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.

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.

Dédiée aux sciences de l'atmosphère, au climat et à d'autres domaines connexes, tels que l'océanographie ou la glaciologie, La Météorologie, révisée par des pairs et publiée en français, s'adresse aux professionnels de la météo et du climat, aux enseignants, aux étudiants, aux amateurs et aux utilisateurs. La Météorologie a succédé en 1925 à l'Annuaire de la Société météorologique de France (1852-1924) qui avait lui-même succédé à l'Annuaire météorologique de la France (1849-1851).

The temperature of the Earth is one of the most important climate parameters. Proxy records of past climate changes, in particular temperature, represent a fundamental tool for exploring internal climate processes and natural climate forcings. Despite the excellent information provided by ice core records in Antarctica, the temperature variability of the past 2000 years is difficult to evaluate from the low-accumulation sites in the Antarctic continent interior. Here we present the results from the Aurora Basin North (ABN) ice core (71∘ S, 111∘ E, 2690 m a.s.l.) in the lower part of the East Antarctic plateau, where accumulation is substantially higher than other ice core drilling sites on the plateau, and provide unprecedented insight into East Antarctic past temperature variability. We reconstructed the temperature of the last 2000 years using two independent methods: the widely used water stable isotopes (δ18O) and by inverse modelling of borehole temperature and past temperature gradients estimated from the inert gas stable isotopes (δ40Ar and δ15N). This second reconstruction is based on three independent measurement types: borehole temperature, firn thickness, and firn temperature gradient. The δ18O temperature reconstruction supports stable temperature conditions within 1 ∘C over the past 2000 years, in agreement with other ice core δ18O records in the region. However, the gas and borehole temperature reconstruction suggests that surface conditions 2 ∘C cooler than average prevailed in the 1000–1400 CE period and supports a 20th century warming of 1 ∘C. A precipitation hiatus during cold periods could explain why water isotope temperature reconstruction underestimates the temperature changes. Both reconstructions arguably record climate in their own way, with a focus on atmospheric and hydrologic cycles for water isotopes, as opposed to surface temperature for gas isotopes and boreholes. This study demonstrates the importance of using a variety of sources for comprehensive paleoclimate reconstructions.

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.

Carbon monoxide (CO) is a regulated pollutant and one of the key components determining the oxidizing capacity of the atmosphere. Obtaining a reliable record of atmospheric CO mixing ratios ([CO]) since preindustrial times is necessary to evaluate climate–chemistry models under conditions different from today and to constrain past CO sources. We present high-resolution measurements of CO mixing ratios from ice cores drilled at five different sites on the Greenland ice sheet that experience a range of snow accumulation rates, mean surface temperatures, and different chemical compositions. An optical-feedback cavity-enhanced absorption spectrometer (OF-CEAS) was coupled with continuous melter systems and operated during four analytical campaigns conducted between 2013 and 2019. Overall, continuous flow analysis (CFA) of CO was carried out on over 700 m of ice. The CFA-based CO measurements exhibit excellent external precision (ranging from 3.3 to 6.6 ppbv, 1σ) and achieve consistently low blanks (ranging from 4.1±1.2 to 12.6±4.4 ppbv), enabling paleoatmospheric interpretations. However, the five CO records all exhibit variability that is too large and rapid to reflect past atmospheric mixing ratio changes. Complementary tests conducted on discrete ice samples demonstrate that these variations are not artifacts of the analytical method (i.e., production of CO from organics in the ice during melting) but are very likely related to in situ CO production within the ice before analysis. Evaluation of the signal resolution and co-investigation of high-resolution records of CO and total organic carbon (TOC) suggest that past atmospheric CO variations can be extracted from the records' baselines with accumulation rates higher than 20 cm w.e.yr-1 (water equivalent per year). Consistent baseline CO records from four Greenland sites are combined to produce a multisite average ice core reconstruction of past atmospheric CO for the Northern Hemisphere high latitudes, covering the period from 1700 to 1957 CE. Such a reconstruction should be taken as an upper bound of past atmospheric CO abundance. From 1700 to 1875 CE, the record reveals stable or slightly increasing values in the 100–115 ppbv range. From 1875 to 1957 CE, the record indicates a monotonic increase from 114±4 to 147±6 ppbv. The ice core multisite CO record exhibits an excellent overlap with the atmospheric CO record from Greenland firn air which spans the 1950–2010 CE time period. The combined ice core and firn air CO history, spanning 1700–2010 CE, provides useful constraints for future model studies of atmospheric changes since the preindustrial period.

Deglaciations are characterized by the largest natural changes in methane (CH4) and nitrous oxide (N2O) concentrations of the past 800 000 years. Reconstructions of millennial- to centennial-scale variability within these periods are mostly restricted to the last deglaciation. In this study, we present composite records of CH4 and N2O concentrations from the EPICA Dome C ice core covering the penultimate deglaciation at temporal resolutions of ∼100 years. Our data permit the identification of centennial-scale fluctuations during the transition from glacial to interglacial levels. At ∼134 000 and ∼129 000 years before present (hereafter ka), both CH4 and N2O increased on centennial timescales. These abrupt rises are similar to the fluctuations associated with the Dansgaard–Oeschger events identified in the last glacial period. In addition, gradually rising N2O levels at ∼130 ka resemble a pattern of increasing N2O concentrations on millennial timescales characterizing the later part of Heinrich stadials. Overall, the events in CH4 and N2O during the penultimate deglaciation exhibit modes of variability that are also found during the last deglaciation and glacial cycle, suggesting that the processes leading to changes in emission during the transitions were similar but their timing differed.

Water-stable isotopes in polar ice cores are a widely used temperature proxy in paleoclimate reconstruction, yet calibration remains challenging in East Antarctica. Here, we reconstruct the magnitude and spatial pattern of Last Glacial Maximum surface cooling in Antarctica using borehole thermometry and firn properties in seven ice cores. West Antarctic sites cooled ~10°C relative to the preindustrial period. East Antarctic sites show a range from ~4° to ~7°C cooling, which is consistent with the results of global climate models when the effects of topographic changes indicated with ice core air-content data are included, but less than those indicated with the use of water-stable isotopes calibrated against modern spatial gradients. An altered Antarctic temperature inversion during the glacial reconciles our estimates with water-isotope observations.

With global change and its amplified impact at high altitudes or in certain regions of the world, mountain glaciers are particularly sensitive to warming. These same glaciers, which have been studied for several decades, have made it possible to reconstruct, through the study of ice cores, unique information on the evolution of the climate or the environment on a regional and global scale since they are located closer to the main regions and sources of aerosol emissions. Unfortunately, these archives are in the process of being altered and are disappearing. It is in this context that the international project ICE MEMORY was initiated in 2015. ICE MEMORY is built on four pillars : 1) Identify, select glacier and extract several complete ice cores, at least two, from sites that have already demonstrated their high scientific potential before they are altered, 2) Analyse one of the cores in order to extract the maximum parameters of information using all currently available technologies and make this data available to the scientific community of today and tomorrow, 3) Store the remaining cores in a naturally adapted site such as the French-Italien Concordia station in Antarctica so that they can be preserved and donated to future generations of scientists, and 4) to set up a sustainable governance system based on an accredited international organization in charge of managing these ice and data archives in the future. This presentation will highlight all the operations, analyses and organization already achieved as well as the future vision and development of ICE MEMORY.

AbstractUsing significant technological breakthroughs and unconventional approaches, the goal of the in situ probing of glacier ice for a better understanding of the orbital response of climate (SUBGLACIOR) project is to advance ice core research by inventing, constructing and testing an in situ probe to evaluate if a target site is suitable for recovering ice as old as 1.5 million years. Embedding a laser spectrometer, the probe is intended to make its own way down into the ice and to measure, in real time and down to the bedrock, the depth profiles of the ice δD water isotopes as well as the trapped CH4 gas concentration and dust concentration. The probe descent is achieved through electromechanical drilling combined with continuous meltwater sample production using a central melting finger in the drill head. A key aspect of the project lies in the design and implementation of an efficient method to continuously transfer to the surface the ice chips being produced by the drill head and from the refreezed water expulsed downstream from the melting finger, into the borehole. This paper presents a detailed calculation and analysis of the flow rates and pressure conditions required to overcome friction losses of the drilling fluid and to effectively transport ice chips to the surface.

Using new and previously published CO2 data from the EPICA Dome C ice core (EDC), we reconstruct a new high-resolution record of atmospheric CO2 during Marine Isotope Stage (MIS) 6 (190 to 135 ka) the penultimate glacial period. Similar to the last glacial cycle, where high-resolution data already exists, our record shows that during longer North Atlantic (NA) stadials, millennial CO2 variations during MIS 6 are clearly coincident with the bipolar seesaw signal in the Antarctic temperature record. However, during one short stadial in the NA, atmospheric CO2 variation is small (∼5 ppm) and the relationship between temperature variations in EDC and atmospheric CO2 is unclear. The magnitude of CO2 increase during Carbon Dioxide Maxima (CDM) is closely related to the NA stadial duration in both MIS 6 and MIS 3 (60–27 ka). This observation implies that during the last two glacials the overall bipolar seesaw coupling of climate and atmospheric CO2 operated similarly. In addition, similar to the last glacial period, CDM during the earliest MIS 6 show different lags with respect to the corresponding abrupt CH4 rises, the latter reflecting rapid warming in the Northern Hemisphere (NH). During MIS 6i at around 181.5±0.3 ka, CDM 6i lags the abrupt warming in the NH by only 240±320 years. However, during CDM 6iv (171.1±0.2 ka) and CDM 6iii (175.4±0.4 ka) the lag is much longer: 1290±540 years on average. We speculate that the size of this lag may be related to a larger expansion of carbon-rich, southern-sourced waters into the Northern Hemisphere in MIS 6, providing a larger carbon reservoir that requires more time to be depleted.

Pulse-like carbon dioxide release to the atmosphere on centennial time scales has only been identified for the most recent glacial and deglacial periods and is thought to be absent during warmer climate conditions. Here, we present a high-resolution carbon dioxide record from 330,000 to 450,000 years before present, revealing pronounced carbon dioxide jumps (CDJ) under cold and warm climate conditions. CDJ come in two varieties that we attribute to invigoration or weakening of the Atlantic meridional overturning circulation (AMOC) and associated northward and southward shifts of the intertropical convergence zone, respectively. We find that CDJ are pervasive features of the carbon cycle that can occur during interglacial climate conditions if land ice masses are sufficiently extended to be able to disturb the AMOC by freshwater input.

We measured the methane mixing ratios of enclosed air in five ice core sections drilled on the East Antarctic Plateau. Our work aims to study two effects that alter the recorded gas concentrations in ice cores: layered gas trapping artifacts and firn smoothing. Layered gas trapping artifacts are due to the heterogeneous nature of polar firn, where some strata might close early and trap abnormally old gases that appear as spurious values during measurements. The smoothing is due to the combined effects of diffusive mixing in the firn and the progressive closure of bubbles at the bottom of the firn. Consequently, the gases trapped in a given ice layer span a distribution of ages. This means that the gas concentration in an ice layer is the average value over a certain period of time, which removes the fast variability from the record. Here, we focus on the study of East Antarctic Plateau ice cores, as these low-accumulation ice cores are particularly affected by both layering and smoothing. We use high-resolution methane data to test a simple trapping model reproducing the layered gas trapping artifacts for different accumulation conditions typical of the East Antarctic Plateau. We also use the high-resolution methane measurements to estimate the gas age distributions of the enclosed air in the five newly measured ice core sections. It appears that for accumulations below 2 cm ice equivalent yr−1 the gas records experience nearly the same degree of smoothing. We therefore propose to use a single gas age distribution to represent the firn smoothing observed in the glacial ice cores of the East Antarctic Plateau. Finally, we used the layered gas trapping model and the estimation of glacial firn smoothing to quantify their potential impacts on a hypothetical 1.5-million-year-old ice core from the East Antarctic Plateau. Our results indicate that layering artifacts are no longer individually resolved in the case of very thinned ice near the bedrock. They nonetheless contribute to slight biases of the measured signal (less than 10 ppbv and 0.5 ppmv in the case of methane using our currently established continuous CH4 analysis and carbon dioxide, respectively). However, these biases are small compared to the dampening experienced by the record due to firn smoothing.

We report the first high-resolution continuous profile of dissolved methane in the shallow water of Lake Kivu, Rwanda. The measurements were performed using an in situ dissolved gas sensor, called Sub-Ocean, based on a patented membrane-based extraction technique coupled with a highly sensitive optical spectrometer. The sensor was originally designed for ocean settings, but both the spectrometer and the extraction system were modified to extend the dynamical range up to 6 orders of magnitude with respect to the original prototype (from nmol L−1 to mmol L−1 detection) to fit the range of concentrations at Lake Kivu. The accuracy of the instrument was estimated to ±22 % (2σ) from the standard deviation of eight profiles at 80 m depth, corresponding to ±0.112 mbar of CH4 in water or ±160 nmol L−1 at 25 ∘C and 1 atm. The instrument was able to continuously profile the top 150 m of the water column within only 25 min. The maximum observed mixing ratio of CH4 in the gas phase concentration was 77 %, which at 150 m depth and under thermal conditions of the lake corresponds to 3.5 mmol L−1. Deeper down, dissolved CH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results are in good agreement with discrete in situ measurements conducted with the commercial HydroC® sensor. This fast-profiling feature is highly useful for studying the transport, production and consumption of CH4 and other dissolved gases in aquatic systems. While the sensor is well adapted for investigating most environments with a concentration of CH4 up to a few millimoles per liter, in the future the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved CO2 and total dissolved gas pressure, for exploring settings with very high concentrations of CH4 such as the bottom waters of Lake Kivu.

Central Antarctic Plateau sites display a strong contrast in deep firn gas ages with relatively high accumulation sites (South Pole, EPICA DML) showing very old (about a century) gas ages in the open porosity of deep firn on one side, and very young (few decades) gas ages and an absence of deep firn δ15N plateau (indicative of remaining gas transport) at low accumulation rate sites (Dome C, Dome F, Vostok) on the other side. Multi-tracer results from an intermediate accumulation site named "Lock-in" will be presented. At this fairly low accumulation rate site (~3.6 cm water equivalent / year), very old air ages were obtained in deep firn but the lock-in zone looks narrower than at South Pole. Analytical results, as well as gas transport and densification modelling results will be discussed in terms of variability of gas-trapping characteristics on the central Antarctic Plateau and degree of understanding of the underlying mechanisms.

Nature has been continuously sampling the atmosphere at the surface of Antarctica throughout the ages. Atmospheric gases trapped in Antarctic ice provide the most direct record of changes in greenhouse gas levels during the past 800,000 years. The best-documented and reliable trace-gas records are for CO2 and CH4, and Antarctic ice is the key player in recording past atmospheric CO2. They are archives of the past and a window to the present and future of the interplay between greenhouse gases and climate. We discuss the pioneering work of glaciologists measuring CO2 in the air extracted from Antarctic ice, which confirmed Arrhenius' prediction about the role of atmospheric carbon dioxide in ice age climate. We detail here how the ice core record has been progressively extended to four and then to eight glacial-interglacial cycles (i.e., over the last 800,000 years). The Antarctic ice record highlights the tight coupling of atmospheric CO2 and Antarctic climate on the timescales of glacial-interglacial cycles for the entire 800,000-year interval. This close linkage suggests that glacial-interglacial variations of CO2 explain a large fraction of glacial-interglacial climate changes observed in the Antarctic ice record, which is consistent with modeling results. We present more recent works showing a near synchronous phasing between Antarctic temperature and CO2 during the last deglaciation and pinpointing the important role of oceanic circulation in both heat transport and CO2 outgassing. We also briefly explore the prospect for investigating Antarctic ice older than 1 million years to document what is often called the enigma of the Mid-Pleistocene Transition around 1 million years ago.

Simultaneous measurement of C2H6 and CH4 concentrations, and of the δ13C-CH4 isotope ratio is demonstrated using a cavity-enhanced absorption spectroscopy technique in the mid-IR region. The spectrometer is compact and has been designed for field operation. It relies on optical-feedback-assisted injection of 3.3 µm radiation from an interband cascade laser (ICL) into a V-shaped high-finesse optical cavity. A minimum absorption coefficient of 2.8×10-9 cm−1 is obtained in a single scan (0.1 s) over 0.7 cm−1. Precisions of 3 ppbv, 11 ppbv, and 0.08 ‰ for C2H6, CH4, and δ13C-CH4, respectively, are achieved after 400 s of integration time. Laboratory calibrations and tests of performance are reported here. They show the potential for the spectrometer to be embedded in a sensor probe for in situ measurements in ocean waters, which could have important applications for the understanding of the source and fate of hydrocarbons from the seabed and in the water column.