In this work we report a compact, low-cost instrument based on Open Path - Incoherent BroadBand Cavity Enhanced Absorption Spectroscopy (OP-IBBCEAS) at 445 nm for the detection of iodine oxide (IO), nitrogen oxide (NO2) and glyoxal (CHOCHO) in the atmosphere. We provide a comparison between OP-IBBCEAS and the closed-path setup on which our instrument is based, previously described in Barbero et al. (2020). The OP system achieved a noise-equivalent absorption sensitivity (NEAS) of 5.7 & times;10-10 cm-1Hz-1/2 per spectral element, only 3 times higher than the closed-path system. Automated performance of regular measurements without absorbance ensures good long-term accuracy of the system. The OP-IBBCEAS is robust and simple to install in the field, allowing quantitative measurements of NO2, CHOCHO, and IO with precisions of +/- 150, +/- 150, and +/- 7 ppt (pmolmol-1, 2 sigma) in 50 s of measurement.

Abstract Methane (CH4) accumulation in the oxic metalimnion is a common yet poorly resolved feature of deep stratified lakes. In Lake Geneva, lateral transport from the Rhône delta has been suggested but lacks spatial validation. Here, we quantify the role of lateral advection in shaping metalimnetic CH4 using high‐resolution in situ CH4 measurements, δ13C‐CH4 analyses, and a three‐dimensional Lagrangian particle‐tracking model. Elevated metalimnetic CH4 (up to 1143 nM) near the Rhône delta becomes isotopically enriched, likely from prolonged residence and partial oxidation during summer stratification. While additional sources may contribute, comparisons of in situ measurements with particle‐tracking simulations suggest circulation gyres may distribute deltaic CH4. Moreover, isotope‐ and transport‐based indicators suggest more coherent coupling in the epilimnion than in the metalimnion. These results demonstrate that horizontal advection can be an important driver of metalimnetic CH4 and highlight the importance of circulation‐driven lateral transport for assessing CH4 fluxes from large, stratified lakes.

Climate-driven retreat of Greenland’s marine-terminating glaciers in rapidly transforming fjord systems has potential implications for marine methane cycling. Because the Greenland ice sheet and its basal meltwaters have been identified as sources of methane, it has been hypothesized that fjords fed by marine-terminating glaciers should be associated with elevated dissolved methane concentrations and enhanced sea-air exchange. Here, we test this hypothesis by comparing two adjacent fjords in southwest Greenland (Narsaq region), one currently fed by a marine-terminating glacier, and one supplied by a pro-glacial river draining a glacier that has retreated inland. Measurements collected during the summers of 2023 and 2024 reveal remarkably similar dissolved methane concentrations, vertical distributions and saturation levels in both fjords. Methane concentrations were highest in surface meltwaters and fjord waters and lower in intruding oceanic waters into the fjords, with no evidence for significant methane inputs from the seafloor. Overall, the observed methane concentrations (4.94 – 9.30 nM) correspond to a surface layer (0-3m) saturation of 129-226% resulting in small atmospheric fluxes from 0.49 to 3.31 µmol m-2 d-1. High-resolution measurements using a membrane inlet laser spectrometer (MILS) further revealed no detectable fine-scale methane enrichment near the glacier front. We attribute the similarity in dissolved methane distributions between the two fjords to the dominance of surface meltwater discharge during summer, even in the presence of a marine-terminating glacier, suggesting that southern Greenland fjords represent a relatively weak source of atmospheric methane under summer conditions. Our results suggest that ongoing retreat from marine-terminating to land-terminating glaciers is unlikely to substantially enhance methane emission from Greenland fjords under present and near-future summer conditions. Wintertime measurements remain needed to fully constrain annual methane budgets in these fjords.

The Mid-Pleistocene Transition (MPT; 1.25 – 0.8 Myr) marks a transition from the “41-kyr world”, in which the Earth alternated between cold and warm periods about every 41,000 years, to the “100-kyr world” in which the Earth remained predominantly under glacial conditions but was punctuated every 80,000 to 120,000 years by interglacial periods. Variations in orbital forcing, the “pacemaker of the ice ages”, are stable across the MPT and thus cannot be invoked as a driver of this transition. Thus, most hypotheses call upon a forcing that drives a secular change, a feedback in the Earth system that changes/emerges, or a combination of the two.One hypothesis for the MPT suggests that a long-term decline in (glacial) atmospheric CO2 levels led to a cooling, facilitating the formation of extensive ice sheets in North America and a sea-level drop of approximately 70 m (Bintanja & van de Wal, 2008). Accordingly, decreasing atmospheric CO2 concentration may have played a central role in driving this global cooling. Despite recent advances in marine and ice core CO₂ reconstructions (Nuber et al., 2025; Marks Peterson et al., 2025) the change of greenhouse gas forcing across the MPT remains uncertain for CO2.In order to investigate the role of atmospheric CO2 across the MPT, greenhouse gases and the stable carbon isotopic composition of CO2 (δ13C-CO2) were measured on discrete ice core samples from the Beyond EPICA ice core. For this purpose, a coupled Laser induced sublimation extraction – Quantum Cascade laser Absorption Spectrometer (LISE-QCLAS) was used, allowing the simultaneous and semi-continuous extraction and measurement of CO2, CH4 and N2O as well as δ13C-CO2 on air samples of only 1 – 2 mL, corresponding to 10 – 15 g of ice.This talk will present the first ice core data capable of capturing glacial–interglacial variations in atmospheric CO₂ across the MPT. Additionally, we will present the first unconditionally pristine measurements of δ13C-CO2 during the 41-kyr world. These data will allow us to explore the underlying biogeochemical processes that may be responsible for the new modes of atmospheric CO2 variability we have observed.

Antarctic ice cores are a preferred climate archive to study global carbon cycle changes at multi-centennial timescales as they provide the only direct reconstructions of past atmospheric CO2 changes. Here we present a new atmospheric CO2 record from the EPICA Dome C ice core spanning Termination III (TIII) and Marine Isotope Stage 7 (MIS 7) (~260-190 ka). 203 ice samples were measured using a ball mill dry extraction system and gas chromatography at IGE. With a temporal resolution of about 300 years on average, our new record improves by a factor of three the existing CO2 record that had been measured on the Vostok ice core over this time interval. Based on our new record, we identified seven centennial-scale releases of atmospheric CO2, also referred as Carbon Dioxide Jumps (CDJ). Combining these new results with previously published ones, we evidenced that 18 of the 22 CDJs identified over the past 500 thousand years occurred under a context of high obliquity. New simulations performed with the LOVECLIM model, an Earth system model of intermediate complexity, point toward both the continental biosphere and the Southern Ocean as the two main carbon sources during CDJs connected to Heinrich events. Notably, the continental biosphere appears to be the obliquity-dependent CO2 source for these rapid events. For the first time, we demonstrate that the long-term external forcing directly impacts past abrupt atmospheric CO2 variations.

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 (CH4) and nitrous oxide (N2O) are potent greenhouse gases (GHGs) and are involved in ozone depletion. They are the second and third most significant GHGs contributing to climate change, with global warming potentials approximately 25 times and 300 times greater than CO2. The contribution of freshwater lakes to CH4 and N2O emissions remains debatable. Oxygen-rich subsurface waters are recognized as hotspots of metalimnetic CH4 flux, while oxygen-driven diffusion in deep water columns is expected to enhance hypolimnetic N2O production in deep freshwater lakes. Therefore, understanding the seasonal dynamics of water-column CH4 and N2O in freshwater ecosystems is crucial for predicting their impact on future climate projections. Traditionally, both GHG gases have been thought to be primarily produced in sediments, with higher emissions occurring during the mixing events. However, the seasonal dynamics and characteristics of both water-column gases, including production in the water column which may contribute to atmospheric emissions, are often overlooked. Here, we hypothesize that both metalimnetic CH4 and hypolimnetic N2O might be present and emitted from deep lakes year-round. We will present our preliminary measurements of seasonal variations in CH4 and N2O concentrations in the deep Swiss alpine Lake Geneva. They relied on laser spectrometric probes called SubOcean, allowing in-situ and real-time observations of CH4 and N2O along the water column, with a remarkable depth resolution.

Le manteau neigeux de l'Antarctique a longtemps été considéré comme un matériau inerte où les espèces chimiques transportées sur de longues distances étaient archivées sans subir de transformation. Cette hypothèse est en partie vraie, comme en témoignent les enregistrements de 800 000 ans des isotopes de l'eau, des teneurs en CO2 et CH4 obtenues grâce à l'analyse des carottes de glace du plateau Antarctique. Cependant, au cours des deux dernières décennies, le paradigme de la neige en tant que matériau inerte a été remis en question par la découverte d'une activité substantielle au sein du manteau neigeux et, en particulier, de sa teneur élevée en oxydes d'azote. Une meilleure compréhension des mécanismes générant ces émissions est fondamentale pour améliorer la connaissance de la chimie atmosphérique qui se produit dans ces régions. Il est donc important d'obtenir des observations des espèces azotées malgré le défi lié à leurs très faibles concentrations. For a long time, the Antarctic snowpack was considered an inert material in which chemical species transported over long distances were archived without undergoing any transformation. This hypothesis is partly true, as shown by the 800,000-year records of water isotopes, CO2 and CH4 content obtained by analysing ice cores from the Antarctic plateau. However, over the last two decades, the paradigm of snow as an inert material has been challenged by the discovery of substantial activity within the snowpack and, in particular, its high content of nitrogen oxides. A better understanding of the mechanisms that generate these emissions is therefore fundamental for a better understanding of the atmospheric chemistry that occurs in these regions. It is therefore important to obtain observations of nitrogen species despite the challenge posed by their very low concentrations.

Centennial-scale increases of atmospheric carbon dioxide, known as carbon dioxide jumps, are identified during deglacial, glacial and interglacial periods and linked to the Northern Hemisphere abrupt climate variations. However, the limited number of identified carbon dioxide jumps prevents investigating the role of orbital background conditions on the different components of the global carbon cycle that may lead to such rapid atmospheric carbon dioxide releases. Here we present a high-resolution carbon dioxide record measured on an Antarctic ice core between 260,000 and 190,000 years ago, which reveals seven additional carbon dioxide Jumps. Eighteen of the 22 jumps identified over the past 500,000 years occurred under a context of high obliquity. Simulations performed with an Earth system model of intermediate complexity point towards both the Southern Ocean and the continental biosphere as the two main carbon sources during carbon dioxide jumps connected to Heinrich ice rafting events. Notably, the continental biosphere appears as the obliquity-dependent carbon dioxide source for these abrupt events. We demonstrate that the orbital-scale external forcing directly impacts past abrupt atmospheric carbon dioxide changes. Centennial-scale releases of atmospheric CO2 occurred during periods of high obliquity over the past 500,000, suggesting a link between external forcing and atmospheric CO2 variations, according to a record from an Antarctic ice core.

We describe a novel compact autonomous in situ sensor for semi-continuous measurement of water isotopes (delta D, delta 18O, and delta 17O) in liquid water. The sensor relies on a dual-inlet water vapor injection system based on the pervaporation through a semi-permeable membrane, and on the water vapor composition analysis using a dedicated optical feedback cavity enhanced absorption spectrometer. The sensor has dimensions of 165 mm diameter and 550 mm long, for a weight of similar to 8 kg. A titanium casing allows applications down to 6000 m deep for a total effective weight of 45 (23) kg in air (water). It has a power consumption of similar to 40 W, and an autonomy of 10-12 h which is ensured by a dedicated Li-ion battery pack. The sensor is equipped with single-pair high-speed digital subscriber line communication for telemetry purposes. The instrument provides an accuracy of 0.3 parts per thousand (2 sigma) for all water isotopes with a 9-min integration time. The instrument is suitable for investigating the freshwater cycle in the ocean, and in particular the transformation of ocean water masses related to iceberg and ice shelf melting.

The oxygen (Δ17O) and nitrogen (δ15N) isotopic compositions of atmospheric nitrate (NO3-) are widely used as tracers of its formation pathways, precursor (nitrogen oxides (NOx) ≡ nitric oxide (NO) + nitrogen dioxide (NO2)) emission sources, and physico-chemical processing. However, the lack of observations on the multi-isotopic composition of NO2 perpetuates significant uncertainties regarding the quantitative links between the isotopic composition of NOx and NO3-, which ultimately may bias inferences about NO3- formation processes and the distribution of sources, particularly in winter urban atmospheres. We report here on the first simultaneous atmospheric observations of Δ17O and δ15N in NO2 (n=16) and NO3- (n=14). The measurements were carried out at sub-daily (∼3 h) resolution over 2 non-consecutive days in an Alpine city in February 2021. A strong diurnal signal is observed in both NO2 and NO3- multi-isotopic composition. Δ17O of NO2 and NO3- ranges from 19.6 ‰ to 40.8 ‰ and from 18.3 ‰ to 28.1 ‰, respectively. During the day and night, the variability in Δ17O(NO2) is mainly driven by the oxidation of NO by ozone, with a substantial contribution from peroxy radicals in the morning. NO3- mass balance equations, constrained by observed Δ17O(NO2), suggest that during the first day of sampling, most of the NO3- was formed locally from the oxidation of NO2 by hydroxyl radicals by day and via heterogeneous hydrolysis of dinitrogen pentoxide at night. For the second day, calculated and observed Δ17O(NO3-) do not match, particularly daytime values; the possible effects on Δ17O(NO3-) of a Saharan dust event that occurred during this sampling period and of winter boundary layer dynamics are discussed. δ15N of NO2 and NO3- ranges from −10.0 ‰ to 19.7 ‰ and from −4.2 ‰ to 14.9 ‰, respectively. Consistent with theoretical predictions of N isotope fractionation, the observed variability in δ15N(NO2) is explained by significant post-emission equilibrium N fractionation. After accounting for this effect, vehicle exhaust is found to be the primary source of NOx emissions at the sampling site. δ15N(NO3-) is closely linked to δ15N(NO2) variability, bringing further support to relatively fast and local NOx processing. Uncertainties in current N fractionation factors during NO2 to NO3- conversion are underlined. Overall, this detailed investigation highlights the potential and necessity of simultaneously using Δ17O and δ15N in NO2 and NO3- in order to better constrain quantitative inferences about the sources and formation chemistry of NO3- in urban environments in winter.

Biomass burning plays an important role in climate-forcing and atmospheric chemistry. The drivers of fire activity over the past two centuries, however, are hotly debated and fueled by poor constraints on the magnitude and trends of preindustrial fire regimes. As a powerful tracer of biomass burning, reconstructions of paleoatmospheric carbon monoxide (CO) can provide valuable information on the evolution of fire activity across the preindustrial to industrial transition. Here too, however, significant disagreements between existing CO records currently allow for opposing fire histories. In this study, we reconstruct a continuous record of Antarctic ice core CO between 1821 and 1995 CE to overlap with direct atmospheric observations. Our record indicates that the Southern Hemisphere CO burden ([CO]) increased by 50% from a preindustrial mixing ratio of ca. 35 ppb to ca. 53 ppb by 1995 CE with more variability than allowed for by state-of-the-art chemistry-climate models, suggesting that historic CO dynamics have been not fully accounted for. Using a 6-troposphere box model, a 40 to 50% decrease in Southern Hemisphere biomass-burning emissions, coincident with unprecedented rates of early 20th century anthropogenic land-use change, is identified as a strong candidate for this mismatch.

To improve our understanding of critical environmental processes, high resolution measurements with acceptable accuracy are essential. Unfortunately, the high spatiotemporal variability often associated with seabed seepage environments is prone to mischaracterization due to limitations in contemporary measurement techniques. This is particularly true for dissolved methane, which are often measured by labor- and time-intensive discrete water sampling and subsequent laboratory analysis. This often yields data with inadequate spatiotemporal resolution. A potential solution to this issue is using in-situ sensors in towing, profiling, mooring/observatory or glider operations. However, typical off-the-shelf sensors with adequate payload and power requirements currently lack the response time necessary for these applications. We offer a new, easy-to-implement, laboratory and field-tested post-processing tool for retrieving fast response data from commercially available methane sensors with slow response times. The tool is based on the framework of statistical inverse theory which in practice enables the user to obtain data with quantified, explicit (modeled) measurement uncertainty and at the resolution (i.e. response time) where the sensor can provide data with a respectable level of accuracy. The user needs no input besides the raw data, sensor accuracy, and response time. In our field experiment, we successfully retrieved data corresponding to a response time of 55 s using a sensor with a stated response time of 29 minutes. Being able to obtain high-resolution data from these types of sensors can considerably enhance the capacity to properly resolve the variability within methane seep sites and comprehend associated environmental processes.

Lakes and reservoirs are a significant source of atmospheric methane (CH4), with emissions comparable to the largest global CH4 emitters. Understanding the processes leading to such significant emissions from aquatic systems is therefore of primary importance for producing accurate projections of emissions in a changing climate. In this work, we present the first deployment of a novel membrane inlet laser spectrometer (MILS) for fast simultaneous detection of dissolved CH4, ethane (C2H6) and the stable carbon isotope of methane (delta 13CH4). During a 1-day field campaign, we performed 2D mapping of surface water of Lake Aiguebelette (France). Average dissolved CH4 concentrations and delta 13CH4 were 391.9 +/- 156.3 nmol L-1 and -67.3 +/- 3.4 parts per thousand in the littoral area and 169.8 +/- 26.6 nmol L-1 and -61.5 +/- 3.6 parts per thousand in the pelagic area. The dissolved CH4 concentration in the pelagic zone was 50 times larger than the concentration expected at equilibrium with the atmosphere, confirming an oversaturation of dissolved CH4 in surface waters over shallow and deep areas. The results suggest the presence of CH4 sources less enriched in 13C in the littoral zone (presumably the littoral sediments). The CH4 pool became more enriched in 13C with distance from shore, suggesting that oxidation prevailed over epilimnetic CH4 production and it was further confirmed by an isotopic mass balance technique with the high-resolution data. This new in situ fast response sensor allows one to obtain unique high-resolution and high-spatial coverage data sets within a limited amount of survey time. This tool will be useful in the future for studying processes governing CH4 dynamics in aquatic systems. High-resolution mapping of surface methane and its isotopic signature enables accurate characterization of aquatic systems and discrimination of biochemical processes at work. At Lake Aiguebelette, this new in situ tool allowed us to conclude that methane present at the surface comes mainly from shallow littoral areas, where sediments, which are a source of methane, are closer to the surface. During lateral transport of water masses from the littoral zone, the change in isotopic signature reveals that methane oxidation prevails over local in situ production. Comparison with previous studies validates the importance of high-resolution measurements (particularly to capture the high variability in the littoral zone) and showed that smaller lakes experience stronger methane isotopic signature changes for a given methane concentration variation. This can be explained by the fact that the smaller lake has a larger littoral-to-total surface area. This new tool will be useful in the nearby future to study the processes governing CH4 dynamics in aquatic systems. Fast in situ measurements of dissolved methane and its stable carbon isotopeHigh-spatial resolution mapping of dissolved methane and its stable carbon isotopeImproved production/oxidation process identification over discrete sampling

Carbon monoxide (CO) is a naturally occurring atmospheric trace gas, a regulated pollutant, and one of the main components determining the oxidative capacity of the atmosphere. Evaluating climate–chemistry models under different conditions than today and constraining past CO sources requires a reliable record of atmospheric CO mixing ratios ([CO]) that includes data since preindustrial times. Here, we report the first continuous record of atmospheric [CO] for Southern Hemisphere (SH) high latitudes over the past 3 millennia. Our continuous record is a composite of three high-resolution Antarctic ice core gas records and firn air measurements from seven Antarctic locations. The ice core gas [CO] records were measured by continuous flow analysis (CFA), using an optical feedback cavity-enhanced absorption spectrometer (OF-CEAS), achieving excellent external precision (2.8–8.8 ppb; 2σ) and consistently low blanks (ranging from 4.1±1.2 to 7.4±1.4 ppb), thus enabling paleo-atmospheric interpretations. Six new firn air [CO] Antarctic datasets collected between 1993 and 2016 CE at the DE08-2, DSSW19K, DSSW20K, South Pole, Aurora Basin North (ABN), and Lock-In sites (and one previously published firn CO dataset at Berkner) were used to reconstruct the atmospheric history of CO from ∼1897 CE, using inverse modeling that incorporates the influence of gas transport in firn. Excellent consistency was observed between the youngest ice core gas [CO] and the [CO] from the base of the firn and between the recent firn [CO] and atmospheric [CO] measurements at Mawson station (eastern Antarctica), yielding a consistent and contiguous record of CO across these different archives. Our Antarctic [CO] record is relatively stable from −835 to 1500 CE, with mixing ratios within a 30–45 ppb range (2σ). There is a ∼5 ppb decrease in [CO] to a minimum at around 1700 CE during the Little Ice Age. CO mixing ratios then increase over time to reach a maximum of ∼54 ppb by ∼1985 CE. Most of the industrial period [CO] growth occurred between about 1940 to 1985 CE, after which there was an overall [CO] decrease, as observed in Greenland firn air and later at atmospheric monitoring sites and attributed partly to reduced CO emissions from combustion sources. Our Antarctic ice core gas CO observations differ from previously published records in two key aspects. First, our mixing ratios are significantly lower than reported previously, suggesting that previous studies underestimated blank contributions. Second, our new CO record does not show a maximum in the late 1800s. The absence of a [CO] peak around the turn of the century argues against there being a peak in Southern Hemisphere biomass burning at this time, which is in agreement with (i) other paleofire proxies such as ethane or acetylene and (ii) conclusions reached by paleofire modeling. The combined ice core and firn air [CO] history, spanning −835 to 1992 CE, extended to the present by the Mawson atmospheric record, provides a useful benchmark for future atmospheric chemistry modeling studies.

RV Helmer Hanssen is the only vessel operating so far north (81 N) in October. One of the research questions addressed during this cruise is: Where do gas hydrate exist in the seabed and how much methane does it actually release? The CAGE 15-6 cruise explored potential gas hydrate charged sub-seabed environments and gas release zones at Storfjordrenna, Vestnesa and Svyatogor Ridge, Yermak Plateau, Sofia Basin and west of Prins Karls Foreland. Our route was traceable by www.sailwx.com. The weather forecasts from www.windyty.comwere used to prepare and adjust our cruise activities. In these Barents Sea-Arctic areas we carried out seismic profiling (mini GI 15/45 in 3 and large GI45/105 in 3) and 6 m long gravity coring for detecting gas hydrate, acoustic profiling (18, 38 and 120 kHz) for detecting gas flares in the water column, and CTD water sampling for gas analyses. Multibeam bathymetry mapping (EM300) data was collected on route for the entire cruise. We also ran echo sounder profiles along the NW Svalbard margin, across Yermak Plateau from west to east into the Sofia Basin and upslope towards the northern Svalbard margin. We reached the upper gas hydrate stability (GHSZ) theoretical outcrop zone but no acoustic evidence for extended gas release activity was found along the GHSZ outcrop zone at Yermak Plateau. The cruise may be known as: CAGE15_6

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.

Previous Antarctic summer campaigns have shown unexpectedly high levels of oxidants in the lower atmosphere of the continental plateau and at coastal regions, with atmospheric hydroxyl radical (OH) concentrations up to 4 × 106 cm−3. Such high reactivity in the summer Antarctic boundary layer results in part from the emissions of nitrogen oxides (NOx ≡ NO + NO2) produced during photo-denitrification of the snowpack, but its underlying mechanisms are not yet fully understood, as some of the chemical species involved (NO2, in particular) have not yet been measured directly and accurately. To overcome this crucial lack of information, newly developed optical instruments based on absorption spectroscopy (incoherent broadband cavity-enhanced absorption spectroscopy, IBBCEAS) were deployed for the first time at Dome C (−75.10 lat., 123.33 long., 3233 m a.s.l.) during the 2019–2020 summer campaign to investigate snow–air–radiation interaction. These instruments directly measure NO2 with a detection limit of 30 pptv (parts per trillion by volume or 10−12 mol mol−1) (3σ). We performed two sets of measurements in December 2019 (4 to 9) and January 2020 (16 to 25) to capture the early and late photolytic season, respectively. Late in the season, the daily averaged NO2:NO​​​​​​​ ratio of 0.4 ± 0.4 matches that expected for photochemical equilibrium through Leighton's extended relationship involving ROx (0.6 ± 0.3). In December, however, we observed a daily averaged NO2:NO ratio of 1.3 ± 1.1, which is approximately twice the daily ratio of 0.7 ± 0.4 calculated for the Leighton equilibrium. This suggests that more NO2 is produced from the snowpack early in the photolytic season (4 to 9 December), possibly due to stronger UV irradiance caused by a smaller solar zenith angle near the solstice. Such a high sensitivity of the NO2:NO ratio to the sun's position is of importance for consideration in atmospheric chemistry models.

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.

Measurements of the water vapor absorption cross-sections at two spectral points of the 2.1 µm and 4.0 µm transparency windows are performed by optical feedback cavity enhanced absorption spectroscopy (OFCEAS).The self-continuum cross-section, CS, is measured for temperature values of 30 and 47°C (303 and 320 K) at the 2853 cm−1 spectral point, corresponding to the lowest opacity region of the 4.0 µm transparency window. The CS values derived from the pressure squared dependence of the self-continuum, are found consistent with previous CEAS measurements in the considered window but significantly smaller than measurements by Fourier transform spectroscopy (FTS). The CS temperature dependence is discussed in relation with FTS measurements at high temperature.Foreign-continuum cross-sections, CF, are newly obtained from OFCEAS spectra of moist air in flow regime at the 4302 cm−1 spectral point of the low energy edge of the 2.1 µm window. After subtraction of the monomer and self-continuum contributions, CF values are derived from the linear variation of the foreign-continuum absorption with the product of the water vapor and air partial pressures. The measurements were performed for temperature values of 34 and 47°C (307 and 320 K) and no significant temperature dependency was observed. The present CF value at 4302 cm−1 is gathered with previous CEAS measurements at seven spectral points of the 2.1 µm window. This consistent set of CF values is used to derive from a polynomial fit, the empirical frequency dependence of CF(ν) over the 4250-5000 cm−1 range. Overall, the semi-empirical MT_CKD_3.5 values of CF are significantly underestimated in the centre of the considered window.