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

The Black Sea experiences widespread methane-rich gas emissions and elevated hydrogen sulfide concentrations, extending from coastal to deep basin areas. The toxicity of hydrogen sulfide and the powerfull greenhouse gas methane, can lead to local acidification, posing threats to ecosystems. Monitoring these compounds is essential for economic growth tied to Black Sea ecosystem services. Also, the rise in seawater temperatures due to climate change increases the risk of the Black Sea releasing methane stored under gas hydrates form into the atmosphere, potentially becoming a important carbon source.The significant release of methane from the seafloor accentuates environmental apprehensions, playing a role in the creation of the Earth's largest anoxic water body (Kosarev, 2007; Riboulot et al., 2017). This underscores the necessity of investigating the dynamics of these gases and closely monitoring their concentrations to gain a comprehensive understanding of their environmental repercussions.This presentation outlines the fieldwork conducted under the European Project H2020 - DOORS, comprising two field campaigns, METZE and METZE2 (Methane dynamics at Varna lakes and the Zelenska coastal Seeps), which investigated methane dynamics in distinct Black Sea environments, namely Varna Lake and Zelenka gas-seeps. They took place in September 2022 and March 2023. The purpose of this research is to enhance our understanding of the environmental challenges associated with methane concentrations in this unique marine setting. The work initiated a study focused on comprehending methane emissions off the coast of Varna by mapping gas emission sites and measuring their flow rates, taking into account environmental factors such as seasonality and extreme events affecting methane flow rate variability. So, in situ methane sensor deployed for several months at Varna lake indicaticates that the coastal methane fluxes exhibit a noteworthy variability, with daily processes exerting discernible influences on fluxes levels. Also, preliminary findings suggest discernible seasonal fluctuations in both the molecular and isotopic compositions in the water column and sediment pore waters, alongside variations in emission flow rates. AcknowledgementsThe authors thank the different projects and programs for their financial supports: DOORS by the EU Project number 101000518, ORAGGE by Interdisciplinary graduate School for the Blue planet (ANR-17-EURE-0015 and "Investissements d'Avenir"), SEAMLESS by INSU LEFE Programme 2022.ReferencesKosarev, A. N., 2007, The Black Sea Environment, Springer.Riboulot, V., Cattaneo, A., Scalabrin, C., Gaillot, A., Jouet, G., Ballas, G., Marsset, T., Garziglia, S., and Ker, S., 2017, Control of the geomorphology and gas hydrate extent on widespread gas emissions offshore Romania: Bulletin De La Societe Geologique De France, v. 188, no. 4.

This study presents the development and application of a novel high-pressure microfluidic system for investigating CO2 hydrate nucleation and growth, with applications for carbon capture and storage (CCS) technologies. Two distinct microchip geometries-a capillary channel chip (serpentine-shaped) and an advanced droplet trap chip- were respectively designed and evaluated. These microchips enable the generation, trapping, and observation of CO2 droplets or bubbles within aqueous systems under static and dynamic conditions. The capillary channel chip allows droplet storage in a single serpentine channel, whereas the droplet trap chip offers superior immobilization and control, preventing droplet/bubble displacement during CO2 hydrate formation. High-resolution optical imaging, coupled with precise pressure and temperature regulation and control, facilitated real-time visualization of CO2 hydrate crystallization at CO2-water interfaces under varying temperature and pressure conditions. Experimental results reveal the influence of geometry, flow dynamics, and hydrodynamics on hydrate morphology and growth. The high-pressure microfluidic setup provides an adaptable and scalable approach for studying hydrate behavior, offering valuable insights for investigating CO2 storage in geological formations.

Sedimentary pyrite iron and sulphur isotope compositions (delta 56FePYR, delta 34SPYR, Delta 33SPYR) are commonly used to reconstruct global ocean properties and the evolving oxidation state of Earth's surface, motivating exploration of impacts of diagenesis on pyrite-based proxies. Along with auxiliary petrographic and porewater data, we present coupled microscale delta 56FePYR-delta 34SPYR-Delta 33SPYR in accumulating sediments on the oxic margin of the Black Sea. The coevolution of microscale delta 56FePYR-delta 34SPYR-Delta 33SPYR distributions provides insight into porewater S species production, consumption, and buildup on pyritization pathways. "Early" pyrite is characterized by low delta 56FePYR and delta 34SPYR values consistent with microbially-mediated iron and sulphate reduction and iron (oxyhydr)oxide sulphidization at low sulphide-to-iron ratios. In contrast, "sulphidic zone" pyrite displays distinct late-stage morphologies and higher delta 56FePYR and delta 34SPYR, which reflect sulphide accumulation at the sulphate-methane transition zone and direct sulphidization of residual iron phases. We propose that coupled delta 56FePYR-delta 34SPYR-Delta 33SPYR distributions constrain pyritization pathways and microbial and physico-chemical aspects of the depositional environment.

In the Western High of the Sea of Marmara, where pervasive gas hydrate and hydrocarbon gas seepage occurs, late Pleistocene-Holocene sediments are composed of a lower lacustrine and an upper marine unit. The sedimentary evolution, gypsum formation and carbonate anomaly, under the complex sedimentary environment at cold seep sites on the Western High, have not been systematically well explained. The transition from the lacustrine to marine unit occurs at around 7.5 m below the sea floor (mbsf), with a sulfidization front present in the lacustrine sediments just below it, and the marine unit includes a sapropel layer between 5.20 and 7.20 mbsf. The sedimentary sequence is characterized by authigenic minerals including pyrite, carbonates, and gypsum in the upper 0.50 mbsf, corresponding to the present-day sulfate-methane transition zone (SMTZ) near the seafloor. Mineralogical, chemical and isotopic compositions (C-13, O-18, S-34, Mg-26) of the sediments in the core studied show their close relationship with intensive sulfate-driven anaerobic oxidation of methane (AOM) and other hydrocarbons. The dissolution of carbonate, related to the gas and oil migration, and organic matter enrichment might be the main cause of carbonate deficiency in the sapropel unit. Heavy carbon isotopic composition (delta C-13(Carb) = 6.98 parts per thousand) of carbonate at 4.30 mbsf implies that the carbon might be the residual by-product of subsurface biodegradation of seeping petroleum. The formation of secondary gypsum and partial dissolution of carbonate in the Sea of Marmara is commonly considered of being associated with the aerobic oxidation of pyrite. However, the euhedral authigenic gypsum studied might be with different origins, possibly including anaerobic oxidation of Fe-sulfides, carbonate deficiency in the sapropel unit and AOM. Gypsums below SMTZ are with the size of hundreds of microns. Linear pits and grooves are developed on these gypsum surfaces, probably due to corrosion by the methane-rich fluid of latest hydrocarbon seepage. Platy hexagonal gypsum in SMTZ, without any corrosion on crystal surface, might be associated with the changing flux of the acidic composition (e.g. CO2) in the seepage. Depleted-Mg-26 by about 1 parts per thousand in the SMTZ might be caused by the upward seepage of hydrocarbons. Magnesium isotope of authigenic carbonate could be a valid proxy for the recognition of SMTZ in a hydrate geo-system.

Natural gas hydrate deposits (NGHD) have been investigated for decades and represent one of the major methane reservoirs on Earth. They are encountered in sediment of both the continental margins and the permafrost region; areas considered to host amongst the most climate-sensitive ecosystems on Earth. With worldwide temperature increases affecting continental margins and the permafrost, it is important to raise concern about the fate of the NGHD in the coming centuries. Thus, this review presents an overview of the potential consequences of hydrate decomposition on its surrounding areas. It compiles and discusses hydrate-derived methane fluxes measured or inferred from in situ data at several sites by considering both dissociation and dissolution. Depending on the magnitude and the duration of hydrate decomposition, the amounts of methane released can affect to varying degrees the seafloor and the microbial communities that sustain the methane cycle and regulate its transfer from the sediment to the water column; and that aspect is addressed in this review. Here, we also considered the transfer of methane from NGHDs and more broadly from marine emissions to the atmosphere, as it is assumed that such transfer will likely increase in the future. Finally, multi-scale monitoring in space and time is a key element to evaluate the impacts of natural and anthropic perturbations on NGHDs. We thus propose potential engineering solutions for the monitoring of NGHD, mainly based on the long-term deployment of sensor systems.

On Earth, natural hydrates are mostly encountered in clay‐rich sediments. Yet their formation processes in such matrices remain poorly understood. Achieving an in‐depth understanding of how methane hydrates accumulate on continental margins is key to accurately assess (a) their role in sustaining the development of some chemosynthetic communities at cold seeps, (b) their potential in terms of energy resources and geohazards, and (c) the fate of the methane releases, a powerful greenhouse gas, in this changing climate. This study investigated the formation of methane hydrates and their gas storage capacity (GSC) in clay‐rich sediments. A set of hydrate experiments were performed in matrices composed of sand, illite‐rich clay, and montmorillonite‐rich clay at different proportions aiming to determine the role of mineralogy on hydrate formation processes. The experiments demonstrate that a clay content of 10% in a partially water saturated sand/clay mixture increases the induction time by ∼60%, irrespective of the nature of the clay used. The increase in water saturation in the two matrices promotes hydrate formation. Micro‐Raman spectroscopic analyses reveal that increasing the clay content leads to a decrease in the hydrate small‐cage occupancy, with an impact on the storage capacity. Finally, the analyses of collected natural samples from the Black Sea (off Romania) enable us to estimate the GSC of the deposit. Our estimates is different from previous ones, and supports the importance of coupling multiscale properties, from the microscale to the geological scale, to accurately assess the total amount of methane hosts in hydrate deposits worldwide.

The anaerobic oxidation of methane, a key geochemical process that is involved in the cycling of sulfate and iron (oxyhydr)oxides in marine sediments, results in the formation of iron sulfides. Although ferrimagnetic iron sulfides have been identified in seepage systems, the link between iron migration and sediment magnetic properties remains poorly understood. Here, we investigate two cores from the Sea of Marmara to evaluate biogeochemical iron cycling and iron sulfide mineralogy in gas hydrate-bearing sediments. Magnetic analyses indicate the presence of greigite and pyrrhotite in a core from a hydrate-rich site with a high hydrocarbon flux, which contrasts with a lack of these minerals in a core characterized by only mild seepage. This is supported by the results of rock magnetic and scanning electron microscope analyses of the sediments. The presence of authigenic greigite is critical for assessing local redox records and together with the occurrence of monoclinic pyrrhotite may suggest specific diagenetic processes in gas hydrate environments. Our analysis demonstrates the usefulness of these ferrimagnetic minerals, with a high saturation isothermal remanent magnetization to mag-netic susceptibility ratio (SIRM/chi > 15 kAm-1) and a high index of hysteresis parameters (DJH > 0.2) indicative of magnetic mineralogy changes, for evaluating variability in the intensity of seepage fluxes and for estimating gas hydrate distributions.

A range of plumbing systems has been identified in deep-water Nigeria by interpreting multi-scale seismic data sets (reprocessed 3D exploration seismic reflection data and 2D very high-resolution near-bottom seismic reflection data) as well as sedimentological and geochemical data obtained from a long sediment core recovered by the seafloor drill rig MARUM-MeBo70. The plumbing systems are characterized by diverse fault networks and lie above tectonic features including thrust/fold structures and strike-slip faults linked to gravity-driven deformation caused by underlying over-pressured shales. The plumbing systems are associated with fluid seeping structures at the seafloor such as pockmarks, mud volcanoes and seafloor zonations colonized by living benthic macrofauna typical of active fluid flow. The comparison of seismic stratigraphic sequences with climate and environmental proxies determined for sediments including lithology, element ratios such as Ca/Fe and Zr/Rb, sedimentation rates, and planktonic foraminifera δ18O records shows a control of short-term (0.1–0.4 Myr) and long-term (around 1 Myr) sedimentary cycles. During short-term fluctuations, the recurrence of fluid seeping structures is controlled by the lithology (coarse-grained versus fine-grained sediments) linked to glacial-interglacial fluctuations and a monsoon regime. Conditions favorable for fluid flow are related to the deposition of coarse-grained layers, glacial periods, sea-level lowstands, and low sedimentation rates. We hypothesize that cyclic gravity-driven sediment deformations controlled by glacial-interglacial and long-term variations in accommodation and sedimentation, have led to the cyclic evolution of the plumbing systems and associated seeping structures since the Plio-Pleistocene.

Understanding the formation and dissociation mechanisms of gas hydrate in porous media is important for the development of new technologies related to cold storage as they provide significant latent heat and energy density at suitable phase change temperature. In this work, we investigated CO2 hydrate formation and dissociation in two different porous materials: sand and silica gels. A calorimetric approach is applied to study both the CO2 hydrate formation kinetics, particularly the induction time, and the amount of hydrate formed in both porous materials. The present work is focused on assessing the effect of key factors like water saturation, particle size and the morphology of porous media on CO2 hydrate formation and dissociation processes. Interestingly, the results obtained with mesoporous silica gel showed a higher amount of hydrate formed compared to those with sand for similar initial pressure, temperature and water content conditions.

The global ocean is a net source of CH4 to the atmosphere. Large uncertainties remain on marine emissions that deserves effort to improve current estimates, and eventually predict their trajectories in a changing climate. Ocean CH4 emissions can either be CH4 emanating from seafloor sediments or in situ production in surface water linked to primary productivity. Sediment input into the water column can be either CH4 emanating from hydrate dissociation or free gas rising through the sediment. Ultimately, CH4 enters the atmosphere across the sea-air interface either from bubbles rising from the seafloor or by diffusion from dissolved gas. Estimates of global marine emissions diverge widely due to very large uncertainties linked to limited data coverage, methodological differences and the difficulty to capture the environmental factors that lead to high variability of the emissions.As the world’s largest natural anoxic waterbody, the semi-enclosed Black Sea (BS) is characterized by widespread seafloor CH4 emissions from the shallow coast to the deep basin. The evolution of the anoxic properties of the BS is strongly linked to the amount of CH4 discharged and the supply of organic matter from the connected large rivers. Therefore, it is crucial to estimate the BS CH4 budget and understand the transfer mechanism to the atmosphere to better understand the impact of climate change. During the GHASS2 (Gas Hydrates, fluid Activities and Sediment deformations in the black Sea) cruise in September 2021, CH4 transfer to the atmosphere has been investigated at water depths ranging from 60 m to 1200m in the Western sector of the BS. CH4 partial pressures were measured in the surface water and in the atmosphere using optical spectrometers, respectively the SubOcean membrane inlet laser spectrometer (Grilli et al., 2021, https://doi.org/10.3389/feart.2021.626372) and an ICOS-calibrated commercial analyzer (Picarro model G2401). We report eddy covariance measurements using an open-path CH4 analyzer Li-7700 and a H2O-CO2 analyzer 7200RS from LiCor, a Gill 3D sonic anemometer, and an inertial navigation sensor (Lord). We compare flux estimates obtained from partial pressure gradient by the diffusive method under various schemes with the experimental eddy covariance set-up, applying available corrections for ship movement and interference with airflow. We also compare our results with previous reports for the area and conclude on the respective challenges and relative basin-scale representativity of the various measurement techniques.

The global ocean is a net source of CH4 to the atmosphere. Among the natural processes, marine emissions are significant contributors with large uncertainties that deserves effort to improve current estimates, and eventually predict their trajectories in a changing climate. Oceanic CH4 emissions to the atmosphere can either be transported from seafloor or in situ produced in surface waters. Seafloor emissions include both CH4 emanating from CH4 hydrate degradation and from free gas in the sediment. Ultimately, CH4 enters the atmosphere across the sea-air interface either from bubbles rising from the seafloor or by diffusion of dissolved gas. Estimates of global marine emissions diverge widely due to very large uncertainties linked to limited data coverage, seasonal and methodological differences and the difficulty to capture the environmental factors that lead to high variability of the emissions. As the world’s largest natural anoxic waterbody, the semi-enclosed Black Sea (BS) is very sensitive to human and climate perturbations. It is characterized by widespread seafloor CH4 emissions from the shallow coast to the deep basin. One of the major issues that arises on the BS methane dynamics is the determine to what extent and in which quantity part of the urge amount of dissolved methane stored in the anoxic bottom water layer is transferred to the atmosphere. During the GHASS2 (Gas Hydrates, fluid Activities and Sediment deformations in the black Sea) cruise in September 2021, CH4transfer to the atmosphere has been investigated in the Western sector of the BS at sites with water depth ranging from 60 m to 1200m. CH4 partial pressures were measured in the surface water and in the atmosphere using optical spectrometers, respectively the SubOcean membrane inlet laser spectrometer (Grilli et al., 2021, https://doi.org/10.3389/feart.2021.626372) and an ICOS-calibrated commercial analyzer (Picarro G2401). We have also developed an open-path setup dedicated to shipborne measurement composed by an open-path CH4 analyzer Li-7700, a H2O-CO2 analyzer 7200RS from LiCor, a Gill 3D sonic anemometer, and an inertial navigation sensor (Lord). An inox structure was specifically designed to protrude by 1m the front mast of the R/V Pourquoi Pas? to install the open-path sensor. We present preliminary flux estimates comparison obtained from partial pressure gradient by the diffusive method with the experimental eddy covariance set-up. We also discuss our preliminary results in comparison with previous reports for the area and conclude on the respective challenges and relative basin-scale representativity of the various measurement techniques.

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.

Understanding the formation and dissociation mechanisms of gas hydrate in porous media is important for the development of new energy-efficient and environmentally friendly technologies related to cold storage as they provide significant latent heat and energy density at suitable phase change temperature. The challenge is to understand the interactions between gas hydrates and the chosen storage media in order to assess the operating conditions likely to optimize time and energy consumption in cold produc-tion and storage systems. In this work, CO2 hydrates formation and dissociation are investigated in two morphologically different porous materials: sand and silica gels. A calorimetric approach is applied to study both the CO2 hydrate formation kinetics, particularly the induction time, and the amount of hydrate formed for each of the two porous materials. The experiments are performed using a differential thermal analysis device with two identical measuring cells. The present work is focused on assessing the effect of key factors like water saturation, particle size and the morphology of porous media on CO2 hydrate formation and dissociation processes. Overall, the results do not show a statistically significant correlation between these factors and the induction time. Interestingly, the results obtained with dual porous silica gel showed a higher amount of hydrate formed compared to those with sand for similar ini-tial pressure, temperature and water content conditions. This result may be due to the fact that silica gels provide higher surface area due to their smaller particle size (20-45 lm vs 80-450 lm for sand), and the presence of internal pore volume in silica gel particles.(c) 2022 Elsevier Ltd. All rights reserved.

[研究目的]在末次冰期,全球气候变化以千年尺度的快速、大幅度温度波动旋回为特征,这种波动变化在两极冰芯、深海沉积、中国黄土和洞穴石笋等诸多地质样品中均有记录.黑海位于北大西洋与东亚季风区过渡带,具有极有代表性的沉积记录.本文旨在通过对黑海沉积序列的研究,建立起其区域环境变化与北大西洋及东亚季风气候域气候变化的联系.[研究方法]研究对取自黑海西北部罗马尼亚陆坡区多瑙河峡谷北侧GAS-CS12钻孔的长22.0 m的岩芯样品,进行了粒度、矿物成分、主量元素、有机碳、总氮及碳氮同位素等分析.[研究结果]揭示出该段岩芯沉积于末次冰期中后期"Neoeuxine"湖相阶段,可划分为5个沉积单元,对应于北大西洋H4、H3、H1气候变化事件、末次冰盛期(LGM)及Bolling-Allerod气候变暖事件.[结论]建立起了其沉积序列及区域环境变化与北大西洋及东亚季风气候域气候变化的联系,印证了末次冰期千年尺度的气候变化事件在北大西洋、东亚季风区及两者过渡带上具有高度的一致性.

Pore water and sediment geochemistry in the western Black Sea were investigated on long Calypso piston core samples. Using this type of coring device facilitates the recovery of the thick sediment record necessary to analyze transport-reaction processes in response to the postglacial sea-level rise and intrusion of Mediterranean salt water 9 ka ago, and thus, to better characterize key biogeochemical processes and process changes in response to the shift from lacustrine to marine bottom water composition. Complementary data indicate that organic matter degradation occurs in the upper 15 m of the sediment column. However, sulfate reduction coupled with Anaerobic Methane Oxidation (AOM) is the dominant electron-accepting process and characterized by a shallow Sulfate Methane Transition Zone (SMTZ). Net silica dissolution, total alkalinity (TA) maxima and carbonate peaks are found at shallow depths. Pore water profiles clearly show the uptake of K+, Mg2+ and Na+ by, and release of Ca2+ and Sr2+ from the heterogeneous lacustrine sediments, which is likely controlled by chemical reactions of silicate minerals and changes in clay mineral composition. Iron (Fe2+) and manganese (Mn2+) maxima largely coincide with Ca2+ peaks and suggest a close link between Fe2+, Mn2+ and Ca2+ release. We hypothesize that the Fe2+ maxima below the SMTZ result from deep Fe3+ reduction linked to organic matter degradation, either driven by DOC escaping from the shallow sulfate reduction zone or slow degradation of recalcitrant POC. The chemical analysis of dissolved and solid iron species indicates that iron is essentially associated with clay minerals, which suggests that microbial iron reduction is influenced by clay mineral composition and bioavailability of clay mineral-bound Fe(III). Overall, our study suggests that post glacial seawater intrusion plays a major role in shaping redox zonation and geochemical profiles in the lacustrine sediments of the Late Quaternary.

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.

The ocean redox structure and nitrogen utilization are investigated in the Holocene sapropel depositions in the Sea of Marmara, based on inorganic, organic, and isotopic analyses and grain-size distribution of the sediments of a piston core from Cinarcik Basin. Two sapropel units are identified in the core studied, the lower (main) sapropel of the early Holocene and the upper sapropel of the middle-late Holocene. Relatively high C/N ratios (10-11) and smaller delta C-13(org) values (similar to-26 parts per thousand) in the lower Holocene sapropel unit indicate that the organic matter is mainly of terrestrial origin. Up to 5.5 parts per thousand delta N-15 values and Mo concentration (6.5 ppm) above crustal values in the bottom of the lower sapropel unit strongly suggest that bottom-water conditions were denitrifying and suboxic-dysoxic, suitable for the preservation and burial of the organic matter. Such bottom-water conditions were induced by water stratification that resulted from the transgression from the Aegean Sea coupled with a riverine influx from the Black Sea that provided the terrestrial organic matter. A progressive upward decrease of C/N ratios and increase of delta C-13(org) values in the upper sapropel unit might have been caused by the additional supply of organic matter of mainly marine origin and improved ventilation of the lower water, which resulted in the oxidation of the organic nitrogen pool and a decrease of delta N-15 values (below 3 parts per thousand) under nitrate-rich conditions. Mn enrichment in the upper sapropelic sediments also supports a relatively oxidative environment.