Pore Fluid Chemistry Research Articles

Interstitial fluid chemistry of sediments underlying the North Atlantic gyre and the influence of subsurface fluid flow

The western flank of the Mid-Atlantic Ridge is a region underlying the oligotrophic waters of the central Atlantic. The seafloor along portions of this ridge is characterized by sediment-filled depressions, which are surrounded by steep basaltic outcrops. We present pore fluid and sediment solid-phase chemical data from fourteen gravity cores from “North Pond”, a sediment pond where previous drilling work indicated directed flow of seawater within the basement. Sediment lithology is broadly characterized as a nannofossil pelagic sediment containing varying amounts of clay, foraminifers, and Mn-micronodules and typically contains less than 0.3% organic carbon and ~70% calcium carbonate. Consistent with its location within an oligotrophic ocean gyre, oxygen and nitrate penetrated deeply into the sediment package. However there is significant spatial variability in the pore fluid nitrate and oxygen profiles, with oxygen generally lower and nitrate higher toward the center of the basin as compared to the edges. In addition, oxygen increased with sediment depth at a number of sites toward the edges of the pond, where sediment cover was thinnest. We interpret these oxygen distributions to indicate that there is upward diffusion of dissolved oxygen from the underlying basaltic basement fluid and the sediment package, and this process appears to be regionally pervasive. Pore fluid molybdenum generally decreases with depth and exhibits spatial variability similar to dissolved oxygen and nitrate. Molybdenum is likely being taken up at depth via adsorption onto manganese oxides, as these sediments are rich in manganese (~300–3000ppm Mn) and molybdenum (~2–14ppm Mo). The strong geographical variations in pore fluid chemistry coupled with the co-variation between molybdenum and oxygen, two species that we would not necessarily expect to be coupled, suggest that diffusion of dissolved constituents into the sediment package from below plays an important role in determining the chemistry of the overlying sediment.

Use of Complex Resistivity Tomography for Moisture Monitoring in a Flooded Masonry Specimen

Moisture ingress is one of the major deteriorating factors for building materials. Today, the only approved way to assess such damage is the gravimetric Darr method, which is essentially destructive. Substantial progress has been made using the geophysical complex-resistivity method, which can be applied non-destructively and provides spatial information along two-dimensional sections, rather than punctual along one borehole. Considerable advantages of complex resistivity are its sensitivity to textural properties, as well as the pore-fluid chemistry of wet, porous media. In a comprehensive laboratory study, and later in field scale experiments, it could be shown that complex resistivity may even be able to distinguish between salt content and saturation degree in a single measurement. A comparison with complementary nondestructive testing techniques points to the benefit and further research to be explored in multimethodical approaches.

Physicochemical controls on initiation and evolution of desiccation cracks in sand–bentonite mixtures: X-ray CT imaging and stochastic modeling

The shrink-swell behavior of active clays in response to changes in physicochemical conditions creates great challenges for construction of geotechnical barriers for hazardous waste isolation, and is of significant importance for management of agricultural and natural resources. Initiation and evolution of desiccation cracks in active clays are strongly dependent on physicochemical initial and boundary conditions. To investigate effects of bentonite content (20, 40, 60%), pore fluid chemistry (0.05 and 0.5 M NaCl) and drying rates (40 and 60 °C) on cracking behavior, well-controlled dehydration experiments were conducted and X-ray Computed Tomography (CT) was applied to visualize and quantify geometrical features of evolving crack networks. A stochastic model based on the Fokker-Plank equation was adopted to describe the evolution of crack aperture distributions (CAD) and to assess the impact of physicochemical factors on cracking behavior. Analyses of crack porosity and crack specific surface area showed that both clay content and temperature had larger impact on cracking than pore fluid concentration. More cracks formed at high bentonite contents (40 and 60%) and at high drying rate (60 °C). The drift, diffusion and source terms derived from stochastic analysis indicated that evaporative demand had greater influence on the dynamics of the CAD than solution chemistry.

Laboratory Model Test on Improving the Properties of Soft Clay by Electrokinetics

This experimental study was carried out in two test series to investigate the feasibility of decreasing the water content and increasing the shear strength and axial load capacity of laboratory-prepared soft clay by electrokinetic treatment. The focus of the investigations is the influence of pore fluid chemistry (fresh or highly saline water) on the gained improvement and on the energy consumption. The results showed that electrokinetics was effective in improving the properties of the soft clay with fresh and saline water. The degree of improvement, however, was superior in tests with freshwaters along with a lower energy consumption. The minimum water content and the maximum shear strength after the treatment were reported near the anode (28% ± 3.6 and 99.3 kPa ± 15.4 compared to 49.7% ± 3.1 and 12.1 kPa ± 1.7 in the control). The maximum axial load capacity of the foundation model after the treatment was 416 N compared to 28 N in the control. The energy consumption varied between 69.1 and 1994.6 Whr.

Microbial community structure of hydrothermal deposits from geochemically different vent fields along the Mid‐Atlantic Ridge

To evaluate the effects of local fluid geochemistry on microbial communities associated with active hydrothermal vent deposits, we examined the archaeal and bacterial communities of 12 samples collected from two very different vent fields: the basalt-hosted Lucky Strike (37°17'N, 32°16.3'W, depth 1600-1750 m) and the ultramafic-hosted Rainbow (36°13'N, 33°54.1'W, depth 2270-2330 m) vent fields along the Mid-Atlantic Ridge (MAR). Using multiplexed barcoded pyrosequencing of the variable region 4 (V4) of the 16S rRNA genes, we show statistically significant differences between the archaeal and bacterial communities associated with the different vent fields. Quantitative polymerase chain reaction (qPCR) assays of the functional gene diagnostic for methanogenesis (mcrA), as well as geochemical modelling to predict pore fluid chemistries within the deposits, support the pyrosequencing observations. Collectively, these results show that the less reduced, hydrogen-poor fluids at Lucky Strike limit colonization by strict anaerobes such as methanogens, and allow for hyperthermophilic microaerophiles, like Aeropyrum. In contrast, the hydrogen-rich reducing vent fluids at the ultramafic-influenced Rainbow vent field support the prevalence of methanogens and other hydrogen-oxidizing thermophiles at this site. These results demonstrate that biogeographical patterns of hydrothermal vent microorganisms are shaped in part by large scale geological and geochemical processes.

Evaluation of the impact of pore fluid chemistry on the hydromechanical behaviour of clay-based sealing materials

The results of an experimental program undertaken to evaluate the impact of pore fluid salinity on the hydromechanical performance of light and dense backfill materials are presented. Light and dense backfills are engineered barrier materials that are being examined in the Canadian concept for storage of spent fuel in a deep geological repository. The current research investigates the impact of pore fluid chemistry on the swelling, compressibility, stiffness, and hydraulic conductivity parameters of light and dense backfills that are required as material parameters for analysis and design. In these tests, pore fluid chemistry was selected to represent groundwater within potential host units including granite and limestone rock. Results show that the performance of light backfill is significantly affected by changes in pore fluid chemistry. The swell potential of light backfill decreases with increasing salinity of the solution. The hydraulic conductivity decreases with increasing effective montmorillonite dry density and specimens saturated with saline solution have higher hydraulic conductivity than those saturated with distilled water. Conversely, the behaviour of dense backfill is governed mainly by the crushed granite component and therefore changes to the pore fluid chemistry have relatively little effect. Results of dense backfill tests confirm the material performance as a sealing material.

Effects of pore fluid flow and chemistry on compaction creep of calcite by pressure solution at 150°C

Uni-axial compaction creep experiments were performed on crushed limestone and analytical grade calcite powders at 150°C, a pore fluid pressure of 20 MPa, and effective axial stresses of 30 and 40 MPa. Previous experiments have shown that compaction under these conditions is dominated by intergranular pressure solution (IPS). The aim of the present tests was to determine the inter-relationship between pore fluid chemistry, compaction rate and the rate-controlling process of IPS. Intermittent flow-through runs conducted using CaCO3 solution showed no effect on creep rate at low strains (<4–5%) but a major acceleration at high strains (5–10%). Measurements of the Ca concentration present in fluid samples revealed the build-up of a high super-saturation of CaCO3 during compaction under zero flow conditions, especially at high strains. Active flow-through led to a drop in Ca concentration, which corresponded with creep acceleration. Addition of NaCl to the pore fluid, at a concentration of 0.5 m, increased the creep rate of the analytical grade calcite samples roughly in proportion to the enhancement of calcite solubility. Addition of Mg2+ and to the pore fluid, in concentrations of 0.05 and 0.001 m, respectively, caused major retardation of compaction creep. Integrating our mechanical, flow-through and chemical data points strongly to diffusion-controlled IPS being the dominant deformation mechanism in the calcite-water system under closed-system (zero flow) conditions at low strains (<4–5%), giving way to precipitation control at higher strains. Our fluid composition data suggest that this transition is because of accumulation of impurities in the pore fluid. As Mg2+ and phosphate ions are common in natural pore fluids, it is likely that retarded precipitation will be the rate-limiting step of IPS in carbonates in nature. To quantify diagenetic compaction and porosity-permeability reduction rates by IPS in carbonates needs to account for this.

Soils Reveal Widespread Manganese Enrichment from Industrial Inputs

It is well-known that metals are emitted to the air by human activities and subsequently deposited to the land surface; however, we have not adequately evaluated the geographic extent and ecosystem impacts of industrial metal loading to soils. Here, we demonstrate that atmospheric inputs have widely contaminated soils with Mn in industrialized regions. Soils record elemental fluxes impacting the Earth's surface and can be analyzed to quantify inputs and outputs during pedogenesis. We use a mass balance model to interpret details of Mn enrichment by examining soil, bedrock, precipitation, and porefluid chemistry in a first-order watershed in central Pennsylvania, USA. This reveals that ∼ 53% of Mn in ridge soils can be attributed to atmospheric deposition from anthropogenic sources. An analysis of published data sets indicates that over half of the soils surveyed in Pennsylvania (70%), North America (60%), and Europe (51%) are similarly enriched in Mn. We conclude that soil Mn enrichment due to industrial inputs is extensive, yet patchy in distribution due to source location, heterogeneity of lithology, vegetation, and other attributes of the land surface. These results indicate that atmospheric transport must be considered a potentially critical component of the global Mn cycle during the Anthropocene.

Pore fluid chemistry of the North Anatolian Fault Zone in the Sea of Marmara: A diversity of sources and processes

As part of the 2007 Marnaut cruise in the Sea of Marmara, an investigation of the pore fluid chemistry of sites along the Main Marmara Fault zone was conducted. The goal was to define the spatial relationship between active faults and fluid outlets and to determine the sources and evolution of the fluids. Sites included basin bounding transtensional faults and strike-slip faults cutting through the topographic highs. The basin pore fluids are dominated by simple mixing of bottom water with a brackish, low-density Pleistocene Lake Marmara end-member that is advecting buoyantly and/or diffusing from a relatively shallow depth. This mix is overprinted by shallow redox reactions and carbonate precipitation. The ridge sites are more complex with evidence for deep-sourced fluids including thermogenic gas and evidence for both silicate and carbonate diagenetic processes. One site on the Western High displayed two mound structures that appear to be chemoherms atop a deep-seated fluid conduit. The fluids being expelled are brines of up to twice seawater salinity with an exotic fluid chemistry extremely high in Li, Sr, and Ba. Oil globules were observed both at the surface and in cores, and type II gas hydrates of thermogenic origin were recovered. Hydrate formation near the seafloor contributes to increase brine concentration but cannot explain their chemical composition, which appears to be influenced by diagenetic reactions at temperatures of 75°C–150°C. Hence, a potential source for fluids at this site is the water associated with the reservoir from which the gas and oil is seeping, which has been shown to be related to the Thrace Basin hydrocarbon system. Our work shows that submerged continental transform plate boundaries can be hydrologically active and exhibit a diversity of sources and processes.

Pore fluid chemistry, stress–strain behaviour, and yielding in reconstituted highly plastic clay

Extensive deposits of proglacial Lake Agassiz clay in the Red River valley in southern Manitoba, Canada are smectite-rich, expansive, strain-softening, and cemented by gypsum (CaSO 4.2H 2O). Mechanical properties of the clay are affected by pore fluid chemistry. They control the engineering behaviour of projects such as shallow foundations, riverbank instabilities, landfill liners and other engineered barriers. Reconstituted Lake Agassiz clay is being considered as a component of the sealing system for a Canadian repository for used nuclear fuel. The project that stimulated this study involved intermittent instabilities of retention dykes for the reservoir (forebay) of a hydroelectric generating station. Testing for cation concentrations suggested that natural seepage beneath the dykes removed gypsum cementation and reduced the strength of the foundation soils. To examine this, blocks of reconstituted clay were prepared using different pore fluids. Specimens that were saturated with gypsum displayed similar yield stresses and peak strengths, even though there were other differences in their pore fluid chemistry. Specimens that were under-saturated with gypsum yielded at lower stresses. The results suggest that early straining up to 1 to 2% axial strain was controlled by the presence of gypsum at near-saturation concentrations.

海床水文地质的最新进展: 集中在基底-沉积物相互作用、俯冲带及大陆斜坡

Focused, interdisciplinary research in the subseafloor environment has advanced our understanding of fluid flow paths and the interrelationship between fluid pressure and deformation, heat flow, and diagenesis. Recent investigations in the Pacific Ocean, the Gulf of Mexico, and the North Sea provide examples of high rates of fluid flow in the permeable basement, with the flow affecting properties of overlying sediments. There are also some indications that flow could continue into subduction. Studies on continental slopes provide evidence for sedimentation-driven overpressures that can destabilize slopes. These studies particularly emphasize the contributions of lateral flow driven by uneven loading. Similarly, evidence continues to build that stratigraphic horizons and faults help to dewater subduction zones. A recent development is the monitoring of pore pressure, temperatures, flow, and fluid chemistry, which aids the understanding of transient behavior and response to strain events.

Experiment and modeling to evaluate the effects of proppant-pack diagenesis on fracture treatments

Observed reductions in the permeability of propped hydraulic fractures are examined by considering the role of mechanical stresses and the chemistry of pore fluids at elevated temperatures as agents of proppant diagenesis. Stress-enhanced dissolution of proppant increases the density of grain packing and reprecipitation of mineral matter further occludes pores — together these mechanisms additively reduce porosity and permeability. Experiments and analyses are presented which explore the evolution of porosity and permeability in proppant packs subjected to reservoir conditions of stresses to 65 MPa and temperatures to 177 °C. Experiments are completed in two modes: in flow-through reactors absent intergranular stresses to evaluate rates of dissolution and reprecipitation on proppant surfaces, and in uniaxially stressed reactors with stagnant fluids to evaluate the relative role of stress in mediating dissolution and porosity reduction. Lumped parameter models are used to evaluate rates of dissolution and chemical compaction in a range of proppants. Mechanisms include mineral dissolution, transport, and reprecipitation of the resulting products in the particle interstices, resulting in a loss of intergranular porosity. The model uses thermodynamic data derived from the reactor experiments to constrain the projected loss of permeability for the mineralogical composition of available proppants. Evaluated silica dissolution rates vary with temperature but are of the order of 1.1 × 10 −11 to 3.1 × 10 −4 mol/m 2 s in the range of 100 °C to 288 °C. Experimental results show chemical compactions of the order of 10 millistrains over 40 days and these project modeled reductions in porosity of the order of 25% over 1000 days. Rates of porosity loss increase both with an increase in temperature and a decrease in mean grain size of the granular proppant although ultimate porosities at the conclusion of compaction are similar. Rates and magnitudes of ultimate reductions in porosity map directly onto reductions in permeability and for proppants in the range 100 to 600 μm, result in permeability reductions in the range of 25% (163 °C) to 75% (191 °C). Such magnitudes (75%) of permeability loss over short periods (1000 days) are consistent with many field observations, with the mechanistic models providing a rational method of scaling such observations.

Mg isotope constraints on soil pore-fluid chemistry: Evidence from Santa Cruz, California

Mg isotope ratios ( 26Mg/ 24Mg) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant 26Mg/ 24Mg ratio (expressed as δ 26 Mg ) at −0.79 ± 0.05‰, identical to seawater δ 26 Mg . Detrital smectite (from the base of the soil profile, and therefore unweathered) has a δ 26 Mg value of 0.11‰, potentially enriched in 26Mg by up to 0.3‰ compared to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for δ 26 Mg , ranging from −0.99‰ near the surface to −0.43‰ at the base of the profile. Shallow pore-waters (<1 m) have δ 26 Mg values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as <32%, calculated using the average Mg isotope enrichment factor between grass and rain ( δ 26 Mg grass - δ 26 Mg rain ) of 0.21‰. The deep pore-waters (1–15 m deep) have δ 26 Mg values that are intermediate between the smectite and rain, ranging from −0.76‰ to −0.43‰, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation inputs. A consistent interpretation is only possible if Mg isotope ratios are fractionated either by the precipitation of a secondary Mg bearing phase, not detected by conventional methods, or selective leaching of 24Mg from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering.

Effect of the water–steam phase transition on the electrical conductivity of porous rocks

The effect of the water–steam phase transition on electrical conductivity was experimentally investigated in volcanic and sandstone samples to support the interpretation of resistivity data to determine changes in steam saturation in geothermal reservoirs. The measurements were performed at simulated in situ conditions with controlled pore fluid chemistry, temperature, and confining and pore pressures. At constant temperature (150 °C) and confining pressure, pore fluid was withdrawn from the sample by steadily increasing the volume of the pore fluid system. At the vapor saturation pressure, the pore water progressively boiled to steam, resulting in a continuous conductivity decrease by a factor of approximately 20. The study showed that: (1) for rocks in which conduction is controlled by the pore fluid, the concurrent changes in both electrical conductivity and pore (vapor) pressure are defined by the pore size distribution; the changes in liquid–steam saturation are approximately proportional to those in conductivity and can thus be quantified; and (2) for rocks in which surface conduction is predominant there is no direct relation between conductivity, pore pressure and drained fluid volume; this implies that the conduction mechanism controls the pattern of electrical conductivity variations as steam saturation changes.

Complex resistivity and radar investigation of building material: First results of field scale measurements

Moisture ingress is one of major damaging factors for masonry buildings. As the complex resistivity (CR) is sensitive to textural properties as well as to the pore fluid chemistry of wet porous media, its non-destructive application can provide helpful information for conservators. In a comprehensive laboratory study it has been shown that CR might even be able to distinguish between salt content and saturation degree in only one measurement. The combined use of electrical and electromagnetic measurement techniques in two field-scale flooding experiments has shown some unexpected differences. Possible reasons are discussed and it is shown that bringing together the information of both methods leads to a clearer picture.

The Use of Neural Networks for the Prediction of Zeta Potential of Kaolinite

The sign and the magnitude of the zeta potential must be known for many engineering applications. For clay soils, it is usually negative, but it is strongly dependent on the pore fluid chemistry. However, measurement of zeta potential time is time-consuming and requires special and expensive equipment. In this study, the prediction of zeta potential of kaolinite has been investigated by artificial neural networks (ANNs) and multiple regression analyses (MRAs). To achieve this, ANN and MRA models based on zeta potential measurements of kaolinite in the presence of salt and heavy metal cations at different pH values have been developed. The results of the models were compared with the experimental results. The performance indices, including coefficient of determination, root mean square error, mean absolute error, and variance, were used to assess the performance of the prediction capacity of the models developed in this study. The obtained indices make it clear that the constructed ANN models were able to predict zeta potential of kaolinite quite efficiently and outperformed the MRA models. Results showed that ANN models can be used satisfactorily to predict zeta potential of kaolinite as a rapid inexpensive substitute for laboratory techniques.

Measuring Temporal Variability in Pore-Fluid Chemistry To Assess Gas Hydrate Stability: Development of a Continuous Pore-Fluid Array

A specialized pore-fluid array (PFA) sampler was designed to collect and store pore fluids to monitor temporal changes of ions and gases in gas hydrate bearing sediments. We tested the hypothesis that pore-fluid chemistry records hydrate formation or decomposition events and reflects local seismic activity. The PFA is a seafloor probe that consists of an interchangeable instrument package that houses OsmoSamplers, long-term pore-fluid samplers, a specialized low-dead volume fluid coupler, and eight sample ports along a 10 m sediment probe shaft. The PFA was deployed at Mississippi Canyon 118, a Gulf of Mexico hydrate site. A 170 day record was acquired from the overlying water and 1.3 m below seafloor (mbsf). Fluids were measured for dissolved chloride, sulfate, and methane concentrations and dissolved inorganic carbon and methane stable carbon and deuterium isotope ratios. Chloride and sulfate did not change significantly over time, suggesting the absence of gas hydrate formation or decomposition events. Over the temporal record, methane concentrations averaged 4 mM at 1.3 mbsf, and methane was thermogenic in origin (delta13C-CH4 = -32.4 +/- 3.4 per thousand). The timing of an anomalous 14 mM methane spike coincided with a nearby earthquake (Mw = 5.8), consistent with the hypothesis that pore-fluid chemistry reflects seismic events.

Fluid–Sediment Interaction in a Marine Shallow‐Water Hydrothermal System in the Wakamiko Submarine Crater, South Kyushu, Japan

AbstractThe Wakamiko submarine crater is a small depression located in Kagoshima Bay, southwest Japan. Marine shallow‐water hydrothermal activity associated with fumarolic gas emissions at the crater sea floor (water depth 200 m) is considered to be related with magmatic activity of the Aira Caldera. During the NT05‐13 dive expedition conducted in August 2005 using remotely operated vehicle Hyper‐Dolphine (Japan Agency for Marine‐Earth Science and Technology), an active shimmering site was discovered (tentatively named the North site) at approximately 1 km from the previously known site (tentatively named the South site). Surface sediment (up to 30 cm) was cored from six localities including these active sites, and the alteration minerals and pore fluid chemistry were studied. The pore fluids of these sites showed a drastic change in chemical profile from that of seawater, even at 30 cm below the surface, which is attributed to mixing of the ascending hydrothermal component and seawater. The hydrothermal component of the North site is estimated to be derived from a hydrothermal aquifer at 230°C based on the hydrothermal end‐member composition. Occurrence of illite/smectite interstratified minerals in the North site sediment is attributed to in situ fluid–sediment interaction at a temperature around 150°C, which is in accordance with the pore fluid chemistry. In contrast, montmorillonite was identified as the dominant alteration mineral in the South site sediment. Together with the significant low potassium concentration of the hydrothermal end‐member, the abundant occurrence of low‐temperature alteration mineral suggests that the hydrothermal aquifer in the South site is not as high as 200°C. Moreover, the montmorillonite is likely to be unstable with the present pore fluid chemistry at the measured temperature (117°C). This disagreement implies unstable hydrothermal activity at the South site, in contrast to the equilibrium between the pore fluid and alteration minerals in the North site sediment. This difference may reflect the thermal and/or hydrological structure of the Wakamiko Crater hydrothermal system.

Marine shallow-water hydrothermal activity and mineralization at the Wakamiko crater in Kagoshima bay, south Kyushu, Japan

Submarine hydrothermal fluid emanation from a small sediment mound associated with bubbling gas was observed in the Wakamiko crater that is located 5 km offshore at 200 m depth in Kagoshima bay, southern Kyushu, Japan. In 2003 and 2005, surface sediments (up to 30 cm) from inside and outside of the hydrothermal mound were sampled using the submersible ROV Hyper-Dolphine (JAMSTEC). Chemistry of pore fluids inside the mound showed a clear difference from those collected from outside the mound, which is explained by mixing of the ascending hydrothermal component and seawater. Estimated chemical composition of the hydrothermal end member suggests that the fluid experienced hydrothermal interaction at a temperature range of about 175 to 200 °C. This is consistent with the observed mound temperature of 137 °C. Hydrothermal minerals such as barite, stibnite, and realgar were identified in the mound sediment, suggesting their precipitation due to mixing of the ascending hydrothermal fluid with seawater. Isotopic composition of the hydrothermal endmember can be explained by mixing product of seawater, magmatic water and meteoric water. The marine shallow-water hydrothermal system in the Wakamiko crater is considered to be driven by a magmatic heat source beneath the crater, which also contributes elements such as arsenic, antimony, mercury and magmatic water to the hydrothermal fluid.

Cold seeps along the main Marmara Fault in the Sea of Marmara (Turkey)

The main Marmara Fault exhibits numerous sites of fluid venting, observed during previous cruises and in particular with R.O.V. VICTOR during the MARMARASCARPS cruise (2002). Long CALYPSO cores were recovered near active vents and at reference sites during the MARMARA-VT cruise (2004), together with echosounder sub-bottom profiles (frequency of 3.5 kHz). We compiled R.O.V. video observations from MARMARASCARPS cruise and show that all known seeps occur in relationship with strike-slip faults, providing pathways for fluid migration. Among the main active sites, a distinction is made between gas seeps and water seeps. At gas seeps, bubble emissions at the seafloor or disturbed echofacies on sounder profiles demonstrate the presence of free methane gas at a shallow depth within the sediment. Most cores displayed gas-related expansion, most intense for cores taken within the gas plumes. On the other hand, authigenic carbonate chimneys characterize the water seeps and visible water outflow was observed at two sites (in the Tekirdağ and Central basins). The pore fluid chemistry data show that the water expelled at these sites is brackish water trapped in the sediment during lacustrine times (before 14 cal kyr BP), in relation with the paleoceanography in the Sea of Marmara. The chimney site in the Tekirdağ Basin is located at the outlet of a canyon feeding a buried fan with coarse sandy turbidites. Pore fluid composition profiles indicate that the sand layers channel the brackish fluids laterally from the basin into the fault zone at less than 20 m depth. However, a deeper gas source cannot be excluded.