature of the seismicity triggering process.¶From this point of view we analyze the spatio-temporal distribution of the late aftershocks of the Antofagasta (northern Chile) 1995 earthquake. These aftershocks were concentrated in a plane, an approximately 3 km-thick spatial zone. This thin seismogenic layer is a part of the South American subduction zone. The time-distance distribution of the aftershocks along this layer indicates that they could be triggered by a diffusion-like process. Possibly, such a process is the relaxation of the pressure perturbation caused in the pore fluid by the main Antofagasta event. We estimated the permeability required to explain the spatio-temporal distribution of the aftershocks by such a triggering mechanism. The obtained value, 60 mD, is very large. However, it is realistic for a long-time existing and large-scale fault zone.
TL;DR: In fine-grained cohesive beds, bottom currents and sediment microtopography shape the diffusive boundary layer and locally produce areas where the interfacial solute fluxes are increased or reduced as mentioned in this paper.
Abstract: Boundary layer flow characteristics and sediment permeability control pathways and magnitude of material exchange in the surface layer of aquatic sediments. In fine-grained cohesive beds, bottom currents and sediment microtopography shape the diffusive boundary layer and locally produce areas where the interfacial solute fluxes are increased or reduced. Where sediment permeabilities exceed 10−12 m2, advective pore water flows driven by boundary flow—topography interaction dominate the sediment—water exchange of matter, with transport rates that exceed those of molecular diffusion by two orders of magnitude and more. The curved paths of the advective pore flows through the surface layers of such sandy beds generate complex three-dimensional biogeochemical patterns with extreme spatial and temporal variability ranging from millimeters to decimeters and seconds to seasons. High filtration rates, a bacterial community firmly attached to the mineral grains, rapidly changing biogeochemical zonations and winnowing of the sediment surface layers by frequent resuspension convert these beds into effective biocatalytical filler systems.
TL;DR: In this paper, the authors proposed three methods to obtain the shape and range of possible depths of the water table from the study of the self-potential distribution recorded at the ground surface.
Abstract: [1] The electrical potential field passively recorded at the ground surface of the Earth (and termed self-potential) can be analyzed to determine the shape and the depth of the piezometric surface. The coupling between hydraulic flow and electrical current density is electrokinetic in nature. The electrokinetic coupling coefficient entering into the integral equation relating the depth of the water table to self-potential signals is analyzed for various types of porous materials. It is simply related to the electrical conductivity of the pore water. In steady state conditions each element of the water table can be seen as an elementary dipole with an inclination locally perpendicular to the water table and strength proportional to the water table elevation. Then, we propose three methods to obtain the shape and range of possible depths of the water table from the study of the self-potential distribution recorded at the ground surface. The nonuniqueness of the solution is removed if one knows either the electrokinetic coupling coefficient or the water table at one location and under the assumption of the spatial homogeneity of the electrokinetic coupling coefficient. Two field cases are discussed to show the success of the proposed methods for estimating the shape and depth of the water table at two different scales of investigations. They concern the study of self-potential signals associated with the shape of the water table in the vicinity of a pumping well and in the flank of the Kilauea volcano.
TL;DR: Paull et al. as mentioned in this paper used the Egeberg and Dickens model to estimate hydrate concentration and distribution in the subsurface of the Blake Ridge gas-hydrate field in the West Atlantic.
TL;DR: In this article, a combination of field and laboratory tests was carried out recently to investigate the triggering effect of rainfall on shallow slips in alpine moraine slopes, with particle sizes varying from silts to boulders.
Abstract: In comparison with saturated conditions, negative pore water pressures arising from partial saturation increase the available shear strength on a potential slip surface. This additional contribution is lost progressively during infiltration of rainfall, leading to instabilities, sometimes before full saturation is reached. In such cases, reliable prediction of the safety factor may be achieved, by taking the suction history of the soil into account. A combination of field and laboratory tests was carried out recently to investigate the triggering effect of rainfall on shallow slips in alpine moraine slopes. The problem is complicated by the strong heterogeneity of these soils, with particle sizes varying from silts to boulders. The data highlight the influence of suction on the peak shear strength, and allow for the calibration of simple models, which take into account the dependence of shear strength on the saturation degree. An infinite slope stability analysis is performed. The results are compared wit...
TL;DR: In this paper, the authors analyzed the 5 and 6 May 1998 event which affected the Pizzo d'Alvano (Campania, southern Italy) and showed that more than 400 slides affecting shallow pyroclastic deposits were triggered by intense and prolonged but not extreme rainfall.
Abstract: . Pyroclastic soils mantling a wide area of the Campanian Apennines are subjected to recurrent instability phenomena. This study analyses the 5 and 6 May 1998 event which affected the Pizzo d’Alvano (Campania, southern Italy). More than 400 slides affecting shallow pyroclastic deposits were triggered by intense and prolonged but not extreme rainfall. Landslides affected the pyroclastic deposits that cover the steep calcareous ridges and are soil slip-debris flows and rapid mudflows. About 30 main channels were deeply scoured by flows which reached the alluvial fans depositing up to 400 000 m3 of material in the piedmont areas. About 75% of the landslides are associated with morphological discontinuities such as limestone cliffs and roads. The sliding surface is located within the pyroclastic cover, generally at the base of a pumice layer. Geotechnical characterisation of pyroclastic deposits has been accomplished by laboratory and in situ tests. Numerical modelling of seepage processes and stability analyses have been run on four simplified models representing different settings observed at the source areas. Seepage modelling showed the formation of pore pressure pulses in pumice layers and the localised increase of pore pressure in correspondence of stratigraphic discontinuities as response to the rainfall event registered between 28 April and 5 May. Numerical modelling provided pore pressure values for stability analyses and pointed out critical conditions where stratigraphic or morphological discontinuities occur. This study excludes the need of a groundwater flow from the underlying bedrock toward the pyroclastic cover for instabilities to occur.
TL;DR: In this paper, logging-while-drilling (LWD) data, downhole physical properties data, and laboratory consolidation tests from the Costa Rican, Nankai, and Barbados subduction zones were combined to document the development and downsection variability of effective stress and pore pressure within underthrust sediments as they are progressively loaded by subduction.
Abstract: [1] At subduction zones, pore pressure affects fault strength, deformation style, structural development, and potentially the updip limit of seismogenic faulting behavior through its control on effective stress and consolidation state. Despite its importance for a wide range of subduction zone processes, few detailed measurements or estimates of pore pressure at subduction zones exist. In this paper, I combine logging-while-drilling (LWD) data, downhole physical properties data, and laboratory consolidation tests from the Costa Rican, Nankai, and Barbados subduction zones, to document the development and downsection variability of effective stress and pore pressure within underthrust sediments as they are progressively loaded by subduction. At Costa Rica, my results suggest that the lower portion of the underthrust section remains nearly undrained, whereas the upper portion is partially drained. An inferred minimum in effective stress developed within the section ∼1.5 km landward of the trench is consistent with core and seismic observations of faulting, and illustrates the important effects of heterogeneous drainage on structural development. Inferred pore pressures at the Nankai and northern Barbados subduction zones indicate nearly undrained conditions throughout the studied intervals, and are consistent with existing direct measurements and consolidation test results. Slower dewatering at Nankai and Barbados than at Costa Rica can be attributed to higher permeability and larger compressibility of near-surface sediments underthrust at Costa Rica. Results for the three margins indicate that the pore pressure ratio (λ) in poorly drained underthrust sediments should increase systematically with distance landward of the trench, and may vary with depth.
TL;DR: There are important differences between the two investigated temperature modes in the predicted evolution of fluid compositions and mineral alteration around drifts, but these differences are largely within to the model uncertainty and the variability of water compositions at Yucca Mountain.
TL;DR: In this article, the effect of wheeling on soil deformation and stress/strain distribution was investigated in a soil bin which contained Hiwassee clay at the NSDL, Auburn.
Abstract: The discussion about the effect of repeated short time wheeling on long-term changes in soil structure and pore functioning reveals a great uncertainty. On the one hand it is told that soil structure elements are rigid and do not undergo intense changes in pore functions as a consequence of the short loading interval during each single wheeling. On the other hand, the complete deterioration of the structure elements and pore functions is assumed to occur, which also results in changes of the shrinkage pattern, soil strength including even strength regain. Consequently, the effect of wheeling on soil deformation and stress/strain distribution was investigated in a soil bin which contained Hiwassee clay at the NSDL, Auburn. If the soil is very strong due to aggregation, plow pan formation or dryness, soil stress applied by repeated wheeling results in an increased primarily vertical soil particle displacement in the Hiwassee clay soil while during repeated wheeling (up to 10×) a more pronounced displacement linked with a more intense movement of particles can be proofed. With increasing number of wheeling events, new platy or again coherent structure elements are formed, which create a very different pore system. The more intense is soil wheeling, the smaller is the saturated hydraulic conductivity and the higher is the unsaturated one at a given pore water pressure value. Such changes are the more pronounced the more completed is the rearrangement of the still existing aggregates into new units like plates. Due to shear because of the three-dimensional soil displacement even under dry conditions such aggregates can be redisturbed and a coherent but very compacted soil horizon can be formed. Under those conditions the values of bulk density are even higher than the Proctor density.
TL;DR: In this article, the wave velocities and quality factors of gas-hydrate-bearing sediments were obtained as a function of pore pressure, temperature, frequency and partial saturation.
Abstract: We obtain the wave velocities and quality factors of gas-hydrate-bearing sediments as a function of pore pressure, temperature, frequency and partial saturation. The model is based on a Biot-type three-phase theory that considers the existence of two solids (grains and gas hydrate) and a fluid mixture. Attenuation is described with the constant-Q model and viscodynamic functions to model the high-frequency behaviour. We apply a uniform gas/water mixing law that satisfies Wood's and Voigt's averages at low and high frequencies, respectively. The acoustic model is calibrated to agree with the patchy-saturation theory at high frequencies (White's model). Pressure effects are accounted by using an effective stress law for the dry-rock moduli and permeabilities. The dry-rock moduli of the sediment are calibrated with data from the Cascadia margin. Moreover, we calculate the depth of the bottom simulating reflector (BSR) below the sea floor as a function of sea-floor depth, geothermal gradient below the sea floor, and temperature at the sea floor.
TL;DR: In this paper, a geomechanics model that is fully coupled to diffusive transport processes is employed to analyze induced stresses and formation damage, and a closed-form solution including heat conduction and convection is obtained for the stresses near a cylindrical wellbore.
TL;DR: In this paper, the measurement of gauge pore water pressure lower than −100 kPa has been demonstrated, which is the first measurement of water pressure below −100kPa for matric suction.
Abstract: Recent advances in the technology for measuring matric suction have permitted the direct measurement of gauge pore water pressures lower than −100 kPa—that is, absolute tensions. This significant a...
TL;DR: This work illuminates the important role of flow transients in colloid mobilization and, through the derivation of a model that couples pore water flow characteristics with colloid mass transfer kinetics, provides a means for quantifying the phenomenon.
Abstract: [1] Contaminant-scavenging colloids are mobilized in the vadose zone during infiltration events, characterized by transient flow regimes. In the research reported here we develop a mathematical model in order to investigate the influences of flow transients on colloid mobilization in unsaturated media. The model solves coupled equations for unsteady pore water flow, colloid release, advective-dispersive colloid transport, and redeposition of pore water colloids. The immobile-phase colloid population is discretized into a series of compartments, each of which are assigned a value of critical moisture content (Θcr) according to a piecewise linear density function. A compartment is activated when the model-calculated moisture content exceeds the Θcr for the compartment, whereupon colloids are released at a rate proportional to the product of the pore water velocity and the colloid concentration within the compartment. We fit solutions of the model equations to data on the mobilization and transport of silica colloids within columns of unsaturated quartz sand. The model accurately reproduces measured pulse-type colloid releases induced by successive step-change increases in flow rate and moisture content. This work illuminates the important role of flow transients in colloid mobilization and, through the derivation of a model that couples pore water flow characteristics with colloid mass transfer kinetics, provides a means for quantifying the phenomenon.
TL;DR: In this article, the authors used numerical models for the formation of gas hydrate to assess the influence of methane supply on observable features of hydrate occurrences, including pore fluid chlorinity and free gas.
TL;DR: In this paper, the authors performed X-ray diffraction measurements of water confined inside the cylindrical pores of SBA-15 with a pore radius of 3.9 nm as a function of temperature and pore filling.
Abstract: To examine the freezing and melting behavior of water in partially filled pores of porous silica, we performed X-ray diffraction measurements of water confined inside the cylindrical pores of SBA-15 with a pore radius of 3.9 nm as a function of temperature and pore filling. The freezing temperature increased continuously with increasing pore filling even in the region of capillary condensation. The results are related to the different states of the pore water depending on the degree of pore filling. On the other hand, the melting of the frozen pore water took place at a well-defined temperature of 256 K, independent of the level of pore filling. The X-ray diffraction patterns show that the freezing of the pore water results in formation of ice microcrystals with almost the same structure and size, irrespective of the different states of the pore water.
TL;DR: In this article, the role of pore water ionic strength in the transport of silica colloids through water-saturated and unsaturated porous media is investigated, and it is shown that the dominant colloid-immobilization mechanism transitions from straining, to air-water interface capture, and finally, to mineral-grain attachment.
Abstract: [1] Contaminant migration through the vadose zone may be influenced by the presence of mineral colloids that are mobilized during infiltration events. In this work, we report model calculations and experimental data on the role of pore water ionic strength in the transport of silica colloids through water-saturated and unsaturated porous media. The transport model solves the advection-dispersion equation, together with kinetics equations for straining, air-water interface capture, and mineral-grain attachment. Parameters for air-water and solid-water interfacial area required by the transport model are derived through the application of a published liquid-vapor configuration model that accounts for dual-phase (i.e., air and water) occupancy of pore spaces. Comparison of experimental and modeled results reveals that the dominant colloid-immobilization mechanism transitions from straining, to air-water interface capture, and finally, to mineral-grain attachment as the ionic strength increases from 2 × 10−4 M to 0.2 M. The results of this research provide a basic framework for interpreting how interactions between moisture content and pore water chemistry affect colloid mobility in the vadose zone.
TL;DR: In this article, a nonlinear unsaturated shear strength relationship with the matric suction in a hyperbolic form was formulated, where conventional saturated soil parameters (c′, φ′) and an ultimate increment of apparent cohesion (Cmax) are required.
TL;DR: In this paper, the authors developed a physicochemical theory for describing shale deformation, which considers chemical and poroelastic processes and couples ion transfer in the mud/shale system to formation stresses and pore pressure.
TL;DR: The critical Rayleigh number for convective flow of pore water through a permeable spherical body is derived in this article, which affords constraints on carbonaceous chondrite planetesimal sizes and rates of heat production.
01 Jan 2003
TL;DR: In this article, the extent of post-depositional oxidation of organic carbon as well as the dissolution and reprecipitation of minerals across these glacial terminations was shown to depend on the overall sedimentation rate and the magnitude of change encountered in the various depositional and geochemical factors.
Abstract: Nonsteady-state conditions — induced by changes in the fluxes of electron donors and acceptors and environmental conditions — are shown to have been and to be still widespread in sediments of the equatorial and South Atlantic Ocean. Typical diagenetic phenomena initiated under such nonsteady-state conditions comprise the fixation and downward progression of redox boundaries and reaction fronts. Intervals most severely altered by diagenetic overprint often occur cyclically within the sedimentary record and are mostly associated with full glacial/interglacial transitions. The extent of post-depositional oxidation of organic carbon as well as the dissolution and reprecipitation of minerals across these glacial terminations was shown to depend on the overall sedimentation rate and the magnitude of change encountered in the various depositional and geochemical factors. A sedimentation rate of about 2 cm/kyr was confirmed to be the critical value below which no significant amounts of non-refractory organic carbon are preserved. The influence of climatically induced variations in environmental conditions is not restricted to the geochemical boundaries in the vicinity of the sediment surface (e.g. oxic/post-oxic and Fe redox boundary) but well extends into much deeper sediment sections — namely into the zone of anaerobic oxidation of methane (AOM). In this way, processes within the zone of AOM can produce a further profound diagenetic alteration of the sediment composition up to hundreds of thousands of years after initial deposition and thus a significantly delayed chemical log-in. The long-term utility of all primary and secondary signals — also those formed and initially preserved across the oxic/post-oxic and Fe redox boundaries — is ultimately controlled by the geochemical processes within and below the sulfate/methane transition (SMT). While dissolution of authigenic and productivity-related barite takes place in sulfate-depleted sediment sections, iron sulfides as well as sulfurized organic matter and associated trace elements have a high potential to survive burial below the SMT. Nonsteady-state diagenesis can be triggered not only by changes in conditions at the sediment/water interface like TOC input, sedimentation rate or O2 content of bottom water but also by processes in the underlying sediment — namely the formation and/or liberation of methane. Apart from the distinct alteration of the solid-phase composition, variations in the upward flux of methane also have a considerable impact on the shape of sulfate pore water profiles. Modelling the effects of such variations in methane flux on sulfate profiles has illustrated that considering possible nonsteady-state situations in the sediment/pore water system is of utmost importance for the interpretation of pore water data.