The Pompeii hydrothermal field was discovered in July 2022 during the HERMINE2 cruise [1]. It is located on an inside corner high (21°20 N) at the northern end of a “doomed” ridge segment [2] located just south of the TAMMAR propagating rift. The inside corner high is a domal bathymetric high with gentle slope ridgewards and spreading-parallel lineations (corrugations) characteristic of an oceanic core complex (OCC). The OCC is dissected by several faults including a ridge-perpedicular fault and a series of smaller ridge-parallel faults.The main Pompeii hydrothermal site is located on the corrugated surface atop a spreading-perpendicular rubble ridge. The mound is about 150 m in diameter and 30-40 m high and mainly composed of sulfide-bearing rocks partly covered by Fe-Mn hydrothermal crusts. Sulfide-bearing mineralization mainly consist of quartz and pyrite and are characterized by low copper and zinc concentrations (i.e.

Methane is widely found on continental margins. It originates from either microbial processes at shallow sedimentary depth or thermal cracking of organic matter at deep depth, and occurs as disolved or free gas, or hydrates. It is the main chemical compound found both in natural gas hydrate deposits and seafloor gas emissions at the cold-seep.There are extensive methane manifestations both in the sedimentary and water columns of the Black Sea. This stratified sea is characterized by large quantities of methane bubbles discharged at the seafloor from the very shallow coastal shelf to the deep basin (Riboulot et al., 2017), contributing to the high concentration level measurement in the water column. Hydrate-bearing sediments are also widely distributed within the sediment on the continental slope, and Riboulot et al. (2018) showed that the seawater infiltration make them vulnerable and prone to dissociation since the reconnection of the Atlantic Ocean via the Sea of Marmara.The expeditions Ghass 2 in September 2021 allowed the investigation of several methane emission sites from the continental shelf to the deep basin in the Romanian sector of the Black Sea, including hydrate-bearing sites. The water column was probed to measure in situ dissolved methane concentration using a commercial methane sensor and the prototype laser spectrometer SubOcean and sampled from CTD-Rosette. A ~6m-length hydrate-bearing core was collected from a long Calypso piston corer from which a high-resolution sampling of hydrates was performed to estimate the influence of geological factors on their cage occupancy.The presentation aims to provide further background on methane dynamics in the Black Sea. ReferencesAgnissan Constant Art-Clarie, Guimpier Charlène, Terzariol Marco, Fandino Olivia, Chéron Sandrine, Riboulot Vincent, Desmedt Arnaud, Ruffine Livio (2023). Influence of Clay-Containing Sediments on Methane Hydrate Formation: Impacts on Kinetic Behavior and Gas Storage Capacity . Journal Of Geophysical Research-solid Earth , 128(9).Riboulot Vincent, Ker Stephan, Sultan Nabil, Thomas Yannick, Marsset Bruno, Scalabrin Carla, Ruffine Livio, Boulart Cedric, Ion Gabriel (2018). Freshwater lake to salt-water sea causing widespread hydrate dissociation in the Black Sea . Nature Communications , 9(117), 1-8 Acknowledgements The authors thank the different projects and programs for their financial supports DOORS by the EU Project number 101000518, ENVRIPLUS by EC Project number 654182, Blame ANR-18-CE01-0007, ORAGGE by Interdisciplinary graduate School for the Blue planet (ANR-17-EURE-0015 and "Investissements d'Avenir"), SEAMLESS by INSU LEFE Programme 2022

Methane clumped isotope signatures of abiogenesis may be diagnostic of the origin of methane on Earth and other planetary bodies. We performed synthesis of abiogenic methane in hydrothermal conditions between 130 and 300°C and determined δ13C, δD, Δ13CH3D, and Δ12CH2D2. The experiments were performed by heating water in the presence of Fe0 powder and CO. The reduction of water on metallic iron led to the formation of H2. CO was reacted with both H2 and H2O, generating both CH4 and CO2. Methane δ13C values are isotopically depleted by ∼25‰ relative to the CO starting material. This is consistent with carbon isotopic equilibrium between methane, carbon monoxide and carbon dioxide in our experiments. In contrast, D/H ratios are inconsistent with equilibrium isotopic fractionation, as illustrated by δD values of methane fractionated by ∼500‰ relative to starting H2O. This suggests that under our experimental conditions, hydrogen additions to carbon may be governed by kinetics. Δ13CH3D values track experimental temperature, with values between +1.5‰ and +5.0‰ for most samples. In contrast, Δ12CH2D2 values are displaced from equilibrium. We find exclusively negative Δ12CH2D2 values, showing deficits down to 40‰ relative to thermodynamic equilibrium. We interpret the data as evidence for distinct, kinetically induced D/H pools contributing to methane assembly, that is, a combinatorial effect. The cumulative D/H fractionations associated with CO hydrogenation explain the direction and magnitude of Δ12CH2D2 values during abiotic methane formation. We suggest that near equilibrium Δ13CH3D with negative Δ12CH2D2 signatures will help identify methane formed abiotically in nature.

Introduction: Although, there is an increasing focus on inactive or extinct seafloor massive sulfide (SMS) deposits driven by the possibility of marine mining, only few studies have been devoted to them so far. The Trans-Atlantic Geotraverse (TAG) hydrothermal field is probably one of the best-studied hydrothermal systems even if the relict SMS deposits known since the mid-1980s have not been thoroughly explored.Objectives: The main objective of this study was to describe the characteristics of these so-called inactive sites.Methods: During four different expeditions, we acquired high-resolution acoustic data and performed numerous human occupied vehicle (HOV) dive operations including extensive rock sampling and in-situ temperature measurements.Results and Discussion: We discovered thirteen new hydrothermal mounds including six large (i.e. > 5,000 m2) deposits making the TAG hydrothermal field one of the largest accumulation of hydrothermal materials (21.1 Mt) known on the seafloor. However, copper and zinc grades of the largest SMS deposits remain low (i.e. < 1.4 wt%) even compared to on-land volcanogenic massive sulfide deposits. Additionally, eight areas of diffuse hydrothermal fluid flow were identified challenging the presumed inactivity of these SMS deposits and, for the first time, emphasizing the importance of low temperature (LT) hydrothermal activity in whole the TAG field. Inactive and weakly active SMS deposits exhibit a large diversity of surface mineralization (e.g. sulfides, Fe-Mn mineralization, jasper) illustrating complexity of hydrothermal activities but also different ageing history. Several mounds no longer have visible sulfide chimneys and are covered by a widespread layer of manganese and iron oxyhydroxides attesting the longevity of diffuse fluid flow at specific locations even long after last high-temperature (HT) hydrothermal activity has ceased. This contrasts with SMS deposits that are devoid of extensive LT precipitates but characterized by standing or topped sulfide chimney indicating a relatively abrupt cessation of HT hydrothermal activity.Conclusion: Together these results allow us to propose evolution models to explain the diversity of active, weakly active and inactive SMS deposits in the TAG hydrothermal field.

About 80% of Earth volcanic activity occurs underwater, releasing deep carbon to submarine environments and impacting Earth’s climate over geological timescales. The CO2 emitted during submarine eruptions and/or hydrothermal degassing creates local ocean acidification, affecting the seawater carbonate equilibrium and oceanic ecosystems at large regional scales. Here, we report for the first time the existence of a major CO2 hydrates field at the seafloor offshore Mayotte Island (Indian Ocean) associated with liquid CO2 venting, following the submarine eruption that occurred in 2018. Using detailed acoustic surveys and in situ Raman spectroscopy, we reveal multiple hydrate mounds and seep zones distributed over an area of 0.06 km². We show that the gas seeps are mainly composed of CO2, with minor contributions of CH4 and H2, with noble gas ratios and stable and radio-carbon isotopes clearly demonstrating their magmatic origin. Estimates of the CO2 emitted over the entire area represent about 0.5% of the global magmatic carbon flux. Our discovery also suggests that CO2 hydrates may potentially be stable at the seafloor at the right pressure-temperature conditions, bringing new prospects into CO2 sequestration and decarbonization pathways in the ocean, in particular regarding kinetics of hydrates dissolution and environmental impacts.

We present a method for measuring the clumped isotope composition of molecular hydrogen (H2) using a high -resolution mass spectrometer, the Thermo 253 Ultra, improved to address subtle artifacts arising from instrument baselines and non-linear responses. We also present methods for purification and concentration of H2 from natural and experimental samples, tailored to this measurement. We document the accuracy of the method through comparison to established methods for the determination of delta D values, and through measurements of H2 gases of widely varying D content that were driven to isotopic equilibrium with respect to their distributions of isotopologues by heating in the presence of a catalyst. Experimental reproducibility of delta D and Delta DD values over months averages +/- 0.5 and +/- 6.9 %o, respectively (1a) - both small fractions of common natural variations. We explore methods of gas purification and handling, and show that preferred methods result in low (0-4 %o) changes in delta D and undetectable changes in Delta DD. Our methods and data processing procedures were further tested by comparing measurements of mixtures of H2 gases that varied widely in delta D and Delta DD with a model describing proportions of isotopologues in such mixtures. Application of these methods to H2 that is residual to laboratory consumption by cultured methanogens shows that metabolic 'back reaction' (metabolic production of H2 from water-derived protons during net H2 consumption) is responsible for driving the Delta DD value of residual H2 toward equilibrium at environmental temperatures. Finally, we report the first measurements of the clumped isotope composition of molecular hydrogen in natural geological samples collected from high and low temper-ature submarine hydrothermal vents (Lost city, Rainbow, Ashadze) and an intracontinental natural reservoir in Mali; initial findings suggest that Delta DD of H2 generally records temperatures of fluid venting or long-term storage, even in cases where the delta D of H2 has not equilibrated with water at those temperatures. This study establishes the first clumped isotope systematics of molecular hydrogen based on both experimental and natural samples, including key processes in the biogeochemical cycle of H2.

Submarine volcanic activity releases large amounts of gases and metals in the water column, affecting biogeo-chemical cycles and ecosystems at a regional and local scale. In 2018, Fani Maore ' submarine volcano erupted 50 km offshore Mayotte Island (Comoros Archipelago, Indian Ocean). Active eruptive plumes were observed in May 2019 at and around the summit with acoustic plumes rising 2 km into the water column coupled to strong geochemical anomalies. Between May 2019 and October 2020, three research cruises monitored the eruptive activity. Here, we report spatial and temporal variability of water column chemistry above the volcano, focusing on dissolved gases, trace metal concentrations, and physico-chemical parameters. In May 2019, concentrations above 800 nM in CH4 and H2 were measured throughout the water column, with Total Dissolvable Mn and Total Dissolvable Fe concentrations above 500 nM, and CO2 values of 265 mu M. Strong water column acidification was measured (0.6 pH unit) compared to the regional background. From May 2019 to October 2020, we observed a general decrease in gas concentrations, and an evolution of the TDMn/TDFe ratios similar to previously reported values in other submarine volcanic contexts, and consistent with a decrease of the eruptive activity at the vol-cano. In October 2020, a rebound of high H2 concentrations resulted from new lava flows, which were identified by seafloor observation using deep-towed camera, 5 km further than the volcano summit. During 2 years timespan of our observations (2019-2020), He, CO2 and CH4 concentrations correlate highlighting a magmatic origin of dissolved gases. delta 13C-CH4 values of -34 parts per thousand vs. vPDB might suggest magma/sediments interaction during the magma ascent, and potential thermal cracking of organic matter, although abiotic methane generation cannot be ruled out given the volcanic context. Weak correlations between H2 and excess of 3He suggest complex processes of H2 from magmatic degassing, lava/seawater interaction, and oxidation processes in the water col-umn. Strong and correlated Fe, Mn and Si water column anomalies are also consistent with fluid-rock reactions induced by acidic fluids rich in magmatic volatiles. Water column acidification appears to be associated with the release of CO2-rich fluids. A year after the main eruptive event, the system seems to be back to steady-state highlighting the buffer capacity and resilience of the seawater column environment.

&lt;p&gt;Since 1977 and the discovery of the first high temperature (HT) hydrothermal vent, more than 300 sites are known (about 600 including inferred ones). Among these hydrothermal sites, the talc-rich deposit is the most recent class of hydrothermal system discovered on the seafloor [1]. Only three talc-rich deposits have been described so far: (i) the active Von Damm Vent Field (VDVF), (ii) the inactive St Paul&amp;#8217;s and (iii) Conrad fracture zones deposits [2]. These hydrothermal sites are associated with lower crustal rocks and/or serpentinized peridotites and might be widespread at slow or ultraslow spreading ridge. However, no clear spatial or temporal relationship of this new class of hydrothermal system and the &amp;#8220;black smoker&amp;#8221;-like system has been highlighted.&lt;/p&gt; &lt;p&gt;&amp;#160;During the HERMINE (March-April 2017) and HERMINE2 (July-August 2022) cruises [3], [4], two hydrothermal areas with talc-rich deposits have been discovered during Nautile HOV dives. The first one (23&amp;#176;N) is an inactive hydrothermal area located 28km northwest of the Snake Pit vent field (25km west of the axial rift). At least two deposits have been observed: (i) a talc-silica deposit and (ii) a fully oxidized SMS-type deposit characterized by copper concentrations up to 3.3wt.%. The second hydrothermal area (26&amp;#176;N) is composed of one large and weakly-active deposit composed of silica-sulfides rocks and at least two small talc-silica deposits. To our knowledge, this is the first time that such a spatial relationship has been described between these two classes of deposits. The preliminary results on these newly discovered hydrothermal field will be presented here.&lt;/p&gt; &lt;p&gt;&amp;#160;&lt;/p&gt; &lt;p&gt;[1] Hodgkinson et al. (2015) &lt;em&gt;Nat.. Commun&lt;/em&gt; &lt;em&gt;6:10150&lt;/em&gt;&lt;/p&gt; &lt;p&gt;&lt;em&gt;doi: 10.1038/ncomms10150 .&lt;/em&gt;&lt;/p&gt; &lt;p&gt;[2] D&amp;#8217;Orazio et al. (2004) &lt;em&gt;Eur. J. Mineral.&lt;/em&gt; 16, 73-83&lt;/p&gt; &lt;p&gt;[3] Fouquet and Pelleter (2017), https://doi.org/10.17600/17000200&lt;/p&gt; &lt;p&gt;[4] Pelleter and Cathalot (2022),&lt;/p&gt; &lt;p&gt;https://doi.org/10.17600/18001851&lt;/p&gt;

Microbial methane oxidation - or methanotrophy - is a key control of the global methane budget on Earth, and perhaps in other planetary systems. Here, we explore the potential role of mass-18 isotopologues of methane, expressed as Δ13CH3D and Δ12CH2D2 values, in tracking both aerobic and anaerobic methanotrophy in nature. We examine two well documented methanotrophic environments: the Lake Pavin (France) water column, where methane degradation is dominated by aerobic methanotrophy (AeOM), and the Black Sea sediments (offshore Romania), dominated by anaerobic methanotrophy (AOM) coupled to sulfate-reduction. In both settings, lighter isotopologues are preferentially consumed, generating elevated 13CH4/12CH4, 12CH3D/12CH4, 13CH3D/12CH4 and 12CH2D2/12CH4 ratios. This results in increasing of δ13C and δD values in the residual methane for both settings, as observed commonly in systems dominated by methanotrophy. As a result, AeOM and AOM cannot be easily distinguished by the development of δ13C and δD. In contrast, the Δ13CH3D and Δ12CH2D2 (departure from stochastic) values have opposite trajectories, with minimal decreases in the case of the AeOM-dominated system, but dramatic increases in the case of AOM, with Δ13CH3D and Δ12CH2D2 reaching values as high as 15.7 ‰ and 76.6 ‰, respectively. This contrasting behavior of clumped isotopologues signatures illustrates fundamental distinction between the two processes and the way they segregate methane isotopologues. These data demonstrate that both AeOM and AOM have distinctive kinetic isotope effects in natural settings, consistent with preliminary laboratory work. In particular, we find that γ-values (which measure the deviation to the product of ‘normal’ bulk isotope fractionation factors) are close to unity in the case of AeOM (i.e. a negligible clumped isotope effect), but significantly below unity in the case of AOM (i.e. strong clumped isotope effect). In addition, our data also illustrate how AOM under low-sulfate conditions may promote methane isotopologue equilibration. Taken together, we suggest these data and apparent isotopologue fractionation factors extrapolated from these two environments may help refine the potential bio-signatures of methane affected by methanotrophy.

Distinguishing biotic compounds from abiotic ones is important in resource geology, biogeochemistry, and the search for life in the universe. Stable isotopes have traditionally been used to discriminate the origins of organic materials, with particular focus on hydrocarbons. However, despite extensive efforts, unequivocal distinction of abiotic hydrocarbons remains challenging. Recent development of clumped-isotope analysis provides more robust information because it is independent of the stable isotopic composition of the starting material. Here, we report data from a 13C-13C clumped-isotope analysis of ethane and demonstrate that the abiotically-synthesized ethane shows distinctively low 13C-13C abundances compared to thermogenic ethane. A collision frequency model predicts the observed low 13C-13C abundances (anti-clumping) in ethane produced from methyl radical recombination. In contrast, thermogenic ethane presumably exhibits near stochastic 13C-13C distribution inherited from the biological precursor, which undergoes C-C bond cleavage/recombination during metabolism. Further, we find an exceptionally high 13C-13C signature in ethane remaining after microbial oxidation. In summary, the approach distinguishes between thermogenic, microbially altered, and abiotic hydrocarbons. The 13C-13C signature can provide an important step forward for discrimination of the origin of organic molecules on Earth and in extra-terrestrial environments.

In addition to high concentrations of CH4 and H-2, abundant dissolved N-2 is found in subsurface fracture fluids in Precambrian cratons around the world. These fracture fluids have hydrogeological isolation times on order of thousands to millions and even billions of years. Assessing the sources and sinks of N-2 and related (bio) geochemical processes that drive the nitrogen cycle in these long isolated systems can shed insights into the nitrogen cycle on early Earth with implications for other planets and moons. In this study, we collected dissolved gas samples from deep subsurface fracture fluids at seven sites (Kidd Creek, LaRonde, Nickel Rim, Fraser, Copper Cliff South, Thompson, and Birchtree) in the Canadian Shield. Multiple gas components (e.g., H-2, O-2 and Ar) were integrated with delta N-15(N2) values to characterize the N-2 signatures. Results show that the dissolved N-2 in deep subsurface fracture fluids from the Canadian Shield sites are more N-15-enriched than those from the Fennoscandian Shield and the Witwatersrand Basin in the Kaapvaal Craton. The nitrogen isotopic signatures of the Canadian Shield samples coupled with their hydrogeological framework indicate the N-2 was sourced from fixed ammonium in silicate minerals in host rocks and was generated by metamorphic devolatilization. Modeling of nitrogen devolatilization from host rocks supports this interpretation, but also suggests that a second process, likely abiotic N-2 reduction, is required to account for the observed N-15 enrichment in the N-2 samples from the Canadian Shield. A 10-year monitoring study for one of the boreholes, at 2.4 km of the Kidd Creek Observatory, shows a steady decrease in delta N-15(N2) values with time, which coincides with the temporal isotopic evolution of some other gas components in this borehole. Although it cannot be confirmed at this time, this isotopic shift in N-2 may be potentially attributed to microbial processes (e.g., anaerobic oxidation of ammonium). Nevertheless, the large N-15 enrichments for the majority of the samples in this study suggest that the nitrogen cycle in the deep saline fracture fluids in the Canadian Shield is dominated by abiotic processes. This is in contrast to the nitrogen cycles in the subsurface fracture fluids in the Fennoscandian Shield and the Witwatersrand Basin, which have been shown to be strongly affected by extant microbial ecosystems discovered in those fracture waters.

Understanding the dynamics and fate of methane (CH 4 ) release from oceanic seepages on margins and shelves into the water column, and quantifying the budget of its total discharge at different spatial and temporal scales, currently represents a major scientific undertaking. Previous works on the fate of methane escaping from the seafloor underlined the challenge in both, estimating its concentration distribution and identifying gradients. In April 2019, the Envri Methane Cruise has been conducted onboard the R/V Mare Nigrum in the Western Black Sea to investigate two shallow methane seep sites at ∼120 m and ∼55 m water depth. Dissolved CH 4 measurements were conducted with two continuous in-situ sensors: a membrane inlet laser spectrometer (MILS) and a commercial methane sensor (METS) from Franatech GmbH. Additionally, discrete water samples were collected from CTD-Rosette deployment and standard laboratory methane analysis was performed by gas chromatography coupled with either purge-and-trap or headspace techniques. The resulting vertical profiles (from both in situ and discrete water sample measurements) of dissolved methane concentration follow an expected exponential dissolution function at both sites. At the deeper site, high dissolved methane concentrations are detected up to ∼45 m from the seabed, while at the sea surface dissolved methane was in equilibrium with the atmospheric concentration. At the shallower site, sea surface CH 4 concentrations were four times higher than the expected equilibrium value. Our results seem to support that methane may be transferred from the sea to the atmosphere, depending on local water depths. In accordance with previous studies, the shallower the water, the more likely is a sea-to-atmosphere transport of methane. High spatial resolution surface data also support this hypothesis. Well localized methane enriched waters were found near the surface at both sites, but their locations appear to be decoupled with the ones of the seafloor seepages. This highlights the need of better understanding the processes responsible for the transport and transformation of the dissolved methane in the water column, especially in stratified water masses like in the Black Sea.

In order to examine the seawater-seafloor sediment interactions that influence the chemical composition of seawater through time, we examined hundreds of pore fluid geochemical analyses from 13 clay-rich sedimentary successions drilled by the ODP-IODP. Chemical trends such as monotonous increases in Ca2+, and decreases in Mg2+ and delta O-18 with depth are traditionally interpreted to result from water-rock interaction. In this view, the release of Ca2+ into fluids and the uptake of Mg2+ and O-18 mainly results from the formation of low-temperature clays in the sediment and within underlying basalts. Chloride concentration profiles and isotopic compositions, however, suggest that different processes may influence pore water geochemistry. The data examined here show relatively constant chloride contents but with a systematic decrease in delta Cl-37 of chlorides with depth from 0 permil (the seawater value) down to -8.5 permil. The delta Cl-37 data are highly correlated with delta O-18 (with delta O-18 down to -5.7 permil). The delta Cl-37-depletions of pore fluid chlorides are found in all studied sedimentary piles regardless of tectonic or sedimentary history. These trends cannot be explained by water-rock exchange reactions because minerals formed at low temperature have Cl contents that are too low to compensate for delta Cl-37 depletions observed in pore fluids. Accordingly, we hypothesize that fluid-specific processes are responsible for the delta Cl-37-depletions of the fluids and that delta Cl-37-enriched chlorides were expelled out of the sediments into the ocean. After reviewing the fluid-specific processes that are known to change the chlorine isotope ratios in chlorides, we rule out diffusion and gravitational isotope fractionations of chlorides could generate this isotope pattern. The flow of a delta Cl-37-depleted fluid from the underlying basaltic basement into the sediments could explain the delta Cl-37 data. But the mechanism that produces depletion in delta Cl-37 of the fluid remains unknown. It cannot be chloride exchanges between fluids and rocks. Here we show that compaction-induced ion filtration of chlorides through clay-rich membranes can produce the observed pore fluid delta Cl-37-depletions, with isotope fractionation factors ranging from 1.000 to 1.008 between the chlorides of the expelled fluid (the permeate) and those of the residual fluid (the retentate). We find that smectite-rich sediments are associated with higher isotopic fractionation factors, while illite/chlorite-rich sediments are associated with intermediate values and with clay-poor sediments associated with lower values. This suggests that chlorine isotope fractionation might be controlled by surface charge associated with specific clay minerals. Our calculations show that compaction-induced filtration has the capacity to produce O-18-depletion for oxygen isotope fractionation factors between the expelled fluid and the retentate ranging from 1.000 to 1.005. O-18-enrichment in the expelled fluid is in agreement with the experimental data of Haydon and Graf (1986).Overall, although further experimental work on both chlorine and oxygen isotopes is certainly needed, the results of this study indicate that ion-filtration should be considered as a potential mechanism for fractionating isotopic species in sediment pore waters, particularly for oxygen isotope ratios whose variations are often commonly attributed to water-rock exchange. (C) 2021 Elsevier Ltd. All rights reserved.

Ca-Na-Cl fluids with high concentrations of dissolved reduced gases reside within fractures in crystalline Precambrian rocks around the world, and have been most intensively studied within South Africa, Fennoscandia and the Canadian Shield. In contrast to surface waters, shallow groundwaters, sedimentary basin brines and metamorphic fluids, the δ18O and δ2H values for these Ca-Na-Cl fluids typically plot to the left/above of the Global Meteoric Water Line (GMWL). To date, most interpretive frameworks for these fracture fluids have focused on their production via water-rock alteration reactions that affect both δ18O and δ2H values, resulting in co-variation of water isotope values above the GMWL. Such alteration processes include silicate hydration coupled with formation of secondary minerals, radiolytic H2 formation, isotopic exchange with a H2-rich gas, or isotope exchange with O and/or H-bearing minerals. This study presents the first compiled global isotopic dataset for these types of fluids, integrating a large amount of unpublished data with the previously published literature in order to investigate these fracture fluid systems on a global scale. Importantly this global perspective allows differentiation between fluids impacted by late-stage mixing with meteoric waters, from fluids that reflect the most saline end-members stored in the host rocks in hydrogeologic isolation from the surface hydrologic cycle. The most saline fluids are shown to occupy a more restricted range of δ18O-δ2H space than previously recognised, with end-member fluids from all of the Precambrian rock settings investigated occupying a range of δ2H δ18O isotope space within which there is no co-variation in these values. These findings suggest a set of common processes may define the isotopic signatures of these most saline end-member fluids in Precambrian settings around the world – creating common signatures identifiable in these fluids, despite differences in geologic setting. This study identifies the important role of oxygen isotopic exchange between primary fluids (associated with hydrothermal/metamorphic activity) and the host rocks, taking place under low temperature, low-volume, water-rock ratios over long (Ma) geologic timescales. This process results in progressive 18O depletion in the fluids over time, while δ2H values remain less affected. For each site the specific isotopic signature of the fracture fluid end-member depends on initial hydrothermal/metamorphic fluid composition, rates of isotopic exchange, water-to-rock ratios, and in-situ residence times. We suggest the often-observed co-variation in both δ18O-δ2H above the GMWL primarily results from late stage mixing of the fracture fluid end-members with (paleo)-meteoric water, resulting in isotopic regression back towards the GMWL, with end-points defined by the local meteoric-climatic conditions for each site.

This study applies a combined isotope and doubly-substituted isotopologue ('clumped') methane approach to samples collected over a 9-year long-term experiment at the Kidd Creek scientific observatory located 2.4 and 2.9 km depth below surface, combined with previously published data from 2.1 km below surface. The observatory is located in a fractured rock system within Kidd Creek Mine in Timmins, Ontario, Canada, situated within a 2.7 Ga Volcanogenic Massive Sulphide (VMS) deposit on the Canadian Shield. Isotope and isotopologue methane data suggest a temporal variation in the various sources of methane within the fracture fluids system between 2.1 and 2.9 km below surface. Predominantly abiogenic methane is identified in samples collected from the deepest level of the mine (2.9 km). Comparing new data from the 2.4 km level with previous data from 2.1 km suggests addition of a small component of microbially-generated methane to the fracture water systems at 2.4 km. The temporal evolution of the methane isotopologue signatures suggest an additional process is occurring within these waters. Specifically, methane in samples from 2.4 km (and some from 2.9 km) approach low-temperature thermodynamic equilibrium in clumped isotopologue space, which is not consistent with kinetically-controlled methane production (either microbial or abiogenic). Anaerobic Oxidation of Methane (AOM) during microbial methanotrophy is shown to be the most likely process to drive such re-equilibration via isotopic bond re-ordering. This study provides an unprecedented high-resolution temporal record over more than a decade for methane in a deep subsurface crystalline environment and demonstrates the advantages of clumped isotopologue studies to identify multiple processes controlling the methane cycle in these systems including both abiotic and biotic methane production and methanotrophy. (C) 2020 Elsevier Ltd. All rights reserved.

Methane is an important greenhouse gas and an energy resource. Methane in sea water can originate from microbially-mediated organic matter (OM) degradation processes at shallow depth within the sediments, or from thermal cracking of refractory OM at deeper depth. On continental margins, this methane is stored in specific sedimentological bodies or as gas hydrates, or is released at the seafloor as submarine geological seeps followed by its oxidation in the water mass. However, methane released at the seafloor may not entirely be oxidized in the water column and a fraction of it may ultimately reach the atmosphere. The factors that govern the magnitude of methane transfer through the water column to the atmosphere remain poorly known. It has been identified that the amount of methane transferred to the atmosphere is strongly dependent on sites, and the thickness of the water column plays a critical role. The Black Sea shelf and margin are known to host a large number of strong methane seepages. It has therefore been identified as a perfect candidate to investigate the fate of methane released from the seafloor to the atmosphere. This area can also act as a proxy for investigating the fate of methane in potential scenarios of hydrate destabilization in a changing climate, which can become a societal problem in the future. In the frame of ENVRIplus H2020 project (www.envriplus.eu) we developed a joint pilot experiment to measure methane transfer from the seafloor to the atmosphere, in a pilot study involving European research infrastructures ICOS, Eurofleets, EMSO and ACTRIS. We investigated the influence of depth by mapping CH4 concentration and bubble distribution at two different sites, at 60m and 100m water depth, respectively. The pilot experiment developed joint monitoring strategy for methane detection at various levels starting from the seafloor and moving across the water column, the water/air interface and the atmosphere. An EK80 echosounder was used to identify emission areas through massive bubble plumes. The methodology applied integrates (1) sampling from the geosphere, hydrosphere and atmosphere for laboratory measurements of methane concentration by well-proven standard methods together with δ13CH4 analysis, (2) in situ measurements of methane concentration into the water column and the atmosphere, and (3) the deployment of a seafloor observatory for a short monitoring period (4-5 days) to evaluate the temporal variability of gas fluxes. During the cruise we found several occurrences of bubble plumes extending near the surface. Our measurements indicate that dissolved methane concentration drastically decreases from the seafloor to the water surface, highlighting its degradation and dispersion along the pathway to the atmosphere. The atmospheric data suggests a consistent input of marine methane to the atmosphere at the shallower site,. Our study highlights the observational challenges both for the measurement of methane from in situ and laboratory methods, and for the estimation of sea surface fluxes.