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We report evidence of active seafloor uplift and gas emissions several kilometers offshore from the port of Naples (Italy).Pockmarks, mounds and craters are features of the seafloor.These formations represent the tops of shallow crustal structures, including pagodas, faults and folds that affect the seabed today.They recorded the rise, pressurization and release of helium and carbon dioxide in decarbonization reactions of mantle melts and crustal rocks.These gases are likely similar to those that feed the hydrothermal systems of Ischia, Campi Flegre and Soma-Vesuvius, suggesting a mantle source mixed with crustal fluids below the Gulf of Naples.Subsea expansion and rupture caused by the gas lift and pressurization process requires an overpressure of 2-3 MPa.Seafloor uplifts, faults, and gas emissions are manifestations of non-volcanic upheavals that may herald seafloor eruptions and/or hydrothermal explosions.
Deep-sea hydrothermal (hot water and gas) discharges are a common feature of mid-ocean ridges and convergent plate margins (including submerged parts of island arcs), whereas cold discharges of gas hydrates (chlatrates) are often characteristic of continental shelves and passive margins1, 2,3,4,5.The occurrence of seafloor hydrothermal discharges in coastal areas implies heat sources (magma reservoirs) within the continental crust and/or mantle.These discharges may precede the ascent of magma through the uppermost layers of the Earth’s crust and culminate in the eruption and in-placement of volcanic seamounts6.Therefore, identification of (a) morphologies associated with active seabed deformation and (b) gas emissions close to populated coastal areas such as the volcanic region of Naples in Italy (~1 million inhabitants) is critical for assessing possible volcanoes.Shallow eruption.Furthermore, while morphological features associated with deep-sea hydrothermal or hydrate gas emissions are relatively well known due to their geological and biological properties, the exceptions are morphological features associated with shallower waters, except those occurring in In Lake 12, there are relatively few records.Here, we present new bathymetric, seismic, water column, and geochemical data for an underwater, morphologically and structurally complex region affected by gas emissions in the Gulf of Naples (Southern Italy), approximately 5 km from the port of Naples.These data were collected during the SAFE_2014 (August 2014) cruise aboard the R/V Urania.We describe and interpret the seafloor and subsurface structures where gas emissions occur, investigate the sources of venting fluids, identify and characterize the mechanisms that regulate gas rise and associated deformation, and discuss volcanology impacts.
The Gulf of Naples forms the Plio-Quaternary western margin, the NW-SE elongated Campania tectonic depression13,14,15.EW of Ischia (ca. 150-1302 AD), Campi Flegre crater (ca. 300-1538) and Soma-Vesuvius (from <360-1944) The arrangement confines the bay to the north AD)15, while the south borders the Sorrento Peninsula (Fig. 1a).The Gulf of Naples is affected by the prevailing NE-SW and secondary NW-SE significant faults (Fig. 1)14,15.Ischia, Campi Flegrei and Somma-Vesuvius are characterized by hydrothermal manifestations, ground deformation, and shallow seismicity16,17,18 (eg, the turbulent event at Campi Flegrei in 1982-1984, with uplift of 1.8 m and thousands of earthquakes).Recent studies19,20 suggest that there may be a link between the dynamics of Soma-Vesuvius and that of Campi Flegre, possibly associated with ‘deep’ single magma reservoirs.Volcanic activity and sea-level oscillations in the last 36 ka of Campi Flegrei and 18 ka of Somma Vesuvius controlled the sedimentary system of the Gulf of Naples.The low sea level at the last glacial maximum (18 ka) led to the regression of the offshore-shallow sedimentary system, which was subsequently filled by transgressive events during the Late Pleistocene-Holocene.Submarine gas emissions have been detected around the island of Ischia and off the coast of Campi Flegre and near Mount Soma-Vesuvius (Fig. 1b).
(a) Morphological and structural arrangements of the continental shelf and the Gulf of Naples 15, 23, 24, 48.Dots are major submarine eruption centers; red lines represent major faults.(b) Bathymetry of the Bay of Naples with detected fluid vents (dots) and traces of seismic lines (black lines).The yellow lines are the trajectories of seismic lines L1 and L2 reported in Figure 6.The boundaries of the Banco della Montagna (BdM) dome-like structures are marked by blue dashed lines in (a,b).The yellow squares mark the locations of the acoustic water column profiles, and the CTD-EMBlank, CTD-EM50 and ROV frames are reported in Fig. 5.The yellow circle marks the location of the sampling gas discharge, and its composition is shown in Table S1.Golden Software (http://www.goldensoftware.com/products/surfer) uses graphics generated by Surfer® 13.
Based on data obtained during the SAFE_2014 (August 2014) cruise (see Methods), a new Digital Terrain Model (DTM) of the Gulf of Naples with 1 m resolution has been constructed.DTM shows that the seafloor south of the Port of Naples is characterized by a gently sloping south-facing (slope ≤3°) surface interrupted by a 5.0 × 5.3 km dome-like structure, locally known as Banco della Montagna (BdM).Fig. 1a,b).BdM develops at a depth of about 100 to 170 meters, 15 to 20 meters above the surrounding seafloor.The BdM dome displayed a mound-like morphology due to 280 subcircular to oval mounds (Fig. 2a), 665 cones, and 30 pits (Figs. 3 and 4).The mound has a maximum height and circumference of 22 m and 1,800 m, respectively.The circularity [C = 4π(area/perimeter2)] of the mounds decreased with increasing perimeter (Fig. 2b).Axial ratios for mounds ranged between 1 and 6.5, with mounds with an axial ratio >2 showing a preferred N45°E + 15° strike and a more dispersed secondary, more dispersed N105°E to N145°E strike ( Fig. 2c). Single or aligned cones exist on the BdM plane and on top of the mound (Fig. 3a,b).The conical arrangements follow the arrangement of the mounds on which they are located.Pockmarks are commonly located on the flat seabed (Fig. 3c) and occasionally on mounds.The spatial densities of cones and pockmarks demonstrate that the predominant NE-SW alignment delimits the northeast and southwest boundaries of the BdM dome (Fig. 4a,b); the less extended NW-SE route is located in the central BdM region.
(a) Digital terrain model (1 m cell size) of the dome of Banco della Montagna (BdM).(b) Perimeter and roundness of BdM mounds.(c) Axial ratio and angle (orientation) of the major axis of the best-fit ellipse surrounding the mound.The standard error of the Digital Terrain model is 0.004 m; the standard errors of perimeter and roundness are 4.83 m and 0.01, respectively, and the standard errors of axial ratio and angle are 0.04 and 3.34°, respectively.
Details of identified cones, craters, mounds and pits in the BdM region extracted from the DTM in Figure 2.
(a) Alignment cones on a flat seabed; (b) cones and craters on NW-SE slender mounds; (c) pockmarks on a lightly dipped surface.
(a) Spatial distribution of detected craters, pits, and active gas discharges.(b) Spatial density of craters and pits reported in (a) (number/0.2 km2).
We identified 37 gaseous emissions in the BdM region from ROV water column echo sounder images and direct observations of the seafloor acquired during the SAFE_2014 cruise in August 2014 (Figures 4 and 5).The acoustic anomalies of these emissions show vertically elongated shapes rising from the seafloor, ranging vertically between 12 and about 70 m (Fig. 5a).In some places, acoustic anomalies formed an almost continuous “train.”The observed bubble plumes vary widely: from continuous, dense bubble flows to short-lived phenomena (Supplementary Movie 1).ROV inspection allows for visual verification of the occurrence of seafloor fluid vents and highlights small pockmarks on the seabed, sometimes surrounded by red to orange sediments (Fig. 5b).In some cases, ROV channels reactivate emissions.The vent morphology shows a circular opening at the top with no flare in the water column.The pH in the water column just above the discharge point showed a significant drop, indicating more acidic conditions locally (Fig. 5c,d).In particular, the pH above the BdM gas discharge at 75 m depth decreased from 8.4 (at 70 m depth) to 7.8 (at 75 m depth) (Fig. 5c), whereas other sites in the Gulf of Naples had pH values ​​between 0 and 160 m in the depth interval between 8.3 and 8.5 (Fig. 5d).Significant changes in seawater temperature and salinity were lacking at two sites inside and outside the BdM area of ​​the Gulf of Naples.At a depth of 70 m, the temperature is 15 °C and the salinity is about 38 PSU (Fig. 5c,d).Measurements of pH, temperature, and salinity indicated: a) the participation of acidic fluids associated with the BdM degassing process and b) the absence or very slow discharge of thermal fluids and brine.
(a) Acquisition window of the acoustic water column profile (echometer Simrad EK60).Vertical green band corresponding to the gas flare detected on the EM50 fluid discharge (about 75 m below sea level) located in the BdM region; the bottom and seafloor multiplex signals are also shown (b) collected with a remote-controlled vehicle in the BdM region The single photo shows a small crater (black circle) surrounded by red to orange sediment.(c,d) Multiparameter probe CTD data processed using SBED-Win32 software (Seasave, version 7.23.2).Patterns of selected parameters (salinity, temperature, pH and oxygen) of the water column above the fluid discharge EM50 (panel c) and outside the Bdm discharge area panel (d).
We collected three gas samples from the study area between August 22 and 28, 2014.These samples showed similar compositions, dominated by CO2 (934-945 mmol/mol), followed by relevant concentrations of N2 (37-43 mmol/mol), CH4 (16-24 mmol/mol) and H2S (0.10 mmol/mol) -0.44 mmol/mol), while H2 and He were less abundant (<0.052 and <0.016 mmol/mol, respectively) (Fig. 1b; Table S1, Supplementary Movie 2).Relatively high concentrations of O2 and Ar were also measured (up to 3.2 and 0.18 mmol/mol, respectively).The sum of the light hydrocarbons ranges from 0.24 to 0.30 mmol/mol and consists of C2-C4 alkanes, aromatics (mainly benzene), propene and sulfur-containing compounds (thiophene).The 40Ar/36Ar value is consistent with air (295.5), although sample EM35 (BdM dome) has a value of 304, showing a slight excess of 40Ar.The δ15N ratio was higher than for air (up to +1.98% vs. Air), while the δ13C-CO2 values ​​ranged from -0.93 to 0.44% vs. V-PDB.R/Ra values ​​(after correcting for air pollution using the 4He/20Ne ratio) were between 1.66 and 1.94, indicating the presence of a large fraction of mantle He.By combining the helium isotope with CO2 and its stable isotope 22, the source of the emissions in BdM can be further clarified.In the CO2 map for CO2/3He versus δ13C (Fig. 6), the BdM gas composition is compared to that of the Ischia, Campi Flegrei and Somma-Vesuvius fumaroles.Figure 6 also reports theoretical mixing lines between three different carbon sources that may be involved in BdM gas production: dissolved mantle-derived melts, organic-rich sediments, and carbonates.The BdM samples fall on the mixing line depicted by the three Campania volcanoes, that is, mixing between mantle gases (which are assumed to be slightly enriched in carbon dioxide relative to classical MORBs for the purpose of fitting the data) and reactions caused by crustal decarbonization The resulting gas rock.
Hybrid lines between mantle composition and end members of limestone and organic sediments are reported for comparison.Boxes represent the fumarole areas of Ischia, Campi Flegrei and Somma-Vesvius 59, 60, 61.The BdM sample is in the mixed trend of the Campania volcano.The endmember gas of the mixed line is of mantle source, which is the gas produced by the decarburization reaction of carbonate minerals.
Seismic sections L1 and L2 (Figs. 1b and 7) show the transition between BdM and the distal stratigraphic sequences of the Somma-Vesuvius (L1, Fig. 7a) and Campi Flegrei (L2, Fig. 7b) volcanic regions.BdM is characterized by the presence of two major seismic formations (MS and PS in Fig. 7).The top one (MS) shows subparallel reflectors of high to moderate amplitude and lateral continuity (Fig. 7b,c).This layer includes marine sediments dragged by the Last Glacial Maximum (LGM) system and consists of sand and clay23.The underlying PS layer (Fig. 7b–d) is characterized by a chaotic to transparent phase in the shape of columns or hourglasses.The top of the PS sediments formed seafloor mounds (Fig. 7d).These diapir-like geometries demonstrate the intrusion of PS transparent material into the uppermost MS deposits.Uplift is responsible for the formation of folds and faults that affect the MS layer and overlying present-day sediments of the BdM seafloor (Fig. 7b–d).The MS stratigraphic interval is clearly delaminated in the ENE portion of the L1 section, while it whitens towards BdM due to the presence of a gas-saturated layer (GSL) covered by some internal levels of the MS sequence (Fig. 7a).Gravity cores collected at the top of the BdM corresponding to the transparent seismic layer indicate that the uppermost 40 cm consists of sand deposited recently to the present; )24,25 and pumice fragments from the explosive eruption of Campi Flegrei of “Naples Yellow Tuff” (14.8 ka)26.The transparent phase of the PS layer cannot be explained by chaotic mixing processes alone, because the chaotic layers associated with landslides, mud flows and pyroclastic flows found outside the BdM in the Gulf of Naples are acoustically opaque21,23,24.We conclude that the observed BdM PS seismic facies as well as the appearance of the subsea outcrop PS layer (Fig. 7d) reflect the uplift of natural gas.
(a) Single-track seismic profile L1 (navigation trace in Fig. 1b) showing a columnar (pagoda) spatial arrangement.The pagoda consists of chaotic deposits of pumice and sand.The gas-saturated layer that exists below the pagoda removes the continuity of the deeper formations.(b) Single-channel seismic profile L2 (navigation trace in Fig. 1b), highlighting incision and deformation of seafloor mounds, marine (MS), and pumice sand deposits (PS).(c) The deformation details in MS and PS are reported in (c,d).Assuming a velocity of 1580 m/s in the uppermost sediment, 100 ms represents about 80 m on the vertical scale.
The morphological and structural characteristics of BdM are similar to other subsea hydrothermal and gas hydrate fields globally2,12,27,28,29,30,31,32,33,34 and are often associated with uplifts (vaults and mounds) and gas Discharge (cones, pits).BdM-aligned cones and pits and elongated mounds indicate structurally controlled permeability (Figures 2 and 3).The spatial arrangement of mounds, pits and active vents suggests that their distribution is partly controlled by the NW-SE and NE-SW impact fractures (Fig. 4b).These are the preferred strikes of fault systems affecting the Campi Flegrei and Somma-Vesuvius volcanic areas and the Gulf of Naples.In particular, the structure of the former controls the location of the hydrothermal discharge from the Campi Flegrei crater35.We therefore conclude that faults and fractures in the Gulf of Naples represent the preferred route for gas migration to the surface, a feature shared by other structurally controlled hydrothermal systems36,37.Notably, BdM cones and pits were not always associated with mounds (Fig. 3a,c).This suggests that these mounds do not necessarily represent precursors to pit formation, as other authors have suggested for gas hydrate zones32,33.Our conclusions support the hypothesis that disruption of dome seafloor sediments does not always lead to the formation of pits.
The three collected gaseous emissions show chemical signatures typical of hydrothermal fluids, namely mainly CO2 with significant concentrations of reducing gases (H2S, CH4 and H2) and light hydrocarbons (especially benzene and propylene)38,39, 40, 41, 42, 43, 44, 45 (Table S1).The presence of atmospheric gases (such as O2), which are not expected to be present in submarine emissions, may be due to contamination from air dissolved in seawater coming into contact with gases stored in plastic boxes used for sampling, as ROVs are extracted from the ocean floor to the sea to revolt.Conversely, positive δ15N values ​​and a high N2/Ar (up to 480) significantly higher than ASW (air-saturated water) suggest that most of the N2 is produced from extra-atmospheric sources, in agreement with the predominant hydrothermal origin of these gases.The hydrothermal-volcanic origin of the BdM gas is confirmed by the CO2 and He contents and their isotopic signatures.Carbon isotopes (δ13C-CO2 from -0.93% to +0.4%) and CO2/3He values ​​(from 1.7 × 1010 to 4.1 × 1010) suggest that the BdM samples belong to a mixed trend of fumaroles around the Gulf of Naples’ mantle end members and decarbonization The relationship between the gases produced by the reaction (Figure 6).More specifically, the BdM gas samples are located along the mixing trend at approximately the same location as the fluids from the adjacent Campi Flegrei and Somma-Veusivus volcanoes.They are more crustal than the Ischia fumaroles, which are closer to the end of the mantle.Somma-Vesuvius and Campi Flegrei have higher 3He/4He values ​​(R/Ra between 2.6 and 2.9) than BdM (R/Ra between 1.66 and 1.96; Table S1).This suggests that the addition and accumulation of radiogenic He originated from the same magma source that fed the Somma-Vesuvius and Campi Flegrei volcanoes.The absence of detectable organic carbon fractions in BdM emissions suggests that organic sediments are not involved in the BdM degassing process.
Based on the data reported above and results from experimental models of dome-like structures associated with subsea gas-rich regions, deep gas pressurization may be responsible for the formation of kilometer-scale BdM domes.To estimate the overpressure Pdef leading to the BdM vault, we applied a thin-plate mechanics model33,34 assuming, from the collected morphological and seismic data, that the BdM vault is a subcircular sheet of radius a larger than a deformed soft viscous deposit The vertical maximum displacement w and thickness h of the (Supplementary Fig. S1).Pdef is the difference between total pressure and rock static pressure plus water column pressure.At BdM, the radius is about 2,500 m, w is 20 m, and the h maximum estimated from the seismic profile is about 100 m.We calculate Pdef 46Pdef = w 64 D/a4 from the relation, where D is the flexural stiffness; D is given by (E h3)/[12(1 – ν2)], where E is the Young’s modulus of the deposit, ν is Poisson’s ratio (~0.5)33.Since the mechanical properties of BdM sediments cannot be measured, we set E = 140 kPa, which is a reasonable value for coastal sandy sediments 47 similar to BdM14,24.We do not consider the higher E values ​​reported in the literature for silty clay deposits (300 < E < 350,000 kPa)33,34 because BDM deposits consist mainly of sand, not silt or silty clay24.We obtain Pdef = 0.3 Pa, which is consistent with estimates of seafloor uplift processes in gas hydrate basin environments, where Pdef varies from 10-2 to 103 Pa, with lower values ​​representing low w/a and/or what.In BdM, stiffness reduction due to local gas saturation of the sediment and/or the appearance of pre-existing fractures may also contribute to failure and consequent gas release, allowing the formation of the observed ventilation structures.The collected reflected seismic profiles (Fig. 7) indicated that PS sediments were uplifted from the GSL, pushing up the overlying MS marine sediments, resulting in mounds, folds, faults, and sedimentary cuts (Fig. 7b,c).This suggests that the 14.8 to 12 ka old pumice has intruded into the younger MS layer through an upward gas transport process.The morphological features of the BdM structure can be seen as the result of the overpressure created by the fluid discharge produced by the GSL.Given that active discharge can be seen from the seafloor up to over 170 m bsl48, we assume that the fluid overpressure within the GSL exceeds 1,700 kPa.Upward migration of gases in the sediments also had the effect of scrubbing material contained in the MS, explaining the presence of chaotic sediments in gravity cores sampled on BdM25.Furthermore, the overpressure of the GSL creates a complex fracture system (polygonal fault in Fig. 7b).Collectively, this morphology, structure, and stratigraphic settlement, referred to as “pagodas”49,50, were originally attributed to secondary effects of old glacial formations, and are currently interpreted as the effects of rising gas31,33 or evaporites50 .At the continental margin of Campania, evaporative sediments are scarce, at least within the uppermost 3 km of the crust.Therefore, the growth mechanism of BdM pagodas is likely to be controlled by gas rise in the sediments.This conclusion is supported by the transparent seismic facies of the pagoda (Fig. 7), as well as gravity core data as previously reported24, where present-day sand erupts with ‘Pomici Principali’25 and ‘Naples Yellow Tuff’26 Campi Flegrei.Furthermore, PS deposits invaded and deformed the uppermost MS layer (Fig. 7d).This structural arrangement suggests that the pagoda represents an uprising structure and not just a gas pipeline.Thus, two main processes govern the formation of the pagoda: a) the density of the soft sediment decreases as gas enters from below; b) the gas-sediment mixture rises, which is the observed folding, faulting and fracture Cause MS deposits (Figure 7).A similar formation mechanism has been proposed for pagodas associated with gas hydrates in the South Scotia Sea (Antarctica).BdM pagodas appeared in groups in hilly areas, and their vertical extent averaged 70–100 m in two-way travel time (TWTT) (Fig. 7a).Due to the presence of MS undulations and considering the stratigraphy of the BdM gravity core, we infer the formation age of the pagoda structures to be less than about 14–12 ka.Furthermore, the growth of these structures is still active (Fig. 7d) as some pagodas have invaded and deformed the overlying present-day BdM sand (Fig. 7d).
The pagoda’s failure to cross the present-day seabed indicates that (a) gas rise and/or local cessation of gas-sediment mixing, and/or (b) possible lateral flow of gas-sediment mixture does not allow for a localized overpressure process.According to the diapir theory model52, the lateral flow demonstrates a negative balance between the rate of supply of the mud-gas mixture from below and the rate at which the pagoda moves upward.The reduction in the supply rate may be related to the increase in the density of the mixture due to the disappearance of the gas supply.The results summarized above and the buoyancy-controlled rise of the pagoda allow us to estimate the air column height hg.The buoyancy is given by ΔP = hgg (ρw – ρg), where g is gravity (9.8 m/s2) and ρw and ρg are the densities of water and gas, respectively.ΔP is the sum of the previously calculated Pdef and the lithostatic pressure Plith of the sediment plate, ie ρsg h, where ρs is the sediment density.In this case, the value of hg required for the desired buoyancy is given by hg = (Pdef + Plith)/[g (ρw – ρg)].In BdM, we set Pdef = 0.3 Pa and h = 100 m (see above), ρw = 1,030 kg/m3, ρs = 2,500 kg/m3, ρg is negligible because ρw ≫ρg.We get hg = 245 m, a value representing the depth of the bottom of the GSL.ΔP is 2.4 MPa, which is the overpressure required to break the BdM seafloor and form vents.
The composition of the BdM gas is consistent with mantle sources altered by the addition of fluids associated with decarbonization reactions of crustal rocks (Fig. 6).Rough EW alignments of BdM domes and active volcanoes such as Ischia, Campi Flegre, and Soma-Vesuvius, along with the composition of the gases emitted, suggest that gases emitted from the mantle below the entire Naples volcanic region are mixed More and more crustal fluids move from west (Ischia) to east (Somma-Vesuivus) (Figs. 1b and 6).
We have concluded that in the Bay of Naples, a few kilometers from the port of Naples, there is a 25 km2 wide dome-like structure that is affected by an active degassing process and caused by the placement of pagodas and mounds.Currently, BdM signatures suggest that non-magmatic turbulence53 may predate embryonic volcanism, ie the early discharge of magma and/or thermal fluids.Monitoring activities should be implemented to analyze the evolution of phenomena and to detect geochemical and geophysical signals indicative of potential magmatic disturbances.
Acoustic water column profiles (2D) were acquired during the SAFE_2014 (August 2014) cruise on the R/V Urania (CNR) by the National Research Council Institute of Coastal Marine Environment (IAMC).Acoustic sampling was performed by a scientific beam-splitting echo sounder Simrad EK60 operating at 38 kHz.Acoustic data was recorded at an average speed of about 4 km.The collected echosounder images were used to identify fluid discharges and accurately define their location in the collection area (between 74 and 180 m bsl).Measure physical and chemical parameters in the water column using multiparameter probes (conductivity, temperature and depth, CTD).Data were collected using a CTD 911 probe (SeaBird, Electronics Inc.) and processed using SBED-Win32 software (Seasave, version 7.23.2).A visual inspection of the seabed was performed using a “Pollux III” (GEItaliana) ROV device (remotely operated vehicle) with two (low and high definition) cameras.
Multibeam data acquisition was performed using a 100 KHz Simrad EM710 multibeam sonar system (Kongsberg).The system is linked to a differential global positioning system to ensure sub-metric errors in beam positioning.The acoustic pulse has a frequency of 100 KHz, a firing pulse of 150° degrees and an entire opening of 400 beams.Measure and apply sound velocity profiles in real time during acquisition.Data were processed using PDS2000 software (Reson-Thales) according to the International Hydrographic Organization standard (https://www.iho.int/iho_pubs/standard/S-44_5E.pdf) for navigation and tide correction.Noise reduction due to accidental instrument spikes and poor-quality beam exclusion was performed with band editing and de-spiking tools.Continuous sound velocity detection is performed by a keel station located near the multi-beam transducer and acquires and applies real-time sound velocity profiles in the water column every 6-8 hours to provide real-time sound velocity for proper beam steering.The entire dataset consists of approximately 440 km2 (0-1200 m depth).The data was used to provide a high-resolution digital terrain model (DTM) characterized by a 1 m grid cell size.The final DTM (Fig. 1a) was done with terrain data (>0 m above sea level) acquired at the 20 m grid cell size by the Italian Geo-Military Institute.
A 55-kilometer high-resolution single-channel seismic data profile, collected during safe ocean cruises in 2007 and 2014, covered an area of ​​approximately 113 square kilometers, both on the R/V Urania.Marisk profiles (eg, L1 seismic profile, Fig. 1b) were obtained by using the IKB-Seistec boomer system.The acquisition unit consists of a 2.5 m catamaran in which the source and receiver are placed.The source signature consists of a single positive peak that is characterized in the frequency range 1-10 kHz and allows to resolve reflectors separated by 25 cm.Safe seismic profiles were acquired using a 1.4 Kj multi-tip Geospark seismic source interfaced with Geotrace software (Geo Marine Survey System).The system consists of a catamaran containing a 1–6.02 KHz source that penetrates up to 400 milliseconds in soft sediment below the seabed, with a theoretical vertical resolution of 30 cm.Both Safe and Marsik devices were obtained at a rate of 0.33 shots/sec with a vessel velocity <3 Kn.Data were processed and presented using Geosuite Allworks software with the following workflow: dilation correction, water column muting, 2-6 KHz bandpass IIR filtering, and AGC.
The gas from the underwater fumarole was collected on the seafloor using a plastic box equipped with a rubber diaphragm on its upper side, placed upside down by the ROV over the vent.Once the air bubbles entering the box have completely replaced the seawater, the ROV is back to a depth of 1 m, and the diver transfers the collected gas through a rubber septum into two pre-evacuated 60 mL glass flasks equipped with Teflon stopcocks in which One was filled with 20 mL of 5N NaOH solution (Gegenbach-type flask).The main acid gas species (CO2 and H2S) are dissolved in the alkaline solution, while the low solubility gas species (N2, Ar+O2, CO, H2, He, Ar, CH4 and light hydrocarbons) are stored in the sampling bottle headspace.Inorganic low solubility gases were analyzed by gas chromatography (GC) using a Shimadzu 15A equipped with a 10 m long 5A molecular sieve column and a thermal conductivity detector (TCD) 54.Argon and O2 were analyzed using a Thermo Focus gas chromatograph equipped with a 30 m long capillary molecular sieve column and TCD.Methane and light hydrocarbons were analyzed using a Shimadzu 14A gas chromatograph equipped with a 10 m long stainless steel column packed with Chromosorb PAW 80/100 mesh, coated with 23% SP 1700 and a flame ionization detector (FID).The liquid phase was used for the analysis of 1) CO2, as, titrated with 0.5 N HCl solution (Metrohm Basic Titrino) and 2) H2S, as, after oxidation with 5 mL H2O2 (33%), by ion chromatography (IC) (IC) (Wantong 761).The analytical error of titration, GC and IC analysis is less than 5%.After standard extraction and purification procedures for gas mixtures, 13C/12C CO2 (expressed as δ13C-CO2% and V-PDB) was analyzed using a Finningan Delta S mass spectrometer55,56.The standards used to estimate external precision were Carrara and San Vincenzo marble (internal), NBS18 and NBS19 (international), while analytical error and reproducibility were ±0.05% and ±0.1%, respectively.
δ15N (expressed as % vs. Air) values ​​and 40Ar/36Ar were determined using an Agilent 6890 N gas chromatograph (GC) coupled to a Finnigan Delta plusXP continuous flow mass spectrometer.The analysis error is: δ15N±0.1%, 36Ar<1%, 40Ar<3%.The He isotope ratio (expressed as R/Ra, where R is 3He/4He measured in the sample and Ra is the same ratio in the atmosphere: 1.39 × 10−6)57 was determined at the laboratory of INGV-Palermo (Italy) 3He, 4He and 20Ne were determined using a dual collector mass spectrometer (Helix SFT-GVI)58 after separation of He and Ne.Analysis error ≤ 0.3%.Typical blanks for He and Ne are <10-14 and <10-16 mol, respectively.
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