Description: Presented is an example of the transport-reaction code (TRACTION) applied to the simulation of pore water species in the seawater mixing zone at Mercator Mud volcano in the Gulf of Cadiz. TRACTION was specifically designed to account for non-ideal transport effects in the presence of thermodynamic (e.g. salinity or temperature) gradients. The model relies on the most fundamental concept of solute diffusion, which states that the chemical potential gradient (Maxwell's model) rather than the concentration gradient (Fick's law) is the driving force for diffusion. In turn, this requires accounting for species interactions by applying Pitzer's method to derive species chemical potentials and Onsager coefficients instead of using the classical diffusion coefficients. Electrical imbalances arising from varying diffusive fluxes in multicomponent systems, like seawater, are avoided by applying an electrostatic gradient as an additional transport contribution.

Description: This dataset comprises porosity data from undisturbed reference sites sampled by a multiple corer (MUC) and disturbed push-cores (PUC), which were retrieved using the ROV Kiel 6000 (GEOMAR) during RV SONNE expedition SO239 in 2015. During this cruise, different European contract areas for the exploration of polymetallic nodules in the area of the Clarion-Clipperton Zone (CCZ) were visited (BGR, IOM, GSR, IFREMER), which are currently being explored in light of potential future deep-sea mining. The PUCs were taken in several small-scale disturbance tracks, which were created 1 day to 37 years before sampling for the simulation of deep-sea mining activities in the different exploration areas. The sampled disturbance tracks include 1-day- and 3-year-old EBS tracks (created with an Epibenthic sledge), 8-months- and 37-year-old dredge tracks, and a 20-year-old track from the Benthic Impact Experiment in the IOM area (IOM-BIE). The dataset was produces in the laboratories of the Alfred Wegener Institute (AWI) Bremerhaven, Germany.

Description: This dataset comprises solid-phase total organic carbon contents (TOC) and bulk sediment manganese contents from disturbed push-cores (PUC), which were retrieved using the ROV Kiel 6000 (GEOMAR) during RV SONNE expedition SO239 in 2015. During this cruise, different European contract areas for the exploration of polymetallic nodules in the area of the Clarion-Clipperton Zone (CCZ) were visited (BGR, IOM, GSR, IFREMER), which are currently being explored in light of potential future deep-sea mining. The PUCs were taken in several small-scale disturbance tracks, which were created 1 day to 37 years before sampling for the simulation of deep-sea mining activities in the different exploration areas. The sampled disturbance tracks include 1-day- and 3-year-old EBS tracks (created with an Epibenthic sledge), 8-months- and 37-year-old dredge tracks, and a 20-year-old track from the Benthic Impact Experiment in the IOM area (IOM-BIE). The dataset was produces in the laboratories of the Alfred Wegener Institute (AWI) Bremerhaven, Germany.

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: Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by a well-defined subsurface seismic chimney structure ca. 80,500 m2 in area. Sub-seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure. Reaction-transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas (up to 3761 mmol m−2 yr−1) dwarfed the rate of methanogenesis within the simulated sediment column (2.6 mmol m−2 yr−1). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) at the base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximum rates in the central part of the chimney (946 and 2420 mmol m−2 yr−1, respectively). A seep-wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of AOM rates, gas hydrate formation and benthic dissolved methane fluxes of 3.68 × 104 mol yr−1, 73.85 × 104 mol yr−1and 1.19 × 104 mol yr−1, respectively. A much larger flux of methane probably escapes in gaseous form through focused bubble vents. The approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales.

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.