Methane release from open leads and new ice following an Arctic winter storm event

We examine an Arctic winter storm event, which led to ice break–up, the formation of open leads, and the subsequent freezing of these leads. The methane (CH4) concentration in under–ice surface water before and during the storm event was 8–12 nmol L−1, which resulted in a potential sea–to–air CH4 flux ranging from +0.2 to +2.1 mg CH4 m−2 d−1 in open leads. CH4 ventilation between seawater and atmosphere occurred when both open water fraction and wind speed increased. Over the nine days after the storm, sea ice grew 27 cm thick. Initially, CH4 concentrations in the sea ice brine were above the equilibrium with the atmosphere. As the ice grew thicker, most of the CH4 was lost from upper layers of sea ice into the atmosphere, implying continued CH4 evasion after the leads were ice–covered. This suggests that wintertime CH4 emissions need to be better constrainedMethane release from open leads and new ice following an Arctic winter storm eventacceptedVersio

Methane release from open leads and new ice following an Arctic winter storm event

We examine an Arctic winter storm event, which led to ice break–up, the formation of open leads, and the subsequent freezing of these leads. The methane (CH4) concentration in under–ice surface water before and during the storm event was 8–12 nmol L−1, which resulted in a potential sea–to–air CH4 flux ranging from +0.2 to +2.1 mg CH4 m−2 d−1 in open leads. CH4 ventilation between seawater and atmosphere occurred when both open water fraction and wind speed increased. Over the nine days after the storm, sea ice grew 27 cm thick. Initially, CH4 concentrations in the sea ice brine were above the equilibrium with the atmosphere. As the ice grew thicker, most of the CH4 was lost from upper layers of sea ice into the atmosphere, implying continued CH4 evasion after the leads were ice–covered. This suggests that wintertime CH4 emissions need to be better constrained

The impact of dissolved organic carbon and bacterial respiration on pCO2 in experimental sea ice

Previous observations have shown that the partial pressure of carbon dioxide (pCO2) in sea ice brines is generally higher in Arctic sea ice compared to those from the Antarctic sea ice, especially in winter and early spring. We hypothesized that these differences result from the higher dissolved organic carbon (DOC) content in Arctic seawater: Higher concentrations of DOC in seawater would be reflected in a greater DOC incorporation into sea ice, enhancing bacterial respiration, which in turn would increase the pCO2 in the ice. To verify this hypothesis, we performed an experiment using two series of mesocosms: one was filled with seawater (SW) and the other one with seawater with an addition of filtered humic-rich river water (SWR). The addition of river water increased the DOC concentration of the water from a median of 142 µmol Lwater-1 in SW to 249 µmol Lwater-1 in SWR. Sea ice was grown in these mesocosms under the same physical conditions over 19 days. Microalgae and protists were absent, and only bacterial activity has been detected. We measured the DOC concentration, bacterial respiration, total alkalinity and pCO2 in sea ice and the underlying seawater, and we calculated the changes in dissolved inorganic carbon (DIC) in both media. We found that bacterial respiration in ice was higher in SWR: median bacterial respiration was 25 nmol C Lice-1 h-1 compared to 10 nmol C Lice-1 h-1 in SW. pCO2 in ice was also higher in SWR with a median of 430 ppm compared to 356 ppm in SW. However, the differences in pCO2 were larger within the ice interiors than at the surfaces or the bottom layers of the ice, where exchanges at the air–ice and ice–water interfaces might have reduced the differences. In addition, we used a model to simulate the differences of pCO2 and DIC based on bacterial respiration. The model simulations support the experimental findings and further suggest that bacterial growth efficiency in the ice might approach 0.15 and 0.2. It is thus credible that the higher pCO2 in Arctic sea ice brines compared with those from the Antarctic sea ice were due to an elevated bacterial respiration, sustained by higher riverine DOC loads. These conclusions should hold for locations and time frames when bacterial activity is relatively dominant compared to algal activity, considering our experimental conditions

Air-ice carbon pathways inferred from a sea ice tank experiment

Air-ice CO2 fluxes were measured continuously using automated chambers from the initial freezing of a sea ice cover until its decay. Cooling seawater prior to sea ice formation acted as a sink for atmospheric CO2, but as soon as the first ice crystals started to form, sea ice turned to a source of CO2, which lasted throughout the whole ice growth phase. Once ice decay was initiated by warming the atmosphere, the sea ice shifted back again to a sink of CO2. Direct measurements of outward ice-atmosphere CO2 fluxes were consistent with the depletion of dissolved inorganic carbon in the upper half of sea ice. Combining measured air-ice CO2 fluxes with the partial pressure of CO2 in sea ice, we determined strongly different gas transfer coefficients of CO2 at the air-ice interface between the growth and the decay phases (from 2.5 to 0.4 mol m−2 d−1 atm−1). A 1D sea ice carbon cycle model including gas physics and carbon biogeochemistry was used in various configurations in order to interpret the observations. All model simulations correctly predicted the sign of the air-ice flux. By contrast, the amplitude of the flux was much more variable between the different simulations. In none of the simulations was the dissolved gas pathway strong enough to explain the large fluxes during ice growth. This pathway weakness is due to an intrinsic limitation of ice-air fluxes of dissolved CO2 by the slow transport of dissolved inorganic carbon in the ice. The best means we found to explain the high air-ice carbon fluxes during ice growth is an intense yet uncertain gas bubble efflux, requiring sufficient bubble nucleation and upwards rise. We therefore call for further investigation of gas bubble nucleation and transport in sea ice

Determination of air‐sea ice transfer coefficient for CO2: Significant contribution of gas bubble transport during sea ice growth

Air‐ice CO2 fluxes were measured continuously from the freezing of a young sea‐ice cover until its decay. Cooling seawater was as a sink for atmospheric CO2 but asthe ice crystalsformed,sea ice shifted to a source releasing CO2 to the atmosphere throughout the whole ice growth. Atmospheric warming initiated the decay, re‐shifting sea‐ice to a CO2 sink. Combining these CO2 fluxes with the partial pressure of CO2 within sea ice, we determined gas transfer coefficients for CO2 at air‐ice interface for growth and decay. We hypothesize that this difference originates from the transport of gas bubbles during ice growth, while only diffusion occurs during ice melt. In parallel, we used a 1D biogeochemical model to mimic the observed CO2 fluxes. The formation of gas bubbles was crucial to reproduce fluxes during ice growth where gas bubbles may account for up to 92 % of the upward CO2 fluxes

The future of Arctic sea-ice biogeochemistry and ice-associated ecosystems

The Arctic sea-ice-scape is rapidly transforming. Increasing light penetration will initiate earlier seasonal primary production. This earlier growing season may be accompanied by an increase in ice algae and phytoplankton biomass, augmenting the emission of dimethylsulfide and capture of carbon dioxide. Secondary production may also increase on the shelves, although the loss of sea ice exacerbates the demise of sea-ice fauna, endemic fish and megafauna. Sea-ice loss may also deliver more methane to the atmosphere, but warmer ice may release fewer halogens, resulting in fewer ozone depletion events. The net changes in carbon drawdown are still highly uncertain. Despite large uncertainties in these assessments, we expect disruptive changes that warrant intensified long-term observations and modelling efforts

Dynamiques du CO2 et du N2O dans le système océan - glace de mer - atmosphère

Greenhouse gases such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are well known to be indirectly responsible for many changes in the sea ice cover in the polar regions, as these regions are sensitive to global warming. The objective of this manuscript is to look at the two climatic gases addressed (CO2 and N2O) and their behaviour in the ocean – sea ice – atmosphere system in the actual warming climate, thus more specifically in the Arctic. On the one hand, the dynamic of CO2 has been studied through artificial sea ice during an experiment performed on two series of mesocosms: one was filled with seawater (SW) and the other one with seawater with added filtered humic-rich river water (SWR). The addition of river water almost doubled the dissolved organic carbon (DOC) concentration in SWR and consequently affected the partial pressure of carbon dioxide (pCO2). This experiment supports previous observations showing that the pCO2 in sea ice brines is generally higher in Arctic sea ice compared to that from the Southern Ocean, especially in winter and early spring. Indeed, DOC is larger in the Arctic seawater: higher concentrations of DOC would be reflected in a greater DOC incorporation in sea ice, enhancing bacterial respiration, which in turn would increase the pCO2 in the ice. Within the same experiment, air–ice CO2 fluxes were measured continuously using automated chambers from the initial freezing of a sea ice cover until its decay. Cooling seawater prior to sea ice formation acted as a sink for atmospheric CO2, but as soon as the first ice crystals started to form, sea ice turned to a source of CO2, which lasted throughout the whole ice growth phase. Once ice decay was initiated, sea ice shifted back again to a sink of CO2. Combining measured air–ice CO2 fluxes with the pCO2 in the air and sea ice, we determined two strongly different gas transfer coefficients of CO2 at the air–ice interface between the growth and the decay phases (2.5 mol m−2 d−1 atm−1 and 0.4 mol m−2 d−1 atm−1 respectively). In the other hand, we present in this work the first winter to spring N2O observations in the sea ice and the upper ocean of the western Nansen Basin (Arctic Ocean). In the seawater, a general N2O undersaturation with respect to the atmosphere was identified at the surface, with a clear enrichment north of 82°N originating from the water masses passing through the Arctic polar circle. We show that the main source of N2O enrichment originates from the East Siberian Arctic Shelf, a key place for benthic denitrification and sea ice formation rejecting brine – including N2O – in the seawater. Sea ice shows N2O oversaturation throughout winter and spring in both first-year (FYI) and second-year ice (SYI). Further, SYI, with expected lower salinity than FYI, is enriched in N2O compared to the dilution curve for salinity. This non-conservative N2O content of SYI with salinity is likely due to (i) SYI formation further east than FYI, (ii) in situ biological activity, (iii) flushing of N2O-rich ice surface meltwater (i.e. brine skim) through decaying FYI and (iv) strongly reduced permeability in SYI. Finally, we suggest that the high N2O concentrations observed in the snow cover results from a combination of brine rejection at the top of the ice (brine skim) and chemical N2O production (under the process of chemodenitrification).CO2 and N2O dynamics in the ocean - sea ice - atmosphere syste

CO2 and N2O dynamics in the ocean-sea ice-atmosphere system

La glace de mer, principalement située dans les régions polaires, est un milieu sensible au réchauffement climatique, et de manière accentuée en Arctique où la réduction significative de l’étendue et de l’épaisseur de la glace de mer sont actuellement en cours. Ce réchauffement est causé par des gaz à effet de serre comme le dioxyde de carbone (CO2), le méthane (CH4) et l’oxyde nitreux (N2O), dont les concentrations augmentent avec l’industrialisation. Ces mêmes gaz sont par ailleurs incorporés dans les saumures liquides et les poches gazeuses de la glace de mer, et par conséquent, leurs concentrations se trouvent affectées par les fluctuations biogéochimiques de la glace de mer. L’objectif de cette thèse est d’étudier la dynamique de deux de ces gaz biologiquement actifs – le CO2 et le N2O – au sein de la glace de mer et aux interfaces avec l’océan et l’atmosphère.La dynamique du CO2 a été étudiée lors d’une expérience sur de la glace de mer artificielle présentant deux types de mésocosmes :l’un rempli avec de l’eau de mer (SW), l’autre rempli avec un mélange d’eau de mer et de rivière (SWR). L’addition d’eau de rivière a presque doublé la concentration en carbone organique dissous (DOC) dans le SWR, affectant la pression partielle en CO2 (pCO2). Cette expérience confirme d’autres études montrant que la pCO2 mesurée dans les saumures de la glace de mer est plus élevée en Arctique qu’en Antarctique. En effet, l’Océan Arctique a un contenu en DOC plus important ;une plus grande concentration en DOC dans l’eau de mer mène à une plus grande incorporation de DOC dans la glace de mer lors de sa formation, renforçant la respiration bactérienne qui, en retour, augmente la pCO2 dans la glace. Lors de cette même expérience, des mesures en continu de flux de CO2 air–glace ont été réalisées, de la formation à la fonte de la glace de mer. Le refroidissement de l’eau de mer a d’abord agi comme un puits de CO2 pour l’atmosphère, une situation qui s’est inversée lors de la formation des premiers cristaux de glace, devenant alors une source de CO2 pour l’atmosphère durant toute la période de croissance de la glace. Enfin, lors de la fonte de la glace, celle-ci s’est repositionnée en puits de CO2 envers l’atmosphère. En combinant les flux air–glace de CO2 avec la pCO2 mesurée dans l’air et dans la glace de mer, deux coefficients de transfert de gaz distincts ont été déterminés ;K = 2.5 mol m−2 d−1 atm−1 pour la phase de croissance et K = 0.4 mol m−2 d−1 atm−1 pour la phase de fonte. Quant à la dynamique du N2O, celle-ci a été étudiée à travers un set de données innovant de mesures de N2O réalisées sur une période de six mois dans la glace et l’eau de mer de l’Océan Arctique, à l’ouest du Basin de Nansen. Une sous-saturation générale est observée à la surface de l’océan par rapport à l’atmosphère, majoritairement due à l’origine Atlantique de ces masses d’eaux. Cependant un enrichissement en N2O est observé au nord de 82°N, également dû à l’origine de ces masses d’eaux, qui elles proviennent du plateau arctique de la Sibérie orientale, un lieu intense de dénitrification benthique et de formation de glace de mer, rejetant d’importantes quantités de sels et de gaz dans l’eau sous-jacente. Les mesures dans la glace de mer montrent une sursaturation tout au long de l’étude, dans les deux types de glace étudiées ;celle de première année (FYI) et celle de seconde année (SYI). Cette dernière présente des salinités plus faibles dues au lessivage des saumures qui a lieu en fin du premier cycle de croissance, cependant les concentrations en N2O sont similaires à celles de la FYI. Ce comportement non conservatif par rapport à la salinité peut être dû :(i) à la formation plus à l’est de la SYI, (ii) à de la potentielle activité biologique, (iii) au lessivage de la surface de la glace enrichie en N2O (iv) à la faible perméabilité de la SYI. Enfin, il est suggéré que les fortes concentrations en N2O rencontrées à la surface de la glace sont dues au rejet des saumures vers la surface, combiné à un processus chimique de production de N2O.Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

Assessing the O2 budget under sea ice: An experimental and modelling approach

The objective of this study was to assess the O2 budget in the water under sea ice combining observations and modelling. Modelling was used to discriminate between physical processes, gas-specific transport (i.e., ice-atmosphere gas fluxes and gas bubble buoyancy) and bacterial respiration (BR) and to constrain bacterial growth efficiency (BGE). A module describing the changes of the under-ice water properties, due to brine rejection and temperature-dependent BR, was implemented in the one-dimensional halo-thermodynamic sea ice model LIM1D. Our results show that BR was the dominant biogeochemical driver of O2 concentration in the water under ice (in a system without primary producers), followed by gas specific transport. The model suggests that the actual contribution of BR and gas specific transport to the change in seawater O2 concentration was 37% during ice growth and 48% during melt. BGE in the water under sea ice, as retrieved from the simulated O2 budget, was found to be between 0.4 and 0.5, which is in line with published BGE values for cold marine waters. Given the importance of BR to seawater O2 in the present study, it can be assumed that bacteria contribute substantially to organic matter consumption and gas fluxes in ice-covered polar oceans. In addition, we propose a parameterization of polar marine bacterial respiration, based on the strong temperature dependence of bacterial respiration and the high growth efficiency observed here, for further biogeochemical ocean modelling applications, such as regional or large-scale Earth System model