Advective Pore Water Flow Research Articles

Dimethylsulfide (DMS) fluxes from permeable coral reef carbonate sediments

Dimethylsulfide (DMS) is a biogenic volatile organic compound of high interest as it can influence the Earth's climate by contributing to cloud formation in the atmosphere. Although DMS occurs at high concentrations in coral reef ecosystems, the role of permeable coral reef carbonate sediments as a source and sink of DMS to the surrounding seawater is unknown. Here, we report for the first time fluxes of dissolved DMS from permeable coral reef carbonate sediments over a full diel cycle obtained from benthic chamber deployments on the reef flat of Heron Island, southern Great Barrier Reef. Carbonate coral reef sediments were a substantial net diel source of DMS to the water column (mean 85.4 ± 13.2; range 42.3 ± 5.1 to 114 ± 4.7 nmol m−2 h−1), with rates similar to saltmarsh sediments. DMS effluxes were higher in the light under advective porewater flow, suggesting that DMS production in carbonate coral reef sediments is associated with photosynthesis by benthic microalgae. However, DMS effluxes decreased with advection at night, suggesting either greater DMS consumption or less DMS production under dark conditions. Similarly, dimethylsulfoniopropionate (DMSP), which is generally considered the main precursor of DMS, was mainly produced during the day. Net and dark methane (CH4) effluxes negatively correlated with net DMSP and dark DMS fluxes, respectively; suggesting that CH4 production via methanogenesis could be a sink for DMS and DMSP in carbonate coral reef sediments, especially under dark anoxic conditions. On Heron Island the permeable sands were estimated to contribute from about 22% to 44% of the benthic DMS emitted from the reef to the water column, which could represent an important portion of the yearly sea-to-atmosphere DMS fluxes. Permeable carbonate sediments should be considered further as a source of benthic DMS emitted from coral reefs and the broader continental shelf.

Solute transport into the Jiulong River estuary via pore water exchange and submarine groundwater discharge: New insights from 224Ra/228Th disequilibrium

Pore water exchange (PEX) and submarine groundwater discharge (SGD) represent two mechanisms for solute transport from the seabed into the coastal ocean. However, their relative importance remains to be assessed. In this study, we pursued the recently developed 224Ra/228Th disequilibrium approach to quantify PEX fluxes of 224Ra into the Jiulong River estuary, China. By constructing a full mass balance of water column 224Ra, we were allowed to put various source terms, i.e., SGD, diffusive and advective pore water flow (PEX), and river input in a single context. This led to the first quantitative assessment of the relative importance of PEX vs. SGD in the delivery of solutes into an estuary. We carried out two surveys in the Jiulong River estuary: one in January 2014 (winter survey), the other in August 2014 (summer survey). By virtue of a 1-D mass balance model of 224Ra in the sediment column, we demonstrated that PEX fluxes of 224Ra were highly variable, both temporally and spatially, and can change by 1–2 orders of magnitude in our study area. Moreover, we identified a strong correlation between 224Ra-based irrigation rate and 234Th-based sediment mixing rate. Our results highlighted irrigation as the predominant PEX process for solute transfer across the sediment–water interface.Total PEX flux of 224Ra (in 1010dpmd−1) into the Jiulong River estuary was estimated to be 22.3±3.0 and 33.7±5.5 during the winter and summer surveys, respectively. In comparison, total SGD flux of 224Ra (in 1010dpmd−1) was 11.3±8.6 and 49.5±16.3 in the respective seasons. By multiplying the PEX fluxes of 224Ra by the ratio of the concentration gradients of component/224Ra at the sediment–water interface, we quantified the total PEX fluxes of dissolved inorganic carbon (DIC) and nutrients (NH4+, NO3−, and H4SiO4) into the Jiulong River estuary. In the meantime, net export of DIC and nutrients via SGD were estimated by multiplying the SGD fluxes of 224Ra by the DIC (nutrients)/224Ra ratios in the SGD end-members around this area. Our results revealed that PEX-driven fluxes of solutes rival net SGD input and river input in an estuary. An additional new finding is that water column NO3− in the surface estuary was effectively sequestered due to SGD, probably as a result of intense denitrification occurring in the anoxic subterranean estuary.

Recoding of the stop codon UGA to glycine by a BD1-5/SN-2 bacterium and niche partitioning between Alpha- and Gammaproteobacteria in a tidal sediment microbial community naturally selected in a laboratory chemostat.

Sandy coastal sediments are global hotspots for microbial mineralization of organic matter and denitrification. These sediments are characterized by advective porewater flow, tidal cycling and an active and complex microbial community. Metagenomic sequencing of microbial communities sampled from such sediments showed that potential sulfur oxidizing Gammaproteobacteria and members of the enigmatic BD1-5/SN-2 candidate phylum were abundant in situ (>10% and ~2% respectively). By mimicking the dynamic oxic/anoxic environmental conditions of the sediment in a laboratory chemostat, a simplified microbial community was selected from the more complex inoculum. Metagenomics, proteomics and fluorescence in situ hybridization showed that this simplified community contained both a potential sulfur oxidizing Gammaproteobacteria (at 24 ± 2% abundance) and a member of the BD1-5/SN-2 candidate phylum (at 7 ± 6% abundance). Despite the abundant supply of organic substrates to the chemostat, proteomic analysis suggested that the selected gammaproteobacterium grew partially autotrophically and performed hydrogen/formate oxidation. The enrichment of a member of the BD1-5/SN-2 candidate phylum enabled, for the first time, direct microscopic observation by fluorescent in situ hybridization and the experimental validation of the previously predicted translation of the stop codon UGA into glycine.

Nitrogen‐cycling bacteria and archaea in the carbonate sediment of a coral reef

In the coarse-grained carbonate sediments of coral reefs, advective porewater flow and the respiration of organic matter establish redox zones that are the scene of microbially mediated transformations of N compounds. To investigate the geobiology of N cycling in reef sediments, the benthic microbiota of Checker Reef in Kaneohe Bay, Hawaii, were surveyed for candidate nitrate reducers, ammonifying nitrite reducers, aerobic and anaerobic ammonia oxidizers (anammox) by identifying phylotypes of their key metabolic genes (napA, narG, nrfA, amoA) and ribotypes (unique RNA sequences) of anammox-like 16S rRNA. Putative proteobacteria with the catalytic potential for nitrate reduction were identified in oxic, interfacial and anoxic habitats. The estimated richness of napA (≥202 in anoxic sediment) and narG (≥373 and ≥441 in oxic and interfacial sediment, respectively) indicates a diverse guild of nitrate reducers. The guild of nrfA hosts in interfacial reef sediment was dominated by Vibrio species. The identified members of the aerobic ammonium oxidizing guild (amoA hosts) were Crenarchaeota or close relatives of Nitrosomonadales. Putative anammox bacteria were detected in the RNA pool of Checker Reef sediment. More than half of these ribotypes show ≥90% identity with homologous sequences of Scalindua spp., while no evidence was found for members of the genera Brocadia or Kuenenia. In addition to exploring the diversity of these four nitrogen-cycling microbial guilds in coral reef sediments, the abundances of aerobic ammonium oxidizers (amoA), nitrite oxidizers (nxrAB), ammonifying nitrite reducers (nrfA) and denitrifiers (nosZ) were estimated using real-time PCR. Representatives of all targeted guilds were detected, suggesting that most processes of the biogeochemical N cycle can be catalyzed by the benthic microbiota of tropical coral reefs.

Transport and degradation of a dinoflagellate bloom in permeable sublittoral sediment

Filtration of planktonic algal cells from the water column into permeable sublittoral sediment and the fate of the cells in the shallow sands were studied during a red tide produced by the dinoflagellate Peridinella catenata at Hel Peninsula/Baltic in May 2004. Advective porewater flows associated with ripple topography of the bed caused rapid transport of algal cells down to 5 cm sediment depth. Sedimentary concentrations of algal cells mirrored algal concentrations in the overlying water column with increases and decreases within the upper 3 cm of the bed occurring within a few hours. Sedimentary algal uptake and release significantly differed between stations only 15 m laterally apart. Laboratory sediment-column experiments with 13C-labeled algal cells revealed algal decomposition at rates of up to 0.2% 13C h–1 in percolated sands originating from the study site. This was 2 orders of magnitude lower than observed decreases in sediment algal cell C abundance of up to 23% C h–1 after a drop in cell concentrations in the water column. Because bioturbation and ripple migration were negligible, we conclude that advective flushing of the uppermost sediment layer could rapidly remove cells from the sediment. Our results demonstrate close spatial and temporal coupling between algal cell concentrations in the boundary layer and those in the upper 6 cm of permeable sand sediment, and suggest that permeable beds can act as short-term storage buffer for phytoplankton. During passage through the sediment, planktonic algae may benefit from the higher nutrient concentrations available in the porewater.

Phytoplankton deposition to permeable sediments under oscillatory flow: Effects of ripple geometry and resuspension

Pressure-gradient driven, advective flows through permeable sediments are recognized as an important mechanism by which dissolved and particulate materials are exchanged between the near-bottom water and the seabed. Here, we focus on the deposition of particulate organic matter in wave-formed, rippled beds representative of permeable coastal ocean sediments. In addition, we examine the release of previously deposited material when sediment transport is initiated. Laboratory experiments were conducted in an oscillatory water tunnel using cultured diatoms (4–6 μm dia.) as a tracer for particulate organic matter. Using different ripple geometries on a coarse sand bed (median dia.=575 μm) and following deposition under controlled flow conditions, we cored extensively to determine the distribution and flux of diatoms to the bed. Results showed greater diatom deposition (3.5×) and penetration depths (4–5 cm) in ripple troughs in comparison to ripple crests (<2 cm), qualitatively in agreement with past model predictions and experimental results. Both this pattern and the total diatom flux were consistent over a 3-fold variation in ripple wavelength (17–50 cm) and a 4-fold variation in ripple height (1.4–7.7 cm). The total diatom flux was only marginally enhanced by the presence of ripples (∼1.6×) compared to a flat bed. Our data suggest that the total diatom flux may be less controlled by rippled bed topography than solute exchanges, including but not limited to advective porewater flows. Repeatability between replicate depositional runs allowed us to conduct resuspension experiments in which rippled beds previously impregnated with diatoms were eroded and allowed to establish a new ripple configuration. Approximately half (45–60%) of the embedded diatoms were released during reconfiguration of the ripples, a process that lasted 20 min. Remaining diatoms were buried comparatively deeper in isolated pockets, the location a function of ripple position both before and after sediment resuspension. Our results clearly indicate that small particle deposition to a rippled bed is a simple pattern predicted by models and prior experiments but that the total flux is not strongly determined by ripple geometry. However, the net depositional pattern in the real ocean will be determined by the combined effects of local sediment erosion and deposition, and thus these two processes act in tandem to lead to substantial, ripple-scale heterogeneity of organic matter input to shelf sediments in both the horizontal and vertical dimensions.

Growth and Population Dynamics of Anaerobic Methane-Oxidizing Archaea and Sulfate-Reducing Bacteria in a Continuous-Flow Bioreactor

The consumption of methane in anoxic marine sediments is a biogeochemical phenomenon mediated by two archaeal groups (ANME-1 and ANME-2) that exist syntrophically with sulfate-reducing bacteria. These anaerobic methanotrophs have yet to be recovered in pure culture, and key aspects of their ecology and physiology remain poorly understood. To characterize the growth and physiology of these anaerobic methanotrophs and the syntrophic sulfate-reducing bacteria, we incubated marine sediments using an anoxic, continuous-flow bioreactor during two experiments at different advective porewater flow rates. We examined the growth kinetics of anaerobic methanotrophs and Desulfosarcina-like sulfate-reducing bacteria using quantitative PCR as a proxy for cell counts, and measured methane oxidation rates using membrane-inlet mass spectrometry. Our data show that the specific growth rates of ANME-1 and ANME-2 archaea differed in response to porewater flow rates. ANME-2 methanotrophs had the highest rates in lower-flow regimes (mu(ANME-2) = 0.167 . week(-1)), whereas ANME-1 methanotrophs had the highest rates in higher-flow regimes (mu(ANME-1) = 0.218 . week(-1)). In both incubations, Desulfosarcina-like sulfate-reducing bacterial growth rates were approximately 0.3 . week(-1), and their growth dynamics suggested that sulfate-reducing bacterial growth might be facilitated by, but not dependent upon, an established anaerobic methanotrophic population. ANME-1 growth rates corroborate field observations that ANME-1 archaea flourish in higher-flow regimes. Our growth and methane oxidation rates jointly demonstrate that anaerobic methanotrophs are capable of attaining substantial growth over a range of environmental conditions used in these experiments, including relatively low methane partial pressures.

Role of pelletization in mineralization of fine-grained coastal sediments

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 291:23-33 (2005) - doi:10.3354/meps291023 Role of pelletization in mineralization of fine-grained coastal sediments Christian Wild1,3,*, Hans Røy1, Markus Huettel1,2 1Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany2Department of Oceanography, Florida State University, Tallahassee, Florida 32306-4320, USA3Present address: Ocean Sciences Section, Intergovernmental Oceanographic Comission, UNESCO, 1 rue Miollis, 75015 Paris, France *Email: c.wild@lrz.uni-muenchen.de ABSTRACT: This study investigates how pellet deposition caused by the head-down deposit feeding polychaete Heteromastus filiformis enhances sediment surface layer permeability and thereby the degradation of organic matter contained in that layer. The permeability of pellet accumulations on the sediment surface was 2 orders of magnitude higher than that of the non-pelletized sediment surface, which also had a 30% lower porosity. Freshly deposited organic-rich pellets initially consumed approximately 10 times more O2 than an equivalent volume of the ambient, oxic, surface sediment. As a consequence of the deposition of the relatively large (400 to 500 µm) grain-like pellets in small mounds, topographical structures are generated on the originally rather flat surface, which cause advective pore water flow through the pellet mounds as soon as the latter are exposed to boundary layer flows. At a flow velocity of 70 mm s–1 at 5 mm above the sediment, pore water flow velocity through pellet accumulations ranged from 1 to 2 mm s–1, transporting oxygenated water to the pellets. Incubation experiments in flow-through columns showed that the O2 consumption of pellet accumulations increases by approximately 100 µmol (g dry mass)–1 d–1 when the flow rate is increased from 10 to 20 ml h–1. A faster degradation of reduced organic C and N contained in the fecal pellets was observed under the influence of flow (70% of C and 68% of N degraded) compared to stagnancy (44% of C and 40% of N degraded) in a 3 wk flume incubation. Degradation of organic C and N in the surrounding, flat, surface sediment, which was not flushed by water flow, was not detectable during the same time period. We conclude that pelletization enhances organic matter turnover in fine-grained deposits through generation of a high secondary permeability of the sediment surface layer that permits advective pore water exchange, enhancing mineralization of the deposited organic-rich pellets. KEY WORDS: Heteromastus filiformis · Fecal pellets · Permeability · Advective flow · Oxygen profiles · Organic matter degradation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 291. Online publication date: April 28, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Hyporheic Exchange with Gravel Beds: Basic Hydrodynamic Interactions and Bedform-Induced Advective Flows

Stream-subsurface exchange processes are important because of their role in controlling the transport of contaminants and ecologically relevant substances in streams. Laboratory flume experiments were conducted to examine solute exchange with gravel streambeds. Two morphologies were studied: flat beds and beds covered by dune-shaped bedforms. High rates of exchange were observed with flat beds under a wide range of stream flow conditions, indicating that there was considerable turbulent coupling of stream and pore water flows. The presence of bedforms produced additional exchange under all flow conditions. The exchange with bedforms could be represented well by considering solute flux caused by bedform-induced advective pumping. Pumping exchange was enhanced by inertial effects, including non-Darcy flow and turbulent diffusion. For the flat bed case, dye injections showed that exchange also occurred by a combination of advective pore water flow and turbulent diffusion near the stream-subsurface interface. The relative effects of advective and diffusive transport processes could not be separated due to the complex nature of the induced flows in the gravel bed. However, exchange was found to scale with the square of the stream Reynolds number in all cases. Comparison of these results with those obtained with coarser and finer sediments demonstrated that the exchange rate is also proportional to the square of the characteristic bed sediment size. These scaling relationships can be used to improve interpretation of solute transport observed in natural rivers.

Rapid wave-driven advective pore water exchange in a permeable coastal sediment

In this study we present in-situ measurements of pore water flow velocities in a coastal sandy sediment (permeability=3.65×10 −10 m 2). The advective pore water flows were driven by the interaction of oscillating boundary flows with sediment wave ripples, (amplitude=7 cm, wavelength=30 to 50 cm). The measurements were carried out in the Mediterranean Sea at 50 to 70 cm water depth during a phase of very low wave energy (max. wave amplitude=10 cm). An optode technique is introduced that permits direct pore water flow measurements using a fluorescent tracer. Near the sediment surface (0.5 cm depth) pore water reached velocities exceeding 40 cm h −1. Thus, advective transport exceeded transport by molecular diffusion by at least 3 orders of magnitude. Based on the pore water velocity measurements and ripple spacing, we calculate that 140 L m −2 d −1 are filtered through the sediment. Pore water visualisation experiments revealed a flow field with intrusion of water in the ripple troughs and pore water release at the ripple crests. The wave-driven water flow through the sediment, thus, was directly linked to the wave-generated sediment topography, and its spatial dimensions. These results show that surface waves cause water filtration through permeable sediments at water depths smaller than half the wavelength. We conclude that surface gravity waves constitute an important hydromechanical process that may convert large areas of the continental shelves into expansive filter systems. Surface gravity waves thereby could affect suspended particle concentration and cycling of matter in the shelf.

A physicochemical model for colloid exchange between a stream and a sand streambed with bed forms

Fine sediment exchange between a stream and the surrounding subsurface influences downstream contaminant transport and stream ecology. Fundamental models for this exchange were developed on the basis of (1) the hydraulics of bed form‐driven advective pore water flow and (2) subsurface colloid transport processes. First, a model was developed to predict the advective flow induced in a sand bed by stream flow over bedforms. The resulting “pumping” exchange rate was calculated based on the streamflow conditions, bed form geometry, and bed depth. The pumping exchange of suspended sediment was then calculated by superimposing advective transport and particle settling in the bed and including the effect of physicochemical filtration by bed sediment. The filtration coefficient approach was used to predict the reduction in the concentration of transported particles. Both settling and filtration cause colloids to be trapped in stream beds, producing a higher net exchange rate relative to conservative solutes. When transported particles are completely trapped in a single pass through the bed, the exchange calculation is simplified because only the particle flux to the bed must be considered. In this case, the net exchange rate may be adequately represented by an effective piston velocity (flux/concentration) or loss rate to the bed in the advection‐dispersion equation for the stream. Solute and colloid exchanges are predicted by the models without the use of fitting coefficients; only measurable hydraulic and particle parameters were used as model inputs. Simulations are presented which show the effect of stream parameters, settling, and filtration on net particle exchange. This fundamental approach to modeling stream‐subsurface exchange potentially has great utility for understanding and predicting the transport and fate of reactive substances in streams.

Advective Transport Affecting Metal and Nutrient Distributions and Interfacial Fluxes in Permeable Sediments

Our laboratory flume experiments demonstrate that advective porewater flows produce biogeochemical reaction zones in permeable sediments, leading to specific and reproducible complex patterns of Fe, Mn, and nutrients. Oxygenated water, forced into the sediment when boundary flows were deflected by protruding sediment structures, generated distinct zones of nitrification and ferric iron precipitation. This inflow was balanced by ammonium-rich porewater ascending from deeper sediment layers, thereby creating an anoxic channel where dissolved Fe2+ and Mn2+ could reach the surface. Between the zones of ferric iron precipitation and Fe2+ upwelling, a layer with increased manganese oxide and solid phase Fe(II) concentrations formed, indicating redox reaction between these components. The establishment of topography on the previously smooth sediment surface reversed the net interfacial flux of solutes. While the smooth control core was found to be a sink for metals and nutrients, the sediment with mounds acted as a source for these substances. Our experiments show that in sandy sediment with an oxidised surface layer, reduced metal species can be released to the water column by flow-topography interactions. We conclude that advective transport processes constitute an important process controlling biogeochemical zonations and fluxes in permeable sea beds.

Impact of boundary layer flow velocity on oxygen utilisation in coastal sediments

Small pressure gradients generated by boundary flow-topography interactions cause advective pore water flows in permeable sediments. Advective pore water exchange enhances the flux of solutes between the sediment and the overlying water, thus generating conditions for an increased utilisation of oxygen. We compared a less permeable (k = 5 X 10-l2 m2) with a permeable sediment (k = 5 X 10- m') typical for coastal and shelf sediments. Total oxygen utilisation (TOU) in incubated sediment cores was measured in 10 laboratory experiments using recirculating flow tanks (33 runs). TOU was a function of flow velocity in permeable sediment where advective pore water flow occurred. TOU increased with the increasing volume of sediment flushed with oxygenated water. We found that TOU increased by 91 2 23% in coarse sand when flow increased from 3 to 14 cm S-' (38 mounds m-', height 10 to 30 mm, flow measured 8 cm above the sediment). Additlon of fresh algal material caused a stronger stimulation of TOU in the coarse sand than in the fine sand (4 additional flume runs). After the addition, intensive oxygen consumption reduced the oxygen penetration depth in the advectively flushed zone of the coarse sediment. However, counteracting this process, advectlve flow maintained an oxic sediment volume still larger than that in the less permeable sediment. Flow-enhanced oxygen utilisation is potentially effective in permeable beds of coastal and shelf regions, in contrast to the situation in cohesive sediments limited by predominantly diffusive oxygen supply.

Impact of biogenic sediment topography on oxygen fluxes in permeable seabeds

Boundary layer flows, interacting with roughness elements at the sediment surface, alter the small-scale flow reglme. Consequently, pressure d~fferences are generated that are the drivlng forces for advective pore-water flow. We ~nvestigated topography-induced transport of oxygen in a permeable coastal sediment from the Mediterranean Sea (Isola del Giglio, Italy). The sediment surface was characterized by a high abundance (120 m-2) of sediment mounds (average height: 4 cm) built by the mud shrimp Callianassa truncata (Decapoda, Thalassinidea). Boundary layer flow velocities recorded in situ ranged between 2 and 16 cm S-' Detailed experiments were performed in a recirculating laboratory flow channel A natural sediment core, 20 cm deep with a surface area of 0 3 m was exposed to a unidirectional flow of varying current velocity (3, 6, 10 cm S S ' ) . The alteration of the small-scale flow reglme a t a sediment mound was documented by vertical velocity profiles measured in 1 mm resolution with temperature-compensated thermistor probes. Oxygen distribution in the sediment was investigated with Clark-type microelectrodes. At a smooth surface, oxygen penetration depth in the permeable sediment did not exceed 4 mm, independent of flow velocity. In contrast, the topography-induced advective oxygen transport increased significantly with current speed. Oxygen reached down to almost 40 mm at the upstream foot of a 1 cm high sediment mound at a flow velocity of 10 cm S-' Thus, the oxic sediment volume increased locally by a factor of 4.8 compared to the oxic zone underneath a smooth surface. At a natural abundance of 120 mounds m-? the oxic sediment volume per mZ seabed was calculated to be 3.3-fold higher than in a seabed with a smooth surface. In a parallel experiment, advective solute transport was also demonstrated in a less permeable sediment (k = 5 r 10-12 m') from the North Sea intertidal flat. Due to the lower permeability the effect on O2 transport was less than in the Mediterranean sand, but oxygen penetration depth increased locally 2-fold a t a sediment mound under a flow velocity of 10 cm S-' The experiments showed the high spatial and temporal variability of oxygen distnbution in a coastal seabed depending on sediment surface topography, boundary layer flow velocities and sediment permeability.

Solute release mechanisms from confined sediment cores in stirred benthic chambers and flume flows

Interfacial solute fluxes from sediment cores enclosed in stirred chambers were compared with solute fluxes from a reference core inserted In a flume simulating in si tu conditions with unidirectional boundary-layer flow. Utilizing Rhodamine-WT dye as conservative solute, and quantifying the hydrodynamic driving forces by hot film anemometry and a highly sensitive wet/wet d~fferential pressure transducer, the pathways of solutes in chamberand flume-confined cores are ~dentified. The critical parameter limiting confined-core flux techniques is the sediment permeability, with equivalency between experimental techniques and natural, smooth bottoms found for impermeable sediments only, where bottom stress and Schmidt number act as the controlling parameters permitting use of diffusive laws. In contrast, pressure gradients, induced by edge and sidewall effects in cores under unidirectional flow and by circular flow patterns in stirred chambers, being ineffective for impermeable sediments, generate non-negligible advective porewater flows in permeable sediments. For fine sandy sediment (4 = 2.2; permeability K = 15 Darcy, D), stirred-chamber circular flow enhanced solute release up to 6-fold over that from the reference core exposed to 2-dimensional flow, with intruding supernatant water reaching up to 8-fold deeper ~ n t o the porespace. Consequences of these advective exchange processes include release of anoxic porewater and increased oxygen uptake in permeable cores, with shifts in redox isolines affecting biological and geochemical processes in the sediment. Results suggest that chamber fluxes are both siteand device-dependent and cannot be automatically equated with In si tu fluxes.

Carbonate recrystallization in basal sediments: evidence for convective fluid flow on a ridge flank

In many Deep Sea Drilling Project (DSDP) drill holes, chalks and/or limestones and even dolomites overlie oceanic basement rocks. These occurrences have often been interpreted as the result of increased calcium carbonate dissolution and reprecipitation as cement and overgrowths (recrystallization) at higher prevailing temperatures, either near an oceanic ridge system or at greater burial depths (burial diagnesis). We report here detailed chemical and isotopic analyses of carbonate sediments recovered from a drill hole on the western flank of the East Pacific Rise during Leg 92 of DSDP. This hole was drilled on 4.6-Myr old oceanic crust and recovered chalks directly overlying oceanic basement at a depth as shallow as 8 m below the sediment/water interface. Oceanic basement was not reached in this hole because of the hardness of deeper chalks, or, more likely, limestones. The chemical and isotopical evidence, detailed below, shows that considerable calcite recrystallization did occur in an aqueous medium similar to normal seawater and at relatively low temperature. This suggests that the observed recrystallization of calcareous ooze to chalk and limestone took place not as a result of burial diagenesis at higher temperatures, but as a result of extensive advective pore water flow through the sediments, which would allow recrystallization of calcium carbonate to take place. This process was, and apparently still is, widespread in the south-east Pacific Ocean.

Diffusion and mass balance of Mg during early dolomite formation, Monterey Formation

Diffusion from overlying seawater is capable of supplying enough Mg to account for all of the early diagenetic dolomite observed in the Monterey Formation (0.5–20 vol %). The amount formed depends on the Mg flux magnitude into the sediment, which is proportional to the rate of dolomite precipitation and inversely proportional to the sediment accumulation rate. The amount of dolomite formed is calculated for accumulation rates of 50 and 600 m/Ma, using reaction rate constants obtained from the Mg concentration profile of DSDP sites 479, 533 and 147, and proposed profiles that assume significant dolomite formation in the uppermost tens of meters of sediment. Advective pore water flow from compaction decreases the Mg flux and becomes increasingly significant as the accumulation rate increases and the reaction rate decreases. Advection alone during compaction supplies sufficient Mg to account for a realistic maximum of 2 vol % dolostone within a compacted section. An approximate evaluation of the Rayleigh number suggests that the low permeability of siliceous ooze prevents convective fluid flow at shallow burial depths. The Mg mass balance is calculated from dolomite abundance in Monterey Formation sections, Santa Maria basin area, California. Pore water and solid particle velocities are calculated as a function of sediment depth using a porosity-depth relation for average siliceous sediment based on a composite of DSDP data. The bulk sediment diffusion coefficient increases with sediment depth at a rate proportional to the geothermal gradient. At shallow burial depths seawater is probably the only significant source of Mg; dolomite, volcanic ash, and clay minerals are the only significant sinks. Clay mineral, organic matter, and silica diagenesis may provide additional Mg at deeper burial depths. The more or less regular spacing of dolomitic layers may be controlled by the residence time of the dolomite forming layer within the uppermost sediment, where the Mg flux is a maximum.