Description: At many cold vent sites authigenic carbonates precipitate due to the release of carbonate alkalinity during the anaerobic oxidation of methane. Carbonate precipitation often induces the formation of massive crusts at the sediment surface or within surface sediments. The range of physical and biogeochemical conditions allowing for the formation of carbonate crusts is largely unknown so that the significance of these widespread manifestations of fluid flow is unclear. Here, we use numerical modeling to investigate the conditions that induce carbonate crust formation in the sediment and the effect of crust formation on sediment porosity and fluid flow rate. Starting with the conditions prevailing at a previously investigated reference site located on Hydrate Ridge, off Oregon, several parameters are systematically varied in a number of numerical experiments. These parameters include coefficients of bioturbation and bioirrigation, sedimentation rate, fluid flow velocity, methane concentration in the ascending vent fluids, and pH and saturation state at the sediment–water interface. The simulations show that carbonate crusts in the sediments only form if the fluids contain sufficient dissolved methane (〉50 mM) and if bioturbation coefficients are low (〈0.05 cm2 a−1). Moreover, high sedimentation rates (〉50 cm ka−1) inhibit crust formation. Bioirrigation induces a downward displacement of the precipitation zone and accelerates the formation of a solid crust. Crusts only form over a rather narrow range of upward fluid flow velocities (20–60 cm a−1), which is somewhat enlarged (up to 90 cm a−1) if the overlying bottom waters are supersaturated with respect to calcite. At higher flow rates, methane is rapidly exported into the water column so that methane oxidation and carbonate precipitation cannot proceed within the surface sediment. The formation of a several centimeters thick carbonate crust in surface sediments is typically completed after a few hundred years (100–500 a). Crust formation reduces the supply of methane to surface sediments which imposes a strong resistance against diffusive and advective methane transport. Therefore, rates of anaerobic methane oxidation and sulfide production are diminished and thus the density and metabolism of chemosynthetic biological communities is limited by crust formation. Due to the moderate flow rates and the slow diffusive transport, only very little methane escapes into the bottom water overlying carbonate-encrusted vent areas.

Description: Significant sediment–ocean chemical fluxes are produced by the expulsion of sedimentary fluids at continental margins. Although such fluxes could play a role in global geochemical cycles, few quantitative estimates of their global, or even regional, significance exist. We carried out a pore water geochemical study of fluids expelled from the Dvurechenskii mud volcano (DMV) in the Black Sea, with the aim of understanding the role played by mud volcanoes in Black Sea geochemical cycles. The DMV is presently expelling highly saline fluids particularly enriched in geochemically important species such as Li+ (1.5 mM), B (2.17 mM), Ba2+ (0.57 mM), Sr2+ (0.79 mM), I (0.4 mM) and dissolved inorganic nitrogen (DIN) (22 mM). A combination of geochemical indicators shows that this geochemical signature was acquired via organic matter and silicate alteration processes in the subsurface down to 3-km depth and near-surface gas hydrate formation. We used a simple transport model to estimate the benthic fluxes of these solutes at the DMV. Our results show that the DMV is expelling fluids at a rather low seepage rate (8–25 cm year−1) resulting in a total water flux of 9.4×10−5 km3 year−1. This gentle regime of fluid expulsion results in Li+, B, Sr2+, I and DIN fluxes between 3.8×104 and 2.1×106 mol year−1. Surface biogeochemical processes affect the benthic fluxes of Ba2+ such that the deep Ba2+ flux is completely consumed through the precipitation of authigenic barite (BaSO4) in surface sediments. The Black Sea I cycle is likely to be affected by mud volcanism, if the 50 known Black Sea mud volcanoes share the rather sluggish activity of the DMV. Mud volcano fluxes of Li, B, Sr and DIN, instead, are too small to affect Black Sea geochemical cycles. On a global scale, mud volcanism could play a role in the marine cycles of Li, B, Sr, I and DIN if current estimates of mud volcano abundance are correct.

Description: Depth profiles of the stable sulfur isotopic composition of dissolved sulfate in near-surface sediments were measured at five stations of the deep Arabian Sea between 1918 and 4426 m water depth (WAST, WAST-Top, CAST, SAST, NAST), sampled in April 1997. The results clearly indicate that net microbial sulfate reduction took place in the sediments at stations WAST and NAST below about 12 cm depth. Sulfate reduction at WAST was more pronounced compared to station NAST, most likely due to higher organic carbon content in turbiditic sediments. No net sulfate reduction took place within the upper 10 cm of the surface sediments at all stations, and no significant isotopic indication for sulfate reduction was found down to 30 cm bsf at station SAST. Results are in accordance with accumulation of reduced isotopically light sulfur species below about 6 cm bsf at station WAST. It is concluded that the sulfur isotopic composition of remaining sulfate is more sensitive to net sulfate reduction than the [SO4]/[Cl] ratio. The sulfur isotopic composition of a vertical profile for dissolved sulfate through the water column at station WAST was essentially constant (250–4047 m: Full-size image (〈1 K)‰ vs. V-CDT n=8). A similar constancy (20–4565 m water depth Full-size image (〈1 K)‰ vs. V-CDT n=15) was found for the station BIOTRANS in the northeastern Atlantic (47°11′ 19°33W), indicating that the oxygen minimum zone in the Arabian Sea has no influence on the sulfur isotopic composition of dissolved sulfate.

Description: Mud extrusion is frequently observed as a dewatering phenomenon in compressional tectonic settings such as subduction zones. Along the Middle American Trench, several of these features have been recently discovered. This paper presents a heat flow study of actively venting Mound Culebra, offshore Nicoya Peninsula, and is complemented by data from geophysical surveys and coring. The mud diapir is characterised by methane emission and authigenic carbonate formation at its crest, and is composed of overconsolidated scaly clays and clast-bearing muds. Compared with the conductive background heat flow, the flux through the mud dome is elevated by 10–20 mW/m2, possibly related to advection of heat by fluids rising from greater depth. Decreased chlorinity in the pore waters from gravity cores may support a deep-seated fluid origin. Geothermal measurements across the mound and temperature measurements made with outriggers on gravity corers were corrected for the effects of thermal refraction, forced by the topography of the mound. Corrected values roughly correlate with the topography, suggesting advection of heat by fluids rising through the mound, thereby generating the prominent methane anomaly over the dome and nurturing vent biota. However, elevated values occur also to the southeast of the mound. We believe that the overconsolidated clays and carbonates on the crest form an almost impermeable lid. Fluids rising from depth underneath the dome are therefore partially channelled towards the flanks of the mound.

Description: Understanding the hydrology of cold seep environments is crucial to perform accurate estimates of fluid and chemical fluxes at sedimentary wedges. Shallow convection processes may affect fluid flux estimates and could favor the destabilization of gas hydrate accumulations, increasing the sediment-ocean methane flux. Evidence for the occurrence of convection at cold seeps, however, is still limited. We use the concentration of 14C (D14C) in carbonate crusts formed at cold seeps of the eastern Mediterranean Sea as a tracer for convective recirculation of seawater-derived fluids. A numerical model is applied to investigate the controls on 14C incorporation in cold seep carbonates. Our simulations show that increased amounts of CH4 in the expelled fluids result in elevated crust D14C, while high Ca2+ and HCO3− concentrations produce the opposite effect. Convection is the only transport process that can significantly increase crust D14C. Advection, bioirrigation, eddy diffusion and bioturbation instead, have little effect on, or produce a decrease of, crust D14C. In addition, the presence of old or modern carbon (MC) in host sediments prior to cementation and the 14C-decay associated to the time needed to form the crust contribute in defining the D14C of carbonate crusts. We then use the model to reproduce the 14C content of the eastern Mediterranean Sea crusts to constrain the chemical and hydrological conditions that led to their formation. Some crusts contain relatively low amounts of 14C (−945.0〈D14C ‰〈−930.2) which, assuming no ageing after crust formation, can be reproduced without considering convection. Other crusts from two sites (the Amsterdam and Napoli mud volcanoes), instead, have a very high 14C-content (−899.0〈D14C ‰〈−838.4) which can only be reproduced by the model if convection mixes deep fluids with seawater. Order-of-magnitude calculations using the Rayleigh criterion for convection suggest that the slow seepage (about 10 cm year−1) of low salinity (20‰) fluids at the Amsterdam sites could trigger haline convection there. On the Napoli mud volcano, where high-density brines are expelled, density-driven convection cannot take place and other processes, possibly involving the rapid movement of free gas in the sediment, could be important.

Description: A numerical model was applied to investigate and quantify the biogeochemical processes fueled by the expulsion of barium and methane-rich fluids in the sediments of a giant cold-seep area in the Derugin Basin (Sea of Okhotsk). Geochemical profiles of dissolved Ba2+, Sr2+, Ca2+, SO42−, HS−, DIC, I− and of calcium carbonate (CaCO3) were fitted numerically to constrain the transport processes and the kinetics of biogeochemical reactions. The model results indicate that the anaerobic oxidation of methane (AOM) is the major process proceeding at a depth-integrated rate of 4.9 μmol cm−2 a−1, followed by calcium carbonate and strontian barite precipitation/dissolution processes having a total depth-integrated rate of 2.1 μmol cm−2 a−1. At the low seepage rate prevailing at our study site (0.14 cm a−1) all of the rising barium is consumed by precipitation of barite in the sedimentary column and no benthic barium flux is produced. Numerical experiments were run to investigate the response of this diagenetic environment to variations of hydrological and biogeochemical conditions. Our results show that relatively low rates of fluid flow (〈∼5 cm a−1) promote the dispersed precipitation of up to 26 wt% of barite and calcium carbonate throughout the uppermost few meters of the sedimentary column. Distinct and persistent events (several hundreds of years long) of more vigorous fluid flow (from 20–110 cm a−1), instead, result in the formation of barite-carbonate crusts near the sediment surface. Competition between barium and methane for sulfate controls the mineralogy of these sediment precipitates such that at low dissolved methane/barium ratios (〈4–11) barite precipitation dominates, while at higher methane/barium ratios sulfate availability is limited by AOM and calcium carbonate prevails. When seepage rates exceed 110 cm a−1, barite precipitation occurs at the seafloor and is so rapid that barite chimneys form in the water column. In the Derugin Basin, spectacular barite constructions up to 20 m high, which cover an area of roughly 22 km2 and contain in excess of 5 million tons of barite, are built through this process. In these conditions, our model calculates a flux of barium to the water column of at least 20 μmol cm−2 a−1. We estimate that a minimum of 0.44 × 106 mol a−1 are added to the bottom waters of the Derugin Basin by cold seep processes, likely affecting the barium cycle in the Sea of Okhotsk.

Description: [1] A new model is developed and applied to simulate the Phanerozoic evolution of seawater composition (dissolved Ca, Sr, dissolved inorganic carbon, alkalinity, pH, δ18O), marine carbonates (Sr/Ca, 87Sr/86Sr, δ13C, δ18O), atmospheric CO2 and surface temperature. The marine carbonate records (Sr/Ca, 87Sr/86Sr, δ13C) are used to reconstruct changes in volcanic/tectonic activity and organic carbon burial over the Phanerozoic. Seawater pH is calculated assuming saturation with respect to calcite and considering the changing concentration of dissolved Ca documented by brine inclusion data. The depth of calcite saturation is allowed to vary through time and the effects of changing temperature and pressure on the stability constants of the carbonate system are considered. Surface temperatures are calculated using the GEOCARB III approach considering also the changing flux of galactic cosmic radiation (GCR). It is assumed that GCR cools the surface of the Earth via enhanced cloud formation at low altitudes. The δ18O of marine carbonates is calculated considering the changing isotopic composition of seawater, the prevailing surface temperatures and seawater pH. Repeated model runs showed that the trends observed in the marine δ18O record can only be reproduced by the model if GCR is allowed to have a strong effect on surface temperature. The climate evolution predicted by the model is consistent with the geological record. Warm periods (Cambrian, Devonian, Triassic, Cretaceous) are characterized by low GCR levels. Cold periods during the late Carboniferous to early Permian and the late Cenozoic are marked by high GCR fluxes and low pCO2 values. The major glaciations occurring during these periods are the result of carbon cycling processes causing a draw-down of atmospheric CO2 and a coevally prevailing dense cloud cover at low-altitudes induced by strong GCR fluxes. The two moderately cool periods during the Ordovician - Silurian and Jurassic - early Cretaceous are characterized by both high pCO2 and GCR levels so that greenhouse warming compensated for the cooling effect of low-altitude clouds. The very high Jurassic δ18O values observed in the geological record are caused by low pH values in surface waters rather than cold surface conditions.

Description: Two cores recovered from the Discovery Basin and one reference core from a location outside the Basin were investigated in detail in order to decipher the influence of hypersaline brines on sediment geochemistry. The cores contain a tephra layer (presumable Y-5) and carbonate microfossils which permit a tentative chrono- and lithostratigraphic correlation. A layer containing up to 60 wt% biogenic opal and 6.6 wt% organic carbon was identified in one basin core, which probably represents the best preserved example of eastern Mediterranean sapropel S-1. The basin is filled with a concentrated solution of MgCl2 which is enriched in dissolved sulfate and has the highest salinity ever encountered in the marine environment. Pore water profiles demonstrate that this brine dissolves sedimentary calcite to form secondary carbonate- and sulfate-bearing minerals. Of these, dolomite, magnesite and gypsum were identified by X-ray diffractometry; thermodynamic calculations show that these phases form in equilibrium with the anomalous brine composition.

Description: Extensive methane hydrate layers are formed in the near-surface sediments of the Cascadia margin. An undissociated section of such a layer was recovered at the base of a gravity core (i.e. at a sediment depth of 120 cm) at the southern summit of Hydrate Ridge. As a result of salt exclusion during methane hydrate formation, the associated pore waters show a highly elevated chloride concentration of 809 mM. In comparison, the average background value is 543 mM. A simple transport-reaction model was developed to reproduce the Cl- observations and quantify processes such as hydrate formation, methane demand, and fluid flow. From this first field observation of a positive Cl- anomaly, high hydrate formation rates (0.15–1.08 mol cm-2 a-1) were calculated. Our model results also suggest that the fluid flow rate at the Cascadia accretionary margin is constrained to 45–300 cm a-1. The amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water. Thus, most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from CH4 dissolved in the pore water.

Description: [1] The role of sediment diagenesis in the marine cycles of Li and B is poorly understood. Because Li and B are easily mobilized during burial and are consumed in authigenic clay mineral formation, their abundance in marine pore waters varies considerably. Exchange with the overlying ocean through diffusive fluxes should thus be common. Nevertheless, only a minor Li sink associated with the low-temperature alteration of volcanic ash has been observed. We describe a low-temperature diagenetic environment in the Black Sea dominated by the alteration of detrital plagioclase feldspars. Fluids expelled from the Odessa mud volcano in the Sorokin Trough originate from shallow (≈100–400 m deep) sediments which are poor in volcanic materials but rich in anorthite. These fluids are depleted in Na+, K+, Li+, B, and 18O and enriched in Ca2+ and Sr2+, indicating that anorthite is dissolving and authigenic clays are forming. Using a simple chemical model, we calculate the pH and the partial pressure of CO2 (PCO2) in fluids associated with this alteration process. Our results show that the pH of these fluids is up to 1.5 pH units lower than in most deep marine sediments and that PCO2 levels are up to several hundred times higher than in the atmosphere. These conditions are similar to those which favor the weathering of silicate minerals in subaerial soil environments. We propose that in Black Sea sediments enhanced organic matter preservation favors CO2 production through methanogenesis and results in a low pore water pH, compared to most deep sea sediments. As a result, silicate mineral weathering, which is a sluggish process in most marine diagenetic environments, proceeds rapidly in Black Sea sediments. There is a potential for organic matter-rich continental shelf environments to host this type of diagenesis. Should such environments be widespread, this new Li and B sink could help balance the marine Li and Li isotope budgets but would imply an apparent imbalance in the B cycle.