Description: Due to the strong interest in geochemical CO2-fluid-rock interaction in the context of geological storage of CO2 a growing number of research groups have used a variety of different experimental ways to identify important geochemical dissolution or precipitation reactions and – if possible – quantify the rates and extent of mineral or rock alteration. In this inter-laboratory comparison the gas-fluid-mineral reactions of three samples of rock-forming minerals have been investigated by 11 experimental labs. The reported results point to robust identification of the major processes in the experiments by most groups. The dissolution rates derived from the changes in composition of the aqueous phase are consistent overall, but the variation could be reduced by using similar corrections for changing parameters in the reaction cells over time. The comparison of experimental setups and procedures as well as of data corrections identified potential improvements for future gas-fluid-rock studies.

Description: The formation of mud volcanoes in the Gulf of Cadiz is closely linked to diapirism in the deep subsurface. The Mercator mud volcano (MMV) is a rare example where diapiric emplacement, in addition to being key for upward fluid migration, is also an important zone for fluid and mineral diagenesis. The most intriguing findings in the near-surface muds of the MMV are extremely high salinities of up to 5.2 M of NaCl from diapiric and evaporitic halite dissolution and the occurrence of authigenic gypsum and anhydrite crystals, both of which have not been observed to date in the Gulf of Cadiz. Employing a thermodynamic model we elucidate how the interplay of temperature pulses, strong salinity gradients, and fluid flow dynamically drive mineral dissolution and re-formation. The strong increase in salinity in the pore fluids has important implications for thermodynamic equilibria by significantly lowering the activity of water, thereby raising the gypsum–anhydrite transition zone from 〉1 km to about 400 m sediment depth at the MMV. This transition is further shifted to immediately below the seafloor during intervals of active mud and fluid expulsion when the MV surface temperature is heated up to at least 30 °C. As a consequence, precipitation of authigenic gypsum near the sediment surface (1–2 mbsf) has been linked to the dissolution of evaporites below the MMV. More precisely, the mechanisms generating supersaturation in the ascending gypsum-saturated MMV fluids are (1) the slow and constant cooling of these fluids along the geothermal gradient during their ascent leading to formation of ubiquitous micro-crystals and (2) the more rapid cooling after a heat pulse or transport from greater and warmer depth during an active mud volcano phase leading to the precipitation of cm-scale gypsum crystals or even fist-size concretions. The MMV fluids approaching the salt diapir from farther below have experienced a genesis similar to those of other mud volcanoes in the Gulf of Cadiz located above deep-rooted faults. These processes include clay mineral dewatering, thermogenic degradation of organic matter and deep high-temperature leaching of terrigenous sediments or continental crust.

Description: The accumulation of gas hydrates in marine sediments is essentially controlled by the accumulation of particulate organic carbon (POC) which is microbially converted into methane, the thickness of the gas hydrate stability zone (GHSZ) where methane can be trapped, the sedimentation rate (SR) that controls the time that POC and the generated methane stays within the GHSZ, and the delivery of methane from deep-seated sediments by ascending pore fluids and gas into the GHSZ. Recently, Wallmann et al. (2012) presented transfer functions to predict the gas hydrate inventory in diffusion-controlled geological systems based on SR, POC and GHSZ thickness for two different scenarios: normal and full compacting sediments. We apply these functions to global data sets of bathymetry, heat flow, seafloor temperature, POC input and SR, estimating a global mass of carbon stored in marine methane hydrates from 3 to 455 Gt of carbon (GtC) depending on the sedimentation and compaction conditions. The global sediment volume of the GHSZ in continental margins is estimated to be 60–67 × 1015 m3, with a total of 7 × 1015 m3 of pore volume (available for GH accumulation). However, seepage of methane-rich fluids is known to have a pronounced effect on gas hydrate accumulation. Therefore, we carried out a set of systematic model runs with the transport-reaction code in order to derive an extended transfer function explicitly considering upward fluid advection. Using averaged fluid velocities for active margins, which were derived from mass balance considerations, this extended transfer function predicts the enhanced gas hydrate accumulation along the continental margins worldwide. Different scenarios were investigated resulting in a global mass of sub-seafloor gas hydrates of ~ 550 GtC. Overall, our systematic approach allows to clearly and quantitatively distinguish between the effect of biogenic methane generation from POC and fluid advection on the accumulation of gas hydrate, and hence, provides a simple prognostic tool for the estimation of large-scale and global gas hydrate inventories in marine sediments.

Description: Geochemical data (CH4, SO42−, I−, Cl−, particulate organic carbon (POC), δ13C-CH4, and δ13C-CO2) are presented from the upper 30 m of marine sediment on a tectonic submarine accretionary wedge offshore southwest Taiwan. The sampling stations covered three ridges (Tai-Nan, Yung-An, and Good Weather), each characterized by bottom simulating reflectors, acoustic turbidity, and different types of faulting and anticlines. Sulfate and iodide concentrations varied little from seawater-like values in the upper 1–3 m of sediment at all stations; a feature that is consistent with irrigation of seawater by gas bubbles rising through the soft surface sediments. Below this depth, sulfate was rapidly consumed within 5–10 m by anaerobic oxidation of methane (AOM) at the sulfate-methane transition. Carbon isotopic data imply a mainly biogenic methane source. A numerical transport-reaction model was used to identify the supply pathways of methane and estimate depth-integrated turnover rates at the three ridges. Methane gas ascending from deep layers, facilitated by thrusts and faults, was by far the dominant term in the methane budget at all sites. Differences in the proximity of the sampling sites to the faults and anticlines mainly accounted for the variability in gas fluxes and depth-integrated AOM rates. By comparison, methane produced in situ by POC degradation within the modeled sediment column was unimportant. This study demonstrates that the geochemical trends in the continental margins offshore SW Taiwan are closely related to the different geological settings.

Description: Hydrocarbon-rich fluids expelled at mud volcanoes (MVs) may contribute significantly to the carbon budget of the oceans, but little is known about the long-term variation in fluid fluxes at MVs. The Darwin MV is one of more than 40 MVs located in the Gulf of Cadiz, but it is unique in that its summit is covered by a thick carbonate crust that has the potential to provide a temporal record of seepage activity. In order to test this idea, we have conducted petrographic, chemical and isotopic analyses of the carbonate crust. In addition a 1-D transport-reaction model was applied to pore fluid data to assess fluid flow and carbonate precipitation at present. The carbonate crusts mainly comprise of aragonite, with a chaotic fabric exhibiting different generations of cementation and brecciation. The crusts consist of bioclasts and lithoclasts (peloids, intraclasts and extraclasts) immersed in a micrite matrix and in a variety of cement types (microsparite, botryoidal, isopachous acicular, radial and splayed fibrous). The carbonates are moderately depleted in 13C (δ13C = − 8.1 to − 27.9‰) as are the pore fluids (δ13C = − 19.1 to − 28.7‰), which suggests that their carbon originated from the oxidation of methane and higher hydrocarbons, like the gases that seep from the MV today. The carbonate δ18O values are as high as 5.1‰, and it is most likely that the crusts formed from 18O-rich fluids derived from dehydration of clay minerals at depth. Pore fluid modelling results indicate that the Darwin MV is currently in a nearly dormant phase (seepage velocities are 〈 0.09 cm yr− 1). Thus, the thick carbonate crust must have formed during past episodes of high fluid flow, alternating with phases of mud extrusion and uplift. Highlights ► Results of pore fluid modelling indicate low seepage activity at localised sites. ► Pore fluids are supersaturated with respect to hydrocarbons of thermogenic origin. ► AOM supports vent fauna and results in the formation of authigenic carbonates. ► The carbonate crust has a brecciated appearance and mainly consists of aragonite. ► The crust formation seems to be regulated by changes in fluid and mudflow activity.

Description: This study focused on biogeochemical processes and microbial activity in sediments of a natural deep-sea CO2 seepage area (Yonaguni Knoll IV hydrothermal system, Japan). The aim was to assess the influence of the geochemical conditions occurring in highly acidic and CO2 saturated sediments on sulfate reduction (SR) and anaerobic methane oxidation (AOM). Porewater chemistry was investigated from retrieved sediment cores and in situ by microsensor profiling. The sites sampled around a sediment-hosted hydrothermal CO2 vent were very heterogeneous in porewater chemistry, indicating a complex leakage pattern. Near the vents, droplets of liquid CO2 were observed emanating from the sediments, and the pH reached approximately 4.5 in a sediment depth 〉 6 cm, as determined in situ by microsensors. Methane and sulfate co-occurred in most sediment samples from the vicinity of the vents down to a depth of 3 m. However, SR and AOM were restricted to the upper 7-15 cm below seafloor, although neither temperature, low pH, nor the availability of methane and sulfate could be limiting microbial activity. We argue that the extremely high subsurface concentrations of dissolved CO2 (1000-1700 mM), which disrupt the cellular pH homeostasis, and lead to end-product inhibition. This limits life to the surface sediment horizons above the liquid CO2 phase, where less extreme conditions prevail. Our results may have to be taken into consideration in assessing the consequences of deep-sea CO2 sequestration on benthic element cycling and on the local ecosystem state.

Description: A simple prognostic tool for gas hydrate (GH) quantification in marine sediments is presented based on a diagenetic transport-reaction model approach. One of the most crucial factors for the application of diagenetic models is the accurate formulation of microbial degradation rates of particulate organic carbon (POC) and the coupled formation of biogenic methane. Wallmann et al. (2006) suggested a kinetic formulation considering the ageing effects of POC and accumulation of reaction products (CH4, CO2) in the pore water. This model is applied to data sets of several ODP sites in order to test its general validity. Based on a thorough parameter analysis considering a wide range of environmental conditions, the POC accumulation rate (POCar in g/m2/yr) and the thickness of the gas hydrate stability zone (GHSZ in m) were identified as the most important and independent controls for biogenic GH formation. Hence, depth-integrated GH inventories in marine sediments (GHI in g of CH4 per cm2 seafloor area) can be estimated as: GHI=a ·POCar·GHSZb ·exp(−GHSZc/POCar/d)+e with a = 0.00214, b = 1.234, c = −3.339, d = 0.3148, e = −10.265. The transfer function gives a realistic first order approximation of the minimum GH inventory in low gas flux (LGF) systems. The overall advantage of the presented function is its simplicity compared to the application of complex numerical models, because only two easily accessible parameters need to be determined.

Description: Lithium concentration and isotope data (δ7Li) are reported for pore fluids from 18 cold seep locations together with reference fluids from shallow marine environments, a sediment-hosted hydrothermal system and two Mediterranean brine basins. The new reference data and literature data of hydrothermal fluids and pore fluids from the Ocean Drilling Program follow an empirical relationship between Li concentration and δ7Li (δ7Li = −6.0(±0.3) · ln[Li] + 51(±1.2)) reflecting Li release from sediment or rocks and/or uptake of Li during mineral authigenesis. Cold seep fluids display δ7Li values between +7.5‰ and +45.7‰, mostly in agreement with this general relationship. Ubiquitous diagenetic signals of clay dehydration in all cold seep fluids indicate that authigenic smectite–illite is the major sink for light pore water Li in deeply buried continental margin sediments. Deviations from the general relationship are attributed to the varying provenance and composition of sediments or to transport-related fractionation trends. Pore fluids on passive margins receive disproportionally high amounts of Li from intensely weathered and transported terrigenous matter. By contrast, on convergent margins and in other settings with strong volcanogenic input, Li concentrations in pore water are lower because of intense Li uptake by alteration minerals and, most notably, adsorption of Li onto smectite. The latter process is not accompanied by isotope fractionation, as revealed from a separate study on shallow sediments. A numerical transport-reaction model was applied to simulate Li isotope fractionation during upwelling of pore fluids. It is demonstrated that slow pore water advection (order of mm a−1) suffices to convey much of the deep-seated diagenetic Li signal into shallow sediments. If carefully applied, Li isotope systematics may, thus, provide a valuable record of fluid/mineral interaction that has been inherited several hundreds or thousands of meters below the actual seafloor fluid escape structure.

Description: Several known gas seep sites along the Hikurangi Margin off the east coast of New Zealand were surveyed by marine controlled source electromagnetic (CSEM) experiments. A bottom-towed electric dipole–dipole system was used to reveal the occurrence of gas hydrate and methane related to the seeps. The experiments were part of the international multidisciplinary research program “New Vents” carried out on German R/V Sonne in 2007 (cruise SO191) to study key parameters controlling the release and transformation of methane from marine cold vents and shallow gas hydrate deposits. Two CSEM lines have been surveyed over known seep sites on Opouawe Bank in the Wairarapa region off the SE corner of the North Island. The data have been inverted to sub-seafloor apparent resistivity profiles and one-dimensional layered models. Clearly anomalous resistivities are coincident with the location of two gas seep sites, North Tower and South Tower on Opouawe Bank. A layer of concentrated gas hydrate within the uppermost 100 m below the seafloor is likely to cause the anomalous resistivities, but free gas and thick carbonate crusts may also play a role. Seismic data show evidence of fault related venting which may also indicate the distribution of gas hydrates and/or authigenic carbonate. Geochemical profiles indicate an increase of methane flux and the formation of gas hydrate in the shallow sediment section around the seep sites. Takahe is another seep site in the area where active venting, higher heat flow, shallow gas hydrate recovered from cores, and seismic fault planes, but only moderately elevated resistivities have been observed. The reasons could be a) the gas hydrate concentration is too low, even though methane venting is evident, b) strong temporal or spatial variation of the seep activity, and c) the thermal anomaly indicates rather temperature driven fluid expulsion that hampers the formation of gas hydrate beneath the vent.

Description: Mud volcanoes are seafloor expressions of focused fluid flow that are common in compressional tectonic settings. New high-resolution 3-D seismic data from the Mercator mud volcano (MMV) and an adjacent buried mud volcano (BMV) image the internal structure of the top 800 m of sediment at both mud volcanoes, revealing that both are linked and have been active episodically. The total volumes of extruded mud range between 0.15 and 0.35 km3 and 0.02–0.05 km3 for the MMV and the BMV, respectively. The pore water composition of surface sediment samples suggests that halokinesis has played an important role in the evolution of the mud volcanoes. We propose that erosion of the top of the Vernadsky Ridge that underlies the mud volcanoes activated salt movement, triggering deep migration of fluids, dissolution of salt, and sediment liquefaction and mobilization since the end of the Pliocene. Since beginning of mud volcanism in this area, the mud volcanoes erupted four times while there was only one reactivation of salt tectonics. This implies that there are other mechanisms that trigger mud eruptions. The stratigraphic relationship of mudflows from the MMV and BMV indicates that the BMV was triggered by the MMV eruptions. This may either be caused by loading-induced hydrofracturing within the BMV or due to a common feeder system for both mud volcanoes. This study shows that the mud volcanoes in the El Arraiche mud volcano field are long-lived features that erupt with intervals of several tens of thousands of years.